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Research Reports |
MW Tsai, PT, PhD, is Lecturer, Institute and Faculty of Physical Therapy, National Yang-Ming University, Taipei, Taiwan. This research was one part of her doctoral thesis in the Institute of Epidemiology, College of Public Health, National Taiwan University, Taipei, Taiwan
WC Chie, MD, PhD, is Professor, Department of Public Health and Graduate Institute of Preventive Medicine, College of Public Health, National Taiwan University
TBJ Kuo, MD, PhD, is Director and Professor, Institute of Neuroscience, Tsu Chi College of Medicine and Humanities, Hualien, Taiwan
MF Chen, MD, is Professor, Department of Internal Medicine, National Taiwan University Hospital
JP Liu, PhD, is Professor, Division of Biometry, Department of Agronomy, National Taiwan University
THH Chen, PhD, is Associate Professor, Department of Public Health and Graduate Institute of Preventive Medicine, College of Public Health, National Taiwan University
YT Wu, PT, PhD, is Director and Associate Professor, School and Graduate Institute of Physical Therapy, College of Medicine, National Taiwan University, No. 1, Sec. 1, Jen-Ai Rd, Taipei 10020, Taiwan
(ytw{at}ntu.edu.tw). Address all correspondence to Dr Wu
Submitted February 14, 2005;
Accepted November 29, 2005
 | Abstract |
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Subjects and Methods. A total of 84 subjects who had undergone PTCA were recruited for this study. The subjects (age [mean±SD]=57.0±9.3 years) were randomly assigned to either an exercise group to undergo an 8-week outpatient exercise program or a control group to undergo usual care. Heart rate variability was measured for 5 minutes in the supine resting position at baseline and at the end of 8 weeks. Results. The parasympathetically modulated HRV of the subjects in the exercise group increased significantly compared with the HRV of subjects in the control group. The effects of training on HRV were independent of angioplasty type (balloon or stent) and were unrelated to whether the subjects had received previous PTCA. Discussion and Conclusion. Exercise training can increase parasympathetic modulation of cardiac function in people after they have undergone successful PTCA. Our results suggest that analysis of HRV can be extended to assess the effect of exercise training on cardiac autonomic dysfunction in people after coronary angioplasty.
Key Words: Exercise training • Heart rate variability • Percutaneous transluminal coronary angioplasty
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Heart rate variability commonly is analyzed in the time and frequency domains.2, 3 Time domain methods use simple mathematical techniques to measure the amount of variability in a continuous electrocardiogram (ECG) in a prespecified time period. Most time domain indices, such as the standard deviation of intervals between normal beats and the mean of successive normal R wave peak intervals (the mean of R-R intervals), reflect the overall autonomic modulation of heart rate but provide no information regarding sympathetic and parasympathetic activities individually.2 In contrast, the frequency domain method, which involves power spectral analysis of HRV, can partially separate parasympathetic effects from sympathetic effects on the heart.2, 3 It is generally accepted that overall HRV is mediated mainly by vagal activity.
Heart rate oscillations at high frequency (HF) are equivalent to respiratory sinus arrhythmia and are considered a marker of cardiac parasympathetic activity, whereas heart rate oscillations at low frequency (LF) are a marker of both sympathetic and parasympathetic nerve activities.2–4 However, the LF in normalized units, representing LF relative to the total power, is considered by some investigators to reflect sympathetic modulations.2–4 The ratio of LF to HF (LF/HF) indicates sympathovagal balance, that is, the balance between the sympathetic system and the parasympathetic system.2–4
Some people with cardiac abnormalities show altered HRV, such as a reduction in overall HRV or a shift in the sympathovagal balance toward a sympathetic predominance instead of a vagal predominance. Alterations in HRV have been observed consistently in patients with acute myocardial infarction (AMI) and chronic heart failure.1, 5–8 Reduced HRV has been linked to an increased risk of cardiovascular mortality and cardiac events.5–9 This increased risk may be a result of electrical instability in the heart rate because of an autonomic environment dominated by sympathetic nervous system influences.1, 2, 5, 8 Thus, it has been proposed that HRV can be used as a prognostic factor for risk stratification and therapeutic intervention for patients with cardiac disease.1, 2
Percutaneous transluminal coronary angioplasty (PTCA) is an effective and minimally invasive intervention for coronary revascularization.10–14 Several reports15–19 have shown that HRV, especially parasympathetically modulated indices, is reduced in patients after PTCA. Some studies15, 16 have shown that the reduction in HRV after coronary angioplasty is only a transient phenomenon, whereas other studies17–19 have shown that the HRV remains lower than that in control subjects who are healthy for more than 6 months after PTCA, especially in people with mixed multiple-vessel disease or comorbidities. Alterations in HRV also have been found to be associated with the degree of revascularization and a greater risk of restenosis after PTCA.16, 19
Besides the well-known benefits of exercise-based cardiac rehabilitation for recovery after cardiac events,20, 21 studies22–25 have shown that it also can benefit HRV and modify altered HRV in a prognostically favorable direction for patients with cardiac conditions. Data from studies of patients after myocardial infarction demonstrated that exercise training, performed for either 8 weeks or 3 months, modified HRV toward a persistent increase in parasympathetically modulated cardiac function.22, 23 Even a 2-week residential exercise training program increased HRV and baroreflex sensitivity in patients with coronary artery bypass grafting.24 Finally, exercise training also could be effectively performed under home-based conditions.25 To our knowledge, however, there are no data from trials with subjects who have undergone PTCA. Although cardiac rehabilitation for people with PTCA has been proposed to be similar to that for other patients with cardiac disease, the exact effect of cardiac rehabilitation on patient outcome has yet to be investigated fully.20, 21, 26 It is still unclear whether exercise training modifies the attending autonomic alterations in people with PTCA.
