Cerebral response to haemodilution during cardiopulmonary bypass in dogs: the role of nitric oxide synthase

During cardiopulmonary bypass, haemodilution is standard practice and isaccompanied by increases in cerebral blood flow (CBF). We investigated ifchanges in cerebral vascular resistance (CVR) during cardiopulmonarybypass-haemodilution are dependent on nitric oxide synthase. The cerebralresponse to haemodilution in nine dogs treated with the nitric oxidesynthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), wascompared with a control group (n = 8). Both groups underwent serialisovolaemic haemodilution (target packed cell volumes 0.39, 0.26, 0.19 and0.14) using 6% dextran 70. CBF, CVR and cerebral metabolic rate for oxygen(CMRO2) were measured. While initial CVR was different in the two groups,haemodilution-dependent reductions in CVR were equivalent and the curvesdescribing the packed cell volume- CVR relationship were parallel incontrol and nitric oxide synthase inhibition groups. Our data indicate thatnitric oxide synthase does not play a primary role in the cerebral responseto haemodilution.