['Implantable cardioverter defibrillators (ICDs) save lives, but the defibrillation shocks delivered by these devices produce substantial pain, presumably due to skeletal muscle activation. In this study, we tested an electrode system composed of epicardial panels designed to shield skeletal muscles from internal defibrillation, but allow penetration of an external electric field to enable external defibrillation when required.', 'Eleven adult mongrel dogs were studied under general anesthesia. Internal defibrillation threshold (DFT) and shock-induced skeletal muscle force at various biphasic shock strengths were compared between two electrode configurations: (1) a transvenous coil placed in the right ventricle (RV) as cathode and a dummy can placed subcutaneously in the left infraclavicular fossa as anode (control configuration) and (2) RV coil as cathode and the multielectrode epicardial sock with the panels connected together as anode (sock-connected). External DFT was also tested with these electrode configurations, as well as with the epicardial sock present, but with panels disconnected from each other (sock-disconnected). Internal DFT was higher with sock-connected than control (24 +/- 7 J vs. 16 +/- 6 J, P < 0.02), but muscle contraction force at DFT was greatly reduced (1.3 +/- 1.3 kg vs. 10.6 +/- 2.2 kg, P < 0.0001). External defibrillation was never successful, even at 360 J, with sock-connected, while always possible with sock-disconnected.'] Does novel electrode design for potentially painless internal defibrillation also allow for successful external defibrillation?