An arterial blood gas shows pH 7.28, PaCO2 58 mmHg, HCO3− 24 mEq/L. What is the acid-base disorder?

• A. Acute respiratory acidosis.
• B. Metabolic acidosis.
• C. Acute respiratory alkalosis.
• D. Metabolic alkalosis.

Answer: (A)

When interpreting ABG results, the starting point is the pH: a value below 7.35 indicates acidemia, above 7.45 indicates alkalemia. Here the pH is 7.28, so we’re dealing with acidemia. The next step is to look at the PaCO2 and HCO3− to identify the primary disorder and any compensation. PaCO2 is 58 mmHg, which is elevated, pointing to a respiratory cause of the acidemia. HCO3− is 24 mEq/L, which is normal and suggests there hasn’t been metabolic compensation yet.

This pattern—a low pH with an elevated PaCO2 and a normal bicarbonate—fits acute respiratory acidosis. In acute cases, the kidneys haven’t had time to increase bicarbonate to compensate, so bicarbonate stays near normal. If the issue were metabolic, you’d see a low bicarbonate; if it were respiratory alkalosis, the PaCO2 would be low and pH would be high; if metabolic alkalosis, the bicarbonate would be high with a high pH.