Heart rate variability (HRV) measures provide valuable insights into physiology, however, gaps remain in understanding circadian patterns in heart rate dynamics. We aimed to explore day-night differences in heart rate dynamics in chronic cardiopulmonary patients compared to healthy controls.

Using 24-hour heart rate data from patients with chronic obstructive pulmonary disease (COPD) and/or heart failure (HF) (n=16) and healthy adult controls (older group, ≥50yrs, n=42; younger group, 20-49 yrs, n=136), we compared day-night differences in conventional time and frequency domain HRV indices and a multiscale-entropy-based complexity index (CI1-20) of HRV among the three groups.

24-hour HRV showed significant day-night differences (marked with “△”) among younger healthy (mean age: 34.5 years), older healthy (mean age: 61.6 years), and cardiopulmonary patients (mean age: 68.4 years), including change in percentage of adjacent intervals that differ > 50 ms (△pNN50), high frequency (△HF), normalized low frequency (△nLF), ratio (△LF/HF), and △CI1-20. Among these, △LF/HF (2.13±2.35 vs 1.1±2.47 vs -0.35±1.25, p<0.001) and △CI1-20 (0.15±0.24 vs 0.02±0.28 vs -0.21±0.27, p<0.001) were significant in each pairwise comparison following ANOVA tests. Average CI1-20 was highest in younger healthy individuals and lowest in cardiopulmonary patients (1.37±0.12 vs 1.01±0.27, p<0.001). Younger healthy subjects showed a heart rate complexity dipping pattern (night<day), older healthy subjects showed non-dipping, and cardiopulmonary patients showed reverse dipping (night>day).

As measures of 24-hour variability, traditional and complexity-based metrics of HRV exhibit large day-night differences in healthy individuals; these differences are blunted, or even reversed, in subjects with cardiopulmonary pathology. Measures of diurnal dynamics may be useful indices of reduced adaptive capacity in patients with cardiopulmonary conditions.