ECG Sex Index in children and adolescents.

The electrocardiographic sex index (ESI) was developed in adults to capture continuous electrocardiogram (ECG) features associated with biological sex, but its behaviour in children, whose ECG morphology changes substantially through growth and puberty, has not been characterized. We applied the adult-derived ESI model to 61 930 paediatric ECGs from the University of Tennessee Health Science Center clinical population (ages 0-18 years). ESI values were examined by single-year age and stratified by sex and race. Age-related patterns were summarized using mean estimates with 95% confidence intervals, distributional analyses across broader age groups, and the prevalence of extreme sex-typical values (ESI ≥0.8 in males; ≤0.2 in females). In early childhood, ESI values were tightly clustered around 0.5 in both sexes. Beginning in late childhood and becoming more pronounced through adolescence, ESI increased in males and decreased in females, producing a widening separation that plateaued in mid-to-late adolescence, a pattern that was consistent across racial subgroups. Distributional analyses showed parallel shifts in ESI distributions, with older males enriched for higher values and older females enriched for lower values, and threshold-based analyses revealed similar divergence, with the proportions of males with ESI ≥0.8 and females with ESI ≤0.2 both increasing sharply during adolescence. Although derived from adult data, ESI thus exhibits age and sex-dependent patterns in children that align with known developmental trajectories of ECG morphology, indicating that ESI tracks physiological maturation across childhood and adolescence and providing a foundation for evaluating its clinical and biological significance in younger populations.