A 19-year-old girl who took an unknown number of tablets presents two hours later with nausea, tinnitus, vertigo, sweating, and deep rapid breathing. Arterial blood gas shows pH 7.50, PaCO2 28 mmHg, bicarbonate 21 mEq/L. Which pattern best describes this stage of her intoxication?

• A Respiratory alkalosis from direct medullary stimulation ✓
• B Uncompensated metabolic acidosis from lactate accumulation
• C Mixed respiratory alkalosis and metabolic acidosis
• D Pure respiratory acidosis from central depression

Explanation

Early salicylate poisoning directly stimulates the respiratory center, producing hyperventilation with respiratory alkalosis, accompanied by salicylism symptoms such as tinnitus and vertigo. Later stages show a mixed picture as accumulating organic acids and uncoupling of oxidative phosphorylation produce high anion gap metabolic acidosis superimposed on respiratory alkalosis. Pure respiratory acidosis occurs only in terminal central depression. The early isolated respiratory alkalosis is the classic exam finding.

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

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