By Joshua D. Moss, MD

Associate Professor of Clinical Medicine, Cardiac Electrophysiology, Division of Cardiology, University of California, San Francisco

Dr. Moss reports no financial relationships relevant to this field of study.

SYNOPSIS: A system incorporating an entirely subcutaneous implantable cardioverter defibrillator that can command a novel leadless anti-tachycardia pacemaker unidirectionally showed success and promise in an early, short-term animal trial.

SOURCE: Tjong FVY, Brouwer TF, Koop B, et al. Acute and 3-month performance of a communicating leadless antitachycardia pacemaker and subcutaneous implantable defibrillator. J Am Coll Cardiol EP 2017. doi: 10.1016/j.jacep.2017.04.002.

In 2012, the FDA approved the subcutaneous implantable cardioverter defibrillator (S-ICD) for use in primary and secondary prevention of sudden cardiac death, and registry follow-up data collected since 2013 have shown high implantation success rates with a low incidence of short-term complications. With the ICD pulse generator implanted subcutaneously along the mid-axillary line and a shock coil tunneled along the anterior surface of the sternum, no intravascular hardware is required, eliminating both perioperative and long-term risks associated with transvenous endocardial leads.

Other than brief post-shock transthoracic pacing, the principal disadvantages of the S-ICD, when compared to the more traditional transvenous ICD, is its lack of pacing capabilities. Not only does that make the device inadequate for patients with any symptomatic bradycardia, but it precludes the ability to terminate many ventricular arrhythmias with pain-free ventricular overdrive pacing (antitachycardia pacing, or ATP). The only therapy available is a high-energy shock, which is highly effective but potentially painful (and perhaps even harmful). More recently, leadless pacemakers (LP) have been introduced as an alternative for ventricular pacing, with a self-contained capsule-like pacing device implanted directly against the endocardium. In this early trial of 40 animals (eight sheep, five pigs, and 27 dogs), the authors combined a novel LP with the S-ICD. The prototype LP was implanted in the right ventricle using a percutaneous, transfemoral approach, and it had the ability to deliver ATP as well as standard pacing. The implanted S-ICD had updated firmware to enable conducted communication directly to the pacemaker, allowing it to command ATP delivery. The LP was implanted successfully in 39 of 40 animals, with satisfactory pacing parameters; one implant attempt was aborted because of prototype catheter malfunction. The S-ICD was implanted successfully without cardiac adverse events in all 39 of these animals, and attempted communication from the S-ICD to the LP (to trigger delivery of ATP) was acutely successful in 99% of attempts. LP pacing did not impede the ability of the S-ICD to correctly discriminate abnormal heart rhythms.

Pacing parameters remained satisfactory at three months, although several animals were excluded from analysis because of early LP prototypes without steroid-eluting drug on the electrode or prototype programmer software coding errors. S-ICD to LP communication was successful in 100% of attempts, with appropriate translation of the conducted communication signal into ATP delivery by the LP. The authors estimated conservatively that 1,000 bursts of ATP per year would result in an approximately a 1.5-week decrease in LP battery longevity, and one hour of conductive telemetry per year would result in an approximately two-month decrease in LP battery longevity. First-in-human trials are planned.


Although still relatively early in its development and clinical testing, this combined system holds considerable promise. To serve its purpose (and justify the risks associated with its implantation and long-term presence), any implanted defibrillator at a minimum must be able to reliably detect dangerous ventricular arrhythmias and reliably terminate those ventricular arrhythmias. Beyond that, advances in ICD design and implementation have focused on better discriminating life-threatening tachyarrhythmias, such as ventricular fibrillation, from more acutely benign arrhythmias, such as atrial fibrillation with rapid ventricular rates, thereby reducing the need for shock therapies in favor of spontaneous arrhythmia termination or painless overdrive pacing while improving hardware integrity and battery longevity. As the authors outlined, this study demonstrated several important findings for a planned system that is completely wireless: A novel ATP-capable leadless pacemaker could be implanted safely and successfully, a standard S-ICD with updated firmware could communicate reliably with the LP and command delivery of ATP, and LP pacing did not impede successful rhythm discrimination by the S-ICD. The modular nature of the system could allow for the addition of an LP for a patient who already has an S-ICD, and vice versa, as clinical requirements dictate.

Clearly, more study will be necessary before the system is ready for clinical implementation; the safety and efficacy requires longer-term demonstration in human subjects. However, it may not be long before such a system becomes an attractive tool in our armamentarium, as the S-ICD component already has proven itself to meet those minimum requirements for an ICD, and the LP mainly serves to augment its safety and efficacy. Reliable sensing and pacing from leadless devices also has already been demonstrated, so the primary feature requiring further long-term study is communication from the S-ICD to the LP to enable coordinated therapy. Conducted communication (intrabody device-device electrical communication using the body tissue as a conductor) poses inherent risks: failed communication (because of inadequate signal transmission or electromagnetic interference) that could result in failure to deliver ATP, and perhaps more worrisome, miscommunication resulting in inadvertent delivery of ATP. The authors discussed two common sense safety features to minimize the risk of communication interference between the S-ICD and the LP, and rigorous testing will be necessary to ensure reliability.

Ultimately, by addressing the most important limitation of the current S-ICD, this combined system may offer the potential advantages of a leadless device to a much broader population of patients.