Purpose: Chronic obstructive pulmonary disease (COPD) negatively impacts autonomic control of the heart rate, as assessed by time and frequency domains of heart rate variability (HRV) analysis. However, it is unknown whether symbolic dynamic analysis may identify cardiac autonomic impairment, and whether such nonlinear indices may be associated with disease severity, prognostic markers, perceived dyspnea and functional capacity in patients with COPD. The current study assessed cardiac autonomic modulation by symbolic analysis of HRV in patients with COPD compared with healthy controls.
Methods: We recruited 54 COPD patients and 20 healthy controls. The interval between two successive R-wave peaks was calculated in the resting supine position. HRV was analyzed using symbolic markers and Shannon entropy (SE). The six-minute walk test (6MWT) was applied in a 30-m corridor.
Results: We found a lower 6MWT distance in patients with COPD compared with healthy controls (p < 0.05). We found increased SE and decreased percentage of no variation patterns (0V%) in COPD patients compared with the control group (p = 0.001). Significant correlations were found between the percentage of one variation pattern (1V%) and the Medical Research Council dyspnea scale (r = 0.38, p = 0.01), BODE index (r = 0.38, p = 0.01), forced expiratory volume in the first second (FEV1) [L] (r = -0.44, p = 0.003) and FEV1 [%] (r = -0.35, p = 0.02). It was found that SE was inversely associated with 0V% (r = -0.87, p < 0.0001).
Conclusion: COPD patients present with depressed sympathetic modulation of HR and higher SE compared with healthy controls. This increased irregularity was inversely associated with 0V%. These results suggested that COPD patients seem to have a cardiac control shifted towards a parasympathetic predominance compared with controls. Symbolic dynamic and complexity index of HRV are related to disease severity, symptoms and functional impairment in these patients.
Keywords: Conditional entropy; Functional capacity; Respiratory disease; Symbolic analysis.