Effects of anesthesia and stimulus intensity on posterior tibial nerve somatosensory evoked potentials

Under anesthesia peak latencies occurring up to 75 milliseconds after stimulus onset upon somatosensory evoked potential testing of the somatosensory evoked potential testing of the posterior tibial nerve were not affected by stimulus intensity (between 5 and 19 ma) or by length of time under isoflurane and nitrous oxide up to over 2 hours. When pre- and postoperative tests on patients who were not under anesthesia were compared with results under anesthesia, no significant latency differences were found in relation to stimulus intensity for peaks N30, P40 and N50. For peaks P60 and N75, however, significantly increased latencies were seen during anesthesia, more pronounced and consistent for N75. Amplitudes, however, were affected by both stimulus intensity and anesthesia duration. A curvilinear relationship was found during early anesthesia. Maximum amplitudes were found at 7 or 11 ma stimulus intensity levels, depending upon which peak was analyzed, with lesser amplitudes occurring at both lower and higher stimulus intensity levels. Stimulus intensity and anesthesia interacted such that maximum amplitude occurred, in general, at 11 ma after short duration anesthesia (6') and at 7 ma after long duration anesthesia (125'). Under long duration anesthesia amplitudes were significantly diminished, mostly at the 11 ma intensity level. At 15 and 19 ma intensity levels peak amplitudes remained relatively constant regardless of anesthesia duration and therefore are the intensities to use to monitor changes during prolonged surgeries. When preoperative during prolonged surgeries. When preoperative and postoperative tests were compared to tests under anesthesia, there was a decrease in amplitude under anesthesia, greater for long than short duration anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)