If an arterial blood gas sample collected in a heparinized syringe is left exposed to room air, which change is most likely?

• A. Increase in pH
• B. Decrease in pH
• C. No Change in pH
• D. Increase in PaO2

Answer: (A)

When an arterial blood gas sample is left exposed to room air, gases can diffuse between the blood and the air. Carbon dioxide diffuses out of the blood into the air more readily than oxygen diffuses in. As CO2 leaves the sample, the dissolved CO2 concentration and carbonic acid in the blood drop, shifting the bicarbonate buffering system toward fewer H+ ions. This raises the pH, producing an apparent alkalemia. While some oxygen may diffuse in and PaO2 can rise, the most consistent and clinically recognized change from room-air exposure is an increase in pH due to CO2 loss.