Respiratory Alkalosis
Respiratory alkalosis is a condition of elevated blood pH resulting from excessive alveolar ventilation that leads to a decrease in arterial carbon dioxide (PaCO₂) below normal levels.
Respiratory alkalosis is a primary acid-base disorder characterised by a decrease in arterial partial pressure of carbon dioxide (pCO₂) resulting from alveolar hyperventilation that exceeds the body's metabolic CO₂ production. This drives the arterial pH upward (towards alkalinity).
- Respiratory alkalosis = hyperventilation → ↓pCO₂ → shifts pH up [1][2]
- Alkalosis = an ongoing process causing removal of plasma [H⁺]; alkalaemia = arterial pH > 7.45, the result when compensation is insufficient [1][2]
- A patient can have respiratory alkalosis without alkalaemia if metabolic compensation is adequate — this is the key distinction between the process (alkalosis) and the end-result (alkalaemia)
Breaking down the terminology:
- "Respiratory" → the primary derangement is in the respiratory (CO₂) component of the Henderson-Hasselbalch equation
- "Alkalosis" → from Arabic al-qalī (ashes of a plant), referring to basic/alkaline; the process drives pH upward
The Henderson-Hasselbalch Equation — First Principles
Mnemonic: Acidity = Bicarb / Carbon dioxide (A = B/CD) [2]
- In respiratory alkalosis, the denominator (CO₂) falls because the lungs blow off CO₂ faster than metabolism produces it → the ratio rises → pH rises
- The metabolic component is represented by HCO₃⁻ concentration; the respiratory component is represented by CO₂ concentration [2]
- Normal pH is 7.4 (range 7.35–7.45), corresponding to [H⁺] of 40 nmol/L [1]
- Lethal pH levels: < 7.1 or > 7.7 — all cellular function becomes disrupted [1]
High Yield — Compensation in Respiratory Alkalosis
Compensation for primary respiratory alkalosis = compensatory metabolic acidosis → kidney excretes more HCO₃⁻ → reduced [HCO₃⁻] [1][2]
- Respiratory compensation occurs immediately and reaches maximum in 12 hours [2]
- Metabolic compensation occurs via: (1) acute buffering (modest) and (2) chronic renal HCO₃⁻ excretion (significant, takes several days) [2]
- Rule of thumb: ONLY in chronic respiratory alkalosis may there be complete compensation (i.e. normal pH) [1][2] — This is the one acid-base disorder where full compensation back to normal pH is expected. In all other primary acid-base disturbances, compensation is incomplete.
Why is chronic respiratory alkalosis the only disorder with complete compensation? Because the kidney can very effectively dump bicarbonate when given enough time (days). The sustained low pCO₂ reduces proximal tubular HCO₃⁻ reabsorption (less CO₂ available for intracellular carbonic anhydrase → less H⁺ secretion → less HCO₃⁻ reclaimed) and reduces distal nephron H⁺ secretion. The net effect is a substantial reduction in serum HCO₃⁻ that can fully normalise the HCO₃⁻/CO₂ ratio, restoring pH to ~7.40. Other acid-base disorders cannot fully compensate because their compensatory mechanisms have inherent limitations (e.g., you can't hypoventilate indefinitely because hypoxia limits it).
- Respiratory alkalosis is the most common acid-base disturbance encountered in hospitalised patients [3]
- Why? Because many common hospital conditions (pain, anxiety, sepsis, pneumonia, PE, mechanical ventilation) all drive hyperventilation
- It is also very common in the community — anxiety-related hyperventilation is frequent in young adults
- In Hong Kong, relevant epidemiological considerations:
- High prevalence of anxiety disorders in the urban population → psychogenic hyperventilation
- High burden of pneumonia (especially in the elderly, and during influenza/COVID-19 seasons) → hypoxia-driven hyperventilation
- Significant prevalence of chronic liver disease (hepatitis B carrier rate ~7–8% of the population) → hepatic encephalopathy/liver failure commonly causes a respiratory alkalosis
- Sepsis in ICU patients — respiratory alkalosis is often the earliest laboratory finding in gram-negative sepsis
- High-altitude exposure is less relevant in HK itself but important for travellers (e.g., trips to Tibet/Nepal)
- No significant sex or ethnic predilection for respiratory alkalosis per se — it depends on the underlying cause
- Age distribution: any age, but the cause profile differs:
- Young adults: anxiety, hyperventilation syndrome, salicylate poisoning
- Elderly: pneumonia, PE, sepsis, heart failure, liver failure
Risk factors are essentially the conditions that predispose to hyperventilation:
| Category | Risk Factors |
|---|---|
| Pulmonary | Pneumonia, asthma, PE, interstitial lung disease, early ARDS |
| Cardiovascular | Congestive heart failure, severe anaemia |
| Hepatic | Chronic liver disease (HBV/HCV cirrhosis — very relevant in HK), acute liver failure |
| CNS | Stroke, meningitis/encephalitis, brain tumours, traumatic brain injury |
| Metabolic/Endocrine | Sepsis/SIRS, thyrotoxicosis, pregnancy (progesterone effect) |
| Drug/Toxin | Salicylate poisoning, progesterone, methylxanthines (theophylline, caffeine), catecholamines |
| Psychogenic | Anxiety disorders, panic disorder, pain |
| Iatrogenic | Mechanical ventilation (over-ventilation — excessive tidal volume or respiratory rate set too high) |
| Environmental | High altitude (> 2500 m) |
| Pregnancy | Normal pregnancy (progesterone stimulates respiratory centre → chronic mild respiratory alkalosis is physiological in pregnancy) |
Anatomy and Physiology of CO₂ Homeostasis
To understand respiratory alkalosis from first principles, you need to understand how CO₂ is regulated:
- CO₂ is a byproduct of aerobic cellular metabolism (Krebs cycle)
- Normal CO₂ production: ~200 mL/min at rest
- CO₂ is transported in blood in three forms:
- Dissolved CO₂ (~5–10%)
- Carbaminohaemoglobin (~20–30%) — bound to Hb
- Bicarbonate (~60–70%) — via the reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (catalysed by carbonic anhydrase in RBCs)
- CO₂ is eliminated entirely through the lungs
- Alveolar ventilation (V_A) is the key determinant of pCO₂:
- Therefore: ↑V_A → ↓pCO₂ → respiratory alkalosis
- Minute ventilation = Tidal volume × Respiratory rate; but only the alveolar portion matters (dead space ventilation is wasted)
- Medullary respiratory centre (dorsal and ventral respiratory groups): generates rhythmic breathing
- Central chemoreceptors (ventral medulla): respond to CSF pH (which closely reflects pCO₂ because CO₂ freely crosses the blood-brain barrier)
- ↑pCO₂ → ↓CSF pH → stimulates ventilation
- ↓pCO₂ → ↑CSF pH → reduces ventilatory drive (this is the brake that normally prevents excessive CO₂ washout)
- Peripheral chemoreceptors (carotid body, aortic body): respond primarily to ↓pO₂ (also ↑pCO₂ and ↓pH)
- Activated when pO₂ < ~60 mmHg → stimulates hyperventilation
- Higher cortical centres: can override the brainstem → voluntary hyperventilation (anxiety, pain)
- Pulmonary J-receptors (juxtacapillary receptors): stimulated by interstitial oedema, pulmonary congestion, microemboli → reflex tachypnoea/hyperventilation
- The kidney compensates for respiratory alkalosis by excreting more HCO₃⁻ and reducing H⁺ excretion [1][2]
- Acute phase (minutes to hours): buffering by intracellular buffers (proteins, haemoglobin, phosphate); H⁺ shifts out of cells in exchange for K⁺ and Na⁺; HCO₃⁻ drops modestly (~2 mEq/L per 10 mmHg drop in pCO₂)
- Chronic phase (2–5 days): kidney reduces proximal tubular HCO₃⁻ reabsorption and distal H⁺ secretion; HCO₃⁻ drops more substantially (~4–5 mEq/L per 10 mmHg drop in pCO₂)
Etiology (with Focus on Hong Kong Context)
The causes of respiratory alkalosis are best organised by the mechanism of hyperventilation:
Detailed Etiological Categories
When pO₂ falls < ~60 mmHg, carotid body chemoreceptors fire → ↑ventilation → pCO₂ drops
| Cause | HK Relevance & Pathophysiology |
|---|---|
| Pneumonia | Very common in HK (bacterial, viral including influenza and COVID-19). V/Q mismatch and shunt → hypoxaemia → hyperventilation |
| Pulmonary embolism | Significant cause. Dead space ↑, V/Q mismatch → hypoxaemia; also J-receptor stimulation by microemboli |
| Interstitial lung disease | Diffusion impairment → hypoxaemia, especially on exertion |
| Asthma (early/moderate) | Airway obstruction with preserved drive → hyperventilation of unaffected alveoli → ↓pCO₂ (late severe asthma → T2RF with ↑pCO₂) |
| High altitude | Less relevant within HK but important for travellers to Tibet/Nepal |
| Severe anaemia | Reduced O₂ carrying capacity → tissue hypoxia → ↑ventilatory drive |
| R-to-L cardiac shunts | Deoxygenated blood bypasses lungs → systemic hypoxaemia |
| Cause | HK Relevance & Pathophysiology |
|---|---|
| Sepsis / SIRS | Extremely important — respiratory alkalosis is often the earliest sign of sepsis, even before fever. Endotoxins and cytokines (IL-1, TNF-α) directly stimulate the medullary respiratory centre. Gram-negative organisms are classic |
| Hepatic failure / Cirrhosis | Very relevant in HK given high HBV prevalence. Mechanism: ↑circulating toxins (ammonia, progesterone, endogenous benzodiazepine-like substances) stimulate the respiratory centre; portosystemic shunting allows these to bypass liver detoxification |
| CNS pathology | Stroke (especially brainstem), meningitis/encephalitis, brain tumours, head trauma — direct irritation of respiratory centres |
| Salicylate (aspirin) poisoning | Direct stimulation of medullary respiratory centre (this is why salicylate toxicity classically gives a mixed acid-base picture: respiratory alkalosis and metabolic acidosis simultaneously) |
| Progesterone | Stimulates central ventilatory drive; explains physiological respiratory alkalosis of pregnancy (pCO₂ drops to ~30 mmHg in normal pregnancy) |
| Methylxanthines | Theophylline, caffeine — stimulate respiratory centre |
| Thyrotoxicosis | ↑metabolic rate → ↑CO₂ production + direct central stimulation of ventilation |
High Yield — Salicylate Poisoning and Acid-Base
Salicylate poisoning produces a characteristic mixed disturbance: early respiratory alkalosis (direct stimulation of respiratory centre) + later metabolic acidosis (uncoupling of oxidative phosphorylation, accumulation of organic acids). In adults the respiratory alkalosis often predominates initially; in children the metabolic acidosis tends to be more prominent. This is a favourite exam question.
| Cause | Pathophysiology |
|---|---|
| Heart failure (pulmonary congestion) | Pulmonary oedema stimulates J-receptors → reflex tachypnoea/hyperventilation. Also contributes to hypoxaemia via shunting |
| Pulmonary embolism | J-receptors activated by microemboli and pulmonary infarction; also V/Q mismatch → hypoxaemia |
| Pneumothorax | Stimulation of lung receptors + hypoxaemia |
| Cause | Notes |
|---|---|
| Anxiety / Panic disorder | Very common, especially in young adults. Cortical override of brainstem centres. Often presents dramatically with paraesthesias, carpopedal spasm — the "hyperventilation syndrome" |
| Pain | Acute pain drives hyperventilation via cortical and limbic inputs |
| Voluntary hyperventilation | Rare as a sustained cause; used in provocative testing |
| Cause | Notes |
|---|---|
| Mechanical over-ventilation | Common ICU problem — excessive tidal volume or respiratory rate on ventilator → ↓pCO₂. Must monitor ABG and adjust settings |
- Pregnancy (normal physiological state — chronic respiratory alkalosis with pCO₂ ~28–32 mmHg, fully compensated)
- Fever (↑metabolic rate → ↑CO₂ production, but ventilatory response overshoots)
- Heat stroke
- Recovery phase of metabolic acidosis (ventilatory drive persists after acidosis corrected → transient respiratory alkalosis)
Classification
Respiratory alkalosis is classified by chronicity, which determines the degree of compensation:
- Duration: minutes to hours (< 24–48 hours)
- Compensation: only cellular buffering (H⁺ released from intracellular buffers, modest)
- Expected: HCO₃⁻ drops by ~2 mEq/L for every 10 mmHg fall in pCO₂ [3]
- Minimum HCO₃⁻ typically does not fall below ~18 mEq/L
- pH is elevated (incomplete compensation) — frank alkalaemia
- Duration: > 2–5 days
- Compensation: renal HCO₃⁻ excretion (substantial)
- Expected: HCO₃⁻ drops by ~4–5 mEq/L for every 10 mmHg fall in pCO₂ [3]
- Minimum HCO₃⁻ may drop to ~12–15 mEq/L
- pH may be completely normal — the only acid-base disorder where this occurs [1][2]
| Parameter | Acute Resp Alkalosis | Chronic Resp Alkalosis |
|---|---|---|
| pCO₂ | ↓↓ | ↓↓ |
| HCO₃⁻ | Mildly ↓ (buffering only) | Significantly ↓ (renal excretion) |
| pH | ↑ (alkalaemic) | Normal or near-normal |
| Expected compensation | ΔHCO₃⁻ = 2 × (ΔpCO₂/10) | ΔHCO₃⁻ = 4–5 × (ΔpCO₂/10) |
Exam Trap — Normal pH with Low pCO₂
A normal pH with low pCO₂ and low HCO₃⁻ could be:
- Fully compensated chronic respiratory alkalosis (most common interpretation)
- Mixed metabolic acidosis + respiratory alkalosis (the two derangements cancel each other's pH effects)
You must use the clinical context to distinguish. If the patient has chronic liver disease and a stable low pCO₂ for weeks, it's chronic respiratory alkalosis. If the patient has sepsis with lactic acidosis AND tachypnoea, it could be a mixed disorder.
