PLASMA FENTANYL CONCENTRATIONS AND CLINICAL OBSERVATIONS DURING AND AFTER OPERATION

Thirty patients undergoing major abdominal gynaecological surgery were anaesthetized with a standard technique including nitrous oxide in oxygen and myoneural blockade. The lungs were ventilated with large tidal volumes (10 ml kg-1) but Paco2, remained within normal limits. Fentanyl was given as a loading dose of 10 μg kg-1 or 25 μg kg-1 i.v. at the start of the anaesthetic. The lower dose was followed by either an infusion of fentanyl 2 μg min-1 or by fentanyl given as increments of 100 μg after 30 min and 50 μg after 1 h. Significant (2% level) second peaks of plasma fentanyl (increases greater than 0.49 ng ml-1) occurred within 45 min of the end of surgery in 60% of patients receiving the infusion, in 40% of patients receiving the high dose and in 20% of patients receiving increments of fentanyl. Analgesia was required in the first 3 h after operation in 70% of patients in the infusion group, in 60% in the high dose group and in 80% in theincrement group. The mean times for administration of postoperative analgesia in these three groups were 102, 80 and 55 min respectively. After operation PaCO2was less than 8 kPa in 60-67% of patients. It is suggested that particular care should be given to patients who are breathing air and recovering from anaesthesia in which fentanyl in large or recurring amounts has been used, since administration of analgesics after operation may produce additive effects and further arterial hypoxaemia. © 1979 Macmillan Journals Ltd.