[Plasma concentration of fentanyl during and after its administration at constant flow].

Using the technique of the radio-immunological estimation with fentanyl-H3, a study was made in sixteen adults anaesthetised by the administration at a constant rate of alfadione and fentanyl, of plasma concentrations of fentanyl during and after anaesthesia. Anaesthesia was induced by the administration of 4.2ml of alfadione and 0.084mg of fentanyl. The maintenance dose was 0.147 ml/kg/hour of alfadione and 2.95 microgram/hg/hour of fentanyl. Five minutes after induction, the concentration of fentanyl was 2.7 microgram/l. The level fell significantly to 2.56 microgram/l at the 45th minute. From this point onwards, it increased regularly up to the 120th minute, when it reached a level of 3.7 microgram/l. When the infusion was stopped, the level first decreased rapidly, the excretion curve then becoming flattened out. At the 120th minute, a level of 1 microgram/l persisted. This study indicates that the administration of fentanyl at a constant rate is not accompanied by a constant blood concentration up to the 120th minute, the point at which the study was terminated. The residual level found after administration and in the absence of any clinical effect implies the need for a reduction in dose at the time of any complementary administration of fentanyl during the postoperative period.