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...