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

Step 1: Read the values. pH is 7.5, which is high, so the blood is alkalotic. We must decide whether the primary driver is respiratory or metabolic. Normal ranges: pH 7.35 to 7.45, pCO2 35 to 45 mmHg, HCO3 22 to 26 meq/L.

Step 2: Look at pCO2. It is 30 mmHg, which is low. A low pCO2 raises pH, so a low CO2 fits and explains the alkalosis. Look at HCO3. It is 16 meq/L, which is low. A low HCO3 by itself would lower pH, so the bicarbonate change cannot be the cause of an alkalosis.

Step 3: The change that matches the alkalemia is the low CO2. So the primary problem is a respiratory alkalosis from hyperventilation. The low bicarbonate is the kidney compensating by excreting bicarbonate to pull the pH back toward normal.

Step 4: Confirm with the expected compensation. For acute respiratory alkalosis, HCO3 falls about 2 meq/L for every 10 mmHg fall in pCO2; for chronic, about 4 to 5 meq/L per 10 mmHg. pCO2 fell roughly 10 mmHg below normal, and HCO3 fell well below 24, which fits a chronic respiratory alkalosis with renal compensation.

Step 5: Note on the key. The printed answer was D, metabolic acidosis, but the values do not support that. In metabolic acidosis pH would be LOW, not 7.5. With an alkalemic pH driven by a low pCO2, the correct primary disorder is respiratory alkalosis. The recalled key is corrected to A on medical grounds.

Answer: A. Respiratory alkalosis.