Supine hypotensive syndrome is a drop in blood pressure occurring in late pregnancy when the gravid uterus compresses the inferior vena cava while the patient is lying supine, reducing venous return and cardiac output.

Exam domain	One-glance essentials
Definition / diagnosis	*Supine hypotensive syndrome is maternal hypotension from gravid-uterus compression of the inferior vena cava, usually in later pregnancy* ^[1]^[2] Aortic compression may also reduce uteroplacental perfusion ^[2]
Epidemiology / risks	Risk is greater with advanced gestation, multiple pregnancy, polyhydramnios, obesity, and prolonged supine positioning, including during procedures ^[1]^[2] Not every pregnant patient becomes symptomatic because collateral venous return varies ^[1]
Core mechanism	Supine uterus compresses the IVC → reduced venous return → reduced preload and cardiac output → maternal hypotension ^[1]^[2] Reduced maternal output and possible aortic compression lower uteroplacental blood flow ^[1]
Clinical picture	*Dizziness, nausea, pallor, sweating, tachycardia, and hypotension while supine that improve with lateral positioning are typical* ^[1]^[2] Persistent symptoms, chest pain, dyspnoea, bleeding, or neurological signs require assessment for other causes of shock or syncope ^[1]
Investigations	Diagnosis is clinical from position-related symptoms and rapid response to left uterine displacement ^[1]^[2] Check maternal observations and fetal wellbeing; investigate haemorrhage, PE, arrhythmia, sepsis, or pre-eclampsia when the pattern is atypical or unresolved ^[1]
Management	*Immediately turn the patient left lateral or manually displace the uterus, then support airway, breathing, and circulation* ^[1]^[2] During resuscitation or anaesthesia, maintain uterine displacement and treat persistent hypotension according to cause ^[2]
Complications	Untreated severe compression can cause maternal syncope, cardiovascular collapse, fetal hypoxia, and fetal bradycardia ^[1]^[2] Aspiration or injury may follow loss of consciousness ^[2]
Prevention / follow-up	Avoid prolonged flat supine positioning in later pregnancy; use left lateral tilt during procedures and transport ^[1]^[2] Teach patients to change position if symptoms arise rather than relying on a fixed sleeping rule ^[1]
Exam traps	*Rapid improvement with uterine displacement supports the diagnosis, but persistent hypotension is not benign supine syndrome* ^[1]^[2] Do not delay evaluation for haemorrhage, pulmonary embolism, arrhythmia, or sepsis when symptoms do not resolve ^[1]

References

^[1] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p. 39 — supine positioning and pregnancy) ^[2] Senior notes: Maksim Surgery Notes.pdf (p. 298 — aortocaval compression in obstetric anaesthesia)

Supine Hypotensive Syndrome

Supine hypotensive syndrome (SHS) — also called aortocaval compression syndrome — is a condition occurring predominantly in late pregnancy (typically after 20 weeks' gestation, most pronounced in the third trimester) in which the gravid uterus compresses the inferior vena cava (IVC) and, to a lesser extent, the abdominal aorta when the patient lies in the supine position. This mechanical compression reduces venous return to the heart, decreases cardiac output, and produces maternal hypotension with associated symptoms of dizziness, nausea, and faintness ^[1]^[2].

Breaking down the name:

Supine = lying flat on the back (Latin supinus = "lying face upward")
Hypotensive = low blood pressure (hypo- = below, tension = pressure)
Syndrome = a collection of signs and symptoms occurring together

So the name literally tells you: "a syndrome of low blood pressure caused by lying on one's back" — and the clinical context is pregnancy.

Aortocaval compression (supine hypotension syndrome): reduced by left lateral uterine displacement ^[1]

Key Distinction

Not every pregnant woman lying supine becomes symptomatic. Radiographic studies show that nearly all women at term have some degree of IVC compression when supine, but only 8–10% develop clinically significant hypotension. The difference lies in the adequacy of compensatory mechanisms (paravertebral venous plexus collaterals, sympathetic vasoconstriction).

Epidemiology and Risk Factors

Incidence of IVC compression (radiographic): Nearly 90–100% of women at term show some degree of IVC compression when supine on venography/ultrasound.
Incidence of symptomatic supine hypotension: Approximately 8–10% of pregnant women in the third trimester develop clinically significant symptoms ^[3].
Onset: Rarely before 20 weeks' gestation; most commonly manifests from 28 weeks onwards as the uterus becomes large enough to exert significant compression.
Peak risk: At term (36–40 weeks) and during labour (especially with regional anaesthesia, which ablates compensatory sympathetic tone).

Risk Factor	Mechanism
Advanced gestational age (especially > 28 weeks)	Larger uterus → greater compressive force on IVC/aorta
Multiple gestation (twins, triplets)	Even larger uterine volume → earlier and more severe compression
Polyhydramnios	Excessive amniotic fluid → increased uterine size and weight
Macrosomia (large fetus)	Greater uterine distension
Obesity	Increased abdominal mass adds to compression; reduced collateral venous flow
Regional anaesthesia (spinal/epidural)	Sympathetic blockade removes compensatory vasoconstriction → unable to maintain venous return via increased peripheral resistance
General anaesthesia	Loss of muscle tone, loss of sympathetic reflexes
Prolonged supine positioning	Sustained compression without positional relief
Supine position during caesarean section or procedures	Mandatory supine position + anaesthesia = double hit

High Yield – Anaesthesia Link

This is why supine hypotensive syndrome is a critical concern in obstetric anaesthesia. Regional anaesthesia (spinal or epidural) blocks sympathetic outflow, removing the compensatory vasoconstriction that normally maintains blood pressure despite IVC compression. This is why left lateral uterine displacement is a standard manoeuvre during caesarean section under regional anaesthesia ^[1].

Anatomy and Function

Understanding supine hypotensive syndrome requires a solid grasp of the anatomical relationships in the retroperitoneum and the cardiovascular physiology of pregnancy.

Relevant Vascular Anatomy

The IVC is a large, thin-walled, low-pressure venous vessel that runs to the right of the vertebral column in the retroperitoneum.
It receives blood from the lower limbs (via common iliac veins), the renal veins, hepatic veins, and lumbar veins.
It ascends through the caval hiatus of the diaphragm (at T8) to drain into the right atrium.
Because it is a low-pressure system (5–15 mmHg), it is highly compressible — unlike the thick-walled, high-pressure aorta.

The aorta descends slightly to the left of midline along the vertebral column.
It is a high-pressure vessel (mean arterial pressure ~90 mmHg in pregnancy), so it is much more resistant to compression.
However, partial aortic compression can occur, particularly at the aortic bifurcation (L4), reducing blood flow to the uterine arteries (which arise from the internal iliac arteries) → fetal compromise even without maternal symptoms.

When the IVC is compressed, blood can return to the heart via collateral pathways:
- Paravertebral (Batson's) venous plexus → azygos and hemiazygos veins → SVC → right atrium
- Superficial abdominal wall veins (epigastric veins)
- Ovarian veins → renal veins
The adequacy of these collaterals determines whether a woman becomes symptomatic or not.

The non-pregnant uterus is a small pelvic organ.
By 20 weeks, the uterine fundus reaches the umbilicus.
By 36–40 weeks, it reaches the xiphisternum and occupies much of the abdominal cavity.
The pregnant uterus naturally dextrorotates (rotates to the right) because the sigmoid colon occupies the left side of the pelvis — this means it sits preferentially over the IVC.

To understand why SHS is so important, you need to appreciate normal cardiovascular adaptations in pregnancy:

Parameter	Change in Pregnancy	Magnitude
Blood volume	↑	+30–50% (peaks at 32–34 weeks)
Cardiac output	↑	+30–50%
Heart rate	↑	+10–20 bpm
Stroke volume	↑	+20–30%
Systemic vascular resistance (SVR)	↓	−20–30% (progesterone-mediated vasodilation)
Blood pressure	↓ slightly in 2nd trimester, returns to baseline in 3rd	Nadir at 24 weeks
Venous pressure (lower limbs)	↑	Due to IVC compression + increased blood volume

These adaptations mean that the pregnant cardiovascular system is volume-dependent: cardiac output is maintained by high preload (large blood volume + high venous return). Anything that reduces venous return — like IVC compression — has a disproportionately large effect on cardiac output.

The placental bed has no autoregulation to compensate for a drop in BP ^[1]

Critical Concept

The uteroplacental circulation is a maximally dilated, low-resistance vascular bed with no autoregulation. This means it cannot compensate for drops in maternal blood pressure by vasodilating further — blood flow is directly proportional to maternal arterial pressure. Therefore, maternal hypotension = reduced uteroplacental perfusion = fetal hypoxia. This is why supine hypotensive syndrome threatens both mother AND fetus.

Etiology and Pathophysiology

The sole cause of supine hypotensive syndrome is mechanical compression of the IVC (and to a lesser extent, the aorta) by the gravid uterus in the supine position.

This is not a disease per se — it is a physiological-positional phenomenon that becomes pathological when compensatory mechanisms fail.

Detailed Pathophysiology

When a pregnant woman lies supine in the third trimester:

The heavy, enlarged uterus (which at term weighs ~5–6 kg including fetus, placenta, and amniotic fluid) falls posteriorly under gravity.
It compresses the IVC against the lumbar vertebral bodies (primarily L3–L5).
The IVC, being a thin-walled, low-pressure vessel, is easily occluded — sometimes almost completely.
This dramatically reduces venous return from the lower body to the right atrium.

By the Frank-Starling mechanism, reduced venous return → reduced right atrial filling → reduced right ventricular end-diastolic volume → reduced stroke volume → reduced cardiac output (can fall by 25–30% in the supine position).
This is the primary mechanism of hypotension.

In most women, the following compensatory mechanisms maintain blood pressure despite IVC compression:

Collateral venous drainage: Blood diverts through the paravertebral venous plexus → azygos system → SVC → right atrium. If these collaterals are well-developed, venous return is maintained.
Sympathetic vasoconstriction: Baroreceptor-mediated increase in sympathetic tone → arteriolar vasoconstriction → maintains SVR and MAP.
Tachycardia: Compensatory increase in heart rate to maintain cardiac output (CO = HR × SV).

Compensation fails when:

Collateral venous pathways are inadequate (anatomical variation, obesity compressing collaterals)
Regional anaesthesia (spinal or epidural) blocks sympathetic outflow → loss of compensatory vasoconstriction and tachycardia
General anaesthesia abolishes muscle tone and autonomic reflexes
Hypovolaemia (e.g., haemorrhage, dehydration) → already reduced preload, so any further reduction is catastrophic
Very large uterus (multiple gestation, polyhydramnios) → more complete IVC occlusion

In addition to IVC compression, the gravid uterus can partially compress the aorta at its bifurcation.
This reduces blood flow distal to the compression, particularly to the uterine arteries (branches of the internal iliacs).
The result is reduced uteroplacental perfusion → fetal hypoxia.
Importantly, aortic compression can cause fetal distress without maternal hypotension — the mother may have a normal brachial BP (measured in the upper limb, proximal to the compression) while the lower body including the uterus is hypoperfused.
This is called the "hidden aortic compression" or Poseiro effect: uterine contractions or supine positioning compress the aorta → late decelerations on CTG without maternal symptoms.

