Epidural-evoked potentials: A more specific indicator of spinal cord ischemia

Abstract

The purpose of this experimental study was to examine the differences between peripheral nerve stimulation and direct spinal stimulation in generating cortical somatosensory-evoked potential (SEP) responses for monitoring spinal cord ischemia during thoracic aorta cross-clamping. Adult mongrel dogs ( n = 6) were placed under general anesthesia and a left thoracotomy was performed. A conventional stimulating electrode was placed over the posterior tibial nerve (PN-SEP), and a special bipolar electrode was placed epidurally over the spinal cord at L1-2 (SC-SEP). The aorta was cross-clamped proximal to the left subclavian artery. Stimulations were alternately performed through both electrodes, and SEP responses were continuously monitored. The cross-clamp was released after one hour and the animal was observed for another hour prior to sacrifice. Excellent SEPs were obtained with six stimuli over 3 sec via the SC-SEP stimulus in contrast to the 200 stimulations over 90 sec required for the PN-SEP stimulus. Aortic cross-clamping resulted in a significantly longer mean time to loss of SEPs for SC-SEP (mean ± SEM, 13.7 ± 1.0 min for SC-SEP vs 11.3 ± 0.7 min for PN-SEP, P < 0.05). Likewise, unclamping of the aorta consistently resulted in a shorter mean time to return of SEPs for SC-SEP compared with PN-SEP. These data indicate that direct epidural stimulation for evoked cortical responses is a more sensitive means of determining the adequacy of posterior spinal cord blood flow as reflected by posterior spinal cord function. Thus, direct stimulation of the spinal cord to produce cortical-evoked potentials may enable the surgeon to differentiate more easily between distal hypotension or factors associated with a preexisting peripheral neuropathy and the inadvertent exclusion of critical intercostals during SEP monitoring for intraoperative spinal cord ischemia.