Recovery from opioid anesthesia: the clinical implication of context-sensitive half-times

The context-sensitive half-time, the time required for a 50% decrease in drug concentration, has been proposed to predict the speed of recovery after infusions of i.v. anesthetics. We studied 40 patients to compare the clinical recovery characteristics of alfentanil and sufentanil. Patients were randomly allocated to receive either sufentanil/propofol (Group 1) or alfentanil/propofol (Group 2) total i.v. anesthesia by target-controlled infusions (TCI), assuming an equipotency ratio of 500:1. After discontinuation, times to tracheal extubation and to discharge from the postanesthesia care unit were measured, as were drug concentrations up to 24 h. The TCI bias was -17.1% for sufentanil and -16.9% for alfentanil. We found no difference in mean extubation times between the groups (48.7 min in Group 1 versus 46.4 min in Group 2), whereas discharge criteria were fulfilled significantly (P = 0.039) earlier after alfentanil (99.5 min) compared with sufentanil (131.3 min). The relative decrement values to tracheal extubation were 62.1% for sufentanil and 48.0% for alfentanil, compared with 75.7% and 65.0% for discharge, respectively. Based on a difference in propofol requirements, we suggest an actual sufentanil to alfentanil equipotency ratio of 1:300. We conclude that the decay in pharmacodynamic effect is not only the result of pharmacokinetics.

Computer simulations may help to anticipate the clinical behavior of anesthetic drugs. In a clinical setting, we tested whether the recovery characteristics after i.v. anesthesia could be explained by a pharmaco-kinetic value, which describes the decline of drug concentrations in the body. This was not fully achieved.