Somatosensory and Auditory Evoked Responses Recorded Simultaneously: Differential Effects of Nitrous Oxide and Tsoflurane

Abstract

Auditory (AER) and somatosensory evoked responses (SSER) were recorded simultaneously in eight patients under anaesthesia before surgery. We studied the effects of equi-MAC end-expiratory concentrations of isoflurane (0.65-0.75%) and nitrous oxide (60-65%). The anaesthetics were changed at random in three consecutive 10-min periods so that each patient received both drugs. From the AER recorded from the vertex and inion, Pa and Nb latency and amplitude were measured. N13, P20 latency and N13 amplitude were measured from SSER recordings from the neck and P15, N20, P25, N35, P45 latency and P15-N20, N20-P25, P25-N35 and N35-P45 amplitude from the scalp over the hand area of the sensory cortex. Compared with nitrous oxide, isoflurane significantly increased the latencies of the AER waves Pa (P = 0.02) and Nb (P = 0.02), and the SSER waves N20 (P = 0.001) and P25 (P = 0.04). We were unable to demonstrate significant differences in Pa and Nb amplitude between isoflurane and nitrous oxide that we had seen previously. However, the amplitude of the SSER wave N20 was reduced significantly by nitrous oxide compared with isoflurane (P = 0.0004). This wave (N20) is thought to emanate from the thalamo-cortical radiations, and our findings may be explained by an analgesic effect of nitrous oxide mediated by endogenous opioids.

Comparison of the effects of four anaesthetic agents on somatosensory evoked potentials in the rat

“…The effects of medetomidine on SEPs do not appear to have been previously investigated. Isoflurane has consistently been shown to increase the latencies and decrease amplitudes of cortical SEPs (Sebel et al 1986, Thornton et al 1992. These changes are similar to those reported for other volatile anaesthetics (Thornton et al 1992).…”

“…An SEP is the electrical response of the nervous system produced by activation of a peripheral nerve, and consists of a series of peaks or waves, which are characterized by their polarity, latency and amplitude. SEPs can be useful in repeated longitudinal studies in the same animal, but anaesthetic agents can significantly affect their size and timing lAngeI & Gratton 1982, Sloan et al 1990, Thornton et al 1992). In order to carry out repeated periodic studies, it…”

“…The SEP is influenced by the hypnotic properties of an anaesthetic (Thornton et al 1992) and is, therefore, likely to be affected to varying extents by different anaesthetic agents. Ketamine-xylazine and pentobarbitone have been previously compared in the rat and the former was found to have a lesser effect on SEPs (Goss-Sampson & Kriss 1991).…”

Median nerve evoked responses and explicit memory during recovery from isoflurane/nitrous oxide anesthesia

“…18,19 In eight patients Thornton et al identified a subsequent negative (N35) and positive deflection (P45) following N20-P25 complex during anesthesia. 12 They documented statistically different latency changes of N20 and P25 induced by isoflurane compared to nitrous oxide, while the latencies of two subsequent components were affected similarly by the two anesthetic agents. Isoflurane reduced amplitudes of P25-N35 and N35-P45 more profoundly, while the effect of nitrous oxide on the amplitude of the P15-N20 and the N20-P25 response was stronger.…”

A hazard of heated humidifiers

“…Indeed, Thornton rt al. have used MLAEP in combination with somatosensory evoked responses to differentiate the hypnotic and analgesic effects of nitrous oxide [4]. We have embarked on a study recently and our preliminary findings do suggest that clonidine potentiates anaesthetic depth as indicated by MLAEP parameters during isoflurane anaesthesia, in patients undergoing gynaecological procedures.…”