Estimating the rate of thiopental blood-brain equilibration using pseudo steady state serum concentrations.

The equilibration between drug serum concentration and drug effect under non-steady state concentrations has been classically modeled using an effect compartment where the transfer from the serum to the effect compartment is considered to be a first-order process. The purpose of the present study was to examine whether an effect compartment with first-order transfer was adequate for describing thiopental serum concentration-EEG pharmacodynamics. The study has two facets: (i) Successive pseudo steady state serum concentrations of thiopental having a square wave shape were produced and maintained in six human subjects by means of a computer-driven infusion pump. An aperiodic wave form transformation of the electroencephalogram (EEG) was used as a continuous measure of thiopental EEG drug effect. The time course of the EEG effect following each thiopental serum concentration square wave showed an exponential pattern. The first-order rate constant for equilibration of the effect site concentration with the drug serum concentration (keo) was estimated by fitting a monoexponential model to the effect vs. time data resulting from the pseudo steady state thiopental serum concentration profile. (ii) In a second experiment, data were obtained from a classical design, i.e., a zero-order intravenous infusion of thiopental. The same subjects were studied. The keo was estimated by means of a semiparametric iterative method using convolution (effect compartment, transfer of drug from serum to site of action is assumed to be a first-order process). The mean pseudo steady state value for keo of 0.51 min-1 was not different from the mean value of 0.46 min-1 from the semi parametric approach when data from a linear portion of the drug concentration vs. effect curve were examined. The pseudo steady state technique gave inaccurate estimates of keo in the nonlinear portion of the thiopental concentration vs. response curve, i.e., at the peak of the biphasic concentration-effect relationship.