(Choice A) Primary metabolic acidosis is characterized by decreased pH, PaCO2, and bicarbonate. Metabolic acidosis can occur when vomiting is accompanied by diarrhea as significant amounts of bicarbonate are lost in the stool. Potassium and chloride are also lost in diarrhea.
(Choice B) Primary respiratory acidosis is characterized by decreased pH with elevated PaCO2 and bicarbonate. This occurs in respiratory depression (eg, narcotic overdose) and hypoventilation syndromes (eg, neuromuscular disease). PaCO2 retention in pyloric stenosis is compensatory to the primary metabolic alkalosis.
(Choice C) A normal acid-base status is characterized by pH of 7.35-7.45, PaCO2 of 35-45 mm Hg, and bicarbonate of 22-26 mEq/L. Laboratory values may be normal initially in patients with pyloric stenosis but are unlikely in this patient with prolonged vomiting, dehydration, and abnormal vital signs.
Explanation
This infant has vomiting with feeds and signs of dehydration (ie, sunken fontanelle, dry mucous membranes) concerning for infantile hypertrophic pyloric stenosis. This condition is most common in first-born boys age 3-5 weeks. The hypertrophied pylorus muscle (see image) obstructs the gastric outlet, resulting in nonbilious, projectile emesis. Although many patients have a palpable, olive-shaped abdominal mass, its absence does not exclude the diagnosis.
If diagnosis is delayed, protracted vomiting can result in hypochloremic, hypokalemic metabolic alkalosis due to hydrochloric acid lost in the emesis. Hypovolemia also activates the renin-angiotensin-aldosterone system and potassium is excreted by the kidneys in response to aldosterone. The respiratory system responds to alkalosis with hypoventilation, which results in increased PaCO2 and a compensatory respiratory acidosis.
A thick, elongated pylorus on abdominal ultrasonography is diagnostic, and pyloromyotomy is the treatment of choice.
Educational objective: Pyloric stenosis presents at age 3-5 weeks with nonbilious, projectile vomiting after each feed. Protracted vomiting produces a hypochloremic, hypokalemic metabolic alkalosis.
