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NEW ORLEANS -- An investigational extravascular implantable cardioverter-defibrillator (ICD) continues to demonstrate it can appropriately shock patients and prevent sudden cardiac death, extended pivotal trial follow-up showed.

After 17 months of follow-up, 21 episodes requiring a shock were recorded among 299 patients (6.8%) who had the extravascular ICD implant -- and all were deemed appropriate and effective, said Paul Friedman, MD, chief of cardiology at the Mayo Clinic in Rochester, Minnesota, here at the annual meeting of the Heart Rhythm Society.

In addition, the device has the capability of anti-tachycardia pacing, and in 67% of cases (35 of 52 episodes), the device was able to abort the tachycardia, reducing the need for shocks in many of those episodes.

"So in the course of this study, we concluded that in the longer term follow-up period, the device remained effective and free from system and procedure-related complications and that shocks were eliminated in about half of all episodes," Friedman said.

However, the inappropriate shock rate was still 11.7% over the extended follow-up (35 patients with 93 episodes), with a 1-year rate of 10.2%. When previously reporting the 6 month rate of 8.5%, the researchers had it "exceeds that of current ICDs but is similar to that of early-generation transvenous and subcutaneous systems."

Approximately half of the inappropriate shocks were due to cardiac oversensing, which Friedman noted had decreased with experience in the trial such that no new oversensing shocks occurred after the primary results were reported.

"For those patients with coronary disease and who have scars, they respond very well in many cases to anti-tachycardia pacing, which would then obviate the need for a shock. For that reason, having a device in that specific population where you can do anti-tachycardia pacing and some pacing there after is obviously incredibly important," commented Fred Kusumoto, MD, of the Mayo Clinic in Jacksonville, Florida, and immediate past president of the Heart Rhythm Society. He was not involved in the study.

Friedman suggested the new device would be suitable for anyone who fits criteria for ICD implant. However, the study excluded patients who had cardiac bypass graft surgery, so other devices might be suitable for that population.

While available subcutaneous and transvenous ICDs "are both excellent and effective," there's room for further innovation, Kusumoto acknowledged to MedPage Today. "The subcutaneous ICD has clearly the advantage of not requiring a transvenous route but has a shorter lifetime. That has huge implications for a patient who is going to need a device over the arc of their lifetime. Having a lead in a more efficient place will then allow for greater device longevity."

Friedman elaborated at a press conference where he discussed his late-breaker presentation, noting risk of cardiac perforation, pneumothorax, hemothorax, and vascular occlusion with transvenous devices. "That led to the development of the subcutaneous defibrillator, which is effective but does not offer any form of pacing support other than briefly after shock delivery and it doesn't have any anti-tachycardia pacing. Additionally, because defibrillation thresholds are higher, it necessitates a larger device and that does impact longevity of the device."

Extravascular ICD leads are implanted under the sternum with fluoroscopic guidance, Friedman added. "Due to the juxtaposition of the lead to cardiac tissue, the defibrillation threshold is lower, the device size is [similar to] a traditional transvenous ICD, and it delivers 40 joules as well as integrated anti-tachycardia pacing and pause prevention pacing and post-shock pacing."

About 3.9% of patients received anti-tachycardia pacing at 18 months in the single-arm trial.

"The long-term effectiveness and safety profile essentially persisted for the longer follow-up period," Friedman said. At 12 months the freedom from system or procedure-related complications was around 90%, he illustrated.

At 6 months, as previously reported, the defibrillation effectiveness was roughly 99%, while the 6-month freedom from procedure-related complications or any complications was 92.6%.

The patients were enrolled from September 2019 through October 2021 in 17 countries in North America, Europe, Australia, and Asia. About three-quarters of the patient population was male, and the average age of the cohort was 54 years. Average left ventricular ejection fraction was 39%.

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