Circadian Variation of Beat‐to‐Beat QT Interval Variability in Patients with Prior Myocardial Infarction and the Effect of β‐Blocker Therapy

BACKGROUND: Recent studies have demonstrated that increased QT interval variability (QTV) is associated with a greater susceptibility to ventricular arrhythmias and that patients with prior myocardial infarction (MI) were prone to ventricular arrhythmias during the daytime. The goal of the present study was to investigate the circadian variation of the QTV and to determine whether beta-blocker therapy improves the temporal fluctuation of the ventricular repolarization in patients with MI. METHODS: The study population consisted of 15 MI patients who had not received beta-blocker therapy, 11 MI patients who had received beta-blocker therapy, and 12 healthy subjects. Twenty-four hour Holter monitoring was obtained, and the RR and QT intervals were calculated automatically from 512 consecutive sinus beats for every 2 hours. RESULTS: In the daytime, the QT-SD was significantly greater in the MI group than in the healthy subjects (P<0.01), but there was no difference in the QT-SD when comparing the beta-blocker group to the control group. Moreover, the QT variability index and the QT variance normalized for the mean QT were similar pattern with QT-SD. The heart rate variability did not significantly differ when compared between the three study groups. CONCLUSION: These data indicate that the QTV increases during the daytime in patients with MI and that this circadian effect is prevented by beta-blocker therapy. Thus, beta-blocker therapy may reverse the maladaptation of the ventricular repolarization to the change in the heart rate and may thereby reduce the ventricular arrhythmias and decrease the mortality in patients with MI.