Effect of age, gender and anaesthetic technique on the pharmacodynamics of atracurium.

We have measured in 38 patients the plasma concentration profile of atracurium and its effect on the electromyographic first response of the train-of-four. One of three techniques was used to supplement anaesthesia with 66% nitrous oxide in oxygen, 0.9% isoflurane (end-tidal), 0.5% halothane (end-tidal) or midazolam 3-10 mg. A four-parameter threshold pharmacodynamic model was fitted to the data in each patient. Compared with a group of patients anaesthetized with an i.v. technique, the steady-state plasma concentration producing 50% block (Cpss50) was reduced by halothane, and to a greater extent by isoflurane. The rate constant for exit from the effect compartment (k(eo)) correlated negatively with age and was greater in female patients, but unaffected by anaesthetic technique. The values of gamma, the slope of the concentration-response curve, and of the threshold (Cpss theta) were not affected significantly by age, sex or anaesthetic technique.