Somatosensory evoked potential surgical monitoring. Observations during combined isoflurane-nitrous oxide anesthesia

Somatosensory evoked potentials are used to monitor the integrity of the of the spinal cord during surgical procedures where the spinal cord is at risk. Changes in somatosensory evoked potential latencies and amplitudes can indicate impending structural damage. This use of somatosensory evoked potentials is complicated by the effects of nonsurgical factors, including anesthetics. Fifty patients were studied during surgery to determine the effects of coadministered isoflurane and nitrous oxide on cortical and subcortical somatosensory evoked potentials potentials following stimulation of the tibial and median nerves. Analysis revealed statistically significant correlations between expired isoflurane concentrations and prolongation of scalp and cervical latencies, and a negative correlation between nitrous oxide concentrations and scalp amplitudes. Scalp somatosensory evoked potential latencies were also positively correlated with procedure duration, unexplained by other variables. These data emphasize the complexity of interpreting somatosensory evoked potential changes during surgical monitoring, and indicate that separate, defined physiologic effects of an anesthetic can be manifest even when more than one anesthetic is administered.