A retrospective analysis of CO2 derived goal-directed perfusion variables under normothermic conditions: do we need to re-evaluate threshold values?

Abstract

This study investigated if current predictive values for increased lactate formation: VCO2i > 60 ml min−1 m−2, respiratory quotient (RQ) > 0.90, and DO2/VCO2 < 5.0, are valid under normothermic conditions. CO2 derived parameters were analyzed in 91 patients undergoing normothermic CABG and related to increase of blood lactate concentrations during bypass. In this study population, 85 patients (93%) had a median VCO2i above 60 ml min−1 m−2 and 53 patients (58%) had a DO2/VCO2 ⩽ 5.0. Eighteen patients (20%) had a median RQ ⩾ 0.90, but RQ remained with a maximum value of 0.94 below the biological threshold of 1.0. Increase of lactate concentrations remained without clinical significance and showed weak correlations with VCO2i (rs = 0.277, p = 0.008) and RQ (rs = 0.346, p = 0.001). The cohort was separated for the different CO2 variables by their median value to compare increase in lactate concentration. Patients with a high VCO2i (⩾70 ml min−1 m−2) and a high RQ (⩾0.82) showed significant higher increase in lactate concentration compared to patients with VCO2i < 70 ml min−1 m−2 (p = 0.004), and a RQ < 0.82 (p = 0.012). Groups separated by a median DO2/VCO2 of 4.8 did not show a difference in increase of lactate concentration in blood. In summary, specific CO2 derived threshold values for the prediction of lactate formation, which have been reported in other studies, cannot be confirmed with our findings. However, a CO2 production ⩾70 ml min−1 m−2 and a RQ ⩾ 0.82 in this study population were correlated with increased lactate formation. Because CO2 production during bypass depends on patient temperature, a different cutoff value, that may take into account the influence of demographic variables, should be determined during normothermic CPB.