TRANSMEMBRANE VOLTAGE CHANGES DURING UNIPOLAR STIMULATION OF RABBIT VENTRICLE

This study tested the prediction of bidomain models that unipolar stimulation of anisotropic myocardium produces transmembrane voltage changes (Delta V(m)s) of opposite signs away from the electrode on perpendicular axes. Stimulation with a strength of 0.1 to 40 mA was applied from a point electrode on the left or right ventricle of isolated perfused rabbit hearts at 37 degrees C to 38 degrees C stained with the potentiometric dye di-4-ANEPPS. A laser scanner system recorded V-m-sensitive fluorescence at 63 spots in an 8x8-mm region around the electrode. Cathodal stimulation in the refractory period produced regions of -Delta V-m 1 to 5 mm away from the electrode on an axis oriented parallel to the fast propagation axis to within 1.8+/-11 degrees (P greater than or equal to.7 for difference versus zero, n=7). Recording spots in these regions underwent +Delta V-m when anodal stimulation was used. At recording spots on the slow propagation axis, cathodal stimulation produced +Delta V-m and anodal stimulation produced -Delta V-m. During diastolic stimulation, early excitation occurred near the electrode for cathodal stimulation or on the fast propagation axis as far as 2.8+/-1 mm away from the electrode for anodal stimulation. A ''dog-bone'' region of +Delta V-m that included tissue near and away from the electrode on the slow propagation axis occurred when cathodal stimulation was given in diastole. Regions of +Delta V-m occurred away from the electrode on the fast propagation axis when anodal stimulation was given in diastole. Thus, Delta V-m differs in regions along and across myocardial fibers, indicating that Delta V-m depends on anisotropic bidomain properties. Sites of early excitation are those where; +Delta V-m occurs, indicating that membrane channel excitation depends on the distribution of Delta V-m.