Pyloric stenosis: correct the alkalosis before surgery
The case
A previously well 5-week-old boy presents with 3 days of progressively projectile, non-bilious vomiting after feeds, fewer wet nappies and fewer bowel motions. Pregnancy and birth were unremarkable. He is lethargic with dry mucous membranes, capillary refill 4 seconds, heart rate 180/min and blood pressure 70/40 mmHg. A firm 1.5 cm 'olive' is palpable in the right upper quadrant. Venous blood gas: pH 7.55 (7.35–7.45), pCO2 58 mmHg (35–45), bicarbonate 42 mmol/L (22–28). Sodium is 132 mmol/L (135–145), potassium 2.7 mmol/L (3.5–5.0) and chloride 84 mmol/L (98–107). Ultrasound shows a pyloric muscle thickness of 4.5 mm and channel length of 18 mm, confirming hypertrophic pyloric stenosis. What is the most appropriate immediate management?
Options
- AIntravenous fluid resuscitation and electrolyte correction before surgery
- BRapid intravenous potassium before fluid resuscitation
- CIntravenous bicarbonate bolus for the alkalosis
- DUrgent pyloromyotomy without resuscitation
- ENasogastric tube and oral rehydration before surgery
Think it through before you read on. Which single option is best, and why are the other four wrong?
Show the answer and explanation
Answer
A. Intravenous fluid resuscitation and electrolyte correction before surgery
Why this is the right answer
Hypertrophic pyloric stenosis presents at 2 to 8 weeks of age with projectile, non-bilious vomiting after feeds in a hungry baby. First-born boys are most often affected. The diagnosis is confirmed by ultrasound: muscle thickness over 3 to 4 mm and channel length over 15 to 17 mm.
The biochemistry is the heart of this question. Repeated vomiting of stomach contents loses hydrogen and chloride ions, causing a hypochloraemic metabolic alkalosis. The kidneys try to hold sodium and water by exchanging them for potassium and hydrogen, which worsens the hypokalaemia and produces paradoxical aciduria. The raised pCO2 is respiratory compensation. This baby also has signs of shock: tachycardia, prolonged capillary refill and lethargy.
Pyloric stenosis is a medical emergency but a surgical urgency, not a surgical emergency. The operation (Ramstedt pyloromyotomy) is straightforward, but anaesthetising a baby with severe alkalosis is dangerous: alkalosis depresses the respiratory drive and the baby may fail to breathe after the anaesthetic. The Royal Children's Hospital guideline requires the baby to be resuscitated first.
Treatment is an initial bolus of isotonic saline (10 to 20 mL/kg) for shock, then rehydration with 0.9% sodium chloride and 5% glucose with added potassium once urine output is established. Chloride in the saline corrects the alkalosis; no bicarbonate or acid is needed. Surgery proceeds when chloride is above about 100 mmol/L, bicarbonate is below about 28 mmol/L and potassium is normal, which usually takes 24 to 48 hours.
The baby is kept nil by mouth with a nasogastric tube on free drainage in the meantime.
Why the other options are wrong
- B
Potassium is never given as a rapid bolus; it is added to maintenance fluids once the baby is passing urine.
- C
Bicarbonate would worsen the alkalosis; saline corrects it by supplying chloride.
- D
Operating before correcting alkalosis risks post-anaesthetic apnoea and cardiac arrhythmia.
- E
Oral rehydration cannot correct shock and will be vomited; intravenous fluids are required.
High-yield takeaway
Pyloric stenosis: fix the shock and the hypochloraemic hypokalaemic alkalosis with saline and potassium first; surgery waits until chloride and bicarbonate are near normal.
Reference: Royal Children's Hospital Melbourne Clinical Practice Guideline: Pyloric Stenosis (2021)
Common questions
Why is metabolic alkalosis dangerous before surgery?
Alkalosis depresses the respiratory drive, so the baby may stop breathing after the anaesthetic.
What fluids correct the alkalosis?
Isotonic saline with glucose, with potassium added once the baby is passing urine. The chloride does the work; bicarbonate is never needed.
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