Hong Kong Physiotherapy Journal
Volume 30, Issue 1 , Pages 29-35, June 2012

Effect of Acu-TENS on post exercise airway resistance in healthy individuals

Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

published online 25 January 2012.

Article Outline

Abstract 

Transcutaneous electrical nerve stimulation on acupoints (Acu-TENS) is associated with increased exercise duration in healthy individuals and improves forced expiratory volume in 1second (FEV1) in those with respiratory illness. Whether a decrease in airway resistance (AR) is responsible for these respiratory system effects has not been investigated. This pilot study investigated the effect of a single session of Acu-TENS on AR in healthy people. Twenty individuals were invited to the laboratory twice, 1 week apart, to receive in random order either Acu-TENS or placebo-TENS (no electrical output from the TENS unit) over bilateral Lièquē (LU 7) and Dìnchuăn (EX-B1), for 45 minutes before and during a submaximal treadmill exercise test following the Bruce protocol. AR, FEV1, forced vital capacity, rate of perceived exertion and heart rate variability were recorded before, immediately after and 15minutes after exercise. Immediately after exercise the percentage decrease in AR from baseline was greater in the Acu-TENS group (−20.10±4.00%) compared to the placebo-TENS group (−7.99±3.43%) (p=0.029). We conclude that the decrease in AR seen with Acu-TENS in healthy individuals could account for the immediate improvement in FEV1. Acu-TENS may have a role in decreasing AR in patients with airflow limitation.

Keywords: acupoints, airway resistance, exercise, TENS

 

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Introduction 

Traditional acupuncture has been used in China for thousands of years. It is believed that stimulation of acupuncture points can regulate the flow of “Qi” (energy) in the body and consequently improve body function [1]. The analgesic effect of acupuncture has also been reported [2], [3]. Acupuncture, however, is invasive, requiring penetration of the skin with needles to achieve its treatment effect, and adverse consequences are well known, including pneumothorax and infection [4], [5].

Transcutaneous electrical nervous stimulation (TENS), is a non-invasive electrical treatment modality widely used for pain relief by physiotherapists. Application of TENS over the acupuncture point Dìnchuăn (EX-B1; termed Acu-TENS), has been shown to increase forced expiratory volume in the first second (FEV1) and decrease the dyspnoeic sensation in not only healthy individuals after exercise [6], but also in patients with chronic obstructive pulmonary disease (COPD) [7]. Although the exact mechanism of effect of Acu-TENS remains speculative, it has been shown that Acu-TENS is associated with an increase in blood β-endorphin levels in patients with COPD [8]. While Acu-TENS is associated with increase in FEV1, the relationship between Acu-TENS and airway resistance (AR) has not been explored.

The aim of this study is to determine the effect of Acu-TENS on AR in healthy individuals after a submaximal exercise test and whether any concurrent modulation of the autonomic nervous system occurs, by analysis of heart rate variability (HRV).

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Methods 

Ethics approval was granted by the Human Ethics Committee of the Hong Kong Polytechnic University prior to data collection. Individuals aged 20–50 years, with normal health and no known cardiovascular, respiratory, neurological or musculoskeletal disease were recruited through convenience (“snowball”) sampling. The study procedures were explained and written consent was obtained from each participant prior to data collection, and they were also required to complete a questionnaire on risk stratification for cardiovascular disease risk factors [American College of Sports Medicine (ACSM) risk stratification questionnaire]. Only individuals identified as the low risk group by the ACSM questionnaire were included in the study.

Experimental procedure 

Participants were invited to visit the laboratory on two occasions, 1 week apart (as a washout period), and randomized to receive either Acu-TENS or placebo-TENS as the first intervention. The order of randomization was generated by a computer program (Random Allocation Software, version 1.0, M. Saghaei, 2004). Individuals were told to refrain from vigorous exercise, coffee and tea for 24 hours before each visit to the laboratory.

Upon arrival at the laboratory, each participant rested in the sitting posture for a minimum of 15 minutes to establish a stable baseline heart rate (HR) which was then recorded using a Polar heart rate monitor (Polar RS800CX, Finland). AR was measured using the MicroRint II MR5000 (Micro Medical Ltd, Basingstoke, UK). Spirometry lung function parameters including forced vital capacity (FVC) and FEV1 were measured by the Pony spirometer (Pony, Cosmed, Italy). AR and spirometry were repeated three times and the best recorded result was retained for analysis [9].

AR was measured twice in each of five individuals to determine the reliability of the measurements, giving an internal conversion coefficient (ICC) (2,1) of 0.897, 95% CI=0.710 to 0.972 (p<0.001). Flow and pressure calibration procedures for the MicroRint (Micro Medical Ltd) were undertaken prior to data collection for validity purposes.

