Isoarea distributions during ectopic stimulation of isolated rabbit hearts.

These studies were designed to explore the effects of ectopic left ventricular stimulation on the area distributions recorded during the QRS, ST-T and QRST intervals. Twenty rabbit hearts were suspended in a spherical volume conductor, perfused with an electrolyte solution and stimulated from an anterior and/or a posterior subepicardial left ventricular site. ECG signals were sensed from 32 surface electrodes and were processed to compute areas under the three intervals, to construct area maps and to fit the computed areas to a single moving dipole equivalent cardiac generator model. Results demonstrated that control distributions were overtly nondipolar in 62.5% of cases. Second, ectopic pacing increased the dipolarity of the QRS and ST-T intervals significantly by both dipole ranging and isoarea methods. All area distributions became dipolar. Last, the QRST distribution response to ectopic stimulation varied widely, with major as well as minor changes in patterns occurring with alteration in excitation sequence. Thus, (1) ectopic stimulation changed electrical field properties of mean cardiac forces, and (2) QRST area distributions were dependent upon activation sequence. Each of these observations has direct bearing upon the ventricular gradient concept.