A closed-loop system for maintaining constant experimental muscle pain in man.

Reliable experimental models are needed to help improve our knowledge of how the central nervous system adapts to function in the presence of muscle pain in man. We developed a microprocessor-based control system for maintaining a constant level of experimental muscle pain. Pain was induced in the relaxed right masseter of healthy young adults by using an infusion pump to inject an algesic 0.15 mL bolus of 5% hypertonic saline over 15 s. Subjects supplied feedback on their present pain intensity (PPI) via a 10 cm long electronic visual-analog scale (VAS) and a 0.07 Hz zero-order hold. The adaptive controller identified the system dynamic response and proportional-integral-derivative (PID) controller parameters from the subject's initial response to the bolus (pain rise and fall time constants and peak amplitude) as well as his/her response to a 90 s constant infusion. Finally, using the pain feedback the adaptive PID controller was successfully used to adjust the infusion rate to maintain PPI in five out of seven healthy adults at a mean (SD) 4.8(0.9) PPI level with respect to the 5.0 PPI setpoint for periods up to 15 min (when the experiment was arbitrarily terminated). The infusion rate required to maintain the given level of masseter pain was found to increase by approximately 3 to 5%/minute.