A child having projectile vomiting who has diagnosed pyloric stenosis. Which of the following acid base imbalance can be seen in the child?

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

Explanation

Pyloric stenosis is characterized by forceful, non-bilious projectile vomiting.
Vomiting causes the loss of large amounts of gastric fluid, which is primarily hydrochloric acid (HCl).
The loss of hydrogen ions (H⁺, an acid) and chloride ions (Cl⁻) leads to a primary excess of bicarbonate in the blood.
This state, where the body loses acid and becomes more alkaline, is defined as metabolic alkalosis.
It is specifically termed hypochloremic metabolic alkalosis due to the concurrent loss of chloride.

Option A: Metabolic acidosis is incorrect. This condition results from a loss of base or an accumulation of acid.
Option C: Respiratory acidosis is incorrect. This is a primary respiratory problem caused by the retention of carbon dioxide (CO₂) due to hypoventilation.
Option D: Respiratory alkalosis is incorrect. This is a primary respiratory problem caused by blowing off too much carbon dioxide (CO₂) due to hyperventilation.

Visual 1: Flowchart - Illustrating the pathophysiology from projectile vomiting -> loss of HCl -> decreased serum H⁺ and Cl⁻ -> increased serum HCO₃⁻ -> resulting in hypochloremic metabolic alkalosis.
Visual 2: Anatomical Diagram - Showing the hypertrophied pyloric sphincter in an infant's stomach, which causes the gastric outlet obstruction.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Acid-base imbalance associated with pyloric stenosis as background academic context rather than a clinical decision trigger.
A key nursing responsibility is to monitor for signs of dehydration and metabolic alkalosis in infants with suspected pyloric stenosis. This includes assessing for poor skin turgor, sunken fontanelles, and irritability.
Pre-operatively, nurses will administer IV fluids with electrolytes (sodium, potassium, and chloride) as prescribed to correct the dehydration and alkalosis before the infant is cleared for surgery (pyloromyotomy).
What if? If the child presented with severe, watery diarrhea instead of vomiting, the most likely acid-base imbalance would be metabolic acidosis due to the loss of bicarbonate-rich intestinal fluid.

How to Approach the Question

First, identify the core clinical condition: pyloric stenosis.
Next, identify the key symptom described: projectile vomiting.
Connect the symptom to its physiological consequence. Vomiting involves the loss of stomach contents.
Recall the composition of stomach contents: primarily hydrochloric acid (HCl).
Reason through the effect of losing acid: If the body loses acid (H⁺ ions), the relative concentration of base (HCO₃⁻) increases, leading to a rise in blood pH.
Conclude that this process results in metabolic alkalosis. The 'metabolic' part refers to the change in bicarbonate, and 'alkalosis' refers to the increased pH.

Concept Tested & Keywords

Concept Tested: Acid-base imbalance associated with pyloric stenosis.
Stem keywords: pyloric stenosis, projectile vomiting, acid base imbalance
Lead-in keywords: Which of the following
Clinical cues: The diagnosis of pyloric stenosis combined with the symptom of projectile vomiting is a classic presentation pointing to a specific electrolyte and acid-base disturbance.
Negative lead-in flag: false

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Practise the full NORCET 5 Prelims - Sept 2023 (Shift-2)

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