Is there an optimal electrode pad size to maximize intracardiac current in transthoracic defibrillation?

Achieving defibrillation depends on adequate intracardiac current. The purpose of this study was to determine, in advance of administering shocks which parameters of body habitus can be used to select the electrode size that maximizes intracardiac current in transthoracic defibrillation. We administered direct current shocks to 18 mongrel dogs over a wide range of weight and size (weight 10-30 kg with chest circumferences 44-77 cm) using a damped sine wave defibrillator and self-adhesive electrode pairs of various diameters (4 cm, 5.8 cm, 8 cm and 10 cm), placed on the right and left lateral chest walls. The energy levels used were 50, 100, and 150 J. Intracardiac voltage gradient, a parameter of intracardiac current, was determined in three orthogonal planes using an intramyocardial electrode array placed in the interventricular septum. The relation between intracardiac voltage gradient magnitude magnitude of VG and various parameters (body weight, chest, circumference, chest volume, chest radius, and heart weight divided by chest radius) was determined. The correlation (r) with the smallest P value was between magnitude of VG and the heart weight divided by chest radius (HW/R) (r = 0.71). Intracardiac current was highest at intermediate pad sizes. The electrode pads that maximized magnitude of VG tended to be large for the larger HW/R dogs, and smaller HW/R dogs. In none of the HW/R groups did the largest electrode pads yield the highest intracardiac voltage gradient. We conclude that there is no simple way to determine in advance an electrode pad size that maximizes intracardiac current. The HW/R ratio influences but does not determine intracardiac intracardiac current.