Chronic obstructive pulmonary disease (COPD) is considered a preventable and treatable condition which is characterized by not fully reversible airflow chronic obstruction1,2 and systemic manifestations, such as nutritional depletion, functional and structural alterations of peripheral and respiratory muscles and arrhythmias.3 Changes in autonomic nervous system (ANS) are also observed in COPD subjects4,5; this counts as a negative factor, since ANS modulates internal functions of the body; hence, it deserves further attention.

A well recognized method of evaluating the autonomic behavior is the heart rate variability (HRV), the conventionally accepted term to describe the fluctuations in the intervals between consecutive heart beats (RR intervals), which are related to influences of the ANS on the sinusal node.6 Among all methods used to describe geometric methods we may include HRV triangular index (RRtri), triangular interpolation of NN interval (TINN) and Poincaré plot. This method allows us to present RR intervals in geometric pattern and the use of an approach to derive HRV measurement.7

The RRtri and TINN are calculated based on the construction of a histogram density of normal RR intervals which contains the RR intervals length in the x-axis and the frequency at which they occurred in the y-axis. The junction of the histogram columns forms a shape similar to a triangle which is extracted from these indexes.6,8 The Poincaré plot is a graphic representation of two-dimensional correlation between consecutive RR intervals, where each interval is plotted against the next interval.7,9,10 They may be analyzed in a qualitative way, by assessing the shape formed by its attractor, which shows the degree of RR intervals complexity,11,12 or quantitatively, by fitting the ellipse of the figure formed by the attractor from where the indexes are obtained: SD1, SD2 and SD1/SD2 ratio.13,14 The analysis of the Poincaré plot is considered by some authors to be based on nonlinear dynamics.7,15

Reduced HRV in COPD patients at rest has already been observed, compared to control subjects at the same age.16–18 Reductions were reported in variables which correspond to sympathetic and parasympathetic modulations together5 and parasympathetic isolated.18 Those investigations regarding specific alterations in autonomic function in COPD determined the use of linear methods in the time and frequency domain. However, to the best of our knowledge, there are few research studies evaluating HRV in COPD patients by means of geometric indexes. After searching the relevant technical literature, only one study was found, and this work used TINN.19 In this study, they showed lower values of this index in COPD individuals compared to healthy subjects.

In order to bring new elements into the literature about this subject we endeavored to evaluate geometric index of HRV in COPD subjects.

Table 1 presents anthropometric measurements and spirometric values in COPD and control groups. We observed that BMI, forced vital capacity (FVC), forced expiratory volume at the first second (FEV1), ratio of forced expiratory volume, forced vital capacity (FEV1/FVC) and peak expiratory flow (PEF) were significantly lower in COPD subjects compared to the control group.

Table 2 displays RRtri, TINN, SD1, SD2 and SD1/SD2 ratio in control and COPD groups. We noted reduced values of RRtri, TINN, SD1 and SD2 in COPD subjects. However, there was no significant difference between control and COPD groups regarding SD1/SD2 ratio.

Fig. 1 shows a representative example of Poincaré plot standard in one control and one COPD subject. It suggests reduced parasympathetic activity in COPD individuals.

Figure 1.

Poincaré plot visual standard in control (SD1=15.5 and SD2=47.6) and COPD (SD1=8.0 and SD2=30.3) individuals.

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In this study we evaluated geometric index of HRV in COPD subjects. Our results show that the SD1 index is reduced in the COPD group as well as SD2, TINN and RRtri. On the other hand, the SD1/SD2 ratio was similar between control and COPD groups. These findings suggest reduced HRV in COPD subjects.

SD1 index was decreased in COPD individuals compared to control subjects; we suggest reduced parasympathetic activity in COPD patients. This index represents the transverse axis of the Poincaré plot; it corresponds to the standard deviation of the instantaneous variability of heart rate beat-by-beat13,32 and indicates the parasympathetic influence on the sinoatrial node.30,33 Reduction of vagal modulation in COPD patients, as assessed by time and frequency domain was also found by Pantoni et al.5 who observed reduction of RMSSD [square root of the mean squared differences of successive NN intervals] and high frequency (HF) in COPD individuals.

