Forceful Vomiting and a Metabolic Alkalosis in a 5-Week-Old
A 5-week-old boy is brought for vomiting that began 6 days ago and has become forceful, projecting 30-60 cm, after nearly every feed. He is formula fed, takes 90 mL every 3 hours, and is hungry again immediately after vomiting. Birth weight was 3.4 kg; today's weight is 3.6 kg. His mother reports fewer wet diapers and no stool for 2 days. Temperature 36.9°C, pulse 150, respiratory rate 34, and a soft, sunken fontanelle. A gastric wave is visible across the upper abdomen, and a 1.5-cm olive-shaped mass is palpable to the right of the umbilicus. Laboratory values: sodium 134 mmol/L, potassium 3.0 mmol/L, chloride 88 mmol/L, bicarbonate 34 mmol/L, pH 7.49, urine pH 4.5, urine chloride <10 mmol/L. Ultrasound shows pyloric muscle thickness 4 mm and channel length 17 mm. The surgical team has booked the operating room for the next morning.
1. Explain the electrolyte and acid-base findings, including why the urine is acidic and low in chloride.
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Vomiting of gastric fluid removes hydrogen and chloride, and this infant cannot replace them because he is not retaining feeds. The result is a hypochloremic, hypokalemic metabolic alkalosis with chloride 88 and bicarbonate 34. Volume depletion is the second driver: the kidney senses decreased effective circulating volume and responds with aldosterone-mediated sodium retention. Because distal sodium delivery is low, sodium is exchanged for hydrogen and potassium rather than delivered to the chloride-bicarbonate exchanger, so the urine stays acidic at pH 4.5 and urine chloride is under 10 mmol/L. Acidic urine in a metabolically alkalotic patient is the paradoxical aciduria, the signature of a chloride-responsive, volume-depleted alkalosis.
The potassium of 3.0 has two sources: renal potassium wasting in exchange for sodium and vomiting itself. The ultrasound findings - muscle thickness 4 mm and channel length 17 mm - confirm hypertrophic pyloric stenosis. The key point is that the laboratory abnormality, not the diagnosis, dictates the timing of surgery.
2. What is your preoperative plan, and what endpoints must be met before induction of anesthesia?
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He needs correction before anesthesia. Make him NPO, start intravenous fluid with D5 0.45% or 0.9% saline, and add potassium chloride 20-40 mEq/L once urine output is documented - never give potassium as a bolus. Do not give sodium bicarbonate; the alkalosis is chloride-responsive and will correct with saline and potassium, and bicarbonate risks overshoot and paradoxical CSF acidosis. Replace deficits gradually and recheck electrolytes every 6-8 hours. Monitor urine output, weight, and hematocrit.
Endpoints before the operating room: chloride above 100 mmol/L, bicarbonate below 26-28 mmol/L, potassium at least 3.5 mmol/L, and adequate urine output of roughly 1 mL/kg/h. These are not bureaucratic numbers. An alkalotic infant hypoventilates and can become apneic after extubation because respiratory drive is set by a compensated pH; hypokalemia predisposes to arrhythmia under anesthesia. Correcting chloride and volume also restores renal potassium handling, so the potassium often corrects itself once the chloride does.
3. The operating room slot cannot be moved. How do you handle this, and why?
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Delay the case. Pyloric stenosis is urgent, not emergent: the infant is stable, the diagnosis is established, and there is no bowel compromise or perforation. Operating on an alkalotic, hypokalemic infant trades a short delay for a real risk of post-extubation apnea, arrhythmia, and prolonged ventilation. The correct response to an inflexible schedule is to speak directly with the surgical and anesthesia teams, document the laboratory values, and rebook when correction is achieved - typically 24-48 hours later. Meanwhile he receives nothing by mouth, ongoing maintenance and deficit fluids, and serial electrolytes.
If he were in shock or severely dehydrated, that too would be corrected first. The widespread teaching that these infants "need surgery tonight" is wrong in practice, and the willingness to say so is the point of the question. Document the conversation and the plan, because the pressure to proceed usually comes from scheduling rather than from the child's condition.