Oxygen Extraction Rate As A Tool To Control The Effect Of Hemodilution During Cardiopulmonary Bypass

摘要

Introduction Despite recent advances in blood conservation techniques, up to 30% to 80% of patients undergoing open heart operations require allogeneic blood transfusions 1. The following methods of reducing allogeneic blood transfusions are well known: autologous blood donation, isovolemic hemodilution, hemodilution at the beginning and during extracorporeal circulation (ECC), hemofiltration and ultrafiltration to increase hemoconcentration, cell saver autotransfusion, reinfusion of blood from drainages, pharmacological influence on hemostasis, use of coated perfusion systems in extracorporeal perfusion, use of centrifugal pumps in extracorporeal perfusion, etc.A prospective, randomized study was performed to test the effect of withdrawing blood during ECC by the perfusionist while controlling the oxygen extraction rate. The aim was to evaluate if a withdrawal of blood (400 ml) is tolerable concerning the O2-extraction rate since a crucial upper limit of 22% - 30% during ECC has been postulated 2. Methods 40 male patients (62–78 years, mean age 69 years) undergoing CABG-procedures were randomized to either withdrawal of 400 ml blood from the cardiotomy reservoir with simultaneous substitution of cristalloid solutions 10 minutes after cross-clamping of the aorta (group I, n= 20) or performance of coronary artery bypass with standard ECC technique (group II, control group, n = 20). In both groups the oxygen extraction rate was measured.In group I 400 ml of blood were temporarely stored in biopack-bags (Biotrans(r), Holland) during the ECC – with simultaneous substitution of cristalloid solutions – and controlled retransfusion during the rewarming or the post-bypass period was performed if the extraction rate increased up to more than 25%.In the control group (group II) CABG surgery was performed without hemodilution using standard ECC technique.The oxygen extraction rate was calculated with the following formula 3 using a conventional software program (Excel(r), Microsoft(r), USA):(1,39 x Hb) x art. saturation + (PaO2 x 0,003) = ml O2 / 100 ml blood = CaO2(1,39 x Hb) x ven. saturation + (PvO2 x 0,003) = ml O2 / 100 ml blood = CvO2CaO2 x CO x 10 = DO2CaO2 - CvO2 = avDO2 (O2 / 100ml blood)avDO2 x CO x 10 = VO2VO2 / DO2 = oxygen extraction rate in %Abbreviations:PaO2 = arterial O2 pressure (mm Hg) CaO2 = arterial oxygen concentrationPvO2 = venous O2 pressure (mm Hg) CvO2 = venous oxygen concentrationDO2 = capacity of oxygen transport CO = cardiac outputavDO2 = arterio-venous difference in oxygen concentrationVO2 = oxygen consumption / minPoints of measurement in both groups:Point I: at 30°C blood temperature (at cross-clamping of the aorta)Point II: at 30°C blood temperature (10 min after withdrawal of 400 ml blood from the cardiotomy reservoirPoint III: regaining 37°C blood temperature and after the end of the cross clamping period Statistics The hematocrit and the oxygen extraction rate were calculated by analysis of variance (ANOVA) with repeated measurements. In each group the oxygen extraction at different points of measurement was calculated with the student T-test. Results A significant difference (p<0,05) was seen regarding the oxygen extraction rate at point II between both groups. (Fig. 1, 2). In group I a significant difference (p: 0,02) was evident between points I and II (Fig. 1). No significant difference was seen regarding the hematocrit between any points in any of the two groups (Fig. 1, 2).In 15% of the patients (n = 3) the withdrawn blood had to be retransfused to the cardiotomy reservoir due to extraction rate values more than 25%. In 85% the stored biopack bags could be handed over for further use.