Effect of halothane anesthesia on the human cortical visual evoked response

The visual evoked response (VER) was monitored in eight women intraoperatively during anesthesia with halothane in oxygen administered via endotracheal tube. Control measurements were made prior to anesthetic induction in these unpremedicated patients. The latency of the positive peak designated P1 increased progressively from a mean of 113 +/- 10 (SD) msec in the awake state to the following values at indicated end-tidal halothane concentrations: 123 +/- 10 msec at 0.75 per cent; 130 +/- 9 msec at 0.90 per cent; 134 +/- 9 msec at 1.13 per cent. The differences among mean latency values at these anesthetic concentrations were insufficient to conclude that VER latency would be a useful monitor of anesthetic depth. However, significant effects were seen at clinical levels of halothane anesthesia, and the prolongation caused by anesthesia must be considered when using VERs to monitor central nervous system function during neurosurgery.