Effects of hematocrit on cerebral microcirculation and tissue oxygenation during deep hypothermic bypass

Background-One rationale for hemodilution during hypothermic cardiopulmonar y bypass (CPB) has been improved microcirculation. However, the optimal deg ree of hemodilution remains unclear. We therefore studied cerebral microcir culation and tissue oxygenation in a new intravital microscopic model at 3 different hematocrit (Hct) values. Methods and Results-Three groups of 5 piglets with a cranial window over th e parietal cortex underwent cooling at Hct of 10%, 20%, or 30%, followed by 1-hour deep hypothermic circulatory arrest (DHCA) and rewarming on CPB. Fo r assessment of functional capillary density (FCD), plasma was labeled with fluorescein-isothiocyanate-dextran. Rhodamine-stained leukocytes were obse rved in postcapillary venules with analysis for adhesion and rolling. NADH, a natural intracellular fluorophore that increases during ischemia, was me asured densitometric ally during bypass and DHCA. FCD did not significantly differ from baseline during cooling in any group. However, during early re perfusion (5 minutes) after DHCA. the FCD was significantly higher in the H ct 30% group than in the Het 10% group. Leukocyte adherence decreased in al l groups during CPB and was only moderately increased at the end of the exp eriment. However, severe hemodilution (Hct 10%) was associated with a signi ficantly greater number of rolling leukocytes relative to Het 30%. Conclusions-Higher Hct does not impair cerebral microcirculation and reduce s white cell/endothelial activation after deep hypothermic bypass and circu latory arrest. Severe hemodilution (Hct 10%) results in evidence of inadequ ate cerebral tissue oxygenation during the cooling phase of CPB. This study suggests that Hct of 30% is preferable relative to lower Hct values during hypothermic CPB, particularly if DHCA is used.