13th International Congress
THE "NEW FRONTIERS"
OF ARRHYTHMIAS 1998

January 24-31, 1998
Marilleva, Trento, Italy

RT-172

Heart rate dependency of ventricular repolarization in congenital and acquired long QT syndrome: long-term analysis by Holter monitoring

Emanuela H. Locati.
Division of Cardiology, Department of Clinical Medicine, Pathology and Pharmacology, University of Perugia, Italy, Heart Research, Department of Cardiology, University of Rochester, Rochester, USA

Short- and long-term analysis of ventricular repolarization dynamics

Prolonged ventricular repolarization, with abnormal configuration and increased dispersion of QT interval duration, has been associated with an increased risk of malignant arrhythmias in congenital and acquired conditions^1,2. The QT interval is modulated by multiple factors, such as heart rate level, circadian rhythm and autonomic nervous system activity, that cannot fully be evaluated by a brief ECG tracing obtained in basal conditions³.

The long-term modulation of QT interval by heart rate changes can now be explored by new computerized analyses of the 24-hour ECG Holter recordings, and some programs are already available for routine clinical use. However, the automatic measurement of QT interval by Holter techniques has several methodological problems, due to the relatively low sampling rate, the shifting of the isoelectric baseline, the low signal-to-noise ratio, and the difficult determination of the T wave end. To overcome such problems, it has been proposed to utilize the interval from Q wave onset to T wave apex (QTa), since in most cases T wave apex can be more accurately identified than T wave end (QTe)⁴. However, QTa cannot be considered equivalent to the total QT duration, particularly in prolonged QT syndromes, where the QT prolongation may affect specifically the terminal components of the T wave^2,5; in contrast, rate dependency of ventricular repolarization should be mainly located in the early phase, ie QTa⁴.

While beat-to-beat automatic measurements of QT duration are often unreliable, averaging procedures can produce a low-noise template adequate to obtain reliable automatic measurements of both QTa and QTe from Holter monitoring³. Robust measurements of both QTa and QTe from 30-second medianized beats, already available in commercial equipments, can be applied to identify long-term patterns in ventricular repolarization dynamics during ambulatory Holter monitoring. In contrast, beat-to-beat measurements, requiring higher quality tracings, can be applied to identify short-term fluctuations of QT duration and instantaneous changes of T wave morphology by sophisticated analysis in controlled conditions. Of note, new multi-programmable digital Holter recorders will have the flexibility to obtain both brief high quality recordings, to perform sophisticate short-term analyses, and standard ambulatory recordings for routine long-term analyses.