In addition, factors specific to the PTCA procedure, such as the type of angioplasty and the history of angioplasty, have been shown to be associated with the prognosis of patients after PTCA.27–30 For example, angioplasty with a stent resulted in a significantly lower incidence of restenosis than did angioplasty with a balloon.28, 29 However, whether the type of angioplasty procedure influences cardiac autonomic regulation after coronary angioplasty is still unknown. Whether a history of angioplasty is a related factor has not been fully addressed in intervention studies of people after PTCA.16, 18, 31 Therefore, the purposes of the present study were to identify the effects of exercise training on alterations in HRV after PTCA and to delineate whether the type of angioplasty and the history of the PTCA procedure influence the effects of exercise training on HRV.
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Interventions
The exercise group attended an outpatient exercise program of supervised training combined with home exercise for 8 weeks. In the outpatient hospital setting, subjects exercised on a cycle ergometer under the supervision of registered physical therapists. An individualized exercise program was prescribed on the basis of the results of a symptom-limited maximal exercise test on a bicycle ergometer at the beginning of the study. Training intensity was initially about 60% of the heart rate reserve and was close to the anaerobic threshold. It was increased to 85% of the heart rate reserve progressively as the subjects experienced the training effect. Training duration was 40 minutes per session, including the 5- to 10-minute warm-up and cool-down phases, and frequency was 1 to 3 sessions per week. For the home program, subjects were instructed to exercise with a tolerable intensity similar to that in their supervised exercise sessions. They were free to perform any aerobic exercises available to them, such as bicycling, walking, or jogging. Subjects were instructed on how to assess heart rate and on how to rate perceived exertion and to record any adverse events in their exercise log. The data were checked every week by the physical therapists to make sure that the home program was optimal and was being adhered to. Subjects in the control group were advised to maintain their usual daily activities without restriction. Subjects in both groups were given a health information instruction session about coronary disease risk factors, diet, stress management, safety guidelines for daily activities, and energy conservation after measurement of the baseline parameters and before the start of the trial. All subjects received normal medical care by their cardiologists and took their medications as prescribed throughout the course of the study.
