Which acid-base disturbance is most likely given pH 7.25, PaCO2 55 mmHg, PaO2 70, and HCO3- 25 mEq/L?

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

Answer: (A)

This pattern indicates respiratory acidosis. The key is that the pH is acidic and the PaCO2 is elevated, which means the primary problem is too much CO2 in the blood from impaired ventilation. The bicarbonate is still normal, suggesting little to no metabolic compensation yet (acute onset would show only a small rise in HCO3-). The low PaO2 also fits with reduced ventilation causing poorer oxygen exchange. If the issue were metabolic, the bicarbonate would be abnormal (low in metabolic acidosis or high in metabolic alkalosis); if it were respiratory alkalosis, the PaCO2 would be low, not high.