Anesthesiology APPLIED Exam › Pediatric and neonatal

Pyloric stenosis

Pediatric and neonatal·~5 min read· Physician-reviewed✓ Sourced · 6

Stem

A 5-week-old boy, 3.9 kg, born at term, has had projectile non-bilious vomiting for four days. Ultrasound confirms pyloric stenosis. He is lethargic with a sunken fontanelle and has had two wet diapers today. Labs: sodium 132, chloride 84, potassium 2.9 and bicarbonate 38 mEq/L. The surgeon has booked him as the next case.

Question

How will you prepare this infant for surgery, and how will you anesthetize him?

Ideal answer

HeadlineResuscitation first: this is a medical emergency, not a surgical one, and his chloride and bicarbonate tell me he is not ready.¹

0.9% saline boluses of 20 mL/kg, because he is volume-depleted with a hypochloremic, hypokalemic metabolic alkalosis; electrolytes rechecked after.
D5 0.45% saline, 1.5 times maintenance, with potassium chloride only once he is voiding.²
Operate at the endpoints: chloride 100 mEq/L or more, bicarbonate 30 or less, normal potassium, good urine output; usually 12–48 hours.¹^,2
Empty the stomach, then rapid sequence. Large orogastric tube in several positions, pre-oxygenate, propofol and succinylcholine 2 mg/kg, or rocuronium, and intubate.³^,4
No nitrous oxide, minimal or no opioids: acetaminophen and the surgeon's infiltration.³

Watch for: apnea, monitored 12–24 hours after a fully awake extubation, because his CSF stays alkalotic after the serum corrects.¹

Breaking down the answer

Delay for resuscitation — the alkalosis and hypovolemia harm the infant; the pylorus does not. A high bicarbonate means compensatory hypoventilation and a CSF alkalosis that lags the serum, hence postoperative apnea.¹ When the surgeon pushes, I give endpoints and a timeline: the graded judgment.
Saline, not lactated Ringer's — chloride is what the kidney needs to stop exchanging hydrogen for sodium; lactate becomes bicarbonate and worsens the alkalosis. Dextrose stays in, because a fasting infant on saline alone becomes hypoglycemic.
Orogastric suction — the stomach holds formula regardless of the fasting interval; a large multi-orifice tube aspirated supine, in both lateral positions and prone recovers more than 95%.³ I still treat it as a full stomach.
Why not awake — awake intubation in a vigorous infant causes hypoxemia, bradycardia and intracranial pressure surges. A modified technique with gentle low-pressure mask ventilation is acceptable, because infants desaturate fast.¹ Atropine or glycopyrrolate ready for the vagal response.³
Rocuronium caveat — 1.2 mg/kg is an adult RSI dose; the label does not recommend rocuronium for pediatric RSI, so I say I use it off-label or choose succinylcholine.⁵
Why minimal opioids — pyloromyotomy is short and low-pain, and opioids add to central apnea risk. IV acetaminophen is 12.5 mg/kg up to 28 days, 15 mg/kg after.⁶ No hyperventilation, which worsens the CSF alkalosis.³
Awake extubation criteria — the stomach may still hold fluid: eyes open, strong hip flexion and a regular breathing pattern.

Follow-up question

The surgeon says the family has waited long enough and wants to proceed tonight. After the first bolus, repeat labs show chloride 88, bicarbonate 36 and potassium 2.9 mEq/L. What do you say and do?

Follow-up answer

HeadlineAn under-resuscitated infant; I tell the surgeon plainly he is not ready by any accepted endpoint, chloride below 100 and bicarbonate above 30.²

Proceeding risks hypotension on induction, arrhythmia from hypokalemia under volatile anesthesia, and postoperative apnea from the alkalosis. My plan: another 20 mL/kg saline bolus now, maintenance with potassium once he is voiding, a repeat panel after the bolus and a target time for surgery.² I speak to the parents myself: the delay makes the anesthetic safer. When the labs reach the endpoints, I reassess and we go, and I document the discussion.

Pitfalls that cost points

Quoting "metabolic alkalosis" without explaining paradoxical aciduria or why saline corrects it.
Giving fentanyl "for the surgery," then being asked why the infant stopped breathing in recovery.

Rapid-fire

Q: Why paradoxical aciduria? A: Once volume and chloride are depleted, the kidney reabsorbs sodium in exchange for hydrogen to defend volume, excreting acid urine despite alkalemia.¹

Q: Why is the CSF alkalosis the apnea mechanism? A: CSF pH takes hours to equilibrate with plasma, so the central chemoreceptors stay alkalotic after the serum normalizes.¹

Q: Why does potassium fall? A: Vomiting loses little potassium; the kidney wastes it in exchange for sodium under aldosterone, and alkalosis shifts it into cells.

Q: Why avoid nitrous oxide? A: It expands bowel gas during a laparoscopic case.³

Q: Would you use a caudal? A: Not routinely; the incisions are small and brief, and the surgeon's infiltration gives the same benefit.

Numbers to know

Item	Value
Endpoints for the OR	Cl⁻ ≥100, HCO₃⁻ ≤30, K⁺ normal, urine ~2 mL/kg/h²
Fluid bolus	20 mL/kg 0.9% saline, repeated by labs²
Maintenance	D5 0.45% saline + KCl 20 mEq/L at 1.5× once voiding²
Succinylcholine, infant	2 mg/kg IV⁴
IV acetaminophen	12.5 mg/kg (≤28 days); 15 mg/kg (29 days–2 years)⁶
Postoperative apnea monitoring	12–24 h³

Sources

1.Pyloric stenosis · OpenAnesthesia · openanesthesia.org
2.Pyloric Stenosis Clinical Pathway · Johns Hopkins All Children's Hospital, 2021 · hopkinsmedicine.org
3.Laparoscopic Pyloromyotomy (SPA Case Guide) · Society for Pediatric Anesthesia, 2023 · www2.pedsanesthesia.orgGives chloride >90 mEq/L as an approximate, institution-dependent endpoint; OpenAnesthesia and the Johns Hopkins pathway use ≥100.
4.Anectine (succinylcholine) dosage, from FDA labeling · Drugs.com · drugs.com
5.Rocuronium dosage, from FDA labeling · Drugs.com · drugs.com
6.Acetaminophen injection prescribing information · Drugs.com · drugs.com

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.