A patient with severe vomiting has metabolic alkalosis. Which clinical finding is expected?

• A. Hyperventilation leading to decreased CO2
• B. Decreased HCO3- and pH
• C. Hypoventilation leading to elevated PCO2; elevated HCO3- and pH
• D. Metabolic acidosis

Answer: (C)

Metabolic alkalosis from severe vomiting happens because the stomach loses a lot of hydrogen ions (and chloride), so bicarbonate accumulates in the blood, raising both HCO3− and the pH. In response, the respiratory system tries to compensate by reducing ventilation (hypoventilating) to keep more CO2 in the blood. The buildup of CO2 increases carbonic acid and helps bring the pH back toward normal, so you’d expect a higher PCO2 along with elevated HCO3− and a high pH. Hyperventilating would push CO2 down and worsen the alkalosis, and metabolic acidosis would present with low pH and low bicarbonate, which doesn’t fit this scenario.