During the study of the articles used in the preparation of this review, it was observed that patients with COPD have reduced heart rate variability compared to healthy patients. Moreover, some features, such as the non-invasive ventilation can improve the cardiac autonomic control of these individuals suggesting a reduction in the impact of cardiac co-morbidities therein. In addition, we verified that some studies have analyzed the effects of exercises like strength and endurance exercises; six minute walk test or even through pulmonary rehabilitation, and it was observed that all interventions improve heart rate variability in COPD patients, which cautions us to importance of physical exercise to improve the quality of life of these population.
Heart rate variability has been used as a tool to analyze the behavior of the autonomic nervous system on the heart, and compare possible differences between healthy and unhealthy people. Its decrease is related to increase morbidity and mortality [24], which was confirmed the data found in our research on COPD. Studies indicate that there are changes in HRV in a list of cardiorespiratory disorders. COPD is associated with vascular remodeling that modifies the pulmonary circulation. This pathological mechanism is usually caused by hypoxia generated by the disease [25].
According to the study of REIS et al [21], both patients with COPD and patients with congestive heart failure (CHF), exhibit alterations in autonomic modulation of heart rate at rest and during respiratory sinusal arrhythmia maneuver (RSA-M) compared with apparently healthy individuals. During rest, patients with COPD showed a reduction in sympathetic activity compared to the control group. Regarding the CHF it was also observed decreased sympathetic and parasympathetic activity of the subjects. It was indicated a possible relationship to regulatory changes of autonomic centers, the sensitivity of chemoreceptors or respiratory pattern characterized by periodic oscillations presented by these patients. During the RSA-M it was observed that both COPD patients and patients with CHF, through the analysis of HRV indices in the time domain (SDNN and rMSSD) and frequency domain (LF and HF, both in units absolute) had reduced response, revealing a decrease in HRV compared with control subjects. These data presented in the study indicated that it is possible that reduced HRV observed in patients of both groups may be related to changes in lung compliance and response of lung receptors stretches.
A study of Mendes et al [18], evaluated the heart rate, blood pressure and heart rate variability in test forced vital capacity (FVC) in patients with COPD and demonstrated that significant changes in HR, while on BP and HRV it did not occurred the same changes. Throughout the test, HR was undergoing changes as fall during the beginning of the inspiratory phase of the test, followed by an increase until the end of it, with the end of the inspiratory phase HR was reduced below the HR at rest before resuming their baseline. During the expiratory phase, HR was progressively increasing, the authors suggested this to be related to the increased intrathoracic pressure and decreased venous return, and reduction in vagal activity and increased sympathetic activity that occurs during expiration. When assessing the HRV, the absence of significant changes was verified, and the authors highlighted the short duration of the maneuver, the age of the volunteers and a probable autonomic dysfunction of individuals.
By analyzing the geometrical HRV indices, Dias de Carvalho et al [17] illustrated decreased SD1, SD2, TINN and RRtri in the COPD group, while the SD1/SD2 ratio were similar between the two groups. These findings suggest a reduced HRV in patients with COPD. Reducing the SD1 index indicates reduction in vagal activity of these individuals, since the collapse in values of SD2, TINN and RRtri in patients with the disease compared with healthy subjects, indicating overall reduction in autonomic modulation of these individuals, which supports other studies which indicated global autonomic damage in COPD. Through the analysis of the Poincaré plot, the authors found greater dispersion of data related to patients with COPD, which, again, suggests the same reduced in HRV. It was highlighted the importance of supervised exercise in subjects with COPD, since the literature shows these beneficial effects on cardiac autonomic modulation.
In a study by Dias de Carvalho et al [20], it was analyzed the properties of fractal correlation in patients with COPD, with loss or breakdown of short-term fractal correlation properties of HRV (alpha-1) associated with the reduction of sympathetic and parasympathetic activity in patients with COPD. According to the authors, patients with COPD exhibit dynamic changes in HR indicating loss of chaotic response. Equally, linear indexes in time and frequency domain analyzed in the study illustrated a reduction in sympathetic and parasympathetic activity in volunteers of the COPD group compared to the control group. The authors have attributed to this loss several clinical events such as heart failure and acute myocardial infarction. It was also suggested that individuals with COPD exhibit autonomic dysfunction, indicated by the loss of short-term fractal correlation properties and reductions in global activity of HRV, supporting the importance of using these tools for evaluation of morbid states, because the ability to evaluate the loss of homeostasis of patients, allowing better discrimination between individuals with or without physiological change. With respect to long-term exponents (alpha-2) and alpha-1/alpha-2 ratio, there was no difference between the COPD group and the control group.
