Heart rate variability during constant work rate exercise at and above the critical power in patients with severe chronic obstructive pulmonary disease

En-Ting Chang^{a, b}, David Silberstein^b, Mehdi Rambod^b, Janos Porszasz^b, Richard Casaburi^b

^a Department of Chest Medicine, Buddhist Tzu Chi General Hospital, Hualien, Taiwan
^b Rehabilitation Clinical Trials Center, Los Angeles Biomedical Research Institute at Harbor, UCLA Medical Center, Torrance, CA, USA

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Abstract
Objective

Heart rate variability is useful in evaluating cardiac autonomic balance. We investigated cardiac autonomic modulation in patients with severe chronic obstructive pulmonary disease (COPD) during heavy and very heavy exercise at and above the critical power (CP).

Materials and Methods

Nine nonhypoxemic patients with COPD (mean forced expiratory volume in 1 second: 1.2 ± 0.5 L; forced expiratory volume in 1 second/forced vital capacity: 37.6 ± 12%) completed an initial incremental exercise test on a cycle ergometer followed by four constant work rate tests to define their CP. In addition, patients underwent a test at their calculated CP for a target of 20 minutes. The CP test plus a test above CP of sufficient duration to allow heart rate variability analysis were studied further. In these tests, R-R interval data were analyzed for variability in the frequency domain before and during exercise.

Results

Compared with rest, the power of the normalized low-frequency component (nuLF) decreased (73.0 ± 20.9 vs. 54.2 ± 20.1; p = 0.01) and the power of normalized high-frequency component (nuHF) increased (27.0 ± 20.9 vs. 45.8 ± 20.1; p = 0.01) during exercise, resulting in a decrease in LF/HF (4.1 ± 2.3 vs. 1.6 ± 1.2; p = 0.01) suggesting a relative increase in parasympathetic tone during exercise. When compared with exercise at CP, exercise above CP featured significantly higher values for the square root of the mean squared differences of the successive R-R intervals, the baseline width of the minimum square difference triangular interpolation of the highest peak of the histogram of all N-N intervals and the nuHF, as well as significantly lower values for the nuLF and LF/HF. During exercise above and at the CP, both LF and HF negatively correlated with the respiratory rate (r = −0.76 and r = −0.70, p < 0.005) and the Vmax/PEF (ratio of end-exercise peak expiratory flow and peak expiratory flow during a forced maneuver at rest) (r = −0.52 and r = −0.49, p < 0.05).

Conclusion

The decrease in nuLF and LF/HF with increasing nuHF in very heavy exercise domains suggests that the autonomic balance is shifted toward parasympathetic tone dominance in patients with severe COPD. This correlates with increasing respiratory rate and ventilatory effort (Vmax/PEF) during heavy and very heavy exercise in COPD patients.

Keywords
Cardiopulmonary exercise test; Chronic obstructive pulmonary disease; Critical power; Heart rate variability