Clinical Features
The symptoms of respiratory alkalosis arise from two main mechanisms:
- Alkalosis → ↓ionised calcium (Ca²⁺ binds more to albumin at higher pH)
- ↓pCO₂ → cerebral vasoconstriction (CO₂ is a potent cerebral vasodilator; losing it constricts cerebral vessels)
- Hypokalaemia (alkalosis shifts K⁺ intracellularly)
| Symptom | Mechanism |
|---|---|
| Lightheadedness / Dizziness | ↓pCO₂ → cerebral vasoconstriction → ↓cerebral blood flow (can drop by 35–40% with pCO₂ around 20 mmHg) |
| Confusion / Impaired concentration | Same mechanism: cerebral hypoperfusion |
| Syncope / Pre-syncope | Severe cerebral vasoconstriction can cause transient loss of consciousness |
| Perioral numbness and paraesthesias (especially circumoral + fingers/toes) | Alkalosis → ↑albumin-Ca²⁺ binding → ↓ionised [Ca²⁺] [4] — reduced ionised calcium increases neuronal excitability. Sensory neurons fire spontaneously → tingling and numbness |
| Carpopedal spasm (tetany) | Same mechanism: ↓ionised Ca²⁺ → neuromuscular hyperexcitability → involuntary muscle contraction of hands ("main d'accoucheur" — obstetrician's hand appearance) and feet [4] |
| Muscle cramps | ↓ionised Ca²⁺ + ↓K⁺ → muscle hyperexcitability |
| Chest tightness / Dyspnoea | Often the initial complaint that drives the patient to hyperventilate further (especially in anxiety); also bronchoconstriction from hypocapnia |
| Palpitations | ↓K⁺ shift intracellularly (alkalosis → ↓ECF [K⁺] [5]) + sympathetic activation → arrhythmia substrate |
| Dry mouth | Mouth breathing during hyperventilation |
| Anxiety / Sense of doom | Often the cause (psychogenic hyperventilation), but also perpetuated by the symptoms → vicious cycle (cognitive theory: anxiety → physical symptoms → ↑anxiety [6]) |
| Blurred vision | Cerebral vasoconstriction affecting visual cortex perfusion |
| Nausea | Cerebral hypoperfusion |
Why Does Alkalosis Cause Hypocalcaemia Symptoms?
Alkalosis (including acute respiratory alkalosis from hyperventilation) causes ↓ionised calcium [4]. The total serum calcium may be completely normal. Here's why:
At higher pH, albumin becomes more negatively charged (H⁺ dissociates from albumin carboxyl groups) → more Ca²⁺ binds to albumin → the free/ionised fraction drops. Since it's the ionised Ca²⁺ that determines neuromuscular excitability, patients develop symptoms of hypocalcaemia (paraesthesias, tetany) even with a normal total calcium.
This is why checking ionised calcium (not just total calcium) is important in alkalotic patients.
| Sign | Mechanism |
|---|---|
| Tachypnoea / Hyperpnoea | The fundamental cause: increased rate and/or depth of breathing. This IS the hyperventilation |
| ↓pCO₂ on ABG | Direct consequence of alveolar hyperventilation |
| Chvostek's sign (tapping CN VII → facial twitching) | ↓ionised Ca²⁺ → neural hyperexcitability → facial nerve fires with minimal stimulation [4] |
| Trousseau's sign (BP cuff inflation > systolic BP for 3 min → carpopedal spasm) | ↓ionised Ca²⁺ → latent tetany unmasked by ischaemia (cuff occludes arterial flow → local acidosis in tissue would normally counteract, but overall alkalosis + ↓iCa²⁺ tips the balance) [4] |
| Carpopedal spasm | Visible when severe — hands adopt "main d'accoucheur" posture [4] |
| Muscle fasciculations / Hyperreflexia | ↓ionised Ca²⁺ → lower threshold for nerve depolarisation |
| Arrhythmias | Hypokalaemia (K⁺ shifts intracellularly in alkalosis) + ↓iCa²⁺ → prolonged QT, predisposition to atrial and ventricular arrhythmias |
| ECG changes | Hypokalaemia: ST depression, T wave flattening, U waves. Hypocalcaemia: prolonged QT interval [4]. Alkalosis itself can also prolong QT |
| Seizures (rare, in severe cases) | Profound cerebral vasoconstriction → cerebral ischaemia + ↓iCa²⁺ → lowered seizure threshold |
| Central neurological signs | If aetiology is CNS disease (stroke, tumour, etc.) — focal deficits may be present |
| Signs of underlying cause | Fever/rigors (sepsis), jaundice/spider naevi (liver failure), swollen calf (DVT/PE), anxiety features (psychogenic) |
Understanding these shifts is crucial for interpreting lab results and predicting complications:
-
Potassium: Alkalosis → K⁺ shifts intracellularly → ↓ECF [K⁺] (hypokalaemia) [5]
- Mechanism: In alkalosis, H⁺ is scarce extracellularly. To maintain electroneutrality, K⁺ moves into cells as H⁺ moves out (H⁺/K⁺ exchange). Also, alkalosis stimulates Na⁺/K⁺-ATPase directly.
- Approximate shift: ~0.2–0.4 mEq/L drop in serum K⁺ per 0.1 unit rise in pH
- The hypokalaemia is a consequence (or parallel phenomenon) of alkalosis, not the cause [5]
-
Ionised Calcium: ↓ (as explained above — ↑albumin binding at higher pH) [4]
- Total calcium remains normal
- This is a functional/effective hypocalcaemia, not true calcium depletion
-
Phosphate: ↓ (alkalosis stimulates glycolysis by activating phosphofructokinase → phosphate is consumed in glycolysis and shifts intracellularly)
-
Chloride: May ↑ slightly (hyperchloraemia) as kidney retains Cl⁻ while excreting HCO₃⁻ during compensation
This deserves special mention as it is one of the most common presentations of acute respiratory alkalosis:
- Definition: Recurrent episodes of hyperventilation, typically psychogenic, producing symptomatic respiratory alkalosis
- Classic presentation: Young adult (often female) with acute anxiety → hyperventilation → paraesthesias (perioral, fingers, toes) → carpopedal spasm → lightheadedness → panic → further hyperventilation (vicious cycle)
- Key features:
- Sighing respirations, air hunger
- Perioral and acral paraesthesias
- Carpopedal spasm (can be dramatic and frightening)
- Lightheadedness, sometimes syncope
- Chest tightness (can mimic cardiac chest pain)
- Often misdiagnosed as cardiac or neurological emergency
Clinical Pearl — Hyperventilation Syndrome vs Serious Pathology
Never assume hyperventilation is "just anxiety" without ruling out serious causes first! Young patients with PE, early asthma, pneumothorax, or diabetic ketoacidosis can all present with tachypnoea and anxiety. Always check:
- SpO₂ (hypoxaemia points to organic cause)
- ABG (a truly psychogenic hyperventilation should have normal pO₂ or supranormal pO₂ with low pCO₂)
- ECG (rule out arrhythmia, PE signs)
- Consider the clinical context (risk factors for PE, DKA, etc.)
The classic teaching of "breathing into a paper bag" to treat hyperventilation syndrome is no longer recommended in emergency settings because it could be dangerous if the patient actually has a pulmonary embolism or pneumothorax.
| Organ System | Effect | Mechanism |
|---|---|---|
| CNS | Lightheadedness, confusion, syncope, seizures | Cerebral vasoconstriction (↓pCO₂ → ↓CBF, up to 35–40% reduction) |
| CVS | Tachycardia, arrhythmias, ↓cardiac output | Hypokalaemia, ↓iCa²⁺, direct effect of alkalosis on myocardium |
| Respiratory | Bronchoconstriction | Hypocapnia causes airway smooth muscle constriction |
| Neuromuscular | Paraesthesias, tetany, cramps | ↓ionised Ca²⁺, ↓K⁺ |
| Metabolic | Leftward shift of oxygen-haemoglobin dissociation curve | Alkalosis ↑ Hb's affinity for O₂ (Bohr effect) → ↓O₂ delivery to tissues |
| Renal | ↑HCO₃⁻ excretion, ↓H⁺ excretion (compensation) | As described above |
The leftward shift of the oxygen-haemoglobin dissociation curve in alkalosis means that even though PaO₂ may be normal, tissue O₂ delivery is impaired — Hb holds onto O₂ more tightly and releases it less readily at the tissue level. This is particularly important in critically ill patients.
| Parameter | Acute | Chronic |
|---|---|---|
| pH | > 7.45 (↑) | Normal or slightly ↑ (7.38–7.42) |
| pCO₂ | < 35 mmHg (↓) — primary change | < 35 mmHg (↓) — primary change |
| HCO₃⁻ | Mildly ↓ (18–22 mEq/L) | Significantly ↓ (12–18 mEq/L) |
| Expected ΔHCO₃⁻ | ↓2 mEq/L per 10 mmHg ↓pCO₂ | ↓4–5 mEq/L per 10 mmHg ↓pCO₂ |
| pO₂ | Normal or ↓ (depends on cause) | Normal or ↓ (depends on cause) |
| Serum K⁺ | ↓ (mild shift) | May normalise |
| Ionised Ca²⁺ | ↓ | May normalise if chronic |
Compensation Formulas (Exam Essential)
For acute respiratory alkalosis:
- Expected HCO₃⁻ = 24 − 2 × [(40 − measured pCO₂)/10]
For chronic respiratory alkalosis:
- Expected HCO₃⁻ = 24 − 5 × [(40 − measured pCO₂)/10]
If the measured HCO₃⁻ is lower than expected → concurrent metabolic acidosis If the measured HCO₃⁻ is higher than expected → concurrent metabolic alkalosis
High Yield Summary
- Respiratory alkalosis = hyperventilation → ↓pCO₂ → ↑pH — the most common acid-base disturbance in hospitalised patients
- Chronic respiratory alkalosis is the ONLY acid-base disorder where compensation may be complete (normal pH) — because the kidney can effectively dump HCO₃⁻ over days
- Key causes to remember: Anxiety/hyperventilation syndrome, pneumonia, PE, sepsis (often earliest sign!), liver failure (HBV cirrhosis — very HK-relevant), salicylate poisoning, pregnancy, mechanical over-ventilation
- Salicylate poisoning = mixed respiratory alkalosis + metabolic acidosis (unique combination)
- Symptoms arise from: ↓ionised Ca²⁺ (paraesthesias, tetany, Chvostek/Trousseau signs), cerebral vasoconstriction (dizziness, syncope, confusion), and ↓K⁺ (arrhythmias)
- Compensation formulas: Acute ΔHCO₃⁻ = 2 per 10 mmHg ΔpCO₂; Chronic ΔHCO₃⁻ = 4–5 per 10 mmHg ΔpCO₂
- Never dismiss tachypnoea as "just anxiety" — always rule out PE, pneumothorax, DKA, and other serious causes
- Alkalosis shifts K⁺ intracellularly and ↑Hb-O₂ affinity (leftward Bohr shift) → impairs tissue O₂ delivery
Active Recall - Respiratory Alkalosis (Definition through Clinical Features)
[1] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.310, Section 9.1 Acid-base Disorders) [2] Senior notes: Ryan Ho Urogenital.pdf (p.34, Section 2.4.1 Approach to Acid-base Disorders); Block A - Electrolyte and Acid-Base Disorders.pdf (p.2-3) [3] Lecture slides: GC 044. Electrolyte and Acid-Base Disorders.pdf; MBBS IV Electrolytes_2024.pdf; Block A - Electrolyte and Acid-Base Disorders.pdf [4] Senior notes: Ryan Ho Endocrine.pdf (p.45, Section 2.3 Hypocalcemia) [5] Senior notes: Ryan Ho Chemical Path.pdf (p.13, Potassium homeostasis — transcellular shift in alkalosis) [6] Senior notes: Ryan Ho Psychiatry.pdf (p.180, Panic disorder — cognitive theory)
Differential Diagnosis of Respiratory Alkalosis
When you encounter a patient with low pCO₂ and elevated (or near-normal) pH, the clinical task is twofold:
- Confirm that the respiratory alkalosis is the primary disorder (and not compensatory hyperventilation for a metabolic acidosis)
- Identify the underlying cause driving the hyperventilation
These are two distinct questions, and confusing them is one of the most common exam mistakes.
This is the critical first-principles question that must be answered before constructing any differential.
Compensatory respiratory alkalosis = hyperventilation to expel CO₂, reduced pCO₂ occurs in response to a primary metabolic acidosis (loss of HCO₃⁻) [1]. In this situation, the low pCO₂ is appropriate — it is the body's attempt to restore pH. The clinical features will be those of the metabolic acidosis (e.g., DKA, lactic acidosis, renal failure) rather than a primary respiratory problem.
How to distinguish:
| Feature | Primary Respiratory Alkalosis | Compensatory Respiratory Alkalosis (for metabolic acidosis) |
|---|---|---|
| pH | > 7.45 (acute) or normal (chronic, fully compensated) | < 7.35 (pH still acidotic because metabolic acidosis is the primary driver) |
| Primary change | ↓pCO₂ is the initiating event | ↓HCO₃⁻ is the initiating event; ↓pCO₂ follows |
| Direction of pH | Alkalotic (or normal if chronic) | Acidotic |
| HCO₃⁻ | Secondarily low (compensation) | Primarily low (the original problem) |
| Clinical context | Anxiety, PE, sepsis, liver disease, etc. | DKA, lactic acidosis, renal failure, diarrhoea, etc. |
| Characteristic sign | — | Kussmaul's breathing → hyperventilation in an attempt to expel as much pCO₂ as possible [1] |
Three-step approach when interpreting ABG: (1) Look at pH — direction of change often indicates primary/predominant disorder. (2) Pattern recognition — "3 ups/3 downs rule": if ALL 3 components of H-H equation (pH, HCO₃⁻, pCO₂) move in same direction, there must be a simple metabolic disturbance. "Not up, not down rule": if HCO₃⁻ and CO₂ move in different directions, there must be mixed acid-base disturbance. (3) Evaluate compensation to uncover subtle mixed disorders. [2]
Exam Trap — Don't Confuse Compensation with Primary Disorder
A patient with DKA will have low HCO₃⁻ AND low pCO₂. The low pCO₂ is compensatory (secondary hyperventilation), NOT primary respiratory alkalosis. The pH will be acidotic. Do not list respiratory alkalosis as part of the differential for a DKA patient's ABG — it is compensation.
