Pyloric Stenosis Electrolytes — FRCA Final MCQ

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ModeratePaediatric AnaesthesiaPyloric Stenosis ElectrolytesFRCA Final

A 4-week-old term infant weighing 3.5 kg requires pyloromyotomy. An arterial blood gas obtained before correction of dehydration and electrolyte abnormalities shows: pH 7.52, PaCO₂ 6.5 kPa, PaO₂ 10.2 kPa, HCO₃⁻ 38 mmol/L, Na⁺ 132 mmol/L, K⁺ 2.8 mmol/L and Cl⁻ 82 mmol/L. What acid-base disturbance is present?

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Correct answer: D — Metabolic alkalosis with respiratory compensation

The correct answer is D. The raised pH and markedly raised bicarbonate establish a primary metabolic alkalosis. In pyloric stenosis, repeated loss of gastric hydrochloric acid produces chloride depletion and alkalosis, while volume depletion and potassium loss maintain it. The PaCO₂ is increased because compensatory hypoventilation retains carbon dioxide; the predicted PaCO₂ is approximately 6.6 kPa, closely matching the measured 6.5 kPa. Respiratory acidosis is therefore not the primary disorder, and a mixed respiratory alkalosis would require an inappropriately low PaCO₂. The low chloride and potassium support the characteristic depletion pattern. The alkalosis and electrolyte abnormalities should be corrected before pyloromyotomy because persistent alkalosis may contribute to hypoventilation and perioperative apnoea.

Reference: Tröbs RB et al. Carbon Dioxide and Hemoglobin at Presentation with Hypertrophic Pyloric Stenosis—Are They Relevant? Cohort Study and Current Opinions. Children. 2025;12:885. https://pubmed.ncbi.nlm.nih.gov/40723078/