Maternal:

Hypotension → cerebral hypoperfusion → dizziness, nausea, visual disturbance, syncope
In severe/prolonged cases → maternal cardiac arrest (rare but reported)

Fetal:

Reduced uteroplacental blood flow → fetal hypoxia
CTG changes: late decelerations, fetal bradycardia, reduced variability
If prolonged → fetal acidosis, neurological injury, stillbirth (very rare in clinical practice because the condition is rapidly reversible)

Why Regional Anaesthesia Makes This Worse

Under normal circumstances, when IVC compression reduces venous return, the baroreceptors in the carotid sinus and aortic arch detect the drop in blood pressure and trigger a sympathetic response: vasoconstriction (to increase SVR) and tachycardia (to increase cardiac output). Spinal/epidural anaesthesia blocks the sympathetic chain → these reflexes are abolished → the patient cannot compensate → severe hypotension. This is why left lateral uterine displacement is mandatory during caesarean section under regional anaesthesia ^[1]^[2].

Classification

Supine hypotensive syndrome does not have a formal classification system, but it can be conceptually categorized by:

Grade	Description
Subclinical / Compensated	IVC compression present on imaging but no maternal symptoms; compensatory mechanisms (collaterals, sympathetic tone) maintain blood pressure
Mild (Symptomatic)	Dizziness, lightheadedness, nausea, mild pallor when supine; rapidly resolves with position change
Moderate	Significant hypotension (systolic BP < 90 mmHg or > 20% drop from baseline), presyncope, diaphoresis; requires active intervention
Severe	Syncope, maternal cardiovascular collapse, fetal distress (CTG abnormalities); may require emergency resuscitation

Context	Significance
Spontaneous (positional)	Woman lies supine at home, during sleep, or during examination
Iatrogenic (peri-operative)	During caesarean section, regional anaesthesia, or any procedure requiring supine positioning — this is the most dangerous context because anaesthesia removes compensatory mechanisms

Type	Mechanism
IVC compression dominant	Primarily reduced venous return → reduced cardiac output → maternal hypotension
Aortic compression dominant (Poseiro effect)	Aortic compression → reduced uteroplacental flow → fetal distress, often with preserved maternal BP (brachial readings are above the level of compression)
Mixed	Both IVC and aortic compression → maternal hypotension AND fetal distress

Clinical Features

Symptom	Pathophysiological Basis
Dizziness / Lightheadedness	Reduced cardiac output → reduced cerebral perfusion → transient cerebral ischaemia
Nausea and sometimes vomiting	Vagal activation secondary to hypotension (Bezold-Jarisch reflex — vigorous contraction of an underfilled ventricle stimulates ventricular mechanoreceptors → paradoxical vagal response → nausea, bradycardia)
Feeling faint / Presyncope	Cerebral hypoperfusion — the brain is very sensitive to reduced perfusion pressure
Syncope (loss of consciousness)	Severe cerebral hypoperfusion — the brain cannot maintain consciousness below ~50 mmHg mean arterial pressure
Shortness of breath / Dyspnoea	Reduced cardiac output → compensatory tachypnoea; also, the large uterus in the supine position pushes the diaphragm cephalad, reducing functional residual capacity (FRC) and tidal volume
Visual disturbances (blurring, "greying out")	Retinal hypoperfusion — the retina is exquisitely sensitive to reduced perfusion
Anxiety / Feeling of "impending doom"	Catecholamine surge secondary to hypotension; also a feature of the Bezold-Jarisch reflex
Reduced fetal movements (reported by mother)	Fetal hypoxia secondary to reduced uteroplacental perfusion → fetus becomes less active

Recommend lady to move around a lot → if pregnant lady lying down for prolonged period of time, have to be worried about DVT, and reduced venous return if supine ^[3]

Sign	Pathophysiological Basis
Hypotension (systolic BP < 90 mmHg or > 20–30% drop from baseline)	Reduced venous return → reduced cardiac output → reduced MAP
Tachycardia (compensatory)	Baroreceptor reflex: reduced BP detected → sympathetic activation → increased heart rate to try to maintain cardiac output
OR Bradycardia (paradoxical, in severe cases)	Bezold-Jarisch reflex: vigorous contraction of a near-empty left ventricle stimulates ventricular C-fibre mechanoreceptors → paradoxical vagal response → bradycardia. This is the dangerous scenario — the heart rate drops instead of compensating
Pallor	Peripheral vasoconstriction (sympathetic-mediated) to shunt blood to vital organs; also reduced overall perfusion
Diaphoresis (cold sweat)	Sympathetic activation → eccrine sweat gland stimulation
Weak, thready pulse	Low stroke volume → reduced pulse pressure
Altered consciousness (in severe cases)	Cerebral hypoperfusion
CTG abnormalities (fetal)	Reduced uteroplacental blood flow → fetal hypoxia → late decelerations, fetal bradycardia, reduced beat-to-beat variability

Symptoms typically begin within 3–10 minutes of assuming the supine position.
They rapidly resolve (within 1–2 minutes) when the patient is turned to the left lateral position or when the uterus is manually displaced to the left.
This rapid positional response is essentially diagnostic — no other cause of hypotension in pregnancy resolves so quickly with position change.

Important Clinical Scenarios

This is the highest-risk scenario. The patient is supine on the operating table, and spinal anaesthesia has blocked sympathetic outflow (typically T4–S5 for a caesarean). Without compensatory vasoconstriction:

Venous pooling in the lower limbs
IVC compression by the uterus
Combined effect: severe, sudden hypotension ("spinal shock" in the obstetric context)
Management: Vasopressors used intra-op — Ephedrine (alpha + beta), Phenylephrine (pure alpha), Atropine (vagal block) ^[2]

Women in labour may be placed supine for examination or delivery.
Contractions themselves compress the aorta (Poseiro effect) → superimposed on positional IVC compression → fetal distress.

Routine obstetric ultrasound requires supine positioning.
Women in the third trimester may feel faint during prolonged scans.

Pregnant women who sleep supine may experience intermittent symptoms.
Epidemiological studies have associated habitual supine sleeping in late pregnancy with an increased risk of stillbirth (likely related to recurrent episodes of reduced uteroplacental perfusion).

Supine Sleep and Stillbirth Risk

Recent evidence (the MiNESS and CRIBSS studies) has shown that supine sleeping position in the third trimester is associated with a 2–3 fold increased risk of late stillbirth. This is likely due to recurrent aortocaval compression during sleep reducing uteroplacental perfusion. Current guidance (including from RCOG and the New Zealand guidelines) recommends that women sleep on their side (preferably left lateral) from 28 weeks onwards.

The hallmark feature of supine hypotensive syndrome is its positional nature: it occurs in the supine position and resolves with lateral positioning. This distinguishes it from:

Haemorrhagic shock (does not resolve with position change; look for bleeding source)
Septic shock (fever, signs of infection)
High spinal block (respiratory compromise, very high sensory level)
Amniotic fluid embolism (sudden cardiovascular collapse, DIC, respiratory distress)
Anaphylaxis (urticaria, angioedema, bronchospasm)

This topic is intimately linked to obstetric anaesthesia, so let us integrate the key anaesthetic considerations here:

Choice of modality: regional anaesthesia is safer (unless very urgent) ^[2]

Spinal anaesthesia (first line!)

Epidural anaesthesia: usually for labour pain relief

Combined spinal epidural anaesthesia (CSE)

GA: RSI with propofol + suxamethonium +/- nitrous oxide inhalation ^[2]

The risk of supine hypotensive syndrome is amplified under anaesthesia because:

Difficult intubation is 8 times more common than normal patients due to weight gain and oedema, pre-existing obstetric disease (e.g., pre-eclampsia), increased oxygen demand by 20% & reduced oxygen reserve (FRC drop 20%): less apnoea time allowed, delayed gastric emptying and relaxed LES due to progesterone → risk of aspiration even with fasting ^[2]
The supine position is obligatory for surgical access during caesarean section.
Regional anaesthesia ablates sympathetic compensation.

Prevention strategies in the operative setting:

Left lateral uterine displacement (either with a wedge under the right hip, tilting the table 15° to the left, or manual displacement of the uterus to the left) ^[1]^[2]
Pre-loading or co-loading with IV crystalloid
Prophylactic vasopressors (phenylephrine infusion is now standard of care)

Mendelson's syndrome (chemical pneumonitis caused by aspiration of acidic stomach contents during anaesthesia in childbirth): reduced by RSI and antacids (30 mL sodium citrate + H2RA/PPI) ^[2]

In the UK, the leading cause of maternal mortality is thromboembolism ^[3]

The same IVC compression that causes supine hypotensive syndrome also contributes to venous stasis in the lower limbs, which is one of Virchow's triad elements (stasis, endothelial injury, hypercoagulability) predisposing to DVT. This is why:

Recommend lady to move around a lot → if pregnant lady lying down for prolonged period of time, have to be worried about DVT, and reduced venous return if supine ^[3]
Early mobilization, compression stockings, and LMWH thromboprophylaxis are important in at-risk pregnant women.

High Yield Summary

Supine Hypotensive Syndrome — Key Points:

Definition: Hypotension in late pregnancy (> 20 weeks, especially 3rd trimester) caused by mechanical compression of the IVC (and partially the aorta) by the gravid uterus in the supine position.
Pathophysiology: IVC compression → ↓ venous return → ↓ cardiac output → ↓ MAP. Aortic compression → ↓ uteroplacental perfusion → fetal hypoxia. The placental bed has no autoregulation to compensate for a drop in BP.
Who gets symptomatic (8–10%): Those with inadequate collateral venous drainage, or those under regional/general anaesthesia (sympathetic compensation abolished).
Clinical features: Dizziness, nausea, pallor, diaphoresis, hypotension, tachycardia (or paradoxical bradycardia via Bezold-Jarisch reflex), syncope. Fetal distress on CTG (late decelerations).
Pathognomonic feature: Rapid resolution with left lateral positioning.
Highest risk context: Caesarean section under spinal anaesthesia — mandatory left lateral uterine displacement and vasopressor support (Phenylephrine, Ephedrine, Atropine).
Fetal considerations: Uteroplacental circulation is maximally dilated with NO autoregulation — maternal BP directly determines fetal perfusion.
Associated concerns: DVT risk from venous stasis (thromboembolism is the leading cause of maternal mortality in the UK); aspiration risk (Mendelson's syndrome).
Prevention: Avoid supine position after 20 weeks; left lateral sleep position from 28 weeks; left uterine displacement during procedures; IV fluid loading and vasopressors during regional anaesthesia.

Active Recall - Supine Hypotensive Syndrome

Differential Diagnosis of Supine Hypotensive Syndrome

When a pregnant woman in the second or third trimester presents with hypotension, dizziness, or syncope — particularly when supine — your immediate thought may be supine hypotensive syndrome. But you must systematically consider and exclude other causes, some of which are life-threatening. The key clinical discriminator is the positional nature of supine hypotensive syndrome: it occurs supine and resolves rapidly (within 1–2 minutes) with left lateral positioning. If it does not resolve with repositioning, you must think of something else.

Let us approach this logically by asking: What are all the causes of hypotension, presyncope, or syncope in a pregnant woman?

