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 VmS) 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 Vm-sensitive fluorescence at 63 spots in an 8 x 8-mm region around the electrode. Cathodal stimulation in the refractory period produced regions of -delta Vm 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 > or = .7 for difference versus zero, n = 7). Recording spots in these regions underwent + delta Vm when anodal stimulation was used. At recording spots on the slow propagation axis, cathodal stimulation produced + delta Vm and anodal stimulation produced -delta Vm. During diastolic stimulation, early excitation occurred near the electrode for cathodal stimulation or on the fast propagation axis as fas as 2.8 +/- 1 mm away from the electrode for anodal stimulation. A "dog-bone" region of + delta Vm 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 Vm occurred away from the electrode on the fast propagation axis when anodal stimulation was given in diastole. Thus, delta Vm differs in regions along and across myocardial fibers, indicating that delta Vm depends on anisotropic bidomain properties. Sites of early excitation are those where + delta Vm occurs, indicating that membrane channel excitation depends on the distribution of delta Vm.