A 45-year-old man with salicylate poisoning has the following ABG: pH 7.46, PaCO2 22 mmHg, HCO3⁻ 15 mEq/L. Serum anion gap is 20 mEq/L (normal 8–12). What is the CORRECT characterization of this acid-base picture?
- A Mixed respiratory alkalosis and high-anion-gap metabolic acidosis ✓
- B Pure respiratory alkalosis with renal compensation
- C Primary metabolic acidosis with appropriate respiratory compensation
- D Triple acid-base disorder: respiratory alkalosis, metabolic acidosis, and metabolic alkalosis
Explanation
Salicylate poisoning classically produces a mixed disorder. Direct stimulation of the respiratory center causes primary respiratory alkalosis (low PaCO2 22 mmHg, high pH). Simultaneously, salicylate acts as an organic acid and uncouples oxidative phosphorylation, causing high-anion-gap metabolic acidosis (AG = 20, elevated; HCO3⁻ = 15, low). The pH of 7.46 is mildly alkalotic because the respiratory alkalosis dominates, but the simultaneous low HCO3⁻ and elevated AG cannot be explained by renal compensation for respiratory alkalosis alone (which would lower HCO3⁻ by only ~4 mEq per 10 mmHg PaCO2 fall). This is the classic mixed picture of salicylate toxicity. Option D (triple disorder) is not supported without additional evidence.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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