The Relationship of Antitachycardia Pacing versus Shock in an Implantable Cardioverter‐Defibrillator Population and the Risk of Mortality: Just an Association?

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Excerpt

The mortality benefit of implantable cardioverter‐defibrillator (ICD) therapy has been shown for both primary and secondary prevention of sudden cardiac death and in both ischemic and nonischemic cardiomyopathy patients.1 Although ICD shocks may be lifesaving, avoiding shocks by attempting to terminate ventricular arrhythmias with antitachycardia pacing (ATP) may be beneficial as shocks can cause patient discomfort and may themselves be associated with increased mortality.4 Previous studies have not fully investigated the underlying mechanism of ICD shocks and the risk of mortality, but in both small animal and human studies, high‐voltage shocks for ventricular arrhythmias may cause myocardial damage as measured by troponin release or impaired cardiac contractility.5 Other studies have found an association of both appropriate and inappropriate ICD shock therapy with increased mortality.7 Whether this potential risk can be mitigated by the programming of ATP in lieu of or in the hopes of preventing shocks has been a subject of debate.
In this issue of the Journal, Strickberger et al. report their findings after analyzing remote‐monitoring data from 69,368 patients in the St. Jude Medical Merlin.net network implanted with an ICD between 2008 and 2013.9 Patients were categorized into three different groups based on whether and what type of ICD therapy they received and included: (1) no therapy, (2) ATP only, and (3) shock. The authors found that compared to the shock group, those in the no therapy (hazard ratio [HR] 0.60, 95% confidence interval [CI] 0.56–0.64, P < 0.001) and ATP (HR 0.70, 95% CI 0.64–0.77, P < 0.001) groups experienced lower mortality risk. The authors also found that ATP was effective in terminating ventricular tachycardia (VT) episodes in 88.5% of episodes, and that ATP effectiveness was dependent on the rate of ventricular arrhythmia, with slower ventricular arrhythmias more commonly terminating with ATP.
The study has several strengths and important notions that are worth highlighting. Although previous studies have evaluated the risk of mortality with ATP therapy versus shocks,10 no contemporary large‐scale study to date has been performed to test properly the association in a broad population in a real‐world, contemporary setting. Most important, as one of the largest studies of its kind, ample power was available to evaluate the success of ATP therapy in terminating ventricular arrhythmias and investigate the association of ATP versus shock on overall mortality. The authors were quite comprehensive in their analyses by evaluating different subgroups of patients including different age categories, sex, type of device, and atrial fibrillation burden to show that ATP was consistently associated with lower mortality across different subgroups of patients. Additionally, the authors analyzed different rates of ventricular arrhythmias including those <181 bpm, 191–213 bpm, 214–250 bpm, and >250 bpm and found relatively consistent improved survival in patients who received ATP versus shock therapies. Taken altogether, these data provide a cohesive argument for an actual influence of ATP (compared with shock) therapy on decreased risk of mortality. The success rate of ATP in this study of >85% indicates that this therapy seems quite effective in terminating ventricular arrhythmias in a real‐world population. With this knowledge in hand, implanters may be more likely to program an initial VT zone to allow for attempts at ATP therapy to terminate ventricular arrhythmias, rather than only a single VF zone at a higher rate cutoff or more intervals of detection, which has been the focus of recent randomized controlled trials showing mortality benefit of these strategies compared with nominal ICD therapy settings.11
It is interesting to surmise that terminating ventricular arrhythmias with ATP instead of shock would lead to improvement of the ultimate clinical outcome—survival.
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