Dispersion of repolarization, myocardial iso-source maps, and the electrocardiographic T and U waves.

The surface potential at any given electrode location is the net result of simultaneously acting and variously directed electrical forces in the myocardium. The degree to which the electrical forces in the heart are thus opposing each other has been defined as cancellation, and this mechanism plays a major role in the formation of the electrocardiogram (ECG). However, previous studies did not take into account the locations of the electrical sources. In this study, we used a computer model of the left ventricle to study the effect of source locations on cancellation during the T and U waves. The model represents an anatomically stylized cross-sectional slice of the left ventricle, containing 1961 hexagonal cells in a single layer. An action potential (AP) is assigned to each cell. The timing of the APs follows a simulated excitation sequence. The potential differences between the APs of adjacent cells produce time-varying electrical sources, each of which contributes to the potential in an arbitrary point P on the body proportionally to its own, location-dependent, transfer function (lead vector). The ECG at P is the sum of all potential contributions. For each time point in the ECG at P, the contribution of each cell is mapped back onto the slice. Adjacent cells with equal contributions form iso-source strings, together forming iso-source maps. The T-U wave as observed in P will be the sum of positive and negative contributions from the iso-source distributions as they change with time. The iso-source maps for an anteriorly located observation point P at 4.2 cm from the epicardial surface show a continuous interplay of positive and negative contributions. During the near-zero ST segment, cancellation varies between 80% and 100%. In the ascending limb of the T wave, positive contributions substantially increase, giving a decrease in cancellation to about 40%. At the end of the T wave (with almost zero amplitude), the positive contributions are only slightly reduced as compared with those at peak T, but greatly increased negative contributions cancel them out. This is contrary to the generally held view that the end of T signifies the end of the repolarization process. The manifest shape of the T and U waves is the result of complex interactions of varying and often largely canceling contributions. The iso-source maps are helpful to understand the genesis of the T and U waves.