Similarities and Differences Between the Extravascular and the Subcutaneous Implantable Cardioverter-Defibrillator: Sensing, Discrimination, and Complications (Part 2).

PURPOSE OF REVIEW: The subcutaneous ICD (S-ICD) and the extravascular ICD (EV-ICD) both avoid intravascular hardware, but they solve sensing and detection differently. This review compares their sensing architectures, detection and discrimination algorithms, therapy capabilities, and system-specific complications, along with practical considerations including extraction, MRI compatibility, and patient-reported outcomes.

RECENT FINDINGS: Inappropriate shock rates have declined substantially for both platforms through algorithmic refinements, two-zone programming and SMART Pass for S-ICD, and improved implant technique with a P-wave oversensing discriminator for EV-ICD. Oversensing patterns differ: T-wave oversensing predominates in S-ICD, while P-wave oversensing is the primary concern in EV-ICD. Lead fracture, though rare, has distinct mechanisms in each system. Quality-of-life data suggest higher device acceptance and fewer body-image concerns with EV-ICD, while S-ICD demonstrates comparable psychosocial outcomes to transvenous systems with potential advantages in anxiety reduction. Optimal non-transvenous ICD performance requires matching the platform's sensing and discrimination characteristics to the individual patient's arrhythmia substrate and clinical profile. Continued algorithmic development and refined implant techniques are expected to further reduce inappropriate therapy rates for both systems, while growing extraction experience and favorable quality-of-life data support expanding adoption.