Effects of mean arterial pressure on haematocrit during cardiopulmonary bypass - a substudy of the PPCI-trial

Abstract

Introduction: Induction of general anaesthesia is known to cause an increase in plasma volume resulting in decrease in haematocrit. Recently, a randomised study showed preservation of haematocrit if mean arterial pressure (MAP) was maintained at baseline level by means of vasoconstrictors (1). Haemodilution is even more pronounced during cardiopulmonary bypass (CPB) and a low haematocrit is associated with ischaemic complications. The purpose of this substudy of the Perfusion Pressure Cerebral Infarct (PPCI) trial (2) was to investigate whether a higher MAP resulted in a higher level of haematocrit during CPB. Method: The PPCI trial randomised 197 (97 versus 98) patients undergoing coronary artery bypass grafting and/or heart valve surgery, in a 1:1 allocation ratio, to either a high MAP (70-80 mmHg) or low MAP (departmental practice) (40- 50 mmHg) during CPB. The CPB pump flow was fixed at 2.4 l/min/m2 plus 10-20 % in both groups. Bolus phenylephrine to a maximum of 2 mg and a titrated infusion of noradrenaline up to 0.4 μg/kg/min was given to obtain target MAP, if necessary. The endpoint of this substudy was the relative change in haematocrit from initiation until the end of CPB defined as first and last haematocrit measured during CPB at full flow, respectively. Secondary outcomes were intraoperative urine output and fluid balance. Patients who required red blood cell (RBC) transfusions during their primary surgery were excluded from the analysis. Results: Median MAP during CPB was 45 (IQR 42-47) mmHg and 68 (IQR 64-70) mmHg in the low and high MAP group, respectively. Complete data were available for 171 patients. Baseline haematocrit levels were comparable in the two groups, but the high MAP group maintained a higher haematocrit level during CPB with a relative change of +8 % versus -3 % in the low MAP group (P<0.001) (Figure 1A). The median urine output was significantly higher in the high MAP group (700 (IQR 400-1100) mL) compared to the low MAP group (500 (IQR (310-800) mL), (P = 0.01) (Figure 1B). Amounts of fluid administered during the first 24 hours were comparable between groups, as were haematocrit levels and fluid balance on the morning after surgery. Discussion: This PPCI trial substudy showed preservation of haematocrit and a higher urine output in the group randomised to a high MAP during surgery. (Figuer Presented).