Nursing care
Why pyloric stenosis causes hypochloraemic metabolic alkalosis
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
In pyloric stenosis, a thickened pylorus blocks stomach emptying, so the infant vomits gastric contents rich in hydrochloric acid. Losing acid and chloride leaves excess bicarbonate, and dehydration activates aldosterone, which wastes potassium and keeps bicarbonate high. The result is hypochloraemic, hypokalaemic metabolic alkalosis, which is corrected with IV fluids before surgery.
Why the vomit matters: losing stomach acid, not bile
Hypertrophic pyloric stenosis usually presents between about three and six weeks of age. The muscle at the stomach outlet thickens until milk cannot pass easily into the duodenum. Vomiting becomes forceful or projectile shortly after feeds, and the infant often remains hungry. Because the blockage sits before the point where bile enters the bowel, the vomit is nonbilious.
That location explains the lab pattern. Gastric juice contains hydrochloric acid, meaning hydrogen ions and chloride. Every vomit removes both from the body. Vomiting that includes intestinal contents would also lose bicarbonate-rich fluid, but in pyloric stenosis the loss is almost purely gastric, so acid leaves while bicarbonate stays behind and the blood becomes more alkaline.
How the kidneys keep the alkalosis going
Ongoing vomiting also shrinks circulating volume. The renin-angiotensin-aldosterone system responds by encouraging the kidneys to retain sodium and water. Aldosterone does this partly by excreting potassium and hydrogen ions into the urine, which deepens both the hypokalaemia and the alkalosis. Low potassium itself shifts hydrogen ions into cells, raising the blood pH further.
Chloride depletion adds another layer. With little chloride available, the kidneys reabsorb bicarbonate instead and cannot excrete the excess. This is why the disorder is called chloride responsive: until volume and chloride are replaced, the kidneys hold on to bicarbonate. The lungs try to compensate by slowing breathing to retain carbon dioxide, so a sleepy infant with shallow breaths needs close observation.
Assessment findings that track the problem
Look for signs of dehydration: dry mouth, fewer tears, fewer wet diapers, a sunken fontanelle, lethargy and poor weight gain. Visible gastric waves may cross the upper abdomen after a feed, and an olive-shaped mass may be felt in the right upper abdomen, although it is not always palpable. Ultrasound is commonly used to confirm the thickened pylorus.
Laboratory results typically show low chloride, low potassium and raised bicarbonate. An expected trend during treatment is improving urine output with chloride and bicarbonate moving back toward normal. A concerning trend is falling potassium, persistent vomiting, worsening lethargy or apneic pauses. Strict intake and output, daily weights and diaper weights give the nurse the clearest picture of fluid balance.
Why fluids and electrolytes come before pyloromyotomy
Pyloromyotomy is the definitive treatment, but it is not an emergency operation. Taking an alkalotic, dehydrated infant to anaesthesia carries avoidable risk, including breathing problems after surgery, because the body compensates for alkalosis by reducing ventilation. Resuscitation with IV fluid containing sodium chloride, with potassium added once the infant is passing urine as prescribed, restores volume and chloride.
The nurse keeps the infant nil by mouth as ordered, maintains the IV infusion, monitors electrolytes and reports results to the surgical team. A nasogastric tube may be placed to decompress the stomach. Surgery proceeds when the team judges the electrolytes adequately corrected, and feeding is then restarted after the procedure as the surgical team directs.
Work through a hypothetical scenario
A hypothetical four-week-old has had projectile nonbilious vomiting for three days and few wet diapers. Labs show low chloride, low potassium and high bicarbonate. The options are to send the infant straight to theatre, start a thickened formula trial, give a bolus of potassium, or begin prescribed IV isotonic fluids and monitor electrolytes. Starting fluids is correct because volume and chloride drive the alkalosis.
Rushing to surgery is tempting because the diagnosis is clear, but the anaesthesia risk is higher while the infant is alkalotic. Thickened feeds do not relieve a mechanical block. Potassium is added cautiously, usually after urine output is confirmed, so an early bolus is unsafe. The mechanism points directly to volume and chloride replacement first.
Sources and further reading
MSD Manual Professional: Hypertrophic pyloric stenosis. Age of onset, projectile nonbilious vomiting, hypochloraemic hypokalaemic metabolic alkalosis, aldosterone-driven potassium loss, olive mass and fluid correction before pyloromyotomy.
MSD Manual Professional: Metabolic alkalosis. HCl loss, renal maintenance through volume and chloride depletion and hypokalaemia, chloride-responsive alkalosis, saline treatment and respiratory compensation.
MedlinePlus: Pyloric stenosis - infants. Projectile vomiting, ongoing hunger, dehydration signs and electrolyte imbalance on blood tests.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pediatrics practice questions are the closest set to what this page covers.
Common questions
Why is the alkalosis hypochloraemic rather than hyperchloraemic?
Hydrochloric acid in vomit carries chloride out of the body with each episode. Low chloride also stops the kidneys excreting bicarbonate, so chloride loss both causes and sustains the alkalosis.
Why is the infant hungry straight after vomiting?
The problem is a mechanical outlet blockage, not an illness that suppresses appetite. The stomach empties by vomiting, so the infant often wants to feed again until dehydration makes them lethargic.
What finding suggests the alkalosis is improving?
Better urine output with chloride and bicarbonate moving toward normal on repeat labs. Report any persistent low potassium, ongoing vomiting or reduced alertness to the team.