Clinical Scenario: 5 week old | Vomiting | Poor weight gain | ABGs
A 5-week-old male infant is brought to the emergency department with progressive vomiting for the last 10 days. The vomiting is forceful and occurs shortly after feeds but does not contain bile. The mother reports that the infant remains hungry after vomiting but has poor weight gain. On examination, he appears dehydrated with dry mucous membranes and reduced urine output. A small firm mass is palpable in the upper abdomen. Laboratory investigations show serum sodium 132 mmol/L, potassium 2.8 mmol/L, chloride 82 mmol/L, and bicarbonate 38 mmol/L. Arterial blood gas analysis shows:
- pH: 7.52
- PaCO₂: 48 mmHg
- HCO₃⁻: 38 mmol/L
Which of the following best describes his acid–base disturbance?
A. Metabolic alkalosis with appropriate respiratory compensation
B. Metabolic alkalosis with additional respiratory acidosis
C. Respiratory alkalosis with metabolic compensation
D. Normal anion gap metabolic acidosis with respiratory compensation
E. Mixed metabolic alkalosis and metabolic acidosis
Correct answer & Explanation:
Correct Answer
A. Metabolic alkalosis with appropriate respiratory compensation
Detailed Explanation: Hypertrophic pyloric stenosis | Acid base imbalance
This infant has features suggestive of hypertrophic pyloric stenosis:
- Age 2–8 weeks
- Progressive projectile non-bilious vomiting
- Persistent hunger after vomiting
- Poor weight gain
- Dehydration
- Palpable upper abdominal mass
The question is testing recognition of the typical acid–base abnormality associated with repeated gastric vomiting.
Step 1: Identify the primary acid–base disorder
The pH is 7.52, indicating alkalemia.
The bicarbonate level is elevated:
- Normal HCO₃⁻: 22–26 mmol/L
- Patient HCO₃⁻: 38 mmol/L
Therefore, the primary disorder is:
Metabolic alkalosis
Step 2: Assess respiratory compensation
In metabolic alkalosis, expected respiratory compensation can be estimated:
Expected PaCO₂ = 0.7 × increase in HCO₃⁻ + 40 ± 5
Increase in bicarbonate:
38 − 24 = 14 mmol/L
Expected PaCO₂:
0.7 × 14 + 40
= 49.8 mmHg
Expected range:
Approximately 45–55 mmHg
Measured PaCO₂:
48 mmHg
Therefore, respiratory compensation is appropriate.
The final interpretation is:
Metabolic alkalosis with appropriate respiratory compensation
Mechanism of metabolic alkalosis in pyloric stenosis
Repeated vomiting causes loss of gastric hydrochloric acid:
Loss of hydrogen ions (H⁺)
↓
Metabolic alkalosis
Loss of chloride ions (Cl⁻)
↓
Hypochloremia
Volume depletion activates the renin–angiotensin–aldosterone system:
↓
Increased renal sodium retention
↓
Increased potassium and hydrogen ion excretion
↓
Hypokalemia and persistence of alkalosis
Why the other options are incorrect
B. Metabolic alkalosis with additional respiratory acidosis
Incorrect.
The elevated PaCO₂ may suggest respiratory acidosis, but it is actually appropriate compensation for metabolic alkalosis. A respiratory acidosis would have a PaCO₂ higher than the expected compensatory range.
C. Respiratory alkalosis with metabolic compensation
Incorrect.
Respiratory alkalosis requires a reduced PaCO₂. This patient has an elevated PaCO₂.
D. Normal anion gap metabolic acidosis with respiratory compensation
Incorrect.
Metabolic acidosis would produce a low bicarbonate level. This patient has significant bicarbonate elevation.
E. Mixed metabolic alkalosis and metabolic acidosis
Incorrect.
There is no evidence of an additional metabolic acidosis. The bicarbonate level and clinical picture are consistent with pure metabolic alkalosis.
CPSP Pearls
- Hypertrophic pyloric stenosis classically causes hypochloremic, hypokalemic metabolic alkalosis.
- Gastric vomiting causes loss of hydrogen ions and chloride ions.
- Volume depletion activates aldosterone, causing renal potassium and hydrogen ion loss, maintaining alkalosis.
- Before pyloromyotomy, correction of dehydration and electrolyte abnormalities is essential.
- In acid–base questions, always differentiate:
- Primary disorder
- Expected compensation
- Evidence of a mixed disorder
