Relation between transcardiac and transthoracic current during defibrillation in humans. Conceptually, transthoracic defibrillation threshold current can be considered a function of at least two quantities. It is directly proportional to the transcardiac threshold current and inversely proportional to the transcardiac current fraction (FC) or the ratio of transcardiac and transthoracic current. Although experimental and theoretical estimates of FC have been as high as 45%, previous measurements in humans have not been made. This study was designed to quantify FC in humans. During intraoperative testing of the automatic implantable cardioverter defibrillator, transthoracic rescue shocks of 200-400 J were delivered when the device failed to defibrillate. Simultaneous transthoracic voltage (VT) and transcardiac voltage (VC) between two implanted epicardial patch electrodes were measured. The ratio, VC/VT, was 0.04 +/- 0.03 (mean +/- SD) in 10 patients. In 16 dogs, a comparison was made between direct measurement of FC and VC/VT. FC was determined with a specially designed electrode system, which was calibrated to account for field distortion introduced by the electrodes. There was no significant difference between FC and VC/VT, which were both approximately 0.05, suggesting that VC/VT was statistically equivalent to FC. The results of this study, therefore, indicate that during transthoracic defibrillation in humans, approximately 4% of transthoracic current traverses the heart. This relatively small percentage of current results from the existence of parallel pathways, such as the thoracic cage and lungs, which shunt current around the heart. 