A 3-week-old infant presents with persistent projectile vomiting and a palpable olive-shaped mass in the right upper abdomen. Which of the following electrolyte imbalances would be expected, in order of pH, HCO3, and CO2 levels?

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Answer 1

Correct Answer: pH 7.45, HCO3 26 mEq/L, CO2 45

Rationale:

The infant presents with persistent projectile vomiting and a palpable olive-shaped mass in the right upper abdomen, which is classic for pyloric stenosis. This condition involves obstruction of the gastric outlet due to hypertrophy of the pyloric muscle, leading to severe vomiting.
Vomiting of gastric contents results in the loss of hydrogen ions (H+) and chloride (Cl-) from the stomach, leading to a metabolic alkalosis characterized by increased pH and bicarbonate (HCO3) levels.
In metabolic alkalosis, the body may compensate by retaining carbon dioxide (CO2) through hypoventilation, which helps balance the pH. The given option (pH 7.45, HCO3 26 mEq/L, CO2 45) reflects this compensated state of metabolic alkalosis, where pH is slightly elevated but within the normal range due to compensation.
Electrolyte disturbances commonly seen in pyloric stenosis include hypokalemia and hypochloremia, in addition to metabolic alkalosis. These abnormalities result from the loss of gastric fluids.

Explanation of Other Options:

pH 7.50, HCO3 29 mEq/L, CO2 40

Rationale: This represents an uncompensated metabolic alkalosis, as the pH is significantly elevated, and bicarbonate is high without sufficient compensatory CO2 retention. In pyloric stenosis, some degree of compensation typically occurs, making this option less likely.

pH 7.35, HCO3 22 mEq/L, CO2 45

Rationale: This reflects a normal pH with normal bicarbonate and CO2 levels, which does not indicate any acid-base disturbance. This is inconsistent with the expected metabolic alkalosis seen in pyloric stenosis.

pH 7.30, HCO3 20 mEq/L, CO2 40

Rationale: This represents a metabolic acidosis, as the pH is low, and bicarbonate is decreased. Metabolic acidosis is not consistent with pyloric stenosis, where metabolic alkalosis is the hallmark finding.

Additional Information:

Pyloric stenosis is most common in male infants and typically presents between 2 and 8 weeks of age. It often requires surgical intervention (pyloromyotomy) for definitive treatment.
Before surgery, correction of electrolyte imbalances, particularly hypokalemia and hypochloremia, is crucial to stabilize the infant.
Laboratory findings in pyloric stenosis typically include hypochloremic, hypokalemic metabolic alkalosis. The correct answer reflects this condition in its compensated form.

Conclusion:

The given scenario strongly suggests pyloric stenosis, which is associated with compensated metabolic alkalosis. Among the options, pH 7.45, HCO3 26 mEq/L, CO2 45 is the most accurate representation of the electrolyte imbalance seen in this condition.