Conversely, a patient with salicylate poisoning has a TRUE mixed disorder: primary respiratory alkalosis (direct medullary stimulation) PLUS primary metabolic acidosis (organic acid accumulation) [3][2]. Here the actual pCO₂ is lower than expected for pure compensation of the metabolic acidosis — this is how you unmask the mixed picture.
Once you've confirmed ↓pCO₂ with ↑pH (or normal pH if chronic) and the compensation fits:
- Acute: Expected ↓[HCO₃⁻] = 2 mmol/L per 10 mmHg (1.3 kPa) drop in pCO₂; [HCO₃⁻] cannot go lower than ~18 mmol/L [2]
- Chronic: Expected ↓[HCO₃⁻] = 4 mmol/L per 10 mmHg (1.3 kPa) drop in pCO₂; [HCO₃⁻] cannot go lower than ~12 mmol/L (can fully compensate) [2]
If measured HCO₃⁻ is lower than expected → concurrent primary metabolic acidosis exists (mixed disorder) If measured HCO₃⁻ is higher than expected → concurrent primary metabolic alkalosis exists (mixed disorder)
Now the real clinical question: Why is this patient hyperventilating?
The differential is best structured by mechanism of hyperventilation, which maps directly to the pathophysiology:
Detailed Differential Diagnosis Table
These are causes where the hyperventilation is a physiological response to hypoxaemia — the body is trying to improve oxygenation, and in the process blows off CO₂.
| Cause | Key Differentiating Features | Why It Causes Resp Alkalosis |
|---|---|---|
| Pneumonia | Fever, productive cough, crackles, consolidation on CXR, raised WBC/CRP | V/Q mismatch + shunt → hypoxaemia → ↑ventilatory drive; also systemic inflammatory mediators stimulate respiratory centre |
| Pulmonary embolism | Pleuritic chest pain, swollen calf (DVT), risk factors (immobility, OCP, malignancy). ABG: hypoxaemia, hypocapnia, respiratory alkalosis, ↑A-a gradient [4] | Dead space ventilation ↑ (clot blocks perfusion to ventilated areas) → V/Q mismatch → hypoxaemia; also J-receptor stimulation. Type I respiratory failure [4] |
| Acute asthma (early/moderate) | Wheeze, ↓PEFR, history of asthma. NOTE: pCO₂ rising toward normal in acute asthma is DANGER SIGN (exhaustion) | Airway obstruction with preserved respiratory drive → hyperventilation of non-obstructed areas → ↓pCO₂. Late severe attack → fatigue → T2RF |
| Interstitial lung disease | Exertional dyspnoea, fine inspiratory crackles, clubbing, restrictive PFTs | Diffusion impairment → hypoxaemia on exertion → ↑drive; also ↓compliance stimulates stretch receptors |
| ARDS | Acute bilateral infiltrates, severe hypoxaemia, known precipitant (sepsis, trauma, aspiration) | Diffuse alveolar damage → shunt + ↓compliance → ↑respiratory drive |
| High altitude | Travel history (> 2500 m) | ↓Atmospheric pO₂ → hypoxaemia → carotid body stimulation → hyperventilation |
| Severe anaemia | Pallor, fatigue, tachycardia, low Hb | ↓O₂ carrying capacity → tissue hypoxia → ↑ventilatory drive (even though pO₂ may be normal, CaO₂ is low) |
| Cyanotic congenital heart disease / R-to-L shunt | Cyanosis from birth, clubbing, characteristic murmurs | Deoxygenated blood bypasses lungs → systemic hypoxaemia |
High Yield — Pulmonary Embolism ABG Pattern
PE classically presents with Type I respiratory failure: hypoxaemia + hypocapnia + respiratory alkalosis + elevated A-a gradient [4][5]. The A-a gradient is elevated because there is true V/Q mismatch (perfusion lost to embolised segments while ventilation continues). This is a favourite exam ABG question. A normal A-a gradient in the setting of respiratory alkalosis argues AGAINST PE and towards psychogenic hyperventilation.
These are potentially dangerous causes where the respiratory centre is driven despite adequate oxygenation.
| Cause | Key Differentiating Features | Why It Causes Resp Alkalosis |
|---|---|---|
| Sepsis / SIRS | Fever/hypothermia, tachycardia, hypotension, raised WBC/CRP/procalcitonin, source of infection | Endotoxins (especially gram-negative) and pro-inflammatory cytokines (TNF-α, IL-1, IL-6) directly stimulate medullary respiratory centre. Respiratory alkalosis may be the earliest laboratory abnormality in sepsis — before fever, before hypotension |
| Hepatic failure / Cirrhosis | Jaundice, spider naevi, palmar erythema, ascites, encephalopathy, coagulopathy. Very relevant in HK (HBV) | Circulating toxins (ammonia, progesterone, endogenous benzodiazepine-like substances) stimulate respiratory centre; portosystemic shunting allows toxins to bypass liver. Chronic respiratory alkalosis is a characteristic finding in stable cirrhosis |
| CNS pathology | Focal neurological signs, altered consciousness, neck stiffness (meningitis), history of trauma | Direct irritation/damage to brainstem respiratory centres: stroke, tumour, meningitis/encephalitis, subarachnoid haemorrhage, TBI. Central neurogenic hyperventilation (rare but classic) = sustained rapid deep breathing from brainstem lesions |
| Salicylate poisoning | Mixed respiratory alkalosis + metabolic acidosis. ABG: resp alkalosis early on → becomes dominated by HAGMA in late stages [3]. Tinnitus, vertigo, nausea/vomiting, hyperpnoea early; altered mental status, hyperpyrexia later. Suspect salicylate poisoning in acid-base disorder of unknown origin [3] | Direct stimulation of medulla → ↑rate and depth of respiration [3]. Simultaneously causes HAGMA from ketoacidosis, lactic acidosis and salicylate accumulation |
| Progesterone | Pregnancy (physiological), medroxyprogesterone use | Progesterone directly stimulates central chemoreceptors → ↓pCO₂ set point. Normal pregnancy: pCO₂ ~28–32 mmHg. Not pathological in pregnancy but may confuse interpretation |
| Methylxanthines | Theophylline toxicity — nausea, vomiting, tremor, seizures; caffeine excess | Direct stimulation of respiratory centre |
| Thyrotoxicosis | Weight loss, tremor, tachycardia, AF, goitre, exophthalmos | ↑Metabolic rate → ↑CO₂ production + direct central stimulation → net ↓pCO₂ |
| Heart failure | Exertional dyspnoea, orthopnoea, PND, oedema, raised JVP, S3 gallop | Pulmonary congestion → J-receptor stimulation → reflex tachypnoea; also mild hypoxaemia from pulmonary oedema (shunt) |
High Yield — Liver Failure and Respiratory Alkalosis in HK Context
Given HK's high HBV carrier rate (~7–8%), chronic liver disease and cirrhosis are common. Chronic respiratory alkalosis is a hallmark finding in stable cirrhosis — the ABG may show a fully compensated picture (normal pH, low pCO₂, low HCO₃⁻). When these patients decompensate (sepsis, GI bleeding, hepatorenal syndrome), additional acid-base disorders stack on top of the chronic respiratory alkalosis, creating complex mixed pictures. Always establish the patient's baseline ABG when interpreting.
| Cause | Key Differentiating Features | Why It Causes Resp Alkalosis |
|---|---|---|
| Anxiety / Panic disorder | Young adult, episodic attacks, sense of doom, palpitations, trembling. DSM-5: recurrent unexpected panic attacks with ≥ 4 symptoms (palpitations, sweating, trembling, SOB, choking, chest pain, nausea, dizziness, chills, paraesthesias, derealization, fear of losing control, fear of dying) + ≥ 1 month of persistent concern/maladaptive behaviour [6]. Hyperventilation as a somatic feature — dizziness, paraesthesia of extremities, paradoxical feeling of SOB [7] | Cortical/limbic override of brainstem ventilatory control → voluntary/involuntary hyperventilation. Cognitive theory: anxiety → physical symptoms → ↑anxiety (downward spiral) [7] |
| Hyperventilation syndrome | Recurrent episodes, often female, classic perioral/acral paraesthesias + carpopedal spasm, normal pO₂ and normal A-a gradient on ABG | Same as anxiety-driven; may become habitual pattern |
| Pain | Acute injury, post-operative, procedural | Cortical and limbic pain pathways drive hyperventilation |
Critical Point — Psychogenic Hyperventilation is a Diagnosis of EXCLUSION
Never diagnose "hyperventilation syndrome" or "anxiety" as the cause of respiratory alkalosis without first ruling out organic causes. Key red flags that MUST prompt further workup:
- Hypoxaemia (pO₂ < 80 mmHg or SpO₂ < 95%) → cannot be psychogenic
- Elevated A-a gradient → suggests V/Q mismatch, shunt, or diffusion impairment (organic)
- Fever → sepsis, pneumonia
- Unilateral leg swelling → DVT/PE
- Focal neurological signs → CNS pathology
- Metabolic acidosis component → salicylate poisoning, DKA, sepsis-related lactic acidosis
A truly psychogenic cause should show: normal pO₂ (often supranormal — > 100 mmHg), normal A-a gradient, low pCO₂, elevated pH, no other abnormalities.
| Cause | Key Differentiating Features | Mechanism |
|---|---|---|
| Mechanical over-ventilation | ICU patient on ventilator, ABG shows unexpectedly low pCO₂ | Excessive tidal volume or respiratory rate set too high → alveolar ventilation exceeds CO₂ production → ↓pCO₂ |
These are scenarios where respiratory alkalosis is one component of a complex picture:
| Mixed Disorder | Classic Example | How to Recognise |
|---|---|---|
| Respiratory alkalosis + Metabolic acidosis | Salicylate poisoning [3][2] | Actual pCO₂ < expected pCO₂ for the degree of metabolic acidosis [2]. pH may be near-normal (the two disorders have opposing effects on pH). Elevated anion gap clinches metabolic acidosis |
| Respiratory alkalosis + Metabolic acidosis | Sepsis (lactic acidosis + direct respiratory stimulation) | Similar to above; lactic acid elevates AG, and pCO₂ is lower than expected for compensation alone |
| Respiratory alkalosis + Metabolic alkalosis | Hepatic failure (chronic resp alk) + vomiting (met alk) | HCO₃⁻ higher than expected for chronic compensation; pH markedly elevated |
| Chronic respiratory alkalosis + Acute metabolic acidosis | Cirrhotic patient with new GI bleed → lactic acidosis | Baseline chronic resp alk (low pCO₂, low HCO₃⁻, normal pH) then acute metabolic acidosis drops pH |
"Not up, not down rule": if HCO₃⁻ and CO₂ move in different directions, there must be a mixed acid-base disturbance [2]. In respiratory alkalosis, both pCO₂ and HCO₃⁻ move downward (same direction) — this is a "simple" disorder. If HCO₃⁻ is paradoxically normal or elevated while pCO₂ is low, suspect a concurrent metabolic alkalosis.
In paediatrics, unexplained respiratory alkalosis should raise suspicion for inborn errors of metabolism [8]:
- Alarming features: significant metabolic acidosis, unexplained respiratory alkalosis, hypoketotic hypoglycaemia [8]
- Urea cycle defects: hyperammonaemia directly stimulates the respiratory centre → respiratory alkalosis + encephalopathy
- Why: ammonia is a potent central respiratory stimulant; the combination of respiratory alkalosis + high ammonia + encephalopathy in a neonate/infant is classic for urea cycle enzyme defects
- Organic acidaemias: may present with mixed picture (metabolic acidosis from organic acids + respiratory alkalosis from central stimulation)
- Clinical features resemble intoxication but without history of ingestion/exposure; or resemble infection but no organism isolated [8] → think IEM
| Category | Causes | Key Distinguishing Clue |
|---|---|---|
| Hypoxia-driven | Pneumonia, PE, asthma, ILD, ARDS, altitude, anaemia, R→L shunt | ↓pO₂, ↑A-a gradient |
| Sepsis / SIRS | Gram-negative sepsis, any severe infection | Fever, ↑WBC, ↑procalcitonin; resp alk may be earliest sign |
| Hepatic | Cirrhosis (HBV, HCV, alcoholic), acute liver failure | Jaundice, coagulopathy, encephalopathy; chronic resp alk |
| CNS | Stroke, tumour, meningitis, encephalitis, TBI, SAH | Focal neurology, altered GCS, neck stiffness |
| Drug / Toxin | Salicylates, theophylline, caffeine, progesterone | Drug history; salicylate = mixed resp alk + met acidosis |
| Endocrine | Thyrotoxicosis, pregnancy | Thyroid signs or positive pregnancy test |
| Cardiac | Heart failure | Orthopnoea, oedema, raised JVP |
| Iatrogenic | Mechanical over-ventilation | ICU setting, ventilator parameters |
| Psychogenic | Anxiety, panic disorder, hyperventilation syndrome, pain | Diagnosis of exclusion; normal pO₂, normal A-a gradient |
| Metabolic (paediatric) | Urea cycle defects, organic acidaemias | Neonatal/infant encephalopathy, hyperammonaemia, consanguinity |
The alveolar-arterial (A-a) gradient is a powerful tool for separating causes:
| A-a Gradient | Interpretation | Causes of Resp Alkalosis |
|---|---|---|
| Normal (< 15 mmHg, age-adjusted: 2.5 + 0.21 × age) | The lungs themselves are fine; hyperventilation is from a non-pulmonary driver | Psychogenic (anxiety, pain), CNS (stroke, tumour), Drugs (salicylates, progesterone), Hepatic, Pregnancy, Mechanical over-ventilation |
| Elevated | There is a genuine pulmonary gas exchange problem | Pneumonia (shunt), PE (V/Q mismatch), ILD (diffusion impairment), ARDS, Heart failure (pulmonary oedema) |
In PE: ABG shows hypoxaemia, hypocapnia, respiratory alkalosis, and ↑A-a gradient [4]. A normal A-a gradient effectively rules against PE as a cause.
