Measurement of systemic carbon dioxide production during cardiopulmonary bypass: a comparison of Fick’s principle with oxygentor exhaust output

Theoretically, systemic carbon dioxide (VCO 2 ) production should be an alternative means to systemic oxygen uptake (VO 2 ) for estimating the global efficacy of cardiopulmonary bypass (CPB). This study compared two methods of estimating VCO 2 : Fick’s principle and oxygenator exhaust carbon dioxide (CO 2 ) output. Both of these estimates were then compared with VO 2 . Fifty-one patients (39 male and 12 female) undergoing elective cardiac surgery requiring CPB were studied. Blood sampling was performed and measurements recorded during active cooling, environmental cooling/stable hypothermia and during rewarming. Blood samples were measured for CO 2 tension from which content was estimated. VCO 2 was calculated as the product of the arteriovenous difference in CO 2 content and pump flow rate (Fick’s principle), or the fresh gas flow rate and concentration of the oxygenator exhaust CO 2 (output technique). Over all measurements, method comparison analysis revealed a large mean bias of 41 (95% confidence intervals (CI) 32-50) mL/min with very wide limits of agreement (-23, 105 mL/min). Regression analysis found that the bias was also proportional to the size of measurement (β=0.75 (95% CI 0.55, 0.95)). Although both methods of VCO 2 correlated significantly with VO 2 ( p<0.01), regression analysis found that the coefficients (β) of both techniques had wide CI (Fick’s principle: β=1.37 (95% CI 1.20, 1.54); output technique: β=0.58 (95%CI 0.44, 0.71)). In conclusion, both techniques of VCO 2 cannot be used interchangeably, and both are imprecisely related to VO 2 as estimated by Fick’s principle.