Which ABG result would indicate respiratory alkalosis in a patient with aspirin overdose?

Which ABG result would indicate respiratory alkalosis in a patient with aspirin overdose?

• A. pH 7.54, PaCO2 41, PaO2 63, SaO2 91, HCO3 36
• B. pH 7.24, PaCO2 37, PaO2 83, SaO2 95, HCO3 18
• C. pH 7.49, PaCO2 30, PaO2 88, SaO2 92, HCO3 25
• D. pH 7.12, PaCO2 28, PaO2 72, SaO2 93, HCO3 10

Answer: (C)

In respiratory alkalosis the primary problem is a drop in carbon dioxide from increased ventilation, which raises the blood pH. In aspirin overdose, the early effect is stimulation of the respiratory center leading to hyperventilation, so you expect an alkalemic pH with a low PaCO2 and a near-normal bicarbonate.

The pattern shown with pH 7.49, PaCO2 30 mmHg, and HCO3- 25 mEq/L fits this: the pH is elevated, PaCO2 is low, and bicarbonate is essentially normal. This is the classic presentation of a primary respiratory alkalosis without significant metabolic compensation, which aligns with the early hyperventilation seen in aspirin overdose.

Patterns that don’t fit include a high pH with a normal or high PaCO2 or a high bicarbonate, which point to metabolic alkalosis rather than respiratory, or an acidemia with low bicarbonate, which reflects metabolic acidosis, possibly with a compensatory respiratory response, not primary respiratory alkalosis.