When you see a patient with confirmed primary respiratory alkalosis:
-
Check pO₂ and A-a gradient → Is this hypoxia-driven?
- If yes → chest pathology or systemic hypoxia (PE, pneumonia, ILD, anaemia, etc.)
- If no → move to step 2
-
Look for signs of serious organic disease:
- Fever / sepsis markers → sepsis (most dangerous — resp alk may be the only early clue)
- Liver stigmata → hepatic failure (very common in HK)
- Focal neurology / altered GCS → CNS pathology
- Drug history → salicylates (check serum level if any doubt), theophylline
-
Consider endocrine / physiological:
- Pregnancy test in women of reproductive age
- Thyroid function tests if thyrotoxicosis suspected
-
ICU patient? → Check ventilator settings → mechanical over-ventilation
-
Only when ALL organic causes excluded → consider psychogenic / hyperventilation syndrome
High Yield Summary — Differential Diagnosis of Respiratory Alkalosis
- First step: confirm it's primary respiratory alkalosis, not compensatory hyperventilation for metabolic acidosis (look at pH direction)
- Use A-a gradient: elevated → pulmonary pathology (PE, pneumonia, ILD); normal → non-pulmonary (psychogenic, CNS, liver, drugs)
- Top dangerous causes to never miss: PE, sepsis (earliest sign!), salicylate poisoning, CNS pathology
- Salicylate poisoning = mixed respiratory alkalosis + HAGMA — suspect in any acid-base disorder of unknown origin [3]
- PE ABG: hypoxaemia + hypocapnia + respiratory alkalosis + ↑A-a gradient + Type I respiratory failure [4]
- Liver failure (HBV cirrhosis in HK) → chronic respiratory alkalosis, often fully compensated
- Psychogenic is a diagnosis of EXCLUSION — must have normal pO₂ and normal A-a gradient
- In paediatrics: unexplained respiratory alkalosis + encephalopathy → think urea cycle defects (hyperammonaemia) [8]
- Mixed disorders: use compensation formulae and "not up, not down" rule to unmask [2]
Active Recall - Differential Diagnosis of Respiratory Alkalosis
References
[1] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.3, Compensatory mechanisms) [2] Senior notes: Ryan Ho Urogenital.pdf (p.35, Expected compensation table and three-step ABG approach) [3] Senior notes: Ryan Ho Chemical Path.pdf (p.42, Section E Salicylate); Ryan Ho Urogenital.pdf (p.48, Aspirin toxicity) [4] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (p.967, ABG findings in PE) [5] Senior notes: Ryan Ho Respiratory.pdf (p.29, Type 1 respiratory failure causes — PE) [6] Senior notes: Ryan Ho Psychiatry.pdf (p.179, DSM-5 diagnostic criteria for panic disorder) [7] Senior notes: Ryan Ho Psychiatry.pdf (p.173, GAD somatic features — hyperventilation) [8] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.463, Inborn errors of metabolism — alarming features including unexplained respiratory alkalosis)
Diagnostic Criteria for Respiratory Alkalosis
Respiratory alkalosis is not a "disease" with formal diagnostic criteria like lupus or rheumatic fever — it is a laboratory diagnosis made on arterial blood gas (ABG) analysis. However, there is a rigorous, stepwise approach to confirming it, classifying its chronicity, detecting mixed disorders, and identifying the underlying cause. This is effectively the "diagnostic criteria."
Respiratory alkalosis is defined by: [2]
- ↓pCO₂ < 35 mmHg (< 4.7 kPa) — the primary derangement (hyperventilation blowing off CO₂)
- pH > 7.45 (acute) or pH normal 7.35–7.45 (chronic, fully compensated)
- ↓[HCO₃⁻] — secondary/compensatory (kidney excreting bicarbonate)
Key Diagnostic Principle
The Henderson-Hasselbalch equation defines the acid-base behaviour in the body: change in pH follows direction of change of HCO₃⁻:CO₂ ratio [2]. In respiratory alkalosis, CO₂ drops (denominator falls) → ratio rises → pH rises. The low HCO₃⁻ is always secondary compensation — never the primary change.
This distinction is clinically crucial because it determines (a) whether the compensation is appropriate and (b) whether a mixed disorder is hiding underneath.
| Criterion | Acute Respiratory Alkalosis | Chronic Respiratory Alkalosis |
|---|---|---|
| Duration | < 24–48 hours | > 2–5 days |
| pH | Elevated (> 7.45) | Normal or near-normal [2][9] |
| pCO₂ | < 35 mmHg | < 35 mmHg |
| Expected compensation | ↓[HCO₃⁻] = 2 mmol/L per 10 mmHg (1.3 kPa) drop in pCO₂; minimum ~18 mmol/L [2] | ↓[HCO₃⁻] = 4 mmol/L per 10 mmHg (1.3 kPa) drop in pCO₂; minimum ~12 mmol/L [2] |
| Mechanism of compensation | Intracellular buffering only (H⁺ released from proteins, Hb, phosphate) | Renal: ↓proximal HCO₃⁻ reabsorption + ↓distal H⁺ secretion |
Remarks: "1-2-3-4" rule for compensation of acute and chronic respiratory acidosis and alkalosis: Acute resp acidosis = 1, acute resp alkalosis = 2, chronic resp acidosis = 3, chronic resp alkalosis = 4 (mmol/L change in HCO₃⁻ per 10 mmHg change in pCO₂) [10]
Rule of thumb: ONLY in chronic respiratory alkalosis there may be complete compensation (i.e. normal pH) [2][9]
Once you've confirmed respiratory alkalosis, you must check whether compensation is appropriate. If not, a concurrent metabolic disorder is present:
| Finding | Interpretation |
|---|---|
| Measured HCO₃⁻ matches expected | Simple (pure) respiratory alkalosis |
| Measured HCO₃⁻ lower than expected | Concurrent primary metabolic acidosis (e.g., salicylate poisoning, sepsis with lactic acidosis) [2] |
| Measured HCO₃⁻ higher than expected | Concurrent primary metabolic alkalosis (e.g., cirrhotic patient with vomiting) [2] |
Three-step approach when interpreting ABG: [2]
- Look at pH: direction of change often indicates primary/predominant acidosis/alkalosis
- Pattern recognition: "3 ups/3 downs rule" — if ALL 3 components (pH, HCO₃⁻, pCO₂) move in same direction, there must be a simple metabolic disturbance. "Not up, not down rule" — if HCO₃⁻ and CO₂ move in different directions, there must be mixed acid-base disturbance
- Evaluate compensation to uncover subtle mixed disorders
Applying the 3-Step Approach to Respiratory Alkalosis
In pure respiratory alkalosis, pH goes up, pCO₂ goes down, and HCO₃⁻ goes down → pH moves opposite to the other two → this is a simple respiratory disturbance (not metabolic, because in simple metabolic disorders all three move in the same direction). If you see pH up, pCO₂ down, but HCO₃⁻ up or normal → HCO₃⁻ and CO₂ are moving in different directions → mixed acid-base disturbance [2].
Case: F/75, drug overdose. ABG: pH 7.54 (H), pCO₂ 2.83 kPa (L), pO₂ 23.76 kPa (H), HCO₃⁻ 15.7 mmol/L (L), BE -4.8 (L). Salicylate 4.42 mmol/L (> 2.2). [3]
Step-by-step interpretation:
- pH 7.54 → alkalaemic → predominant process is alkalosis
- pCO₂ 2.83 kPa (≈21 mmHg) → very low → primary respiratory alkalosis present
- Check compensation: ΔpCO₂ = 40 - 21 = 19 mmHg. For acute resp alk: expected ΔHCO₃⁻ = 2 × (19/10) = 3.8; expected HCO₃⁻ = 24 - 3.8 ≈ 20.2 mmol/L
- Measured HCO₃⁻ = 15.7 → much lower than expected 20.2 → concurrent metabolic acidosis [3]
- This confirms a mixed respiratory alkalosis + metabolic acidosis → classic salicylate poisoning pattern [3][2]
- Anion gap: Na 138 - Cl (not given but can be estimated) — the elevated AG confirms unmeasured anions (salicylate, lactate, ketoacids)
"In acute respiratory alkalosis, immediate bicarbonate buffer compensation can only result in 2 mmol/L drop in HCO₃⁻ per each 1.33 kPa drop in pCO₂. In this case, the expected HCO₃⁻ is at ~20 mmol/L; we can thus conclude an underlying metabolic acidosis is present. This can be verified by anion gap (↑AG → anion present). Note that base excess does not give extra info over [HCO₃⁻]." [3]
The following algorithm integrates ABG interpretation, chronicity assessment, mixed disorder detection, and aetiological workup:
Investigation Modalities — Systematic Approach
The investigations for respiratory alkalosis serve two purposes:
- Confirm and characterise the acid-base disorder (ABG, electrolytes)
- Identify the underlying aetiology (targeted tests based on clinical suspicion)
A. Confirming the Acid-Base Disorder
ABG is the definitive investigation for diagnosing respiratory alkalosis [7][11].
| Parameter | Normal Range | Finding in Resp Alkalosis | Interpretation |
|---|---|---|---|
| pH | 7.35–7.45 | > 7.45 (acute) or normal (chronic) | Confirms alkalaemia or compensated alkalosis |
| pCO₂ | 35–45 mmHg (4.7–6.0 kPa) | < 35 mmHg (< 4.7 kPa) | Primary derangement — hyperventilation |
| pO₂ | 75–100 mmHg (10–13 kPa) | Variable (↓ if hypoxia-driven, normal/↑ if psychogenic) | Distinguishes pulmonary vs non-pulmonary causes |
| HCO₃⁻ | 22–26 mmol/L | ↓ (18–22 acute; 12–18 chronic) | Compensatory renal excretion |
| Base excess | -2 to +2 | Negative (more so in chronic) | Base excess does not give extra info over [HCO₃⁻] [3] |
Interpret ABG using a stepwise approach: assess oxygenation via PaO₂ and A-a gradient, determine acid-base status through pH, identify primary disorder from PaCO₂ and bicarbonate, then calculate compensation [7]
A-a gradient calculation:
Or simplified: PAO₂ (in kPa) = FiO₂ × 94.5 − PaCO₂ × 1.25 [11]
- Normal A-a gradient: < 15 mmHg (age-adjusted: 2.5 + 0.21 × age), or < 2.7 kPa [11]
- ↑A-a gradient (> 2.7 kPa): due to V/Q mismatch, diffusion abnormalities [11]
| A-a Gradient | Significance for Resp Alkalosis |
|---|---|
| Normal | Lungs are fine; hyperventilation driven by non-pulmonary cause (anxiety, CNS, liver, drugs, pregnancy) |
| Elevated | Pulmonary gas exchange problem → PE, pneumonia, ILD, ARDS, pulmonary oedema |
High Yield — Venous vs Arterial Blood Gas
Note the source of blood sample determines interpretation: [2]
- Arterial BG: pH 7.36–7.44, [HCO₃⁻] 21–27 mmol/L, pCO₂ 36–44 mmHg (4.8–5.9 kPa)
- Peripheral venous BG: usually add 0.02–0.04 to pH, subtract 1–2 mmol/L from [HCO₃⁻] and 3–8 mmHg from pCO₂
- Central venous BG: usually add 0.03–0.05 to pH and 4–5 mmHg from pCO₂
In practice, a venous blood gas (VBG) can screen for acid-base status (pH and HCO₃⁻ are reasonably accurate), but pO₂ on VBG is meaningless — you cannot assess oxygenation or calculate A-a gradient from a VBG. If you need oxygenation data, you must get an ABG.
| Test | Expected Finding | Why |
|---|---|---|
| K⁺ | ↓ (hypokalaemia) | Alkalosis drives K⁺ intracellularly (H⁺/K⁺ exchange) |
| Ionised Ca²⁺ | ↓ | ↑pH → ↑albumin-Ca²⁺ binding → ↓free Ca²⁺ |
| Phosphate | ↓ | Respiratory alkalosis results in ↓cellular CO₂. The resultant high cellular pH stimulates Krebs cycle, resulting in consumption of phosphate [12] — also glycolysis consumes phosphate |
| Na⁺ | Usually normal | May be deranged depending on underlying cause |
| Cl⁻ | May be ↑ (hyperchloraemia) | Kidney retains Cl⁻ as it excretes HCO₃⁻ in compensation |
Hypokalemia is usually associated with alkalosis [13]. Always check serum bicarbonate alongside potassium — if hypoK and metabolic acidosis → indicates renal tubular acidosis; if hypoK and alkalosis → more commonly vomiting, diuretics, or secondary to the alkalosis itself [13][14]
- Anion gap = Na⁺ − Cl⁻ − HCO₃⁻ [1]
- Normal anion gap = 8–14 [1]
- Why calculate it? To detect a concurrent high anion gap metabolic acidosis (HAGMA) hidden behind the respiratory alkalosis
- If AG > 14 in a patient with respiratory alkalosis → there is a concurrent metabolic acidosis with unmeasured anions (e.g., salicylate poisoning: resp alkalosis early → becomes dominated by HAGMA in late stages [3])
Low bicarbonate does not mean metabolic acidosis — could be the compensatory mechanism to a primary respiratory alkalosis (e.g., something causing hyperventilation) [1]. Conversely, if HCO₃⁻ is low AND the AG is elevated, the low HCO₃⁻ is not just compensation — there is a true metabolic acidosis as well.
B. Identifying the Underlying Aetiology
Once respiratory alkalosis is confirmed, targeted investigations depend on clinical suspicion:
- Quick bedside screen for hypoxaemia
- SpO₂ < 94% in a hyperventilating patient → likely hypoxia-driven cause → proceed to CXR, CTPA, etc.