Intervention groups 
Acu-TENS group 

Individuals received TENS over acupuncture points – Dìnchuăn (EX-B1) and Lièquē (LU 7) – bilaterally for 45 minutes prior to, as well as during, a treadmill exercise test. An alcohol swab was used to clean the skin area over the selected acupuncture points. A total of four electrodes (3 M, Seoul, South Korea), 30mm in diameter and filmed with dry gel, were placed over the acupuncture points. Dìnchuăn was located at 0.5 cun lateral to the spinous process of the seventh cervical vertebra (C7). Lièquē is a point 1.5 cun proximal to the radial styloid process and is located at the tip of the top index finger when the thumbs are interlaced at the first web space. ‘Cun’ is a historical measurement unit used to locate acupuncture points and is defined as the distance between the creases of the middle and distal interphalangeal joints of the right middle finger, on the radial side in finger flexion. A portable TENS unit (ITO ES320, ITO Co. Ltd, Tokyo, Japan) was set at a frequency of 2Hz, 200μs pulse width and the intensity increased to the maximum tolerable (without being painful) [6], [7], [8]. Stimulation was maintained for 45 minutes because reported data suggests that stimulation for longer than 40 minutes is necessary to induce an analgesic effect [10]. The sites for electrode attachment are illustrated in Fig. 1.

Placebo-TENS group 

The procedure of application was identical to the Acu-TENS group except that no electrical current was delivered by the TENS machine to the subject. The output intensity display and the intervention time countdown were activated to enhance placebo credibility. The individuals were told two different frequencies of TENS were being tested and they might or might not feel any stimulation.

Sub-maximal exercise test 

After electrical stimulation for 45 minutes, the individuals were asked to run on a treadmill, adopting the Bruce protocol, until their heart rate reached 85% of age-predicted maximum. They were asked to stop exercising if they found it too difficult to continue [point 17 on the Borg rate of perceived exertion (RPE) scale]. Application of the TENS intervention was continued during the exercise test.

AR and lung function parameters were measured in a sitting posture, before and immediately after the exercise, and at 15 minutes post exercise. RPE was recorded at the termination of exercise. HR was measured continuously throughout the procedure. Laboratory room temperature and relative humidity were constant over both visits.

The above procedure was repeated after at least 1 week. Those individuals who received Acu-TENS on the first visit, received placebo-TENS on the second visit, and vice versa. The flow of the study protocol is displayed in Fig. 2.

The recorded HR was subjected to HRV analysis. The Fast Fourier Transform frequency analysis was adopted at 5-minute intervals using the software aHRV (version 11.1.0, Nevrokard, Slovenia). The ratio of low frequency to high frequency (LF/HF) was used to determine sympathovagal balance [11].

Statistical analysis 

The RPE reached at the end of exercise on the two visits were compared by Mann–Whitney U test. AR, HRV, FEV1 and FVC at each time point (baseline, immediately after and at 15 minutes post exercise) were compared between the Acu-TENS and placebo-TENS groups using 2-way repeated measures ANOVA followed by post hoc contrast analysis. Percentage changes of AR, FEV1 and FVC across different time periods were compared between groups using independent t-test and the Mann–Whitney U test. Percentage change was compared because AR varies with the lung volume at which it is measured, thus a normalization procedure is necessary [12]. All data were analyzed using the statistical software (SPSS for Windows version 18 SPSS Inc., Chicago, IL, USA). Type I error level was set at α=0.05.

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Results 

Twenty individuals (nine males) participated in the study but one member of the placebo group dropped out at the second visit for personal reasons and was excluded from the analysis. The mean age (± SEM) and BMI of the 19 remaining participants were 20.5±0.2 years old and 20.6±0.7 kg/m2, respectively. The mean cun was 2.00±0.07cm. All individuals were non-smokers and had basal FEV1 and FVC more than 70% of the predicted value and an FEV1/FVC ratio >75%. There was no between-group difference in baseline pre-intervention variables between the two visits (p>0.05) (Table 1). No adverse effect was reported.

Table 1. Demographic data and baseline variables of the two intervention groups
Acu-TENS (n=19)Placebo-TENS (n=19)p
Age (years)20.5±0.2N/A
Male/female gender (n)7/9N/A
Height (m)1.66±0.03N/A
Weight (kg)56.7±2.83N/A
BMI (kg/m2)20.6±0.69N/A
Cun (cm)2.00±0.07N/A
Airway resistance (kPa/L/s)0.31±0.020.32±0.030.867
FEV1 (L)3.30±0.183.34±0.190.880
FVC (L)3.85±0.223.88±0.240.921
HRV (LF/HF)3.70±0.693.02±0.410.400

Data are presented as mean±SEM unless otherwise indicated. FEV1=forced expiratory volume in first one second; FVC=forced vital capacity; HRV=heart rate variability, LF/HF=low frequency/high frequency.