Measurements
Short-term HRV, which has been successfully demonstrated to estimate tonic ANS functions and to have good reproducibility,4, 33, 34 was assessed at baseline and at the end of 8 weeks. Each subject underwent HRV measurements during the same time of day on repeat testing. Electrocardiogram tracings were taken over a 5-minute period after the subjects had rested for at least 10 minutes. Each subject was tested in a supine position while breathing normally. The pericardial ECG signals were recorded and amplified with a gain of 1,000 and band-pass filtered (0.68–16 Hz). Signals were digitized by use of an analog-to-digital converter with a sampling rate of 256 Hz. The digitized ECG signals were analyzed online and were stored on a hard disk for off-line verification. Signal acquisition, storage, and processing were performed on an IBM-compatible portable personal computer. Our computer algorithm identified each QRS complex and rejected each ventricular premature complex or noise according to the likelihood that it would fit a standard QRS template. Stationary R-R intervals were resampled and interpolated at a rate of 7.11 Hz to obtain continuity in the time domain. Frequency domain analysis was performed by use of a nonparametric method (fast Fourier transformation). The direct-current component was deleted, and a Hamming window was used to attenuate the leakage effect.33 For each time segment (288 seconds; 2,084 data points), our algorithm estimated the power spectrum density on the basis of fast Fourier transformation. The resulting power spectrum was corrected for attenuation resulting from the sampling and the Hamming window. The power spectrum was quantified into various frequency bands as standards.2 The HRV was reported in terms of both time and frequency domain indices, including the mean of R-R intervals, the variance of the mean of R-R intervals (variance), HF power (0.15–0.4 Hz), LF power (0.04–0.15 Hz), LF in normalized units (LF%), and LF/HF (Tab. 1). Variance, HF, LF, and LF/HF were logarithmically transformed to correct for severe skewness before statistical analysis.4
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Data Analysis
Statistical analysis was conducted with SPSS* for Windows,
version 10.0. The intention-to-treat principle was applied to the analysis of outcome for all subjects on the basis of their assigned treatments. Baseline information was compared for the 2 groups (exercise and control) by independent t tests and chi-square tests, taking into account the measurement scales. The alpha level was set at .05 for these tests. An analysis of covariance adjusted for stratified factors was used to compare the differences between groups with regard to the change from the baseline for each of the HRV indices. The adjustments for multiple comparisons used a P value of .008 because of cross-correlation among HRV indices.33 Furthermore, the effects of previous PTCA treatment and the type of angioplasty on the changes in HRV after interventions were assessed by analysis of covariance (the exercise and control groups were used as covariants). In addition, we explored the possible effects of clinical characteristics on the changes in the HRV indices by using subgroup analyses.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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2=0.085; df=1,7; P=.71) (Tab. 3).
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Effects of Exercise Training on HRV
At baseline, the HRV indices did not differ significantly between the groups (Tab. 4); however, after interventions, the HRV of the subjects in the exercise group changed significantly compared with the HRV of subjects in the control group (Tab. 4). We found a 10% increase in HF, a 5% increase in the mean of R-R intervals, and a 5% increase in the variance in the exercise group, whereas decreases in these HRV indices were found in the control group. There were no significant differences between the groups with regard to changes in LF, LF%, and LF/HF.
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| Discussion |
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Our subjects appeared to have lower parasympathetic modulations of heart rate (HF power) and relatively higher sympathetic activity (LF%) at the time of the baseline measurements (4.0±1.2 weeks after PTCA) than did a comparable healthy population in a previous study.4 The pattern of HRV after PTCA in our subjects was similar to the pattern reported in other studies.16–18 In the present study, the relevant clinical variables and HRV indices were comparable between the exercise group and the control group at baseline. The use of drug therapies was not statistically significantly different between the groups during the interventions. Thus, the changes observed in parasympathetically modulated HRV in the exercise group most likely were attributable to exercise training.
We found significant increases in parasympathetic modulations of HRV indices in subjects who had undergone PTCA after 8 weeks of exercise training. Our results are similar to the findings of previous studies of subjects with myocardial infarction and coronary bypass.22–25 However, in the present study, an average increase of 10% in HF after exercise training was relatively lower than increases of approximately 7% to 27% reported for other populations with cardiac disease.22–25 A subgroup analysis by Tygesen et al25 showed that subjects with coronary bypass achieved more pronounced HRV gains than did subjects with myocardial infarction when using the same exercise program. The authors of that analysis suggested that the factors and clinical features underlying heart disease, such as a low-risk status, could explain the lower increase in HRV in their subjects with myocardial infarction. In addition, different basal states of cardiac autonomic tone under various study conditions also may have contributed to the discrepancy in HRV gains.3
Previous studies1, 2, 5–7, 9 demonstrated that the risk for cardiovascular mortality and further cardiac events is higher among subjects who have more depressed HRV. However, the degree of increases in HRV that may produce a truly protective effect is still unknown. Evidence from animal studies has indicated that the risk for ventricular fibrillation or sudden cardiac death after acute myocardial ischemia is reduced markedly by interventions that increase vagal activity.36–38 These studies suggested that the improvement in cardiac electrical stability and adaptations in peripheral and central neural pathways may contribute to the training effect of exercise on vagal activity. Our results showed that exercise training up-regulated a lower HRV after PTCA, especially for HF and parasympathetically modulated HRV indices. This finding may reflect an improved cardiac autonomic environment and may provide a favorable situation for cardiovascular health.
