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

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.