Heart Rate Influences the Relationship Between Heart Rate Variability and Ventricular Repolarization in Children: A 24-h Holter Study

Background/Objectives: Heart rate variability (HRV) and ventricular repolarization are key noninvasive markers of cardiac autonomic regulation and electrical stability. Their physiological relationship in healthy pediatric populations remains incompletely understood. This study evaluated the association between HRV parameters and ventricular repolarization indices and established age-specific reference values for HRV. Methods: This cross-sectional study included 254 healthy children (5–18 years) who underwent 24 h Holter monitoring. HRV parameters (SDNN, SDANN, RMSSD, pNN50, LF, HF, LF/HF ratio) and repolarization indices (QT, QTc, QTd, QTcd) were analyzed. Age-stratified percentile distributions were generated, and multivariable analyses were performed, adjusting for age, sex, and heart rate. Results: All HRV parameters increased significantly with age (p < 0.001), accompanied by a decrease in heart rate. In unadjusted analyses, the QT interval showed positive correlations with HRV indices, particularly SDNN (r = 0.434, p < 0.001). However, these associations were no longer significant after adjustment. QTd remained modestly associated with SDNN, whereas QTc showed no association with HRV; sex was the only independent predictor of QTc. Conclusion: In healthy children, the apparent association between HRV and the QT interval appears to be largely explained by heart rate rather than by an independent physiological relationship. These findings highlight the importance of considering heart rate when interpreting associations between autonomic and ventricular repolarization parameters in pediatric studies. Age-specific HRV reference values may support the clinical interpretation of pediatric autonomic and electrophysiological assessments.