Effect of sodium nitroprusside on spinal cord perfusion and paraplegia during aortic cross-clamping

To evaluate the effects of sodium nitroprusside (SNP) on hemodynamics, cerebrospinal fluid dynamics, and neurological outcome after 30 minutes of thoracic aortic occlusion, we monitored proximal and distal blood pressure, cerebrospinal fluid pressure, spinal cord blood flow, and somatosensory evoked potentials. In group 1 (n = 6), no attempts were made to control proximal hypertension, whereas in group 2 (n = 6), proximal blood pressure was controlled with intravenous infusion of SNP. There was no significant difference in proximal or distal blood pressure or cerebrospinal fluid pressure between the two groups at baseline. During the crossclamp interval, the mean proximal aortic pressure rose from 108 ± 21 to 146 ± 14 mm Hg (p < 0.001) in the control group, whereas the mean blood pressure in the SNP group was maintained at 99.8 ± 12 mm Hg (p = not significant compared with baseline blood pressure). Mean distal aortic pressure decreased from systemic values to 23 ± 7 mm Hg in control animals and to 11 ± 5 mm Hg in the SNP group (p < 0.005). In the latter group, cerebrospinal fluid pressure increased significantly from 10.6 ± 1.9 to 20.1 ± 5.5 mm Hg (p < 0.005). In animals receiving SNP, spinal cord blood flow was decreased in the lower spinal cord segments and increased in the upper cord segments. When compared with controls, this difference did not reach significance. The use of SNP was associated with an earlier loss of somatosensory evoked, potentials (3 minutes 32 seconds versus 11 minutes 1 second). All animals in the SNP group had negative spinal cord perfusion pressure (-9.9 ± 6.8 mm Hg) and sustained spastic paraplegia, whereas control animals with positive spinal cord perfusion pressure (+10.3 ± 7.6 mm Hg) were free from injury (p < 0.05). This study indicates that SNP can increase the risk of neurological injury during aortic operations by reducing spinal cord perfusion pressure. Alternative means of controlling proximal hypertension are recommended.