Effects of β‐Blocker Therapy on the Dynamic QT/RR Relation in Patients with Long QT Syndrome During 24‐Hour Holter ECG Monitoring

Objective: Patients with long QT syndrome (LQTS) have an abnormality in the dynamic relationship between the QT and RR intervals (QT/RR relation) assessed by 24‐hour Holter ECG monitoring. The clinical efficacy of β‐blocker therapy in LQTS patients has been established, but its antiarrhythmic mechanism has not been fully elucidated. In the present study, the influences of β‐blocker therapy on the QT/RR relation were investigated in LQTS patients with no arrhythmic event during β‐blocker therapy. Methods: Holter ECG recordings before and after the therapy were obtained from seven LQTS patients (all female, age range 10–45 years). The QT/RR relation was analyzed by our original computer algorithm with automatic measurement of the Q‐aT interval (time interval from the QRS onset to the T wave apex. The correlation coefficient (r), and the slope of the best fit linear regression line between the Q‐aT and RR intervals (Q‐aT/RR slope) after β‐blocker therapy were compared with those before the therapy. To determine whether the dose of β‐blockers was sufficient for suppression of the sympathetic nervous activity, a frequency‐domain analysis of the heart rate variability was performed (low frequency power [LF]: 0.04–0.15 Hz, high frequency power [HF]: 0.15–0.40 Hz). Results: The LF component of the heart rate variability decreased and the HF component increased during the daytime after β‐blocker therapy. However, the Q‐aT/RR relation after β‐blocker therapy was the same as that before the therapy (mean slope before = 0.20 ± 0.07, mean slope after = 0.19 ± 0.08; P = not significant; mean r before = 0.82 ± 0.17; mean r after = 0.76 ± 0.15; P = not significant). Conclusion: β‐Blocker therapy did not affect the unique Q‐aT/RR relation, which reflects an intrinsic abnormality of the ventricular repolarization in LQTS patients.