Purpose: This experiment was designed for the determination of the optimal epidural cooling temperature for the allowance of spinal cord protection with minimal side effects during an aortic occlusion-induced spinal cord ischemia model in rabbits.
Methods: Spinal cord ischemia was induced in rabbits with infrarenal aortic occlusion for 40 minutes. Spinal cord cooling was effected with epidural infusion of normal saline solution at the following different temperatures: group 1, 17 degrees C (n = 6); group 2, 24 degrees C (n = 6); group 3, 32 degrees C (n = 6); and group 4, 39 degrees C (n = 3). Sham-operated rabbits without aortic occlusion were used as controls with epidural infusion at healthy body temperature (39 degrees C; n = 3). Motor function was assessed at 48 hours with Tarlov's criteria, and the animals were killed. The spinal cord was sectioned into multiple segments, and semiquantitative histologic scoring (0 to 5) was used to grade ischemic injury.
Results: Cooling solution and spinal cord temperatures showed linear correlation (r = 0.95). All the rabbits in groups 1 (except one with mild weakness), 2, and 3 were neurologically intact, and all in group 4 had paraplegia develop (P < .001). One rabbit in group 1 died from increased intracranial pressure (ICP). Mean blood pressure, ICP, and body temperature were similar among the groups. Histology correlated with the clinical findings. In groups 1 and 2, minimal histologic changes were noted. Low-grade ischemic changes were present in group 3 in the low-lumbar and mid-lumbar segments. Severe ischemic injury occurred at the same segments in group 4 (P < .05).
Conclusion: These study results suggest that in rabbits satisfactory spinal cord protection during aortic occlusion can be achieved at moderate regional hypothermia (24 degrees C). Large volume infusion for the achievement of profound hypothermia may cause deleterious effects of increased ICP and is not warranted.