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Contribution of Myocardium Responsible for Ventricular Tachycardia to Abnormalities Detected by Analysis of Signal‐Averaged ECGs

BackgroundCurrent methods of signal-averaged ECG analysis interrogate the terminal 40 msec of the QRS complex and/or the ST segment and have a low positive-predictive accuracy for detecting vulnerability to sustained ventricular tachycardia (VT). The extent to which abnormalities detected during these ECG intervals are generated by myocardial tissue responsible for VT has not been well defined. The purpose of this study was to determine when, during sinus rhythm, myocardium responsible for VT is activated. Methods and ResultsThree-dimensional ventricular activation maps were analyzed during sinus rhythm and during 10 VTs in eight patients with healed myocardial infarctions undergoing arrhythmia surgery for sustained monomorphic VT. The mechanism of VT was focal in five instances and macroreentrant in five. During sinus beats, myocardium responsible for all focal VTs activated 43 ± 38 msec before the onset of the terminal 40-msec interval of the QRS complex. During sinus rhythm, activation of the myocardium critical to macroreentrant VT began 72 ± 13 msec before the onset of the terminal QRS interval and in only three instances extended 2-25 msec into the terminal 40 msec of the QRS complex. Electrograms recorded during the ST segment represented late activation of epicardial sites overlying zones of infarction that were temporally and spatially remote from tissue critical to VT. ConclusionsCurrent methods of signal-averaged ECG analysis limiting interrogation to the terminal QRS/ST segment exclude detection of >95% of the signals generated by myocardium responsible for sustained VT. These results establish a pathophysiological basis for expanding signal-averaged ECG analysis to include more of the cardiac cycle.