A 19-year-old girl who ingested an unknown quantity of enteric-coated aspirin six hours ago presents with tinnitus, nausea, fever and hyperventilation. Arterial blood gas shows pH 7.44, PaCO2 28 mmHg, bicarbonate 19 mEq/L. This early pattern reflects:
- A Pure metabolic acidosis from lactic acid accumulation
- B Respiratory alkalosis from direct medullary stimulation, partially compensated ✓
- C Mixed metabolic and respiratory acidosis indicating severe poisoning
- D Respiratory acidosis from salicylate-induced hypoventilation
Explanation
Salicylates directly stimulate the medullary respiratory centre, causing hyperventilation and a fall in PaCO2, producing respiratory alkalosis as the earliest acid-base disturbance. Renal compensation lowers bicarbonate, giving the mildly reduced value seen here. Only later, with larger doses, does salicylate uncouple oxidative phosphorylation and accumulate organic acids, adding a high anion gap metabolic acidosis, typically in adults with severe poisoning. Hypoventilation is a late, ominous sign signalling impending respiratory failure, not the presenting pattern.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.
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