New concepts in pharmacokinetics and pharmacodynamics.

Pharmacokinetic and dynamic drug description aim to optimize drug dosing to a desired duration and intensity of effect. In anesthesia practice the therapeutic window is narrow and versatile. Rapid adjustments and rapid reversal of drug effect are mandatory. The simplified traditional pharmacokinetic parameters lack precision to guide dosing and to predict duration of drug effect in routine clinical practice and are therefore restrictive for the popularity of intravenous anesthesia techniques. The context sensitive half-time is the time required for drug concentration to decrease by half of its value after a given drug infusion duration. It is a better comparative predictor for drug concentration decay than the traditional parameters. Compared to opiates, the context sensitive half-times of hypnotics are significantly shorter at any infusion duration. Hysteresis in the concentration-affect relationship is responsible for the lag time between plasma drug concentration and effect. Hysteresis delays initiation of effect as well as recovery from effect. The development of microprocessor aided infusion devices will simplify and ameliorate to a significant extend, the sound application of intravenous drugs in anesthesia.