Cardiac electrical activity is a complex process that is both time dependent and spatially distributed throughout the heart. Standard ECG and multi-electrode body-surface potential mapping (BSPM) only record a very low resolution projection of this activity on the torso surface. Therefore, locations of cardiac events (e.g. activation initiation sites; regions of conduction slowing or block) and spatial details (e.g. number and locations of activation fronts; the reentry pathway underlying an arrhythmia) cannot be determined from such measurements. As a result, invasive electrophysiological mapping from the epicardial and endocardial surfaces of the heart has become an important experimental tool in the study of cardiac excitation and arrhythmogensis. It has also become an essential clinical tool for diagnosis of arrhythmias, guidance of intervention (e.g., ablation) and evaluation of treatment outcome.
From the experimental perspective, most of our knowledge of arrhythmia mechanisms stems from animal models, which are designed to resemble disease states in humans. For example, arrhythmias associated with myocardial infarction (MI) have been studied in a canine model, in which MI was produced by 5-day ligation of the left anterior descending coronary artery (LAD). While the benefit of such animal models cannot be overestimated, it is human electrophysiology that we are trying to understand, and human arrhythmias that we are trying to prevent and cure. The arrhythmogenic substrate in the human heart can vary considerably from that in animal models. For example, the 5-day canine infarct is probably very different in its characteristics from an infarct in the elderly patient with 20 years history of coronary disease. It is essential, therefore, to characterize the electrophysiological substrates and to study the mechanisms of arrhythmias in the human heart.
Electrocardiographic imaging (ECGI) is a novel, noninvasive tool for imaging cardiac arrhythmia and defining electrophysiologic properties. ECGI combines multi-electrode body surface ECG recordings with three-dimensional anatomical heart-torso imaging to reconstruct an epicardial electroanatomic map. ECGI images can be presented as epicardial potential maps, electrograms, isochrones (activation sequences), or repolarization patterns during activation and repolarization of the heart.
Until the recent development of noninvasive ECGI in our laboratory, studies of human cardiac electrophysiology involved invasive cardiac mapping and were, consequently, very limited. With the successful application and validation of ECGI in humans, we can now begin to study noninvasively the electrophysiology of the human heart, its modification by disease processes and the associated arrhythmias. From the clinical perspective, ECGI has potential applications in the following: (1) screening people with genetic predisposition (e.g. the long QT syndrome or Brugada syndrome) or with altered myocardial substrate (e.g. post-MI or cardiomyopathy) for risk of life-threatening arrhythmias, in order to take prophylactic measures; (2) specific diagnosis of arrhythmia mechanism to determine most suitable interventions; (3) determination of cardiac location for optimal localized intervention (ablation, pacing, targeted drug delivery or targeted gene transfer); (4) evaluation of efficacy and guidance of therapy over time.