Effect of varying frequency, intensity, and pulse duration of transcutaneous electrical nerve stimulation on primary hyperalgesia in inflamed rats

Objectives: To examine the effect of varying frequency, intensity, and pulse duration of transcutaneous electrical nerve stimulation (TENS) on primary hyperalgesia (increased response to a noxious stimuli) to heat and mechanical stimuli induced by carrageenan paw inflammation in rats. Design: Inflammation was induced by injection of 3% carrageenan into the hindpaw. Two frequencies (high, 100Hz; low, 4Hz), 2 intensities (high, motor; low, sensory), and 2 pulse durations (100 mu sec, 250 mu sec) were applied for 20 minutes to the inflamed paw. The paw withdrawal latency (PWL) to radiant heat, threshold to mechanical stimuli, and spontaneous pain-related behaviors were measured before and 4 hours after induction of inflammation, after TENS, and at 8, 12, and 24 hours after inflammation. A 3-factor (frequency, intensity, pulse duration) repeated-measures (time) design was used to analyze the changes in PWL. Mechanical threshold and spontaneous pain-related behaviors were compared for frequency, intensity, and pulse duration with a Kruskal-Wallis analysis of variance. Results: For changes in PWL to heat, there was an effect for time (p = .0001) and frequency (p = .0001), but not for intensity (p = .45) or pulse duration (p = .21). For changes in mechanical threshold, there was also an effect for frequency (p = .007), but not for intensity (p = .055) or pulse duration (p = .058), after treatment with TENS. High-frequency TENS significantly reduced the primary hyperalgesia to heat and mechanical stimuli when compared with controls receiving no TENS or treatment with low-frequency TENS. High-frequency motor TENS also reduced spontaneous pain-related behaviors for 1 day after treatment. Conclusion: High-frequency TENS reduces primary hyperalgesia to heat and mechanical stimuli for up to 1 day after treatment. In contrast, low-frequency TENS is ineffective in reducing primary hyperalgesia. Varying intensity or pulse duration had no effect on the degree of antihyperalgesia produced by high-frequency TENS.