Introduction
Congenital long QT syndrome (LQTS) is a hereditary disorder characterized by prolonged QT interval and fatal ventricular arrhythmias.1,2 The clinical course and the treatment consideration in the congenital LQTS are genotype-specific. The most frequent types of LQTS are LQT1 and LQT2, caused by mutations in genes of the potassium channels. Cardiac events are often associated with a sympathetic response by physical stress in LQT1 patients, and beta-blockers are more effective than those in LQT2 patients.3,4 Therefore, the differential diagnosis between LQT1 and LQT2 is important but can be difficult with standard 12-lead ECG. The QT–RR relationship using Holter ECG recordings is a novel method for evaluating QT adaptation to the heart rate change, and it has been reported to be useful for detecting LQTS. Patients with LQTS showed an abnormal prolongation of the QT intervals at lower heart rate, resulting in a steeper QT/RR slope than in controls.5, 6 Furthermore, previous studies suggested that the heart rate dependence of QT interval was steeper in LQT2 than in LQT1, and QT intervals at slower heart rate were longer in LQT2 patients than those in LQT1 patients.7, 8 Therefore QT/RR relationship obtained from Holter monitoring may be useful for differential diagnosis between LQT1 and LQT2.
In the present study, we aimed to further evaluate the utility of QT/RR slope by 24-hour Holter monitoring by examining that separately at daytime and at night time for differential diagnosis between LQT1 and LQT2.