Leadless Pacemakers: Current and Future Applications Applications

Leadless pacemakers: a brand new technology with a low rate of lead and pocket complications compared to traditional transvenous pacing. What it is known until now, current issues and future perspectives.


Introduction
Since its first prototype proposed in the 1970s [ The main advantage of leadless pacing technology over traditional pacing technology should essentially be to avoid leadand pocket-related complications. The incidence of post implant complications has been estimated as high as 10% [2] and it accounts for short term complications (such as pneumothorax, cardiac tamponade, pocket hematoma and lead dislodgement) and long term complications, mostly due to the vulnerable component of the system, the transvenous leads (such as insulation breaches and lead fractures), but also skin erosions, pocket or systemic infections, venous obstruction, tricuspid regurgitation and endocarditis.

Characteristics of Currently Available Leadless Pacemakers
The Nanostim LPS received a CE mark in October 2013, but it is still awaiting for the Food and Drug Administration (FDA) approval, while the Micra TPS was CE certified in 2015 and approved by the FDA in April 2016. The two self containing leadless devices share some common features, with some differences: a) Discrete volume (1 cm3 for nanostim LPS versus 0,8 cm3 for Micra TPS); Nanostim is longer and narrower than Micra (42mm x 5.99mm vs 25.9mm x 6.7mm respectively). b) Delivery system via a dedicated deflectable sheath (18F/21F, inner/outer diameter, for the LPS and 23F/27F for the TPS) advanced through the femoral vein under local anaesthesia and fluoroscopic guidance.
c) A small amount of contrast media is injected through the sheath to assess the correct position of the device inside the right ventricle. d) Stimulation mode (VVI/R) with an integrated algorithm for rate response modality, based on a temperature sensor for Nanostim while Micra employs a three-axis accelerometer. e) Retrievability of the device: the proximal end of both devices has been designed to recapture the system. Nanostim has a specially designed catheter with a single-loop snare system. Nanostim retrieval has a high success rate (up to 88%), and it remains this high also in older devices in selected cases. Micra retrieval requires the use of a goose-neck catheter introduced inside a steerable sheath or through the delivery system. The success rate appears to be 60-80%. There is some concern, and poor documentation, about retrieval of older devices due to early encapsulation so abandonment seems to be a suggested strategy.
The Micra Global Clinical Trial was a prospective, non randomized, single arm worldwide trial, which aimed to evaluate safety and efficacy of the Micra TPS mid-term (6 months) and longterm (12 months), in terms of freedom from major complication and electrical performance [11], comparing the Micra cohort with a historical "VVI modality" cohort. In brief, a safety endpoint was achieved without major complications (a rate of 96% complicationfree at 12 months and a 48% reduction in 1-year complication rate, with an 82% decrease in revision procedures, a 48% lower 1-year complication rate). Electrical performance was excellent throughout the 24 months. To date, no long term performance and safety data on leadless pacemakers is available and RCTs comparing this new technology with traditional transvenous/epicardial pacing are lacking. Retrievability of leadless systems is easily feasible during the implantation procedure, before unlocking the tethering lead, and the procedure is well described by the manufacturer.
Further concerns have been raised upon retrieval after the "acute" phase, when fibrosis develops around the distal end of the device.
Few cases of retrieval of the Micra TPS have been described [12], demonstrating that retrieval is feasible during an early phase (a range of 1-61 days) post implant using a percutaneous gooseneck snare. Our group described a successful retrieval of a Micra TPS after 40-days from implantation, due to increased pacing threshold secondary to micro-dislodgement [13].

Future Perspectives
Single chamber pacemakers serve a minority of PM  [14,15].
Another possible application of leadless pacemakers is within defibrillation therapy, for instance combined with subcutaneous ICD (s-ICD), with the aim to erogate antitachycardia pacing. Further investigation in these fields is needed.

Conclusion
Leadless pacemakers are a new promising technology, the use of which is spreading in the clinical practice due to the possibility of avoiding leads-and pocket-related complications of transvenous pacing. It is mandatory to further investigate long term performance, safety and retrievability of these devices in ad hoc randomized clinical trials, in comparison with conventional endocardial pacing. Their field of application is still confined to single-chamber pacing. In the future, multicomponent, wireless, leadless systems could possibly enable integration between multisite pacing, resynchronization therapy, defibrillator therapy and even heart failure monitoring.