Measurement of QT Interval Dispersion Hanne Elming, 1 Li Jun, 1 Christian Torp-Pedersen, 1 Lars Køber, 1 and Marek Malik 2 1 Gentofte University Hospital, Dept. of Cardiology, Copenhagen, Denmark, 2 St. George’s Hospital, Cardiological Sciences, London, England Invasive studies have demonstrated that regional dif- ferences in ventricular recovery time are re_ected as differences in QT intervals in leads that correspond to different parts of the myocardium [1,2]. This heteroge- neity is called QT interval dispersion [3]. Increased QT interval dispersion decreases the threshold for ven- tricular tachycardia and is associated with a risk of ventricular arrhythmias [4]. At one time conceived as a recording artefact [5], QT interval dispersion is cur- rently being evaluated for its potential as a non-inva- sive and inexpensive marker of arrhythmia risk. Vari- ous investigations have shown that increased QT interval dispersion is associated with increased mortal- ity in relation to anti-arrhythmic drug therapy [6], in cardiomyopathy [7,8], in diabetes mellitus [9], and in the long QT syndrome [10,11]. Particular interest has been focused on ischemic syndromes; many investiga- tors found [12–14] and a few failed to ~nd [15,16] the QT interval dispersion having a prognostic value in pa- tients surviving an myocardial infarction. There are major methodological problems in meas- uring the QT interval dispersion resulting in low repro- ducibility of results from the same investigators. These problems are mainly related to dif~culties in de~ning the end of the T waves. In this article we will describe some of the methodological problems, how the QT in- terval dispersion is currently measured, and some of the alternative approaches. Measurement of QT Interval Nearly all investigations dealing with the QT interval measurement state that the QT interval is measured from the onset of the QRS complex to the end of the T wave. T end is de~ned as the point where the T wave returns to the iso-electric line. In case of visible U wave, T end is de~ned as the nadir between the T wave and the U wave. Great methodological problems are ignored in such statements. First, does the QT interval re_ect the ventricular recovery time? Lepeschin stated in the 1950s [5] that both the beginning and the end of the QT interval could be iso-electric as a result of “silent repolarisation” (cancelled potential differ- ences resulting in a iso-electric line). However, inva- sive animal studies [17] have shown that the QT inter- val in the surface ECG is slightly shorter than ven- tricular recovery time; and it is therefore assumed that, in general, no “silent” repolarisation occurs. Nev- ertheless, as a second problem, the end of the T wave is very dif~cult to de~ne in many ECG leads. One prob- lem is a _at T wave that is either not visible or is so _at that its determination is undetectable. For this reason, leads that might re_ect a ventricular region with a potentially very long or very short duration of recov- ery time may be excluded from measurement. Third, other practical problems arise in the presence of an U wave. It can be impossible to detect whether or not a U wave is hidden in the T wave. A false high dispersion can be measured from an ECG if the T waves and the U waves are clearly separated in some leads but not in others (Figure 1). In light of the problems in deciding where the T wave ends, some investigators have used more easily determined measurement points, such as T peak or the point where the maximal slope of the descending part of the T wave joins the iso-electric line. These measure- ment points do not correlate to any known physiologi- cal event of interest, but are used in the hope of a relation to “dispersion” and because they can be deter- mined more precisely. Unfortunately, T peak can be just as impossible to de~ne in _at T waves as T end, and furthermore, Kautzner et al. [18] found poor correla- tion btween QT end and QT peak measurements. Current De~nitions of QT Interval Dispersion In most clinical studies, QT interval dispersion is de~ned as the maximum difference between the QT interval in any 2 of the 12 leads from a standard ECG (QT max - QT min ). As more leads are included in the measurements, more different areas of the myocar- dium are represented in the measurement value; pos- sibly right-sided and posterior leads should even be Address correspondence to: Hanne Elming, Dept. of Cardiology, Gentofte University Hospital, Niels Andersensvej 65, DK-29000 Hellerup, Denmark. 372 Cardiac Electrophysiology Review 1997;3:372–376 © Kluwer Academic Publishers. Boston. Printed in U.S.A. PIPS# 140999