Cold or warm start of cardiopulmonary bypass- influence on microcirculatory blood flow

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Abstract
In a randomized study of 30 patients undergoing elective aortocoronary bypass grafting, either cold start of cardiopulmonary bypass (CPB) (prime: room temperature [approximately 20°C], n = 15) or normothermic start of CPB (prime: warmed up to the patients' blood temperature, n = 15) were performed. After warm start, CPB was continued using almost normothermia (lowest nasopharyngeal temperature: 35.8 ± 0.4°C), after cold start hypothermia was used (lowest nasopharyngeal temperature: 28.8 ± 0.2°C). Changes in microcirculatory perfusion were assessed by measuring skin capillary blood flow at the patient's forearm and forehead using laser Doppler technique. Laser Doppler flow (LDF) was continuously monitored before onset of CPB (=baseline values), 30 seconds, one, five, 10, 15 and 20 minutes after start of CPB. Mean arterial blood pressure (MAP) and systemic vascular resistance (SVR) were reduced by CPB in both groups, with the more pronounced reduction in the normothermic patients. Haemoglobin and plasma viscosity were without differences between the groups. The lowest blood temperature in the hypothermic patients was 21.0 ± 0.3°C, and the lowest rectal temperature in these patients was 29.0 ± 0.3°C (20 minutes after start of CPB). Forehead- and forearm-LDF increased significantly in both groups by start of CPB. In the hypothermic patients, this increase was significantly lower, and LDFs were already reduced below baseline values five to 10 minutes after onset of CPB (LDF-forehead -18%, LDF-forearm -72%). In the normothermic patients, LDFs remained elevated during the first 20 minutes after the beginning of bypass (LDF- forehead +38%; LDF-forearm +35%) and were significantly higher than baseline ' even after CPB (hypothermia: LDF-forehead -34%; LDF-forearm -31%). Except for the blood temperature, none of the measured haemodynamic and laboratory values could be related significantly to changes of LDFs (analyses of covariance). It is concluded that skin microcirculatory perfusion as assessed by laser Doppler flowmetry was less altered when using warm start and maintenance of perfusion than after cold start and hypothermic CPB. Whether this improvement in microperfusion after warm start of CPB takes place also in other (more vital) organs has to be elucidated in further studies.