Fifty-six subjects participated in this randomized, controlled trial, and no subject withdrew from the trial. There was no difference in age, education, gender, anxiety, and level of belief in acupuncture among the groups (Table 1). There were no differences in the arterial blood pressure or heart rate among the groups before, during, or after EAS.
Because the durations of EAS were different, we only compared the differences in temperature at which volunteers sensed cold pain among groups between 40 and 80 min after the onset of intervention. As illustrated in Figure 3, participants receiving 30 min of EAS had significant hypoalgesia as compared with those receiving 0 min (P = 0.008), 20 min (P = 0.005), and 40 min (P = 0.024). The asynchronous EAS-induced hypoalgesia did not differ significantly between participants receiving 0 min vs 20 min (P = 0.59), 0 min vs 40 min (P = 0.57), and 20 min EAS vs 40 min of EAS (P = 0.69).
At 0 min post-EAS, participants in the 30-min group sensed cold pain at a significantly lower temperature than those who received 0 min (P = 0.01) and 20 min (P = 0.013) but not lower than those who received 40 min of EAS (P = 0.93). However, at 5 min post-EAS, the participants who received 30 minutes of asynchronous EAS reported sensing cold pain at a much lower temperature than those who received 0 min (P = 0.006), 20 min (P = 0.005), and 40 min (P = 0.03). The above phenomenon persisted for 60 min post-EAS (Fig. 3).
Under the conditions of this study, we found that 30 min of asynchronous 2/100 Hz (5 mA) EAS at ST36 and SP6 resulted in significant hypoalgesia compared with 0 min, 20 min, or 40 min. The hypoalgesic effect was sustained for 60 min post-EAS. Similar to previous electrical stimulation studies,3,14 the asynchronous EAS-induced hypoalgesia is not restricted to segmental pain inhibition. As illustrated in Figures 1 and 2, EAS at left ST36 and SP6 (L4, L5, and S2 dermatomal distribution) and the hypoalgesia was tested at the right premarked sites (L3, L4, and S2 dermatomal distribution). A previous manual acupuncture study demonstrated that the peak of hypoalgesia was between 20 and 40 min sustained up to 60 min,15 and the peak hypoalgesia of single frequency EAS also occurred at about 30 min and was sustained up to 75 min during stimulation.4,16 We were not able to demonstrate the similar phenomena as described in these previous studies.4,15,16 In our study, the temperatures at which subjects perceived cold pain after receiving 40 min of asynchronous EAS were significantly higher than the temperatures in those who received 30 min of the same EAS. Future studies should explore the potential mechanism(s) contributing to the development of this phenomenon. Interestingly, the optimal duration for alternating frequencies of EAS as demonstrated in our study is the same as the time setting used in other electrical nerve stimulators that are commercially available to deliver alternating electrical frequency stimulations to the target nerves.5,9 Therefore, we speculate that the analgesia resulting from alternating frequencies of EAS, transcutaneous nerve stimulation, or percutaneous nerve stimulation may share a similar underlying mechanism.
The limitations of this study are as follows: 1) we did not assess the level of hypoalgesia while the stimulation was ongoing, thus we could not establish how the hypoalgesia developed over time, and 2) we did not have an “adequate” duration of CTPT assessment after stimulation. Based on a previous single frequency EAS study, the resulting hypoalgesic effects gradually return to control values within 35 min after termination of stimulation.16 As a result, we decided a priori to assess the CTPTs up to 60 min after EAS was terminated. In addition, the temperatures at which subjects sensed a CTPT were directly recorded into PATHWAY. Thus we did not discover that there was any fading even at 60 min post-EAS until after the study was completed and at the time of data analysis.
In conclusion, the duration of asynchronous 2/100 Hz EAS indeed affects the resultant hypoalgesia, and 30 min of stimulation seems to be the optimal duration.
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*A subjective sensation, described by subjects who received acupuncture treatment, as numbness, aching, soreness, or distension. From the acupuncturist’s perspective, this subjective sensation coincides with the sensation of the needle getting caught during application of acupuncture needle.