Changes in the latency of the maximum positive peak of visual evoked potential during anesthesia

The relationship between the latency of visual evoked potential (VEP) and the anesthetic concentration was investigated in surgical patients in order to examine the applicability of VEP in monitoring of the depth of anesthesia. The VEP was recorded with a standard EEG electrode from the midline parietal region in reference to both earlobes linked to the ground. An array of light-emitting-diodes mounted in opaque goggles was used to stimulate both eyes simultaneously and photic stimuli were delivered at random inter-pulse intervals with uniform distribution ranging from 2 to 5 seconds. Fifty trials of data were averaged to estimate that Pmax latency, i.e., the latent period from the photic stimulus to the maximum positive peak arising after 170 msec. Increases in the Pmax latency following the administration of anesthetics and restorations to preanesthetic values after recovery from anesthesia were found. A significant correlation was demonstrated between the Pmax latency and the inspiratory concentration of enflurane. The latency of the Pmax showed a drastic and a sensitive prolongation from about 200 msec in the awake state up to about 600 msec at the stage where the EEG exhibits large-voltage slow waves. Thus the measurement of the Pmax latency of VEPs was found to be useful for monitoring the depth of anesthesia.