Abstract Body (Do not enter title and authors here): Background: Heart rate variability (HRV) is a sensitive marker of autonomic dysfunction and an independent predictor of mortality in cardiovascular disease (CVD). While structured exercise training is widely used in cardiac rehabilitation, its impact on HRV, particularly with respect to training characteristics, remains unclear. This systematic review examined the effectiveness of structured exercise on HRV in individuals with CVD.
Methods: PubMed, Embase, and CINAHL were screened through February 2025. Eligible studies included clinical trials assessing the effect of exercise training (≥6 weeks) on HRV in adults with CVD. Extracted data included demographics, training characteristics, and HRV acquisition methods. HRV outcomes were grouped as a) time-domain (RMSSD, SDNN) and b) frequency-domain measures (LF, HF). Three reviewers independently screened studies using Covidence and assessed risk of bias with RoB-2 and ROBINS-I tools. Meta-analyses were performed in R using a random effects model to calculate pooled standard mean differences (SMDs) and 95% confidence intervals (CIs).
Results: Twenty studies (1,131 participants; mean age 58.7±2.5 yrs; 89.1% male) were included. Diagnoses included coronary artery disease (n=387), coronary artery bypass grafting (n=300), chronic heart failure (n=203), and acute myocardial infarction (n=241). Most protocols were aerobic (n=16), lasting 6–24 weeks, and included ≥3 sessions/week (n=15), of 20–120 minutes. Intensity ranged from moderate to vigorous (n=9) or low to moderate (n=11). Sixty-five percent of studies used ECG (n=13) with supine recordings (n=14), ≥ 5 minutes (n=18). A general trend toward improved HRV was observed, especially in higher-intensity and longer-duration programs. Meta-analysis of six studies showed a small but statistically significant improvement in SDNN (SMD=0.2, 95%CI=0.08-0.31), suggesting its utility as a marker of autonomic adaptation. Other HRV indices showed no significant change. Methodological heterogeneity was observed in LF and HF outcomes (I² > 50%), HRV acquisition, and training protocols.
Conclusion: Structured exercise training may improve cardiac autonomic function in CVD, primarily through increases in SDNN. However, variability in exercise dosing and HRV methodology likely limits broader conclusions. Future research should use sex-stratified data, standardized exercise prescriptions, and uniform HRV protocols to improve understanding of its role as a prognostic marker.