A 19-year-old girl is brought to the emergency department after ingesting an unknown number of aspirin tablets. Arterial blood gas shows pH 7.38, PaCO2 25 mmHg, bicarbonate 16 mEq/L. Which pattern best explains these findings?

• A Pure metabolic acidosis from lactic acid accumulation
• B Pure respiratory alkalosis from direct medullary depression
• C Respiratory acidosis with compensatory metabolic alkalosis
• D Mixed respiratory alkalosis and metabolic acidosis ✓

Explanation

Salicylates directly stimulate the medullary respiratory centre, producing hyperventilation and respiratory alkalosis early on. They also uncouple oxidative phosphorylation, increasing lactate and ketoacid production, giving a superimposed metabolic acidosis. The near-normal pH with very low PaCO2 and reduced bicarbonate reflects both processes together. Pure respiratory alkalosis ignores the falling bicarbonate, and salicylate intoxication classically presents with this mixed picture rather than a single primary disorder.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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