- SpO₂ normal → non-pulmonary cause more likely
| Finding | Suggests |
|---|---|
| Consolidation / air bronchograms | Pneumonia |
| Bilateral alveolar infiltrates | ARDS [15] |
| Pleural effusion, blunted costophrenic angle | PE (also pneumonia, HF) |
| Peripheral wedge-shaped opacity (Hampton hump) | PE [16] |
| Focal oligaemia (Westermark sign) | PE [16] |
| Pulmonary oedema (upper lobe venous diversion, Kerley B lines) | Heart failure |
| Reticular/honeycombing pattern | ILD |
| Normal CXR | Anxiety, PE (CXR normal in 12–22% of PE [16]), early sepsis, drugs, liver disease |
| Finding | Suggests |
|---|---|
| Sinus tachycardia | Non-specific (anxiety, PE, sepsis, thyrotoxicosis) |
| S1Q3T3 pattern | PE (massive) [16] |
| RV strain: T wave inversion V1–V4, new RBBB, RAD, P pulmonale | PE (massive) [16] |
| ST depression, flat T, U waves | Hypokalaemia (consequence of alkalosis) |
| Prolonged QTc | Hypocalcaemia (↓ionised Ca²⁺ from alkalosis) |
| AF | PE, thyrotoxicosis |
| Investigation | What It Tells You | When to Order |
|---|---|---|
| CBC with differentials | ↑WBC (sepsis, pneumonia), ↓Hb (severe anaemia as cause), ↑eosinophil (anaphylaxis) | Routine |
| CRP / Procalcitonin | Infection/sepsis markers | Fever, suspected sepsis |
| LFT | Deranged LFT → liver failure/cirrhosis causing chronic resp alkalosis | Jaundice, liver stigmata, known HBV |
| Ammonia (NH₃) | ↑↑ in urea cycle defects (paediatric) and hepatic encephalopathy; NH₃ is a potent ventilatory stimulator → respiratory alkalosis [8] | Encephalopathy, neonatal/infant, liver failure |
| RFT | AKI (sepsis-related), CKD (as underlying cause of acidosis in mixed disorder) | Routine |
| Lactate | ↑ in sepsis, shock (lactic acidosis component of mixed disorder) | Suspected sepsis or shock |
| Serum salicylate level | Therapeutic 10–30 mg/dL; toxic > 40 mg/dL [3]. Suspect salicylate poisoning in acid-base disorder of unknown origin [3] | Any unexplained mixed resp alk + met acidosis, drug OD |
| Urine toxicology screen | Identifies salicylates, methylxanthines, other drugs | Suspected poisoning |
| D-dimer | Sensitive (↑↑NPV) but not specific; only used to rule out venous thrombosis if absent [16] | Suspected PE (low-risk Wells score) |
| Thyroid function tests | Thyrotoxicosis | Goitre, tremor, AF, weight loss |
| β-hCG | Pregnancy | Women of reproductive age with unexplained chronic resp alk |
| Blood cultures | Identify causative organism in sepsis | Fever, suspected sepsis |
| Investigation | Indication | Key Findings |
|---|---|---|
| CTPA (CT Pulmonary Angiography) | Suspected PE | Intraluminal filling defects in pulmonary arteries |
| CT Brain | Suspected CNS pathology (stroke, tumour, SAH) | Infarct, haemorrhage, mass, meningeal enhancement |
| HRCT Thorax | Suspected ILD | Ground-glass opacities, honeycombing, traction bronchiectasis |
| Echocardiography | Suspected heart failure or PE (RV strain) | RV dilatation/dysfunction, McConnell's sign (regional RV wall motion abnormality) in PE; EF assessment in HF |
| Lumbar puncture | Suspected meningitis/encephalitis | CSF analysis: cells, protein, glucose, culture, PCR |
When unexplained respiratory alkalosis is found in a neonate or infant with encephalopathy [8]:
| Investigation | Finding | Diagnosis |
|---|---|---|
| Plasma ammonia (NH₃) | ↑↑ (> 100 μmol/L in term neonates) | Urea cycle defects [8] |
| ABG pattern | ↑pH, ↓pCO₂, ↓HCO₃⁻ (respiratory alkalosis with 2° metabolic compensation) — because NH₃ is a potent ventilatory stimulator | Urea cycle enzyme defects [8] |
| Plasma amino acids | ↑glutamine, ↑citrulline (distal defects) or ↓citrulline (proximal defects) | Specific urea cycle defect |
| Urine organic acids | Pattern of organic acid accumulation | Organic acidaemias |
| Acylcarnitine profile | Specific patterns | Fatty acid oxidation defects, organic acidaemias |
Contrast with organic acidaemias which give ↓pH, ↓pCO₂, ↓HCO₃⁻ (metabolic acidosis with 2° hyperventilation) [8]
| Phase | Investigation | Purpose |
|---|---|---|
| 1. Confirm disorder | ABG (gold standard) | pH, pCO₂, pO₂, HCO₃⁻, A-a gradient |
| 1. Confirm disorder | Serum electrolytes (K⁺, Ca²⁺, PO₄³⁻, Na⁺, Cl⁻) | Detect consequences + calculate AG |
| 2. Assess severity | Pulse oximetry | Rapid hypoxaemia screen |
| 2. Assess severity | ECG | Arrhythmias from ↓K⁺/↓iCa²⁺; PE signs |
| 3. Identify cause | CXR | Pneumonia, PE signs, ARDS, ILD, HF |
| 3. Identify cause | CBC, CRP, blood cultures, lactate | Sepsis workup |
| 3. Identify cause | LFT, NH₃ | Liver failure / cirrhosis |
| 3. Identify cause | Salicylate level, urine tox screen | Drug poisoning |
| 3. Identify cause | D-dimer, CTPA | PE |
| 3. Identify cause | TFTs, β-hCG | Thyrotoxicosis, pregnancy |
| 3. Identify cause | CT brain, LP | CNS causes |
| 3. Identify cause (paeds) | NH₃, amino acids, organic acids, acylcarnitine | Inborn errors of metabolism |
Normal pH value with ↓HCO₃⁻ and ↓pCO₂ → Metabolic acidosis + Respiratory alkalosis [10]
This is a crucial interpretation. When you see:
- pH normal (~7.40)
- HCO₃⁻ low
- pCO₂ low
It could be either:
- Chronic compensated respiratory alkalosis (the kidney has had time to dump HCO₃⁻, restoring pH) — only in chronic respiratory alkalosis may there be complete compensation [2][9]
- Mixed metabolic acidosis + respiratory alkalosis (the two processes cancel each other's pH effects) — check anion gap: if AG elevated → there IS a metabolic acidosis present [10]
The clinical context and anion gap distinguish them:
- Stable cirrhotic, no acute illness, normal AG → chronic compensated respiratory alkalosis
- Septic patient, elevated lactate, elevated AG → mixed disorder
High Yield Summary — Diagnosis of Respiratory Alkalosis
- ABG is the gold standard: ↓pCO₂ + ↑pH (acute) or normal pH (chronic) + ↓HCO₃⁻
- Use the "1-2-3-4 rule": acute resp alk compensation = 2 mmol/L ↓HCO₃⁻ per 10 mmHg ↓pCO₂; chronic = 4 mmol/L [2][10]
- Always check if compensation is appropriate — if HCO₃⁻ lower than expected → mixed with met acidosis; if higher → mixed with met alkalosis [2]
- A-a gradient separates pulmonary (↑) from non-pulmonary (normal) causes
- Calculate anion gap to unmask hidden HAGMA (especially salicylate poisoning) [1][3]
- Salicylate level should be checked in any unexplained acid-base disorder [3]
- Normal pH + ↓pCO₂ + ↓HCO₃⁻ = either chronic compensated resp alk OR mixed met acidosis + resp alk — use AG and clinical context to distinguish [10]
- In paediatrics: resp alkalosis + hyperammonaemia → urea cycle defects [8]
- Venous BG can screen for pH and HCO₃⁻ but cannot assess oxygenation — need ABG for pO₂ and A-a gradient [2]
Active Recall - Diagnostic Criteria, Algorithm and Investigations for Respiratory Alkalosis
[1] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.3–4, Compensatory mechanisms, Anion gap) [2] Senior notes: Ryan Ho Urogenital.pdf (p.34–35, Approach to Acid-base Disorders, Expected compensation, Three-step approach, VBG vs ABG interpretation) [3] Senior notes: Ryan Ho Chemical Path.pdf (p.42, Salicylate poisoning — mixed resp alk + met acidosis case) [7] Senior notes: Learning_Points_All_Lectures.txt (Section 4, Respiratory Medicine — stepwise ABG interpretation) [8] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.463–465, Inborn errors of metabolism — urea cycle defects vs organic acidaemias ABG patterns) [9] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.310, Rule of thumb — only chronic resp alkalosis may have complete compensation) [10] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.85–90, Expected compensation "1-2-3-4" rule, mixed disorders table, causes of respiratory alkalosis) [11] Senior notes: Maksim Medicine Notes.pdf (p.213, ABG parameters, A-a gradient formula, compensation table) [12] Senior notes: Ryan Ho Chemical Path.pdf (p.27, Hypophosphataemia in respiratory alkalosis — Krebs cycle stimulation) [13] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.3, Hypokalemia associated with alkalosis) [14] Senior notes: Block A - Introduction to Renal Investigations (RFT, urine tests and US kidneys).pdf (p.6, When to suspect renal tubular problems) [15] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.306, ARDS — ABG shows respiratory alkalosis) [16] Senior notes: Ryan Ho Respiratory.pdf (p.134–135, PE investigation findings — CXR, ECG, D-dimer)
The single most important concept in managing respiratory alkalosis is this: you treat the underlying cause, not the alkalosis itself. There is no "anti-alkalosis drug." The low pCO₂ is a consequence of hyperventilation, and hyperventilation is a consequence of something else. Fix that something else, and the pCO₂ normalises on its own.
This is fundamentally different from, say, managing metabolic acidosis where you might directly administer bicarbonate [17]. In respiratory alkalosis, the lungs are working too hard (or being driven too hard) — the fix is to address why.
The body compensates for acid-base disorders via: Lungs → immediate (hyperventilate, hypoventilate); Kidneys → takes several days [1]. In respiratory alkalosis, the kidneys are already compensating by excreting HCO₃⁻. Your job as a clinician is to remove the stimulus for hyperventilation.
Treatment Modalities by Aetiology
1. Treat the Underlying Cause (The Cornerstone)
This is the most important management principle and cannot be overemphasised.
The hyperventilation here is a physiological response to low pO₂. The treatment is to correct the hypoxaemia — once pO₂ is restored, the drive to hyperventilate diminishes.
| Cause | Specific Treatment | Rationale |
|---|---|---|
| Pneumonia | Antibiotics (empiric then targeted), O₂ therapy, supportive care | Eliminate infection → resolve consolidation → improve V/Q matching → ↑pO₂ → ↓hyperventilatory drive |
| Pulmonary embolism | Anticoagulation (LMWH → warfarin/DOAC), thrombolysis if massive, supportive O₂ | Resolve clot → restore perfusion to ventilated areas → ↓V/Q mismatch → ↑pO₂ |
| Asthma (acute) | SABA (salbutamol) ± SAMA (ipratropium), IV/PO corticosteroids (hydrocortisone IV or prednisolone PO) [18]. Second-line: IV aminophylline (methylxanthine, PDE4 inhibitor) if no response [18] | Bronchodilation → ↓airway obstruction → improve gas exchange → ↓hypoxic drive. Caveat: aminophylline itself can stimulate the respiratory centre, so use judiciously |
| ARDS | Treat precipitant (sepsis, trauma), lung-protective mechanical ventilation (low tidal volume 6 mL/kg, PEEP), prone positioning | Minimise ventilator-induced lung injury while maintaining oxygenation |
| ILD | Immunosuppression (if inflammatory), antifibrotics (pirfenidone, nintedanib), supplemental O₂ | Reduce disease progression; O₂ corrects hypoxaemia |
| Severe anaemia | Transfuse PRBCs, treat underlying cause (iron deficiency, bleeding) | Restore O₂ carrying capacity → ↓tissue hypoxia → ↓ventilatory drive |
| High altitude | Descent, supplemental O₂, acetazolamide (for prevention/treatment of altitude sickness) | Acetazolamide induces metabolic acidosis (carbonic anhydrase inhibitor → ↓HCO₃⁻ reabsorption in PCT) → counteracts the respiratory alkalosis and allows better acclimatisation |
Oxygen therapy considerations:
- Target SpO₂ 94–98% in most patients
- In COPD patients: maintain SaO₂ = 88–92%; start with 24% Venturi mask or 1–2 L/min by nasal prongs; avoid high-dose O₂ to prevent removal of hypoxic drive [18]
- Why is this relevant? A patient with COPD and acute pneumonia may present with T1RF and respiratory alkalosis initially. As you treat with O₂, be careful not to remove the hypoxic drive (in chronic CO₂ retainers), which could paradoxically convert them to T2RF with respiratory acidosis
High Yield — O₂ Therapy in Hypoxia-Driven Respiratory Alkalosis
Giving O₂ to a hypoxaemic patient with respiratory alkalosis corrects the stimulus for hyperventilation (low pO₂ at carotid body). As oxygenation improves, the drive to breathe excessively diminishes, pCO₂ rises back toward normal, and the alkalosis resolves. This is why supplemental O₂ is the first-line "treatment" for hypoxia-driven respiratory alkalosis.
| Component | Treatment | Rationale |
|---|---|---|
| Source control | Drain abscesses, remove infected devices, debride necrotic tissue | Eliminate the source of endotoxin/cytokine release that drives respiratory centre stimulation |
| Antibiotics | Empiric broad-spectrum within 1 hour of recognition (e.g., piperacillin-tazobactam + vancomycin if MRSA risk), then narrow based on cultures | Kill the organisms producing the endotoxins |
| Fluid resuscitation | Crystalloid bolus (30 mL/kg initial) | Restore perfusion; if lactic acidosis coexists (mixed disorder), improving perfusion ↓lactate |
| Vasopressors | Noradrenaline if hypotensive despite fluids | Maintain MAP ≥ 65 mmHg to ensure organ perfusion |
| Monitoring | Serial ABGs, lactate, urine output | Track resolution of alkalosis and any concurrent metabolic acidosis |
Respiratory alkalosis in sepsis is often the earliest lab abnormality — it typically resolves as sepsis is treated. If the patient worsens, respiratory alkalosis may progress to metabolic acidosis (lactic acidosis from shock) or T2RF (respiratory muscle fatigue).
