Significant differences can be observed between isotropic and anisotropic tissue models (Figure 4-8) when a voltage point source was modeled (internal cardiversion-defibrillation). In comparison to the results obtained when a surface potential difference was modeled at high- range voltages, the behavior under isotropic and anisotropic tissue properties is clearly different. There is a faster increase of the percentage of nodes above the defined threshold in the anisotropic tissue model than in the isotropic model. 1.2 0 C 0.8 46 0.6 +- Anisotropic -- Isotropic 0.4 M 0.2 E Z 0 - 5 6 7 8 9 10 20 30 40 100 Point Source Voltage Input (V) Figure 4-8. Implantable cardioversion at high range. Normalized number of nodes of heart tissue above 5 volts at different voltage with a point source input. The differences seen at the high-range voltage input are more important when modeling point sources than when modeling surface potential difference. Better visualization of the results is seen on the following graph (Figure 4-9). Eighty percent of anisotropic tissue reaches a voltage threshold of 5 V at approximately 18 V of voltage point source input. When the tissue is isotropic with the same input, eighty percent of its nodes reach above the threshold at approximately 35 V of input. On the other hand, differences are not significant between isotropic point source and isotropic face models. Nevertheless, it can be observed that in the isotropic model, there is a faster increment of number of nodes than in the anisotropic model.