A blood gas sample was sent to the lab on ice, and a bubble was present in the syringe. The blood had been exposed to room air for at least 30 mins. The following change in blood gases will occur:

• A. CO2 content increased/PCO2 decreased
• B. CO2 content and PO2 increased/pH increased
• C. CO2 content and PCO2 decreased/pH decreased
• D. PO2 increased/HCO3 decreased

Answer: (D)

Gas exchange with ambient air after collection alters blood gas values through diffusion and the bicarbonate buffer equilibrium. A bubble provides a reservoir for air, so the blood comes to equilibrium with room air. Oxygen diffuses into the blood, increasing the measured PO2, while carbon dioxide diffuses out into the air, decreasing the PCO2. The loss of CO2 drives the carbonic acid–bicarbonate reaction toward CO2 formation, which lowers the bicarbonate concentration (and raises pH). This combination—higher PO2 with lower bicarbonate—is what would be observed in a sample exposed to room air for an extended period.