Which condition is defined by PCO2 above 45 mmHg due to hypoventilation or impaired gas exchange?

Which condition is defined by PCO2 above 45 mmHg due to hypoventilation or impaired gas exchange?

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

Answer: (C)

The key idea is that CO2 level in the blood reflects ventilation. When PCO2 is higher than about 45 mmHg, it means the lungs aren’t removing CO2 effectively—either due to hypoventilation or impaired gas exchange. CO2 combines with water to form carbonic acid, which dissociates to release hydrogen ions, lowering pH. That’s respiratory acidosis: a primary respiratory problem causing increased acidity. In acute cases, the pH falls quickly; in longer-standing cases, the kidneys can compensate by retaining bicarbonate to raise pH toward normal. The other disorders involve non-respiratory changes in bicarbonate or acid load, with CO2 typically not elevated from ventilation problems, or they show a different primary driver of pH change.

The key idea is that CO2 level in the blood reflects ventilation. When PCO2 is higher than about 45 mmHg, it means the lungs aren’t removing CO2 effectively—either due to hypoventilation or impaired gas exchange. CO2 combines with water to form carbonic acid, which dissociates to release hydrogen ions, lowering pH. That’s respiratory acidosis: a primary respiratory problem causing increased acidity. In acute cases, the pH falls quickly; in longer-standing cases, the kidneys can compensate by retaining bicarbonate to raise pH toward normal. The other disorders involve non-respiratory changes in bicarbonate or acid load, with CO2 typically not elevated from ventilation problems, or they show a different primary driver of pH change.