We can use a pathophysiological framework based on the mechanism of hypotension/syncope, which maps neatly onto the shock classification and syncope differential ^[4]^[5]^[6]:

Detailed Differential Diagnosis Table

Condition	Mechanism of Hypotension	Key Distinguishing Features	Why It Is NOT Supine Hypotensive Syndrome
Obstetric haemorrhage (placenta praevia, placental abruption, uterine rupture, postpartum haemorrhage)	Blood loss → reduced circulating volume → reduced preload → reduced cardiac output	Vaginal bleeding (may be concealed in abruption), uterine tenderness (abruption), rigid "woody" uterus (abruption), signs of shock disproportionate to visible blood loss	Does NOT resolve with repositioning; look for bleeding source; haemoglobin drops
Non-obstetric haemorrhage (GI bleed, ruptured spleen, trauma)	Same mechanism as above	History of trauma, melaena, haematemesis, abdominal distension	Not positional; signs of bleeding from non-obstetric source
Severe dehydration (hyperemesis gravidarum, diarrhoea, inadequate intake)	Reduced intravascular volume → reduced preload	Dry mucous membranes, reduced skin turgor, tachycardia persistent in all positions, concentrated urine, history of vomiting/diarrhoea	Not purely positional; hypotension persists regardless of position (though may worsen upright — orthostatic component)

Concealed Haemorrhage

In placental abruption, bleeding may be entirely concealed behind the placenta with no visible vaginal bleeding. The patient may present with hypotension, abdominal pain, and a tense uterus. Do not be falsely reassured by the absence of external bleeding — the blood is hidden retroplacentally.

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
Peripartum cardiomyopathy (PPCM)	Dilated cardiomyopathy developing in the last month of pregnancy or within 5 months postpartum → systolic dysfunction → reduced cardiac output	Dyspnoea, orthopnoea, PND (paroxysmal nocturnal dyspnoea), bilateral lung crackles, elevated JVP, S3 gallop, peripheral oedema; echocardiography shows dilated LV with reduced EF	Persistent hypotension not related to position; signs of pulmonary congestion (which are ABSENT in SHS)
Arrhythmia (SVT, VT, complete heart block)	Abnormal cardiac rhythm → reduced effective cardiac output	Palpitations, irregular or very fast/slow pulse, ECG abnormalities; may cause syncope in any position	Not positional; ECG diagnostic; persists regardless of position ^[5]^[6]
Valvular heart disease (e.g., severe aortic stenosis, mitral stenosis)	Fixed obstruction to outflow or reduced filling → unable to increase cardiac output with demand	Pre-existing murmur, exertional syncope, symptoms may worsen with exercise rather than position	History of known valvular disease; murmur on auscultation; echocardiography diagnostic
Myocardial infarction (rare in pregnancy, but possible)	Acute coronary occlusion → myocardial pump failure	Chest pain, ST changes on ECG, elevated troponin	Persistent symptoms; ECG and troponin abnormalities; not position-dependent

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
High spinal block / neuraxial anaesthesia-related hypotension	Sympathetic blockade from spinal or epidural anaesthesia → vasodilation (reduced SVR) + venodilation (reduced preload) → reduced BP ^[2]	Occurs after regional anaesthesia administration; high sensory level (above T4); may have respiratory compromise (phrenic nerve involvement if above C3-5); warm, dry peripheries initially	Occurs in the context of recent anaesthesia; high sensory block identifiable on examination; does not resolve with position alone — needs vasopressors. Note: SHS and high spinal can co-exist and compound each other
Septic shock	Infection → systemic inflammatory response → vasodilation + capillary leak + myocardial depression	Fever or hypothermia, rigors, tachycardia, localising source of infection (e.g., chorioamnionitis — foul-smelling liquor, uterine tenderness, maternal tachycardia, fetal tachycardia), elevated WBC, elevated lactate, positive cultures ^[5]	Not positional; sepsis markers present; persistent hypotension requiring vasopressors
Anaphylaxis (e.g., to antibiotics, latex during procedures)	Massive IgE-mediated mast cell degranulation → histamine release → vasodilation + bronchospasm + capillary leak	Urticaria, widespread flushing and pruritus, severe bronchospasm and angioedema ^[5]; exposure history (drug, latex, food); rapid onset after exposure	Urticaria/angioedema/bronchospasm are absent in SHS; temporal relation to allergen exposure; does not resolve with position

Neuraxial Hypotension vs. SHS — They Overlap!

In clinical practice, during a caesarean section, the patient often has BOTH supine hypotensive syndrome AND neuraxial anaesthesia-related hypotension simultaneously. The neuraxial block removes the sympathetic compensation that would normally offset IVC compression. This is why aortocaval compression (supine hypotension syndrome): reduced by left lateral uterine displacement ^[1] and vasopressors are used together — you are treating both problems at once.

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
Massive pulmonary embolism (PE)	Thrombus occludes pulmonary vasculature → right heart cannot pump blood through → reduced LV filling → reduced cardiac output → obstructive shock	Acute dyspnoea, pleuritic chest pain, haemoptysis; tachycardia, distended neck veins (JVP ↑), right heart strain on ECG (S1Q3T3, RBBB, sinus tachycardia); risk factors: pregnancy itself is a hypercoagulable state + immobility ^[3]^[7]	Does not resolve with repositioning; JVP elevated (in SHS, JVP is low/flat); ECG shows RV strain pattern; CTPA diagnostic
Amniotic fluid embolism (AFE)	Amniotic fluid enters maternal circulation → anaphylactoid reaction → vasospasm → right heart failure → DIC	Sudden cardiovascular collapse during labour or immediately postpartum; respiratory distress; DIC with uncontrollable bleeding; extremely high mortality (~20–60%)	Catastrophic, sudden onset; DIC features; does not respond to repositioning; typically occurs during or just after delivery
Tension pneumothorax	Air accumulates in pleural space → mediastinal shift → kinks great vessels → reduced venous return	Absent breath sounds unilaterally, tracheal deviation, distended neck veins, hypotension; history of trauma or line insertion	Unilateral absent breath sounds; tracheal deviation; haemodynamically does not resolve with repositioning

In the UK, the leading cause of maternal mortality is thromboembolism ^[3]

This is why PE must always be high on your differential when a pregnant woman presents with sudden haemodynamic compromise. Pregnancy is a prothrombotic state (elevated clotting factors, venous stasis from IVC compression, reduced protein S). Recommend lady to move around a lot → if pregnant lady lying down for prolonged period of time, have to be worried about DVT, and reduced venous return if supine ^[3].

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
Vasovagal syncope	Emotional/painful stimulus → Bezold-Jarisch reflex → paradoxical vagal activation → bradycardia + vasodilation → hypotension → cerebral hypoperfusion ^[4]^[6]	Precipitated by pain, fear, prolonged standing, venepuncture; prodrome of nausea, warmth, pallor, diaphoresis; recovery rapid once supine	Triggered by emotional/painful stimuli, NOT by supine positioning; in fact, lying supine is the TREATMENT for vasovagal syncope (opposite of SHS!)
Orthostatic hypotension	Failure of baroreceptor-mediated vasoconstriction on standing → venous pooling → reduced venous return ^[4]^[6]	Defined as ↓SBP ≥ 20 mmHg or ↓DBP ≥ 10 mmHg within 3 minutes of standing; occurs on STANDING (upright), not supine; common with antihypertensives, autonomic neuropathy (DM, PD), hypovolaemia	Positional BUT in the opposite direction — orthostatic hypotension occurs on STANDING, whereas SHS occurs when SUPINE. This is a critical distinction
Neurogenic shock (e.g., from high spinal cord injury)	Loss of sympathetic vasomotor tone → profound vasodilation + paradoxically slow heart rate ^[5]	History of CNS injury (SCI, TBI) or neuraxial anaesthesia; warm, dry peripheries; bradycardia (unlike most other forms of shock which cause tachycardia)	History of CNS injury; persistent; does not resolve with position change

Vasovagal vs SHS — Opposite Positional Responses!

A classic exam trap: Vasovagal syncope is TREATED by lying supine (increases venous return to the brain). Supine hypotensive syndrome is CAUSED by lying supine and TREATED by turning lateral. If a pregnant woman faints while standing in a queue → think vasovagal. If she feels faint while lying on her back for an ultrasound scan → think SHS. The position that provokes vs. relieves the symptoms tells you the diagnosis.

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
Eclampsia / Severe pre-eclampsia	Endothelial dysfunction → vasospasm → hypertension (usually), but can present with seizures and subsequent hypotension; HELLP syndrome may cause hepatic rupture → haemorrhagic shock ^[3]	Usually hypertension (not hypotension!); proteinuria; headache, visual disturbance, epigastric pain; seizures in eclampsia; thrombocytopenia, elevated LFTs, haemolysis in HELLP	Typically presents with HYPERTENSION, not hypotension (though cardiovascular collapse can occur in severe cases); does not resolve with repositioning
Uterine rupture	Catastrophic tear in uterine wall → massive intra-abdominal haemorrhage + loss of uterine tone → haemorrhagic shock	Prior caesarean scar; sudden severe abdominal pain; cessation of contractions; recession of presenting part; fetal distress; peritonism	Haemorrhagic shock; does not resolve with positioning; requires emergency laparotomy
Placental abruption	Premature separation of placenta → retroplacental haemorrhage → hypovolaemic shock + consumptive coagulopathy (DIC)	Painful vaginal bleeding (or concealed), tense "woody" uterus, fetal distress, signs of DIC	Haemorrhagic; does not resolve with position change; uterine tenderness is the key sign

Condition	Mechanism	Key Distinguishing Features	Why It Is NOT SHS
Hypoglycaemia	Low blood glucose → inadequate cerebral energy supply → altered consciousness	Tremor, diaphoresis, confusion, seizures; rapid response to glucose administration; BM/fingerprick glucose is low	Not positional; BM diagnostic; responds to glucose, not position change ^[6]
Seizure / Eclamptic fit	Abnormal neuronal electrical activity → LOC with motor phenomena	Tonic-clonic movements, tongue biting, incontinence, prolonged post-ictal confusion ^[6]; in pregnancy, always consider eclampsia (check BP, proteinuria, reflexes)	Motor phenomena present; LOC > 1 minute typically; post-ictal confusion (SHS recovery is immediate)
Hyperventilation / Panic attack	Anxiety → hyperventilation → respiratory alkalosis → reduced ionized calcium → paraesthesias, tetany, lightheadedness	Perioral and distal paraesthesias, carpopedal spasm, tachypnoea without hypoxia, psychological stressor	Not associated with true hypotension; ABG shows respiratory alkalosis; no cardiovascular compromise

Feature	Supine Hypotensive Syndrome	Other Causes
Positional trigger	Supine position specifically	Variable (standing = orthostatic/vasovagal; any position = cardiac/haemorrhagic)
Resolution with position change	Rapid (1–2 min) with left lateral tilt	Does not resolve (or resolves with opposite position, e.g., supine for vasovagal)
Gestational age	> 20 weeks, especially 3rd trimester	Variable
Bleeding	Absent	Present in haemorrhagic causes
Fever / Infection signs	Absent	Present in sepsis
Rash / Wheeze	Absent	Present in anaphylaxis
Neurological signs	Absent (other than from hypoperfusion)	Present in seizure, stroke
JVP	Low / flat (due to reduced venous return)	Elevated in PE, tamponade, tension pneumothorax, cardiogenic shock
Response to IV fluids	Partial (increases preload but does not fix mechanical compression)	Hypovolaemia responds fully; cardiogenic may worsen
ECG	Normal (or sinus tachycardia)	Abnormal in arrhythmia, PE (S1Q3T3), MI

High Yield Summary — Differential Diagnosis

The pathognomonic feature of supine hypotensive syndrome is its rapid resolution with left lateral positioning.

