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Aim: Compare magnitude and test-retest reliability of the CPM effect assessed as changes in reflex responses to an electrical test-stimulus, to changes in the psychophysical response to a thermal test-stimulus. Methods: 24 healthy volunteers participated in two identical experiments separated by minimum 1 week. The electrical test-stimulus was quantified by tracking the threshold of the nociceptive withdrawal reflex elicited by repeated electrical stimulation on the arch of the foot for 120 seconds recorded on tibialis anterior. The thermal test-stimulus was a constant nociceptive heat stimulation of 120 seconds on the subjects' forearm, with continuous ratings of pain intensity on a 0-10 cm visual analogue scale. Conditioning stimulus was a 7°C water bath. Differences in the magnitude and test-retest reliability were investigated with repeated-measures analysis of variance and by relative and absolute reliability indices. Results: The CPM effect was 46% and 4.5% during the thermal and electrical test-stimulus, respectively (p<0.001). Fair relative reliability (intraclass correlation coefficient) was found with the electrical (0.5) and thermal test-stimulus (0.4). Wide limits of agreement indicated poor absolute reliability in both the electrical (-3.4 to 3.8 mA) and the thermal test-stimulus (-3.2 to 3.6 cm). Conclusions: Magnitude of the CPM effect assessed by psychophysical response to a thermal test-stimulus was larger compared to the CPM effect assessed by reflex responses to an electrical test-stimulus. We emphasize that one should be cautious to use the CPM effect as a biomarker or in clinical decision making on an individual level due to large intra-individual variability. [ABSTRACT FROM AUTHOR]

Background: Slowly rising bounded exponential (BE) electrical pulses cause large fiber accommodation and may therefore activate small nociceptive fibers more preferentially than rectangular pulses. The feasibility of using long pulses in pain experiments is however unknown. The aim of this study was to compare the intensity needed to evoke an intense pain response (numerical rating scale, NRS > 4) between BE pulses and standard rectangular pulses (REC). Materials and methods: Electrical stimulation with small diameter pin electrode was applied on the volar forearm of 24 subjects. NRS scores were obtained for 40ms BE and 1ms REC of 11 intensities ranging between 0.1- 20 times perception threshold. Experimental paradigm was imitated by applying pulse trains (10Hz, Is) at each intensity. NRS-score vs. intensity was fitted to sigmoidal curves and the coefficients for the slope, the intensity needed to cause NRS-max/2, and NRS-max were compared between pulse shapes. Results: The results showed an NRS -max (with 95% confidence-intervals) of 4.20 (4.41, 5.42) for BE and 4.52 (4.11, 4.93) REC. The curve slopes were 0.40 (0.29, 0.52) for BE and 1.02 (0.56,1.48) for REC and finally the NRS-max/2 was 6.19 (5.20, 7.19) for BE and 2.44 (1.91, 2.97) for REC. Conclusion: The NRS stimulation intensity relation showed that pain responses were bounded to a similar maximum NRS for BE and REC, indicating saturation of pain perception. The BE pulse, which mainly activates small fibers, is therefore considered feasible for use in pain experiments. Lower intensities were needed to cause NRS-max for REC. [ABSTRACT FROM AUTHOR]