The Paradox of Innovation With Leadless Pacing

Michael Rehorn MD; Albert Y. Sun MD


J Cardiovasc Electrophysiol. 2019;29(12):1705-1706. 

Over the past four decades, Moore's law has accurately predicted that computing power will double every 2 years. Yet, while today's handheld calculators possess more processing ability than the original NASA Apollo Guidance Computer, the basic design and function of a VVI pacemaker have not changed significantly since the 1960s. Nevertheless, transvenous pacemakers have proven to be an indispensable treatment option for patients with symptomatic and often life-threatening bradycardia. This therapy, however, is not without risk. Amongst other issues, pocket hematoma, pneumothorax, hemothorax, lead dislodgement, infection, and vascular compromise are significant causes of morbidity in patients with transvenous devices. Aimed at reducing associated risks by dramatic changes in pacing delivery, the recent development of leadless pacemakers represents one of the most significant design innovations in cardiac pacing since the very first implantable device. Paradoxically, however, the evolution from a modular and expandable transvenous system to a fixed single chamber device is, in some respects, taking a step backward in our programming ability. To date, two separate products have been evaluated and brought to market, the Nanostim LP (Abbott, Abbott Park, IL) and the Micra Transcatheter Pacemaker (Medtronic, Minneapolis, MN).[1,2] While initial results are quite promising, understanding the appropriate application and limitations of transcatheter pacing systems (TPS) will be essential for the safe adoption of this relatively new technology.

In this issue of the journal, Tjong et al studied the health-related quality of life (HRQoL) impact of a TPS.[3] Similar to the standard pacemaker therapy, patient-rated HRQoL was found to be significantly improved 3 months after the implantation of the device. This effect was studied on both a physical and a mental component scale. The significant improvement persisted after 12 months of follow-up and was most pronounced in the role-physical domain. Furthermore, most patients were either very satisfied or satisfied with their recovery, appearance, and level of activity after implantation, and a significant majority of implanting physicians rated the degree of activity restriction as either equally or less restrictive (96%) than the traditional pacemaker therapy. These quality of life (QoL) improvements with TPS should not be undervalued. The decision to implant a pacemaker is often based heavily on the acute rhythm disturbance, with the long-term impact of the device delivery system as more of a secondary afterthought.

Beyond QoL, this study by Tjong et al also highlights the importance of patient selection. Quite notably, during the 12-month follow-up period, there were a total of 52 deaths in the study group (7%). While these deaths were not attributed to complications from either device implantation or dysfunction, patient selection may have contributed to the increased mortality. In this study group, for instance, nearly 18% of the patients had congestive heart failure, a condition in which RV pacing has been associated with worse outcomes, prompting the need for additional investigation.

As described previously, current indications for permanent single ventricular pacing with a leadless device include atrial tachyarrhythmias not expected to benefit from atrioventricular (AV) synchrony, sinus bradycardia with low anticipated pacing burden, low level of baseline physical activity, limited life expectancy, and compromised vascular access.[1,2,4] In a recently published real-world registry of 795 patients who underwent Micra TPS implantation, a majority had permanent or persistent atrial tachycardia (57.7%). Of these 795 patients, nearly 21% had more than one condition that precluded placement of a transvenous system (compromised vascular access, history or risk of infection, cancer, thrombosis, a need to preserve veins for hemodialysis, and valvular issues).[5] In addition, there were only 13 major complications in these patients, with the most common (6 events) being related to the groin access site. Notably, there was one device dislodgement (without embolization) as compared with zero dislodgements in the initial study group.[1] Additional 12-month monitoring data for the Micra device showed a significantly lower rate of complications compared with traditional intravascular devices, while also demonstrating adequate pacing thresholds, R-wave sensing, and battery longevity. While longer-term surveillance will be needed to fully understand the limitations of the device, the initial safety data are quite promising, making TPS a very appealing alternative to standard therapies in the appropriately selected patients.

Several groups of patients appear to be optimal candidates for isolated right ventricular pacing. These include patients in permanent atrial tachyarrhythmia for whom restoring AV synchrony is unnecessary, those with vascular compromise that makes placement of a transvenous system infeasible, patients with elevated risk of infection, patients expected to pace infrequently, and those with limited life expectancy. For example, consider a 70-year-old male with a history of permanent atrial fibrillation, end-stage renal disease on hemodialysis, normal ejection fraction (EF), hypertension, and hyperlipidemia who presents with intermittent periods of bradycardia and associated presyncope. In all likelihood, his pacing burden would be quite low; given the intermittent nature of his symptoms, he has no need for an atrial lead due to his permanent a-fib, and his risk of infection is elevated due to his hemodialysis needs. With all of the above comorbidities and limited life expectancy, he would be an ideal candidate for this technology. Now, consider the same patient but with an EF of 45%. Should he receive a single chamber device or might he benefit from biventricular or His bundle pacing, and at what cost? Were the 52 deaths in the study by Tjong et al simply a result of a high-risk patient population or are we, as operators, doing harm with an "overuse" of VVI pacing given the perceived benefit of a TPS? A single arm clinical study cannot answer these questions.

The lack of AV synchrony, one of the most significant limitations with TPS, may soon be addressed as the recently published MARVEL study suggests.[6] By utilizing the accelerometer feature within the device, an algorithm was developed that allowed effective sensing of E and A waves during diastole. By timing ventricular pacing off of the sensed A wave, the authors were able to restore AV synchrony 87% of the time in this group of 64 patients. While these initial data are somewhat limited in scope, the potential implications are quite exciting and may expand implications for these pacemakers in the future.

In comparing early safety outcomes with standard intravascular pacemaker therapy, the Micra device performs quite strongly. At 6 months, the Micra device had significantly fewer major complications than the traditional control group (4.0% vs 7.4%).[1,7] Therefore, despite the lack of AV synchrony and biventricular pacing options, use of transcatheter devices may be a preferable therapy for those at increased risk of significant complications. Specifically, the extremely limited number of procedure-related infections to date and no infections requiring device removal[5,8] make TPS an attractive option in patients at risk for recurrent infections, including those frequent infections, a history of endocarditis, need for frequent vascular access (eg, hemodialysis), and intravenous drug users. In our experience, it is also reasonable to consider this device for use as a temporary permanent (temp-perm) pacing system in those requiring temporary support, as the exceptionally low rate of dislodgements suggests that the device can be left in place once no longer needed. This may reduce hospitalization time, costs, and associated morbidity as patients who would traditionally require hospitalization for temporary pacing could feasibly be discharged while awaiting future procedures.

Although TPS are in their infancy, the initial data regarding functionality and safety are quite promising. The work by Tjong et al demonstrates that the 12-month QoL improvement is at least comparable to traditional intravascular pacemakers despite the potentially sicker population they initially studied. Given the known limitations, however, patient selection is exceptionally important. Presently, ideal patients will have low anticipated pacing burden, would be unlikely to benefit from restored AV synchrony, would not be in the need for biventricular pacing, and may have increased infection risk, vascular access issues, or limited life expectancy. As this technology continues to improve, it may be possible to restore AV synchrony with the use of a transcatheter system. However, caution should be exercised in patients with depressed EFs or clinical heart failure diagnoses. Ultimately, while TPS has taken us one step back with VVI only pacing, the progress made in reducing complications and improving QOL has led us at least two steps forward, an innovation that has been a long time coming.