Implantable cardioverter defibrillator (ICD) implantation is a common approach in patients at high risk of sudden cardiac death. Verification of defibrillation efficacy by defibrillation threshold (DFT) testing during ICD implantation is the current standard. Traditionally, a safety margin of at least 10 J between the maximum output of the pulse generator and the energy needed for defibrillation has been used because early studies indicate that lower safety margins were associated with high rates of failed defibrillation and sudden cardiac death. Improvements in ICD and lead technology result in marked reductions in defibrillation thresholds and more stable thresholds long term. Despite these improvements, some patients still require system modification during implantation to obtain an adequate safety margin. During DFT testing multiple induction of ventricular fibrillation cause brief transient episodes of cerebral ischemia. These repeated short episodes of circulatory arrest with global cerebral ischemia have been associated with changes in cerebral oxygen uptake and cerebral electrical activity. In addition, minor neurologic injury can occur after ICD implantation and defibrillation testing. This finding needs to be examined in further research.