A 3-week-old infant presents with persistent projectile vomiting and a palpable olive-shaped mass in the right upper abdomen. Which electrolyte imbalances would be seen in order of pH, HCO3 and CO2?

1. A

   7.45, 26 mEq/L, 45

2. B

   7.50, 29 mEq/L, 40

3. C

   7.35, 22 mEq/L, 45

4. D

   7.30, 20 mEq/L, 40

Appeared in: NORCET 8 Prelims-2025

Explanation

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• The clinical presentation is classic for Hypertrophic Pyloric Stenosis (HPS), a condition causing gastric outlet obstruction.
• Persistent vomiting of gastric contents leads to the loss of hydrochloric acid (H+ and Cl− ions).
• The loss of hydrogen ions (H+) causes the blood pH to rise, resulting in metabolic alkalosis.
• The body compensates for chloride loss by retaining bicarbonate (HCO3−), further elevating its level.
• The values pH 7.50 (alkalotic), HCO3− 29 mEq/L (high), and pCO2 40 mmHg (normal) perfectly represent uncompensated metabolic alkalosis, the hallmark imbalance of HPS.

Why Other Options Were Wrong

• Option A: These values (pH 7.45, HCO3− 26, pCO2 45) are at the upper edge of the normal range and do not reflect the significant pathological imbalance caused by persistent projectile vomiting.
• Option C: These values (pH 7.35, HCO3− 22, pCO2 45) are all within the normal physiological range. An infant with the severe symptoms described would have a deranged acid-base status.
• Option D: These values (pH 7.30, HCO3− 20, pCO2 40) indicate metabolic acidosis (low pH, low HCO3−). This is the opposite of what occurs in HPS.

Related Visual

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Interpretation of Arterial Blood Gas (ABG) values in the context of Hypertrophic Pyloric Stenosis (HPS) as background academic context rather than a clinical decision trigger.
• Recognizing the classic signs of HPS and anticipating the resulting metabolic alkalosis is a critical nursing skill for ensuring prompt intervention and preventing complications.
• A key nursing priority is managing fluid and electrolyte replacement pre-operatively. Surgery (pyloromyotomy) is typically delayed until dehydration and electrolyte imbalances are corrected to ensure patient safety under anesthesia.
• Strict monitoring of intake and output, including the volume and frequency of emesis, is essential for assessing the severity of dehydration and the effectiveness of fluid resuscitation.

How to Approach the Question

• First, analyze the clinical vignette to identify the underlying condition. The triad of a 3-week-old infant, non-bilious projectile vomiting, and a palpable 'olive-shaped' mass is pathognomonic for Hypertrophic Pyloric Stenosis (HPS).
• Next, determine the primary pathophysiological consequence. HPS causes forceful vomiting of stomach contents.
• Connect the consequence to its effect on acid-base balance. Vomiting stomach acid (HCl) leads to a loss of acid from the body.
• Deduce the resulting imbalance. Losing acid makes the blood more alkaline, causing metabolic alkalosis, which is defined by a high pH and high bicarbonate (HCO3−).
• Finally, evaluate the options to find the set of ABG values that matches metabolic alkalosis. The option with a pH above 7.45 and an HCO3− above 26 mEq/L is the correct choice.

Concept Tested & Keywords

• Concept Tested: Interpretation of Arterial Blood Gas (ABG) values in the context of Hypertrophic Pyloric Stenosis (HPS).
• Stem keywords: 3-week-old infant, persistent projectile vomiting, palpable olive-shaped mass, electrolyte imbalances
• Lead-in keywords: Which
• Clinical cues: The combination of a young infant (3 weeks), projectile vomiting, and a palpable 'olive-shaped' mass is a classic triad for Hypertrophic Pyloric Stenosis.
• Negative lead-in flag: false

Question ID

Q1lhiogIkkaBxmv8ozmqEN

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Reference Book

E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 58-60

E6 Nursing Fundamentals Potter Perry 12e Part 5 p. 57-59