A 19-year-old woman is brought to the emergency department 8 hours after ingesting a large quantity of aspirin tablets. Arterial blood gas shows pH 7.38, PaCO2 28 mmHg, bicarbonate 16 mEq/L. The primary mechanism by which salicylates produce these early findings is:

• A Central suppression of chemoreceptor sensitivity to carbon dioxide
• B Inhibition of renal carbonic anhydrase causing bicarbonate loss
• C Uncoupling of oxidative phosphorylation in skeletal muscle
• D Direct stimulation of the medullary respiratory centre causing hyperventilation ✓

Explanation

Salicylates directly stimulate the medullary respiratory centre, producing hyperventilation with a fall in PaCO2 and a respiratory alkalosis as the earliest acid-base disturbance. Uncoupling of oxidative phosphorylation contributes later, generating lactic acidosis and the anion gap metabolic acidosis seen at higher levels or later stages. Carbonic anhydrase inhibition is a property of acetazolamide, not salicylates. The mixed picture here reflects early uncompensated respiratory alkalosis beginning to shift toward metabolic acidosis.

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

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Written and medically reviewed by the StethoPrep medical team.