The effects of fentanyl on first and second pain

Twenty people received heat stimuli and rated their intensity before and after an injection of placebo or fentanyl. Subjects rated trains of eight heat pulses each, four before and four after injection. Each pulse began at 41 degrees Celsius and rose to 49 degrees. The conduction processes of A-delta fibers should correspond with pain (first pain) that decreases over the course of a train first pain suppression. On the other hand, the conduction processes of C-fibers should correspond with second pain gradual increases over a course of train (second pain summation). Therefore, the overall rating pattern should consist of a decrease and a final increase. Latency measures confirmed that the A-delta and C-fibers corresponded with first and second pain respectively. Three other patterns of responding were also found. These patterns served as a baseline for assessing the analgesic effects of the narcotic, fentanyl.