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

1. A

   7.45, 26 mEq/L, 45

2. B

   7.50, 29 mEq/L, 45

3. C

   7.35, 22 mEq/L, 45

4. D

   7.30, 20 mEq/L, 45

Appeared in: NORCET 8 Prelims-2025

Explanation

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• The infant's symptoms are classic for Hypertrophic Pyloric Stenosis (HPS).
• HPS causes persistent vomiting of stomach acid (HCl), leading to a loss of hydrogen (H⁺) and chloride (Cl⁻) ions.
• This loss results in hypochloremic metabolic alkalosis, characterized by a high pH (greater than 7.45) and high bicarbonate (HCO₃⁻, greater than 26 mEq/L).
• The values of pH 7.50 and HCO₃⁻ 29 mEq/L clearly indicate metabolic alkalosis.
• The PaCO₂ of 45 mmHg is at the upper limit of normal, showing that respiratory compensation (hypoventilation to retain CO₂) has not yet significantly occurred.

Why Other Options Were Wrong

• Option A: These values (pH 7.45, HCO₃⁻ 26 mEq/L) represent the upper limits of the normal range. While the infant might pass through this stage, the described persistent vomiting would lead to a more pronounced, frank alkalosis.
• Option C: These values (pH 7.35, HCO₃⁻ 22 mEq/L) are perfectly within the normal range for an infant. They do not reflect any acid-base disturbance, which is inconsistent with the severe clinical picture of persistent vomiting.
• Option D: These values (pH 7.30, HCO₃⁻ 20 mEq/L) indicate metabolic acidosis. This is incorrect because vomiting causes a loss of stomach acid, leading to alkalosis, not acidosis.

Related Visual

• Visual 1: Pathophysiology Diagram - A flowchart illustrating how projectile vomiting in HPS leads to the loss of HCl, resulting in hypochloremia, hypokalemia, and metabolic alkalosis.
• Visual 2: Anatomical Illustration - A drawing showing the hypertrophied pyloric muscle in an infant's stomach, visually representing the 'olive-shaped mass'.
• Visual 3: ABG Interpretation Chart - A simple chart that helps differentiate between the four main types of acid-base imbalances (respiratory/metabolic acidosis/alkalosis) based on pH, PaCO₂, and HCO₃⁻ levels.

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) results in the context of Hypertrophic Pyloric Stenosis (HPS) as background academic context rather than a clinical decision trigger.
• Correcting the metabolic alkalosis and dehydration is a critical preoperative nursing priority. Surgery (pyloromyotomy) is typically delayed until fluid and electrolyte balance is restored to ensure patient safety under anesthesia.
• Nurses must meticulously monitor intake, output, and specific gravity of urine to assess the effectiveness of IV fluid resuscitation.
• Postoperatively, the nurse plays a key role in carefully reintroducing feedings, starting with small volumes and observing for any signs of vomiting, which could indicate surgical complications.

How to Approach the Question

• First, analyze the clinical scenario. The key signs are a 3-week-old infant, persistent vomiting, and a 'palpable olive-shaped mass'.
• Recognize these hallmark signs as pointing directly to the diagnosis of Hypertrophic Pyloric Stenosis (HPS).
• Next, recall the primary pathophysiological consequence of HPS: the forceful vomiting leads to a massive loss of gastric acid (HCl).
• Predict the resulting acid-base imbalance. Loss of acid (H⁺) from the body will make the blood more alkaline, causing metabolic alkalosis.
• Define the ABG parameters for metabolic alkalosis: pH will be high (greater than 7.45), and the metabolic component, HCO₃⁻, will also be high (greater than 26 mEq/L).
• Finally, evaluate the given options to find the set of values that matches the profile of metabolic alkalosis.

Concept Tested & Keywords

• Concept Tested: Interpretation of Arterial Blood Gas (ABG) results in the context of Hypertrophic Pyloric Stenosis (HPS).
• Stem keywords: 3-week-old infant, persistent vomiting, palpable olive-shaped mass, electrolyte imbalances
• Lead-in keywords: Which electrolyte imbalances may be seen
• Clinical cues: The combination of projectile vomiting and a palpable 'olive-shaped' mass in an infant is pathognomonic for Hypertrophic Pyloric Stenosis.
• Clinical cues: Persistent vomiting of gastric contents points towards a loss of acid, leading to metabolic alkalosis.

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