Hypoplastic left heart syndrome is a congenital heart defect present at birth in which the left-sided heart structures—including the left ventricle, mitral valve, aortic valve, and ascending aorta—are severely underdeveloped, rendering the heart unable to effectively pump oxygenated blood to the body and requiring intervention in the neonatal period.

Hypoplastic Left Heart Syndrome (HLHS) — Paediatric Cardiology

Hypoplastic Left Heart Syndrome (HLHS) is a spectrum of congenital heart defects characterised by severe underdevelopment of the left-sided cardiac structures, rendering the left ventricle incapable of supporting the systemic circulation ^[1]^[2].

Let's break down the name:

"Hypoplastic" (Greek: hypo = under, plassein = to form) → underdeveloped/undersized
"Left Heart" → refers to the left atrium, mitral valve, left ventricle, aortic valve, and ascending aorta
"Syndrome" → a constellation of related defects occurring together

The key concept: the left ventricle is so small and dysfunctional that it cannot pump blood to the body. The baby is therefore entirely dependent on the right ventricle to supply both the pulmonary AND systemic circulations — a situation that is only possible as long as the ductus arteriosus (DA) remains open and there is an atrial communication (ASD/PFO) to allow pulmonary venous blood to reach the right side.

Although classified as cyanotic CHD, its clinical picture is dominated by severe LVOT obstruction ^[1]. This is a crucial exam point — HLHS presents primarily with heart failure and shock rather than deep cyanosis like Tetralogy of Fallot.

Parameter	Detail
Incidence	2–3 per 10,000 live births ^[1]
Proportion of all CHD	2–3% of all congenital heart disease ^[1]
Sex ratio	Male predominance (~60% M, ~40% F)
Contribution to neonatal cardiac deaths	Most common cause of cardiac death in the first week of life if untreated
Recurrence risk	~2–4% in siblings; higher with affected parent
Hong Kong context	Incidence similar to worldwide figures; increasingly detected on antenatal ultrasound with improving fetal echocardiography screening in Hong Kong public hospitals (QEH, PYNEH, PWH)

High Yield Exam Point

HLHS accounts for ~25% of all neonatal cardiac deaths in the first week of life. Despite being only 2–3% of all CHD, it carries disproportionate mortality because the systemic circulation is entirely duct-dependent.

Risk Factors

Genetic/chromosomal: Associated with Turner syndrome (45,X), Jacobsen syndrome (11q deletion), and trisomy 18; familial clustering with other left-sided obstructive lesions (bicuspid aortic valve, coarctation of aorta) — suggesting a shared genetic basis for "left-sided obstructive lesion spectrum"
Maternal factors: First-trimester viral illness, poorly controlled diabetes mellitus, exposure to certain teratogens
Flow-related hypothesis: Reduced flow through the developing left heart in utero (e.g., from a small foramen ovale) may cause progressive hypoplasia — the "no flow, no grow" principle
Family history: ~8–10% of first-degree relatives have bicuspid aortic valve or other left-sided heart defects, even if subclinical

Anatomy and Normal Function (Understanding What Goes Wrong)

To understand HLHS, you must first understand fetal circulation:

In the fetus, the placenta (not the lungs) is the gas exchange organ → pulmonary vascular resistance (PVR) is very high → most RV output bypasses the lungs
Two key shunts exist:
- Foramen ovale (FO): Oxygenated blood from IVC (via ductus venosus) crosses from RA → LA → LV → ascending aorta → brain and coronary arteries
- Ductus arteriosus (DA): RV output → pulmonary artery → DA → descending aorta → placenta and lower body
At birth: first breath → ↓PVR → ↑pulmonary blood flow → ↑LA pressure → FO closes; ↑PaO₂ + ↓circulating PGE₂ (placenta removed) → DA constricts (10–15 hours, complete closure by 2–3 weeks) ^[3]

Structure	Normal Function
Left atrium (LA)	Receives oxygenated blood from 4 pulmonary veins
Mitral valve (MV)	Allows blood from LA → LV in diastole; prevents backflow in systole
Left ventricle (LV)	Thick-walled pump that generates systemic pressure (120 mmHg in adults)
Aortic valve (AV)	Allows LV → aorta ejection; prevents backflow in diastole
Ascending aorta	First portion of aorta; gives rise to coronary arteries and then brachiocephalic vessels

In HLHS, all of these structures are variably hypoplastic or atretic, making the left heart a non-functional circuit.

Aetiology

HLHS is not a single defect but a spectrum of left-sided underdevelopment. The severity varies, but the common theme is that the left ventricle cannot support systemic output.

Pathology: characterised by underdevelopment of left heart structures ^[1]:

Structure	Abnormality in HLHS
Mitral valve	May be stenotic or atretic ^[1]
Left ventricle	Extremely small, slit-like cavity ^[1] — often < 1 cm in long axis; thick, non-compliant walls with endocardial fibroelastosis
Aortic valve	Usually atretic ^[1] — sometimes severely stenotic
Great arteries	Diminutive ascending aorta (often 2–3 mm diameter) with dilated pulmonary trunk ^[1]

The most critical distinction is the status of the mitral valve and aortic valve:

Subtype	Mitral Valve	Aortic Valve	Prognosis
Mitral atresia / Aortic atresia (MA/AA)	Atretic	Atretic	Variable — coronary perfusion via ventriculo-coronary connections possible
Mitral stenosis / Aortic atresia (MS/AA)	Stenotic (small but patent)	Atretic	Worst prognosis — coronary sinusoids may be present with risk of coronary steal
Mitral stenosis / Aortic stenosis (MS/AS)	Stenotic	Stenotic	Best relative prognosis — some forward flow through LV
Mitral atresia / Aortic stenosis (MA/AS)	Atretic	Stenotic	Rare

Why MS/AA is the Worst Subtype

When the mitral valve is stenotic (allowing some blood into the LV) but the aortic valve is atretic (no forward exit), the LV pressurises in systole with no way out. Blood is forced retrograde through ventriculo-coronary connections (sinusoidal communications). This creates coronary steal where blood flows FROM the coronary arteries INTO the LV in diastole, causing myocardial ischaemia. This explains higher operative mortality.

Intact or restrictive atrial septum (~6% of cases) — a surgical emergency (see pathophysiology below)
Coarctation of the aorta (commonly coexists, given the shared "left-sided obstruction" spectrum)
Endocardial fibroelastosis of the LV
Anomalous pulmonary venous drainage (rare but devastating)

Syndrome/Chromosomal	Association
Turner syndrome (45,X)	Left-sided obstructive lesions including HLHS, CoA, bicuspid AV
Jacobsen syndrome (11q23 deletion)	HLHS is present in ~10%
Trisomy 18 (Edwards syndrome)	Various CHD including HLHS
Familial inheritance	Polygenic; genes involved include NOTCH1, NKX2-5, GJA1 — mutations in NOTCH1 are particularly associated with aortic valve calcification/bicuspid AV and HLHS

Most cases are sporadic without identifiable chromosomal abnormality
Increasingly detected on routine anomaly scan at 18–22 weeks in Hong Kong (the four-chamber view screening picks up HLHS because the LV is tiny or absent)
Hong Kong's universal antenatal screening programme has improved fetal detection, allowing for planned delivery at a tertiary centre (e.g., Queen Mary Hospital, Prince of Wales Hospital) with immediate postnatal management
Some families opt for termination of pregnancy after counselling given the complex surgical pathway and long-term morbidity

Pathophysiology

This is the most important section to understand. Everything in HLHS flows from understanding the haemodynamic consequences of a non-functional left ventricle.

The left ventricle cannot generate systemic cardiac output. Therefore, survival depends on two things:

An atrial communication (ASD or PFO) to allow pulmonary venous blood to reach the right atrium

A patent ductus arteriosus to allow the right ventricle to supply the systemic circulation

Detailed Haemodynamic Explanation

Pulmonary venous blood (oxygenated) returns to the LA
The mitral valve is stenotic/atretic → blood cannot pass into the LV
The only exit for this blood is across the atrial septum (PFO or ASD) into the RA
This is an obligatory shunt — without it, there is no cardiac output at all
The blood in the RA is now a mix of systemic venous return (deoxygenated) and pulmonary venous return (oxygenated) → this explains the mild-to-moderate cyanosis (SpO₂ typically 75–85%)

Restrictive/Intact Atrial Septum — Emergency!

If the atrial septum is intact or highly restrictive (~6% of HLHS), pulmonary venous blood has NO escape route. This causes severe pulmonary venous congestion, pulmonary oedema, and profound hypoxia from birth. These neonates present with immediate cyanosis and respiratory distress at birth and require emergent balloon atrial septostomy (Rashkind procedure) or surgical septectomy within hours. This scenario has the highest mortality in HLHS.

The RV receives all the blood (both pulmonary and systemic venous return + the L-to-R shunt across the ASD)
The RV ejects into the pulmonary artery (PA)
From the PA, blood flows in two directions:
- Forward into the pulmonary arteries → lungs (pulmonary circulation)
- Through the PDA → descending aorta → systemic circulation

Because the aortic valve is atretic, there is no antegrade flow in the ascending aorta
The ascending aorta is perfused retrogradely — blood flows from the PDA → aortic isthmus → backwards up the aortic arch → brachiocephalic vessels (brain) and coronary ostia (heart)
The ascending aorta is diminutive (often only 2–3 mm) and functions merely as a conduit for retrograde coronary and cerebral perfusion
This retrograde flow is entirely dependent on PDA patency and adequate pressure gradient

This is the clinical catastrophe:

As the PDA constricts (usually within hours to days after birth):
- Systemic perfusion drops → hypotension, shock, metabolic acidosis
- Coronary perfusion drops → myocardial ischaemia → further pump failure
- Cerebral perfusion drops → lethargy, seizures
- Renal perfusion drops → oliguria, rising creatinine
- Mesenteric perfusion drops → NEC risk
Without intervention, death occurs rapidly (within days)

The ratio of pulmonary to systemic blood flow (Qp:Qs) is critical:

Qp:Qs	Clinical Consequence
Too high (> 2:1)	Excessive pulmonary blood flow → pulmonary overcirculation → pulmonary oedema + systemic steal (inadequate systemic perfusion despite high total cardiac output)
Balanced (~1:1)	Optimal — adequate systemic and pulmonary perfusion
Too low (< 0.5:1)	Inadequate pulmonary blood flow → severe cyanosis

In the immediate neonatal period, PVR is still relatively high → Qp:Qs is somewhat balanced
As PVR falls naturally over the first days-weeks, Qp:Qs rises → pulmonary overcirculation and systemic underperfusion
This is why managing PVR (avoiding supplemental O₂, maintaining mild hypercapnia) is crucial in preoperative management

Why Too Much Oxygen is Dangerous in HLHS

Supplemental oxygen is a pulmonary vasodilator — it lowers PVR, causing more blood to flow to the lungs (↑Qp) at the expense of systemic perfusion (↓Qs). This seemingly paradoxical situation means that giving oxygen to a cyanotic HLHS baby can worsen shock and cause death by systemic steal. Target SpO₂ is 75–85%, NOT 95–100%. This is one of the most counter-intuitive but critical concepts in neonatal cardiology.

Classification

HLHS is classified as:

Cyanotic CHD (due to mixing of oxygenated and deoxygenated blood in the RA)
Duct-dependent systemic circulation (the systemic blood flow depends entirely on the PDA)
Single-ventricle physiology (only the RV is functional — the ultimate management pathway is single-ventricle palliation)

Type	Examples	Mechanism
Duct-dependent systemic circulation	HLHS, critical coarctation, interrupted aortic arch, critical aortic stenosis	PDA provides systemic blood flow
Duct-dependent pulmonary circulation	Critical pulmonary stenosis, pulmonary atresia, severe Tetralogy of Fallot, tricuspid atresia	PDA provides pulmonary blood flow
Duct-dependent mixing	Transposition of great arteries (d-TGA)	PDA allows mixing between parallel circulations

HLHS exists on a spectrum with other left-sided obstructive lesions:

Condition	Severity	Duct-Dependent?
Bicuspid aortic valve	Mildest	Usually no
Aortic stenosis (mild-moderate)	Mild-moderate	No
Critical aortic stenosis	Severe	Yes (duct-dependent systemic)
Coarctation of aorta	Severe when critical	Yes (duct-dependent systemic) ^[4]
Shone complex (multiple left-sided obstructions)	Severe	Variable
HLHS	Most severe	Yes (duct-dependent systemic)

Clinical Features

Timing: present earlier at birth if ASD restrictive/intact, later with ductus closure if ASD wider ^[1]

Scenario	Timing	Mechanism
Intact/restrictive atrial septum	Minutes after birth	Pulmonary venous blood has no escape → immediate pulmonary congestion and severe cyanosis
Adequate ASD + open PDA	First few hours to days	May appear deceptively well initially; deterioration coincides with PDA closure
Large ASD + open PDA	Days 2–7	Longer "honeymoon period" before ductal closure triggers collapse

The 'Well-Looking Newborn' Trap

HLHS babies may look deceptively well in the first hours of life while the ductus is still open. They may pass the initial newborn examination. This is why pulse oximetry screening (measuring pre-ductal and post-ductal saturations at 24–48 hours) is critical for detecting duct-dependent lesions before clinical deterioration. In Hong Kong, universal pulse oximetry screening is implemented in public hospital neonatal units.

