A Multicenter Study of Shock Pathways for Subcutaneous Implantable Defibrillators

Subcutaneous Defibrillation Introduction A purely subcutaneous implantable cardioverter defibrillator (ICD) requires higher energy but may be an effective alternative to transvenous ICDs to deliver lifesaving therapies. Objective To identify combinations of anteroposterior subcutaneous shock pathways and waveforms with defibrillation efficacy comparable to transvenous ICDs. Methods Defibrillation testing was performed in 141 patients temporarily implanted with an active can emulator and subcutaneous coil electrodes. The patients were subdivided into 5 groups within 2 study phases. In all groups, a posterior electrode was positioned with its tip close to the spine. In the first study phase, 2 different can locations were evaluated: (1) an inframammary pocket (IM‐1–750), or (2) a conventional infraclavicular pocket (IC‐1–750). In both cases, a 70 J biphasic shock was used (peak voltage 750 V; 270 μF capacitance). In the second phase, configuration IC‐1–750 was enhanced by the addition of a second (parasternal) subcutaneous electrode (IC‐2–750). Furthermore, the effects of a different 70 J shock waveform (1,000 V, 160 μF) were evaluated for configurations IM‐1–750 and IC‐2–750 (becoming IM‐1–1000 and IC‐2–1000). Results The proportion of patients satisfying a defibrillation safety margin test of 2 consecutive successes at ≤50 J was 74%, 11%, and 44%, respectively, for the IM‐1–750, IC‐1–750, and IC‐2–750 configurations, and 93% and 86% for the IM‐1–1000 and IC‐2–1000 configurations. Conclusions Defibrillation efficacy comparable to that of a transvenous system was achieved with an anteroposterior subcutaneous ICD configuration, with 160 μF capacitance, 1,000 V, and 70 J output. An infraclavicular pocket location becomes feasible if a parasternal subcutaneous coil is added.