Leiden, the Netherlands - A retrospective, single-center experience supports post hoc analyses of two major randomized trials in finding that inappropriate implantable cardioverter defibrillator (ICD) shocks increase all cause mortality. Moreover, the risk increased the more inappropriate shocks patients experienced, according to the analysis of 1544 patients with ICDs featuring electrogram storyage, from different manufacturers, implanted for either primary or secondary prevention from 1996-2006.
Three-quarters of the inappropriate shocks were triggered by misdiagnosis of supraventricular tachycardias, especially atrial fibrillation (AF), report Dr Johannes B van Rees (Leiden University Medical Center, the Netherlands) and colleagues in the February 1, 2010 issue of the Journal of the American College of Cardiology. 
The cumulative incidence of inappropriate shocks rose from 7% at one year after implantation to 13% at three years and 18% at five years (mean follow-up 41 months).
More than a third of patients with inappropriate shocks, 36%, experienced them on more than one occasion; the time from first to second episode averaged 11 months.
In multivariate analysis, predictors of inappropriate shocks included age >70 years (hazard ratio 1.9, 95% CI 1.3-2.5; p=0.01) and history of AF (HR 2.0, 95% CI 1.5-2.7; p<0.01).
The occurrence of only one inappropriate shock showed an all-cause mortality hazard ratio of 1.6 (95% CI 1.1-2.3, p=0.01) adjusted for history of AF, age, NYHA functional class, renal function, QRS duration, and beta-blocker use.
Each additional inappropriate shock corresponded to an HR of 1.4 (95% CI 1.2-1.7, p=0.01), such that the risk was more than triple after a total of five such shocks.
Subgroup analyses from both MADIT-2  and SCD-HeFT , which entered patients with LV systolic dysfunction getting ICDs for primary prevention, found that inappropriate shocks were independently associated with increased mortality, van Rees et al note. "The present study confirmed this finding in routine clinical practice, outside the setting of a clinical trial."
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