Finite difference model of thoracic fields and current flow due to surface applied defibrillation voltages

A finite difference model with 216000 elements was developed based on magnetic resonance (MR) images to determine an improved electrode size and position for external electrode defibrillation. With 10×10 cm electrodes located in the standard position on the anterior thoracic surface 31% of the current passed through the entire heart but only 8.5% of the current passed through the myocardium. Moving each electrode laterally 3.3 cm reduced the current passing through the heart to 25.4% but improved the uniformity of the current passing the myocardium by nearly a factor of two based on an index developed to assess electrode performance. It appears an improved electrode size and position can be developed but its effect on the output level settings used with commercial defibrillators needs to be carefully considered because of load impedance changes