Key differentials to always consider in a hypotensive pregnant woman:

Haemorrhage (abruption, praevia, rupture) — look for bleeding, tense uterus
Massive PE — pregnancy is a hypercoagulable state; thromboembolism is the leading cause of maternal mortality in the UK ^[3]; elevated JVP, RV strain on ECG
Amniotic fluid embolism — sudden collapse during/after labour, DIC
Sepsis (chorioamnionitis) — fever, foul liquor, fetal/maternal tachycardia
Anaphylaxis — rash, wheeze, allergen exposure
High spinal block — recent neuraxial anaesthesia, high sensory level, respiratory compromise
Eclampsia — usually hypertensive, seizures, proteinuria
Vasovagal — triggered by standing/pain/emotion, relieved by lying supine (opposite to SHS!)
Peripartum cardiomyopathy — heart failure signs, pulmonary congestion

The position that triggers vs. relieves symptoms is your best bedside discriminator.

Active Recall - Differential Diagnosis of Supine Hypotensive Syndrome

References

^[1] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p39) ^[2] Senior notes: Maksim Surgery Notes.pdf (p298 — Obstetrics Anaesthesia section) ^[3] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p39 — Prevention/DVT sidebar) ^[4] Senior notes: Maksim Medicine Notes.pdf (p118 — Syncope) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p15–16 — Shock Classification) ^[6] Senior notes: Ryan Ho Neurology.pdf (p90 — Faints and Fits) ^[7] Senior notes: Ryan Ho Haemtology.pdf (p131 — VTE)

Diagnostic Criteria, Algorithm, and Investigations for Supine Hypotensive Syndrome

Let me be upfront: supine hypotensive syndrome does not have formal published diagnostic criteria in the way that pre-eclampsia, sepsis, or heart failure do. There is no scoring system, no international consensus statement with numbered criteria. Why? Because the diagnosis is fundamentally clinical and positional — it is made at the bedside by demonstrating a clear temporal relationship between the supine position and haemodynamic compromise, with rapid resolution upon repositioning.

That said, we can define a structured diagnostic framework based on physiological principles and clinical practice.

The diagnosis of supine hypotensive syndrome requires ALL of the following:

Criterion	Rationale
1. Pregnancy ≥ 20 weeks' gestation (typically 3rd trimester)	The gravid uterus must be large enough to compress the IVC. Before 20 weeks, the uterus is too small to exert meaningful compression against the vertebral column.
2. Supine positioning as the precipitant	Symptoms must develop while the patient is lying flat on her back. This is the mechanical trigger — gravity pulls the heavy uterus posteriorly onto the IVC.
3. Haemodynamic compromise — defined as: systolic BP drop > 15–20% from baseline, OR absolute systolic BP < 90 mmHg, OR symptomatic hypotension (dizziness, nausea, presyncope, syncope)	This confirms that the mechanical compression is haemodynamically significant, not merely subclinical. A drop in cardiac output must be sufficient to produce symptoms or measurable hypotension.
4. Rapid resolution (within 1–3 minutes) with left lateral positioning or manual left uterine displacement	This is the pathognomonic feature. No other cause of hypotension in pregnancy resolves so quickly and completely with a simple position change. It confirms that the mechanism is mechanical (IVC decompression → restored venous return → restored cardiac output).
5. Absence of another identifiable cause of hypotension (haemorrhage, sepsis, anaphylaxis, PE, cardiogenic shock, high spinal block)	SHS is essentially a diagnosis of positive identification (positional relationship) combined with exclusion of dangerous mimics. You must briefly consider and exclude the serious differentials discussed previously.

The Positional Test IS the Diagnostic Test

Think of the lateral tilt as both a diagnostic manoeuvre and a therapeutic intervention simultaneously. If you suspect supine hypotensive syndrome, turn the patient to the left lateral position. If BP recovers within 1–3 minutes → you have confirmed the diagnosis AND treated the patient in one move. This is one of the few conditions in medicine where diagnosis and treatment are the same action.

The clinical approach is driven by two priorities:

Immediate management — because the patient is hypotensive and potentially compromising fetal perfusion, you must act before you investigate.
Exclusion of dangerous mimics — you must rapidly consider whether this could be something more sinister.

The placental bed has no autoregulation to compensate for a drop in BP ^[1] — so every minute of maternal hypotension is a minute of reduced fetal perfusion. Speed matters.

Act First, Investigate Second

Notice the algorithm starts with an intervention (left lateral tilt), not an investigation. This is deliberate. In an acutely hypotensive pregnant woman, the priority is to restore uteroplacental perfusion immediately. You can always investigate afterwards. The positional response itself is diagnostic.

Investigation Modalities

Investigations in supine hypotensive syndrome serve two purposes:

Confirm the diagnosis (largely clinical — limited role for investigations)
Exclude dangerous differentials (this is where investigations are essential)

Investigation	What You Are Looking For	Key Findings / Interpretation
Blood pressure monitoring (serial, bilateral)	Documenting the haemodynamic event and its resolution	Systolic BP drop > 15–20% from baseline when supine; recovery within 1–3 minutes of left lateral tilt. Document the positional BP change — this is your primary diagnostic evidence. Measure in the upper arm (brachial) noting that this may not reflect uteroplacental perfusion if aortic compression is present (Poseiro effect).
Heart rate monitoring (continuous or serial)	Compensatory tachycardia vs. paradoxical bradycardia (Bezold-Jarisch reflex) ^[8]	Tachycardia = compensatory sympathetic response, reassuring that autonomic reflexes are intact. Bradycardia = Bezold-Jarisch reflex activation, more dangerous — vigorous contraction of underfilled ventricle stimulates vagal afferents → paradoxical bradycardia and vasodilation.
Pulse oximetry (SpO2)	Maternal oxygenation	Usually normal in isolated SHS (the problem is flow, not oxygenation). If SpO2 is low, consider PE, amniotic fluid embolism, high spinal block, or aspiration.
Cardiotocography (CTG) — CRITICAL	Fetal heart rate pattern: assessing for fetal hypoxia secondary to reduced uteroplacental perfusion	Normal/Reassuring CTG: Normal baseline (110–160 bpm), normal variability (5–25 bpm), accelerations present, no decelerations → fetal perfusion adequate despite maternal event. Abnormal CTG suggesting fetal compromise: Late decelerations (onset after peak of contraction, smooth descent, slow recovery — these indicate uteroplacental insufficiency), fetal bradycardia (< 110 bpm sustained), reduced variability (< 5 bpm for > 40 minutes). Late decelerations in the context of maternal supine positioning are highly suggestive of the Poseiro effect (aortic compression reducing uterine arterial flow).
Capillary blood glucose	Exclude hypoglycaemia as a mimic of presyncope/syncope ^[5]	Normal glucose effectively rules out hypoglycaemia as the cause.

Aortocaval compression (supine hypotension syndrome): reduced by left lateral uterine displacement ^[1]

These are indicated when the diagnosis is not clear-cut (e.g., incomplete response to repositioning, or features suggesting an alternative or coexisting pathology).

Investigation	Rationale	Key Findings in Context
Full blood count (FBC)	Screen for anaemia (haemorrhage), leukocytosis (infection/sepsis), thrombocytopenia (DIC, HELLP, pre-eclampsia) ^[5]^[9]	Normocytic anaemia with dropping Hb → consider haemorrhage (note: acute blood loss may not show Hb drop for 24–48h due to proportional loss of plasma and red cells). Thrombocytopenia → consider HELLP, DIC, TTP.
Coagulation profile (PT, APTT, fibrinogen)	Screen for DIC (abruption, AFE, sepsis) and coagulopathy	Prolonged PT/APTT, low fibrinogen, elevated D-dimer → DIC. In pregnancy, fibrinogen is normally elevated (4–6 g/L); a "normal" non-pregnant fibrinogen level (2–4 g/L) in a pregnant woman is actually relatively LOW and concerning.
Renal function tests (U&E, creatinine)	Assess for renal impairment secondary to hypoperfusion; also relevant for pre-eclampsia screening ^[9]	Elevated creatinine → renal hypoperfusion from prolonged hypotension, or renal involvement in pre-eclampsia. Hypokalaemia/hypochloraemia may indicate prolonged vomiting as a cause of dehydration.
Liver function tests (LFTs)	Screen for HELLP syndrome (elevated AST/ALT), hepatic hypoperfusion ("shock liver") ^[9]	Markedly elevated AST/ALT → HELLP syndrome or "shock liver" from prolonged hypotension.
Venous blood gas (VBG) / Arterial blood gas (ABG) + Lactate	Assess tissue perfusion (lactate is a marker of anaerobic metabolism from poor perfusion); acid-base status ^[5]	Elevated lactate ( > 2 mmol/L) → tissue hypoperfusion from any cause. In isolated, brief SHS that resolves quickly, lactate should be normal or only mildly elevated. Persistently elevated lactate suggests an alternative or additional cause of shock.
Cardiac biomarkers (troponin, BNP/NT-proBNP)	Exclude MI as a cause of cardiogenic shock; BNP for heart failure (peripartum cardiomyopathy) ^[5]	Elevated troponin → myocardial ischaemia or injury. Elevated BNP → heart failure (consider peripartum cardiomyopathy if new onset in late pregnancy). Note: BNP may be mildly elevated in normal pregnancy.
D-dimer	Screen for PE/VTE	Caveat: D-dimer is physiologically elevated in pregnancy (levels rise throughout gestation) ^[7]. It has very poor specificity in pregnant women. A negative D-dimer may help exclude PE in some settings, but a positive result is NOT diagnostic and requires further imaging.
Group and screen / Cross-match	Prepare for possible transfusion if haemorrhage is suspected	Standard in any haemodynamically unstable pregnant patient.

D-dimer in Pregnancy

A common mistake: D-dimer is often reflexively ordered when PE is suspected. In pregnancy, D-dimer is physiologically elevated from the first trimester and continues to rise. Most pregnant women at term will have D-dimer levels above the standard cut-off (500 ng/mL). Therefore, a positive D-dimer in pregnancy has very low specificity and should NOT be used to diagnose PE. Trimester-specific cut-offs have been proposed but are not widely validated. If clinical suspicion for PE is high, proceed directly to imaging ^[7]^[10].

Investigation	When to Use	Key Findings
ECG (12-lead)	Always obtain if hypotension persists or if cardiac cause suspected ^[5]	Normal/sinus tachycardia → consistent with SHS (compensatory tachycardia). S1Q3T3, RBBB, RV strain pattern, T-wave inversions V1–V4 → massive PE ^[7]^[10]. ST elevation/depression → myocardial ischaemia. Arrhythmia (AF, SVT, VT, heart block) → arrhythmogenic cause. In isolated SHS, ECG should be normal apart from sinus tachycardia.
Bedside echocardiography (TTE/POCUS)	If cardiogenic or obstructive shock suspected	Hyperdynamic, underfilled LV → consistent with hypovolaemia or SHS (low preload). Dilated RV with RV dysfunction, D-shaped septum → massive PE (acute right heart strain). Dilated LV with reduced EF → peripartum cardiomyopathy. Pericardial effusion → tamponade. Collapsible IVC → hypovolaemia / reduced preload (consistent with SHS). Non-collapsible, dilated IVC → obstructive or cardiogenic shock.
Chest X-ray (CXR)	If respiratory distress, hypoxia, or suspected PE/pneumothorax	Clear lung fields → consistent with SHS (no pulmonary pathology). Wedge-shaped opacity (Hampton hump), focal oligaemia (Westermark sign) → PE ^[10]. Absent lung markings, mediastinal shift → tension pneumothorax. Bilateral infiltrates → aspiration (Mendelson's syndrome) ^[1], pulmonary oedema, ARDS.
V/Q scan (ventilation-perfusion scintigraphy)	If PE suspected in pregnancy — lower radiation, tracer does not cross placenta → safe to use in pregnant patient ^[10]	Mismatched perfusion defects (normal ventilation, absent perfusion) → PE. V/Q scan has similar PPV and NPV to CTPA but with lower radiation (~2 mSv vs 8–10 mSv) and avoids iodinated contrast (which crosses the placenta and can affect fetal thyroid) ^[10].
CT pulmonary angiography (CTPA)	If PE suspected and V/Q not available or non-diagnostic; also if alternative diagnosis sought	Intraluminal filling defect in pulmonary arteries → PE. Relatively high radiation (8–10 mSv) and contrast crosses placenta → risk of CA breast in the mother ^[10]. Use only when V/Q is non-diagnostic or unavailable.
Doppler ultrasound of lower limbs	If DVT suspected as source of PE	Non-compressible vein, absent flow on Doppler → DVT ^[7].
Obstetric ultrasound	Assess fetal wellbeing, placental position, amniotic fluid volume	Exclude placenta praevia, assess fetal growth, check for polyhydramnios (risk factor for more severe SHS due to increased uterine size).

