An exemplary method includes detecting fibrillation, measuring impedance of a defibrillation circuit that includes myocardial tissue, determining one or more defibrillation shock parameters based at least in part on the impedance, delivering a defibrillation shock using the one or more defibrillation shock parameters and, if the shock was unsuccessful, adjusting a membrane time constant and determining one or more new defibrillation shock parameters based at least in part on the adjusted membrane time constant. Various other exemplary methods are disclosed as well as various exemplary devices, systems, etc.