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.