Indexes of the overall autonomic modulation (SD2, TINN and RRtri) were also reduced in COPD subjects. Our data are supported by the literature, which points to a general autonomic damage in COPD.19,34,35 Paschoal et al.16 and Volterrani et al.18 found reduction of global autonomic modulation in COPD patients compared to control group by means of SDNN [standard deviation of normal to normal] index obtained in the time domain, which supports results in the same condition at rest, but using the low frequency (LF) index.17,36

TINN was significantly decreased in COPD compared to control group. These findings are in agreement with Sin et al.,19 who found that the TINN of COPD patients was lower in healthy volunteers (322ms vs. 444ms). It is noteworthy that even the average TINN obtained in this investigation is lower than those found by these authors,19 which may be related to the older age of the study population compared to the average age of the Sin et al. study.22

Concerning the SD1/SD2 ratio analysis, we reported no significant differences between COPD and control groups. This may be explained by the observed reduction of both indexes (SD1 and SD2) in COPD subjects.

Visual analysis (qualitative) of the Poincaré plot allowed us to observe reduction of HRV in COPD patients. In these individuals there was lower dispersion of RR intervals both beat-by-beat and long-term, which characterizes reduction of HRV. In control subjects the intervals between heart beats are irregular, which lead us to visualize a cloud of points in the Poincaré plot. The mechanism by which the autonomic modulation is altered in COPD is not well established. Hypotheses are considered in relation to the prevailing tone in these cases, because the hyperinflation (feature of vagal activity) could generate a pulse change.17 Additionally, hypoxic patients may suffer from abnormal autonomic function34 and oxygen supplementation could partially reverse this dysfunction.4

It is clear in the literature that reduced HRV is indicative of poor prognosis in cardiovascular disease,27 it is also related to arrhythmias37 and sudden death.38 Furthermore, it is well recognized that cardiovascular diseases often coexist with COPD and are a source of morbidity and mortality in COPD patients.2,4,5 Likewise, the beneficial effects of exercise on cardiac autonomic modulation have been reported.13,14,16 Thus, we highlight the importance of pulmonary rehabilitation in COPD treatment, especially for the supervised practice of physical activity.

Our study presents a limitation that should be addressed: the anthropometric characteristics of volunteers should be discussed. It was not possible to maintain the same BMI between the COPD and control groups. COPD is a systemic disease, the increase in inflammatory mediators may contribute to an increased metabolism and imbalance between food intake and energy expenditure, leading to the weight loss observed in these individuals,1,3 reflected by lower BMI in the COPD group. It is well recognized that obesity causes changes in cardiac autonomic modulation.30,39 On the other hand, the effects of the reduction of BMI on HRV in patients with COPD are not known. Further studies evaluating autonomic modulation in COPD and controls are needed.

Another limitation of this study was the absence of specific clinical tests to evaluate the cardiac condition of the volunteers. However, all these volunteers were evaluated by doctors to establish the diagnosis and we understand that if there were any cardiac disease, the volunteer would report this condition. As mentioned above, when some health problem was reported the subject was not included in the sample.

In summary, the analysis by means of geometric indexes suggests that COPD individuals present autonomic dysfunction, characterized by reductions of heart rate variability. Our findings are relevant, since the results indicate that these indexes, obtained by a non-invasive and inexpensive method, may be useful both for clinical manifestations of systemic disease and for risk stratification and monitoring of therapeutic procedures performed with these patients.

In conclusion, geometric indexes of HRV in COPD individuals were reduced compared to control subjects. Therefore, we indicate that COPD patients present reduced HRV.

The authors confirm that there are not conflict of interest to declare.