Fig. 1. Alfentanil and placebo analgesic responses assessed by using two distinct pain models (transcutaneous electrical pain:  A –  C ; thermal pain:  D –  F ) and alfentanil- and placebo-induced sedation (  G –  I ). Top graphs (  A ,  D , and  G ) are the mean alfentanil plasma concentrations ± 95 confidence intervals (CIs); middle graphs are the mean alfentanil responses ± 95% CIs (  B : electrical pain;  E : thermal pain;  H : sedation); bottom graphs are the mean placebo responses ± 95% CIs (  C : electrical pain;  F : thermal pain;  I : sedation). Pain tolerance values (electrical pain model) are relative to baseline. C^P= plasma concentration; Num. = numerical; VAS = visual analog scale score.