  Background-The electric field applied to the heart during defibrillation
causes mechanical forces (electrostriction), and as a result the heart deforms.
This paper analyses the physical origin of the deformation, and how significant
it is. Methods-We represent the heart as an anisotropic cylinder. This simple
geometry allows us to obtain analytical solutions for the potential, current
density, charge, stress, and strain. Results-Charge induced on the heart
surface in the presence of the electric field results in forces that deform the
heart. In addition, the anisotropy of cardiac tissue creates a charge density
throughout the tissue volume, leading to body forces. These two forces cause
the tissue to deform in a complicated manner, with the anisotropy suppressing
radial displacements in favor of tangential ones. Quantitatively, the
deformation of the tissue is small, although it may be significant when using
some imaging techniques that require the measurement of small displacements.
Conclusions-The anisotropy of cardiac tissue produces qualitatively new
mechanical behavior during a strong, defibrillation-strength electric shock.
