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

A microprocessor-based control system for maintaining a constant level of experimental muscle pain was developed. 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. 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