Patient-controlled analgesic therapy, Part II: Individual analgesic demand and analgesic plasma concentrations of pethidine in postoperative pain.

20 survival patients were allowed to self-administer small intravenous doses of pethidine to relieve pain after major abdominal surgery. Pethidine injections were given by means of a programmable drug injector. Pethidine consumption varied from 12 to 50mg/h with a mean +/- SD of 26 +/- 10mg/h. The mean measured plasma concentration of pethidine during self-administration was 551 +/- 182 ng/ml, with a range of 132 to 896 ng/ml. Minimum effective concentrations averaged 455 +/- 174 ng/ml. Individual pethidine consumption and mean plasma concentrations did not differ between men and women, and were not correlated to variations in age, anthropometric factors, plasma clearance or elimination rate constant for pethidine. Individual consumption of pethidine was consistent, with stable plasma concentrations throughout most of the trial period. Pseudo-steady-state plasma concentrations of pethidine were established and maintained at widely different levels. 19 of the 20 patients obtained subjectively satisfactory analgesia.