Differential effects of non-selective and cardio-selective beta-blocker therapy on ECG parameters in long QT syndrome type 1

•Non-selective β-blockers markedly reduce Tpeak/end in LQT1 patients.•Cardio-selective β-blockers show no consistent effect on Tpeak/end.•The nsBB effect persists after exercise, independent of heart rate.•Tpeak/end shortening indicates decreased global repolarization heterogeneity and reduced arrhythmic risk.•Tpeak/end may serve as a novel ECG marker for β-blocker efficacy in LQT1.