Rational: Heat is a well-accepted pain reliever, but questions remain about significant fundamentals such as optimal temperature, time of onset and duration of effect. This study compared the delivery of two levels of thermal energy to a control condition and examined onset, analgesia, and duration.

Objectives: A randomized blinded controlled three arm trial compared the analgesic response to heat delivered via pulses at 4 pulses/minute at 45°C (N=30) versus heat delivered via pulses at 2 pulses/minute at 45°C (N=49) to steady heat at 37°C (N=51) in subjects with longstanding low back pain. Treatment lasted 30 minutes with followup out to four hours. The hypothesis was that the highest energy group (4 pulses/minute) would receive improved analgesia compared to the other groups. Time of onset and duration of effect was also measured.

Findings: Reduction in pain was greater for the both the Initial Group (2 pulses per minute) and the High Energy Group (4 pulses per minute) as compared to the control group (steady heat). The High Energy group (reduced pain for 180 minutes as compared to the Initial Study group that reduced pain for 120 minutes.

Conclusion: High level pulsed heat 45°C at 4 pulses per minute produced significantly longer analgesia as compared to pulsed heat 45°C at 2 pulses per minute, and steady heat at 37°C. Pain relief was rapid, with an onset of analgesia < 5 minutes. The results suggest that there is a dose relationship between thermal energy delivered and duration of analgesia with the upper limit likely defined as the analgesic nociceptive boundary. The results provide some important insights into the analgesic effect of heat in humans.