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Objectives: This study aimed to retrospectively assess cardiac autonomic activity in children with LQTS, considering genotype, symptoms, sex, age, and beta-blocker therapy (BB) and compare it to healthy controls.

Methods: Heart rate variability (HRV), using power spectrum analysis, was analyzed in 575 Holter recordings from 116 children with LQTS and in 69 healthy children. The data were categorized into four age-groups and four heart rate (HR) ranges.

Results: In LQT1 and LQT2, increasing HR corresponded to significantly lower low (LF) and high frequency (HF) compared to controls. Total power (PTOT) was lower in all LQT1 age-groups compared to controls at HR 120–140 bpm (1–15 years: p <.01; 15–18 years: p =.03). At HR 80–100, LQT1 patients aged 1–10 years had lower HF than LQT2 patients (1–5 years: p =.05; 5–10 years: p =.02), and LQT2 patients aged 15–18 years had lower HF than LQT1 patients (p <.01). Symptomatic patients aged 10–15 years had lower PTOT at HR 100–120 bpm than asymptomatic patients (p =.04). LQT1 girls aged 10–15 and 15–18 years had a lower PTOT (10–15 years: p =.04; 15–18 years: p =.02) than boys.

Conclusion: This study shows a correlation between HR and changes in HRV parameters. At higher HRs, LQTS patients generally had lower HRV values than controls, suggesting an abnormal autonomic response. These results may strengthen the link between physical activity and arrhythmias in LQTS.