In order to analyze HRV after two exercise programs in patients with COPD, Camillo and coworkers [19]assessed 40 patients divided into two groups: high intensity, characterized by endurance exercise and strength, and of low intensity, that consisted of calisthenics and breathing exercises for a period of 3 months. Cardiac autonomic control was assessed before and after 3 months of training and, was judged during rest and as a response to an orthostatic stimulus using a head-up tilt test following a previous published protocol. Patients were positioned on an orthostatic table where they initially stayed for 10 minutes in supine position. The table was then lifted 75°, and this was followed by a period of more 10 minutes in orthostatic position. The authors concluded that three months of high-intensity exercise training enable an important improvement in post-training cardiac autonomic function in patients with COPD. Additionally, better baseline values of upper limbs muscle force, physical activity in daily life and total heart rate variability may help predicting those patients who will more likely improve their cardiac autonomic function after a high-intensity exercise training program.
An important study [15] attempted to investigate the effect of pulmonary rehabilitation on heart rate variability during exercise in patients with COPD. Beyond, the effect of pulmonary rehabilitation on health-related quality of life and exercise capacity concurrently. Cardiac autonomic control was measured with a total recording time of 5 minutes at rest and at peak exercise and, were obtained from the ECG signals. The protocol program consisted of 12-week outpatient-based program with two sessions per week. Before all training sessions, patients went through prior educational training, then the lower limb cycle ergometer exercise was performed. The training protocol of the lower limb exercise consisted of warm up of 4 minute, followed by 60-100% peak VO2 for 40 minute and finally cool down of 4 minute. Work rate, SpO2, HR, BP, dyspnea Borg’s scale, and leg fatigue during the exercise training were monitored. They concluded that patients with COPD often have impaired parasympathetic and sympathetic activity and poor exercise capacity, and quality of life. Pulmonary rehabilitation provides significant improvement in HRV with concurrent improvements in HRQL and exercise capacity.
In an attempt to investigate heart rate variability responses to a psychosocial stressor in COPD patients, and the potential role of anxiety as a confounding factor in this relationship, Suh and colleagues [16] analyzed four groups of participants: COPD patients with elevated anxiety, COPD patients without anxiety, healthy individuals with elevated anxiety, and healthy individuals without elevated anxiety with 15 participants each. Participants performed a test to check the level of anxiety and eligible completed the pulmonary function test. The study was divided into three phases: Baseline, Task, and Recovery. At Baseline, all participants completed a packet of self-report questionnaires. Following the questionnaires, participants remained sitting quietly for 5 minutes to obtain a stable measure of heart rate variability at rest. During the Task phase, participants were exposed to the stressor task. A modified version of the Trier Social Stress Test was used for this study. And finally during the Recovery phase, each participant completed a post-stressor questionnaire of state anxiety. The participant then listened to relaxing music for 20 minutes. In this study the authors indicate that the combined effect of anxiety and COPD did not have a cumulative negative effect on autonomic function, contrary to the original hypothesis. However, an atypical pattern of HRV in response to the stressor task for the COPD patients with elevated anxiety and healthy individuals with elevated anxiety compared to their non-anxious counterparts suggests that anxiety may play a mediating role in HRV patterns in response to a stressor.
Hypothesizing that respiratory muscle weakness negatively influences heart rate variability during respiratory maneuvers in patients with COPD; Reis and coworkers [26] aimed to evaluate the influence of respiratory muscle strength on the magnitude of respiratory sinus arrhythmia. All chronic obstructive pulmonary disease patients used short-action bronchodilators, and six used long-action bronchodilators. Subjects in the control group were free of chronic pulmonary, cardiovascular, immune, and/or metabolic disease. The volunteers were kept at rest in the supine position for approximately 10 minutes to ensure that a true resting HR value was achieved. Then, the ECG signal and the instantaneous HR were obtained at rest in the supine position for 15 minutes. Subsequently, the heart rate and RR intervals were recorded during the respiratory sinus arrhythmia maneuver in the supine position in the following order: for one minute at rest with spontaneous breathing; for four minutes while performing the respiratory sinus arrhythmia maneuver; and for one minute at rest with spontaneous breathing. It was concluded that COPD patients showed evidence of impaired autonomic modulation of heart rate at rest and during respiratory sinus arrhythmia maneuver, the relationship between the maximal inspiratory pressure and heart rate variability indices during respiratory sinus arrhythmia maneuver indicates that the inspiratory muscle weakness observed in this population may be associated with cardiac autonomic control.
