Sudden cardiac death: The search for a non-invasive means to detect the electrical substrate for the development of life-threatening cardiac arrhythmias

Cardiovascular diseases remain the major cause of death in the adult North American population. Most of these deaths are sudden, occurring secondary to ventricular tachycardia/fibrillation (VT/VF). Based on the results of recent clinical trials, it seems likely that many of these deaths could be prevented if reliable means were available to select those patients at highest risk. To date, however, no totally satisfactory means to establish risk has been identified. The purpose of this paper is to review the currently utilized techniques and to draw attention to some new and potentially useful technology involving computer processing of surface recorded electrocardiographic (ECG) signals. ECG monitoring during rest and activity is neither sensitive nor specific. Invasive studies, using programmed ventricular stimulation to reproduce clinical arrhythmias, have proven extremely useful in the management of patients with recurrent VT/VF; this technique allows the selection of effective therapy as documented by subsequent inability to induce the arrhythmia. However, this technique does not have broad applicability given the complexity of these studies and the significant potential for morbidity. Recent efforts have focused on the detection of the abnormal electrical substrate giving rise to VT/VF by using body-surface electrocardiographic recording. Two approaches are being investigated. In one, high-gain, signal-averaged ECG signals are being utilized to detect (delayed) depolarization potentials occurring during the repolarization process - an electrophysiologic state known to give rise to re-entrant arrhythmias. Initial results with this technique are encouraging. We have focused on another approach - that of body-surface potential mapping. Preliminary data indicate that patients at high risk of VT/VF have abnormal surface maps believed to reflect heterogeneity of recovery properties. Detailed analysis of such maps leads us to believe that the abnormal patterns also reflect the presence of low level potentials during the early phase of repolarization. Thus, surface recording techniques promise to provide a means to select patients at high risk of developing VT/VF. If this approach proves valuable in determining the effectiveness of a given therapy, these techniques could have major implications to our ability to deal with the problem of sudden cardiac death.