Shock energy for successful defibrillation of atrial tissue during vagal stimulation

Atrial flutter and fibrillation are pathologic conditions which lead to the breakdown of organized electrical activity. Restoring a normal activation sequence can be achieved by delivering an electrical shock. Whether the energy needed for the termination of reentry depends on the degree of disorganization of the activation pattern and which role is played by microscopic inhomogeneities in this process was examined in this study. A three-dimensional bidomain model was used. The intracellular conductivities were varied statistically between neighboring elements to obtain different levels of inhomogeneity. Shocks were applied to a passive tissue slab to determine how the spatial pattern of activation depend on the degree of inhomogeneity. In active tissue slabs with different degrees of inhomogeneity, incorporating human atrial kinetics, an ACh dependent K⁺ current and electroporation, reentry was initiated. Depending on the distribution of ACh, either a single rotor or spiral wave breakup with multiple wavelets were observed. Shocks were delivered to both activation patterns to determine the probability of shock success as a function of the shock strength. Results indicate that 1) inhomogeneities caused a change in transmembrane voltage in the tissue bulk, 2) the defibrillation threshold was lower in presence of inhomogeneities, 3) the defibrillation threshold was significantly higher in presence of multiple wavelets.