Changes in AR 

Immediately after exercise, the percentage drop in AR (compared to baseline) was significantly greater in the Acu-TENS group compared with the placebo-TENS group (−20.1±4.00% and −7.99±3.43%, respectively; p=0.029). The AR then returned close to baseline value at 15 minutes post exercise. There was no difference in AR between baseline and the 15 minute data, for either group (Table 2 and Fig. 3).

Table 2. Outcome measurements at baseline and post exercise
Acu-TENS (n=19)Placebo-TENS (n=19)p
Exercise duration (min)8.00±0.348.02±0.340.974
Rate of perceived exertion14.1±0.3614.6±0.470.685

Airway resistance (kPa/L/s):
Baseline0.31±0.020.32±0.030.867
Immediately post exercise0.24±0.010.29±0.020.071
15 min post exercise0.29±0.020.30±0.02

Change of airway resistance after exercise (%):
Immediately post-ex – baseline−20.1±4.00−7.99±3.430.029 ∗
15 min post ex – baseline−3.90±4.95−4.00±3.900.910

FEV1 (L):
Baseline3.30±0.183.34±0.190.880
Immediately post exercise3.32±0.203.21±0.190.005 ∗
15min post exercise3.35±0.213.24±0.19

Change of FEV1 after exercise (%):
Immediately post ex – baseline0.04±1.37−4.17±1.110.010 ∗
15min post ex – baseline0.80±1.69−3.25±0.920.029 ∗

FVC (L):
Baseline3.85±0.223.88±0.240.921
Immediately post exercise3.83±0.263.84±0.250.620
15min post exercise3.83±0.263.90±0.26

HRV (LF/HF):
Baseline3.70±0.693.02±0.410.400
Immediately post exercise1.22±0.312.24±0.400.104
15min post exercise4.54±1.292.93±0.46

Data are presented as mean±SEM unless otherwise indicated. FEV1=forced expiratory volume in one second; FVC=forced vital capacity; HRV=heart rate variability, LF/HF=low frequency/high frequency.

*p<0.05.

  • View full-size image.
  • Figure 3 

    Airway resistance (mean value and SEM) at baseline, immediately after exercise and 15 minutes post exercise. (—) Acu-TENS group; (- - -) Placebo-TENS group.

Changes in FEV1 

Immediately after exercise, there was a significant drop in FEV1 (–4.17±1.11%) after placebo-TENS but an increase in FEV1 was observed over all time points after exercise in the Acu-TENS group (Fig. 4). While there was no between-group difference at baseline, the percentage change was significantly different (between-group) immediately post exercise (p=0.010) and at 15 minutes post exercise (p=0.029; Table 2, Fig. 4).

  • View full-size image.
  • Figure 4 

    Forced expiratory volume in first one second (FEV1; mean value and SEM) at baseline, immediately after exercise and 15 minutes post exercise. (—) Acu-TENS group; (- - -) Placebo-TENS group.

There was no significant difference in FVC, RPE, exercise duration or LF/HF ratio between the two groups at any time points.

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Discussion 

This study showed that AR decreased immediately after the treadmill exercise irrespective of the intervention. As airway calibre is the main factor affecting the AR, a lower AR after exercise suggests dilation of the airway. Previous reports have shown that exercise has a powerful bronchodilating effect in healthy individuals [13], [14]; however, as oral propranolol was used in those cohorts to preconstrict the airway, the effect of exercise on bronchodilation may have been magnified. It is believed that airway diameter is mainly modulated by the parasympathetic nervous system innervated by the vagus nerve, while sympathetic airway innervation is sparse [15]. At rest, vagal tone is active, but during exercise it is decreased or withdrawn, resulting in a relaxation of the airway smooth muscle, accompanied by bronchodilation and reduction in AR [16]. This study demonstrated a decrease in AR in both intervention groups. The AR reduction in the placebo group is likely to be the effect of exercise on airway diameter; i.e., a reduction in parasympathetic tone induced by exercise leading to relaxation of airway smooth muscle and a reduction in AR. Our study demonstrated a more significant decline in AR in the Acu-TENS group compared to the placebo-TENS group, suggesting the bronchodilation effect was more predominant after Acu-TENS.

Blood β-endorphin levels were reportedly raised, with an increase in FEV1 after 45-minutes of Acu-TENS in individuals with COPD [8]; β-endorphin acts on the μ-receptors [17] located in the rhythm-generating component of the respiratory centre [18], [19]. A slower respiratory rate is induced by stimulation of μ- and δ-opioid receptors [18], [20]. Endorphin is also associated with a suppression of hyperventilation [18], [21]. Fung et al related hyperpnoea to bronchoconstriction [22]. It may be that the lower AR observed in our Acu-TENS group was magnified by an endogenous opioid release triggered by Acu-TENS, over and above the exercise effect. This proposal, however, is speculative as we did not measure β-endorphin levels in this study.

