A 19-year-old presents with tinnitus, fever, and deep rapid breathing after ingesting an unknown quantity of analgesic tablets. ABG: pH 7.46, PaCO2 24 mmHg, HCO3 17 mEq/L. The acid-base disturbance is:

• A Primary respiratory alkalosis only
• B Primary metabolic acidosis with appropriate respiratory compensation
• C Primary metabolic alkalosis with respiratory compensation
• D Mixed respiratory alkalosis and metabolic acidosis ✓

Explanation

Salicylate directly stimulates the medullary respiratory center, producing a primary respiratory alkalosis, while salicylic acid accumulation produces a concurrent metabolic acidosis. Here PaCO2 of 24 is lower than Winter's prediction for HCO3 17 (about 34 to 38 mmHg), proving the hypocapnia is primary, not compensatory. The alkalemic pH with low bicarbonate cannot be explained by either single disorder alone.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.