Differential frequency effects of strong nonpainful transcutaneous electrical nerve stimulation on experimentally induced ischemic pain in healthy human participants

INTRODUCTION: Electrophysiological studies show frequency-dependent effects of transcutaneous electrical nerve stimulation (TENS) in animal models of hyperalgesia. Evidence of frequency-dependent effects of TENS in humans is conflicting. Objective: To assess the effects of low-frequency and high-frequency TENS at a strong nonpainful intensity on experimentally induced ischemic pain. Methods: Submaximal effort tourniquet tests were carried out on 48 healthy human participants before (baseline) and during TENS at 3 pulsed currents per second (pps), 80 pps, and no current (placebo). TENS was switched on for 20 minutes and a submaximal effort tourniquet test was carried out during the final 5 minutes of the intervention. There was a 30-minute washout, with TENS switched off, between the interventions. Results: Repeated measure analysis of variance detected significant effects for pain intensity [100 mm Visual Analog Scale (VAS)] for condition (P<0.001), time (P<0.001), and time×condition (P=0.039). When compared with pre-TENS lower VAS scores were detected for placebo TENS (P=0.026) and 80 pps (P<0.001), but not for 3 pps (P=0.19). There were lower VAS scores for 80 pps than placebo (mean difference, 13.29 mm; 95% CI, 9.71, 16.87; P<0.001) and 3 pps (mean difference, 19.88 mm; 95% CI, 17.20-22.55; P<0.001), yet 3 pps scores were higher than placebo (mean difference, 6.58 mm; 95% CI 3.45, 9.72; P<0.001). There were significantly lower scores for sensory dimensions of the short-form McGill Pain Questionnaire for both 3 pps and 80 pps when compared with the placebo (P<0.001; P=0.005, respectively), but no significant differences between TENS at 80 and 3 pps (P=1.0). There were no significant effects detected for condition (P=0.217) or for condition×sequence interaction (P=0.800) for affective dimensions. Conclusions: Strong nonpainful TENS delivered at 80 pps reduced experimentally induced ischemic pain when compared with TENS delivered at 3 pps.