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BOSTON -- A leadless pacemaker reliably communicated with a subcutaneous implantable cardioverter-defibrillator (S-ICD) to deliver anti-tachycardia (ATP) and bradycardia pacing, the MODULAR ATP study showed.

In terms of safety, fully 97.5% of recipients of the investigational modular system were free from leadless pacemaker-related major complications, exceeding the performance goal of 86%, reported Reinoud E. Knops, MD, PhD, of Amsterdam University Medical Center, at the annual meeting.

The findings were also published in the .

The pacemaker and S-ICD maintained communication in 98.8% of tests in a variety of body positions and had a pacing threshold of 2.0 V or less at a 0.4-msec pulse width for 97.4%, both exceeding performance goals of 88% and 80%, respectively.

The modular system terminated 61.3% of episodes of arrhythmia using ATP; no episodes missed ATP due to a communication failure between devices.

Knops also presented 1-year follow-up data from a second trial at the same session, showing that the dual-chamber Aveir DR leadless pacemaker continued to meet safety and communication performance targets at that point. The device had been approved in 2023 based on 3-month data.

"These data will make S-ICD therapy available for a larger patient population," Knops told attendees at the late-breaking clinical trial session.

Guidelines have contraindicated S-ICD use for patients who need ATP because they haven't had that capability. With the addition of a leadless pacemaker, the system resolves that and offers benefits to patients, the researchers suggested.

"Modular communicating pacemaker-defibrillators enable individual component upgrades or revisions without complete replacement of the hardware, eliminate single points of failure, and allow multichamber and multidevice interaction by means of intrabody communication without transvenous leads, which are a source of long-term complications of transvenous devices," they wrote.

Session study discussant Ratika Parkash, MD, of the Queen Elizabeth II Health Sciences Centre in Halifax, Nova Scotia, called the modular system findings an exciting advance for patients at risk for arrhythmic death. "Compared to current leadless ICD systems, it provides the much desired option of painless termination of VT [ventricular tachycardia] with ATP. ATP can also be effective in limiting inappropriate shocks by terminating 1:1 SVTs [supraventricular tachycardias] or slowing the ventricular response in the case of AF [atrial fibrillation]."

However, some questions remain about safety and efficacy. Parkash raised the issues of battery longevity of the leadless portion, whether it needs to be removed prior to re-implant, whether communication between devices will remain constant over time, and if later complications will arise.

"The future looks bright for a leadless defibrillation system with ATP, but for now transvenous ICDs are here to stay until these issues can be resolved. The modular ATP system is certainly getting closer to solving some of these problems for our patients with high voltage systems," she said.

The multinational, single-group included 293 patients at risk for sudden death from ventricular arrhythmias. Of these patients, 151 of 162 planned for 6-month follow-up reached that endpoint for the interim analysis. Mean age was 60, 16.7% were women, and left ventricular ejection fraction averaged 33.1%. Need for chronic pacing was an exclusion criterion.

All were successfully implanted (14% with one reposition and 13% with multiple repositions) with an investigational modular pacing-defibrillator system comprised of the Emblem S-ICD coupled with the Empower leadless pacemaker. Knops noted that any patient with an existing S-ICD can be upgraded to a modular system when the leadless pacemaker is commercially available. The Empower device acts as a stand-alone pacemaker and can deliver ATP after receiving the request from the S-ICD and confirming that the rate is in the therapy zone.

However, there's currently no communication from the leadless pacemaker back to the S-ICD. "In the future it will be possible to have bidirectional communication," Knops said, "and that will certainly minimize the problem of inappropriate therapy, because then you will have the best of both worlds. You could incorporate transvenous algorithms and morphology algorithms from the subcutaneous ICD, combine them, and further minimize inappropriate therapy."

The system does not delay time to therapy for ATP but does delay time to shock.

Complications seen in the patients included two cases of myocardial perforation with tamponade (1.2%), which the researchers said was "a percentage similar to that reported with implantation of other single-chamber leadless pacemakers, and was resolved with pericardiocentesis."

No pacemaker dislodgements occurred up to 6 months after implantation, "a finding that contrasts with reports for other leadless pacemakers," Knops and team wrote. Two patients had successful device retrieval some time after implantation. In another patient, vasovagal syncope occurred related to the procedure, and in one case the leadless pacemaker was "inadvertently implanted" in the left ventricle through the interarterial septum. Eight patients died (4.9%), but none were deemed to be related to arrhythmias or the implantation procedure.

No pacemaker revisions had to be done for inadequate pacing thresholds. "Pacing never resulted in undersensing of arrhythmia by the subcutaneous ICD during defibrillation testing or during clinical follow-up," the researchers noted.

The 61.3% rate of ATP termination of ventricular arrhythmias "appears similar to the 46 to 72% of pacing attempts that were shown to be successful in large trials of ICDs and the 70% reported in a study of an extravascular ICD," the group added.

However, they acknowledged that "inappropriate shocks were delivered in 4.9% of patients, which is higher than the 2.1% at 6 months that was reported in a recent trial of a subcutaneous ICD. Because most of the inappropriate shocks were delivered in response to slower ventricular arrhythmias, this finding may be attributable to patient selection and device programming."

Limitations of the study included its nonrandomized design with no comparator group, its reliance on performance goals without being able to determine efficacy, and unknown generalizability to patient populations outside of the selected, high-risk group studied, such as those who require ICDs or who have ICDs already implanted.

Knops noted that the study had "a bit of an atypical ICD population," with nearly half implanted for secondary prevention, an older age averaging 60 years, and almost 60% with prior ventricular arrhythmias.

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