Detection of exercise intensity thresholds in patients with chronic heart failure based on correlation properties of heart rate variability

Abstract

Purpose: This study aimed to assess the agreement between ventilatory thresholds (VT1 and VT2), and heart rate variability (HRV) thresholds (HRVT1 and HRVT2) based on the alpha 1 index of detrended fluctuation analysis (DFA a1) in patients with chronic heart failure (CHF). Validating HRV-based thresholds could provide a cost-effective alternative for individualised exercise intensity prescription, improving safety and efficacy in exercise-based cardiac rehabilitation (CR) programmes. Methods: Twenty CHF patients (13 males, 7 females) performed a cardiopulmonary exercise test (CPET) on a cycle ergometer. Ventilatory thresholds were identified using a mixed method, while HRV thresholds were determined at DFA a1 values of 0.75 (HRVT1) and 0.5 (HRVT2). Threshold values for oxygen consumption (VO2), heart rate (HR), and power output (PO) were compared with paired t test or Wilcoxon test. Agreement was assessed using correlation coefficients (Pearson’s r and Spearman’s rho), intraclass correlation coefficient (ICC), and Bland–Altman analysis. Results: HRVT2 showed moderate-to-strong associations with VT2 for VO2 (rho = 0.88, ICC = 0.86), for HR (r = 0.88, ICC = 0.81) and for PO (r = 0.82, ICC = 0.85). Mean biases were small and limits of agreement (LoA) narrow. HRVT1 correlated only modestly with VT1 for VO2 (rho = 0.67, ICC = 0.43) and weakly for HR (r = 0.43, ICC = 0.37) and PO (r = 0.49, ICC = 0.35), with wide LoA. Conclusion: In CHF patients, HRVT2 appears to be a valid, practical surrogate for VT2 and may facilitate personalised intensity prescription where full CPET is unavailable. HRVT1 showed insufficient agreement with VT1 and should be used with caution. Larger cohorts and protocol refinements are warranted to confirm these observations and to explore strategies for improving HRVT1 accuracy.