Symptom	Pathophysiological Basis
Cyanosis	Due to R-to-L shunting via ductus arteriosus ^[1] — mixing of oxygenated and deoxygenated blood in the RA means systemic arterial blood is desaturated (SpO₂ typically 75–85%)
Early onset heart failure with shock ≤1 week	Duct-dependent systemic circulation due to severe LVOT obstruction ^[1] — as PDA narrows, systemic perfusion falls; the RV, which was never designed for sustained systemic pressures, fails
Poor feeding / feeding intolerance	Heart failure → increased metabolic demand + poor perfusion of GI tract → fatigue during feeds, diaphoresis with feeding
Tachypnoea / respiratory distress	Pulmonary overcirculation (as PVR falls) → pulmonary congestion; or pulmonary venous congestion (if ASD restrictive) → ↑work of breathing
Lethargy / reduced activity	Poor cerebral perfusion from low systemic cardiac output → encephalopathy; metabolic acidosis from tissue hypoperfusion → CNS depression
Oliguria / anuria	Poor renal perfusion → pre-renal failure; one of the earliest signs of ductal closure
Grunting	Sign of respiratory distress — grunting maintains positive end-expiratory pressure to keep alveoli open in the setting of pulmonary congestion

Sign	Pathophysiological Basis
Cyanosis: degree dependent on Qp:Qs ^[1]	Mild-moderate cyanosis reflects the ratio of pulmonary to systemic blood flow. More pulmonary flow = more oxygenated blood mixing in RA = less cyanosis, but potentially worse systemic perfusion
Heart failure with shock: with ductus closure ^[1]	As PDA closes, there is acute loss of systemic perfusion → cardiogenic shock with cool peripheries, mottling, prolonged capillary refill time ( > 3 seconds), tachycardia, hypotension
Weak peripheral pulses: depend on ductus size ^[1]	Systemic arterial flow comes through the PDA — as it narrows, pulse volume drops. All four limb pulses will be weak (unlike CoA where upper limb pulses are preserved) ^[4]
RV impulse: as RV supports systemic and pulmonary circulation ^[1]	The RV is doing double duty → volume AND pressure loaded → parasternal heave (left parasternal lift). The apex may be RV-type (tapping) rather than the expected LV-type
Single, loud S2: due to aortic atresia (absent A2) + pulmonary hypertension (loud P2) ^[1]	Normally S2 = A2 + P2 with physiological splitting. In HLHS, there is no A2 (no functioning aortic valve) → only P2 is heard → single S2. P2 is loud because the PA pressure is elevated (RV generates systemic pressures)
Ejection click: due to dilation of pulmonary trunk ^[1]	The massively dilated PA trunk (receiving all cardiac output) creates an audible early systolic ejection click, heard best at the upper left sternal border
No or non-specific heart murmur	e.g., ESM from pulmonary flow murmur, PSM of TR ^[1] — This is a crucial point: HLHS typically has NO dramatic murmur. There is no VSD creating turbulence, no stenotic valve with high-velocity jet across it. There may be a soft ejection systolic murmur from increased flow across the pulmonary valve, or a pansystolic murmur of tricuspid regurgitation (from RV dilation). The absence of a murmur in a sick neonate does NOT exclude critical CHD
Hepatomegaly	RV failure → ↑systemic venous pressure → hepatic congestion. The liver edge may be palpable 3–4 cm below the right costal margin
Gallop rhythm (S3)	Volume-overloaded RV → rapid early diastolic filling → audible S3
Mottled / grey appearance	Poor systemic perfusion → peripheral vasoconstriction → mottling of skin; grey colour indicates a combination of cyanosis + poor perfusion (worse prognostic sign than pure cyanosis)
Metabolic acidosis (clinical correlate: deep/rapid breathing)	Tissue hypoperfusion → anaerobic metabolism → lactic acidosis → compensatory Kussmaul-type breathing (deep, rapid)

Key Clinical Pearl — No Murmur ≠ No Heart Disease

One of the most dangerous misconceptions is that all significant CHD produces a loud murmur. HLHS is the classic example of critical CHD with no significant murmur. A grey, shocked neonate with a single S2, weak pulses, hepatomegaly, and metabolic acidosis should prompt immediate consideration of duct-dependent CHD — even in the absence of a murmur.

When assessing a neonate with suspected HLHS, a systematic approach is essential:

General inspection: Colour (cyanosis? mottling? grey?), respiratory distress (tachypnoea, grunting, nasal flaring, subcostal recession), activity level (lethargic?)
Pulses: Palpate all four limbs — brachial and femoral
- In HLHS: all pulses weak (duct-dependent systemic)
- Compare with CoA: upper limb pulses strong, lower limb pulses weak
Four-limb pulse oximetry: Pre-ductal (right hand) and post-ductal (either foot)
- In HLHS: both may be low (75–85%) with variable differential
- The hyperoxia test (administering 100% O₂ and checking if PaO₂ rises above 150 mmHg) helps differentiate cardiac from pulmonary cyanosis — in HLHS, PaO₂ will NOT significantly rise (positive hyperoxia test = failure to rise = cardiac cause)
Precordium: RV impulse (heave), single loud S2, listen for murmurs and added sounds
Abdomen: Hepatomegaly
Perfusion: CRT, skin temperature, urine output

Feature	HLHS (Duct-Dependent Systemic)	Critical PS/PA (Duct-Dependent Pulmonary)	d-TGA (Duct-Dependent Mixing)
Predominant feature	Shock + mild cyanosis	Severe cyanosis + relatively well perfused	Severe cyanosis + relatively well perfused
Timing	Days 1–7 (with ductal closure)	Hours after birth	Hours after birth
Pulses	Weak all four limbs	Normal or bounding	Normal
S2	Single, loud	Single (absent P2) or soft P2	Single, loud (anterior aorta)
Murmur	Absent/non-specific	Continuous murmur of PDA	Usually absent
CXR	Cardiomegaly, ↑ pulmonary markings	Oligaemic lung fields	Egg-on-string, narrow mediastinum
Response to PGE₁	Improves systemic perfusion	Improves oxygenation	Improves mixing

CXR findings in HLHS ^[1]:

Cardiomegaly with dilated RA/RV
↑ pulmonary vascular markings from ↑ flow (from ASD) or pulmonary venous congestion (if ASD restrictive)

ECG ^[1]:

Right axis deviation
Right atrial enlargement
Right ventricular hypertrophy (dominant R waves in V1, deep S waves in V5–V6)
Absence of LV forces (no normal R wave progression in left precordial leads)
This ECG pattern makes sense: the RV is the only functioning ventricle and is doing all the work

Echocardiography: Definitive diagnostic modality

Small/absent LV cavity
Atretic or severely stenotic mitral and aortic valves
Hypoplastic ascending aorta
Dilated PA trunk
Assess ASD size (restrictive vs. adequate)
Assess PDA size and flow direction
Assess tricuspid valve function (TR → poor prognosis)
Assess RV function

HLHS is detectable on the 18–22 week anomaly scan (four-chamber view) — the LV appears small or absent
Fetal diagnosis allows:
- Parental counselling (including option of termination in some jurisdictions)
- Planned delivery at a tertiary cardiac centre
- Immediate postnatal PGE₁ infusion
- Better outcomes compared to postnatal diagnosis
In Hong Kong, fetal echocardiography is available at QMH, PWH, and PYNEH with paediatric cardiology input

High Yield Summary

Definition: HLHS = underdevelopment of left heart structures (MV, LV, AV, ascending aorta) → LV cannot support systemic circulation.

Epidemiology: 2–3/10,000 live births; 2–3% of CHD; most common cause of cardiac death in first week of life.

Key Pathophysiology:

Obligate L-to-R shunt via ASD/PFO (pulmonary venous blood must cross to right side)
Duct-dependent systemic circulation (RV → PA → PDA → aorta)
Retrograde flow in ascending aorta supplies coronaries and brain
PDA closure = cardiovascular collapse and death

Clinical Features:

Shock with mild cyanosis (not deep cyanosis — shock dominates)
Early onset HF ≤1 week with ductal closure
Weak pulses in all four limbs
Single loud S2 (absent A2, loud P2)
Ejection click (dilated PA)
No or non-specific murmur — critical CHD without a dramatic murmur!
RV impulse (RV doing all the work)

Critical Concepts:

Intact/restrictive atrial septum = emergency (requires urgent septostomy)
Supplemental O₂ can worsen systemic perfusion (pulmonary vasodilation → ↑Qp → systemic steal)
Target SpO₂ is 75–85%, NOT 95–100%
Immediate PGE₁ infusion is life-saving

Active Recall - HLHS: Definition, Epidemiology, Pathophysiology, and Clinical Features

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.228 — HLHS section) ^[2] Lecture slides: GC 147. Heart failure and cyanosis in children acyanotic and cyanotic congenital heart disease - Part 2.pdf ^[3] Senior notes: Ryan Ho Cardiology.pdf (p.189 — Patent Ductus Arteriosus section, fetal circulation review) ^[4] Senior notes: Ryan Ho Cardiology.pdf (p.190 — Coarctation of Aorta section) ^[5] Lecture slides: GC 147. Heart failure and cyanosis in children acyanotic and cyanotic congenital heart disease - Part 1.pdf ^[6] Senior notes: Block A - Inherited Cardiac conditions.pdf ^[7] Lecture slides: Block C - Heart failure and cyanosis in children acyanotic and cyanotic congenital heart disease.pdf ^[8] AOS material: AOS - Paeds.pdf (Question 6 — cyanotic CHD vs respiratory cause)

Differential Diagnosis of Hypoplastic Left Heart Syndrome (HLHS)

Before we build a differential, let's think about how HLHS actually presents — because the DDx list depends on which clinical presentation brings the neonate to attention. As established in the prior section, HLHS can present in three principal ways ^[1]:

Cyanosis in the newborn (mild-to-moderate, SpO₂ 75–85%)
Heart failure with cardiogenic shock (typically days 2–7, coinciding with ductal closure)
Severe respiratory distress from birth (if atrial septum intact/restrictive → pulmonary venous congestion)

The differential diagnosis therefore encompasses conditions that mimic one or more of these presentations in the neonatal period. We will systematically work through:

A. Cardiac causes of neonatal cyanosis
B. Cardiac causes of neonatal shock (duct-dependent systemic lesions)
C. Non-cardiac mimics (respiratory, infective, metabolic)

A. Cardiac Differential Diagnosis — Cyanotic Congenital Heart Disease

This is where HLHS sits nosologically. The classic teaching framework categorises cyanotic CHD by the underlying haemodynamic physiology ^[9]^[10]:

Cyanotic CHD can be categorised according to the underlying physiology: (1) ↓ Pulmonary blood flow, (2) ↑ Pulmonary blood flow, (3) Severe heart failure ^[9]^[10]

These conditions have reduced flow to the lungs → less oxygenated blood returns to the left heart → cyanosis. On CXR: oligaemic lung fields ^[8].

Condition	Ductal Dependence	Key Differentiating Features from HLHS
Tetralogy of Fallot (TOF)	√/X (only if severe RVOT obstruction) ^[9]	Boot-shaped heart on CXR; right aortic arch in 25%; cyanotic spells (tet spells) with squatting in older infants; harsh ESM at LUSB from RVOT obstruction; RVH on ECG. NOT associated with shock — the LV works fine
TOF with pulmonary atresia	√ ^[9]	Continuous murmur of PDA or MAPCAs; severe cyanosis from birth; no pulmonary flow without PDA
Pulmonary atresia with intact ventricular septum (PA/IVS)	√ ^[9]	Small RV (cf. HLHS which has small LV); cyanosis without shock; duct-dependent pulmonary circulation (not systemic)
Critical pulmonary stenosis	√ (only in critical PS) ^[9]	Harsh ESM at LUSB with post-stenotic dilatation of PA; duct-dependent pulmonary circulation; responds to balloon valvuloplasty
Tricuspid atresia	√/X (only if severe RVOT obstruction) ^[9]	Left axis deviation (unusual for cyanotic CHD — a classic exam clue); small RV; obligate R-to-L shunt at ASD level
Ebstein's anomaly	√/X (only if severe RVOT obstruction) ^[9]	Massive RA dilation → "wall-to-wall" heart on CXR; apical displacement of tricuspid valve; WPW pattern on ECG (delta waves); SVT common

Why these differ from HLHS: In all of these, the right-sided outflow is obstructed (reduced pulmonary blood flow), and the ductus is needed to supply the lungs. In HLHS, the left-sided outflow is obstructed, and the ductus is needed to supply the body. The clinical distinction is that HLHS babies develop shock (systemic underperfusion), whereas duct-dependent pulmonary lesions develop deepening cyanosis without initial shock.

These conditions have increased or normal pulmonary flow but abnormal mixing of oxygenated and deoxygenated blood → cyanosis despite adequate (or excessive) pulmonary blood flow. On CXR: plethoric lung fields.

Condition	Ductal Dependence	Key Differentiating Features from HLHS
Transposition of great arteries (d-TGA)	√ ^[9]	The most important DDx in a cyanotic neonate. Deep cyanosis within hours of birth; no shock initially (both ventricles work); "egg-on-string" appearance on CXR with narrow mediastinum; needs PDA and ASD for mixing (duct-dependent mixing, not systemic circulation). The baby is cyanotic but well-perfused — the key difference from HLHS
Truncus arteriosus	X (no mixing required) ^[9]	Single arterial trunk overriding both ventricles; VSD always present; presents with HF + mild cyanosis at 2–4 weeks (as PVR falls); truncal ejection click; a/w DiGeorge syndrome (22q11 deletion)
Total anomalous pulmonary venous return (TAPVR)	X (no mixing required) ^[9]	All PVs drain to systemic venous system; obligate R-to-L shunt through ASD; obstructed TAPVR presents with severe cyanosis + pulmonary oedema from birth (can mimic HLHS with restrictive ASD); unobstructed TAPVR presents with mild cyanosis + HF later; "snowman" or "figure-of-eight" sign on CXR in supracardiac type

HLHS vs d-TGA: The Two Most Important Cyanotic Neonatal Emergencies

Both are duct-dependent and present in the first days of life. The crucial clinical distinction:

d-TGA: Deep cyanosis, good perfusion, normal/strong pulses, no acidosis initially
HLHS: Mild-moderate cyanosis, poor perfusion/shock, weak pulses in all four limbs, metabolic acidosis

Both require urgent PGE₁, but the definitive management differs entirely (arterial switch for TGA vs staged single-ventricle palliation for HLHS).

This is the category where HLHS truly belongs. These are the conditions where the systemic circulation depends on the PDA — ductal closure causes shock ^[9]^[10].

Condition	Ductal Dependence	Key Differentiating Features from HLHS
HLHS	√ ^[9]	Index condition — slit-like LV, atretic AV/MV, diminutive ascending aorta; all pulses weak; single loud S2; no murmur
Critical coarctation of aorta (CoA)	√ (only in critical coarctation) ^[9]	Upper limb pulses present/strong but lower limb pulses weak/absent ^[4]^[11] — the key differentiating sign from HLHS (where ALL four limb pulses are weak). 4-limb BP shows UL > LL gradient ^[11]. LV impulse (not RV). ESM at LUSB radiating to back. A/w bicuspid AV, Turner syndrome. Ascending aorta is of normal size
Interrupted aortic arch (IAA)	√ ^[9]	Complete discontinuity of aortic arch (cf. CoA which is narrowing); strongly a/w DiGeorge syndrome (22q11.2 deletion) — look for absent thymic shadow on CXR, hypocalcaemia; differential cyanosis (pink upper body, blue lower body); presents like severe CoA but typically more critical
Critical aortic stenosis (AS)	√ (only in critical AS) ^[9]	Severe stenosis of an otherwise present aortic valve; LV is present and may be hypertrophied (unlike HLHS where LV is slit-like); harsh ESM at RUSB with radiation to carotids; ejection click; may respond to balloon valvuloplasty in some cases. The ascending aorta is of normal calibre

High Yield — Differentiating the Duct-Dependent Systemic Lesions

The key bedside manoeuvre is four-limb pulse and BP assessment ^[11]:

Finding	Diagnosis Suggested
All four limb pulses weak	HLHS, critical AS, or IAA
Upper limb pulses strong, lower limb pulses weak/absent with radiofemoral delay	Critical coarctation of aorta
Right arm pulse stronger than left arm + both legs weak	CoA proximal to left subclavian or IAA type B
Differential cyanosis (pink right hand, blue feet)	CoA or IAA with PDA supplying descending aorta

A cyanotic, shocked neonate does not always have CHD. The differential must include non-cardiac causes, which can often coexist with or mimic HLHS ^[11].

