EFFECT OF EPICARDIAL PATCH ELECTRODES ON TRANSTHORACIC DEFIBRILLATION

To improve survival rates in patients undergoing surgical ablative procedures for malignant ventricular tachycardia (VT), a frequent practice is to implant epicardial electrodes at the time of map-guided surgery. After operation, patients with inducible VT often receive an automatic internal cardioverter/defibrillator (AICD) implant, whereas patients with noninducible VT usually do not. In the event that spontaneous, hypotensive VT or ventricular fibrillation should subsequently occur in the patient with noninducible VT, however, transthoracic defibrillation and resuscitation may prove difficult, because the patch electrodes are insulated with silicone rubber that can reduce the amount of current traversing the myocardium and thus can increase transthoracic defibrillation threshold (DFT). In this study, DFT was determined in mongrel dogs to test the hypothesis that epicardial patch electrodes elevate threshold. This study was also designed to assess the effect of patch electrode orientation and size on DFT. In the first protocol (perpendicular orientation), small epicardial patch electrodes (surface area, 30 cm2) were sutured to the epicardial surfaces of the anterior right and posterior left ventricles in 15 dogs so that the center axes of the patch electrodes were coincident and perpendicular to the coincident center axes of the transthoracic electrodes. The effect of two large epicardial patch electrodes (surface area, 53 cm2) on transthoracic DFT was also examined in eight of these dogs. In the second protocol (parallel orientation), small patch electrodes were sutured to the lateral surfaces of the right and left ventricles in seven dogs. In this orientation, the center axes of the epicardial and transthoracic electrodes were coincident and passed through the center of the heart. In protocol 1, small epicardial patch electrodes increased DFT from 100 ± 38 J (control) to 141 ± 73 J (p = 0.01). Large patch electrodes increased DFT from 77 ± 48 J (control) to 186 ± 106 J (p < 0.01). Three of the eight dogs could not be defibrillated after the large patches were implanted. In protocol 2, small patch electrodes increased DFT from 117 ± 69 J (control) to 190 ± 98 J (p < 0.01). The mean percent increment in DFT due to small epicardial patches for each dog in protocol 1 was 46 ± 16%, and in protocol 2, the increment was 80 ± 16%. Therefore, AICD epicardial patch electrodes, regardless of orientation or size, can alter or block current flow through the heart during transthoracic defibrillation, thereby markedly increasing defibrillation energy.