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

During cardiopulmonary bypass, haemodilution is standard practice and is accompanied by increases in cerebral blood flow (CBF). We investigated if changes in cerebral vascular resistance (CVR) during cardiopulmonary bypass-haemodilution are dependent on nitric oxide synthase. The cerebral response to haemodilution in nine dogs treated with the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), was compared with a control group (n = 8). Both groups underwent serial isovolaemic haemodilution (target packed cell volumes 0.39, 0.26, 0.19 and 0.14) 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 curves describing the packed cell volume-CVR relationship were parallel in control and nitric oxide synthase inhibition groups. Our data indicate that nitric oxide synthase does not play a primary role in the cerebral response to haemodilution.