Investigation	Context	Explanation
IVC compression assessment by Doppler ultrasound	Research / academic interest	Can directly visualise IVC compression by the gravid uterus in the supine position and demonstrate restoration of IVC calibre with lateral tilt. Used in studies quantifying the degree of aortocaval compression. Not routine in clinical practice.
Non-invasive cardiac output monitoring	Peri-operative (during caesarean section)	Devices such as oesophageal Doppler, LiDCO, or pulse contour analysis can continuously measure cardiac output. Can demonstrate the drop in CO with supine positioning and recovery with tilt. Used in anaesthetic research and some high-risk obstetric cases.
Invasive haemodynamic monitoring (arterial line, CVP, pulmonary artery catheter)	ICU setting for refractory/unexplained shock ^[5]	Central venous pressure (CVP) will be low in SHS (reduced venous return). In cardiogenic shock, CVP is high. In obstructive shock (PE), CVP is high. This distinction is diagnostically useful in the critical care setting. Reduced CVP < 8 mmHg → hypovolaemia or reduced preload ^[5].

Finding	Consistent with SHS	Suggests Alternative Diagnosis
BP drops supine, recovers left lateral	✅ Pathognomonic	—
Normal ECG or sinus tachycardia	✅	S1Q3T3/RBBB → PE; ST changes → MI
CTG: late decelerations supine, resolves lateral	✅ (Poseiro effect)	Persistent decelerations → other cause of fetal distress
Normal FBC, coagulation, LFTs	✅	Anaemia → haemorrhage; ↓platelets → HELLP/DIC
Collapsible IVC on echo	✅ (reduced preload)	Dilated non-collapsible IVC → obstructive shock
Hyperdynamic, underfilled LV on echo	✅	Dilated LV with ↓EF → cardiomyopathy
Elevated lactate	⚠️ Only if brief/mild	Persistently elevated → shock from other cause
Low JVP	✅	Elevated JVP → PE, tamponade, cardiogenic

High Yield Summary — Diagnosis

Supine Hypotensive Syndrome: Diagnostic Approach

Clinical diagnosis — no formal scoring system or biomarker exists. The diagnosis is made by demonstrating:
- Onset of hypotension/symptoms in the supine position in a pregnant woman ≥ 20 weeks
- Rapid resolution (1–3 minutes) with left lateral positioning
- Exclusion of other causes
The positional tilt IS the diagnostic test — turning the patient to the left lateral position is simultaneously diagnostic and therapeutic.
CTG is essential — to assess fetal wellbeing, as the placental bed has no autoregulation to compensate for a drop in BP ^[1].
Investigations are primarily to exclude differentials:
- ECG → exclude arrhythmia, PE, MI
- FBC, coagulation, LFTs → exclude haemorrhage, HELLP, DIC
- V/Q scan (preferred in pregnancy) or CTPA → exclude PE ^[10]
- Bedside echo → exclude cardiogenic or obstructive shock
Key bedside echo finding: Collapsible IVC + hyperdynamic underfilled LV = low preload, consistent with SHS. Dilated non-collapsible IVC = obstructive cause.
D-dimer is unreliable in pregnancy — physiologically elevated; do not use to rule in or rule out PE ^[7]^[10].

Active Recall - Diagnosis of Supine Hypotensive Syndrome

References

^[1] Senior notes: Maksim Surgery Notes.pdf (p298 — Obstetrics Anaesthesia section) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p15–17 — Shock Classification and Approach) ^[7] Senior notes: Ryan Ho Haemtology.pdf (p131 — VTE) ^[8] Senior notes: Ryan Ho Cardiology.pdf (p66 — Neurocardiogenic Syncope / Bezold-Jarisch Reflex) ^[9] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p25 — Baseline Investigations for Pre-eclampsia) ^[10] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p62 — CT Angiography vs V/Q Scan in Pregnancy)

Management of Supine Hypotensive Syndrome

Let me frame this clearly: supine hypotensive syndrome is one of the most gratifying conditions to manage in all of medicine. Why? Because the treatment is simple, immediate, and almost universally effective — turn the patient on her side. The challenge is not in treating SHS itself, but in:

Preventing it from happening in the first place (the real goal)
Managing it in the operative setting where supine positioning is unavoidable
Recognising when it is NOT SHS and something more dangerous is occurring

Management can be divided into:

Immediate management (acute episode)
Preventive management (avoiding recurrence / anticipatory management)
Peri-operative management (the highest-risk context — caesarean section under anaesthesia)

A. Immediate Management of an Acute Episode

This is the single most important intervention. It is both diagnostic and therapeutic.

Intervention	Technique	Mechanism
Left lateral position	Turn the patient fully onto her left side	Gravity pulls the uterus away from the IVC and aorta → immediately decompresses both vessels → restores venous return → restores cardiac output → restores MAP → restores uteroplacental perfusion
Left lateral tilt (15° wedge)	Place a firm wedge (or folded blanket/pillow) under the patient's right hip to tilt the pelvis 15° to the left	Achieves partial uterine displacement while keeping the patient nearly supine — useful in peri-operative settings where full lateral position is not feasible for surgical access
Manual left uterine displacement	An assistant places both hands on the right side of the uterus and pushes it to the patient's left	Direct mechanical displacement of the uterus off the IVC; faster than repositioning the whole patient; ideal in the operative setting where the patient cannot be moved

Aortocaval compression (supine hypotension syndrome): reduced by left lateral uterine displacement ^[1]

Why LEFT Lateral, Not Right?

The IVC runs to the right of the vertebral column. The pregnant uterus naturally dextrorotates (rotates to the right) because the sigmoid colon is on the left. Therefore, left lateral positioning moves the uterus away from the IVC most effectively. Right lateral positioning would push the already dextrorotated uterus further onto the IVC. Additionally, left lateral positioning avoids aortic compression (aorta is slightly left of midline, but much more resistant to compression due to higher intraluminal pressure).

Measure	Rationale
Call for help	Even though SHS is benign, it can co-exist with more dangerous pathology; in the operative setting, it may herald cardiovascular collapse
Supplemental oxygen (if available)	While the primary problem is flow (not oxygenation), supplemental O₂ increases the oxygen content of the blood that IS reaching the placenta → maximises fetal oxygen delivery during the hypotensive episode
IV access (if not already in situ)	In case IV fluids or vasopressors are needed; also necessary if the diagnosis turns out to be something else
Assess fetal wellbeing (CTG)	The placental bed has no autoregulation to compensate for a drop in BP ^[1] — must confirm that the fetus has not been compromised during the hypotensive episode

If blood pressure does not recover within 1–3 minutes of left lateral positioning, this is NOT isolated supine hypotensive syndrome. Initiate the ABCDE approach to shock:

Airway: Ensure patent; consider the need for intubation if consciousness impaired
Breathing: High-flow O₂ (15 L/min via non-rebreather mask); assess for respiratory compromise (high spinal block, PE, aspiration)
Circulation:
- Two large-bore IV cannulae (14–16G)
- IV crystalloid bolus (500–1000 mL Ringer's lactate or normal saline)
- Vasopressors if hypotension persists despite fluid loading (see below)
- Group and screen / cross-match blood
Disability: GCS, blood glucose, pupil examination
Exposure: Full examination to identify source of bleeding, infection, etc.

B. Pharmacological Management (Peri-operative Context)

Pharmacological agents are primarily used in the peri-operative setting — during caesarean section or other procedures under anaesthesia where supine positioning is unavoidable and regional anaesthesia abolishes sympathetic compensation.

Vasopressors used intra-op: Ephedrine (alpha + beta), Phenylephrine (pure alpha), Atropine (vagal block) ^[1]

Drug	Mechanism	Dose	Indications	Contraindications / Cautions	Why This Drug?
Phenylephrine ("phenyl" = phenyl ring, "ephrine" = adrenaline derivative)	Pure α₁-adrenergic agonist → arteriolar vasoconstriction → ↑SVR → ↑MAP	Bolus: 50–100 mcg IV. Infusion: 25–50 mcg/min (titrate to BP)	First-line vasopressor for anaesthesia-related hypotension during CS. Prophylactic infusion is now standard of care	Maternal bradycardia (can worsen; use ephedrine instead if HR < 60). Caution in severe cardiac disease	Phenylephrine does NOT cross the placenta as readily as ephedrine → less fetal acidosis. Provides predictable, dose-dependent vasoconstriction. Associated with better neonatal acid-base status in RCTs vs ephedrine
Ephedrine ("eph" from Ephedra plant)	Mixed α + β agonist → vasoconstriction (α₁) + ↑HR and contractility (β₁) + mild bronchodilation (β₂)	Bolus: 6–12 mg IV, repeat Q3–5 min prn	Second-line vasopressor. Preferred over phenylephrine when maternal heart rate is low (bradycardia) because β₁ effect ↑HR	Tachycardia (avoid if HR already elevated). Crosses placenta more readily → may cause fetal acidosis (β-mediated increase in fetal metabolism → ↑lactate production)	Historically was first-line, but RCTs showed phenylephrine produces better neonatal umbilical cord pH → phenylephrine now preferred. Still useful when bradycardia is present
Atropine ("atropa" from Atropa belladonna, the deadly nightshade plant)	Muscarinic (M₂) receptor antagonist → blocks vagal (parasympathetic) input to the SA node → ↑HR	Bolus: 0.3–0.6 mg IV (can repeat to max 3 mg)	Severe bradycardia (HR < 50 bpm) — particularly if Bezold-Jarisch reflex is suspected	Tachycardia. Glaucoma (relative). Crosses placenta (but usually single dose is safe)	Addresses the paradoxical Bezold-Jarisch reflex — where vigorous contraction of an underfilled ventricle triggers vagal activation → bradycardia. Atropine blocks this vagal response
Noradrenaline (norepinephrine)	Strong α₁ + moderate β₁ agonist → vasoconstriction + modest inotropy	Infusion: 0.05–0.3 mcg/kg/min	Refractory hypotension not responding to phenylephrine/ephedrine; more commonly used in ICU setting for distributive/septic shock ^[11]	Must be given via central line (risk of tissue necrosis if extravasates peripherally)	Reserved for refractory cases; not first-line for obstetric anaesthesia-related hypotension

Phenylephrine vs. Ephedrine — The Modern Evidence

Multiple RCTs (including the landmark Ngan Kee et al. studies) have demonstrated that phenylephrine is associated with better fetal acid-base status (higher umbilical artery pH) compared to ephedrine during spinal anaesthesia for CS. The reason: ephedrine crosses the placenta more readily due to its higher lipophilicity and stimulates fetal β-adrenergic receptors → increases fetal metabolic rate → increases fetal oxygen consumption → relative fetal acidosis. Phenylephrine, as a pure α-agonist, has minimal placental transfer and does not stimulate fetal metabolism. Current consensus (2024–2026): Phenylephrine infusion is first-line for prophylaxis and treatment of spinal hypotension during CS.

