The spatial dipole moment, M, of the isolated perfused rabbit heart at the center of a spherical container was determined during the cardiac cycle from three sets of bipolar voltage measurements. Average peak values for 12 experiments were 72,355, and 128 µa-cm for P, QRS, and T waves, respectively. Rates of change of M and spatial angle were also calculated. Successive excitation vectors are described by a sequence of numbers indicating directions in space. Comparisons between frontal plane vectors of in vivo and isolated hearts showed that, in QRS, average mean values of αf were 51° and 57°; for T these angles were 73° and –137°, indicating a reversal of direction. For P, the angles were 67° and –33°. The hypothesis is advanced that the spread of excitation in the atria is faster along the endocardial surface due to the relatively low resistivity of intracardiac blood; this results in a substantial dipole component perpendicular to the atrial wall.
Equations are developed for the potential due to a source and sink, or dipole, inside a circle of finite resistivity surrounded by a two-dimensional infinite medium of different resistivity. When the resistivity of the external medium is allowed to increase to infinity, the resulting equations give identical results to those presented by Bayley for the dipole potential in an insulating circle. A method of defining the zero of potential for a bounded medium is proposed.
Normal rabbit electrocardiograms were taken using high sensitivity, recording speed, and frequency response. The signals were recorded on magnetic tape and replayed with a time expansion of 32 to 1. Bipolar and augmented unipolar limb leads and two unipolar chest leads were recorded. Mean values, ranges, and standard deviations for the P, QRS, and T voltages were obtained, as well as mean frontal plane vectors. Similar data for durations were found. The ranges of mean axes were 44° to 86° for P, 55° to 103° for T, and –79° to +156° for QRS. The wide range in ventricular electrical axis may be due to changes in heart position. Notching of P and QRS complexes was due to the high resolution used.
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