Energy levels for defibrillation: What is real clinical importance?

Today, transthoracic and intracardiac defibrillation offer a well-accepted and widely used form of therapy for patients with life-threatening ventricu lar arrhythmias. Despite the wide clinical use of defibrillators, the mecha nisms by which an electrical shock hairs fibrillation are still not complet ely understood. During a shock, different amounts of current flow through t he different parts of the heart and the current distribution is highly unev en. This current distribution is affected by changes in the shock potential gradient through the heart, changes in fiber orientation, and changes in m yocardial conductivity caused by connective tissue barriers. It would be id eal if the potential gradient distribution throughout the ventricles could be measured directly for each individual patient during defibrillator impla ntation and follow-up and the shock strength could be programmed based on t his measurement, but so far this is not possible. A more feasible approach is to determine, by trial and error, the magnitude of the shock strength de livered through the defibrillation electrodes for successful defibrillation . There is no distinct threshold value above which all shocks succeed and b elow which ail shocks fail to defibrillate. Rather, increasing shock streng th increases the likelihood the shock will succeed. Therefore, instead of a distinct defibrillation threshold, a probability of success curve exists. However, increasing the shock strength above an optimal range can actually decrease the success rate for defibrillation. One possible explanation is c aused by such large that the high voltage gradients caused shacks damage ce lls and result in postshock arrhythmias that may reinitiate fibrillation. A nother problem that can affect the probability of defibrillation success fo r a particular programmed energy setting is that the shock strength require d for defibrillation may Increase over time due to (1) the growth of fibrot ic tissue around the defibrillation electrode; (2) migration of the lead; ( 3) acute ischemia; or (4) other changes in the underlying cardiac disease ( e.g., worsening of heart failure). Such possible increases In the defibrill ation shock strength requirement should be compensated for before they occu r by adding a margin of safety to the shock strength needed for effective d efibrillation. When programming an implantable defibrillator, it is importa nt to keep in mind that the defibrillation shock should be (1) strong enoug h to defibrillate at least 98% of the time with the first shock; (2) weak e nough not to cause severe postshock arrhythmias or reinitiation of fibrilla tion; but (3) strong enough to compensate for changes of defibrillation ene rgy requirements over time. This usually can be accomplished by setting the defibrillator 7-10 J higher than the defibrillation threshold determined b y a standard step-down protocol. (C) 1999 by Excerpta Medica, Inc.