A patient with acute respiratory failure and rising CO2 has which acid-base disturbance?

• A. Metabolic acidosis
• B. Respiratory alkalosis
• C. Metabolic alkalosis
• D. Respiratory acidosis

Answer: (D)

Rising CO2 in the setting of acute respiratory failure points to respiratory acidosis. When CO2 accumulates, it combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. The extra hydrogen ions lower the pH, causing acidosis. In an acute scenario, the kidneys haven’t had time to compensate by increasing bicarbonate, so the primary disturbance is the elevated CO2 driving acidemia. If this were a chronic issue, you’d expect renal compensation with a higher bicarbonate level reducing the acidosis somewhat. Metabolic disturbances involve changes in bicarbonate independent of CO2, which isn’t the primary driver here.