A 70 kg patient undergoing a 4-hour laparotomy has received 4 litres of 0.9% saline intraoperatively. Postoperatively, ABG shows pH 7.28, PaCO2 38 mmHg, HCO3 17 mEq/L, Na+ 140, Cl- 116 mEq/L. What is the PRIMARY cause of this acid-base disturbance?

• A Lactic acidosis from intraoperative hypoperfusion
• B Respiratory acidosis from hypoventilation
• C Dilutional hyponatraemia with metabolic acidosis
• D Hyperchloraemic non-anion gap metabolic acidosis from normal saline ✓

Explanation

Large volumes of 0.9% saline (154 mEq/L Cl-) cause hyperchloraemic metabolic acidosis because the excess chloride load suppresses renal bicarbonate reabsorption. The anion gap = 140 - (116 + 17) = 7 mEq/L, which is normal (not elevated), excluding lactic acidosis. PaCO2 is normal, ruling out respiratory acidosis. This is a well-recognised complication of saline-based resuscitation; balanced crystalloids (Ringer's lactate, Plasmalyte) are preferred for large-volume replacement to avoid this.

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

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