DDx of cyanosis in neonates ^[11]:

Category	Conditions	How to Differentiate from HLHS
Respiratory disorders	Surfactant deficiency (RDS), meconium aspiration, pulmonary hypoplasia ^[11]	Tachypnoea and respiratory distress are prominent; hyperoxia test positive (PaO₂ rises > 150 mmHg on 100% O₂) unlike cardiac cyanosis; CXR shows lung parenchymal disease (ground-glass, air bronchograms, meconium aspiration pattern); responds to CPAP/surfactant; pulses are normal
Persistent pulmonary hypertension of the newborn (PPHN)	PPHN ^[11]	R-to-L shunting through PDA and/or FO; pre-post ductal SpO₂ difference > 10% (right hand higher than foot); echocardiography shows structurally normal heart with elevated PA pressures and TR jet; responds to inhaled NO; may coexist with CDH or MAS
Infection	Group B Streptococcal septicaemia ^[11]	Risk factors: maternal GBS colonisation, prolonged ROM, prematurity; presents with temperature instability, poor feeding, respiratory distress, metabolic acidosis; raised CRP/procalcitonin; positive blood culture; may cause secondary myocardial dysfunction mimicking cardiogenic shock
Metabolic	Inborn errors of metabolism (IEM) → metabolic acidosis and shock ^[11]	Severe metabolic acidosis with high anion gap; may have hyperammonaemia; poor feeding, lethargy, seizures; structurally normal heart on echo (though secondary myocardial dysfunction possible); positive newborn metabolic screen
Congenital diaphragmatic hernia (CDH)	Left-sided Bochdalek hernia (90%) ^[12]	Respiratory distress, mediastinal shift, absent breath sounds on ipsilateral chest, scaphoid abdomen ^[12]; CXR diagnostic (bowel in thorax); may cause PPHN and mimic cyanotic CHD

The Hyperoxia Test — Distinguishing Cardiac from Pulmonary Cyanosis

The hyperoxia (nitrogen washout) test is the critical bedside tool:

Administer 100% FiO₂ for 10 minutes
Measure right radial artery PaO₂ (pre-ductal)
PaO₂ > 150 mmHg → likely respiratory cause (pulmonary disease can be overcome by high FiO₂)
PaO₂ < 100 mmHg → likely cardiac cause (fixed intracardiac shunt cannot be overcome by supplemental O₂)
PPHN may give intermediate results (100–150 mmHg) and pre-post ductal difference

A radiograph showing an oligaemic lung field due to reduced pulmonary blood flow SUPPORTS cyanotic CHD rather than respiratory disease ^[8] — this was tested in the AOS past paper.

For completeness (and for the echo-literate student), some conditions share features with HLHS on initial assessment:

Condition	Similarity to HLHS	Key Distinction
Shone complex	Multiple left-sided obstructive lesions (supramitral ring, parachute MV, subaortic stenosis, CoA)	LV is present and may be of reasonable size; obstruction is at multiple levels but the LV is not slit-like
Unbalanced AVSD (left-dominant or right-dominant)	Right-dominant AVSD → small LV, similar physiology to HLHS	AVSD anatomy with common AV valve; strongly a/w Down syndrome; primum ASD component
Critical aortic stenosis with evolving LV hypoplasia	May progress to HLHS physiology in utero	On fetal echo, there may be a borderline LV that is "on the cusp" — serial fetal echocardiography determines whether biventricular repair is feasible or single-ventricle pathway is needed

D. Differential Diagnosis by Clinical Scenario

DDx	Key Features
d-TGA	Deep cyanosis, good perfusion, egg-on-string CXR
HLHS (early, PDA still open)	Mild cyanosis, may appear well initially; single S2, weak pulses may already be present
TAPVR (unobstructed)	Mild cyanosis, RV impulse, fixed split S2
Tricuspid atresia	Left axis deviation on ECG
Ebstein's anomaly	Massive cardiomegaly on CXR, WPW on ECG

Cardiogenic shock occurring at Day 4–7 after delivery usually occurs in ductal-dependent congenital heart disease ^[11].

DDx	Key Differentiating Feature
HLHS	All four limb pulses weak; single loud S2; no murmur; slit-like LV on echo
Severe CoA ^[11]	UL pulses present, LL pulses absent; 4-limb BP ^[11]; LV impulse
Interrupted aortic arch ^[11]	DiGeorge features; absent thymic shadow; differential cyanosis
Critical AS	LV present but hypertrophied; harsh ESM at RUSB; ejection click
Neonatal sepsis (GBS)	Temperature instability; risk factors; raised inflammatory markers; normal heart on echo
IEM	High anion gap acidosis; hyperammonaemia; normal heart on echo

DDx	Key Features
HLHS with intact/restrictive atrial septum	Immediate severe cyanosis + pulmonary congestion; echo shows small LV and restrictive ASD
Obstructed TAPVR	Similar presentation (pulmonary oedema + cyanosis); echo shows no direct PV drainage to LA; often "ground-glass" appearance on CXR mimicking RDS
PPHN	Structurally normal heart; pre-post ductal SpO₂ difference; responds to inhaled NO

Feature	HLHS	Critical CoA	Critical AS	IAA	d-TGA
Primary presentation	Shock + mild cyanosis	Shock + LL ischaemia	Shock	Shock	Deep cyanosis
Upper limb pulses	Weak	Strong	Weak	Variable*	Normal
Lower limb pulses	Weak	Absent/weak	Weak	Absent	Normal
S2	Single, loud	Normal/single	Paradoxically split or single	Variable	Single, loud (anterior aorta)
Murmur	Absent/soft	ESM → back	Harsh ESM RUSB	Variable	Usually absent
CXR	Cardiomegaly, plethoric	Cardiomegaly, pulmonary oedema	Cardiomegaly	No thymic shadow	Egg-on-string, narrow mediastinum
ECG	RVH, RAE, absent LV forces	LVH	LVH (strain)	RVH	RVH (RV generates systemic pressure)
Echo	Slit-like LV, atretic MV/AV	Discrete narrowing, LV present	Stenotic AV, LV hypertrophied	Arch discontinuity	Great artery discordance
Genetic assoc.	Turner, Jacobsen	Turner, bicuspid AV	Williams (if supravalvar)	DiGeorge (22q11)	—

*In IAA type B (most common, between left CCA and left SCA): right arm and both carotids have strong pulses; left arm and both legs have weak pulses.

High Yield Summary — Differential Diagnosis of HLHS

Key teaching points for exams:

HLHS presents primarily with shock + mild cyanosis, not deep cyanosis ^[1] — this distinguishes it from conditions like d-TGA (deep cyanosis, good perfusion) and duct-dependent pulmonary lesions (deep cyanosis, no shock)
Cyanotic CHD is categorised by: ↓ pulmonary blood flow, ↑ pulmonary blood flow, or severe heart failure ^[9]^[10] — HLHS falls in the "severe heart failure" category
Four-limb pulse assessment is the critical bedside differentiator ^[4]^[11]:
- All pulses weak → HLHS, critical AS, IAA
- UL strong / LL weak → critical CoA
An oligaemic lung field on CXR supports cyanotic CHD over respiratory disease ^[8]
Cardiogenic shock at Day 4–7 = think duct-dependent systemic lesion ^[11] — HLHS, critical CoA, IAA, critical AS
The hyperoxia test differentiates cardiac from pulmonary cyanosis — PaO₂ remains < 100 mmHg in cardiac causes
Echocardiography is definitive — identifies the slit-like LV, atretic valves, and diminutive ascending aorta of HLHS

Active Recall - Differential Diagnosis of HLHS

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.228 — HLHS section) ^[4] Senior notes: Ryan Ho Cardiology.pdf (p.190 — Coarctation of Aorta section) ^[8] AOS material: AOS - Paeds.pdf (Question 6 — cyanotic CHD vs respiratory cause) ^[9] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.280 — Cyanotic CHD classification) ^[10] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.593 — Cyanotic CHD classification) ^[11] Senior notes: Maksim Paediatric Notes.pdf (p.55–56 — DDx of cyanosis and cardiogenic shock in neonates) ^[12] Senior notes: Maksim Surgery Notes.pdf (p.332 — Congenital diaphragmatic hernia) ^[13] Senior notes: Ryan Ho Cardiology.pdf (p.185 — Syndromes associated with CHD)

Diagnostic Criteria, Algorithm and Investigations for Hypoplastic Left Heart Syndrome (HLHS)

Diagnostic Criteria

Unlike many medical conditions with formalised diagnostic criteria (e.g., Jones criteria for rheumatic fever, Framingham criteria for heart failure), HLHS does not have a consensus diagnostic scoring system. The diagnosis is anatomical and echocardiographic — you see it and name it. Nevertheless, we can define the diagnostic criteria by the structural features that must be present.

HLHS is diagnosed when echocardiography demonstrates a constellation of underdeveloped left heart structures ^[1] that renders the left ventricle incapable of supporting the systemic circulation:

Criterion	Description	Why It Must Be Present
Hypoplastic LV: extremely small, slit-like cavity ^[1]	LV internal dimension is far below the normal Z-score for gestational/postnatal age (typically Z-score < −5); LV is non-apex-forming	The LV cannot generate meaningful stroke volume
Hypoplastic MV: may be stenotic or atretic ^[1]	Mitral valve annulus Z-score far below normal; valve is either completely absent (atretic) or severely narrowed (stenotic)	Prevents or severely limits filling of the LV from the LA
Hypoplastic AV: usually atretic ^[1]	Aortic valve is absent or severely stenotic with minimal forward flow	Prevents or severely limits ejection from the LV
Diminutive ascending aorta with dilated pulmonary trunk ^[1]	Ascending aorta diameter often 2–4 mm (cf. normal ~8–10 mm in neonates); serves merely as a conduit for retrograde coronary/cerebral flow	Ascending aorta cannot carry forward systemic output; PA dilated because it receives the entire cardiac output

Key concept: The diagnosis is made when these structures are so hypoplastic that biventricular repair is not feasible — the left heart cannot be "recruited" to support the systemic circulation. This is what separates HLHS from milder left-sided obstructive lesions (e.g., moderate aortic stenosis with a viable LV).

Beyond the core four criteria, the echocardiographer must also characterise:

Feature	Clinical Importance
Atrial septum status (intact, restrictive, or adequate ASD/PFO)	Present earlier at birth if ASD restrictive/intact ^[1]; restrictive atrial septum = surgical emergency requiring urgent septostomy
Ductus arteriosus patency and size	Ductal-dependent systemic circulation: solely dependent on DA ^[1]; assessment of PDA calibre determines adequacy of systemic perfusion
Tricuspid valve function (degree of TR)	The tricuspid valve is the sole systemic AV valve — significant TR leads to reduced effective forward output and worse surgical prognosis
RV function (qualitative and quantitative)	The RV will become the permanent systemic ventricle — baseline function is critical for surgical planning and prognostication
Aortic arch anatomy (coarctation, hypoplasia of transverse arch)	Commonly coexists; affects surgical reconstruction at Stage I (Norwood)
Coronary artery pattern (ventriculo-coronary connections)	Present in MS/AA variant; coronary steal may cause ischaemia — affects candidacy for transplant vs. Norwood
Pulmonary venous drainage	Anomalous drainage (rare) worsens prognosis

Borderline LV — When Is It HLHS vs. 'Rescuable'?

Some neonates have borderline left heart structures that could potentially support a biventricular repair. Deciding whether to pursue single-ventricle palliation (HLHS pathway) or attempt biventricular repair is one of the most difficult decisions in paediatric cardiology. Published criteria to predict success of biventricular repair include:

Rhodes criteria: LV long-axis Z-score, indexed LV mass, MV annulus Z-score, aortic root size
Congenital Heart Surgery Nomenclature: HLHS requires "aortic and/or mitral atresia or severe hypoplasia with a non-apex-forming LV"

These borderline cases are managed in multidisciplinary team discussions at tertiary centres.

Diagnostic Algorithm

The diagnostic pathway for HLHS depends on when the diagnosis is made: antenatal (fetal echocardiography) or postnatal (symptomatic presentation). We will cover both pathways.

Step-by-Step Postnatal Diagnostic Pathway

Early recognition and emergent stabilisation is critical to ensure optimal outcome in patients with cyanotic congenital heart disease ^[9]^[10].

The neonate may come to attention via:

Universal newborn pulse oximetry screening (performed at 24–48 hours in Hong Kong)
- Screening positive if: SpO₂ < 95% in either limb, OR pre-post ductal difference > 3%
- Why this works: HLHS produces SpO₂ typically 75–85% from RA-level mixing; even with a wide PDA, saturations rarely exceed 90%
Clinical recognition of cardiogenic shock (Day 2–7)
- Cardiogenic shock at Day 4–7 → think duct-dependent lesion ^[11]
- Grey, mottled, tachypnoeic neonate with weak pulses, hepatomegaly, metabolic acidosis
- First presentation may be oliguria (or even anuria) — kidney is first organ affected ^[11]
Incidental clinical finding (less common with modern screening)
- Abnormal S2, cyanosis noted during routine examination

Assessment	Action	Interpretation in HLHS
Four-limb SpO₂	Pre-ductal (right hand) and post-ductal (either foot)	Both low (75–85%); variable differential
Four-limb BP	Measure with age-appropriate cuff	Low systemic BP in all limbs (cf. CoA: UL > LL gradient)
Pulse palpation	Brachial and femoral	Weak peripheral pulses ^[1] in all four limbs
Hyperoxia test	100% FiO₂ for 10 min → right radial arterial blood gas	PaO₂ remains < 100 mmHg (fixed intracardiac shunt)
Blood glucose	Heel-prick or arterial	Rule out hypoglycaemia as contributing factor
Blood gas	Arterial or capillary	Metabolic acidosis (↑lactate from tissue hypoperfusion)

The Hyperoxia Test — Explained from First Principles

Why does PaO₂ not rise in HLHS?

