Differences in cardiac activation times for endocardium and epicardium in response to external electric shock

Externally applied electric shock E_a, which are used to terminate cardiac defibrillation, operate by altering the cardiac transmembrane potential through a mechanism that is not yet understood. The effect of E_a on endocardium and epicardium was studied on Langendorff-perfused, di-4-ANEPPS stained isolated rabbit right ventricles (RV) to which a 10-40 V/cm E_a was applied from plate electrodes inside a Tyrode´s bath. Field stimulation of the flat RV preparation caused immediate distribution depolarization followed by a rapid, non-propagative activation of the entire RV wall. For every E _a strength tested, the activation time T_a, of the epicardium was shorter than epicardial T_a. This difference in T_a may be attributed to a higher degree of anatomical heterogeneities on the endocardial surface