During cardiopulmonary resuscitation of a patient who sustained a cardiac arrest, the anesthesia provider asks a colleague to insert an arterial catheter and draw blood for blood-gas analysis. Shortly thereafter, the results are obtained, and the oxygen tension value is reported as 51 mmHg. The anesthesia provider says to the assistant “You must have drawn a venous sample.” The assistant asserts with confidence that it was an arterial sample. He is right but how can he be so certain? The answer lies in the following analysis.

Consider the blood-gas values in a normal , healthy patient breathing room air, assuming a total partial pressure of one atmosphere or 760 mmHg (Table 18.1).