A 70 kg patient undergoing open abdominal aortic aneurysm repair (estimated 3 hours) receives 4.2 L of normal saline (0.9% NaCl) intraoperatively. Postoperatively his arterial blood gas shows pH 7.28, PaCO₂ 38 mmHg, HCO₃⁻ 17 mEq/L, Cl⁻ 115 mEq/L. The acid-base disorder is:

• A Lactic acidosis from bowel ischaemia
• B Renal tubular acidosis type 1
• C Mixed metabolic and respiratory acidosis
• D Dilutional (hyperchloraemic) non-anion-gap metabolic acidosis ✓

Explanation

Large-volume 0.9% normal saline infusion causes dilutional hyperchloraemic metabolic acidosis. Saline contains 154 mEq/L of both Na⁺ and Cl⁻ — far more chloride than plasma (95–105 mEq/L). The excess chloride suppresses the strong ion difference, lowering bicarbonate. The resulting acidosis is non-anion-gap (hyperchloraemic type), distinguishable from lactic acidosis by a normal anion gap and elevated chloride. This is a well-recognised complication of large-volume normal saline resuscitation; balanced crystalloids (Ringer's lactate, Hartmann's, PlasmaLyte) have lower chloride content and are preferred for large-volume fluid therapy.

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

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Written and medically reviewed by the StethoPrep medical team.