In respiratory disease, the cyanosis is due to V/Q mismatch or alveolar disease. Flooding the alveoli with 100% O₂ overcomes this → PaO₂ rises dramatically ( > 150 mmHg).

In HLHS, the cyanosis is due to obligate mixing of oxygenated and deoxygenated blood in the RA. No matter how much O₂ you deliver to the alveoli, the blood returning from the lungs (now fully saturated) still mixes with deoxygenated systemic venous blood in the RA. The fixed anatomical shunt limits the maximum achievable PaO₂ → typically stays < 100 mmHg.

Moreover, in HLHS, giving 100% O₂ is actually dangerous because it lowers PVR, increasing pulmonary blood flow (Qp) at the expense of systemic flow (Qs) → worsening shock. Use the hyperoxia test diagnostically but discontinue high-flow O₂ once cardiac cause is confirmed.

Echocardiography is the gold standard for diagnosing HLHS. It is the single most important investigation and should be performed urgently whenever duct-dependent CHD is suspected.

CXR, ECG, blood tests — these support the diagnosis and assess end-organ function but do NOT replace echo.

Management: maintain ductal patency by PGE₁ ^[11] — this should be started on clinical suspicion alone, BEFORE the echocardiogram is obtained. Do not wait for confirmatory imaging in a shocked neonate.

Investigation Modalities — Detailed Findings and Interpretation

The single most important investigation. Performs both diagnosis and preoperative assessment.

Echo Finding	Description	Pathophysiological Basis
Small/slit-like LV	LV internal dimension Z-score far below normal (typically < −5); endocardial fibroelastosis may be visible as bright echogenic endocardium	LV has received minimal flow in utero ("no flow, no grow"); endocardial fibroelastosis results from chronic LV wall stress and ischaemia
Atretic/stenotic mitral valve	MV annulus tiny or completely absent; no flow across MV on colour Doppler (if atretic) or severely restricted flow (if stenotic)	MV malformation prevents LA → LV filling
Atretic/stenotic aortic valve	AV not visible or tiny; no antegrade flow in ascending aorta (if atretic); severely restricted flow (if stenotic)	AV malformation prevents LV → aorta ejection
Diminutive ascending aorta ^[1]	Measured at 2–4 mm (cf. normal ~8–10 mm); retrograde flow visible on colour/pulsed-wave Doppler	Ascending aorta supplied by retrograde flow ^[1] from PDA for coronary and cerebral perfusion
Dilated pulmonary trunk ^[1]	PA trunk markedly dilated (receives entire cardiac output)	RV ejects its entire output into PA; all systemic blood must pass through PDA
Atrial septum assessment	2D and colour Doppler: determine if ASD/PFO is adequate (unrestricted L→R flow) or restrictive (high-velocity jet, LA dilation, pulmonary venous congestion)	Restrictive ASD → present earlier at birth ^[1]; requires emergent septostomy
PDA assessment	Size, flow direction, and velocity	Large PDA with R→L (PA to aorta) flow = adequate systemic perfusion; narrowing PDA = impending collapse
Tricuspid valve and RV function	TR jet velocity (estimates RV/PA pressure); RV FAC (fractional area change) or TAPSE	Significant TR reduces effective cardiac output; poor RV function → poor surgical candidate
Aortic arch anatomy	Transverse arch hypoplasia, coarctation, isthmal narrowing	Affects Norwood reconstruction planning
Coronary artery assessment	Ventriculo-coronary connections (sinusoids)	Present in MS/AA variant; coronary steal → myocardial ischaemia; may preclude Norwood and favour transplant

Echo Report Must-Haves in HLHS

When reading or requesting an echo for HLHS, ensure the following are characterised:

MV and AV morphology (stenotic vs. atretic) → determines subtype
LV size and function → confirms non-viability for biventricular repair
Ascending aorta diameter → surgical planning
ASD adequacy → determines need for septostomy
PDA size and flow → determines adequacy of current systemic perfusion
TV function and RV function → prognostic
Aortic arch anatomy → Norwood planning
Coronary anatomy → sinusoidal connections?

CXR findings in HLHS ^[1]:

Finding	Description	Pathophysiological Basis
Cardiomegaly with dilated RA/RV ^[1]	Cardiothoracic ratio > 0.6 in neonates (normal neonatal CTR up to 0.6; > 0.6 is pathological); right-sided cardiac silhouette prominence	RV is volume AND pressure overloaded (supports both circulations); RA dilated from receiving both systemic venous return AND redirected pulmonary venous return via ASD
↑ Pulmonary vascular markings from ↑ flow (from ASD) ^[1]	Plethoric lung fields — vessels visible to lung periphery	Pulmonary blood flow is increased because there is no RVOT obstruction; blood flows freely from RV → PA → lungs. As PVR falls, pulmonary flow increases further
Pulmonary venous congestion (if ASD restrictive) ^[1]	Upper lobe pulmonary venous distension, perihilar haze, Kerley B lines (rare in neonates), pleural effusions	If the atrial septum is restrictive, oxygenated blood cannot escape from LA → backs up into pulmonary veins → pulmonary oedema. This mimics left heart failure patterns seen in adults
Absent/small aortic knuckle	The normal aortic knuckle on PA CXR is diminished or absent	Diminutive ascending aorta; the aortic arch may be hypoplastic
Dilated PA segment	Prominent PA shadow at left heart border	Dilated pulmonary trunk ^[1] receiving the entire cardiac output

How to differentiate from other conditions on CXR:

CXR Pattern	Condition Suggested
Cardiomegaly + plethoric lungs	HLHS, large VSD, AVSD, d-TGA, truncus arteriosus
Cardiomegaly + oligaemic lungs	TOF, PA/IVS, critical PS, tricuspid atresia
"Egg-on-string" + narrow mediastinum	d-TGA
"Wall-to-wall" heart	Ebstein anomaly
"Snowman" / "figure-of-eight"	Supracardiac TAPVR
"Boot-shaped" heart	TOF
Absent thymic shadow	DiGeorge syndrome (IAA, truncus)

The ECG in HLHS reflects the fact that the RV is the dominant (and essentially only) functioning ventricle ^[1].

ECG Finding	Description	Pathophysiological Basis
Right axis deviation	QRS axis typically +90° to +180° (may be even further rightward)	Normal neonatal axis is right-deviated, but HLHS shows persistent/exaggerated rightward axis due to RV dominance and absent LV forces
Right atrial enlargement (RAE)	Peaked P waves > 3 mm in lead II ("P pulmonale"); tall P in V1	RA receives increased volume (systemic venous + pulmonary venous via ASD); RA dilated and hypertrophied
Right ventricular hypertrophy (RVH)	Tall R in V1 (> 98th centile for age), deep S in V5–V6; upright T in V1 after 48 hours of age (pathological — normally inverted in neonates after Day 2); qR pattern in V1	RV generates systemic pressures (pumping against systemic vascular resistance via PDA) → pressure and volume hypertrophy
Absent or diminished LV forces	Absent R wave progression in left precordial leads (V5, V6); no normal LV dominance developing in the weeks after birth	The LV is slit-like and generates no meaningful electrical activity
ST-T changes	ST depression or T wave inversion in lateral leads (rare)	May indicate myocardial ischaemia from coronary steal (in MS/AA variant with ventriculo-coronary connections)

ECG in Neonates — Age-Appropriate Normal Values

Remember that neonatal ECGs are very different from adult ECGs:

Normal neonatal axis: +90° to +180° (right-deviated)
Dominant R wave in V1 is NORMAL in neonates (RV dominant at birth)
T wave in V1 should be upright at birth, then inverts by Day 2–3 and stays inverted until adolescence
Persistent upright T in V1 after Day 2 = pathological RVH

In HLHS, the ECG findings of RVH may overlap with normal neonatal patterns, making isolated ECG interpretation tricky. The absence of LV forces (no R progression in V5–V6) and RAE are more specific clues.

Blood tests do not diagnose HLHS but are essential for assessing end-organ damage and guiding resuscitation.

Investigation	Expected Findings in HLHS	Clinical Significance
Arterial blood gas (ABG)	Metabolic acidosis: pH < 7.25, ↑ lactate ( > 4 mmol/L), base excess < −5; PaO₂ typically 35–50 mmHg	Reflects tissue hypoperfusion from inadequate systemic cardiac output; lactate is the single most important marker of adequacy of systemic perfusion
Serum lactate	Elevated (often > 4–8 mmol/L)	Rising lactate = worsening systemic perfusion; serial monitoring guides resuscitation. Target < 2 mmol/L after PGE₁
Renal function (Cr, urea, electrolytes)	↑ Creatinine, ↑ urea, ↓ urine output	Kidney is first organ affected ^[11]; pre-renal AKI from poor renal perfusion
Liver function tests	↑ ALT, ↑ AST, ↑ LDH; possibly ↑ INR	Hepatic congestion (RV failure) + ischaemic hepatitis (poor hepatic arterial perfusion)
Complete blood count	Relative polycythaemia in some; may show thrombocytopenia if DIC develops	Neonatal polycythaemia → higher deoxyHb → more visible cyanosis; DIC from shock indicates severe end-organ dysfunction
Coagulation (PT, APTT, fibrinogen)	May be deranged if DIC develops in severe shock	Guide need for FFP, cryoprecipitate before surgical intervention
BNP / NT-proBNP	Markedly elevated	Reflects ventricular wall stress — mainly from the overloaded RV. Useful for serial monitoring but NOT for diagnosis in neonates (BNP is physiologically elevated in the first 48 hours)
Blood glucose	May be low (neonatal hypoglycaemia)	Stressed neonates have high glucose demands; hypoglycaemia worsens neurological outcome
Blood culture	Should be sent if sepsis cannot be excluded	Neonatal sepsis (GBS) is a differential; empirical antibiotics often started alongside PGE₁ until cultures negative
Calcium, magnesium	Hypocalcaemia may be present	Important if DiGeorge syndrome is considered (IAA is a differential); also, sick neonates are prone to hypocalcaemia

Parameter	Detail
Technique	Administer 100% FiO₂ via head box or non-rebreather for 10 minutes; measure right radial arterial PaO₂ (pre-ductal)
Interpretation	PaO₂ > 150 mmHg → pulmonary cause; PaO₂ < 100 mmHg → cardiac cause; 100–150 mmHg → PPHN or mixed
In HLHS	PaO₂ typically 35–50 mmHg and does not rise significantly with 100% FiO₂ (fixed intracardiac shunt)
Caution	Prolonged high FiO₂ is dangerous in HLHS — lowers PVR → ↑Qp → systemic steal → worsening shock. Use the test diagnostically then discontinue high FiO₂

Parameter	Detail
Timing	Routine anomaly scan at 18–22 weeks; fetal echo at specialised centre
Key finding	Abnormal four-chamber view: absent or tiny LV cavity; hypoplastic MV/AV; disproportion between great arteries (large PA, tiny aorta); reversed flow in aortic arch
Detection rate	Four-chamber screening detects ~50–70% of HLHS cases; addition of outflow tract views increases sensitivity to > 90%
Impact	Fetal diagnosis improves outcomes by allowing planned delivery at a tertiary cardiac centre, immediate PGE₁, and avoidance of postnatal collapse. In Hong Kong, fetal cardiac cases are concentrated at QMH, PWH, and PYNEH
Serial monitoring	Some cases of critical AS show progressive LV hypoplasia on serial fetal echo → fetal intervention (balloon aortic valvuloplasty) may be considered to prevent evolution to HLHS

Parameter	Detail
Role	Rarely needed for initial diagnosis (echo is sufficient); main role is interventional
Interventional use	Emergent balloon atrial septostomy (Rashkind procedure) if restrictive/intact atrial septum → creates adequate interatrial communication
Haemodynamic assessment	May be used preoperatively to measure PVR, assess coronary anatomy (ventriculo-coronary connections), and evaluate branch PA anatomy before Stage II or III surgery

Modality	Role in HLHS
Cardiac MRI	Not used in the acute neonatal period; valuable in interstage assessment (between surgical stages) to evaluate RV function, aortic arch anatomy, and branch PA anatomy. Provides accurate volumetric RV data
CT angiography	Used when echo windows are limited or when detailed 3D anatomical reconstruction is needed (e.g., aortic arch, coronary anatomy); faster than MRI (important in unstable patients); involves radiation

Test	Indication
Chromosomal microarray / karyotype	If syndromic features present (e.g., Turner syndrome features — short stature, webbed neck, widely spaced nipples → a/w HLHS) ^[13]
22q11.2 FISH / MLPA	If DiGeorge features present (primarily for IAA/truncus, but may co-occur)
Exome sequencing / gene panels	NOTCH1 mutation screening in familial cases; increasingly offered for prognostication and family planning

Investigation	HLHS	Critical CoA	d-TGA	PA/IVS
CXR	Cardiomegaly, plethoric/congested lungs	Cardiomegaly, pulmonary oedema	Egg-on-string, narrow mediastinum, plethoric	Normal/slight cardiomegaly, oligaemic lungs
ECG	RAE + RVH + absent LV forces	LVH (LV is present and hypertrophied)	RVH (RV at systemic pressure)	RAE + RVH (small RV forces if severe)
Echo	Slit-like LV, atretic MV/AV, tiny ascending aorta	Discrete narrowing at isthmus, LV present	Great artery discordance, parallel circulations	Small RV, atretic PV, intact septum
4-limb SpO₂	Both low (75–85%)	Post-ductal lower than pre-ductal	Both very low (50–70%)	Both low
4-limb BP	Low in all four limbs	UL > LL gradient	Normal or slightly low	Normal
Hyperoxia test	PaO₂ < 100 mmHg	PaO₂ may rise modestly (not a primary cyanotic lesion)	PaO₂ < 100 mmHg	PaO₂ < 100 mmHg

High Yield Summary — Diagnosis of HLHS

Echocardiography is the definitive diagnostic test — identifies slit-like LV, atretic/stenotic MV and AV, diminutive ascending aorta with retrograde flow, and dilated PA trunk
Diagnosis is anatomical: there is no "points-based" scoring system. HLHS = left heart structures too hypoplastic for biventricular repair
Fetal diagnosis is possible at 18–22 weeks on the four-chamber view → improves outcomes by allowing planned delivery and immediate PGE₁
Newborn pulse oximetry screening detects HLHS before clinical deterioration (SpO₂ < 95%)
The hyperoxia test differentiates cardiac from pulmonary cyanosis — PaO₂ remains < 100 mmHg in HLHS
CXR: cardiomegaly with dilated RA/RV; ↑ pulmonary vascular markings or PV congestion ^[1]
ECG: RAD, RAE, RVH, absent LV forces — reflects single-ventricle (RV) physiology
Blood tests assess end-organ damage — lactate is the most important marker of systemic perfusion adequacy
ASD adequacy must be assessed on echo — restrictive/intact atrial septum requires emergent septostomy
Start PGE₁ on clinical suspicion BEFORE confirmatory echo ^[11] — do not delay life-saving treatment for diagnosis

Active Recall - Diagnosis and Investigations in HLHS

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.228 — HLHS section) ^[9] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.280 — Cyanotic CHD classification) ^[10] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.593 — Cyanotic CHD classification) ^[11] Senior notes: Maksim Paediatric Notes.pdf (p.55–56 — DDx of cyanosis and cardiogenic shock in neonates) ^[13] Senior notes: Ryan Ho Cardiology.pdf (p.185 — Syndromes associated with CHD) ^[14] Senior notes: Ryan Ho Cardiology.pdf (p.184 — Congenital Heart Disease classification and fetal circulation)

Management Algorithm and Treatment Modalities for Hypoplastic Left Heart Syndrome (HLHS)

HLHS management is one of the most complex journeys in all of paediatric medicine. The fundamental problem is that the left ventricle cannot be repaired or grown — it is irreversibly hypoplastic. Therefore, the management goal is:

Convert the single functional ventricle (RV) into the permanent systemic pump, and route systemic venous blood passively to the lungs without a subpulmonary ventricle — a Fontan circulation.

