A 19-year-old girl who ingested a large number of aspirin tablets presents with tinnitus, fever, and hyperventilation. Arterial blood gas shows pH 7.46, PaCO2 25 mmHg, HCO3- 17 mEq/L. What acid-base disturbance is present?
- A Mixed respiratory alkalosis and metabolic acidosis ✓
- B Primary metabolic acidosis with respiratory compensation
- C Primary respiratory alkalosis alone
- D Primary metabolic alkalosis with respiratory compensation
Correct answer: A. Mixed respiratory alkalosis and metabolic acidosis
Explanation
Salicylate directly stimulates the medullary respiratory centre causing hyperventilation and respiratory alkalosis, while salicylate itself impairs oxidative phosphorylation and accumulates as salicylic acid, producing metabolic acidosis. The combination of low PaCO2 with low bicarbonate and alkalemic pH is the classic mixed picture of early salicylate toxicity. Pure respiratory alkalosis would show a normal bicarbonate after applying the acute compensation rule of a 2 mEq/L fall per 10 mmHg PaCO2 fall.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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