Cerebral blood flow does not change following sodium nitroprusside infusion during hypothermic cardiopulmonary bypass

Changes in cerebral blood flow (CBF) associated with decreases in mean arterial pressure (MAP) produced by sodium nitroprusside (SNP) infusion were measured by intra-aortic injection of ¹³³Xe in 17 patients during hypothermic cardiopulmonary bypass (CPB). In each patient, CBF was determined as baseline and then again following SNP-induced reduction of MAP. Two groups were studied. In Group I (n = 9), PaCO₂ was maintained near 42 mm Hg uncorrected for nasopharyngeal temperature (NPT). In Group II (n = 8), PaCO₂ was maintained near 60 mm Hg, uncorrected for NPT. Nasopharyngeal temperature, MAP, pump oxygenator flow, PaO₂, and hematocrit were maintained within a narrow range in each patient during both studies. Since the baseline CBF determinations were conducted at the higher MAP in all subjects, we corrected post-SNP CBF data for the spontaneous decline that occurs over time during CPB. In Group I, a reduction in MAP from 76 ± 9 mm Hg (mean ± SD) to 50 ± 6 mm Hg was associated with a reduction in CBF from 17 ± 5 to 13 ± 3 ml·100 g·min^-1 (P < 0.01), a decrease that became statistically insignificant once the time correction factor had been applied (16 ± 4 ml·100 g^-1·min^-1). In Group II, MAP declined from 75 ± 5 mm Hg to 54 ± 5 mm Hg, and CBF declined from 25 ± 10 to 17 ± 7 ml·100 g·min^-1 (P < 0.01), but, again, after time correction, the CBF decline was statistically insignificant (22 ± 8 ml·100 g^-1·min^-1). We conclude that SNP infusion is associated with a decrease in CBF during hypothermic nonpulsatile CPB but that the decrease is not significant when corrected for the duration of cardiopulmonary bypass.