A large conditioned pain modulation response is not related to a large blood pressure response: A study in healthy men

BACKGROUND: Endogenous pain modulation has been studied with the conditioned pain modulation (CPM) paradigm with large differences in the magnitude of the CPM effect. We hypothesized that differences in CPM effects might be associated with differences in blood pressure responses to the conditioning stimulus when comparing the CPM effects using two different conditioning stimuli. METHODS: A single-blind repeated-measures design with block-randomization was applied on 25 healthy male subjects. The test stimulus (TS; tonic heat pain for 120 s) was first presented alone, thereafter in parallel with a conditioning stimulus (CS). Conditioning stimuli were either a cold pressor test (CPT) or equally painful ischaemic muscle pain (ISC), both lasting 120 s. Finger blood pressure and heart rate were recorded continuously. Data were analysed in a linear mixed model framework with CS type (CPT or ISC) and conditioning (TS or TS + CS) as independent factors. RESULTS: An inhibitory CPM effect was found for both types of conditioning (p < 0.001). The CPM effect was larger during CPT conditioning compared with ISC conditioning (p = 0.001). No association with the concomitant cardiovascular response (blood pressure and heart rate) was found (p > 0.34). CONCLUSION: Cold pressor pain CS induces larger CPM effects than ischaemic pain CS. The larger CPM effect is, however, not associated with a larger blood pressure response. Other factors related to the CS should be investigated to understand why different CS modalities give different CPM effects.