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Saline Hyperchloraemic Acidosis — FRCA Final MCQ

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ModeratePerioperative MedicineSaline Hyperchloraemic AcidosisFRCA Final

A 70-year-old man (ASA III) undergoes a radical cystectomy and ileal conduit formation. The procedure takes 6 hours. Post-operatively in ICU his blood gas shows: pH 7.28, PaCO2 4.8 kPa, PaO2 12 kPa, HCO3⁻ 16 mmol/L, Cl⁻ 118 mmol/L, Na⁺ 140 mmol/L, K⁺ 5.8 mmol/L. Anion gap is 6 (normal). What is the most likely cause of this hyperchloraemic metabolic acidosis?

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Correct answer: B — Respiratory compensation failure

Excessive administration of 0.9% normal saline (NaCl 154 mmol/L of each) during prolonged surgery is the most common cause of hyperchloraemic (normal anion gap) metabolic acidosis in the perioperative setting. The mechanism: the high chloride load exceeds the kidney's ability to excrete it, displacing bicarbonate and causing acidosis (Stewart approach: increased strong ion difference). Balanced crystalloids (Hartmann's, Plasma-Lyte) have a chloride concentration closer to plasma (98-112 mmol/L) and cause significantly less hyperchloraemic acidosis. The SPLIT and SMART trials support balanced crystalloids over saline.

Reference: Semler MW et al – 2018 – SMART trial; NICE – 2017 – CG174 IV fluid therapy