| Component | Treatment | Rationale |
|---|---|---|
| Chronic respiratory alkalosis in stable cirrhosis | No specific treatment needed — this is a well-compensated, chronic state | The kidney has already fully compensated; pH is normal. Treating the alkalosis directly would be harmful |
| Hepatic encephalopathy | Lactulose (↓NH₃ production/absorption), rifaximin (reduce gut bacteria that produce NH₃) [19] | NH₃ is a potent ventilatory stimulator → ↓NH₃ → ↓respiratory drive → pCO₂ normalises |
| Decompensated liver failure | Supportive ICU care, treat precipitants (infection, GI bleeding), consider liver transplant if King's College Criteria met [19] | Ultimate treatment of the failing liver removes the source of toxin accumulation |
| ↑ICP in acute liver failure | Mannitol, hyperventilation, barbiturate coma (if refractory) [19] | Note: here, deliberate hyperventilation (maintaining resp alkalosis) is actually used therapeutically — ↓pCO₂ → cerebral vasoconstriction → ↓ICP. This is one of the few situations where you intentionally maintain respiratory alkalosis |
Paradox — Deliberate Respiratory Alkalosis in Raised ICP
Mx of ↑ICP: mannitol, hyperventilation, barbiturate coma (if refractory) [19]. In acute liver failure with raised intracranial pressure, controlled hyperventilation is used therapeutically to lower pCO₂ → cerebral vasoconstriction → ↓cerebral blood volume → ↓ICP. This is a situation where you deliberately induce/maintain respiratory alkalosis. However, this must be done cautiously and temporarily (target pCO₂ ~30–35 mmHg), as prolonged severe hypocapnia causes excessive cerebral ischaemia.
| Cause | Treatment |
|---|---|
| Stroke | Thrombolysis (if ischaemic, within window), thrombectomy, neurosurgical decompression (if haemorrhagic with mass effect) |
| Meningitis / Encephalitis | IV antibiotics (ceftriaxone + ampicillin ± dexamethasone for bacterial), acyclovir for HSV encephalitis |
| Brain tumour | Dexamethasone (↓peri-tumoural oedema), surgery ± radiotherapy ± chemotherapy |
| TBI | Neurosurgical management, ICP monitoring |
Resolving the CNS pathology removes the direct stimulus on brainstem respiratory centres.
E. Drug / Toxin-Induced Respiratory Alkalosis
Treatment of aspirin toxicity: [3]
- Resuscitation if unstable vitals
- GI decontamination by activated charcoal to bind GI aspirin (most effective within 1–2 hours of ingestion)
- Alkalinisation of serum and urine by sodium bicarbonate:
- Alkalinisation of serum → ionic trapping into blood → ↓intracellular salicylate level (salicylate in ionised form cannot cross cell membranes → trapped in blood → ↓tissue toxicity)
- Alkalinisation of urine → ionic trapping into tubular fluid → ↑salicylate excretion (same principle: ionised salicylate in urine cannot be reabsorbed → excreted)
- Supplemental glucose if altered mental status (salicylates can cause neuroglycopaenia even with normal blood glucose, because they ↓CNS glucose transport)
- Haemodialysis if: [3]
- Contraindication to NaHCO₃ (fluid overload)
- Renal failure (cannot excrete aspirin)
- Severe intoxication (salicylate level > 100 mg/dL, or refractory acidosis, or end-organ damage)
Why Alkalinise in Salicylate Poisoning — Not Acidify?
This seems counterintuitive: the patient already has respiratory alkalosis, so why make them more alkaline with bicarbonate? The answer lies in ionic trapping. Salicylic acid (pKa ~3.0) is a weak acid. In alkaline urine (pH > 7.5), salicylate becomes ionised (charged) and cannot be reabsorbed across the tubular epithelium back into blood → it stays in the urine and gets excreted. Similarly, alkaline blood keeps salicylate ionised in the intravascular space, preventing it from crossing into cells (especially the brain) where it does damage.
NEVER intubate a salicylate-poisoned patient without extreme caution: if you sedate and mechanically ventilate them, you may lose their compensatory hyperventilation → pCO₂ rises → pH drops → more salicylate becomes unionised → crosses into brain and other tissues → catastrophic worsening. If intubation is unavoidable, maintain their pre-intubation minute ventilation.
| Drug | Management |
|---|---|
| Theophylline/methylxanthine toxicity | Stop drug, supportive care, activated charcoal (multi-dose), β-blockers for tachycardia, consider haemodialysis/haemoperfusion for severe toxicity |
| Progesterone | No treatment needed if physiological (pregnancy). Discontinue exogenous progesterone if causing symptoms |
This is an ICU-specific problem and is entirely preventable.
| Intervention | How It Works |
|---|---|
| ↓Respiratory rate on ventilator | Fewer breaths per minute → ↓minute ventilation → ↑pCO₂ |
| ↓Tidal volume | Less volume per breath → ↓alveolar ventilation → ↑pCO₂ |
| Add mechanical dead space | Inserting extra tubing between the ventilator circuit and the patient → patient rebreathes some CO₂ → ↑pCO₂. Used cautiously |
| Monitor with ABG | Check ABG 30–60 minutes after any ventilator adjustment to confirm pCO₂ is normalising |
Check ABG 30–60 mins after initiation of non-invasive ventilation; do not delay intubation and mechanical ventilation if no improvement [18]. The same principle applies in reverse — after adjusting ventilator settings to correct respiratory alkalosis, recheck ABG to ensure you haven't over-corrected into respiratory acidosis.
Lung-protective ventilation (6 mL/kg tidal volume, as used in ARDS) may actually cause permissive hypercapnia (mild respiratory acidosis) — this is considered acceptable and even beneficial in ARDS management, representing the opposite end of the spectrum from respiratory alkalosis.
| Intervention | Details | Rationale |
|---|---|---|
| Reassurance | Calm, authoritative explanation that symptoms (tingling, cramps, dizziness) are from overbreathing and are not dangerous | Breaks the cognitive spiral: anxiety → physical symptoms → ↑anxiety [6]. Patient understanding reduces fear |
| Breathing retraining | Coach slow, controlled breathing (e.g., diaphragmatic breathing, 6–8 breaths/min, prolonged exhalation) | Reduces minute ventilation → allows pCO₂ to normalise → symptoms resolve |
| Anxiolytics (short-term, acute) | Benzodiazepines (e.g., lorazepam 0.5–1 mg PO/SL/IV) for acute severe panic attacks | ↓CNS excitability → ↓cortical drive for hyperventilation. Use with caution — not for chronic use |
| CBT (long-term) | Cognitive behavioural therapy for panic disorder/anxiety | Addresses "safety behaviours" and catastrophic cognitions that perpetuate the cycle [6]. Gold-standard psychological intervention |
| SSRIs/SNRIs (long-term) | Sertraline, paroxetine, venlafaxine for panic disorder | First-line pharmacotherapy for panic disorder if recurrent |
| Psychiatric referral | If recurrent episodes, significant functional impairment | Formal diagnosis and structured treatment plan |
Paper Bag Rebreathing — Do NOT Recommend
The classic teaching of "breathe into a paper bag" to raise pCO₂ is no longer recommended in emergency settings. Reasons:
- If the patient actually has a PE, pneumothorax, or cardiac disease causing their tachypnoea, reducing O₂ intake with a paper bag could be fatal
- It may worsen anxiety in some patients
- The same effect can be achieved more safely with coached slow breathing
Only consider in a clearly psychogenic setting where organic causes have been definitively excluded — and even then, breathing retraining is preferred.
2. Symptomatic Management of Electrolyte Consequences
Even as you treat the underlying cause, the electrolyte shifts caused by alkalosis may require direct correction:
| Severity | Treatment | Rationale |
|---|---|---|
| Mild (paraesthesias only) | Reassurance + treat underlying cause; no calcium replacement needed | Resolving the alkalosis will restore normal ionised Ca²⁺ as pH normalises |
| Moderate (carpopedal spasm, Chvostek/Trousseau positive) | IV calcium gluconate 10% 10–20 mL over 10–20 min with cardiac monitoring | Directly repletes ionised Ca²⁺ to stop neuromuscular hyperexcitability. Use calcium gluconate (not chloride) peripherally because CaCl₂ causes tissue necrosis if extravasated |
| Severe (seizures, laryngospasm, arrhythmia) | IV calcium gluconate bolus + infusion, treat underlying cause urgently | Life-threatening emergency |
Important: this is functional hypocalcaemia (total Ca usually normal, ionised Ca low due to pH-dependent albumin binding). Once the alkalosis resolves, ionised Ca normalises without needing ongoing calcium supplementation.
| Severity | Treatment | Rationale |
|---|---|---|
| Mild (K⁺ 3.0–3.5) | PO KCl supplementation (40–80 mEq/day) if symptomatic | Replete K⁺ stores; alkalosis drives K⁺ intracellularly so total body K⁺ may actually be normal — correction of alkalosis alone may restore serum K⁺ |
| Moderate (K⁺ 2.5–3.0) | PO or IV KCl (rate ≤ 20 mEq/hour via peripheral line, ≤ 40 mEq/hour via central line) with cardiac monitoring | ECG changes (ST depression, U waves) warrant more aggressive replacement |
| Severe (K⁺ < 2.5 or ECG changes or arrhythmia) | IV KCl via central line with continuous cardiac monitoring | Hypokalemia complications: generalised weakness (especially proximal musculature), respiratory failure, fatal arrhythmia [13] |
Alkalosis very often is NOT the cause of hypoK, but rather its consequence, or a consequence of a common cause (e.g., vomiting, excessive renal K loss) [5]. In respiratory alkalosis specifically, the K⁺ shift is usually modest (~0.2–0.4 mEq/L per 0.1 pH rise) and rarely life-threatening on its own. But in mixed disorders (e.g., vomiting causing both metabolic alkalosis AND K⁺ loss), hypoK can be severe.
- Usually mild and self-correcting as alkalosis resolves
- Respiratory alkalosis → ↓cellular CO₂ → high cellular pH → stimulates Krebs cycle → consumption of phosphate [12]
- Only treat if severe (PO₄ < 0.3 mmol/L) or symptomatic (muscle weakness, respiratory failure): IV sodium/potassium phosphate
3. Ventilatory Support — When and How
In respiratory alkalosis, the problem is too much ventilation, not too little. Ventilatory support is therefore directed at the underlying cause (e.g., O₂ for hypoxaemia, NIV for associated conditions), not at the alkalosis itself.
NIV may be relevant when respiratory alkalosis coexists with an underlying condition requiring ventilatory support:
Strongest evidence for NIV use: [20]
- Acute COPD exacerbation with hypercapnic acidosis (PaCO₂ > 6.0 kPa or pH < 7.35)
- Hypercapnic respiratory failure secondary to OHS, chest wall deformity, or neuromuscular disease
- Cardiogenic pulmonary oedema
- Facilitate weaning from invasive ventilation (especially in COPD)
- Acute respiratory failure in immunocompromised patients
Contraindications to NIV: [20]
- Absolute: Lack of spontaneous breathing/cardiopulmonary arrest; gasping; anatomical/functional airway obstruction; severe facial deformity/trauma/burns; GI bleeding or ileus
- Relative: Haemodynamic instability; severe acidosis (pH < 7.15); inability to protect airway (GCS < 8); inability to cooperate in agitated status; excessive secretions; recent upper airway or GI surgery
Contraindications to NIV (from respiratory notes): [21]
- Respiratory arrest and medical instability
- Inability to protect airway and copious secretions
- Uncooperative or agitated status and unfitting mask
- Recent upper airway or GI surgery
Indications for invasive ventilation: [18]
- Patients unable to tolerate or failed NIV
- Impaired consciousness, massive aspiration, or unable to clear secretions
- Severe haemodynamic instability without response to fluid and vasopressors
- Severe ventricular arrhythmia, cardiac or respiratory arrest
When a patient with respiratory alkalosis requires intubation (e.g., for airway protection in severe encephalopathy), the ventilator settings must be carefully chosen to match the patient's pre-intubation minute ventilation. This is especially critical in salicylate poisoning (as discussed above).
