A tonic heat test stimulus yields a larger and more reliable conditioned pain modulation effect compared to a phasic heat test stimulus

Introduction: The interest in conditioned pain modulation (CPM) as a clinical tool for measuring endogenously induced analgesia is increasing. There is, however, large variation in the CPM methodology, hindering comparison of results across studies. Research comparing different CPM protocols is needed in order to obtain a standardized test paradigm. Objectives: The aim of the study was to assess whether a protocol with phasic heat stimuli as test-stimulus is preferable to a protocol with tonic heat stimulus as test-stimulus. Methods: In this experimental crossover study, we compared 2 CPM protocols with different test-stimulus; one with tonic teststimulus (constant heat stimulus of 120-second duration) and one with phasic test-stimuli (3 heat stimulations of 5 seconds duration separated by 10 seconds). Conditioning stimulus was a 7˚C water bath in parallel with the test-stimulus. Twenty-four healthy volunteers were assessed on 2 occasions with minimum 1 week apart. Differences in the magnitude and test–retest reliability of the CPM effect in the 2 protocols were investigated with repeated-measures analysis of variance and by relative and absolute reliability indices. Results: The protocol with tonic test-stimulus induced a significantly larger CPM effect compared to the protocol with phasic teststimuli (P , 0.001). Fair and good relative reliability was...