Changes in the auditory evoked potentials index by induction doses of four different intravenous anesthetics

Many studies have investigated the electroencephalographic changes during the induction and maintenance of anesthesia. However, no comparative studies have been performed on the effects of intravenous anesthetics on the auditory evoked potentials index (AAI). The present study was performed to compare the changes in AAI caused by induction doses of thiopental, propofol, midazolam and ketamine.

Eighty females, aged 30-70 years, referred for mastectomy, had anesthesia induced with thiopental 4 mg/kg, propofol 2 mg/kg, midazolam 0.1 mg/kg or ketamine 1 mg/kg (each 20 patients). The response to verbal command and the AAI were measured every minute for 5 min.

The AAI decreased to less than 40 within 1 min with thiopental and propofol. The AAI increased after 3 min with thiopental, but remained low with propofol. The AAI gradually decreased to less than 40 within 4 min with midazolam, but was higher than the AAI with propofol or thiopental. The AAI increased significantly with ketamine. The AAIs at the loss of verbal command were 19 +/- 7 with thiopental, 21 +/- 8 with propofol, 31 +/- 10 with midazolam and 92 +/- 2 with ketamine.

The AAI correlated with changes in hypnotic level, as measured by the response to verbal command, with induction doses of thiopental, propofol and midazolam, but not with ketamine. The AAI decreased to lower levels with propofol and thiopental than with midazolam at the induction of anesthesia.