Funding Acknowledgements Type of funding sources: Private company. Main funding source(s): Boston Scientific Corporation Background The current Subcutaneous ICD (S-ICD) model incorporates SMART Pass (SP) to improve sensing and discrimination capabilities to reduce inappropriate shocks (IAS). SP status is programmable but may also be disabled automatically in the setting of low amplitude signals or low heart rate in order to avoid under-sensing of VT/VF. Objective To evaluate SP impact on IAS, appropriate shocks (AS), complications and mortality in the UNTOUCHED S-ICD trial. Methods Primary prevention patients (pts, n=1111) with ejection fraction ≤35% and no pacing requirement were followed for up to 18 months. SP status during a study visit was programmed ON or OFF and status between visits was either consistently OFF, ON, or automatically disabled (DIS). The impact of SP status on pt outcomes was evaluated using Kaplan-Meier (K-M) analysis. Multivariable proportional hazard analysis identified predictors of IAS and SP disable events. Results Percent of pts with SP always ON, always OFF, ON with DIS, and OFF then ON with no DIS were 56, 16, 15, and 13%, respectively. At least one SP DIS occurred in 177 pts, but only 13% had 2 or more, mostly due to PVCs and low EGM amplitudes. Significant multivariable predictors of SP disable events are history of atrial fibrillation (hazard ratio (HR) 2.49, odds ratio (OR) (1.49-4.16); p=.0005), only one passing vector at S-ICD screening, (HR 1.85, OR (1.10-3.10; p=.0202) and lower left ventricular ejection fraction (HR 1.05, OR (1.01-1.08); p=.0074). K-M IAS rates were highest for pts experiencing DIS (fig 1) and lowest for SP ON. While neither AS (p=0.58) nor complication (p=0.58) rates varied significantly according to SP status, mortality was lower for pts with SP ON during any duration of time (p=0.044) by univariate analysis. Further analysis is planned to better understand the relationship between SP status and mortality. Conclusion Patients in the UNTOUCHED trial with SMART Pass (SP) consistently ON had significantly fewer inappropriate shocks, with no impact on appropriate therapy for VT/VF. Patients with history of atrial fibrillation, lower left ventricular ejection fraction, and only one passing vector at S-ICD screening are at higher risk of SP disable events; therefore, care should be taken for these patients to assess SP status and their higher risk for inappropriate shocks.

Funding Acknowledgements Type of funding sources: Private company. Main funding source(s): Boston Scientific Corporation Background The PRAETORIAN trial demonstrated that the subcutaneous ICD (S-ICD) is non-inferior to the transvenous ICD (TV-ICD) with regard to inappropriate shocks (IAS) and complications. Inappropriate therapy is an undesirable side effect of ICD therapy. Purpose This pre-specified secondary analysis evaluates all inappropriate therapy in the PRAETORIAN trial and subsequent action to reduce recurrence of IAS. Methods The PRAETORIAN trial is an international, multicentre, randomised trial, which included patients with an indication for ICD therapy. In total, 849 patients were randomised to receive an S-ICD (N=426) or TV-ICD (N=423). ICD programming was mandated by protocol. Inappropriate therapy was defined as any ICD therapy on a different rhythm than ventricular tachycardia or ventricular fibrillation. A day with inappropriate therapy was defined as all device episodes on the same day. Mechanism, actions and recurrence rate were determined using days with inappropriate therapy. Results In the S-ICD group, 42/426 (10%) patients received inappropriate therapy, compared to 42/423 (10%) patients in the TV-ICD group (P=0.97). In total, 41 patients in the S-ICD and 29 patients in the TV-ICD group received at least one IAS (P=0.14). The total number of IAS in the S-ICD and TV-ICD group is comparable (124 vs. 130, P=0.88). The most common underlying mechanism of inappropriate therapy was T-wave oversensing in the S-ICD group (45%) and a supraventricular tachycardia (SVT) in a therapy zone in the TV-ICD group (93%). When no action was undertaken after the first IAS, the recurrence rate of an IAS of the same etiology was 56% (5/9) in the S-ICD group and 50% (4/8) in the TV-ICD group. An action, such as a change in medication, a change in programming, an invasive action or lifestyle advise resulted in a recurrence rate of 23% (7/30) in the S-ICD group and 30% (6/20) in the TV-ICD group. The recurrence rate was significantly higher when no action was undertaken versus any action (P=0.04) (Figure 1.). A change in programming resulted in a lower recurrence rate than a change in medication (S-ICD, 40% vs. 27% and TV-ICD, 44% vs. 17%). After an invasive action no recurrence of IAS occurred in both groups. Invasive actions were more common in the S-ICD group (7/41, vs. 1/29). In the S-ICD group the most common action was a change in programming (17/41), mainly when the underlying mechanism of IAS was cardiac oversensing. In the TV-ICD group the most common action was a change in medication (12/29), mainly and only when the underlying mechanism of IAS was an SVT (Figure 2.). Conclusion The total number of IAS and total patients receiving IAS are not statistically different between the S-ICD group and the TV-ICD group. However, the underlying mechanism and action after IAS differ and are associated with the different sensing of the ICDs. The recurrence rate after a first IAS was significantly higher when no action was undertaken.