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Vector magnitude derived QT variability following myocardial ischemia

Myocardial ischemia alters ventricular repolarization process through a variety of mechanisms. Studies show that QTVI, an index of QT variability, changes during acute myocardial ischemia and cardiomyopathy. Preliminary studies from our laboratory show that the length of the QT interval varies by less than 14 ms in healthy subjects and QT interval frequently varies by an amount that exceeds 14 ms in ischemia. The purpose of this study was to test the hypothesis that myocardial ischemia causes the length of resting QT intervals to vary more than 14 ms. Data were obtained from the PTB database (www.physionet.org). Exclusion criteria were subjects with frequent ectopic beats or arrhythmias. The Frank X, Y, Z leads were used to calculate vector magnitude. The QT interval was determined from the vector magnitude channel. Results from 20 healthy and 20 acute myocardial ischemia subjects support our preliminary studies. Specifically, QT intervals for healthy subjects fell within a 14 ms window and QT intervals for MI patients exceeded the 14 ms window. Furthermore, our algorithm was able to differentiate between the healthy controls and acute MI patients using as few as 20 consecutive beats. Arrhythmias and/or ectopic beats complicate interpretation of healthy subjects. These findings may provide an alternative index to assess ventricular repolarization during myocardial ischemia.