A child having projectile vomiting who has diagnosed pyloric stenosis. Which of the following acid base imbalance can be seen in the child?

Appeared in: NORCET 5 Prelims - Sept 2023 (Shift-2)

Explanation

Pyloric stenosis leads to forceful, non-bilious projectile vomiting.
This vomiting causes a significant loss of gastric fluid, which is rich in hydrochloric acid (HCl).
The loss of hydrogen ions (H+) and chloride ions (Cl−) from the body decreases the overall acidity of the blood.
This net loss of acid results in an increase in serum bicarbonate and an elevated blood pH, a condition known as metabolic alkalosis.
The accompanying loss of chloride results in hypochloremia, making hypochloremic metabolic alkalosis the classic finding.

Option A: Metabolic acidosis results from a loss of base or an accumulation of acid. Vomiting causes a loss of acid, not a loss of base.
Option C: Respiratory acidosis is caused by the lungs' inability to remove enough carbon dioxide (CO2), a condition known as hypoventilation. It is a primary respiratory problem, not a metabolic one caused by vomiting.
Option D: Respiratory alkalosis is caused by the lungs expelling too much carbon dioxide (CO2), a condition known as hyperventilation. The cause in this scenario is vomiting, not a change in breathing rate.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology of acid-base imbalance in pyloric stenosis as background academic context rather than a clinical decision trigger.
Nurses must closely monitor infants with suspected pyloric stenosis for signs of dehydration (e.g., sunken fontanelle, poor skin turgor, decreased urine output) and electrolyte imbalance.
A key nursing priority is managing fluid and electrolyte replacement before surgery (pyloromyotomy). Correcting the metabolic alkalosis and hypokalemia is crucial for a safe surgical outcome.
What if? If the infant's primary symptom was severe, watery diarrhea instead of vomiting, the expected acid-base imbalance would be metabolic acidosis due to the loss of bicarbonate from the lower gastrointestinal tract.

How to Approach the Question

First, identify the key elements in the question: a child with pyloric stenosis and projectile vomiting.
Recall the primary function of the stomach and the composition of its contents: it produces acid (hydrochloric acid, HCl).
Connect the symptom (projectile vomiting) to its direct consequence: loss of large volumes of stomach acid.
Analyze the effect of losing acid on the body's pH. Losing an acid makes the body more alkaline (basic).
Determine if the cause is metabolic or respiratory. Since the problem originates from the gastrointestinal system (a metabolic process), the imbalance is metabolic alkalosis.
Evaluate the options based on this reasoning. Eliminate the respiratory options first, as the cause is not related to breathing. Then, differentiate between metabolic acidosis (loss of base/gain of acid) and metabolic alkalosis (loss of acid).

Concept Tested & Keywords

Concept Tested: Pathophysiology of acid-base imbalance in pyloric stenosis
Stem keywords: child, projectile vomiting, pyloric stenosis, acid base imbalance
Lead-in keywords: Which of the following
Clinical cues: Projectile vomiting is a hallmark sign of pyloric stenosis and directly points to the loss of gastric acid.
Negative lead-in flag: false

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E6 Nelson Textbook of Pediatrics(2024) — Volume 1 pp. 563-565, 559-561

E6 Ghai Essential Pediatrics(pp 26-904 of 913) p. 70-72

Practise the full NORCET 5 Prelims - Sept 2023 (Shift-2)

Attempt every question from this paper in a timed mock, then review the full solution for each one.