Adaptive Capacities and Complexity of Heart Rate Variability in Patients With Chronic Obstructive Pulmonary Disease Throughout Pulmonary Rehabilitation

Abstract

Introduction: The complexity of bio-signals, like R-R intervals, is considered a reflection of the organism's capacity to adapt. However, this association still remains to be consolidated. We investigated whether the complexity of R-R intervals at rest and during perturbation [6-minute walking test (6MWT)], yielded information regarding adaptive capacities in Chronic Obstructive Pulmonary Disease (COPD) patients during pulmonary rehabilitation (PR).

Methods: In total, 23 COPD patients (64 ± 8 years, with forced expiratory volume in 1 s of 55 ± 19% predicted) were tested three times at the start (T1), middle (T2), and end (T3) of 4 weeks PR. Each time, R-R intervals were measured at rest and during 6MWT. The complexity of R-R intervals was assessed by evenly spaced Detrended Fluctuations Analysis and evaluated by the fractal exponent α and deviation from maximal complexity |1-α|.

Results: The 6MWT distance was significantly increased at T2 and T3 compared to T1. Neither α nor |1-α| at rest and during perturbation significantly changed throughout PR, nor were they consistently associated with 6MWT distances at each time. Throughout the PR program, complexity during the 6MWT was significantly lower compared to the rest. The level of α during 6MWT at T1 was positively correlated with the improvement of the 6MWT distance throughout the PR program.

Discussion: Reduced complexity in COPD patients during acute perturbation at the beginning of PR supports a decreased improvement of the 6MWT distance throughout PR. This result seems consistent with the notion that the complexity reflects the patients' adaptive capacities and could therefore become a clinical indicator in an applied perspective.

Keywords: 6-minute walking test; chronic disease; exercise capacities; fractal; perturbation; resilience.

FIGURE 1

Dot plot of individual distances, mean heart rates, dyspnea levels during the 6MWT, and MFI20 scores throughout the PR program. (A) 6MWT distance (m); (B) mean heart rate (beats.min^–1); (C) dyspnea level; (D) MFI-20 score; (n = 23 for A–D). Mean and standard deviation are represented by a dot and a line both in gray. Repeated ANOVA revealed a significant difference for the 6MWT distance, mean heart rate and perceived fatigue throughout the PR program [F(2,44) = 38.2, p < 0.05, η² = 0.11; F(2,44) = 5.11, p < 0.05, η² = 0.2; and F (2,44) = 23.74, p < 0.001, η² = 0.21, respectively]. Post hoc analyses revealed that 6MWT distances at T3 and T2 were significantly higher compared to T1 (p < 0.001), mean heart rate during the 6MWT at T3 was significantly higher compared to T1 (p < 0.001) and MFI-20 scores at T3 (45.04 ± 10.5) and T2 (45.9 ± 11.7) were significantly lower compared to T1 (58.3 ± 13.6, p < 0.001).

FIGURE 2

Box plot of R-R interval fractal exponent values during PR. (A) Evolution of the level of complexity (α) at rest (blue dot) and during 6MWT (yellow triangle) throughout the PR program (n = 23); (B) evolution of the deviation from maximal complexity (|1-α|) at rest (blue dot) and during the 6MWT (yellow triangle) throughout the PR program (n = 23). In each boxplot, the data are represented from bottom to top by the minimum value, the first quartile, the median, the third quartile, and the maximum value. A two-way ANOVA revealed a significant difference between complexity at rest and during the 6MWT for the two fractal exponents [F(1,21) = 0.09, p = 0.91, η² = 0.001 and F(1,21) = 2.51, p = 0.09, η² = 0.03, respectively].

FIGURE 3

Correlations between fractal exponents at T1 and the evolution of 6MWT distances throughout the PR program. (A) Level of complexity (α) at rest; (B) level of complexity during the 6MWT; (C) deviation from maximal complexity (|1-α|) at rest; (D) deviation from maximal complexity during the 6MWT (n = 23 for A–D). Dashed lines represent the regression lines, with confidence intervals in gray. Pearson correlation tests revealed one significant positive correlation between the exponent α of the R-R interval series during the 6MWT at T1 and the improvement in 6MWT distance over PR (r = 0.47, p < 0.05).