When respiratory alkalosis is due to hyperammonaemia from urea cycle defects [8]:
Emergency treatment of ↑↑NH₃: [8]
- Stop protein intake (including milk) — but only for a short period; prolonged ↓protein may ↑catabolism and worsen the condition
- 10% dextrose ± insulin/lipids to ↓catabolism — provides alternative energy substrate to prevent protein breakdown
- Sodium benzoate/phenylbutyrate to ↓NH₃ — these are ammonia scavengers that provide alternative pathways for nitrogen excretion (benzoate conjugates with glycine → hippurate, excreted by kidney; phenylbutyrate conjugates with glutamine → phenylacetylglutamine, excreted by kidney)
- Extracorporeal detoxification to ↓NH₃ if encephalopathy ± NH₃ > 300 mmol/L — haemodialysis or continuous veno-venous haemodiafiltration
- Enzyme replacement to replenish deficient enzymes — e.g., arginine, citrulline, Carbaglu (carglumic acid for N-acetylglutamate synthase deficiency), carnitine (↑free-state transporter)
- Forced diuresis to promote urinary excretion
Indications for haemodialysis (mnemonic: AEIOU): [17]
- A — Acidosis: metabolic acidosis with pH < 7.1 refractory to bicarbonate infusion
- E — Electrolyte imbalance: hyperK > 6.5 or rapidly rising K refractory to medical Rx
- I — Intoxication: drug removal in overdose (salicylate, methanol, ethylene glycol, lithium)
- O — Overload: fluid overload refractory to diuretics
- U — Uraemia: features of uraemia (pericarditis, neuropathy, ↓mental status)
In the context of respiratory alkalosis, dialysis is relevant for:
- Salicylate poisoning: removes salicylate directly when alkalinisation/charcoal are insufficient or contraindicated
- Concurrent renal failure: if AKI develops (e.g., in sepsis), the kidney cannot compensate or excrete toxins
- Concurrent severe metabolic acidosis: if mixed disorder with refractory metabolic acidosis
| Aetiology | Primary Treatment | Supportive Measures |
|---|---|---|
| Pneumonia | Antibiotics, O₂ | Electrolyte correction, fluids |
| PE | Anticoagulation ± thrombolysis, O₂ | Monitoring, haemodynamic support |
| Asthma | Bronchodilators, steroids | O₂, monitor for fatigue → T2RF |
| Sepsis | Antibiotics, source control, fluids, vasopressors | Serial ABG, lactate, electrolytes |
| Liver failure | Treat precipitant, lactulose/rifaximin for encephalopathy, transplant | Hyperventilation for ↑ICP [19], electrolyte correction |
| Salicylate OD | Activated charcoal, NaHCO₃ (serum + urine alkalinisation), haemodialysis [3] | Do NOT suppress hyperventilation |
| CNS pathology | Treat underlying (surgery, antibiotics, steroids) | ICP management if needed |
| Over-ventilation | Adjust ventilator (↓RR, ↓TV, add dead space) | Recheck ABG 30–60 min |
| Anxiety / Panic | Reassurance, breathing retraining, anxiolytics, CBT/SSRIs long-term | Exclude organic causes first |
| Altitude | Descent, O₂, acetazolamide | Hydration |
| Pregnancy | None — physiological; pCO₂ 28–32 mmHg is normal | Reassurance |
| Urea cycle defects | Stop protein, dextrose, ammonia scavengers, dialysis [8] | Enzyme replacement, genetic counselling |
High Yield Summary — Management of Respiratory Alkalosis
- The treatment is the underlying cause — there is no "anti-alkalosis" drug for respiratory alkalosis
- Hypoxia-driven: O₂ therapy is the first-line "anti-alkalosis" measure; treating the pulmonary pathology resolves the alkalosis
- Salicylate poisoning: activated charcoal + NaHCO₃ alkalinisation + haemodialysis for severe cases [3]. NEVER suppress the patient's hyperventilation (it's keeping salicylate out of the brain)
- Mechanical over-ventilation: ↓RR and ↓TV on the ventilator; recheck ABG in 30–60 min
- Psychogenic: reassurance, breathing retraining, anxiolytics acutely, CBT/SSRIs long-term. Paper bag rebreathing is no longer recommended in emergency settings
- Electrolyte consequences: IV Ca gluconate for symptomatic ↓iCa²⁺; KCl for significant hypoK; phosphate replacement rarely needed
- Deliberate respiratory alkalosis is used therapeutically for ↑ICP in acute liver failure: hyperventilation → ↓pCO₂ → cerebral vasoconstriction → ↓ICP [19]
- Chronic compensated respiratory alkalosis (e.g., stable cirrhosis, pregnancy): no treatment needed — the kidney has fully compensated
- Haemodialysis indications (AEIOU): Acidosis, Electrolyte, Intoxication, Overload, Uraemia [17] — relevant for salicylate poisoning and concurrent renal failure
Active Recall - Management of Respiratory Alkalosis
[1] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.2–3, Compensation mechanisms) [3] Senior notes: Ryan Ho Chemical Path.pdf (p.42, Salicylate poisoning); Ryan Ho Urogenital.pdf (p.48, Aspirin toxicity treatment) [5] Senior notes: Ryan Ho Chemical Path.pdf (p.18, Footnote 20 — alkalosis is often not the cause of hypoK) [6] Senior notes: Ryan Ho Psychiatry.pdf (p.173, Cognitive theory of anxiety spiral) [8] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.463–465, Emergency treatment of hyperammonaemia, urea cycle defects) [12] Senior notes: Ryan Ho Chemical Path.pdf (p.27, Hypophosphataemia in respiratory alkalosis) [13] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.3, Hypokalemia complications) [17] Senior notes: Ryan Ho Critical Care.pdf (p.26, Haemodialysis indications AEIOU, management of metabolic acidosis with NaHCO₃) [18] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.226–228, Acute COPD exacerbation management — O₂ therapy, NIV, invasive ventilation indications) [19] Senior notes: Ryan Ho GI.pdf (p.207, Acute liver failure management — hyperventilation for ↑ICP, lactulose, rifaximin) [20] Lecture slides: Handbook of Internal Medicine 2024.pdf (p.421, NIV evidence levels and contraindications) [21] Senior notes: Ryan Ho Respiratory.pdf (p.33, NIV contraindications and use)
Complications of Respiratory Alkalosis
The complications of respiratory alkalosis arise from the downstream effects of low pCO₂ (hypocapnia) and elevated pH (alkalaemia) on multiple organ systems. They can be broadly divided into:
- Direct consequences of the alkalosis and hypocapnia (electrolyte shifts, cerebral vasoconstriction, impaired O₂ delivery)
- Complications of the renal compensation (chronic changes)
- Complications of the underlying cause (which often dominate the clinical picture)
It is important to understand that isolated respiratory alkalosis is rarely fatal on its own — lethal pH levels are < 7.1 or > 7.7 [1][22]. However, even moderate alkalosis produces clinically significant complications that can worsen the patient's overall condition, and in critically ill patients these effects compound other organ dysfunction.
1. Neuromuscular Complications
These are the most immediately apparent complications and often what prompts the patient to seek medical attention.
- Mechanism: Alkalosis → ↓ionised Ca²⁺ (Ca²⁺ binds more tightly to negatively-charged albumin at higher pH) → lowered threshold for neuronal and muscular depolarisation → spontaneous firing of sensory and motor nerves
- Clinical features:
- Perioral numbness and acral paraesthesias (tingling around the mouth, fingertips, toes) — sensory neurons fire spontaneously
- Carpopedal spasm — involuntary hand/foot contraction ("main d'accoucheur" posture)
- Positive Chvostek's and Trousseau's signs — latent tetany elicited on examination
- Severity: Usually self-limiting if the alkalosis resolves; severe or prolonged alkalosis can progress to generalised muscle spasm
- Why perioral specifically? The sensory nerve endings around the mouth (branches of CN V) have a particularly low threshold for depolarisation, making this region the earliest and most sensitive to detect ↓ionised Ca²⁺
- Mechanism: Two synergistic factors:
- Cerebral vasoconstriction from ↓pCO₂ → cerebral ischaemia → lowered seizure threshold
- ↓Ionised Ca²⁺ → neuronal hyperexcitability → lowered seizure threshold
- Clinical significance: Seizures from respiratory alkalosis alone are uncommon but can occur when pH exceeds ~7.6 or when superimposed on pre-existing epilepsy or CNS pathology
- Type: Usually generalised tonic-clonic, but focal seizures can occur depending on the degree of regional cerebral ischaemia
- Mechanism: Hypokalemia complications include generalised weakness (especially of the proximal musculature), respiratory failure, and fatal arrhythmia [13]. The hypokalaemia of alkalosis (K⁺ shifts intracellularly) reduces the resting membrane potential of muscle cells, impairing their contractile function
- Paradox: alkalosis causes both tetany (from ↓iCa²⁺ → hyperexcitability) and weakness (from ↓K⁺ → impaired contraction). These are not contradictory — they affect different pathways. Tetany is from nerve hyperexcitability at the neuromuscular junction level; weakness is from impaired muscle cell repolarisation
2. Cardiovascular Complications
- Mechanism: Alkalosis produces multiple pro-arrhythmic changes:
- Hypokalaemia → flattened T waves, U waves, ST depression, prolonged QT → predisposition to re-entrant tachycardias, ventricular ectopics, and potentially torsades de pointes (TdP)
- ↓Ionised Ca²⁺ → prolonged QT interval (Ca²⁺ is needed for the plateau phase of the cardiac action potential; ↓iCa²⁺ prolongs it)
- Alkalosis directly alters cardiac ion channel kinetics (shifts voltage-gating thresholds)
- Clinical significance: Particularly dangerous in patients with pre-existing cardiac disease, on digoxin (alkalotic hypokalaemia potentiates digoxin toxicity), or with concurrent electrolyte abnormalities
- Both hypo- and hyperkalemia have lethal consequences [14][22] — in the context of respiratory alkalosis, the hypokalaemia is usually modest, but in combination with other causes of K⁺ loss (vomiting, diuretics), it can become life-threatening
Hypokalemia: U wave, flat T wave, long QTc — these ECG changes should be actively monitored in patients with significant respiratory alkalosis [13]
- Mechanism: Alkalosis causes smooth muscle constriction in coronary arteries → ↓coronary blood flow → risk of myocardial ischaemia, especially in patients with pre-existing coronary artery disease
- In severe alkalosis (pH > 7.55), this can precipitate angina or even acute coronary syndrome
- Peripheral vasoconstriction also occurs, though this is usually clinically less significant than the coronary effects
- Mechanism: Severe alkalosis → ↓cardiac contractility (direct effect of extreme pH on myocardial function) + vasodilation in some vascular beds → reduced cardiac output
- This is primarily seen at lethal pH levels (> 7.7) [1] and is a pre-terminal event
3. Central Nervous System Complications
- Mechanism: CO₂ is a potent cerebral vasodilator. When pCO₂ drops, cerebral arterioles constrict. This is a well-characterised physiological response:
- Every 1 mmHg drop in pCO₂ → approximately 2% decrease in cerebral blood flow (CBF)
- At pCO₂ of ~20 mmHg, CBF can be reduced by 35–40% compared to baseline
- Clinical consequences:
- Lightheadedness, dizziness, visual disturbances (blurred vision, tunnel vision)
- Impaired concentration, confusion
- Syncope or near-syncope
- In extreme cases: ischaemic cerebral damage (especially in patients with pre-existing cerebrovascular disease)
- Clinical relevance: This is why deliberate hyperventilation for ↑ICP must be used cautiously — while it effectively lowers ICP by reducing cerebral blood volume, prolonged severe hypocapnia (pCO₂ < 25 mmHg) can cause cerebral ischaemia and worsen neurological outcome
- Ranges from mild confusion to obtundation
- In critically ill patients (e.g., sepsis with respiratory alkalosis), it may be difficult to separate the CNS effects of the alkalosis from those of the underlying disease
4. Respiratory Complications
This is one of the most clinically important complications, especially in critically ill patients.
- Mechanism: Alkalosis → ↑Hb affinity for O₂ → leftward shift of the O₂-Hb dissociation curve → Hb holds onto O₂ more tightly at the tissue level → ↓O₂ release to tissues
- Clinical significance: Even though PaO₂ may be normal or elevated (especially in psychogenic hyperventilation where PaO₂ can be supranormal), tissue oxygen delivery is impaired. This is a "hidden hypoxia" — the blood looks well-oxygenated on ABG and pulse oximetry, but the tissues are receiving less O₂ than expected
- This effect compounds other causes of tissue hypoxia (e.g., in sepsis with both respiratory alkalosis AND lactic acidosis, the leftward shift further worsens an already compromised oxygen delivery)
- Mechanism: Hypocapnia causes airway smooth muscle constriction → ↑airway resistance
- Clinical significance: In patients with pre-existing asthma or reactive airway disease, this can worsen bronchospasm. It can also paradoxically worsen dyspnoea, perpetuating the cycle of hyperventilation (especially in psychogenic hyperventilation)
- If the cause of hyperventilation persists (e.g., worsening pneumonia, PE), the respiratory muscles may eventually fatigue
- This converts the initial respiratory alkalosis picture into Type 2 respiratory failure (T2RF): ↓pO₂ + ↑pCO₂ → respiratory acidosis [5]
- Hypoxia initially leads to increased RR and respiratory alkalosis; eventually respiratory muscle fatigue will lead to respiratory acidosis [23]
- Clinical pearl: In acute asthma, a "normalising" pCO₂ (from low → normal) is actually a danger sign — it means the patient is tiring and can no longer maintain the compensatory hyperventilation. The next step is a rapidly rising pCO₂, respiratory acidosis, and potentially respiratory arrest
High Yield — Normalising pCO₂ in Acute Asthma Is a Danger Sign
A patient with an acute severe asthma exacerbation who initially has respiratory alkalosis (low pCO₂) and whose pCO₂ is now rising toward normal is NOT improving. They are fatiguing. A normal or rising pCO₂ in the setting of acute asthma should prompt consideration of ICU transfer and preparation for intubation.
