A new method for estimating spinal cord function: Refractory period of conductive spinal cord evoked potentials

Study Design. This study was designed to examine the possibility of a new spinal cord monitoring method using measurement of the refractory period to monitor spinal cord function. Objectives. To determine whether measuring the refractory period and the recovery rate of conductive spinal cord evoked potential is a useful method for estimating spinal cord function. Background. Measuring the refractory period and constructing the recovery curve have been used to investigate peripheral nerve function. Spinal cord evoked potential elicited by the single stimulus usually is used to evaluate spinal cord function, and it has been said that 50% attenuation of the amplitude is the critical alarm level. Methods. In anesthetized cats, amplitude, area, and latency were measured on a personal computer from subtracted data collected with a paired-stimulation technique. The authors constructed recovery curves of ascending and descending conductive spinal cord evoked potentials and measured the refractory period during spinal cord compression. Results. When the amplitude of the ascending spinal cord evoked potential began to decrease during spinal cord compression, the amplitude of the response elicited by the second stimulus with interstimulus intervals of 0.8 msec and 1.0 msec decreased more significantly. When the amplitude of the ascending spinal cord evoked potential decreased to 50% of the precompression amplitude, the mean value of the absolute refractory periods of the ascending and descending spinal cord evoked potentials became prolonged from 0.40 ± 0.007 msec to 0.53 ± 0.014 msec, and the mean values of their amplitude and area recovery rates decreased from 75% ± 1% to 35% ± 2% (interstimulus interval, 0.8 msec) and from 81% ± 1% to 46% ± 2% (insterstimulus interval, 1.0 msec). Conclusions. The change of the responses elicited by the paired stimuli is more sensitive than those elicited by the single stimulus in the spinal cord evoked potentials. The absolute refractory periods and the recovery rate during 50% attenuation of the precompression amplitude is the critical alarm level in spinal cord monitoring.