Temperature-Related Differences in Mean Expired Pump and Arterial Carbon Dioxide in Patients Undergoing Cardiopulmonary Bypass

Objective: To investigate the relationship between arterial carbon dioxide (PaCO2) and mean expired pump CO2 during cardiopulmonary bypass (PeCPBCO2) in patients undergoing cardiac surgery with CPB during steady state, cooling, and rewarming phases of CPB.Design: Consenting patients, prospective study.Setting: University-affiliated hospital.Participants: Twenty-nine patients.Interventions: Patients aged 22 to 81 years were enrolled. An α-stat acid-base regimen was performed during CPB. The PeCPBCO2 was measured by an infrared multigas analyzer with the sampling line connected to the scavenging port of the oxygenator. Values for PaCPBCO2 from the arterial outflow to the patient and PeCPBCO2 during CPB at various oxygenator arterial temperatures were collected and compared. Data were analyzed by analysis of variance with 1-way repeated measures and post hoc pair-wise Tukey testing when appropriate. The differences between PaCPBCO2 and PeCPBCO2 were linearly regressed against temperature. A p value <0.05 was considered significant.Measurements and Main Results: Three to 5 data sets during CPB were collected from each patient. The mean gradient between PaCPBCO2 and PeCPBCO2 was positive 12.4 ± 10.0 mmHg during the cooling phase and negative 9.3 ± 9.9 mmHg during the rewarming phase, respectively. On regression of the data, the difference between PaCPBCO2 and PeCPBCO2 shows a good correlation with the change in temperature (r2 = 0.79). The arterial CO2 ± x mmHg can be predicted by the formula PaCPBCO2 = (−2.17x + 69.2) + PeCPBCO2, where x is temperature in °C.Conclusions: Monitoring the mean expired CO2 value from the CPB oxygenator exhaust scavenging port with a capnography monitor provides a continuous and noninvasive data source to aid in sweep flow CPB circuit management during CPB.