ABG pH 7.51, PO2 90, PCO2 20, HCO3 24. This represents the early stages of a patient with

• A. Carbon monoxide poisoning
• B. Salicylate overdose
• C. Oxygen toxicity
• D. Aspirin overdose

Answer: (D)

Recognizing an early respiratory alkalosis is key here. The ABG shows an alkaline pH (7.51) with a very low PCO2 (20) and a normal HCO3− (24). That combination points to primary respiratory alkalosis, not a metabolic one. In aspirin (salicylate) overdose, the first physiologic effect is stimulation of the medullary respiratory center, which causes tachypnea and blowing off CO2. The result is alkalemia driven by low CO2, while bicarbonate hasn’t yet compensated or changed, so it remains near normal. This pattern is classic for the early stage of salicylate/aspirin toxicity. Over time, salicylates disrupt cellular respiration and lactate production, leading to a metabolic acidosis that can mix with the respiratory alkalosis.

So, this ABG picture fits the early stages of aspirin overdose. It wouldn’t align with carbon monoxide poisoning or oxygen toxicity, which produce different clinical and ABG patterns.