An analysis of the critical level of oxygen delivery during cardiopulmonary bypass

Abstract

　Introduction : A decrease in oxygen content and/or low blood flow during cardiopulmonary bypass (CPB) can lead to inadequate oxygen delivery (DO₂) and subsequently cause anaerobic metabolism in cells. This deficiency is considered a main cause of hyperlactemia, referred to as type A hyperlactemia. Yet, some reports have indicated that if DO₂ does not decrease below a threshold termed the “critical level”, oxygen consumption (VO₂) remains unchanged. Given that the critical level is well defined and that hyperlactemia during CPB results from DO₂ under this threshold, oxygen supply above the critical DO₂ level can be the target value for CPB management. We conducted a prospective observational study to determine whether the critical DO₂ level correlated with hypoxic blood lactate increase. Method : We studied 52 consecutive patients who underwent cardiac surgery with CPB. Indexed DO₂ (DO₂I), indexed VO₂ (VO₂I), and lactate were measured every 30 min during CPB. To evaluate any correlations between the parameters, univariate and regression analyses were applied. Results: There was a weak correlation between DO₂I and VO₂I, and the critical level of DO₂ was 303 ml/min/m². Blood lactate levels increased with CPB time, but did not correlate with DO₂I.

　Conclusion : We identified a critical level of DO₂ during CPB. However, there was no significant correlation between DO₂I and blood lactate levels.