Physiological Characterisation of Severe Heart Failure Using Long-Duration Ambulatory ECG: A Retrospective Exploratory Analysis

BACKGROUND: Heart failure is associated with progressive autonomic and cardiorespiratory dysregulation that is incompletely captured by episodic clinical assessment. Continuous ambulatory ECG offers a unique opportunity to describe longitudinal physiological dynamics, particularly in patients with advanced disease. AIM: The aim of this study is to provide a descriptive and exploratory physiological characterisation of cardiac and respiratory dynamics in patients with severe heart failure using long-duration ambulatory ECG recordings. METHODS: This retrospective secondary analysis used the publicly available Beth Israel Deaconess Medical Center (BIDMC) Congestive Heart Failure database, comprising approximately 20 hours of two-channel ambulatory ECG recordings from 15 patients with NYHA class III-IV heart failure. Automated ECG annotations were used to derive heart rate (HR), time- and frequency-domain heart rate variability (HRV), and ECG-derived respiration (EDR). Analyses focused on inter-individual variability and within-subject temporal patterns using descriptive statistics and longitudinal visualisation. RESULTS: Marked heterogeneity was observed across subjects in mean HR and HRV metrics, with generally depressed HRV consistent with advanced autonomic dysfunction. Longitudinal analyses revealed substantial within-subject variability and temporal trends in HR and EDR across recording periods, including systematic differences between early and late segments. HR and HRV showed partial dissociation across individuals, underscoring the limitations of single-parameter assessment. CONCLUSION: Long-duration ambulatory ECG enables rich descriptive characterisation of cardiac and respiratory physiology in severe heart failure. These findings are exploratory and hypothesis-generating, providing a physiological foundation for future studies that integrate clinical covariates, outcomes, and contemporary wearable technologies.