A baby who was diagnosed with pyloric stenosis has continued to have projectile vomiting. With prolonged vomiting, the infant is prone to?

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

Explanation

Prolonged projectile vomiting, a hallmark of pyloric stenosis, causes a significant loss of gastric contents.
Gastric fluid is rich in hydrochloric acid (HCl), so its loss leads to a depletion of hydrogen ions (H+) and chloride ions (Cl-) from the body.
The loss of acid (H+) raises the blood pH, leading to an alkaline state. Since the origin is gastrointestinal (metabolic), the condition is termed metabolic alkalosis.
This condition is classically described as hypochloremic (low chloride), hypokalemic (low potassium) metabolic alkalosis, as confirmed by laboratory tests.

Option A: This is caused by inadequate breathing (hypoventilation), which leads to the retention of carbon dioxide (CO2). The primary problem in pyloric stenosis is vomiting, not a respiratory issue.
Option B: This results from excessive breathing (hyperventilation), which causes too much CO2 to be blown off. It is not related to the loss of stomach acid.
Option C: This occurs from a loss of bicarbonate (base), typically through diarrhea, or an accumulation of other acids (e.g., lactic acid, ketones). Vomiting causes a loss of acid, not base.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology of acid-base imbalances in infants with pyloric stenosis as background academic context rather than a clinical decision trigger.
Recognizing the signs of metabolic alkalosis is crucial for nurses, as it is a life-threatening condition that must be corrected before surgical intervention (pyloromyotomy) to prevent complications.
Nursing priorities include strict monitoring of intake and output, assessing for signs of dehydration (e.g., sunken fontanelle, poor skin turgor), and preparing for IV fluid resuscitation with solutions containing sodium, potassium, and chloride.
What if? If the infant had severe diarrhea instead of vomiting, the primary acid-base disturbance would be metabolic acidosis due to the loss of bicarbonate-rich fluid from the lower gastrointestinal tract.

How to Approach the Question

First, identify the key elements of the clinical scenario: an infant with pyloric stenosis and projectile vomiting.
Next, determine the contents of the fluid being lost. Vomiting expels stomach contents, which are highly acidic (hydrochloric acid).
Consider the effect of losing large amounts of acid from the body. The loss of hydrogen ions (H+) will make the blood more alkaline (higher pH).
Categorize the origin of the problem. Since the issue stems from the gastrointestinal system (a metabolic function), not the lungs, it is a 'metabolic' disturbance.
Combine the findings: a metabolic origin leading to an alkaline state results in 'metabolic alkalosis'. Evaluate the options to find the correct match.

Concept Tested & Keywords

Concept Tested: Pathophysiology of acid-base imbalances in infants with pyloric stenosis.
Stem keywords: pyloric stenosis, projectile vomiting, prolonged vomiting, infant
Lead-in keywords: prone to
Clinical cues: Diagnosis: Pyloric stenosis - indicates an upper GI obstruction.
Clinical cues: Symptom: Projectile vomiting - signifies forceful loss of acidic gastric contents.

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

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