Direct spinal cord stimulation and recording in hemorrhagic stock

Somatosensory evoked potentials (SSEPs) have been used extensively to evaluate the central nervous system. A limitation of using SSEPs is that the peripheral nervous system is intrinsically linked to the production of these potentials. Peripheral nerve ischemia and neuropathies have been shown to result in changes in SSEPs that may be misinterpreted as showing a pathological condition in the central nervous system. Our experimental paradigm was designed to study the practicality and possible benefits of obtaining spinal evoked potentials (SEPs) by direct cord stimulation and recording. Twelve lambs underwent laminectomies at C-7 and L-1. Epidural electrodes were placed at each level. Each lamb was then bled to produce mean arterial pressures of 80, 60, 40, and less than or equal to 30 mm Hg. At each of these designated pressures, SEPs were obtained by direct cord stimulation and recording. At the same time, spinal cord blood flow was measured using the radioactive microsphere technique. The spinal cord blood flow remained constant down to a mean arterial pressure of 40 mm Hg, whereas blood flow to the peripheral nerve and renal cortex decreased with the mean arterial pressure. SEPs produced by this method did not vary in terms of latency or amplitude over mean arterial pressures ranging from 100 to 30 mm Hg. We conclude that SEPs produced by direct cord recording and stimulation constitute a method of assessing cord viability free of the variables attributable to the peripheral nervous system.