A 70 kg patient undergoing a 4-hour laparotomy receives 3 L of normal saline. Postoperative ABG shows pH 7.28, PaCO2 38 mmHg, HCO3 18 mEq/L, Na 138 mEq/L, Cl 116 mEq/L. What is the primary acid-base disturbance?

• A Metabolic acidosis with high anion gap due to lactic acidosis
• B Mixed respiratory and metabolic acidosis due to inadequate ventilation
• C Dilutional hyponatraemia causing pseudometabolic acidosis
• D Hyperchloraemic (normal anion gap) metabolic acidosis due to saline loading ✓

Explanation

Large-volume normal saline (154 mEq/L Cl) causes hyperchloraemic metabolic acidosis by diluting bicarbonate and providing excess chloride. Anion gap = 138 − (116 + 18) = 4 mEq/L (normal), confirming a non-gap acidosis. PaCO2 is normal at 38 mmHg, excluding respiratory contribution. This is a well-recognised complication of excessive 0.9% saline; balanced crystalloids (Hartmann's, PlasmaLyte) are preferred to avoid this phenomenon.

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

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