Infantile hypertrophic pyloric stenosis (IHPS) causes projectile vomiting because it creates a **progressive, near–fixed gastric outlet obstruction** at the pylorus.
- **Mechanical obstruction from muscle hypertrophy/hyperplasia:** The circular (and to a degree longitudinal) pyloric muscle thickens and the pyloric canal lengthens, narrowing the lumen and impeding gastric emptying. As obstruction worsens, the stomach progressively distends after feeds. (1)(2)
- **Markedly increased intragastric pressure + vigorous gastric peristalsis:** With the outlet functionally “closed,” normal postprandial antral contractions become increasingly forceful as the stomach attempts to overcome the resistance. This raises intragastric pressure until gastric contents are expelled **forcefully**, producing the characteristic “projectile” emesis. (1)
- **Non-bilious nature helps localize the level of obstruction:** Because the obstruction is **proximal to the duodenum/ampulla**, vomitus is typically **non-bilious** (bile has not yet mixed with gastric contents). (1)
Clinically, repeated vomiting also drives the classic downstream physiology—**volume depletion and hypochloremic, hypokalemic metabolic alkalosis**—which often accompanies more advanced/persistent obstruction. (1)(2)
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