抄録
Relations between the excitation fronts propagating in the heart and the equivalent dipoles estimated from the body surface potential distributions have been investigated by means of the computer simulation. The equivalent dipoles are determined either by minimizing the square deviation between the measured potential distribution and that generated from the dipoles or by comparing them through their multipole expansion coefficients. These two methods, that will be called the direct and the indirect methods, respectively, were compared in the single moving dipole (SMD) approximation. Furthermore, the direct method was applied to the two-moving dipole (TMD) approximation.
Simulations were performed in the following way: At first, the propagation processes of the excitation fronts were simulated on the three-dimensional heart model composed of 125000 unit cells, to each of which can be assigned different action potential waveform. Next, the body surface potential distributions generated by the resultant EMF sources in the heart were calculated. Then, the equivalent dipole method was applied to them, and the dipole locations thus obtained were compared with the mean locations of the excitation fronts. Since the EMF sources are distributed on the excitation front as an electric double layer, its periphery rather than the shape of the excitation front itself is essential in determining the potential distribution. Hence, the mean location of an excitation front is defined as the gravitational center of its periphery.
When there is a single excitation front with simple shape, the equivalent dipole location obtained from the SMD approximation coincides with the mean location of the excitation front. The coincidence is better with the direct method, but it needs longer calculation time than the indirect method. Through 25 to 30ms after the onset of the ventricular depolarization, the excitation fronts have complicated shapes and the deviations of the equivalent dipole locations from their mean locations become large even if the TMD approximation is used.
Except the early and the last stages of the ventricular depolarization, there are usually a dominant excitation front and smaller ones. One of the equivalent dipoles resulting from the TMD approximation is located near the mean location of the largest excitation front, and the other has a tendency to be located around the center of small excitation fronts.
