Significant Difference in Beat-to-Beat QT Interval Variability among Different Leads

QT interval on the surface electrocardiogram (ECG) reflects the time for repolarization of myocardium, and QTc prolongation is strongly associated with sudden cardiac death. Findings of studies that have examined the dispersion of QT interval in 12-lead ECGs strongly suggest that an increased QT dispersion between different leads is associated with an increased risk of cardiac mortality. Recent studies using novel techniques on beat-to-beat QT interval variability have also shown that the interval is both influenced by the autonomic nervous system and a predictor of sudden cardiac death. However, in these studies the variability of QT was quantified in a single lead, and thus is different from QT dispersion. The present study examined whether there was a significant difference between QT variability (detrended QT variance [QTv]) and Qtvi (index of QT interval variability corrected for mean QT squared divided by heart rate variability corrected for mean heart rate squared) in 17 data sets of continuous ECGs recorded in three channels (leads V(5), V(1), and V(3)) during ambulatory monitoring of ECG digitized at 1,000 Hz. The results showed a highly significant difference between QT variability measures (QTv and QTvi) (P < 0.0001) in two of three lead configurations that were used (V(5) and V(1) versus V(3)). This finding underscores the importance of using the same lead while recording ECG for the calculation of QT variability, though further investigations are clearly warranted before any definitive conclusion can be made. These findings should be evaluated in relation to the issue of local versus global cardiac repolarization of ventricular myocardium.