Question

A patient presents with the following parameters: pH 7.5, pCO2 30 mmHg, pO2 102 mmHg, and HCO3 16 meq/L. Which of the following correctly describes the primary disorder with its compensatory mechanism?

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

Hint:

pH is high, so it is an alkalosis; see which value, CO2 or HCO3, actually raises the pH.

Answer 1

The Correct Option is A

Start by judging the pH. At 7.5 the blood is alkalotic, so the body has either blown off too much acid as CO2 or gained too much bicarbonate. Reference values to keep in mind are pH 7.35 to 7.45, pCO2 35 to 45 mmHg, and HCO3 22 to 26 meq/L.

Now find which parameter explains the high pH. The pCO2 is 30, which is below normal. Since CO2 behaves like an acid, losing it pushes pH up, so a low pCO2 is exactly what an alkalosis needs. The bicarbonate is 16, also below normal, but a low bicarbonate would tend to drop pH, so it cannot be causing this alkalosis. That tells us the respiratory side is primary and the metabolic side is reacting.

So the primary disturbance is respiratory alkalosis, usually from hyperventilation. The kidneys answer by dumping bicarbonate, which is why the HCO3 has fallen to 16, and the expected drop of a few meq per 10 mmHg fall in pCO2 fits a chronic, well-compensated picture.

The recalled key of metabolic acidosis cannot be right, because metabolic acidosis would show a pH below 7.35, not 7.5. With an alkaline pH led by a low pCO2, the diagnosis is respiratory alkalosis. Key corrected accordingly.

$$\boxed{\text{Respiratory alkalosis}}$$