["Analgesic" fentanyl blood concentrations during neuroleptanalgesia]

In 72 patients (ASA I-III) undergoing all kinds of elective surgery under routine neuroleptanalgesia, fentanyl plasma concentrations were determined at the moment of diminishing analgesic effect. Minimum analgesic blood concentrations were found to be approximately log-normally distributed (mean 2.89 ng/ml, c.v. 77.56%). There was no significant difference between sex, age or type of surgery. The finding contradict the common opinion that analgesic threshold concentrations exist or that pharmacodynamic effects of the receptor specific drug fentanyl could easily be predicted by blood pharmacokinetics. It is suggested that individual dosing of fentanyl rather than continuous infusion should be preferred in routine neuroleptanalgesia.