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.