Optimizing Temporal Summation of Heat Pain Using a Constant Contact Heat Stimulator

Parker A Kell,1 Claudia N Vore,1 Burkhart J Hahn,1,2 Michael F Payne,1,3 Jamie L Rhudy1 1Department of Psychology, The University of Tulsa, Tulsa, OK, USA; 2Department of Psychology, Oklahoma State University, Stillwater, OK, USA; 3Department of Pediatrics, Division of Child & Adolescent Psychiatry, Cincinnati Children’s Hospital Medical Center, Cincinnati, OH, USACorrespondence: Jamie L Rhudy, University of Oklahoma Health Sciences Center, TSET Health Promotion Research Center, Department of Health Promotion Sciences, 4502 E. 41st Street, Tulsa, OK, 74104, USA, Tel +1 918-660-3050, Email jamie-rhudy@ouhsc.eduPurpose: Temporal summation (TS) of pain occurs when pain increases over repeated presentations of identical noxious stimuli. TS paradigms can model central sensitization, a state of hyperexcitability in nociceptive pathways that promotes chronic pain onset and maintenance. Many experimenters use painful heat stimuli to measure TS (TS-heat); yet, TS-heat research faces unresolved challenges, including difficulty evoking summation in up to 30– 50% of participants. Moreover, substantial variability exists between laboratories regarding the methods for evoking and calculating TS-heat.Patients and Methods: To address these limitations, this study sought to identify optimal parameters for evoking TS-heat in healthy participants with a commercially available constant contact heat stimulator, the Medoc TSA-II. Working within constraints of the TSA-II, stimulus trains with varying parameters (eg, stimulus frequency, baseline temp, peak temp, peak duration, testing site) were tested in a sample of 32 healthy, chronic pain-free participants to determine which combination best evoked TS-heat. To determine whether TS scoring method altered results, TS-heat was scored using three common methods.Results: Across all methods, only two trains successfully evoked group-level TS-heat. These trains shared the following parameters: site (palmar hand), baseline and peak temperatures (44°C and 50°C, respectively), and peak duration (0.5 s). Both produced summation that peaked at moderate pain (~50 out of 100 rating).Conclusion: Future TS-heat investigations using constant contact thermodes and fixed protocols may benefit from adopting stimulus parameters that include testing on the palmar hand, using 44°C baseline and 50°C peak temperatures, at ≥ 0.33 Hz stimulus frequency, and peak pulse durations of at least 0.5 seconds.Keywords: temporal summation, heat pain, second pain, wind-up, Medoc TSA-II