Figure 1. An overview of the study design. The computer-controlled infusion pump maintained each target concentration for a minimum of 40 min. Target levels were 7, 20, and 60 micro gram/ml (A); 10, 40, and 60 micro gram/ml (B); 14, 40, and 100 micro gram/ml (C); or 24, 50, and 60 micro gram/ml (D). At each occasion, the nociceptive stimuli were repeated three times with 1-3-min intervals. Intubation was performed only once.