Is Ventricular Fibrillation Induction Really Necessary with Implantable Cardioverter-Defibrillators? Clinical Implications of the ASSURE Study

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Citation
US Cardiology, 2007;4(2):74-6

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Implantable cardioverter–defibrillators (ICDs) are being implanted in an ever-increasing number of patients at risk for sudden cardiac death. To ensure the efficacy of the defibrillation system, step-down defibrillation threshold (DFT) testing has traditionally been used to ensure that the ICD can appropriately detect and terminate clinical ventricular fibrillation (VF).
Currently, the most widely accepted method of ‘device testing’ at the time of ICD implantation requires at least two VF inductions, with subsequent demonstration of two conversion successes with shock energies of 10J less than the maximum energy delivered by the ICD. While VF conversion testing is a relatively safe procedure, in rare cases the repeated induction of VF, particularly in patients with marked heart failure, may result in serious complications such as myocardial depression or ischemia, cerebral hypoperfusion, intractable VF, or even death.1–3 A retrospective analysis from the Low Energy Safety Study (LESS) showed that one low-energy VF conversion success at 14J was sufficient to ensure the adequacy of an ICD system at the time of implantation when programming all shocks to the maximum energy of the device at 31J.4,5 Based on these findings and the concern of potential patient cardiac decompensation with repeated VF inductions during ICD implantation, many centers have adopted a single VF induction approach.
Testing for the upper limit of vulnerability (ULV) is an alternative method that may be used to estimate the DFT without the repeated VF inductions required with traditional DFT testing. It is well known that a vulnerable period exists in the cardiac cycle and that a single shock of appropriate strength can induce VF. If the strength of the shock delivered during the vulnerable period is gradually increased, the ventricle will eventually become ‘non-vulnerable’ and VF will not be induced. The ULV is defined as the weakest shock delivered during the vulnerable phase of the cardiac cycle (which correlates to the peak of the T wave) at which VF is not induced. The basis of the ULV approach is that the ULV closely correlates with the DFT and that ICDs programmed on the basis of the ULV reliably defibrillate VF. Thus, the purpose of the Arrhythmia Single Shock Defibrillation Threshold Testing versus Upper Limit of Vulnerability: Risk Reduction Evaluation with Implantable Cardioverter Defibrillator Implantations (ASSURE) Study was, for the first time, to prospectively evaluate in a randomized, controlled, large multicenter study the efficacy of a potentially inductionless ULV screening approach versus a single low-energy VF conversion test at the time of ICD implantation.

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