A 6-week-old male infant presents with progressive non-bilious projectile vomiting, visible peristalsis, and an olive-shaped mass palpable in the epigastrium. Ultrasound shows a pyloric muscle thickness of 4.2 mm and pyloric canal length of 18 mm. Before pyloromyotomy, what metabolic abnormality must be corrected?

• A Hyperchloremic metabolic acidosis
• B Hypochloremic, hypokalemic metabolic alkalosis ✓
• C Hypernatremic dehydration with metabolic acidosis
• D Dilutional hyponatremia from excessive water intake

Explanation

Pyloric stenosis causes loss of gastric HCl and K+ via vomiting, producing hypochloremic, hypokalemic metabolic alkalosis — the classic metabolic disturbance. Paradoxic aciduria occurs late due to renal conservation of bicarbonate at the expense of H+ secretion. Pyloromyotomy is NOT a surgical emergency; it is a metabolic emergency. The infant must be adequately resuscitated with 0.45% NaCl with 20-40 mEq/L KCl until serum chloride >100 mEq/L, potassium >3.5 mEq/L, bicarbonate <30 mEq/L, and adequate urine output is established before anaesthetic induction. Proceeding to surgery in the alkalotic state risks laryngospasm and apnoea during anaesthetic.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.