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BARCELONA -- The early global experience with an extravascular implantable cardioverter-defibrillator (ICD) saw it working safely to prevent sudden cardiac deaths, albeit amid an excess of inappropriate shocks, the EV ICD pivotal study showed.

People receiving the ICD for primary or secondary prevention experienced successful defibrillation in 98.7% out of 316 implantation attempts, the lower limit of the confidence interval allowing the device to meet its 88% performance goal in detecting and terminating induced ventricular arrhythmias.

Freedom from major system- or procedure-related complications was 92.6% at 6 months, and the device also met the safety goal of 79%, reported Ian Crozier, MB, ChB, of Christchurch Hospital in New Zealand, at the European Society of Cardiology (ESC) meeting. The results were published simultaneously in the .

The extravascular ICD features a single lead implanted substernally such that the device can deliver pause-prevention pacing and antitachycardia pacing without any leads in the veins or heart, unlike the conventional transvenous ICD. The extravascular device is also smaller than subcutaneous ICDs and delivers shocks at approximately half the energy.

"Despite concerns or the impression that this might be a dangerous procedure," there was no cardiac injury during implant, no deaths from the procedure, and no unique complications not already seen with transvenous and subcutaneous ICDs, according to Crozier.

There were no observed cases of mediastinitis, sepsis, or endocarditis related to the extravascular ICD. However, there were four extravascular ICDs that were explanted due to infection over follow-up averaging 10.6 months, suggesting a risk similar to what has been observed in subcutaneous ICDs.

What's more, there were 29 people who received 118 inappropriate shocks for 81 arrhythmic episodes. "The frequency of inappropriate shock of 8.5% at 6 months exceeds that of current ICDs but is similar to that of early-generation transvenous and subcutaneous systems," the authors noted.

Their observation that P-wave oversensing was to blame for many of the inappropriate shocks in the study cohort has led to new ICD programming recommendations and the ongoing development of a specific P-wave algorithm to reduce these events, Crozier stated.

During the Q&A portion of the presentation, ESC attendee Maged William, PhD, of Gosford Private Hospital in North Gosford, Australia, brought up the question of extracardiac stimulation and potential discomfort for patients.

Crozier acknowledged that there was patient awareness of pacing, which varied from mild to moderate with "a handful of patients" experiencing greater sensations. The number of people who had antitachycardia pacing disabled because of discomfort reached 5%, he said.

EV ICD was a follow-up to a 2019 that had reported short-term device safety and efficacy.

Crozier and colleagues conducted the present study at 46 participating sites in 17 countries with 356 people with a class I or IIa indication for a primary or secondary prevention ICD. Exclusion criteria included a chronic pacing indication including cardiac resynchronization therapy.

Of those enrolled, 316 ultimately proceeded with an implantation attempt, which was performed by several dozen expert implanters.

Mean age was 53.8, and 25.3% were women. Approximately four out of five people got the extravascular ICD for primary prevention. Nearly half the cohort had ischemic heart disease.

Crozier reported that 302 device recipients completed the defibrillation testing protocol and were counted in the main efficacy analysis.

The antitachycardia pacing of the extravascular ICD, as assessed with generalized estimating equations, was 50.8% successful in terminating spontaneous arrhythmia episodes.

No major intraprocedural complications were reported. By 6 months, the rate of major complications reached 7.3%.

There was one patient with a device software issue that required device replacement. The software was revised and the problem did not resurface in other patients.

There were two late lead fractures, one at 7 months and the other at 11 months, attributed to implantation below the xiphisternum and substantial unanticipated bending conditions. The study sponsor responded by changing the implantation guidance.

"Implantation of the extravascular ICD requires accessing the substernal space, an anatomical location not traditionally approached by cardiologists," Crozier and colleagues wrote. "In our study, which involved a dedicated training program and initial collaboration with a cardiac surgeon, implantation procedures were performed safely in electrophysiology laboratories, which supports both the importance of a comprehensive training program and the generalizability of the practice."

The investigators acknowledged that there was no comparison group receiving transvenous and subcutaneous ICDs in the study. They also cautioned that the relatively young patient population may limit the generalizability of the study's results to older, sicker patients.

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