Electrocardiographic Abnormalities Across Body Mass Index Categories Among 6-Year-Old Mongolian Children: Findings from a Nationwide School-Entry Screening Program

Background/Objectives: Childhood obesity is an important public health concern that may be linked to early cardiovascular alterations. Electrocardiography (ECG) is a simple, non-invasive method for detecting cardiac abnormalities, but evidence on the association between body mass index (BMI) and ECG abnormalities in school-aged children remains limited. We aimed to determine the prevalence of ECG abnormalities and their association with BMI category among Mongolian school-entry children. Methods: We conducted a nationwide, school-based cross-sectional study using data from Mongolia’s school-entry health screening program (2024–2025). Of 70,214 children screened, 60,847 had sufficient anthropometric data for BMI classification, of whom 57,401 also had valid ECG outcome data and formed the final analytic sample. Children were classified as underweight, normal weight, overweight, or obese using WHO age- and sex-specific BMI-for-age percentiles. Overall ECG abnormality and six specific findings were assessed. Associations with BMI category were evaluated using chi-square tests and multivariable logistic regression adjusted for sex and grade level, with obesity as the reference category. Holm–Bonferroni correction was applied across the six secondary outcomes. Results: Among 57,401 children, 22.2% had at least one ECG abnormality, with similar prevalence across BMI categories (21.6–22.4%; χ2 = 2.548, p = 0.467). Electrical axis abnormality was the most common specific finding (15.5%). BMI category showed weak overall evidence of association with overall ECG abnormality (global Wald χ2 = 7.91, p = 0.048), but no individual category differed significantly from obesity. Electrical axis abnormality showed a nominal overall association with BMI category (p = 0.017), but this did not remain significant after Holm correction (Holm-adjusted p = 0.102). No other specific ECG abnormality was associated with BMI category (all Holm-adjusted p ≥ 0.335). Conclusions: ECG abnormalities were common among Mongolian school-entry children, but BMI category showed no robust association with them once multiple testing was accounted for. These findings do not support BMI alone as a predictor of ECG abnormality at school-entry age.