Background: Administration of inhaled anesthetics during cardiopulmonary bypass is nowadaysperform without monitorize their exhaled concentrations and CO2 levels are setting in anintermittent way according to blood gas values of CO2, even though there are severalmodifications of fresh gas flows and circulatory flows between blood samples. The purpose ofthis study was to monitorize the gases behavior along the bypass, data reliability and gradientsthrough membrane oxygenator. Methods: We monitorize during cardiopulmonary bypass inhaled and exhaled concentrations ofvolatile anesthetics, CO2 and oxygen in real time from oxygenator inlet and outlet port, describing the effect of machine circulatory flow in that concentrations among differentoxygenators. Seventy patients over 18 years old undergoing cardiac surgery withcardiopulmonary bypass were enrolled. Results: There was a strong positive correlation between end tidal CO2 and blood arterial gasvalues of CO2 (Rho de Spearman 0, 74, p=0, 00). The Isofluorane gradient through theoxygenator showed a statistically significative difference between Affinity, Fusion and Terumooxygenators (p=0, 015). The mean time to equilibrate Isofluorane concentrations was 493, 9seconds ((SD +/- 164, 98sec, CI 95% 454-532sec). When circulatory flow was reduced to 0. 5l/min the measured exhaled concentration values were statistically higher compared to thosebefore the reduction of flow (Fisher T, p=0, 07). We found statistically significant differencesbetween oxygenators in Sevofluorane washout at beginning of bypass (mean: 117. 5sec). Conclusion: The in line monitorization of inhaled and exhaled gases during cardiopulmonarybypass should be mandatory to make opportune adjustments in order to achieve desiredplasmatic concentrations.