Time Course of Subsequent Shocks After Initial Implantable Cardioverter-Defibrillator Discharge and Implications for Driving Restrictions

IMPORTANCE Although guidelines recommend driving restrictions for 3 to 6 months after appropriate implantable cardioverter-defibrillator (ICD) shocks, contemporary data to support these recommendations are lacking. OBJECTIVE To define the time course of subsequent shocks after an initial ICD discharge. DESIGN, SETTING, AND PARTICIPANTS Retrospective analysis of a nationwide cohort of 14 230 ICD recipients enrolled in a remote monitoring program. Participants underwent ICD implantation from October 1, 2008, to December 31, 2013, and experienced at least 1 shock. The risk of driving after an ICD shock was estimated using the risk for harm (RH) formula, and an annual RH of less than 5 events per 100 000 ICD recipients was deemed safe. The likelihood of loss of consciousness associated with an ICD shock was estimated using a cautious value of 32% and an estimate of 14% based on contemporary data. Data were extracted and analyzed from December 17, 2014, to October 31, 2015. MAIN OUTCOMES AND MEASURES Time course of subsequent shocks after an initial ICD discharge. RESULTS Of 73 503 ICD recipients who underwent remote monitoring, 14 230 (19.4%) experienced at least 1 ICD shock and were included in this analysis (10 870 men [76.4%]; 3360 women [23.6%]; median age at device implantation, 68 years; interquartile range [IQR], 60-76 years). The cumulative incidence of receiving a second shock was 14.5%(IQR, 13.9%-15.1%) at 1 month and 28.7%(IQR, 27.9%-29.5%) at 6 months. The time from implantation to initial shock had an inverse association with the likelihood of receiving a second shock (lowest quartile of time at 6 months, 31.6%[95% CI, 30.2%-33.2]; highest quartile of time at 6 months, 25.3%[95% CI, 23.8%-26.9%]). The number of ICD therapy zones was also significantly associated with the incidence of a second shock (1 therapy zone, 20.8%[95% CI, 19.4%-22.3%] at 3 months to 51.5%[95% CI, 48.5%-53.7%] at 3 years; 3 therapy zones, 26.9%[95% CI, 24.8%-29.0%] at 3 months to 57.3%[95% CI, 54.1%-60.5%] at 3 years). When a likelihood of loss of consciousness of 32% associated with an ICD shock was used, the RH while driving fell below the accepted threshold at 4 to 6 months after an initial shock. However, when a contemporary estimate for loss of consciousness associated with an ICD shock of 14% was used, the RH fell below the threshold at 1 month after an initial shock. CONCLUSIONS AND RELEVANCE In this large cohort of ICD recipients, the incidence of a second shock after an initial ICD discharge was lower than previously reported and depended on several programmed ICD variables. These data, with future research to derive contemporary estimates of the likelihood of fatality resulting from an ICD shock while driving, should support the development of evidence-based guidelines for driving restrictions in ICD recipients.