Influence of arterial carbon dioxide tension on systemic vascular resistance in patients undergoing cardiopulmonary bypass

Background: The effects of induced hypothermia in cardiac surgical patients are not yet fully understood. Despite numerous studies on the effects of acid-base management on organ blood now, only little information is available on the effects of alpha-stat versus pH-stat management on systemic haemodynamics. We therefore compared the effect of alpha-stat and pH-stat acid-base management on systemic haemodynamics in a prospective, controlled, cross-over study. Methods: Twenty patients undergoing coronary artery bypass surgery were included in the study. Cardiac output was measured by thermodilution. Cardiac index and systemic vascular resistance were calculated according to standard formulae. Measurements were performed under hypo-and hypercapnia after induction of anaesthesia. Measurements were repeated at the end of two 30-min periods of pH-stat and alpha-stat acid-base management, respectively. Results: Systemic vascular resistance at the lower PaCO2-levels (hypocapnia and alpha-stat, respectively) was significantly higher than those at the higher level (hypercania and pH-stat, respectively). The periods of different PaCO2-levels were comparable with respect to haematocrit, blood viscosity and temperature. Systemic vascular resistance was not significantly different from the control period. Conclusions: This study demonstrates that during hypothermic cardiopulmonary bypass, systemic vascular resistance under alpha-stat acid-base management is higher than under pH-stat management. As obvious from measurements during the control period, this finding can be completely explained by the difference in PaCO2. (C) Acta Anaesthesiologica Scandinavica 42 (1998).