Abstract

Objectives: To introduce a nomogram of the normal QT interval at various heart rates measured from 24 hour Holter ECG recordings in healthy subjects with respect to age and sex and to use the nomogram to characterise dynamic changes in QT interval in patients with idiopathic ventricular fibrillation (IVF) and the long QT syndrome (LQT).

Methods: The study group consisted of 422 subjects: 249 healthy men ranging in age from 21–88 years (mean (SD) 47 (20) years) and 173 healthy women ranging in age from 21–85 years (47 (19) years). In addition, seven men with IVF ranging in age from 33–53 years (43 (9) years) and five women with LQT ranging in age from 20–55 years (37 (14) years) were studied. For each subject, QT interval and heart rate were determined automatically from 24 hour Holter ECG digital data—namely, QT interval was measured from signal averaged ECG waves obtained by averaging consecutive sinus beats during each 15 second period over 24 hours. Data were grouped and averaged at an interval of 5 beats/min for heart rates ranging from 46–120 beats/min.

Results: In healthy subjects aged < 50 years and ⩾ 50 years QT intervals were longer in women than in men. QT intervals were longer in both men and women aged ⩾ 50 years than in ages < 50 years. From these findings a nomogram of QT interval at varying heart rates adjusted for age (younger group aged < 50 years or older group aged ⩾ 50 years) and sex was determined. In patients with IVF, QT intervals were significantly shorter at slower heart rates than normal values obtained from the nomogram. In patients with LQT, QT intervals were significantly longer at both faster and slower heart rates than normal values.

Conclusions: The nomogram of QT interval at varying heart rates adjusted for sex and age could be used to assess dynamic changes of QT interval of various pathological conditions. For example, patients with IVF had shorter QT interval at slower heart rates, a finding suggestive of arrhythmogenicity of this specific syndrome at night. Patients with LQT had prolonged QT interval at specific heart rate ranges depending on their genotype.