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).
• Persistent vomiting in HPS causes a significant loss of hydrochloric acid (HCl) from the stomach.
• The loss of acid (H+ ions) leads to a compensatory increase in serum bicarbonate (HCO₃⁻), resulting in metabolic alkalosis.
• The correct option shows a high pH (7.50), high bicarbonate (29 mEq/L), and a normal CO2 (40 mmHg), which defines uncompensated metabolic alkalosis.
• This acid-base disturbance is a hallmark laboratory finding in infants with HPS before treatment.

Why Other Options Were Wrong

• Option A: These values (pH 7.45, HCO₃⁻ 26, CO₂ 45) are all within or at the very edge of the normal range. An infant with persistent, forceful vomiting would have a more significant and clear derangement.
• Option C: These values (pH 7.35, HCO₃⁻ 22, CO₂ 45) represent a normal acid-base status, leaning towards the acidic end of the spectrum. This is inconsistent with the loss of stomach acid.
• Option D: These values (pH 7.30, HCO₃⁻ 20, CO₂ 40) indicate metabolic acidosis. This is the opposite of what occurs with vomiting.

Related Visual

• Visual 1: Diagram - Illustration of hypertrophic pyloric stenosis, showing the thickened pyloric muscle obstructing the gastric outlet. This helps visualize the cause of the vomiting.
• Visual 2: Flowchart - A flowchart for ABG interpretation (e.g., ROME mnemonic) to systematically determine the type of acid-base imbalance from pH, PaCO₂, and HCO₃⁻ values.
• Visual 3: Table - A summary table of the four main acid-base disorders (Metabolic/Respiratory Acidosis/Alkalosis) with their expected changes in pH, PaCO₂, and HCO₃⁻.

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Acid-Base Imbalance in Hypertrophic Pyloric Stenosis as background academic context rather than a clinical decision trigger.
• Recognizing the signs of HPS and its associated metabolic alkalosis is critical for nurses, as it is a surgical emergency that requires prompt intervention.
• A key nursing priority is managing fluid and electrolyte imbalances pre-operatively. This involves administering IV fluids to correct dehydration, hypochloremia, and hypokalemia before the infant can safely undergo surgery (pyloromyotomy).
• Monitoring intake, output, and specific gravity of urine is essential to assess the effectiveness of fluid resuscitation.

How to Approach the Question

• Step 1: Analyze the clinical scenario. Identify the key signs: a 3-week-old infant with projectile vomiting and a palpable 'olive' mass.
• Step 2: Formulate a diagnosis. These are classic signs of Hypertrophic Pyloric Stenosis (HPS).
• Step 3: Recall the pathophysiology. HPS involves vomiting of stomach contents, which are rich in hydrochloric acid (HCl).
• Step 4: Determine the expected acid-base imbalance. Loss of acid (H+) leads to metabolic alkalosis.
• Step 5: Define the ABG profile for metabolic alkalosis: high pH (>7.45) and high bicarbonate (HCO₃⁻ > 26 mEq/L).
• Step 6: Evaluate the given options to find the set of values that matches the profile of metabolic alkalosis. Option B (pH 7.50, HCO₃⁻ 29) is the only one that fits.

Concept Tested & Keywords

• Concept Tested: Acid-Base Imbalance in Hypertrophic Pyloric Stenosis
• Stem keywords: 3-week-old infant, persistent projectile vomiting, olive-shaped mass, electrolyte imbalances
• Lead-in keywords: Which electrolyte imbalances
• Clinical cues: The combination of projectile vomiting and a palpable 'olive' in an infant is a pathognomonic sign for Hypertrophic Pyloric Stenosis (HPS).

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Q1lhiogIkkaBxmv8ozmqEN

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