The aim of the study by Borghi-Silva and coworkers [22] was to assess the impact of an aerobic exercise program on autonomic modulation in patients with moderate-to-severe COPD. Forty patients of both sexes were included in the study and all of them received regular treatment consisting of inhaled bronchodilators and steroids and none of them were prescribed oral steroids, antibiotics, antihypertensive or beta-blockers. Heart rate variability data were collected during 10 minutes of rest and throughout the six-minute walk test. Aiming to maintain peripheral oxygen saturation greater than 90%, four patients of the experimental group and three patients in control group received supplemental oxygen. Concluding, the study demonstrates that a 6-week aerobic exercise training program leads to an improvement in exercise tolerance, promotes ventilator and physiological adaptations and favorably impacts the derangements in autonomic modulation of heart rate both at rest and during exercise in patients with COPD.
A group of Italian researchers [23] studied 54 patients with hypoxemic chronic obstructive pulmonary disease. Neuropsychological assessment was performed by The Mini Mental State Examination and the Mental Deterioration Battery and heart rate variability was assessed based on 24-hour Holter ECG recording. It was observed by the researchers, a correlation between drawing impairment and depressed sympathetic modulation of the neuroautonomic tone. Such a relationship seems worthy of reassessment in a larger and more heterogeneous COPD population in the framework of properly designed studies including also a well balanced set of executive and copying tests. Confirming or denying present findings would make physicians aware of whether a further dimension should be added to the many characterizing COPD as a systemic disease.
An important study of Van Gestel and co-workers [13] aimed to evaluate whether there is an association between cardiac autonomic dysfunction and health-related quality of life in COPD patients. They applied a six-minute walk test in all patients and performed on a 30 metre indoor track using standardized encouragement strategy. Breathlessness and leg fatigue were assessed using a conventional Borg scale in German. Analysis of heart rate variability was performed using a Holter-ECG device for a recording period of 5 minutes, with the patients at rest, sitting on a chair. Cardiac autonomic function was performed 30 minutes after pulmonary function was assessed during the midday between 11:00 –15:00. The study shows that resting parasympathetic tone, as measured by HRV, is independently associated with health-related quality of life emphasizing the role of cardiac autonomic dysfunction on health-related quality of life in patients with COPD. Modification of cardiac autonomic dysfunction may therefore be of benefit in the treatment of COPD patients, but this needs to be proven in controlled interventional trials.
Gunduz and colleagues [14] also analyzed the presence of autonomic dysfunction in patients with COPD by heart rate variability and heart rate turbulence analysis and to determine whether the parameters of these in this population are different from the normal population. Spirometric and blood tests were performed in all participants and heart rate variability was assessed by 24-hour Holter monitoring. In the view of authors, the combination of heart rate variability and heart rate turbulence variables may be simple and elegant ways of evaluating cardiac autonomic functions. Such a combination may increase the positive predictivity and lead to a more accurate identification of high risk patients, more aggressive treatment towards preventing sudden death and/or preventing progression of disease to mortality.
By analyzing the studies eligible for our review, unfortunately, we found poor methodological quality of most of them, according to the PEDro scale [11]. We observed that 36% of them had score 3, 27.5% score 4, 27.5% score 5, and only one study with score 6, considered with good methodological quality; it should be emphasized that better designed studies can better represent proposed objectives by their results.
Thus, HRV is an important tool for assessing the autonomic nervous system (ANS), which has an important role in maintaining homeostasis. Its use is diverse and it stands as a predictor of the internal functions of the body, both in normal and pathological conditions, characterizing current instrument evaluation and identification of problems in the health, growth and human development [27]. Health care is conventionally regarded as the diagnosis, treatment, and prevention of disease, illness, injury, and other physical and mental impairments in humans. How we define the quality of public health at any given time must be compatible with future generations enjoying health in an equivalent way [28].
