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

During cardiopulmonary bypass, haemodilution is standard practice and is ac companied by increases in cerebral blood flow (CBF). We investigated if cha nges in cerebral vascular resistance (CVR) during cardiopulmonary bypass-ha emodilution are dependent on nitric oxide synthase. The cerebral response t o haemodilution in nine dogs treated with the nitric oxide synthase inhibit or, Nw-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, C VR and cerebral metabolic rate for oxygen (CMRo(2)) were measured. While in itial CVR was different in the two groups, haemodilution-dependent reductio ns in CVR were equivalent and the curves describing the packed cell volume- CVR relationship were parallel in control and nitric oxide synthase inhibit ion groups. Our data indicate that nitric oxide synthase does not play a pr imary role in the cerebral response to haemodilution.