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

1. A

   Respiratory acidosis

2. B

   Respiratory alkalosis

3. C

   Metabolic acidosis

4. D

   Metabolic alkalosis

Appeared in: NORCET 6 mains-2024

Explanation

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• Prolonged vomiting, characteristic of pyloric stenosis, causes the loss of large amounts of gastric acid (hydrochloric acid or HCl).
• The loss of hydrogen ions (H⁺) from the body leads to a relative increase in the concentration of bicarbonate (a base), causing the blood pH to rise.
• This process is metabolic in origin because it stems from the gastrointestinal system, not the respiratory system.
• The resulting condition is specifically a hypochloremic, hypokalemic metabolic alkalosis due to the concurrent loss of chloride and potassium.

Why Other Options Were Wrong

• Option A: Respiratory acidosis is caused by the retention of carbon dioxide (CO₂) due to inadequate breathing (hypoventilation), not by the loss of stomach acid.
• Option B: Respiratory alkalosis is caused by blowing off too much carbon dioxide (CO₂) during rapid breathing (hyperventilation). Vomiting does not cause hyperventilation.
• Option C: Metabolic acidosis is the opposite of what occurs with vomiting. It is caused by the loss of base (bicarbonate) or the accumulation of other acids in the body.

Related Visual

• Visual 1: Flowchart - A flowchart illustrating the sequence: Pyloric Stenosis → Obstruction → Projectile Vomiting → Loss of HCl → Decreased H⁺, Cl⁻, K⁺ → Increased HCO₃⁻ & pH → Metabolic Alkalosis.
• Visual 2: Diagram - An acid-base balance chart (like a Davenport diagram) showing the four quadrants (Metabolic/Respiratory Acidosis/Alkalosis) and plotting the typical location for a patient with prolonged vomiting.

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 resulting from gastrointestinal losses as background academic context rather than a clinical decision trigger.
• A key nursing priority for an infant with pyloric stenosis is correcting the dehydration and metabolic alkalosis with intravenous (IV) fluids containing electrolytes (sodium, chloride, and potassium) BEFORE the infant can safely undergo corrective surgery (pyloromyotomy).
• Nurses must meticulously monitor the infant's fluid intake and output, daily weight, and serum electrolyte levels (especially potassium and chloride) to assess the response to fluid therapy.
• What if? If the baby had severe diarrhea instead of vomiting, the correct answer would be Metabolic Acidosis. Diarrhea causes the loss of bicarbonate-rich fluid from the lower gastrointestinal tract, leading to a primary deficit of base.

How to Approach the Question

• First, identify the core clinical condition: pyloric stenosis.
• Next, recall the hallmark symptom of this condition: projectile vomiting.
• Analyze the physiological consequence of vomiting: What is being lost from the body? The answer is stomach contents, which are highly acidic (hydrochloric acid).
• Connect the loss of acid to acid-base balance. Losing acid makes the body more alkaline (basic).
• Determine if the cause is metabolic or respiratory. Since the problem originates in the GI tract (a metabolic system), it is a metabolic imbalance.
• Combine these steps to conclude that prolonged vomiting leads to metabolic alkalosis, and then select the corresponding option.

Concept Tested & Keywords

• Concept Tested: Pathophysiology of acid-base imbalances resulting from gastrointestinal losses.
• Stem keywords: pyloric stenosis, projectile vomiting, prolonged vomiting
• Lead-in keywords: prone to
• Clinical cues: The diagnosis of pyloric stenosis is a key clinical cue, as its hallmark symptom is non-bilious projectile vomiting, which directly causes a specific and predictable acid-base disturbance.

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