This requires three staged surgical procedures over the first 2–4 years of life, each building upon the last. Between and around these surgeries, intensive medical management keeps the baby alive and optimised.

Management: maintain ductal patency to re-perfuse abdominal organs and lower limb by PGE₁ ^[11] — this is the immediate life-saving step.

Phase 1: Immediate Stabilisation (Neonatal Period — Hours to Days)

This is the most critical phase — the baby will die within hours to days without intervention. The goals are:

Maintain ductal patency (PGE₁)
Balance Qp:Qs (avoid excessive pulmonary blood flow)
Support end-organ perfusion (correct acidosis, maintain BP)
Ensure adequate atrial communication (septostomy if needed)
Identify and treat precipitating factors (infection, metabolic derangement)

Parameter	Detail
Drug	Prostaglandin E₁ (alprostadil) — "prostag-landin" = from prostate gland where first isolated; E₁ = specific prostaglandin subtype
Mechanism	Relaxes smooth muscle of the ductus arteriosus wall → maintains/reopens the PDA → allows RV output to reach systemic circulation via PA → PDA → aorta
Dose	Initial: 0.01–0.05 mcg/kg/min IV infusion; can increase to 0.1 mcg/kg/min if no response; once PDA is confirmed open on echo, titrate down to lowest effective dose (typically 0.01–0.02 mcg/kg/min) to minimise side effects
Route	Continuous IV infusion via central or peripheral line (ideally central for reliability); must NEVER be interrupted
Indication	All duct-dependent congenital heart disease — start on clinical suspicion BEFORE echo confirmation ^[11]
Side effects	Apnoea (most important — occurs in ~12%; always have intubation equipment ready), fever, hypotension, flushing, diarrhoea, jitteriness, cortical hyperostosis (with prolonged use > 4 weeks) ^[11]
Monitoring	Continuous SpO₂, cardiorespiratory monitor, arterial line for invasive BP, check ductal patency on serial echo

PGE₁ Side Effects — Paediatric Exam Favourite

The three most commonly tested side effects of PGE₁ are: apnoea, fever, and diarrhoea ^[11]. Apnoea is the most dangerous — always be prepared to intubate. Some centres electively intubate neonates on PGE₁ before transfer to a surgical centre. The mnemonic: "A-F-D" — Apnoea, Fever, Diarrhoea.

This is the most conceptually challenging part of HLHS management. The RV is pumping blood into the PA, and that blood can go in two directions:

To the lungs (pulmonary circulation, Qp)
Through the PDA to the body (systemic circulation, Qs)

The ratio Qp:Qs determines outcomes:

Qp:Qs	Problem	Clinical Manifestation
Too high ( > 2:1)	Pulmonary overcirculation → systemic steal	High SpO₂ ( > 90%), but poor systemic perfusion: acidosis, oliguria, rising lactate — paradoxically the baby looks "pink" but is dying
Balanced (~1:1)	Optimal	SpO₂ 75–85%, adequate systemic perfusion, lactate < 2 mmol/L
Too low ( < 0.5:1)	Inadequate pulmonary flow	Severe cyanosis, SpO₂ < 65%

Strategies to balance Qp:Qs:

Strategy	Mechanism	Practical Application
Avoid supplemental O₂	O₂ is a pulmonary vasodilator → ↓PVR → ↑Qp → systemic steal	Target SpO₂ 75–85%, NOT 95–100% ^[15]; room air or even subambient FiO₂ (0.17–0.19 by mixing N₂) in some ICUs
Allow mild hypercapnia	CO₂ is a pulmonary vasoconstrictor → ↑PVR → ↓Qp → more blood to systemic circulation	Target PaCO₂ 40–50 mmHg; avoid hyperventilation (which would lower PaCO₂ and vasodilate pulmonary bed)
Avoid excessive PEEP	High PEEP compresses pulmonary vasculature → ↑PVR → variable effect on Qp:Qs	Keep PEEP low (3–5 cmH₂O) if intubated
Sedation and paralysis	Reduce metabolic demand → ↓systemic O₂ consumption → better Qp:Qs tolerance	Used selectively in unstable neonates
Inotropic support	Maintain systemic vascular resistance and cardiac output	See below

Why a 'Pink' HLHS Baby is Dangerous

If the SpO₂ is > 90% in a neonate with HLHS, this means Qp:Qs is too high — excessive blood is going to the lungs and not enough to the body. The baby looks well-oxygenated ("pink") but the systemic organs are underperfused. This is pulmonary overcirculation with systemic steal. The lactate will be rising. Counter-intuitively, the correct response may be to reduce FiO₂ or even add nitrogen to the inspired gas. This is one of the most important management principles in single-ventricle physiology.

Intervention	Detail
Volume resuscitation	Judicious — 10 mL/kg NS bolus if hypovolaemic, but avoid excessive fluid (the RV is already volume-overloaded)
Inotropes	Milrinone (phosphodiesterase-3 inhibitor) is the agent of choice: provides inotropy + systemic and pulmonary vasodilation ("inodilator") → ↑cardiac output without excessive ↑SVR; dose: 0.25–0.75 mcg/kg/min IV. Dopamine (5–10 mcg/kg/min) or dobutamine (5–20 mcg/kg/min) as alternatives ^[15]
Correct metabolic acidosis	NaHCO₃ only if severe (pH < 7.1); primary strategy is improve perfusion (PGE₁ + inotropes) → lactate clearance
Blood glucose management	Maintain normoglycaemia (2.6–8 mmol/L in neonates); D10W infusion as needed

Parameter	Detail
Indication	Restrictive or intact atrial septum — occurs in ~6% of HLHS; pulmonary venous blood cannot decompress from LA → severe pulmonary congestion and cyanosis from birth
Procedure	Balloon atrial septostomy (Rashkind procedure): percutaneous catheter passed via umbilical or femoral vein → through FO/PFO → balloon inflated and pulled back across septum → tears open the atrial communication. If septum is too thick, may require blade septostomy or surgical septectomy
Timing	Emergent — within hours of birth
Outcome	Creates an adequate ASD allowing pulmonary venous blood to reach RA → RV → systemic circulation; dramatically improves oxygenation and reduces pulmonary congestion

General supportive measures ^[15]:

Bed rest with elevation of bed head → improve lung function ^[15]
Nutritional support: high caloric diet due to ↑metabolic demand ^[15] — in neonates, this means expressed breast milk or high-calorie formula (often fortified to 24–30 kcal/oz) via nasogastric tube if unable to feed orally
Fluid restriction ^[15] — typically 100–120 mL/kg/day (rather than the usual 150 mL/kg/day for neonates)
O₂: caution in large L-to-R shunt (↑PAO₂ → pulmonary vasodilation → ↓PVR → ↑shunting) ^[15]
Monitor urine output closely (target > 1 mL/kg/hr)
Empirical antibiotics until infection excluded (GBS cover: ampicillin + gentamicin)
Thermoregulation in incubator
Minimal handling to reduce metabolic demand

Phase 2: Definitive Management — Three Options

After stabilisation, the family is counselled by a multidisciplinary team (paediatric cardiologist, cardiac surgeon, cardiac anaesthetist, nursing specialist, social worker, clinical psychologist). Three pathways exist:

This is the most commonly pursued pathway worldwide. It consists of three operations that progressively reconstruct the circulation to achieve a Fontan physiology — where the single RV serves as the systemic pump and systemic venous blood passes passively to the lungs.

Listed below as an alternative; availability severely limited by donor organ scarcity (especially in Hong Kong and Asia).

Some families, after informed counselling, elect not to pursue surgical intervention. Palliative supportive care is provided with dignity.

Phase 2A: Staged Surgical Palliation — The Three Stages

This is the most complex and highest-risk of the three operations. The goals are:

Create an unobstructed systemic outflow from the RV
Ensure adequate pulmonary blood flow (but not too much)
Ensure adequate atrial mixing

Component	Surgical Step	Rationale
Neo-aorta reconstruction	The diminutive ascending aorta and aortic arch are augmented using the main PA trunk + homograft patch material → creates a wide "neo-aorta" arising from the RV	The native ascending aorta (2–3 mm) cannot carry systemic output; the PA trunk (large, as it received all fetal output) is anastomosed to provide a wide systemic outflow; the RV now ejects directly into this neo-aorta
Source of pulmonary blood flow	Either: (a) Modified Blalock-Taussig shunt (mBTS): synthetic Gore-Tex graft from innominate artery → right PA; OR (b) Sano shunt (RV-PA conduit): synthetic graft from RV → PA	Since the main PA has been used for the neo-aorta, the lungs now have no direct blood supply. A shunt must provide pulmonary blood flow. mBTS provides continuous flow (diastolic runoff → risk of coronary steal); Sano provides pulsatile flow (no diastolic runoff → potentially better coronary perfusion but requires RV incision)
Atrial septectomy	Wide excision of atrial septum	Ensures unrestricted flow of pulmonary venous blood from LA → RA → RV
PDA ligation	The PDA is ligated and divided	Once the neo-aorta is constructed, the PDA is no longer needed; leaving it open would cause uncontrolled pulmonary overcirculation

mBTS vs. Sano Shunt — Key Comparison:

Feature	Modified BT Shunt	Sano Shunt
Connection	Innominate artery → right PA	RV → PA conduit
Flow pattern	Continuous (systolic + diastolic)	Pulsatile (systolic only)
Diastolic runoff	Yes → potential coronary steal	No → better diastolic coronary perfusion
RV incision	Not required	Required (ventriculotomy) → concern for long-term RV function
Interstage mortality	Historically slightly higher	Historically slightly lower interstage mortality (SVR trial data)
Shunt thrombosis	Risk → listen for continuous murmur on follow-up; blocked BT shunt can kill ^[3]	Risk but larger conduit

Norwood Outcomes:

Hospital survival: ~70–85% in high-volume centres (2020s data)
This is the highest-risk stage — mortality comes from the complexity of the procedure, Qp:Qs imbalance, coronary ischaemia, and RV dysfunction
Hong Kong outcomes: performed at Grantham Hospital / Queen Mary Hospital; volumes are low given the rarity of HLHS in Chinese populations

HLHS — 3rd rare in Chinese, more common in West ^[16]

Why Does the Norwood Work?

The key insight is that by connecting the PA trunk to the ascending aorta (creating a neo-aorta), the RV now ejects directly into the systemic circulation without needing the PDA. The mBTS or Sano shunt provides a controlled, limited amount of pulmonary blood flow (Qp:Qs ~1:1). This eliminates the dependence on the PDA and provides a more stable haemodynamic state — but it is still a parallel circulation with a single ventricle doing all the work.

