Follow-Up Defibrillator Testing for Antiarrhythmic Drugs

Probabilism: The doctrine that probability is a sufficient basis for belief or action . Uncertainty: The condition of being in doubt . It has been rigorously demonstrated that implantable cardioverter-defibrillators (ICDs) prolong life as primary and secondary prevention therapies for the management of malignant ventricular tachyarrhythmias. Despite 20 years of clinical use, however, there exist many unresolved issues in ICD therapy.1 Even the most fundamental aspect of ICD implantation, defibrillation threshold (DFT) testing, is not standardized. Determining a DFT is fundamentally different from determining a pacing threshold.1,2 The latter is essentially a fixed value at a given combination of pulse width and voltage. Clinically, there is a sharp demarcation of pacing voltages that capture and those that do not. In contrast, defibrillation is characterized as a probabilistic event based on our current understanding. The dose-response curve for defibrillation is described as sigmoidal in shape with a higher likelihood of success at higher energies.1,2 The possibility of an “outlier” event, defibrillation at low energy, or failure at higher energy remains present. Defining a true defibrillation threshold, ie, that energy above which defibrillation will always succeed and below which it will always fail, is not the goal of defibrillation testing in the clinical setting. Instead, the clinical approach to ICD implantation defines an energy level delivered from a device that has a reasonable likelihood of success. The physician then programs this energy plus an additional safety margin that he or she trusts will compensate for the probabilistic nature of defibrillation. There are many methods to test defibrillation in clinical practice. Methods in clinical use include step down to failure, 2 or 3 successes without failure at a given energy level (DFT+, DFT++), binary search, Bayesian search, 3-reversal method, verification method (single low-energy success), and testing upper limit of vulnerability …