  Spatial variations in the electrical properties of cardiac tissue can occur
because of cardiac diseases. We introduce such gradients into mathematical
models for cardiac tissue and then study, by extensive numerical simulations,
their effects on reentrant electrical waves and their stability in both two and
three dimensions. We explain the mechanism of spiral- and scroll-wave
instability, which entails anisotropic thinning in the wavelength of the waves
because of anisotropic variation in its electrical properties.