This is a high-risk period with ~5–15% interstage mortality. The baby goes home with:

Component	Detail
Home monitoring	Daily SpO₂ and weight measurements by parents; any drop in SpO₂ < 70% or excessive weight gain triggers urgent review
Nutritional support	High-calorie feeds (often 24–30 kcal/oz formula or fortified breast milk); NG tube feeding if oral intake insufficient; target weight gain > 20 g/day
Medications	Aspirin (antiplatelet for shunt patency, typically 3–5 mg/kg/day); digoxin (sometimes used for RV function, though seldom used due to narrow therapeutic index ^[15]); diuretics (furosemide ± spironolactone) for volume overload; ACEI (captopril/enalapril) sometimes used for afterload reduction — though evidence is mixed
Regular follow-up	Weekly or fortnightly paediatric cardiology review; serial echocardiography
Red flags	Increasing cyanosis (shunt narrowing or thrombosis), poor feeding, excessive weight gain, irritability, tachypnoea

Parameter	Detail
Goal	Remove the volume load on the RV by redirecting SVC blood passively to the lungs
Procedure	Bidirectional Glenn (BDG): SVC is disconnected from RA and anastomosed end-to-side to the right PA. The mBTS or Sano shunt is taken down (no longer needed). Alternatively, Hemi-Fontan: SVC-PA connection with a dam placed in the RA to prevent SVC blood from entering RA
Physiology	Upper body venous blood (SVC) → passes directly to PA → lungs (without passing through the heart). Lower body venous blood (IVC) → still returns to RA → RV → neo-aorta → body. This reduces RV volume load (only pumping IVC return + pulmonary venous return)
Why at 4–6 months?	PVR must be low enough for passive flow to occur. At birth, PVR is high → passive SVC-to-PA flow would be inadequate. By 4–6 months, PVR has fallen sufficiently. Also, the baby needs to be large enough for the anastomosis
SpO₂ after Glenn	75–85% (IVC blood still mixes with pulmonary venous return in RA → cyanosis persists)
Mortality	Low (~1–3%)

Parameter	Detail
Goal	Complete separation of systemic and pulmonary circulations by routing IVC blood to the PA as well
Procedure	Total cavopulmonary connection (TCPC): IVC is connected to PA via an intracardiac lateral tunnel or extracardiac conduit (Gore-Tex tube from IVC → PA). A fenestration (small hole in the conduit/tunnel) is often created to allow R-to-L shunting as a "pop-off" valve
Physiology	Now ALL systemic venous blood (SVC + IVC) passes passively to the lungs. The RV only receives pulmonary venous return and pumps it to the body. The circulations are "in series" rather than "in parallel." The patient's SpO₂ improves to ~90–95% (or ~85–90% if fenestrated)
Why the fenestration?	The Fontan circulation depends on low PVR for passive flow. If PVR rises (e.g., during illness), the fenestration allows decompression of the venous system by shunting some blood R-to-L → maintains cardiac output at the expense of mild cyanosis. Many fenestrations close spontaneously or can be closed by catheter device later
Mortality	Low (~1–2%)

Understanding the Fontan Circulation from First Principles

The Fontan circulation is fundamentally different from normal physiology:

Normal: Two pumps in series (RV for pulmonary, LV for systemic)
Fontan: One pump (RV) for systemic only; pulmonary flow is passive, driven by the pressure gradient between systemic venous pressure (~10–15 mmHg) and LA pressure (~5–8 mmHg)

This means:

Fontan patients have chronically elevated systemic venous pressure (needed to drive pulmonary flow)
They are exquisitely sensitive to PVR — any increase (illness, dehydration, altitude) reduces cardiac output
They have limited cardiac reserve during exercise (cannot increase pulmonary flow by increasing pump output like normal hearts)
Long-term complications are inevitable (see Complications section)

Parameter	Detail
Indication	Primary transplantation as alternative to Norwood; or secondary transplantation for failed staged palliation (e.g., failing Fontan, severe RV dysfunction)
Advantages	Four-chamber heart with biventricular physiology → normal exercise capacity; avoids the inherent limitations of single-ventricle physiology
Disadvantages	Severe donor shortage — especially for neonatal-sized hearts; waiting list mortality ~15–25%; lifelong immunosuppression with all its complications (infection, malignancy, renal toxicity, graft vasculopathy); median graft survival ~15–20 years → likely need re-transplantation in adulthood
Contraindications to transplant	Fixed pulmonary hypertension unresponsive to drugs, systemic illness limiting survival, age < 1 year is relative ^[17]; active infection; significant neurological impairment
Hong Kong context	Neonatal heart transplantation is extremely rare in Hong Kong due to donor scarcity and cultural factors around organ donation. Staged surgical palliation is the standard pathway

Parameter	Detail
When discussed	Offered to ALL families as part of balanced counselling; some families choose this after considering the multi-year surgical journey, mortality risks, and long-term quality of life
What it involves	Discontinuation of PGE₁ and intensive care measures; comfort feeding; pain management; family-centred end-of-life care; involvement of palliative care team, chaplaincy/spiritual care, bereavement support
Ethical framework	This is a valid choice. HLHS management is not curative — it is palliative (the child will never have a normal heart). The family's values, cultural context, and assessment of quality of life must be respected. In Hong Kong, this decision is supported by the paediatric palliative care team

Drug	Class	Mechanism	Dose (Neonatal/Paediatric)	Role in HLHS	Key Side Effects
PGE₁ (Alprostadil)	Prostaglandin	Relaxes ductal smooth muscle → maintains PDA	0.01–0.05 mcg/kg/min IV (max 0.1)	Life-saving: maintains duct patency ^[11]	Apnoea, fever, diarrhoea ^[11]; hypotension, flushing
Milrinone	PDE3 inhibitor (inodilator)	Inhibits phosphodiesterase-3 → ↑cAMP → inotropy + vasodilation	0.25–0.75 mcg/kg/min IV (loading: 50 mcg/kg over 15 min, often omitted in neonates)	Perioperative inotropic support; improves cardiac output without excessive ↑SVR	Hypotension, thrombocytopenia, arrhythmia
Dopamine	Catecholamine	Dose-dependent: D₁ (renal, 1–3 mcg), β₁ (cardiac, 3–10 mcg), α₁ (vasoconstriction, > 10 mcg)	5–10 mcg/kg/min IV	Rescue inotrope for shock	Tachycardia, arrhythmia, tissue necrosis if extravasated
Dobutamine	β₁-agonist	Positive inotropy + mild vasodilation	5–20 mcg/kg/min IV	Alternative inotrope	Tachycardia, arrhythmia
Furosemide	Loop diuretic	Blocks Na-K-2Cl transporter in thick ascending limb → natriuresis	0.5–2 mg/kg/dose IV/PO (up to Q6–8H)	Volume overload, pulmonary congestion	Hypokalaemia, hyponatraemia, ototoxicity, nephrocalcinosis
Spironolactone	MRA (potassium-sparing diuretic)	Aldosterone antagonist at collecting duct	1–3 mg/kg/day PO in 1–2 divided doses	Adjunct to furosemide; K⁺-sparing; anti-remodelling	Hyperkalaemia, gynaecomastia
Captopril	ACE inhibitor	Blocks ACE → ↓angiotensin II → afterload reduction + anti-remodelling	0.05–0.5 mg/kg/dose PO Q8H (start low)	Afterload reduction for RV; evidence mixed in single-ventricle	Hypotension, hyperkalaemia, renal impairment, cough
Aspirin	Antiplatelet	Irreversible COX-1 inhibition → ↓TXA₂ → ↓platelet aggregation	3–5 mg/kg/day PO	Shunt patency (mBTS/Sano)	Bleeding, Reye syndrome (rare at low dose)
Digoxin	Cardiac glycoside	Inhibits Na⁺/K⁺-ATPase → ↑intracellular Ca²⁺ → ↑contractility; vagal tone ↑	5–10 mcg/kg/day PO in 2 divided doses (neonates)	Seldom used due to narrow therapeutic index ^[15]; sometimes for RV support	Toxicity: arrhythmias, nausea, visual disturbance; hypokalaemia potentiates toxicity
Heparin / Enoxaparin	Anticoagulant	AT-III mediated inhibition of factor Xa and thrombin	UFH: 10–25 U/kg/hr IV; Enoxaparin: 1.5 mg/kg/dose SC Q12H (neonates need higher doses)	Perioperative anticoagulation; post-Fontan thromboprophylaxis	Bleeding, HIT (rare in neonates)
Warfarin	Vitamin K antagonist	Inhibits vitamin K-dependent clotting factors (II, VII, IX, X)	Dose adjusted to INR 2.0–3.0	Long-term anticoagulation post-Fontan (some centres prefer aspirin alone for fenestrated Fontan)	Bleeding, teratogenic (not relevant in paediatric age), drug interactions

Stage	Procedure	Age	Goal	Mortality
Immediate	PGE₁ ± septostomy	Birth → days	Maintain PDA + adequate ASD	—
Stage I	Norwood	Day 3–7	Unobstructed systemic outflow from RV; controlled pulmonary blood flow; wide ASD	15–30% (centre-dependent)
Stage II	Bidirectional Glenn / Hemi-Fontan	4–6 months	Passive SVC → PA flow; reduce RV volume load	1–3%
Stage III	Fontan completion (TCPC)	2–4 years	Passive IVC → PA flow; complete separation of circulations	1–2%

Some centres offer a Hybrid Stage I procedure as an alternative to the classic Norwood in very high-risk neonates:

Component	Detail
Procedure	Bilateral PA banding (surgical — limits pulmonary blood flow) + PDA stenting (catheter-based — maintains systemic output) ± atrial septostomy
Rationale	Avoids cardiopulmonary bypass and deep hypothermic circulatory arrest in the neonatal period; delays the complex aortic arch reconstruction to a "Comprehensive Stage II" at 4–6 months when the baby is larger and more resilient
Comprehensive Stage II	Combines Glenn + aortic arch reconstruction + removal of PA bands and PDA stent at ~4–6 months
Advantage	Lower initial operative risk; avoids neonatal CPB
Disadvantage	More complex Stage II; PA band migration/distortion; limited long-term data

Fontan patients require lifelong follow-up — the Fontan circulation is palliative, not curative.

Domain	Management
Cardiology follow-up	Annual echocardiography, cardiac MRI every 2–3 years, Holter monitoring, exercise testing
Anticoagulation/antiplatelet	Aspirin (minimum) or warfarin — centre-dependent; thromboembolism is a major Fontan complication
Hepatology	Fontan-associated liver disease (FALD) develops in virtually all patients → serial liver ultrasound, fibroscan, AFP; may need liver biopsy
Exercise	Balance basic physical activity vs risk from exercise → guidelines available ^[15]; encourage regular moderate exercise; avoid competitive/isometric sports
Nutrition	High-protein diet; protein-losing enteropathy monitoring (serum albumin); medium-chain triglyceride (MCT) supplementation if PLE develops
Reproductive counselling	Pregnancy is extremely high-risk in Fontan patients → requires specialist joint obstetric-cardiology care
Psychological support	Chronic illness in childhood → risk of anxiety, depression, neurodevelopmental delay → regular developmental assessment and psychological support
Transition to adult CHD care	At ~16–18 years, formal transition to adult congenital heart disease (ACHD) service

Aspect	Detail
Antenatal counselling	Balanced, non-directive; explain the three options (staged palliation, transplant, comfort care); involve both parents; offer psychological support; allow time for decision-making
Interstage education	Teach parents: daily SpO₂ and weight monitoring; red flag signs (↓SpO₂, poor feeding, irritability, colour change); when to seek emergency care; CPR training
Informed consent	For each surgical stage: explain risks (including death), benefits, alternatives; use visual aids and diagrams; ensure understanding (teach-back method); document in medical records
Cultural considerations in Hong Kong	Organ donation reluctance (impacts transplant pathway); traditional beliefs may influence acceptance of comfort care; language-appropriate written materials in Chinese; involve hospital chaplaincy/social work as needed
Sibling and family support	Chronic illness impacts entire family; offer sibling support groups; social work assessment for financial and logistical support

High Yield Summary — Management of HLHS

Immediate Stabilisation:

PGE₁ infusion to maintain PDA — start on clinical suspicion, BEFORE echo ^[11]
Side effects: apnoea, fever, diarrhoea ^[11]
Target SpO₂ 75–85%: avoid supplemental O₂ (pulmonary vasodilator → systemic steal)
Emergent balloon atrial septostomy if ASD restrictive/intact
Milrinone as preferred inotrope (inodilator)

Definitive: Staged Surgical Palliation (3 stages):

Stage I — Norwood (Day 3–7): Neo-aorta reconstruction + mBTS or Sano shunt + atrial septectomy + PDA ligation
Stage II — Glenn (4–6 months): SVC → PA (passive); reduce RV volume load
Stage III — Fontan (2–4 years): IVC → PA (passive via conduit); complete separation of circulations

Alternatives: Heart transplantation (limited by donor availability); compassionate care

Long-term: Fontan circulation requires lifelong follow-up — anticoagulation, liver surveillance (FALD), exercise guidance, neurodevelopmental support, transition to adult ACHD care

Key Drugs: PGE₁, milrinone, furosemide, spironolactone, captopril, aspirin, warfarin

Active Recall - Management of HLHS

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.228 — HLHS section) ^[3] Senior notes: Ryan Ho Cardiology.pdf (p.188 — Tetralogy of Fallot management, mBTS) ^[11] Senior notes: Maksim Paediatric Notes.pdf (p.55–56 — Cardiogenic shock, PGE₁ management) ^[15] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.200 — Heart failure management in children) ^[16] Lecture slides: Block C - Heart failure and cyanosis in children: acyanotic and cyanotic congenital heart disease.pdf (p.12 — LVOT obstruction, HLHS rarity in Chinese) ^[17] Senior notes: Ryan Ho Cardiology.pdf (p.79 — Cardiac transplantation indications and contraindications)

Complications of Hypoplastic Left Heart Syndrome (HLHS)

The complications of HLHS can be understood by thinking about when they occur along the treatment timeline and why they happen based on the underlying physiology. HLHS complications arise from five fundamental sources:

The disease itself (before/without treatment) — duct closure, end-organ damage
Preoperative/perioperative — surgical morbidity of each stage
The inherent limitations of single-ventricle physiology — the RV was never designed to be a systemic ventricle
The Fontan circulation — chronically elevated systemic venous pressure without a subpulmonary pump
Neurodevelopmental consequences — from chronic cyanosis, bypass surgery, and the cumulative burden of illness

We'll work through each systematically.

These are the complications that occur if HLHS is not recognised or not treated — essentially what happens as the ductus arteriosus closes.

Complication	Mechanism	Clinical Manifestation
Cardiovascular collapse / cardiogenic shock	PDA closure → complete loss of systemic blood flow (duct-dependent systemic circulation) ^[1]	Acute shock: grey/mottled, tachycardia, hypotension, weak pulses all four limbs, metabolic acidosis, multi-organ failure → death within hours to days
Acute kidney injury	Kidney is first organ affected ^[11] — renal perfusion entirely dependent on PDA → ductal closure → acute renal hypoperfusion → pre-renal AKI	Oliguria/anuria; rising creatinine and urea; hyperkalaemia
Myocardial ischaemia and infarction	Coronary arteries perfused retrogradely via diminutive ascending aorta → dependent on PDA for driving pressure → ductal closure → coronary ischaemia. Worsened in MS/AA variant with ventriculo-coronary connections (coronary steal) ^[1]	RV dysfunction; ST changes on ECG; elevated troponin; further haemodynamic compromise
Hypoxic-ischaemic encephalopathy	Cerebral perfusion via retrograde ascending aortic flow → dependent on PDA → ductal closure → cerebral hypoperfusion + chronic mild hypoxaemia from RA-level mixing	Seizures, lethargy, long-term neurodevelopmental impairment
Necrotising enterocolitis (NEC)	Mesenteric blood supply dependent on PDA → ductal closure → gut ischaemia → NEC. Also exacerbated by diastolic runoff through mBTS in post-Norwood state	Abdominal distension, bloody stools, pneumatosis on AXR; high mortality if surgical NEC
Lactic acidosis	Systemic hypoperfusion → anaerobic metabolism → lactic acid accumulation	Deep, rapid breathing (compensatory); pH < 7.2; lactate > 4–8 mmol/L; worsening organ dysfunction
Pulmonary venous congestion / oedema	If atrial septum is restrictive/intact → pulmonary venous blood cannot decompress into RA → backs up into pulmonary veins → pulmonary oedema ^[1]	Severe respiratory distress from birth; ground-glass appearance on CXR; refractory hypoxia

The Natural History Without Treatment

Before the Norwood procedure was developed (first performed by William Norwood in 1983), HLHS was uniformly fatal — median survival was 4.5 days. Even today, without surgical intervention, mortality approaches 100% within the first month of life. This underscores why immediate PGE₁ and rapid surgical planning are essential.

