Membrane oxygenator exhaust capnography for continuously estimating arterial carbon dioxide tension during cardiopulmonary bypass

Typically, the standard practice for measuring the arterial blood carbon dioxide tension (PaCO2) during cardiopulmonary bypass (CPB) is to take intermittent blood samples for analysis by a bench blood gas analyzer. Continuous inline blood gas monitors are available but are expensive. A potential solution is the capnograph, which was evaluated by determining how accurately the carbon dioxide tension in the oxygenator exhaust gases (PECO2) predicts PaCO2. A standard capnograph monitoring line was attached to the exhaust port of the membrane oxygenator. During CPB, the capnograph reading and arterial blood temperature were recorded at the same time as routine arterial blood gases were taken. One hundred fifty-seven blood samples were collected from 78 patients. A good correlation was found between the PECO2 and the temperature corrected PaCO2 (r2 = 0.833, P < .001). There was also a reasonable degree of agreement between the PECO2 and the temperature corrected PaCO2 during all phases of CPB: accuracy (bias or mean difference between PaCO2 and PECO2) of -1.2 mmHg; precision (95% limits of agreement) of +/- 4.7 mmHg. These results suggest that oxygenator exhaust capnography may be a simple and inexpensive adjunct to the bench blood gas analyzer in continuously estimating PaCO2 of a clinically useful degree of accuracy during CPB.