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 s hocks, which parameters of body habitus can be used to select the elec trode size that maximizes intracardiac current in transthoracic defibr illation. We administered direct current shocks to 18 mongrel dogs ove r a wide range of weight and size (weight 10-30 kg with chest circumfe rences 44-77 cm) using a damped sine wave defibrillator and self-adhes ive electrode pairs of various diameters (4 cm, 5.8 cm, 8 cm, and 10 c m), placed on the right and left lateral chest walls. The energy level s used were 50, 100, and 150 J. Intracardiac voltage gradient, a param eter of intracardiac current, was determined in three orthogonal plane s using an intramyocardial electrode array placed in the interventricu lar septum. The relation between intracardiac voltage gradient magnitu de \V-G\ and various parameters (body weight, heart weight, chest circ umference, chest volume, chest radius, and heart weight divided by che st radius) was determined. The correlation (r) with the smallest P val ue was between \Vc\ and the heart weight divided by chest radius (HW/R ) (r = -0.71). Intracardiac current was highest at intermediate pad si zes. The electrode pads that maximized (VG) tended to be large for the larger HW/R dogs, and smaller for the smaller HW/R dogs. In none of t he HW/R groups did the largest electrode pads yield the highest intrac ardiac voltage gradient. We conclude that there is no simple way to de termine in advance an electrode pad size that maximizes intracardiac c urrent. The HW/R ratio influences but does not determine intracardiac current.