Objective: During surgical general anesthesia, the EEG often reaches a discontinuous burst-suppression (BS) pattern. The relative time spent in suppression expressed as the BS ratio (BSR) is commonly used for monitoring the level of anesthesia. The aim of this study was to investigate the extent to which somatosensory stimuli influence BSR during anesthesia in a surgical setting.
Methods: We measured the changes in BSR during electrical peripheral nerve stimulation used for intraoperative spinal cord monitoring by somatosensory evoked potentials (SSEP) in 9 patients under propofol anesthesia (up to 140 mcg/kg/min). We recorded EEG from Cp3 and Cp4 during bilateral ulnar nerve stimulation at wrists with 3.17 Hz for 60 seconds.
Results: We included 27 trials with BSR up to 77%. During stimulation, the mean BSR transiently dropped from 42% to 35%. The BS reactivity, measured as the change in BSR, decreased with BSR prior to stimulation. For BSR below 30% the reactivity was negligible. The reactivity was preserved in trials with reduced N19/P22 SSEP amplitudes.
Conclusion: Peripheral nerve stimulation during intraoperative SSEP monitoring can transiently reduce BSR.
Significance: Even non-nociceptive peripheral nerve stimulation can reduce the BSR during anesthesia. The level of somatosensory input should be accounted for when monitoring the anesthesia by BSR.