Strategy	Technique	Mechanism	Evidence
Pre-loading	500–1000 mL crystalloid (Ringer's lactate or NS) given before spinal anaesthesia	Expands intravascular volume → increases preload → offsets the venodilation caused by sympathetic blockade	Evidence shows pre-loading alone is insufficient to prevent spinal hypotension — the fluid rapidly redistributes into the interstitium. Must be combined with vasopressors
Co-loading (preferred)	500–1000 mL crystalloid given rapidly at the time of or immediately after spinal injection	Same mechanism, but timing is more effective — fluid is given at the point of maximal sympathetic blockade	Co-loading with vasopressor infusion is now the standard approach. More effective than pre-loading alone
Colloid (e.g., Gelofusine, Voluven)	Oncotic agents that remain in the intravascular space longer	Sustains intravascular volume expansion for longer than crystalloid	Some evidence of modest benefit over crystalloid for pre-loading, but cost-benefit does not favour routine use. Not commonly used in current practice

In the operative setting, the risk of aspiration is intertwined with supine positioning during anaesthesia:

Mendelson's syndrome (chemical pneumonitis caused by aspiration of acidic stomach contents during anaesthesia in childbirth): reduced by RSI and antacids (30 mL sodium citrate + H2RA/PPI) ^[1]

Agent	Mechanism	Dose	When
Sodium citrate 0.3M (30 mL)	Non-particulate antacid → neutralizes gastric acid → raises gastric pH	30 mL orally	Within 30 minutes of induction
Ranitidine (H₂RA) or PPI	Reduces gastric acid secretion → ↓volume and ↓acidity of gastric contents	Ranitidine 150 mg PO or 50 mg IV; or omeprazole 40 mg IV	Given 1–2 hours before procedure
Metoclopramide	Prokinetic → accelerates gastric emptying; also antiemetic (D₂ antagonist)	10 mg IV	Given 30–60 minutes before procedure

C. Preventive Management (Avoiding Occurrence)

Prevention is the cornerstone of management. Most episodes of SHS are entirely preventable.

Advice	Rationale
Avoid lying flat on the back from 20 weeks	Prevents IVC compression; especially important in the 3rd trimester when the uterus is large enough to cause significant compression
Sleep in the left lateral position from 28 weeks	Recent evidence (MiNESS, CRIBSS studies) shows supine sleep position in the 3rd trimester is associated with 2–3× increased risk of late stillbirth due to recurrent uteroplacental hypoperfusion. Current guidelines (RCOG, NZ) recommend lateral sleep position
Use a pillow or wedge behind the right hip if she rolls onto her back during sleep	Prevents inadvertent return to supine position; creates a slight left lateral tilt even if she shifts during sleep
Move around a lot → if pregnant lady lying down for prolonged period of time, have to be worried about DVT, and reduced venous return if supine ^[3]	Mobilisation prevents both venous stasis (DVT risk) and prolonged IVC compression
During ultrasound or examinations requiring supine position — communicate symptoms immediately (dizziness, nausea)	Allows the clinician to adjust position promptly; ultrasound can be performed in semi-reclined or left-lateral tilted position

Setting	Preventive Action
Antenatal clinic / Ultrasound	Position the patient with a wedge under the right hip during any supine examination in the 3rd trimester. Monitor for symptoms. Minimise time in supine position
Labour ward	Avoid supine lithotomy; encourage lateral or upright positions during labour. If supine is necessary (e.g., for vaginal examination, CTG), keep it brief and use left tilt
Operating theatre (CS)	Left lateral uterine displacement is mandatory before and during caesarean section — use a table tilt (15° left), a pelvic wedge, or manual displacement by an assistant ^[1]^[2]
Emergency / Resuscitation	If a pregnant woman > 20 weeks requires CPR, perform it with continuous manual left uterine displacement — without this, chest compressions will be ineffective because the IVC is compressed and venous return cannot be restored by compressions alone. If CPR is unsuccessful after 4 minutes, consider perimortem caesarean section (ideally within 5 minutes) to relieve aortocaval compression and allow effective resuscitation of the mother (as well as delivering the fetus)

Perimortem Caesarean Section

The rationale for perimortem CS is often misunderstood. It is NOT primarily to save the baby (though that is a secondary benefit). It is primarily to save the mother — by removing the gravid uterus's compressive effect on the IVC and aorta, you restore venous return and allow chest compressions to generate effective cardiac output. Studies show that ROSC (return of spontaneous circulation) often occurs immediately after delivery. Guidelines recommend delivery within 5 minutes of cardiac arrest onset if the uterus is palpable above the umbilicus (i.e., ≥ 20 weeks).

The same IVC compression that causes SHS also predisposes to DVT through venous stasis. Preventive measures overlap:

In the UK, the leading cause of maternal mortality is thromboembolism ^[3]

Intervention	Indication	Notes
Early mobilisation	All pregnant women; especially post-CS	Recommend lady to move around a lot ^[3]
Graduated compression stockings (GCS)	All women at moderate DVT risk	Improve venous return from lower limbs; reduce venous stasis
Intermittent pneumatic compression (IPC)	During CS; post-CS until mobile	Mechanical compression of calf → enhances venous return ^[12]
LMWH thromboprophylaxis	High-risk women (previous VTE, thrombophilia, BMI > 40, immobility, pre-eclampsia)	Pregnancy is a prothrombotic state due to ↑coagulation factors + ↑venous stasis ^[7]. LMWH does not cross the placenta → safe in pregnancy. Continue up to 6 weeks postpartum ^[7]

D. Management in Specific Clinical Scenarios

This is the most critical scenario and warrants a detailed protocol:

Step	Action	Rationale
1. Pre-operative	Document any history of supine symptoms; risk stratification; administer antacid prophylaxis (sodium citrate + H₂RA/PPI) ^[1]	Preparation; aspiration prevention
2. Positioning	Left lateral uterine displacement (15° left pelvic tilt with wedge OR manual displacement) — must be applied before spinal injection and maintained throughout surgery ^[1]^[2]	Prevents IVC compression; most important single intervention
3. IV access	Secure large-bore IV (16–18G)	For fluid and vasopressor administration
4. IV fluid co-loading	Start 500–1000 mL crystalloid rapidly at the time of spinal injection	Offsets venodilation from sympathetic blockade
5. Prophylactic vasopressor infusion	Phenylephrine infusion 25–50 mcg/min, started at the time of spinal injection and titrated to maintain systolic BP > 90% of baseline	Prevents hypotension rather than treating it; current gold-standard based on RCT evidence
6. Continuous monitoring	BP every 1–2 minutes (initially), HR, SpO₂, CTG	Early detection of hypotension allows immediate dose adjustment
7. Rescue boluses	Phenylephrine 50–100 mcg IV bolus if breakthrough hypotension; Ephedrine 6–12 mg IV if concurrent bradycardia; Atropine 0.3–0.6 mg IV if severe bradycardia (HR < 50) ^[1]	Tiered approach depending on haemodynamic pattern

Difficult intubation is 8 times more common than normal patients due to weight gain and oedema, pre-existing obstetric disease (e.g., pre-eclampsia), increased oxygen demand by 20% & reduced oxygen reserve (FRC drop 20%): less apnoea time allowed, delayed gastric emptying and relaxed LES due to progesterone → risk of aspiration even with fasting ^[1]

Principle	Action
Positioning	Encourage upright, lateral, or semi-reclined positions during labour. Avoid supine lithotomy position for prolonged periods
If epidural in situ	Monitor BP closely after epidural top-ups (sympathetic blockade can unmask aortocaval compression). Maintain left tilt
Second stage	If supine position is required (e.g., instrumental delivery), apply left tilt with a wedge. Keep duration of supine positioning as short as possible
Fetal monitoring	CTG: watch for late decelerations that improve with position change (Poseiro effect) → change maternal position rather than proceeding to emergency CS

Time	Action
0 minutes	Begin standard CPR (high-quality chest compressions + ventilation). Apply continuous manual left uterine displacement throughout CPR — without this, chest compressions are ineffective ^[11]
1–4 minutes	Standard ALS algorithm (defibrillation if shockable rhythm, adrenaline if non-shockable). Continue left uterine displacement. Consider reversible causes (4 H's and 4 T's)
4–5 minutes	If no ROSC, prepare for perimortem caesarean section. Aim to deliver the fetus within 5 minutes of cardiac arrest onset. This is to relieve aortocaval compression and allow effective maternal resuscitation
Post-delivery	Continue CPR. ROSC often occurs immediately after delivery due to restoration of venous return

The 5-Minute Rule for Perimortem CS

The "5-minute rule" does not mean "wait 5 minutes and then decide." It means the baby should be OUT by 5 minutes from arrest onset. Given that the decision, preparation, and incision all take time, the decision to proceed must be made within the first 1–2 minutes if initial resuscitation is unsuccessful. In practice, this means: start CPR with left uterine displacement → if no ROSC within 4 minutes → deliver immediately. The operation is performed at the bedside, not in the operating theatre.

Context	Key Interventions	Pharmacological
Community / Home	Education: avoid supine; left lateral sleep; pillow behind right hip	None
Antenatal clinic / Ultrasound	Wedge under right hip; minimise supine time; communicate symptoms	None
Labour	Lateral or upright positioning; left tilt if supine needed; CTG monitoring	Vasopressors only if epidural-related hypotension
CS under spinal	Left lateral uterine displacement; IV co-loading; prophylactic phenylephrine infusion; continuous monitoring	Phenylephrine (first-line), Ephedrine (if bradycardic), Atropine (if severe bradycardia) ^[1]
CS under GA	RSI with propofol + suxamethonium; left tilt; rapid delivery to minimise neonatal drug exposure ^[1]	Same vasopressors; anti-aspiration prophylaxis (sodium citrate, H₂RA/PPI)
Cardiac arrest	Manual left uterine displacement during CPR; perimortem CS within 5 minutes if no ROSC	Adrenaline per ALS protocol
Post-event	Document in notes; educate patient; flag for future anaesthesia planning; assess fetal wellbeing	None

High Yield Summary — Management

Key Management Principles of Supine Hypotensive Syndrome:

The treatment IS the diagnosis: Left lateral positioning resolves SHS within 1–3 minutes. If it doesn't resolve → think of something else.
Prevention > Treatment: Avoid supine position from 20 weeks; left lateral sleep from 28 weeks; left uterine displacement is mandatory during CS.
Peri-operative gold standard (CS under spinal):
- Left lateral uterine displacement (15° tilt or manual) ^[1]
- IV crystalloid co-loading (not pre-loading alone)
- Prophylactic phenylephrine infusion (25–50 mcg/min) — now first-line
- Rescue: Phenylephrine bolus, Ephedrine if bradycardia, Atropine if severe bradycardia ^[1]
Phenylephrine > Ephedrine: Phenylephrine causes less fetal acidosis (less placental transfer, no β-adrenergic stimulation of fetal metabolism).
Cardiac arrest in pregnancy: Manual left uterine displacement during CPR. Perimortem CS within 5 minutes if no ROSC — primarily to save the mother by relieving aortocaval compression.
Never forget the fetus: The placental bed has no autoregulation ^[1] — maternal BP directly determines fetal perfusion. Always assess fetal wellbeing with CTG after any hypotensive episode.
Thromboprophylaxis overlap: Venous stasis from IVC compression also predisposes to DVT. Thromboembolism is the leading cause of maternal mortality in the UK ^[3]. Mobilisation, compression stockings, and LMWH for high-risk women.