The type of angioplasty and a history of angioplasty were associated with patient outcome after PTCA27–30; however, their association with the effect of exercise training is not fully understood. In the present study, we used a stratified randomization design to control for possible confounding effects and to identify any different effects in subgroup analyses. Neither the type of angioplasty nor previous PTCA was associated with changes in HRV after exercise training. Because complete revascularization with almost no residual stenosis was observed in most of our subjects (50 of 67 subjects), our findings seem to show that the type of angioplasty may not play a role in the changes in HRV induced by exercise training for people after successful angioplasty.
Many researchers1, 2, 5–7 have reported data showing that HRV tends to decrease in people with clinical evidence of hypertension, diabetes, or myocardial infarction. In the present study, we found that subjects with AMI showed a greater increase in parasympathetically modulated heart rate after exercise training than did subjects without AMI. The disparate patterns of training gains were similar to those previously observed in subjects after AMI.22, 23, 25 This finding suggested that the presence of AMI could influence the effect of exercise training on the autonomic balance in people after PTCA. No other clinical features were found to have significant influences on the baseline values for HRV. The findings may be partly attributable to a relatively small population within each stratum and the exclusion of subjects who had diabetic neuropathy or severe hypertension.
The major limitation of the present study was lack of masking, which may have influenced subjects’ attitudes toward therapy and which also may have influenced the eventual responses. However, it is often not feasible to conduct a double-blind trial for an exercise intervention.39 The potential bias of the subject response may have been minimized with more objective measurements of HRV. In addition, subject adherence to the therapy regimen, such as low participation in exercise for some subjects in the exercise group or having an active lifestyle for some subjects in the control group, may have had some influence on the study results. However, our intention-to-treat analysis could provide a valid assessment of treatment efficacy, as it relates to actual clinical practice.39
Despite the potential underestimation of effect size from this analysis, the findings of the present study still demonstrated that exercise training increased HF and overall HRV in subjects after PTCA. As yet it is not known how large training gains in HRV are clinically meaningful in terms of providing adequate health benefits. In addition, it may be debated as to how long the exercise period should be or how much the exercise capacity should be increased to obtain adequate training gains in HRV.20, 21, 40 Further research is needed to delineate the dose-response relationship between exercise and autonomic functions and how both functions relate to health benefits in patients with cardiac disease.
Clinical Implications
Heart rate variability reflects cardiac autonomic modulation and has provided some insights into the pathophysiological conditions in ischemic heart disease,1, 2 thereby contributing to additional prognostic information beyond traditional outcome measures in physical therapist practice. The HRV is a function of the synergistic action between the 2 branches of the ANS, which act in balance through neural, humoral, and physiological mechanisms to maintain cardiovascular functions in an optimal range. If the dynamics of regulation are disturbed or the activity of 1 of the branches decreases, the probability of diseases, complications, and fatal outcomes increases. On the other hand, augmentation of the ability to control the regulatory systems is helpful in creating favorable situations for producing better prognoses and treatment results. From our experience, analysis of HRV is a simple procedure and requires only minimal cooperation from the patients being tested, especially with the availability of well-developed computer algorithm technology. The present study may help shed light on the importance of understanding ANS responses and may provide new knowledge regarding the effects of exercise training on the behavior of ANS in people undergoing PTCA.
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| Footnotes |
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Microsoft Corp, One Microsoft Way, Redmond, WA 98052-6399.
Dr Tsai, Dr Chie, and Dr Wu provided concept/idea/research design. Dr Tsai and Dr Wu provided writing. Dr Tsai provided data collection, and Dr Tsai, Dr Liu, and Dr THH Chen provided data analysis. Dr Liu provided project management and institutional liaisons. Dr Tsai provided fund procurement. Dr MF Chen provided subjects. Dr Kuo, Dr Wu, and Dr Liu provided facilities/equipment. Dr Chie, Dr Kuo, Dr MF Chen, and Dr THH Chen provided consultation (including review of manuscript before submission). The authors are grateful to Dr Chiau-Suong Liau, Dr Kwan-Lih Hsu, Dr Hsien-Li Kao, Dr Tson-Ming Lee, and Dr Chii-Ming Lee for their support in contacting subjects and to Ms Shu-Fang Hsiao for her assistance in exercise training. The authors also acknowledge Huei-Chen Lin and Li-Ying Kuo for providing clerical support.
This research was approved by the Institutional Review Board of National Taiwan University Hospital, Taipei, Taiwan.
This study was supported, in part, by National Science Council (ROC) Grant NSC89-2320-B-010-036.
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