Background: In previous studies, epicardial patch electrodes decreased transthoracic defibrillation efficacy. We studied the effects of two inactive epicardial 14-cm2 titanium mesh patches on defibrillation energy requirements with nonthoracotomy internal lead configurations. Methods and Results: A 6/6-millisecond biphasic shock waveform was delivered via several electrode configurations 10 seconds after ventricular fibrillation was initiated with a 60-Hz generator. In two series, a total of 16 dogs (weight, 23.3±2.4 kg) underwent an up-down defibrillation protocol. In the first series, the defibrillation threshold (DFT) was determined for each electrode configuration in the presence of two inactive epicardial patches. In the second series, DFTs were determined in the presence of an inactive right ventricular (RV) or left ventricular (LV) patch alone. For several nonthoracotomy lead configurations tested in the first 8 dogs, the mean±SD DFT energy increased 49% to 97% with two inactive patches on the heart compared with no patches on the heart as follows: RV to superior vena caval (SVC) electrode, from 8.9±2.6 to 18.0±14.3 J; RV to SVC plus subcutaneous array electrode, from 7.0±2.4 to 10.7±5.3 J; RV to subcutaneous pectoral plate electrode, from 6.2±1.3 to 11.4±4.0 J (P≤.05). The lowest DFT was achieved by defibrillating between the epicardial patches (3.8±3.3 J). The second series showed that DFT voltage requirements increased significantly for all three nonthoracotomy lead configurations with the inactive LV patch alone (P≤.05) but not with the inactive RV patch alone. Conclusions: Inactive epicardial patches can significantly increase the defibrillation energy requirements for nonthoracotomy lead configurations. This negative impact may be due to an insulating effect of the patches and to a disturbance of the potential gradient field under the patches. If the same holds true in patients, these results have clinical implications. Functioning epicardial patch leads should be incorporated in the defibrillation lead system if already present. If the LV patch is nonfunctioning, such as because of a lead fracture, the marked increase in DFT due to an inactive LV patch calls for thorough DFT testing during surgery and, in selected patients, may necessitate patch removal to produce an effective transvenous-based system.