# A Computer Simulation Research of Two Types of Cardiac Physiological Pacing
## 2 Materials and Methods  
### Rabbit Ventricular Electrophysiological Model with His-Purkinje System  
The electrophysiological model used in this manuscript is an optimized rabbit ventricular model. We completed the optimization based on an anatomically detailed finite-element rabbit ventricular model that was created from magnetic resonance images. The model consisted of 82,619 nodes and 431,990 tetrahedral elements [13]. The myocardium was separated into three layers with equal thicknesses: endocardium, mid-myocardium, and epicardium, according to the transmural distance. The Mahajan cell models [14] were added on three layers of myocardial tissue, with most of the parameters set on default, while several parameters were adjusted to ensure the correct sequence during repolarization, which can be reflected in the action potential (AP) morphology of each myocardial layer and the polarity of the T wave. We changed the slow inactivating transient outward K+ current (I\({}_{\text{to},s}\)) conductance and slow delayed rectifier K+ current (I\({}_{\text{KS}}\)) conductance according to the ratio of those in [15]: g\({}_{\text{to},s}\) = 0.04 mS/\(\upmu\)F for epicardial and M cells, g\({}_{\text{to},s}\) = 0.01 mS/\(\upmu\)F for endocardial cells; and g\({}_{\text{KS}}\) = 0.1386 mS/\(\upmu\)F for endocardial and epicardial cells, and g\({}_{\text{KS}}\) = 0.0347 mS/\(\upmu\)F for M cells. We simulated the electrical activation with the bidomain model of this rabbit ventricle that is commonly used in cardiac electrophysiological simulation presently. The bidomain model considers tissue as a two-phase medium along with extracellular and intracellular spaces [16]. We introduced anisotropic conduction based on the microstructural fiber orientations contained in the model. Extracellular and intracellular conductivities were set according to the study of [PERSON] [17]. Specifically, a conductivity of 0.62 S/m was set along the fibers and 0.24 S/m perpendicular to the fibers in the extracellular space, while the corresponding conductivities in the intracellular space were 0.174 S/m and 0.019 S/m, respectively. We then multiplied all these ratio values by 0.3 times to make the conduction velocity of the simulated myocardium consistent with that of the real rabbit ventricular myocardium. A detailed His-Purkinje system was integrated in the ventricular model. The His-Purkinje system model was generated according to the method proposed by [PERSON] [18], which had been applied in many modeling studies [19; 20]. In this method, the two-dimensional fractal tree network was first generated, and was then projected on the endo-cardium by a two-step projection method to form the Purkinje network on the ventricular model. To conduct the physiological pacing simulations more efficiently, the following details were noted in the construction of the His-Purkinje system in this manuscript: We added an extra branch on the top of the Purkinje network to represent the His-bundle. Previous studies had indicated that the left and right bundle branches have separated within the His-bundle [21]. The longitudinal dissociation is the reason HBP can correct the bundle branch block that occurs within the His-bundle [22]. Therefore, we constructed the structure in which the left and right bundle branches were dissociated within the His-bundle. There are left and right bundle branches, and the left bundle branch divides into the left anterior and the left posterior fascicles. The His-Purkinje system model we generated contained 369 branches and 146 terminals, and its morphology was consistent with its true physiological structure. Figure 1 illustrates the optimized rabbit ventricular model with the His-Purkinje system model. The gray part and the white network represent the myocardium and the His-Purkinje system model, respectively. The endocardi was connected with the Purkinje network model by Purkinje-muscle junctions (PMJ).