A 62-year-old patient is mechanically ventilated after a laparotomy. Arterial blood gas shows: pH 7.28, PaCO2 52 mmHg, PaO2 88 mmHg, HCO3 24 mEq/L on FiO2 0.5. What is the PRIMARY acid-base disturbance?

• A Metabolic acidosis with respiratory compensation
• B Mixed metabolic and respiratory acidosis
• C Acute respiratory acidosis without metabolic compensation ✓
• D Respiratory alkalosis with metabolic compensation

Explanation

The pH is low (acidaemia) and the PaCO2 is elevated (52 mmHg), indicating respiratory acidosis. The bicarbonate is 24 mEq/L — within the normal range (22–26) — confirming there has been no renal metabolic compensation, which would take 3–5 days to manifest as raised bicarbonate. This is therefore an acute respiratory acidosis. In acute respiratory acidosis, pH falls by approximately 0.08 for every 10 mmHg rise in PaCO2 from 40; here PaCO2 has risen 12 mmHg, predicting pH ≈ 7.40 − 0.096 ≈ 7.30, which matches the measured 7.28. The management is to increase minute ventilation.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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