A 19-year-old girl who ingested a large quantity of adult aspirin tablets six hours ago now complains of ringing in her ears and is breathing deeply and rapidly. Arterial blood gas shows pH 7.44, PaCO2 24 mmHg, bicarbonate 17 mEq/L. The earliest acid-base disturbance produced by salicylate toxicity is:
- A Respiratory alkalosis from direct stimulation of the medullary respiratory centre ✓
- B Metabolic acidosis from lactic acid production alone
- C Mixed respiratory and metabolic acidosis
- D Respiratory acidosis with compensatory metabolic alkalosis
Correct answer: A. Respiratory alkalosis from direct stimulation of the medullary respiratory centre
Explanation
Salicylates directly stimulate the medullary respiratory centre, producing hyperventilation and respiratory alkalosis as the initial disturbance, accompanied by tinnitus. Uncoupling of oxidative phosphorylation follows later, generating lactic acid and ketoacids, so adults typically progress to a mixed picture of respiratory alkalosis plus metabolic acidosis, while children decompensate to metabolic acidosis earlier. Pure metabolic acidosis alone and respiratory acidosis do not fit the early presentation described here.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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