This study confirmed that Acu-TENS has a positive effect on FEV1, which agrees with previous reports [23]. FEV1 in our individuals increased at all time points after the treadmill exercise with Acu-TENS. In the study by Ngai et al [23], Acu-TENS for 45 minutes prior to and continued through the treadmill exercise maintained an increased FEV1 for up to 1hour in individuals with asthma. Hasegawa et al found that FEV1 positively correlates with airway calibre in a group of COPD patients, suggesting that FEV1 is higher when the airway calibre is wider [24]. Briscoe and Dubois found that the AR negatively correlates with the radius of the airway, suggesting the area of airway calibre is a factor which affects both FEV1 and AR [25].

This study is the first to demonstrate a reduction in AR accompanying an improved FEV1 after Acu-TENS. A reduction in AR after exercise has been previously reported [26] and we also found a reduction in AR after exercise with placebo-TENS; however, unlike in the Acu-TENS group, the decrease in AR was not accompanied by an improved FEV1.

While exercise induced a bronchodilation effect sufficient to lower the AR, the degree of airway dilation was not sufficient to affect the FEV1. We hypothesise that Acu-TENS lowers AR and improves FEV1 to a greater extent than placebo due to a release of β-endorphin and stimulation of the μ- and δ-opioid receptors. The current study did not measure β-endorphin nor was it intended to investigate the effect of Acu-TENS on the μ- and δ-opioid receptors. Further investigation of this relationship is warranted.

Acu-TENS individuals reported a lower rate of exertion at 85% of their maximal HR compared with the placebo-TENS group, although the difference did not reach statistical significance. Ngai et al suggest that exercise duration is significantly extended by treatment with Acu-TENS compared to placebo-TENS [6]. The maximal HR reached in Ngai’s study was over 90% of the individuals’ maximal HR, and the RPE was around 17 in both groups. It is possible that the exercise intensity adopted in this study was insufficient to induce a significant between-group difference in the exercise duration. Further studies on the effect of Acu-TENS on exercise duration using a standardized maximal exercise protocol are warranted.

To our surprise, there was no significant difference in LF/HF ratio between the two interventions. It was expected that vagal withdrawal and/or raised sympathetic activity induced by exercise would lead to an increase in the LF/HF ratio; however, in both groups the LF/HF ratio decreased after exercise. Our Acu-TENS data showed a trend towards a more dramatic decrease in the LF/HF ratio after exercise compared to the placebo-TENS group, suggesting a possibility of inhibition of sympathetic or accentuation of parasympathetic output, but the changes in LF/HF did not reach statistical significance. We postulate that the exercise intensity was insufficient to demonstrate a significant intervention effect on the modulation of the autonomic nervous system innervation of the airways. Further studies using higher exercise intensity are again warranted.

Clinical implication 

This study showed that 45 minutes of Acu-TENS induced a reduction in AR and an improvement in FEV1 in young healthy individuals. Acu-TENS may have a role in dyspnoea reduction during submaximal exercise in patients with airflow limitation, enhancing acceptability of exercise training. Further investigation of the effect of Acu-TENS on AR in a clinical population(e.g., individuals with asthma or COPD) during submaximal exercise training are warranted.

Limitations of the study 

There are several limitations in our study. First, the sample size was small. As this was the first study investigating the effect of Acu-TENS on AR, we were unable to conduct a power analysis calculation from previous reports. Our data achieved a medium effect size of 0.54 with observed power of 72.6%, and in retrospect the power of the study is considered acceptable. Secondly, data measurements were terminated after 15 minutes post exercise and any long-term effect of Acu-TENS on FEV1 in healthy individuals was not evaluated. Lastly, a further control group subjected to electrical stimulation over nonacupuncture points should have be included. We were thus unable to determine whether the decrease in AR or the increase in FEV1 was an acu-point effect or the result of electrical stimulation. Future studies should include a sham-TENS group with electrical stimulation over nonacupuncture points.

Conclusions 

This study has shown that 45 minutes of Acu-TENS before and during a submaximal exercise test in healthy individuals was associated with a significant decrease in AR and improved FEV1 when compared to a placebo group. Further investigation of the effect of Acu-TENS on AR and FEV1, employing a more intensive exercise protocol and a sham-TENS control group, is warranted.

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PII: S1013-7025(11)00070-4

doi:10.1016/j.hkpj.2011.11.002

Hong Kong Physiotherapy Journal
Volume 30, Issue 1 , Pages 29-35, June 2012

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