Evaluation of defibrillation thresholds in extravascular defibrillators: results from a monocenter prospective study

Abstract Background Extravascular defibrillators (EV ICD) is a promising alternative to transvenous and subcutaneous defibrillators(1). However, defibrillation efficacy (DE) require further investigation in view of the glassed feed through recall. Objective To determine DE during EV ICD implantation. Methods A prospective study, from November 2020 to February 2025, was conducted in our center involving 49 eligible patients with an indication for defibrillator implantation (class I or IIa) in whom the EV ICD was chosen. Access and tunneling were carried out within the retrosternal space, with lead placement at the left sternal border based on the CT scan (Figure 1)(2). An intermuscular pocket for generator placement was created between the latissimus dorsi and serratus anterior muscles at the left mid-axillary line(3). DE was determined using a standardized protocol, with both standard and reverse polarity shocks assessed to determine optimal defibrillation efficacy. Results Implantation was successful in 48 out of 49 patients (12 females; success rate 98%), performed for primary (38/49) or secondary (10/49) prevention, with no procedure-related complications. One implantation failed due to inadequate sensing (0.5 mV) despite multiple tunneling attempts. The cohort had a mean age of 46±15 years, BMI of 24.4 ±4 kg/m², NYHA class of 1.4 ±0.6, and a left ventricular ejection fraction (LVEF) of 46±14%. The predominant etiology was non-ischemic cardiomyopathy (42/49), including 16 cases of arrhythmogenic cardiomyopathy and 8 of hypertrophic cardiomyopathy. DE was tested in all patients (success was 100%) on induced VF: in the first 16 patients (PIVOTAL study cohort (1)) with reverse shock polarity (RDE), while in 31 with standard shock polarity (SDE), and 1 patient tested with both polarities. As show in Figure 2, 15 J shock successfully terminated ventricular fibrillation (VF) in 20 patients (41,7% of total; RDE 70,6%; SDE 25%); while a 20 J shock was effective in another 21 patients (43,8% of total; RDE 23,5%; SDE 53,1%); 30 J shock was required in 6 patients (12,5% of total; RDE 0%; SDE 18,7%); 1 patient failed 40 J SDE but successfully converted at 35 J with RDE. The average DE was 19.5 J (RDE 17,3 J; SDE 21,5 J considering 40 J for the patient who did not respond to SDE). There were no significant differences amongst RDE and SDE patients. Conclusions In our cohort, the EV ICD demonstrated a high implantation success rate, apart one patient with inadequate sensing. While EV ICDs provide reliable DE at outputs comparable to those observed in the PIVOTAL study and in transvenous ICDs, it seems that reverse polarity shock confers a greater safety margin for defibrillation.