Which of the following is an abnormal value for arterial CO2?

• A. 40 mmHg
• B. 35 mmHg
• C. 60 mmHg
• D. 47 mmHg

Answer: (D)

Arterial CO2 is normally tightly regulated, with PaCO2 typically around 35–45 mmHg. Values above this range indicate hypercapnia, meaning CO2 isn’t being eliminated effectively and ventilation is inadequate. A value of 47 mmHg sits just above the upper limit, so it is outside the normal range and classified as abnormal, signaling mild hypercapnia. The normal values in the range, such as 40 or 35, fall within the expected window, while a much higher value would point to more pronounced respiratory imbalance.