Heat hyperalgesia in humans: assessed by different stimulus temperature profiles.

The aim of the present study was to investigate the effect of the rate of temperature increase on the intensity of the evoked pain before and after hyperalgesia induced by topical capsaicin. Further, hyperalgesia to suprathreshold heat stimuli was investigated. Thirteen healthy volunteers were included in the experiment. All stimuli were applied in randomised order within the volar surface of both forearms using a computer-controlled contact stimulator. In one of the forearms, the effect of the rate of temperature change was investigated for 1.0, 5.0, and 8.0 degrees C/s reaching a peak temperature of 30.0, 33.0, 36.0, 39.0, and 42.0 degrees C in the primary hyperalgesic area and reaching a peak temperature of 33.0, 36.0, 39.0, 42.0, 45.0, 47.0, and 49.0 degrees C in the secondary hyperalgesic area before and after the induction of hyperalgesia. In the other forearm, the same procedure was repeated without capsaicin application as a control measurement. After the induction of hyperalgesia, the pain ratings were significantly higher in the arm treated with capsaicin compared with baseline for 36, 39, and 42 degrees C heating rates in the primary hyperalgesic area. The pain ratings were significantly higher with 1 degrees C/s heating rate compared with 5 and 8 degrees C/s for 36, 39, and 42 degrees C in the primary hyperalgesic area. Heat hyperalgesia was also observed within the secondary hyperalgesic area to pin-prick for stimulus temperatures of 45, 47, and 49 degrees C compared with the baseline measurements. Increased ratings were found for all three heating rates in the secondary hyperalgesic area. There were no heat hyperalgesia in the control arm. In conclusion, hyperalgesia to suprathreshold heat stimuli was observed in the secondary hyperalgesic area and C-fibres play an important role in the primary hyperalgesia to heat.