Does electrical stimulation to the hands
(transcutaneous electroacupuncture stimulation, TEAS)
have frequency-specific effects on heart rate variability (HRV)?
David Mayor,1 Tony Steffert,1,2 Deepak Panday,1 Aiste Noreikaite,1 Lidia Zaleczna2
1. University of Hertfordshire; 2. Open University
Heart rate variability (HRV) is increasingly used as a measure of autonomic health (PNS/SNS balance), including in acupuncture research. Our aim here was to ascertain if transcutaneous electroacupuncture stimulation (TEAS) has frequency-specific effects on HRV. With 66 participants attending for four sessions each, to our knowledge this is the largest study published to date that investigates our research question.
Ethics approval was obtained. Following an initial 5-minute baseline recording, TEAS was applied for 20 minutes to each hand between acupuncture point LI4 (hegu) and the ulnar border. In each session, stimulation was at a different frequency – 2.5, 10 or 80 pps (pulses per second) at a ‘strong but comfortable’ amplitude, or as ‘sham’ at 160 pps and zero amplitude. After stimulation, recording was continued for 15 minutes to assess post-stimulation changes (making eight 5-minute recordings for each session). ECG (electrocardiograph) data was collected using a Mitsar-EEG-202 amplifier and down-sampled to 500 Hz. After pre-processing in Matlab and Kubios HRV software, 1988 clean 5-minute recordings were available for analysis in Excel and SPSS using nonparametric methods. Fifty-six different HRV measures were considered as indices of parasympathetic or sympathetic functioning and subgroups of measures selected for further examination according to criteria described in background information available online (see link above).
Significant differences for stimulation frequency were found for many of the measures during successive 5-minute epochs. Most differences were observed during rather than after stimulation, particularly for HRV measures RMSSD, SD2 and DFA alpha1. Other HRV measures showing significant differences for frequency or over time were PNS and SNS indices, absolute LF and HF powers, NNxx and pNNxx, mean RR and multiscale entropy (MSE) at various scales, with other measures – particularly the ratio of LF and HF contributions to the HR spectrum – showing marked changes over time rather than frequency-specific effects.
A finding with potentially useful clinical applications was that stimulation at both 2.5 and 80 pps appeared to increase rather than decrease the stress response, whereas sham and 10 pps did so somewhat less.
Indeed, 10 Hz stimulation reduced the Kubios HRV ‘Stress index’, and changes in several other measures (SNS, HF_abs, RR, pNNxx) suggest that 10 Hz stimulation was experienced as less stressful during and after stimulation than at the other frequencies used.
We had hoped to find definitive differences in the effects of stimulation frequency on HRV, and our new findings are a little disappointing in that we only found a scattering of indications of how the effects of stimulation might vary with frequency (especially 10 Hz), and also – an unwelcome finding – that the overall effect of taking part in the study, whatever the frequency of stimulation, was to stimulate the SNS rather than (or in addition to) the PNS. However, our HRV data only became available a month ago, and further analysis using data already collected is planned to clarify questions raised by the current study, including on the effects of stimulation amplitude, participant preference for stimulation at particular frequencies, measures at baseline, and pulse rate variability (PRV) in addition to HRV.
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