Measured context-sensitive half-times of remifentanil and alfentanil

The context-sensitive half-time, rather than the terminal elimination half-life, has been proposed as a more clinically relevant measure of decreasing drug concentration after a constant infusion of a given duration. The context-sensitive half-time is derived from computer modelling using known pharmacokinetic parameters. The modelled context-sensitive half-time for a 3-h infusion of alfentanil is 50-55 min and is 3 min for remifentanil. The terminal elimination half-life is 111 min for alfentanil and 12-30 min for remifentanil. It has not been tested whether the modelled context-sensitive half-time reflects the true time for a 50% decrease in drug concentration or drug effect.

Thirty volunteers received a 3-h infusion of remifentanil or alfentanil at equieffective concentrations. Depression of minute ventilation to 7.5% ETCO2 was used as a measure of drug effect. Minute ventilation response was measured, and blood samples for drug concentration were taken during and after drug infusion. The recovery of minute ventilation (drug effect) and decrease in blood drug concentration was plotted, and the time for a 50% change was determined.

The measured pharmacokinetic context-sensitive half-time for remifentanil after a 3-h infusion was 3.2 +/- 0.9 min, and its pharmacodynamic offset was 5.4 +/- 1.8 min. Alfentanil's measured pharmacokinetic context-sensitive half-time was 47.3 +/- 12 min, and its pharmacodynamic offset was 54.0 +/- 48 min. The terminal elimination half-life modelled from the volunteers was 11.8 +/- 5.1 min for remifentanil and 76.5 +/- 12.6 min for alfentanil.

The measured context-sensitive half-times were in close agreement with the context-sensitive half-times previously modelled for these drugs. The results of this study confirm the value of the context-sensitive half-time in describing drug offset compared to the terminal elimination half-life.