B. Stage-Specific Surgical Complications

The Norwood is the most complex neonatal cardiac operation. It carries a mortality of 15–30% even in experienced centres (improving to ~10–15% in the highest-volume centres).

Complication	Mechanism	Management
Shunt thrombosis/obstruction	mBTS or Sano conduit is a small-calibre synthetic graft (3–3.5 mm) → susceptible to thrombus formation	Always listen for shunt murmur on follow-up → blocked BT shunt can kill ^[3]; acute desaturation → emergent catheter intervention or surgical revision; aspirin for prophylaxis
Restrictive or obstructed neo-aortic arch	Technical difficulty of reconstructing a tiny ascending aorta + arch → residual/recurrent obstruction	Balloon angioplasty or surgical revision; presents as hypertension in upper body, recurrent HF
Tricuspid regurgitation (TR)	RV dilation from volume/pressure overload → annular dilation → TR; intraoperative TV damage	Medical HF management; TV repair at subsequent stage if severe; severe TR is a major risk factor for death
RV dysfunction	RV not designed for sustained systemic afterload → progressive systolic dysfunction; ischaemia from coronary steal (MS/AA variant); bypass-related myocardial injury	Inotropic support (milrinone); afterload reduction (ACEI); mechanical circulatory support (ECMO) if refractory; transplantation for irreversible failure
Arrhythmias	Atrial suture lines → re-entrant circuits; RV hypertrophy → ventricular ectopy; electrolyte derangement perioperatively	Anti-arrhythmics; pacing for bradycardia; ECMO for refractory VT/VF
Unbalanced Qp:Qs	Shunt too large → pulmonary overcirculation (systemic steal); shunt too small → severe cyanosis	Manipulate PVR (FiO₂, PaCO₂); revise shunt if needed
Bleeding / coagulopathy	Complex surgery on deep hypothermic circulatory arrest (DHCA); neonatal coagulation factor deficiency; bypass-related platelet consumption	Blood products (FFP, cryoprecipitate, platelets); protamine for heparin reversal; surgical haemostasis
Diaphragmatic paresis	Phrenic nerve injury during aortic arch reconstruction	Ventilator support; diaphragm plication if no recovery in 4–6 weeks
Chylothorax	Thoracic duct injury during surgery → lymphatic leak into pleural space	MCT-based feeds; octreotide; thoracic duct ligation if refractory
Infection / sepsis	Open-heart surgery in a neonate with pre-existing end-organ injury; central lines, chest drains	Empirical broad-spectrum antibiotics; source control; careful line management

This period carries a 5–15% mortality rate — the highest outside the operating room.

Complication	Mechanism	Prevention/Management
Sudden death at home	Shunt thrombosis; arrhythmia; intercurrent illness causing dehydration → ↓preload → ↓cardiac output	Home monitoring programme: daily SpO₂ and weight; parental CPR training; low threshold for hospital review
Poor weight gain / failure to thrive	Increased metabolic demand from single-ventricle physiology + residual cyanosis; volume-overloaded RV → ↓feeding tolerance	High-calorie feeds (24–30 kcal/oz); NG tube supplementation; frequent weight checks
Intercurrent illness	Even simple infections (RSV bronchiolitis, gastroenteritis) can be life-threatening in single-ventricle physiology → ↑PVR, dehydration, acidosis	RSV prophylaxis (palivizumab); hand hygiene; early medical review for any illness; low threshold for admission
Progressive TR / RV dysfunction	Ongoing volume and pressure overload → progressive tricuspid annular dilation → worsening TR → further RV dilation (vicious cycle)	Serial echo; early Stage II if deteriorating; TV repair

Generally well-tolerated (mortality 1–3%), but complications include:

Complication	Mechanism
SVC syndrome	SVC-PA anastomosis stenosis → upper body venous congestion → facial oedema, headache, plethora
Pulmonary AV malformations	Hepatic factor (hepatocyte growth factor) normally distributed to both lungs via hepatic veins → IVC blood. After Glenn, IVC blood goes to RA (not lungs) → lungs deprived of hepatic factor → pulmonary AV malformations develop → progressive cyanosis. This is one reason Fontan completion is eventually needed
Persistent cyanosis	SpO₂ remains 75–85% (IVC blood still mixes in RA); decompressing venous collaterals (veno-venous collaterals from SVC territory draining to IVC/RA) → steal from Glenn circuit → worse cyanosis

Why Do Pulmonary AV Malformations Develop After Glenn?

This is a fascinating concept. The liver produces a factor (likely hepatocyte growth factor or another unidentified substance) that is essential for maintaining normal pulmonary vascular architecture. In the Glenn circulation, IVC blood (containing this hepatic factor) bypasses the lungs and goes to the RA → RV → body. Only SVC blood (which does NOT contain hepatic factor) reaches the lungs. This deprivation leads to pulmonary arteriovenous malformation (AVM) formation, causing intrapulmonary right-to-left shunting and progressive cyanosis. Completing the Fontan (routing IVC blood to lungs) reverses this process.

The Fontan circulation is the final palliation but is NOT a cure. It creates a fundamentally abnormal haemodynamic state — chronically elevated systemic venous pressure (CVP ~12–15 mmHg) and chronically reduced cardiac output — that leads to progressive multi-organ complications over decades.

The concept: every organ drained by the systemic veins is exposed to chronically elevated venous pressure, and every organ that depends on cardiac output for perfusion receives less than normal.

C. Fontan-Specific Complications (Long-Term)

These complications define the long-term morbidity and mortality of HLHS survivors. They are increasingly seen in adolescents and young adults as survival improves.

Parameter	Detail
Prevalence	Virtually 100% of Fontan patients develop some degree of liver disease
Mechanism	Chronically elevated hepatic venous pressure (CVP 12–15 mmHg) → hepatic congestion → centrizonal fibrosis → progressive fibrosis → cirrhosis. Additionally, ↓cardiac output → hepatic arterial hypoperfusion → ischaemic hepatopathy. The combination of congestion + ischaemia = "dual insult"
Progression	Congestion → sinusoidal dilation → centrilobular fibrosis → bridging fibrosis → cirrhosis → hepatocellular carcinoma (HCC)
Clinical manifestation	Often asymptomatic for years; then: elevated GGT/ALP (cholestatic pattern); low albumin; coagulopathy (↓factor synthesis); eventually portal hypertension with ascites, varices; HCC (incidence ~1–5% at 20+ years post-Fontan)
Surveillance	Serial LFTs; liver ultrasound ± fibroscan every 1–2 years; AFP for HCC screening; liver biopsy if diagnostic uncertainty; cardiac MRI with liver assessment
Management	Optimise Fontan haemodynamics (reduce CVP); treat HF; Fontan revision/conversion if feasible; liver transplant (with or without heart transplant) for end-stage FALD/HCC

FALD — The Emerging Epidemic in Fontan Survivors

As Fontan survival improves into the 3rd and 4th decades, FALD is becoming the leading cause of late morbidity and mortality. Some experts now recommend combined heart-liver transplant evaluation for all Fontan patients with advanced liver disease. This is an area of active research and a growing challenge for ACHD (adult congenital heart disease) services, including in Hong Kong.

Parameter	Detail
Prevalence	5–12% of Fontan patients; one of the most feared complications
Mechanism	Chronically elevated mesenteric venous pressure → intestinal lymphatic dilation and rupture → protein (albumin, immunoglobulins) lost into gut lumen. Contributing factors include low cardiac output and subclinical intestinal inflammation
Clinical manifestation	Peripheral oedema; ascites; pleural effusions; diarrhoea; growth failure; recurrent infections (immunoglobulin loss); hypoalbuminaemia (serum albumin often < 25 g/L)
Diagnosis	Elevated faecal alpha-1-antitrypsin (α₁-AT) clearance — this protein is neither secreted nor absorbed in the gut, so its presence in stool indicates protein loss; low serum albumin; low immunoglobulins
Management	High-protein, low-fat diet + MCT supplementation (MCTs absorbed directly into portal venous system, bypassing lymphatics); IV albumin infusions for acute episodes; diuretics (carefully — these patients are intravascularly depleted despite oedema); sildenafil or bosentan (pulmonary vasodilators to ↓PVR → improve Fontan haemodynamics); heparin (mechanism unclear but sometimes effective); budesonide (anti-inflammatory in gut mucosa); Fontan revision/conversion; heart transplantation for refractory cases
Prognosis	50% 5-year survival from PLE diagnosis if refractory to treatment

Parameter	Detail
Prevalence	10–20% of Fontan patients over their lifetime
Mechanism	Fontan circuit = low-flow, non-pulsatile venous system → Virchow's triad fulfilled: (1) stasis (low-flow conduit), (2) endothelial injury (synthetic conduit material), (3) hypercoagulability (liver synthetic dysfunction → altered coagulation factor balance; PLE → loss of antithrombin III, protein C, protein S)
Types	Fontan pathway thrombus → can embolise to pulmonary arteries (devastating — blocks passive pulmonary flow); intracardiac thrombus; cerebral thromboembolism (paradoxical embolism through fenestration or Fontan connections); DVT/PE equivalent in Fontan circuit
Prevention	Aspirin (minimum) or warfarin (INR 2.0–3.0); choice remains debated (no definitive RCT); some centres use both
Management	Anticoagulation (heparin → warfarin); thrombolysis for massive acute thrombus; catheter-directed therapy; surgical thrombectomy

Parameter	Detail
Prevalence	40–50% of Fontan patients develop clinically significant arrhythmias by 20 years post-Fontan
Types	Atrial arrhythmias (most common): intra-atrial re-entrant tachycardia (IART) / atrial flutter; atrial fibrillation. Ventricular arrhythmias: VT (less common but life-threatening). Sinus node dysfunction: bradycardia requiring pacing
Mechanism	Atrial suture lines from multiple surgeries → re-entrant circuits; chronic atrial dilation from elevated venous pressure → atrial substrate for arrhythmia; fibrosis; RV dysfunction → ventricular arrhythmia
Clinical significance	Arrhythmias are poorly tolerated in Fontan patients because: (1) loss of AV synchrony reduces already limited cardiac output; (2) tachycardia reduces diastolic filling time (critical when pulmonary flow is passive); (3) can precipitate thrombus formation
Management	Anti-arrhythmic drugs (amiodarone, flecainide — with caution given pro-arrhythmic risk); catheter ablation (technically challenging due to complex anatomy); pacemaker for bradycardia; Fontan conversion (old lateral tunnel → extracardiac conduit + arrhythmia surgery/ablation)

Parameter	Detail
Prevalence	4–14% of Fontan patients (emerging recognition)
Mechanism	Elevated systemic venous pressure → abnormal bronchial lymphatic drainage → lymphatic fluid leaks into airways → forms rubbery casts that obstruct bronchi
Clinical manifestation	Recurrent cough productive of branching casts; dyspnoea; wheezing; hypoxia; can be life-threatening if large cast obstructs main bronchus
Management	Bronchoscopic cast removal (acute); aerosolised tPA; mucolytics; lymphatic interventional procedures (thoracic duct embolisation, lymphatic stenting — cutting-edge); heart transplantation for refractory cases

Parameter	Detail
Mechanism	The morphological RV is inherently less suited for sustained systemic afterload than the LV. It has a crescent-shaped cavity (not designed for pressure generation), different myofibre architecture, and limited coronary reserve. Over decades, it progressively dilates and fails
Contributing factors	Chronic volume overload; chronic pressure overload (systemic afterload); TR (→ further volume overload → vicious cycle); chronic cyanosis-related myocardial injury; arrhythmias; residual arch obstruction increasing afterload
Clinical manifestation	Progressive exercise intolerance; fluid retention (oedema, ascites, pleural effusions); fatigue; syncope
Management	Medical therapy for heart failure ^[15]: ACEI/ARB, beta-blockers (carvedilol), MRA (spironolactone), diuretics; cardiac resynchronisation therapy (limited data); mechanical circulatory support (ECMO, VAD) ^[15] as bridge to transplant; heart transplantation for end-stage failure

Why the RV Fails as a Systemic Ventricle — From First Principles

The RV and LV are fundamentally different structures:

LV: Thick-walled (~10 mm), cylindrical shape, longitudinal + circumferential myofibre layers → generates high pressures (120 mmHg) efficiently
RV: Thin-walled (~3–4 mm), crescent-shaped, predominantly longitudinal myofibres → designed for low-pressure, high-compliance pulmonary circuit (~25 mmHg)

When forced to generate systemic pressures chronically, the RV hypertrophies but eventually cannot maintain contractile efficiency → dilatation → TR → volume overload → further dilatation (vicious cycle). This is the central reason why Fontan patients inevitably develop heart failure over decades.

