A 16-year-old girl is brought to the emergency department after ingesting a large quantity of aspirin tablets several hours ago. Arterial blood gas shows pH 7.48, PaCO2 28 mmHg, bicarbonate 22 mEq/L. Two hours later she develops pH 7.25 with bicarbonate 14 mEq/L. The initial primary acid-base disturbance and its mechanism are:

• A Metabolic acidosis from accumulation of lactic acid
• B Metabolic alkalosis from loss of hydrogen ions in urine
• C Respiratory acidosis from depression of the respiratory centre
• D Respiratory alkalosis from direct stimulation of the medullary respiratory centre ✓

Explanation

Salicylates directly stimulate the medullary respiratory centre, causing hyperventilation and respiratory alkalosis, which is the earliest disturbance in salicylate poisoning. Uncoupling of oxidative phosphorylation later produces lactic acid and ketoacid accumulation, giving the subsequent high anion gap metabolic acidosis seen in the second gas sample. Respiratory centre depression occurs only in terminal severe poisoning, not early.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.