Thiopental pharmacodynamics. I. Defining the pseudo-steady-state serum concentration-EEG effect relationship.

To assess depth of anesthesia for intravenous anesthetics using clinical stimuli and observed responses, it is necessary to achieve constant serum concentrations of drug that result in constant biophase or central nervous system concentrations. The goal of this investigation was to use a computer-controlled infusion pump (CCIP) to obtain constant serum thiopental concentrations and use the electroencephalogram (EEG) as a measure of thiopental's central nervous system drug effect. The number of waves per second obtained from aperiodic waveform analysis was used as the EEG measure. A CCIP was used in six male volunteers to attain rapidly and then maintain for 6-min time periods the following pseudo-steady-state constant serum thiopental target concentrations: 10, 20, 30, and 40 micrograms/ml. The median performance error (bias) of the CCIP using 149 measurements of thiopental serum concentrations in six subjects was +5%, and the median absolute performance error (accuracy) was 16%. Following the step change in serum thiopental concentration, the EEG number of waves per second stabilized within 2-3 min and the remained constant until the target serum thiopental concentration was changed. When the constant serum thiopental concentration was plotted against the number of waves per second for each subject, a biphasic serum concentration versus EEG effect relationship was seen. This biphasic concentration:response relationship was characterized with a nonparametric pharmacodynamic model. The awake, baseline EEG was 10.6 waves/s; at peak activation the EEG was 19.1 waves/s and occurred at a serum thiopental concentration of 13.3 micrograms/ml. At a serum thiopental concentration of 31.2 micrograms/ml the EEG had slowed to 10.6 waves/s (back to baseline) and at 41.2 micrograms/ml was 50% below the baseline, awake value. Zero waves per second occurred at serum thiopental concentrations greater than 50 micrograms/ml. Using a CCIP it is possible to establish constant serum thiopental concentration rapidly and characterize the concentration versus EEG drug effect relationship.