Thiopental sodium is one of the most commonly used intravenous anesthetics, but there exists no reliable report of BIS that can predict the hypnotic state. Our purpose was to compare BIS values after administering different doses of thiopental sodium.

With institutional review board approval and informed consent, sixty adult patients scheduled for elective surgery were studied. The patients were divided into three groups according to induction doses as follows: group 1: 3 mg/kg, group 2: 4 mg/kg, group 3: 5 mg/kg. After induction the BIS was monitored at 15 second intervals to 90 seconds. The number of patients with a BIS of less than 55 was recorded.

The BIS at baseline and 15 seconds after administration were not statistically different in the three groups. The BIS at 30 seconds in group 1 differed from those of groups 2 and 3, but all of the mean BIS values were more than 55. The BIS values at 45 and 90 seconds were statistically different for three groups, and the mean BIS values of groups 2 and 3 were less than 55. The number of patients with a BIS of less than 55 at any point was eight in group 1, and twenty in groups 2 and 3.

BIS monitoring suggests that a dosage above 4 mg/kg of thiopental sodium is sufficient for the reliable induction of anesthesia.