Arterial blood gas of a post-operative patient: pH 7.28, PaCO2 48 mmHg, HCO3 21 mEq/L, Na 138, Cl 115, albumin 2.2 g/dL. What is the PRIMARY acid-base disorder?
- A Simple metabolic acidosis with respiratory compensation
- B Mixed metabolic acidosis and respiratory acidosis
- C Respiratory acidosis with metabolic compensation
- D Hyperchloraemic metabolic acidosis with corrected anion gap normal for hypoalbuminaemia ✓
Explanation
The calculated anion gap = 138 – (115 + 21) = 2, which appears low but must be corrected for hypoalbuminaemia: corrected AG = measured AG + 2.5 × (4.0 – 2.2) = 2 + 4.5 = 6.5 mEq/L (normal). This indicates a non-anion-gap (hyperchloraemic) metabolic acidosis, most likely from large-volume normal saline administration. The slightly elevated PaCO2 (48) represents appropriate respiratory compensation (expected CO2 = 1.5 × HCO3 + 8 ± 2 = 39.5 ± 2, so 48 is slightly above, suggesting a mild concurrent respiratory acidosis). The primary disorder identified by the clinical context and AG correction is hyperchloraemic metabolic acidosis.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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