Active Recall - Management of Supine Hypotensive Syndrome

References

^[1] Senior notes: Maksim Surgery Notes.pdf (p298 — Obstetrics Anaesthesia section) ^[2] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p39) ^[3] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p39 — Prevention/DVT sidebar) ^[7] Senior notes: Ryan Ho Haemtology.pdf (p131–132 — VTE management in pregnancy) ^[11] Senior notes: Ryan Ho Critical Care.pdf (p15–17 — Shock management and vasopressor use) ^[12] Senior notes: Adrian Lui Obstetric Notes.pdf (p123 — Thrombophilia and thromboembolic diseases in pregnancy)

Complications of Supine Hypotensive Syndrome

Supine hypotensive syndrome is typically a self-limiting, rapidly reversible condition. When recognised promptly and managed appropriately (left lateral positioning), it resolves within minutes with no lasting consequences. However, when it is unrecognised, prolonged, or compounded by anaesthesia, serious — even fatal — complications can occur for both mother and fetus.

The complications flow directly from the two fundamental pathophysiological consequences:

Maternal hypotension (from IVC compression → reduced venous return → reduced cardiac output)
Reduced uteroplacental perfusion (from both maternal hypotension AND aortic compression → the Poseiro effect)

The placental bed has no autoregulation to compensate for a drop in BP ^[1]

This single fact is the key to understanding virtually all fetal complications. The uteroplacental circulation is a maximally dilated, low-resistance bed that cannot vasodilate further to compensate for falling perfusion pressure. Blood flow is directly proportional to maternal arterial pressure. Therefore, any sustained maternal hypotension = proportional reduction in fetal oxygen and nutrient delivery.

A. Maternal Complications

Aspect	Detail
Mechanism	Prolonged cerebral hypoperfusion (MAP falls below the autoregulatory threshold for cerebral blood flow, approximately 50–60 mmHg) → loss of consciousness
Why it matters	If the patient faints while standing (having transitioned from supine to upright while still hypotensive), she may fall and sustain injuries — head injury, fractures, abdominal trauma (which can cause placental abruption)
Risk context	More common in community settings where the woman does not recognise the positional trigger — e.g., standing up quickly after lying supine → orthostatic component superimposed on recovering aortocaval compression
Prevention	Patient education: rise slowly from supine; always via the lateral position first, then sit, then stand. Never stand directly from supine in late pregnancy

Aspect	Detail
Mechanism	If SHS causes loss of consciousness → loss of airway protective reflexes → gastric contents (which are acidic, particularly in pregnancy due to delayed gastric emptying and relaxed LES from progesterone) can be aspirated into the tracheobronchial tree → chemical pneumonitis
Why pregnancy is high-risk	Delayed gastric emptying and relaxed LES due to progesterone → risk of aspiration even with fasting ^[1]. Pregnancy increases intra-abdominal pressure (gravid uterus pushes stomach cephalad). Combined effect: higher gastric volume + lower LES tone + increased intra-abdominal pressure = high aspiration risk
Clinical features	Acute onset of cough, wheeze, hypoxia, and bilateral crackles after loss of consciousness. CXR shows bilateral infiltrates (aspirated gastric acid causes diffuse chemical injury to alveolar epithelium)
Severity	Can progress to ARDS and death if severe. Historically a leading cause of maternal anaesthetic mortality
Prevention	Mendelson's syndrome (chemical pneumonitis caused by aspiration of acidic stomach contents during anaesthesia in childbirth): reduced by RSI and antacids (30 mL sodium citrate + H₂RA/PPI) ^[1]. Lateral positioning during recovery from any loss of consciousness

Mendelson's syndrome is named after Curtis Mendelson, who described the syndrome in 1946 after studying 66 cases of aspiration during obstetric anaesthesia. The name literally means: aspiration pneumonitis in the context of childbirth anaesthesia ^[1].

Aspect	Detail
Mechanism	In severe, unrecognised SHS (especially under anaesthesia where compensatory mechanisms are ablated), cardiac output can fall to near-zero → myocardial hypoperfusion → arrhythmia → cardiac arrest. Alternatively, severe Bezold-Jarisch reflex → profound bradycardia → asystole
Why this is rare	In the awake, non-anaesthetised patient, syncope itself is protective — the patient falls to the ground (no longer supine), which relieves IVC compression. Under anaesthesia, the patient cannot change position → IVC compression persists → progressive cardiovascular collapse
Risk factors	General anaesthesia without left tilt; spinal anaesthesia with high block and no vasopressor support; failure to apply left uterine displacement during CPR
Management	CPR with continuous manual left uterine displacement. Perimortem caesarean section within 5 minutes if no ROSC (to relieve aortocaval compression and restore effective venous return for maternal resuscitation)
Prognosis	If recognised and managed with immediate left tilt + CPR + perimortem CS, survival rates are reasonable. If unrecognised, fatal

Cardiac Arrest in Pregnancy

A critical teaching point: standard CPR on a supine pregnant woman is futile without left uterine displacement. The IVC is compressed → no venous return → chest compressions cannot generate forward flow. You MUST either tilt the patient or have someone manually displace the uterus to the left throughout CPR. If this fails within 4 minutes → perimortem CS (primarily to save the mother by removing aortocaval compression, not just the baby).

Aspect	Detail
Mechanism	IVC compression causes venous stasis in the lower limbs and pelvic veins — one element of Virchow's triad. Pregnancy simultaneously provides the other two elements: hypercoagulability (increased fibrinogen, factors VII, VIII, X, XII; decreased protein S) and endothelial activation. Together → thrombogenesis
Connection to SHS	The same mechanical compression that causes SHS also causes the venous stasis that predisposes to DVT. Prolonged supine positioning (e.g., bedrest, prolonged surgery, hospitalisation) exacerbates this
Significance	In the UK, the leading cause of maternal mortality is thromboembolism ^[3]. PE from DVT can cause obstructive shock and maternal death
Prevention	Recommend lady to move around a lot → if pregnant lady lying down for prolonged period of time, have to be worried about DVT, and reduced venous return if supine ^[3]. Compression stockings, intermittent pneumatic compression during CS, LMWH thromboprophylaxis for high-risk women (continue up to 6 weeks postpartum) ^[7]

Pregnancy is a prothrombotic state due to ↑coagulation factors + ↑venous stasis ^[7]

Aspect	Detail
Mechanism	Hypotension triggers the Bezold-Jarisch reflex (vigorous contraction of underfilled ventricle → vagal afferent activation → parasympathetic surge). One of the cardinal features of this reflex is nausea and vomiting. In a supine, semi-conscious pregnant patient with a full stomach, vomiting → aspiration
Clinical significance	Nausea is often the earliest warning symptom of SHS and should trigger immediate repositioning. In the operative setting under regional anaesthesia, nausea during CS is a well-known sign of developing hypotension

Aspect	Detail
Mechanism	Sustained maternal hypotension → reduced renal perfusion pressure → pre-renal AKI (if prolonged). The kidneys receive ~20% of cardiac output; when CO drops significantly, renal perfusion is compromised
Clinical significance	This is a complication of prolonged SHS only (e.g., unrecognised during prolonged surgery). Brief, self-resolving episodes do not cause renal injury. Oliguria (urine output < 0.5 mL/kg/hr) is an early sign
Recovery	Usually fully reversible once haemodynamics are restored

B. Fetal and Neonatal Complications

These are the complications that make SHS truly dangerous. The fetus is exquisitely vulnerable because the placental bed has no autoregulation to compensate for a drop in BP ^[1].

Aspect	Detail
Mechanism	Reduced uteroplacental blood flow (from maternal hypotension + aortic compression) → reduced oxygen delivery to the fetus → fetal hypoxia. The fetus responds with a stress response: redistribution of blood flow to vital organs (brain, heart, adrenals) at the expense of the periphery and gut
CTG findings	Late decelerations (the hallmark): FHR drops after the peak of uterine contraction (or after the onset of supine positioning), with smooth descent and slow recovery. This pattern indicates uteroplacental insufficiency. Also: reduced beat-to-beat variability (< 5 bpm for > 40 min), loss of accelerations, fetal bradycardia (< 110 bpm sustained)
Poseiro effect	Aortic compression reduces uterine artery flow even when maternal brachial BP is maintained (because the brachial reading is taken above the level of compression). This can cause fetal distress without maternal symptoms — making it a particularly insidious complication
Reversibility	In most cases, rapidly reversible with left lateral positioning. If the CTG normalises within minutes of repositioning → diagnosis confirmed, no lasting harm
When it doesn't reverse	Persistent CTG abnormalities after repositioning suggest either: (a) a different cause of fetal distress (abruption, cord compression), or (b) significant fetal compromise that has already occurred. May require urgent delivery

Aspect	Detail
Mechanism	Prolonged fetal hypoxia → anaerobic metabolism → lactic acid production → metabolic acidosis (↓pH, ↓base excess, ↑lactate in fetal blood). This is the same metabolic shift that occurs in any tissue deprived of oxygen
Assessment	Fetal blood sampling (FBS) from the fetal scalp during labour shows ↓pH (< 7.20 is pathological, < 7.25 is borderline). Cord blood gas at delivery: umbilical artery pH < 7.00 with base deficit > 12 mmol/L is associated with significant risk of neonatal encephalopathy
Clinical significance	Mild, transient acidosis (from a brief episode of SHS that resolves with repositioning) is usually well-tolerated by the fetus. Prolonged or severe acidosis → risk of hypoxic-ischaemic encephalopathy (HIE)

Aspect	Detail
Mechanism	Severe, prolonged fetal hypoxia and acidosis → neuronal energy failure → excitotoxicity (excessive glutamate release) → calcium influx → cell death in vulnerable brain regions (particularly the basal ganglia, thalamus, and parasagittal cortex)
When this occurs	Only in cases of severe, prolonged SHS that goes unrecognised — e.g., maternal cardiac arrest without timely resuscitation, or prolonged surgery in the supine position without left tilt under deep anaesthesia
Grades (Sarnat classification)	Grade I (mild): hyperalert, normal tone → good prognosis. Grade II (moderate): lethargy, hypotonia, seizures → variable prognosis. Grade III (severe): coma, flaccidity, absent reflexes → poor prognosis with high mortality or severe disability
Management	Therapeutic hypothermia (cooling to 33.5°C for 72 hours) started within 6 hours of birth for moderate-to-severe HIE → reduces the secondary phase of neuronal injury
Rarity	Extremely rare as a direct complication of SHS alone because the condition is almost always rapidly reversible. HIE from SHS would require a catastrophic, prolonged, unrecognised event

Aspect	Detail
Mechanism	Acute fetal hypoxia → vagal reflex → fetal heart rate drops below 110 bpm. This is a direct fetal response to hypoxia (analogous to the diving reflex in neonates — hypoxia triggers parasympathetic activation to conserve myocardial oxygen)
Significance	Sustained fetal bradycardia (< 100 bpm for > 3 minutes) is a medical emergency requiring immediate repositioning and, if not resolving, urgent delivery
Reversibility	Usually resolves within minutes of left lateral repositioning if the cause is SHS. If it does not resolve → consider other causes (cord prolapse, abruption, uterine rupture)