Parameter	Detail
Prevalence	50–70% of HLHS survivors have some degree of neurodevelopmental impairment
Mechanisms	(1) Pre-operative: chronic hypoxaemia + cerebral hypoperfusion (retrograde flow via tiny ascending aorta); (2) Intra-operative: deep hypothermic circulatory arrest (DHCA) — complete cessation of blood flow at 18°C for aortic arch reconstruction; cardiopulmonary bypass → microemboli and inflammatory response; (3) Post-operative: low cardiac output states; arrhythmias; seizures; prolonged ICU stay; multiple general anaesthetics; (4) Chronic: persistent cyanosis (until Fontan completion); reduced cardiac output → chronic subclinical cerebral hypoperfusion
Spectrum	Motor delays (gross and fine); speech and language delays; attention deficit; learning difficulties (especially maths and visuospatial); behavioural problems; executive function deficits; reduced IQ (mean IQ ~85–90, cf. population mean 100); anxiety and depression
Surveillance	Formal neurodevelopmental assessment at 12 months, 24 months, school entry (4–5 years), and periodically thereafter; early intervention programmes (physiotherapy, speech therapy, occupational therapy); educational support plans
Management	Early intervention services; individualised education plan (IEP); physiotherapy and occupational therapy; psychological support; ADHD management if indicated; family counselling

Parameter	Detail
Mechanism	↑Metabolic demand (single-ventricle physiology) + ↓caloric intake (feeding difficulties, HF) + ↓absorption (PLE, mesenteric venous congestion) + chronic cyanosis → growth failure
Clinical impact	Weight and height often below 3rd centile; catch-up growth may occur after Fontan completion but often remains below population norms
Management	High-calorie feeds; NG tube or gastrostomy if oral intake insufficient; regular growth monitoring (plot on WHO growth charts); dietitian involvement

Parameter	Detail
Mechanism	Chronic low cardiac output → chronic renal hypoperfusion; chronically elevated renal venous pressure (Fontan physiology) → cardiorenal syndrome; nephrotoxic drugs (e.g., cyclosporin post-transplant, contrast agents from multiple catheterisations)
Manifestation	Chronic kidney disease (CKD); proteinuria; reduced GFR — often subclinical until advanced
Surveillance	Annual creatinine, cystatin C, urinalysis; renal ultrasound periodically

Complication	Detail
Risk	HLHS patients (both pre- and post-surgical) are at lifelong risk of infective endocarditis due to prosthetic material (conduits, patches, shunts), residual turbulent flow, and abnormal intracardiac anatomy
Prophylaxis	Antibiotic prophylaxis recommended for ALL dental procedures involving gingival manipulation, periapical procedures, or oral mucosal perforation; standard regimen: amoxicillin 50 mg/kg PO 30–60 min before procedure (clindamycin 20 mg/kg if penicillin-allergic)
Education	Dental hygiene education for family; regular dental follow-up; endocarditis alert card

Timing	Complications
Pre-treatment	Cardiogenic shock, AKI (first organ affected), myocardial ischaemia, HIE, NEC, lactic acidosis, pulmonary oedema (if restrictive ASD)
Stage I (Norwood)	Shunt thrombosis, neo-aortic arch obstruction, TR, RV dysfunction, arrhythmia, bleeding, diaphragmatic paresis, chylothorax, infection
Interstage	Sudden death (shunt blockage), poor weight gain, intercurrent illness, progressive TR/RV dysfunction
Stage II (Glenn)	SVC syndrome, pulmonary AV malformations, persistent cyanosis, venous collaterals
Stage III (Fontan) and long-term	FALD (→ cirrhosis → HCC), PLE, thromboembolism, arrhythmias, plastic bronchitis, progressive RV failure ("failing Fontan"), neurodevelopmental impairment, growth failure, CKD, IE

Metric	Data
Without treatment	100% fatal within the first month; median survival 4.5 days
Norwood survival (Stage I)	70–85% hospital survival (centre-dependent)
Overall 5-year survival (all stages)	~60–70%
Fontan survival at 20 years	~70–80% in modern era (improving)
Quality of life	Most survivors attend regular school; reduced exercise capacity; neurodevelopmental challenges in ~50–70%; growing population of Fontan adults requiring lifelong specialist care
Long-term outlook	Mean age at Fontan failure requiring transplant or death: ~25–30 years post-Fontan. Heart transplant remains the ultimate option for failing Fontan — but graft survival is finite (~15–20 years)

High Yield Summary — Complications of HLHS

Pre-treatment: Cardiogenic shock from ductal closure; kidney is first organ affected ^[11]; myocardial and cerebral ischaemia from loss of retrograde aortic perfusion; NEC

Surgical complications: Shunt thrombosis — blocked BT shunt can kill ^[3]; neo-aortic arch obstruction; TR; RV dysfunction; arrhythmias; chylothorax

Interstage: 5–15% mortality; sudden death at home; failure to thrive; intercurrent infection

Long-term Fontan complications (the "big five"):

FALD — virtually universal; progression to cirrhosis and HCC
PLE — 5–12%; hypoalbuminaemia, oedema, immunodeficiency; 50% 5-year survival if refractory
Thromboembolism — 10–20% lifetime risk; Virchow's triad fully met in Fontan circuit
Arrhythmias — 40–50% by 20 years; poorly tolerated due to passive pulmonary flow
Progressive RV failure — the RV was never designed for systemic afterload; inevitable over decades

Neurodevelopment: 50–70% have some impairment; mean IQ ~85–90; requires formal surveillance and early intervention

Lifelong IE prophylaxis required for all dental procedures

Active Recall - Complications of HLHS

References

^[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.228 — HLHS section) ^[3] Senior notes: Ryan Ho Cardiology.pdf (p.188 — Tetralogy of Fallot management, mBTS shunt complications) ^[11] Senior notes: Maksim Paediatric Notes.pdf (p.55–56 — Cardiogenic shock in neonates, kidney as first organ affected) ^[15] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.200 — Heart failure management in children) ^[16] Lecture slides: Block C - Heart failure and cyanosis in children: acyanotic and cyanotic congenital heart disease.pdf (p.12 — LVOT obstruction classification) ^[18] Senior notes: Ryan Ho Cardiology.pdf (p.186 — Eisenmenger syndrome complications) ^[19] Lecture slides: GC 147. Heart failure and cyanosis in children acyanotic and cyanotic congenital heart disease - Part 1.pdf (p.23 — HLHS slide)

High Yield Summary

Definition: HLHS = underdevelopment of left heart structures (MV, LV, AV, ascending aorta) → LV cannot support systemic circulation.

Epidemiology: 2–3/10,000 live births; 2–3% of CHD; most common cause of cardiac death in first week of life.

Key Pathophysiology:

Obligate L-to-R shunt via ASD/PFO (pulmonary venous blood must cross to right side)
Duct-dependent systemic circulation (RV → PA → PDA → aorta)
Retrograde flow in ascending aorta supplies coronaries and brain
PDA closure = cardiovascular collapse and death

Clinical Features:

Shock with mild cyanosis (not deep cyanosis — shock dominates)
Early onset HF ≤1 week with ductal closure
Weak pulses in all four limbs
Single loud S2 (absent A2, loud P2)
Ejection click (dilated PA)
No or non-specific murmur — critical CHD without a dramatic murmur!
RV impulse (RV doing all the work)

Critical Concepts:

Intact/restrictive atrial septum = emergency (requires urgent septostomy)
Supplemental O₂ can worsen systemic perfusion (pulmonary vasodilation → ↑Qp → systemic steal)
Target SpO₂ is 75–85%, NOT 95–100%
Immediate PGE₁ infusion is life-saving

High Yield Summary — Differential Diagnosis of HLHS

Key teaching points for exams:

HLHS presents primarily with shock + mild cyanosis, not deep cyanosis ^[1] — this distinguishes it from conditions like d-TGA (deep cyanosis, good perfusion) and duct-dependent pulmonary lesions (deep cyanosis, no shock)
Cyanotic CHD is categorised by: ↓ pulmonary blood flow, ↑ pulmonary blood flow, or severe heart failure ^[9]^[10] — HLHS falls in the "severe heart failure" category
Four-limb pulse assessment is the critical bedside differentiator ^[4]^[11]:
- All pulses weak → HLHS, critical AS, IAA
- UL strong / LL weak → critical CoA
An oligaemic lung field on CXR supports cyanotic CHD over respiratory disease ^[8]
Cardiogenic shock at Day 4–7 = think duct-dependent systemic lesion ^[11] — HLHS, critical CoA, IAA, critical AS
The hyperoxia test differentiates cardiac from pulmonary cyanosis — PaO₂ remains < 100 mmHg in cardiac causes
Echocardiography is definitive — identifies the slit-like LV, atretic valves, and diminutive ascending aorta of HLHS

High Yield Summary — Diagnosis of HLHS

Echocardiography is the definitive diagnostic test — identifies slit-like LV, atretic/stenotic MV and AV, diminutive ascending aorta with retrograde flow, and dilated PA trunk
Diagnosis is anatomical: there is no "points-based" scoring system. HLHS = left heart structures too hypoplastic for biventricular repair
Fetal diagnosis is possible at 18–22 weeks on the four-chamber view → improves outcomes by allowing planned delivery and immediate PGE₁
Newborn pulse oximetry screening detects HLHS before clinical deterioration (SpO₂ < 95%)
The hyperoxia test differentiates cardiac from pulmonary cyanosis — PaO₂ remains < 100 mmHg in HLHS
CXR: cardiomegaly with dilated RA/RV; ↑ pulmonary vascular markings or PV congestion ^[1]
ECG: RAD, RAE, RVH, absent LV forces — reflects single-ventricle (RV) physiology
Blood tests assess end-organ damage — lactate is the most important marker of systemic perfusion adequacy
ASD adequacy must be assessed on echo — restrictive/intact atrial septum requires emergent septostomy
Start PGE₁ on clinical suspicion BEFORE confirmatory echo ^[11] — do not delay life-saving treatment for diagnosis

High Yield Summary — Management of HLHS

Immediate Stabilisation:

PGE₁ infusion to maintain PDA — start on clinical suspicion, BEFORE echo ^[11]
Side effects: apnoea, fever, diarrhoea ^[11]
Target SpO₂ 75–85%: avoid supplemental O₂ (pulmonary vasodilator → systemic steal)
Emergent balloon atrial septostomy if ASD restrictive/intact
Milrinone as preferred inotrope (inodilator)

Definitive: Staged Surgical Palliation (3 stages):

Stage I — Norwood (Day 3–7): Neo-aorta reconstruction + mBTS or Sano shunt + atrial septectomy + PDA ligation
Stage II — Glenn (4–6 months): SVC → PA (passive); reduce RV volume load
Stage III — Fontan (2–4 years): IVC → PA (passive via conduit); complete separation of circulations

Alternatives: Heart transplantation (limited by donor availability); compassionate care

Long-term: Fontan circulation requires lifelong follow-up — anticoagulation, liver surveillance (FALD), exercise guidance, neurodevelopmental support, transition to adult ACHD care

Key Drugs: PGE₁, milrinone, furosemide, spironolactone, captopril, aspirin, warfarin

High Yield Summary — Complications of HLHS

Pre-treatment: Cardiogenic shock from ductal closure; kidney is first organ affected ^[11]; myocardial and cerebral ischaemia from loss of retrograde aortic perfusion; NEC

Surgical complications: Shunt thrombosis — blocked BT shunt can kill ^[3]; neo-aortic arch obstruction; TR; RV dysfunction; arrhythmias; chylothorax

Interstage: 5–15% mortality; sudden death at home; failure to thrive; intercurrent infection

Long-term Fontan complications (the "big five"):

FALD — virtually universal; progression to cirrhosis and HCC
PLE — 5–12%; hypoalbuminaemia, oedema, immunodeficiency; 50% 5-year survival if refractory
Thromboembolism — 10–20% lifetime risk; Virchow's triad fully met in Fontan circuit
Arrhythmias — 40–50% by 20 years; poorly tolerated due to passive pulmonary flow
Progressive RV failure — the RV was never designed for systemic afterload; inevitable over decades

Neurodevelopment: 50–70% have some impairment; mean IQ ~85–90; requires formal surveillance and early intervention

Lifelong IE prophylaxis required for all dental procedures

Hypoplastic Left Heart Syndrome

Definition

Epidemiology

Anatomy and Normal Function (Understanding What Goes Wrong)

Normal Fetal Circulation (Review)

Normal Left Heart Structures

Aetiology

Structural Basis

Anatomical Subtypes (Important for Surgical Planning)

Associated Anomalies

Genetic Associations

Aetiology in the Hong Kong Context

Pathophysiology

Core Problem

Step-by-Step Pathophysiology

Detailed Haemodynamic Explanation

1. Obligate L-to-R shunt via ASD/PFO as LV inflow is obstructed [1]

2. Ductal-dependent systemic circulation: NO cardiac output from LV → solely dependent on DA [1]

3. Ascending aorta and its branches (coronary and cranial arteries) supplied by retrograde flow [1]

4. What Happens When the Ductus Closes?

5. The Qp:Qs Balance

Classification

Within the Spectrum of Congenital Heart Disease

Classification of Duct-Dependent Lesions (for Context)

Comparison with Other Left-Sided Obstructive Lesions

Clinical Features

Timing of Presentation

Symptoms

Signs

Physical Examination Approach in the Neonate

Comparison of Presentations of Duct-Dependent Lesions

Investigations (Brief Preview)

Foetal Diagnosis and Implications

Summary of Pathophysiology Linking to Clinical Features

Active Recall - HLHS: Definition, Epidemiology, Pathophysiology, and Clinical Features

References

Framing the Clinical Problem

A. Cardiac Differential Diagnosis — Cyanotic Congenital Heart Disease

1. Cyanotic CHD with ↓ Pulmonary Blood Flow

2. Cyanotic CHD with ↑ Pulmonary Blood Flow (The "3 T's")

3. Cyanotic CHD Presenting with Severe Heart Failure / Shock (Duct-Dependent Systemic Circulation)

B. Non-Cardiac Differential Diagnosis

C. Within-HLHS Differential: Conditions That May Be Confused with HLHS on Echocardiography

D. Differential Diagnosis by Clinical Scenario

Scenario 1: Neonate (Day 1–3) with Mild Cyanosis, No Shock

Scenario 2: Neonate (Day 3–7) with Shock, Metabolic Acidosis, ± Mild Cyanosis

Scenario 3: Neonate with Severe Cyanosis + Pulmonary Oedema from Birth

Diagnostic Approach — Decision Tree

Summary Table: Key Differentiating Features

Active Recall - Differential Diagnosis of HLHS

Diagnostic Criteria

Echocardiographic Diagnostic Definition

Additional Anatomical Features to Define on Echo

Diagnostic Algorithm

Algorithm Overview

Step-by-Step Postnatal Diagnostic Pathway

Step 1: Recognition — Clinical Suspicion

Step 2: Bedside Assessment — Immediate

Step 3: Echocardiography — Definitive Diagnosis

Step 4: Supportive Investigations (See Detailed Section Below)

Step 5: Start PGE₁ BEFORE Echo if Clinical Suspicion is High

Investigation Modalities — Detailed Findings and Interpretation

1. Echocardiography (Definitive Investigation)

2. Chest X-Ray (CXR)

3. Electrocardiogram (ECG)

4. Blood Investigations

5. Hyperoxia (Nitrogen Washout) Test

6. Fetal Echocardiography (Antenatal Diagnosis)

7. Cardiac Catheterisation

8. Cardiac MRI and CT Angiography

9. Genetic Testing

Integrated Diagnostic Approach — Mermaid Diagram

Key Investigation Comparison Table — HLHS vs. Differentials

Active Recall - Diagnosis and Investigations in HLHS

Overview: The Management Paradigm

Management Algorithm — Overview Mermaid Diagram

Phase 1: Immediate Stabilisation (Neonatal Period — Hours to Days)

1A. Prostaglandin E₁ (PGE₁) — Alprostadil

1B. Balancing Qp:Qs — The Art of Managing Parallel Circulations

1C. Haemodynamic Support