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 puls e duration of transcutaneous electrical nerve stimulation (TENS) on primary hyperalgesia (increased response to a noxious stimuli) to heat and mechani cal stimuli induced by carrageenan paw inflammation in rats. Design: Inflammation was induced by injection of 3% carrageenan into the hi ndpaw. 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 ra diant heat, threshold to mechanical stimuli, and spontaneous pain-related b ehaviors were measured before and 4 hours after induction of inflammation, after TENS, and at 8, 12, and 24 hours after inflammation. A 3-factor (freq uency, intensity, pulse duration) repeated-measures (time) design was used to analyze the changes in PWL. Mechanical threshold and spontaneous pain-re lated 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 = .000 1) and frequency (p = .0001), but not for intensity (p = .45) or pulse dura tion (p = .21). For changes in mechanical threshold, there was also an effe ct for frequency (p = .007), but not for intensity (p = .055) or pulse dura tion (p = .058), after treatment with TENS. High-frequency TENS significant ly reduced the primary hyperalgesia to heat and mechanical stimuli when com pared 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 me chanical stimuli for up to 1 day after treatment. In contrast, low-frequenc y TENS is ineffective in reducing primary hyperalgesia. Varying intensity o r pulse duration had no effect on the degree of antihyperalgesia produced b y high-frequency TENS.