[Pharmacokinetic simulations of remifentanil].

Remifentanil has short half-lives: the values of alpha and beta decay are about 2 and 15 min, respectively. Therefore, the time for remifentanil concentration to reach its steady state concentration (Css) is shorter than those of other anesthetic drugs such as propofol and fentanyl. The Css and the time course of plasma concentration as well as effect-site concentration (Ce) during the continuous infusion of remifentanil could be helpful in our clinical practice. Css is strongly affected by weight and age with Minto's pharmacokinetic parameters of remifentanil. Css increases by a factor of 1.8 when the weight doubles, and Css in a 75-year-old is 1.5 times higher than Css in a 25-year-old. We tried to simulate the time courses of remifentanil Ce and fentanyl Ce during induction, maintenance, and emergence phases of anesthesia. Because of the short half-lives, remifentanil concentration could be well-controlled. However, remifentanil concentration does not reach the Css immediately with constant rate infusion. For example, it will take about 5 min for remifentanil effect-site concentration to decrease from Css to a half of Css. It is considered that anesthetic management with remifentanil is superior to that without it.