Metabolic emergencies - alkalosis
Description
- Metabolic alkalosis: primary rise in bicarbonate, classified by chloride responsiveness - guides cause and treatment
- Respiratory alkalosis: primary fall in CO2 from alveolar hyperventilation
Epidemiology
- Metabolic alkalosis common in hospitalised patients - vomiting, nasogastric suction, diuretic use
- Respiratory alkalosis common in anxiety/panic, but also a marker of serious underlying illness (PE, sepsis, salicylate toxicity, hypoxia)
Aetiopathogenesis
Metabolic alkalosis
- Chloride-responsive (urine Cl- <20): vomiting/NG loss, diuretics (loop/thiazide), post-hypercapnic states - responds to volume/chloride repletion
- Chloride-resistant (urine Cl- >20): mineralocorticoid excess (primary aldosteronism, Cushing's, liquorice), severe hypokalaemia, Bartter/Gitelman syndrome
- Mechanism: H+ loss (vomiting) or renal H+/K+ wasting (mineralocorticoid excess, diuretics) with contraction alkalosis from volume depletion maintaining it
Respiratory alkalosis
- Hyperventilation (anxiety, pain) - benign but a diagnosis of exclusion
- Serious causes: hypoxia (PE, pneumonia), sepsis (early), salicylate toxicity (direct medullary respiratory centre stimulation), pregnancy (physiological), hepatic failure, high altitude
Diagnosis
- ABG/VBG for primary disturbance and compensation; urine chloride to classify metabolic alkalosis mechanism
- Assess volume status, potassium (correlates with severity in both types)
- Consider salicylate level if respiratory alkalosis with metabolic acidosis (mixed picture) - classic salicylate toxicity pattern
Management
Metabolic alkalosis
- Chloride-responsive: IV normal saline (0.9%) repletes volume and chloride, corrects the alkalosis; replace potassium (often depleted, and hypokalaemia itself perpetuates alkalosis)
- Chloride-resistant: treat the underlying cause (spironolactone for aldosteronism, correct hypokalaemia which is severe)
- Stop/reduce causative diuretic where feasible; treat vomiting cause
Respiratory alkalosis
- Treat the underlying cause - do not simply reassure "anxiety-related" without excluding hypoxia, PE, sepsis, salicylate toxicity first
- Address hyperventilation directly (breathing techniques) only once serious causes excluded
Associations
- Hypokalaemia (both causes and results from metabolic alkalosis), tetany/paraesthesia from reduced ionised calcium with alkalosis
- Arrhythmia risk with severe hypokalaemia
- Underlying primary aldosteronism/Cushing's if chloride-resistant and hypertensive
Natural history & complications
- Chloride-responsive metabolic alkalosis resolves promptly with volume/chloride/potassium repletion
- Chloride-resistant alkalosis persists until the underlying endocrine/genetic cause is treated
- Respiratory alkalosis from a serious underlying cause (PE, sepsis, salicylate) will not resolve until that cause is addressed - it is a clue, not a diagnosis in itself
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