A 3-week-old male infant presents with projectile non-bilious vomiting after every feed. He appears hungry after vomiting. Ultrasound shows pyloric muscle thickness of 4.5 mm and channel length of 18 mm. Pre-operative management before pyloromyotomy must include correction of which metabolic derangement?
- A Hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria ✓
- B Hyperchloremic metabolic acidosis
- C Dilutional hyponatremia from formula feeding
- D Respiratory acidosis from aspiration pneumonia
Explanation
Hypertrophic pyloric stenosis causes loss of gastric HCl via persistent vomiting, resulting in hypochloremic metabolic alkalosis with hypokalemia. Paradoxical aciduria occurs because the kidney initially conserves Na+ by exchanging H+ (generating acidic urine despite systemic alkalosis); as K+ depletion worsens, K+ is conserved and NH4+ excretion increases. Pre-operative correction with IV 0.9% NaCl + KCl infusion is MANDATORY — pyloromyotomy under alkalosis increases risk of apnea from reduced respiratory drive and anesthesia complications. The operation is NOT a surgical emergency; it is a medical emergency requiring metabolic correction first. Target serum chloride >95 mEq/L and bicarbonate <30 mEq/L before surgery.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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Written and medically reviewed by the StethoPrep medical team.