Association Between Cardiorespiratory Fitness and a Composite Index of Cardiac Autonomic Control: Potential Implications for Exercise Training Evaluation

Background/Objectives: Cardiorespiratory fitness (CRF), measured as VO₂peak during cardiopulmonary exercise testing (CPX), represents a fundamental marker of cardiovascular health. Heart Rate Variability (HRV) assesses cardiac autonomic regulation (CAR) non-invasively, though technical issues limit clinical use. Combining CRF and CAR may provide complementary information on cardiovascular and autonomic function. We examined the relationship between CRF and HRV using a composite autonomic index, the Autonomic Nervous System Index (ANSI), which incorporates multiple HRV variables accounting for age/sex-related influences. Methods: This cross-sectional study involved 200 adults undergoing preventive health evaluation without overt cardiometabolic disease. CRF was expressed as VO₂peak and as VO₂peak percentile from the FRIEND registry. CAR was assessed at rest using HRV. Results: We observed significant correlations between VO₂peak and several HRV variables, with ANSI showing the strongest association (r = 0.457, p < 0.001). In simple regressions, ANSI was linked to VO₂peak (R² = 0.175, p < 0.001) and VO₂peak percentile (R² = 0.209, p < 0.001). In multiple regression, several autonomic variables were retained for VO₂peak, whereas ANSI was the only autonomic variable retained for VO₂peak percentile. This association remained significant after adjustment for age, sex, smoking, and anthropometric measures. Trend analyses (p < 0.001) and ordinal regression analyses (p < 0.001) further substantiated these findings. A 10-point increase in ANSI corresponded to an OR of 1.39 for belonging to a higher CRF percentile category. Conclusions: Multiple HRV indices were associated with CRF, with ANSI showing the strongest association. These findings highlight the potential of ANSI as a simple, integrated measure of CAR in individuals with different levels of CRF.