Most diagnostic methods used in clinical electrocardiography (ECG) are statistically based forms of pattern recognition. Over the years, the selection of the signal features used in these methods, such as timing, amplitude, and duration of wavelets, has been guided by knowledge gathered through linking clinical observations with ECG waveforms, along with insight gained from invasive electrophysiology. These methods have reached a diagnostic accuracy of up to 90% in some categories of cardiac disease. However, in other categories, the performance is much lower. Moreover, the manifestation in the ECG of some types of abnormality remains poorly understood. Examples of these problematic domains are the diagnosis of left ventricular hypertrophy, the interpretation of ST changes during acute ischemia, the electric manifestation of the Brugada syndrome, and the long QT syndrome. What these examples have in common is that the major features that play a role in the related ECG analysis are their waveforms, which are the result of the electric depolarization and repolarization processes of the membranes of cardiac myocytes, rather than rhythm abnormalities. Since Einthoven's day (late nineteenth century/early twentieth century),1,2 the development of diagnostic ECG criteria has been accompanied by the development of biophysical models aimed at linking the electrophysiology of cardiac function with the waveforms of the ECG signals observed on the body surface. In such an approach, two aspects of the bioelectric generator need to be specified—a source model of the cardiac electric activity, and a volume conductor model, which is a model for describing the passive effects on the observed data of the body tissues that surround the active electric sources.
This chapter focuses on these model-based explanations of ECG morphology, in particular on the ECG components that reflect the electric activity of the ventricles. It does not include an analysis of cardiac rhythm. The emphasis of the approach lies on the ventricular activity. However, it applies in a similar manner to the electric activity of the atria.
The topic is illustrated by simulations from a computer program ECGSIM,3 which allows the user to interactively change the major source parameters: the timing of depolarization and repolarization on the ventricular surface and the local source strength. The program is available, free of charge, from the Web site http://www.ecgsim.org. The program was designed specifically to serve both as a tool in teaching the basic link between electrophysiology and ECG morphology and as an instrument in research applications.
In ECGSIM, the expression of the cardiac sources in terms of electric potential fields and signals (the handling of the so-called forward problem) is worked out in a realistic volume conductor model that accounts for the spreading out of the electric currents in a three-dimensional (3D) representation of the tissues surrounding the heart. Images are presented of the distribution of these parameter values over the ventricular surface as well as those of the accompanying potentials on the ventricular surface and on the body ...