5. Electrolyte and Metabolic Complications
- Mechanism: Alkalosis → H⁺ leaves cells (to buffer the extracellular alkalosis) → K⁺ enters cells (to maintain electroneutrality) → ↓serum K⁺
- Approximate shift: ~0.2–0.4 mEq/L drop per 0.1 unit pH rise
- Hypokalemia is usually associated with alkalosis [13]
- Complications of hypokalaemia: generalised weakness (especially proximal musculature), respiratory failure, fatal arrhythmia [13]
- In respiratory alkalosis alone, the K⁺ shift is usually modest. But if combined with other causes of K⁺ loss (diarrhoea, diuretics, vomiting), significant life-threatening hypokalaemia can develop
- Mechanism: Respiratory alkalosis results in ↓cellular CO₂. The resultant high cellular pH stimulates Krebs cycle, resulting in consumption of phosphate [12]. Additionally, alkalosis stimulates glycolysis (activates phosphofructokinase), which also consumes intracellular phosphate
- Clinical significance: Usually mild and self-limiting, but severe hypophosphataemia (PO₄ < 0.3 mmol/L) can cause:
- Muscle weakness (including respiratory muscles → can worsen respiratory failure)
- Impaired myocardial contractility
- Rhabdomyolysis (rare)
- Haemolytic anaemia (↓RBC 2,3-DPG → fragile RBCs)
- Most commonly seen in patients who are already phosphate-depleted (malnourished, alcoholics, refeeding syndrome)
- Mechanism: ↑pH → ↑negative charges on albumin → ↑Ca²⁺ binding → ↓free/ionised Ca²⁺
- Total calcium remains normal — this is NOT true hypocalcaemia but a redistribution effect
- Resolves completely when pH normalises
- Complications: as above (paraesthesias, tetany, seizures, long QT, arrhythmias)
- Mechanism: Alkalosis stimulates glycolysis by activating phosphofructokinase → ↑pyruvate production → ↑lactate generation. Additionally, the leftward shift of the O₂-Hb curve means tissues release less O₂ → tissue hypoxia → anaerobic metabolism → lactate production
- Clinical significance: This creates a paradoxical situation where respiratory alkalosis can generate a concurrent metabolic acidosis. In sepsis, this compounds the lactic acidosis already present from poor perfusion
- Mixed disorders: M. acidosis + R. alkalosis → salicylate overdose, sepsis, liver + renal failure [11] — the lactic acidosis in sepsis may be partly driven by the alkalosis itself
6. Renal Complications (from Chronic Compensation)
- In chronic respiratory alkalosis, the kidney dumps HCO₃⁻ as compensation → serum HCO₃⁻ drops to 12–18 mmol/L
- Clinical significance: The patient now has a reduced buffer reserve. If they develop an acute metabolic acidosis (e.g., sepsis with lactic acidosis, acute kidney injury), they have less buffering capacity to withstand the acid load → pH drops more precipitously than it would in someone with normal HCO₃⁻ stores
- This is particularly relevant in cirrhotic patients (chronic respiratory alkalosis) who develop acute-on-chronic liver failure with lactic acidosis — they are extremely vulnerable to acute pH drops
- As the kidney excretes HCO₃⁻, it retains Cl⁻ to maintain electroneutrality → mild hyperchloraemic state
- Usually clinically insignificant but contributes to a non-anion-gap metabolic acidosis component that can complicate ABG interpretation
In clinical practice, the complications of the underlying cause usually dominate over those of the alkalosis itself. These are not strictly "complications of respiratory alkalosis" but are essential to consider:
| Underlying Cause | Major Complications |
|---|---|
| Sepsis | Multi-organ failure, DIC, ARDS, death |
| PE | RV failure, obstructive shock, death |
| Hepatic failure | Hepatic encephalopathy, variceal bleeding, hepatorenal syndrome, coagulopathy |
| Salicylate poisoning | Metabolic acidosis (late stage), non-cardiac pulmonary oedema, cerebral oedema, coma, death [3] |
| Panic disorder | Functional impairment, agoraphobia, depression (long-term psychiatric sequelae) |
| Mechanical over-ventilation | Haemodynamic compromise (↓venous return from excessive positive pressure), barotrauma, ventilator-induced lung injury |
8. Post-Hyperventilation Complications — When Respiratory Alkalosis Resolves
- If the cause of hyperventilation is abruptly removed (e.g., sedation of an anxious patient, correction of ventilator settings), pCO₂ rises rapidly
- However, the renal compensation (↓HCO₃⁻) takes days to reverse → the patient now has a low HCO₃⁻ with a rising pCO₂ → pH drops below normal → transient metabolic acidosis or mixed disorder
- This is analogous to post-hypercapnic metabolic alkalosis (where rapid correction of chronic respiratory acidosis unmasks the renal compensation as a primary alkalosis [22]) — but in reverse
- During the alkalosis, K⁺ was shifted intracellularly. As pH normalises, K⁺ shifts back out → if K⁺ was aggressively replaced during the alkalosis, the patient may now be at risk of hyperkalaemia as the intracellular K⁺ returns to the ECF
- Conversely, if K⁺ was not adequately replaced and the underlying cause also depleted total body K⁺ (e.g., vomiting), the patient may remain hypokalaemic
| Organ System | Complication | Mechanism |
|---|---|---|
| Neuromuscular | Paraesthesias, tetany, carpopedal spasm | ↓Ionised Ca²⁺ |
| Neuromuscular | Seizures | ↓iCa²⁺ + cerebral hypoperfusion |
| Neuromuscular | Muscle weakness | Hypokalaemia |
| CNS | Dizziness, confusion, syncope | Cerebral vasoconstriction from hypocapnia |
| CVS | Arrhythmias (VT, TdP, AF) | Hypokalaemia + ↓iCa²⁺ → prolonged QT |
| CVS | Coronary vasoconstriction → ischaemia | Direct effect of alkalosis on coronary smooth muscle |
| Respiratory | ↓Tissue O₂ delivery | Leftward shift O₂-Hb curve (Bohr effect) |
| Respiratory | Bronchoconstriction | Hypocapnia → airway smooth muscle constriction |
| Respiratory | Progression to T2RF | Respiratory muscle fatigue |
| Metabolic | Hypokalaemia | Intracellular K⁺ shift |
| Metabolic | Hypophosphataemia | ↑Glycolysis and Krebs cycle activity consume PO₄ |
| Metabolic | Functional hypocalcaemia | ↑Albumin-Ca²⁺ binding at high pH |
| Metabolic | Paradoxical lactic acidosis | ↑Glycolysis + ↓tissue O₂ delivery |
| Renal | ↓HCO₃⁻ buffer reserve (chronic) | Renal compensation → bicarbonate wasting |
| Post-correction | Rebound acidosis | Persistent low HCO₃⁻ after pCO₂ normalises |
High Yield Summary — Complications of Respiratory Alkalosis
- Neuromuscular: paraesthesias, carpopedal spasm, and tetany from ↓ionised Ca²⁺ (total Ca normal); seizures from cerebral hypoperfusion + ↓iCa²⁺
- Cardiovascular: arrhythmias (prolonged QT from ↓iCa²⁺ and ↓K⁺ → risk of TdP); coronary vasoconstriction → ischaemia in susceptible patients
- CNS: cerebral vasoconstriction → ↓CBF up to 35-40% → lightheadedness, syncope, confusion
- Tissue oxygenation: leftward shift of O₂-Hb dissociation curve → "hidden hypoxia" — tissues receive less O₂ despite adequate PaO₂
- Hypokalaemia complications: generalised weakness, respiratory failure, fatal arrhythmia [13]
- Hypophosphataemia from ↑Krebs cycle activity consuming phosphate [12]
- In acute asthma: normalising pCO₂ (from low toward normal) = DANGER SIGN of respiratory muscle fatigue → impending T2RF
- Chronic respiratory alkalosis reduces HCO₃⁻ buffer reserve → the patient is vulnerable to superimposed metabolic acidosis
- Post-correction: risk of rebound acidosis and K⁺ shifts — monitor electrolytes closely
- Lethal pH levels: < 7.1 or > 7.7 [1] — while respiratory alkalosis alone rarely reaches pH > 7.7, combined with metabolic alkalosis it can approach this
Active Recall - Complications of Respiratory Alkalosis
References
[1] Senior notes: Block A - Electrolyte and Acid-Base Disorders.pdf (p.2, Lethal pH levels) [3] Senior notes: Ryan Ho Chemical Path.pdf (p.42, Salicylate poisoning complications) [5] Senior notes: Ryan Ho Fundamentals.pdf (p.230, Respiratory failure types and clinical features); Ryan Ho Respiratory.pdf (p.29, T1RF and T2RF causes) [11] Senior notes: Maksim Medicine Notes.pdf (p.213, Mixed disorders — M. acidosis + R. alkalosis combinations) [12] Senior notes: Ryan Ho Chemical Path.pdf (p.27, Hypophosphataemia in respiratory alkalosis — Krebs cycle mechanism) [13] Senior notes: Block A - Two cases of polyuria and polydipsia.pdf (p.3, Hypokalemia complications) [14] Senior notes: Block A - Introduction to Renal Investigations (RFT, urine tests and US kidneys).pdf (p.6, Both hypo and hyperK have lethal consequences) [22] Lecture slides: Introduction-kidney-Ix.pdf (p.27); Nephrology - ntroduction to Renal Investigation.pdf (p.27, Acid-base disorders definitions, lethal pH) [23] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.92, Causes of respiratory alkalosis — hypoxic hyperventilation progression)
High Yield Summary
- Respiratory alkalosis = hyperventilation → ↓pCO₂ → ↑pH — the most common acid-base disturbance in hospitalised patients
- Chronic respiratory alkalosis is the ONLY acid-base disorder where compensation may be complete (normal pH) — because the kidney can effectively dump HCO₃⁻ over days
- Key causes to remember: Anxiety/hyperventilation syndrome, pneumonia, PE, sepsis (often earliest sign!), liver failure (HBV cirrhosis — very HK-relevant), salicylate poisoning, pregnancy, mechanical over-ventilation
- Salicylate poisoning = mixed respiratory alkalosis + metabolic acidosis (unique combination)
- Symptoms arise from: ↓ionised Ca²⁺ (paraesthesias, tetany, Chvostek/Trousseau signs), cerebral vasoconstriction (dizziness, syncope, confusion), and ↓K⁺ (arrhythmias)
- Compensation formulas: Acute ΔHCO₃⁻ = 2 per 10 mmHg ΔpCO₂; Chronic ΔHCO₃⁻ = 4–5 per 10 mmHg ΔpCO₂
- Never dismiss tachypnoea as "just anxiety" — always rule out PE, pneumothorax, DKA, and other serious causes
- Alkalosis shifts K⁺ intracellularly and ↑Hb-O₂ affinity (leftward Bohr shift) → impairs tissue O₂ delivery
High Yield Summary — Differential Diagnosis of Respiratory Alkalosis
- First step: confirm it's primary respiratory alkalosis, not compensatory hyperventilation for metabolic acidosis (look at pH direction)
- Use A-a gradient: elevated → pulmonary pathology (PE, pneumonia, ILD); normal → non-pulmonary (psychogenic, CNS, liver, drugs)
- Top dangerous causes to never miss: PE, sepsis (earliest sign!), salicylate poisoning, CNS pathology
- Salicylate poisoning = mixed respiratory alkalosis + HAGMA — suspect in any acid-base disorder of unknown origin [3]
- PE ABG: hypoxaemia + hypocapnia + respiratory alkalosis + ↑A-a gradient + Type I respiratory failure [4]
- Liver failure (HBV cirrhosis in HK) → chronic respiratory alkalosis, often fully compensated
- Psychogenic is a diagnosis of EXCLUSION — must have normal pO₂ and normal A-a gradient
- In paediatrics: unexplained respiratory alkalosis + encephalopathy → think urea cycle defects (hyperammonaemia) [8]
- Mixed disorders: use compensation formulae and "not up, not down" rule to unmask [2]
High Yield Summary — Diagnosis of Respiratory Alkalosis
- ABG is the gold standard: ↓pCO₂ + ↑pH (acute) or normal pH (chronic) + ↓HCO₃⁻
- Use the "1-2-3-4 rule": acute resp alk compensation = 2 mmol/L ↓HCO₃⁻ per 10 mmHg ↓pCO₂; chronic = 4 mmol/L [2][10]
- Always check if compensation is appropriate — if HCO₃⁻ lower than expected → mixed with met acidosis; if higher → mixed with met alkalosis [2]
- A-a gradient separates pulmonary (↑) from non-pulmonary (normal) causes
- Calculate anion gap to unmask hidden HAGMA (especially salicylate poisoning) [1][3]
- Salicylate level should be checked in any unexplained acid-base disorder [3]
- Normal pH + ↓pCO₂ + ↓HCO₃⁻ = either chronic compensated resp alk OR mixed met acidosis + resp alk — use AG and clinical context to distinguish [10]
- In paediatrics: resp alkalosis + hyperammonaemia → urea cycle defects [8]
- Venous BG can screen for pH and HCO₃⁻ but cannot assess oxygenation — need ABG for pO₂ and A-a gradient [2]
High Yield Summary — Management of Respiratory Alkalosis
- The treatment is the underlying cause — there is no "anti-alkalosis" drug for respiratory alkalosis
- Hypoxia-driven: O₂ therapy is the first-line "anti-alkalosis" measure; treating the pulmonary pathology resolves the alkalosis
- Salicylate poisoning: activated charcoal + NaHCO₃ alkalinisation + haemodialysis for severe cases [3]. NEVER suppress the patient's hyperventilation (it's keeping salicylate out of the brain)
- Mechanical over-ventilation: ↓RR and ↓TV on the ventilator; recheck ABG in 30–60 min
- Psychogenic: reassurance, breathing retraining, anxiolytics acutely, CBT/SSRIs long-term. Paper bag rebreathing is no longer recommended in emergency settings
- Electrolyte consequences: IV Ca gluconate for symptomatic ↓iCa²⁺; KCl for significant hypoK; phosphate replacement rarely needed
- Deliberate respiratory alkalosis is used therapeutically for ↑ICP in acute liver failure: hyperventilation → ↓pCO₂ → cerebral vasoconstriction → ↓ICP [19]
- Chronic compensated respiratory alkalosis (e.g., stable cirrhosis, pregnancy): no treatment needed — the kidney has fully compensated
- Haemodialysis indications (AEIOU): Acidosis, Electrolyte, Intoxication, Overload, Uraemia [17] — relevant for salicylate poisoning and concurrent renal failure
High Yield Summary — Complications of Respiratory Alkalosis
- Neuromuscular: paraesthesias, carpopedal spasm, and tetany from ↓ionised Ca²⁺ (total Ca normal); seizures from cerebral hypoperfusion + ↓iCa²⁺
- Cardiovascular: arrhythmias (prolonged QT from ↓iCa²⁺ and ↓K⁺ → risk of TdP); coronary vasoconstriction → ischaemia in susceptible patients
- CNS: cerebral vasoconstriction → ↓CBF up to 35-40% → lightheadedness, syncope, confusion
- Tissue oxygenation: leftward shift of O₂-Hb dissociation curve → "hidden hypoxia" — tissues receive less O₂ despite adequate PaO₂
- Hypokalaemia complications: generalised weakness, respiratory failure, fatal arrhythmia [13]
- Hypophosphataemia from ↑Krebs cycle activity consuming phosphate [12]
- In acute asthma: normalising pCO₂ (from low toward normal) = DANGER SIGN of respiratory muscle fatigue → impending T2RF
- Chronic respiratory alkalosis reduces HCO₃⁻ buffer reserve → the patient is vulnerable to superimposed metabolic acidosis
- Post-correction: risk of rebound acidosis and K⁺ shifts — monitor electrolytes closely
- Lethal pH levels: < 7.1 or > 7.7 [1] — while respiratory alkalosis alone rarely reaches pH > 7.7, combined with metabolic alkalosis it can approach this