Aspect	Detail
Mechanism	This is a theoretical and epidemiological association rather than a direct acute complication. Recurrent episodes of aortocaval compression (e.g., from habitual supine sleeping) cause intermittent episodes of reduced uteroplacental perfusion → chronic suboptimal oxygen and nutrient delivery → impaired fetal growth
Evidence	Observational studies (Auckland Stillbirth Study, MiNESS, CRIBSS) have shown associations between habitual supine sleep position in the 3rd trimester and adverse fetal outcomes including IUGR and late stillbirth. The mechanism is presumed to be recurrent subclinical aortocaval compression
Prevention	Left lateral sleep position from 28 weeks; pillow/wedge behind right hip to prevent supine rolling

Aspect	Detail
Mechanism	Extreme end of the spectrum of chronic uteroplacental hypoperfusion from recurrent aortocaval compression during supine sleep. Prolonged or recurrent fetal hypoxia → fetal death in utero
Evidence	Meta-analyses of case-control studies show a 2–3 fold increased risk of late stillbirth (after 28 weeks) associated with maternal supine sleep position. The adjusted OR is approximately 2.3–2.6. These studies (MiNESS 2017, CRIBSS 2019) led to public health campaigns recommending lateral sleep in the 3rd trimester
Important caveat	This is an association, not a proven causation. There may be confounding factors. However, the biological plausibility is strong (aortocaval compression → reduced uteroplacental perfusion), and the intervention (sleeping on one's side) is simple, free, and harmless → widely recommended

Supine Sleep and Stillbirth — Population-Level Impact

Modelling studies suggest that if all women adopted non-supine sleep from 28 weeks, approximately 6% of late stillbirths could be prevented. While the absolute risk to any individual is small, the population-level impact is significant given the devastating nature of the outcome. This is why "Sleep on your side" campaigns (e.g., Tommy's charity in the UK) have been widely promoted.

C. Complications in the Peri-operative Context

The operative setting introduces iatrogenic risks that compound the inherent risks of SHS:

Aspect	Detail
Mechanism	Regional anaesthesia (spinal/epidural) blocks sympathetic outflow → loss of compensatory vasoconstriction and tachycardia. Combined with IVC compression from supine positioning → double hit: reduced venous return (mechanical) + inability to compensate (pharmacological) → severe hypotension → potential cardiovascular collapse
Risk magnification	Difficult intubation is 8 times more common than normal patients ^[1] — if the patient requires emergency conversion from regional to general anaesthesia due to cardiovascular collapse, airway management is extremely challenging in the pregnant patient (weight gain and oedema, increased oxygen demand by 20% & reduced oxygen reserve (FRC drop 20%): less apnoea time allowed) ^[1]
Prevention	Left lateral uterine displacement ^[1]; prophylactic phenylephrine infusion; IV co-loading; continuous BP monitoring

Aspect	Detail
Mechanism	Hypotension during CS (from SHS + neuraxial sympathetic block) triggers the Bezold-Jarisch reflex → nausea and vomiting. Also: intestinal handling during surgery + peritoneal traction → vagal stimulation → emesis. In the awake patient (under regional anaesthesia), this is extremely distressing
Risk	Aspiration of vomitus in the lateral or supine position → Mendelson's syndrome
Management	Treat the hypotension (vasopressors → once BP is restored, IONV usually resolves). Antiemetics: ondansetron 4 mg IV (5-HT₃ antagonist — safe in pregnancy), dexamethasone 4 mg IV (acts on chemoreceptor trigger zone). Ensure the patient is positioned with slight head-up tilt

Aspect	Detail
Mechanism	If SHS leads to maternal cardiac arrest and CPR is performed without left uterine displacement → chest compressions cannot generate effective forward flow because there is no venous return (IVC is compressed) → CPR fails → maternal and fetal death
Prevention	Training all healthcare workers in maternal resuscitation: left uterine displacement during CPR is non-negotiable. Perimortem CS within 5 minutes if no ROSC

Complication	Frequency	Severity	Reversible?	Key Prevention
Maternal dizziness, nausea, presyncope	Very common (~8–10% of pregnant women at term when supine)	Mild	Rapidly reversible with left lateral positioning	Avoid supine position
Maternal syncope	Uncommon	Moderate	Rapidly reversible	Position education; slow position changes
Fall-related injury	Rare	Variable (mild to severe)	Depends on injury	Education to avoid sudden position changes
Fetal heart rate changes (late decelerations)	Common when supine (often subclinical)	Moderate	Usually rapidly reversible with repositioning	Left lateral positioning; CTG monitoring
Fetal bradycardia	Uncommon	Moderate to severe	Usually reversible; may require urgent delivery if persistent	Same as above
Fetal acidosis	Rare (only with prolonged events)	Severe	Partially reversible; depends on duration	Prompt recognition and management
DVT/PE	Related (shared mechanism)	Potentially fatal	PE may be fatal if massive	Mobilisation; thromboprophylaxis ^[3]^[7]
Aspiration pneumonitis (Mendelson's)	Rare (peri-operative)	Severe (can be fatal)	Partially reversible with treatment	RSI; antacid prophylaxis ^[1]
Maternal cardiac arrest	Very rare	Critical	Potentially reversible with left tilt + CPR + perimortem CS	Left uterine displacement ^[1]; vasopressor support
HIE / Neonatal brain injury	Extremely rare (only with prolonged unrecognised events)	Devastating	Irreversible damage; therapeutic hypothermia may mitigate	Prompt recognition and management
Late stillbirth (from chronic supine sleep)	Rare but significant at population level	Devastating	Irreversible	Left lateral sleep from 28 weeks

In the vast majority of cases, SHS has no long-term consequences for either mother or fetus because it is rapidly recognised and treated. However:

Population	Potential Long-Term Issue	Mechanism
Mother	Anxiety about future anaesthesia; important to document the event for future anaesthetic planning	Psychological impact; practical importance for safe obstetric anaesthesia in subsequent pregnancies
Fetus/Neonate	If HIE occurred: cerebral palsy, developmental delay, epilepsy, intellectual disability	Irreversible neuronal death from prolonged hypoxia-ischaemia
Population level	Recurrent subclinical aortocaval compression (supine sleep) associated with IUGR and late stillbirth	Chronic intermittent uteroplacental hypoperfusion

High Yield Summary — Complications

Maternal complications: Syncope → fall injury; aspiration pneumonitis (Mendelson's syndrome) if LOC; cardiac arrest (rare, under anaesthesia); DVT/PE (shared mechanism of venous stasis — thromboembolism is the leading cause of maternal mortality in the UK ^[3]); renal hypoperfusion (prolonged episodes only).

Fetal complications: Fetal hypoxia → late decelerations → fetal bradycardia → fetal acidosis → HIE (only if prolonged/unrecognised). Recurrent subclinical aortocaval compression (supine sleep) associated with IUGR and 2–3× increased risk of late stillbirth.

The central principle: The placental bed has no autoregulation to compensate for a drop in BP ^[1]. This means ALL fetal complications are directly proportional to the depth and duration of maternal hypotension. Prompt recognition and repositioning prevents virtually all complications.

Peri-operative complications are the most dangerous because anaesthesia removes compensatory mechanisms. Left lateral uterine displacement is mandatory ^[1]. If cardiac arrest occurs → left uterine displacement during CPR + perimortem CS within 5 minutes.

Almost all complications are preventable with: (1) avoidance of supine position; (2) left lateral sleep from 28 weeks; (3) left uterine displacement during procedures; (4) prophylactic vasopressors during spinal anaesthesia for CS.

Active Recall - Complications of Supine Hypotensive Syndrome

References

^[1] Senior notes: Maksim Surgery Notes.pdf (p298 — Obstetrics Anaesthesia section) ^[3] Lecture slides: Block C - Hypertension and Pregnancy (CFB WCS in 2023_24).pdf (p39 — Prevention/DVT sidebar) ^[7] Senior notes: Ryan Ho Haemtology.pdf (p131–132 — VTE management in pregnancy)

Supine Hypotensive Syndrome

Definition

Epidemiology and Risk Factors

Epidemiology

Risk Factors

Anatomy and Function

Relevant Vascular Anatomy

Inferior Vena Cava (IVC)

Abdominal Aorta

Collateral Venous Pathways

Uterine Position

Cardiovascular Physiology of Pregnancy (Relevant Context)

Etiology and Pathophysiology

Primary Etiology

Detailed Pathophysiology

Step 1: Mechanical Compression of the IVC

Step 2: Reduced Cardiac Preload and Output

Step 3: Compensatory Mechanisms (Why Not Everyone Is Symptomatic)

Step 4: Decompensation (Why 8–10% Become Symptomatic)

Step 5: Aortic Compression (Poseiro Effect)

Step 6: Consequences

Classification

By Severity

By Context

By Predominant Mechanism

Clinical Features

Symptoms (What the Patient Reports)

Signs (What the Clinician Finds)

Temporal Pattern

Important Clinical Scenarios

1. During Caesarean Section Under Spinal Anaesthesia

2. During Labour (Supine Lithotomy Position)

3. During Ultrasound or Other Procedures

4. During Sleep

Distinguishing from Other Causes of Hypotension in Pregnancy

Connection to Obstetric Anaesthesia

Connection to Venous Thromboembolism

Active Recall - Supine Hypotensive Syndrome

References

Framework for Differential Diagnosis

Detailed Differential Diagnosis Table

A. Hypovolaemic Causes (Reduced Preload from Volume Loss)

B. Cardiogenic Causes (Pump Failure → Reduced Cardiac Output)

C. Distributive Causes (Vasodilation → Reduced SVR)

D. Obstructive Causes (Impeded Cardiac Filling or Outflow)

E. Reflex / Neurogenic Causes

F. Pregnancy-Specific Causes

G. Metabolic Mimics (Non-Circulatory Causes of LOC/Presyncope)

Clinical Approach to Differentiation: Key Discriminating Features

High-Yield Differentiating Algorithm

Active Recall - Differential Diagnosis of Supine Hypotensive Syndrome

The Nature of the Diagnosis

Diagnostic Criteria (Clinical Consensus)

Diagnostic Algorithm

Investigation Modalities

A. Bedside Investigations (Immediate, No Delay)

B. Blood Tests (To Exclude Differentials)

C. Imaging (If Differentials Need Exclusion)

D. Specialised Investigations (Rarely Needed for SHS Itself)

Putting It All Together: Interpretation Framework

Active Recall - Diagnosis of Supine Hypotensive Syndrome

Management Philosophy

Management Algorithm

A. Immediate Management of an Acute Episode

Step 1: Positional Correction (THE Treatment)

Step 2: Basic Supportive Measures

Step 3: If No Response to Repositioning

B. Pharmacological Management (Peri-operative Context)

Vasopressors

IV Fluid Therapy

Anti-aspiration Prophylaxis (Related, Not Directly for SHS)

C. Preventive Management (Avoiding Occurrence)

1. Patient Education

2. Healthcare Provider Awareness

3. Thromboprophylaxis (Related Preventive Measure)

D. Management in Specific Clinical Scenarios

Scenario 1: Caesarean Section Under Spinal Anaesthesia (Highest Risk)

Scenario 2: Labour (Vaginal Delivery)

Scenario 3: Cardiac Arrest in Pregnancy

E. Summary Table: Management by Context