Sodium nitroprusside infusion does not dilate cerebral resistance vessels during hypothermic cardiopulmonary bypass

This study determined whether sodium nitroprusside (SNP) changes cerebral vascular resistance during stable, hypothermic cardiopulmonary bypass (CPB). Cerebral blood flow (CBF) was measured using Xenon clearance in 39 patients anesthetized with fentanyl. In 25 patients (group 1), CBF was measured before and during infusion of SNP at a rate sufficient to reduce mean arterial pressure (MAP) ~ 20%. In 14 other patients (group 2), CBF was measured before and during simultaneous infusion of SNP and phenylephrine; SNP was continued at a rate that had reduced MAP ~20% while phenylephrine was added in a dose sufficient to restore MAP to preinfusion levels. Patients within each group were randomized to maintenance of Pa(CO2) ~ 40 mmHg (groups 1a and 2a), uncorrected for body temperature, or to maintenance of Pa(CO2) ~ 50 mmHg (groups 1b and 2b). The following variables were maintained within a narrow range: nasopharyngeal temperature (26-29° C), pump oxygenator flow (1.7-2.5 1·min^-1·m^-2), Pa(O2) (150-300 mmHg), and Hct (22-28 vol%). In each patient, controlled variables varied no more than ±5% between measurements. In group 1a (Pa(CO2) ~ 40 mmHg), MAP was 86 ± 9 mmHg (mean ± SD) before and 65 ± 8 mmHg during SNP infusion (P < 0.0001). CBF was 12 ±3 ml·100 g^-1·min^-1 before and 10 ± 2 ml·100^-1·min^-1 during SNP infusion (P < 0.01). In group 1b (Pa(CO2) ~ 55 mmHg), MAP was 86 ± 11 mmHg before and 66 ± 13 mmHg during SNP infusion (P < 0.0001). CBF changed from 22 ± 10 to 16 ± 6 ml·100 g^-1·min^-1 (P < 0.05). In group 2a (Pa(CO2) ~ 40 mmHg), MAP was 71 ± 10 mmHg before and 73 ± 9 mmHg during the combined SNP-phenylephrine infusion (P = not significant). CBF was 12 ± 2 ml·100 g^-1·min^-1 before and 10 ± 1 ml·100 g^-1·min^-1 during the combined infusion (P < 0.05). In group 2b (Pa(CO2) ~ 50 mmHg), MAP was 74 ± 8 mmHg before and 71 ± 4 mmHg during the combined infusion (P = ns). CBF was 18 ± 5 ml·100 g^-1·min^-1 before and 15 ± 5 ml·100 g^-1·min^-1 during the combined infusion (P < 0.01). The decrease in CBF was statistically similar in each group and was comparable to that previously reported to occur as a function of the duration of stable hypothermic CPB. Restoration of MAP by phenylephrine during continued SNP infusion did not result in either a relative or absolute CBF increase. During nonpulsatile, hypothermic CPB, SNP does not produce primary cerebral vasodilation in humans anesthetized with fentanyl.