Given the early in-human experience with leadless pacemakers, safety and effectiveness of chronic retrieval of such devices is still under question. Reinoud Knops (Amsterdam, The Netherlands) reviews what is known so far on this subject.
By Reinoud Knops
Since the introduction of the first pacemaker in 1958, significant refinements and advancements have been made to these devices. Improved longevity, rate response and further miniaturisation of the pulse generator have improved their performance and usability. However, the implantation of conventional transvenous pacemakers has not changed. It still requires a surgical procedure to create a pocket in the chest and the introduction of one or more leads into the cardiac chambers to deliver pacing therapy.
The recent introduction of leadless pacing systems from St Jude Medical and Medtronic has changed this paradigm forever. The Nanostim Leadless Cardiac Pacemaker (St Jude Medical) is a cylindrical device 42mm long and 6mm (18F) in diameter. It uses a single-turn helix and secondary tines to provide secure fixation. The device uses temperature to provide rate response and has magnet mode. The Micra Transcatheter Pacing System (Medtronic) is also a cylindrical device measuring 26mm in length and 6.7mm (20F) in diameter. The pacemaker utilises four electrically inactive, curved tines to provide attachment to the endocardium. The three-axis accelerometer provides rate response. Both leadless pacemakers are delivered via the femoral vein with a steerable, deflectable catheter and can be repositioned if electrical parameters are deemed to be inadequate.
Early experience with the Nanostim and Micra devices in prospective, multicentre single-arm clinical studies showed positive results with both systems meeting the pre-specified safety and performance criteria.1,2 Although the early performance of these leadless pacemakers has been positive it has also raised questions about how to approach these pacemakers at end of service or if there are other reasons to remove the system such as infection or need for a device upgrade. In conventional transvenous pacemakers, the subcutaneous position makes the generator readily accessible for removal and replacement at end of service. Unlike conventional pacemakers, leadless pacemakers are totally self-contained residing entirely in the right ventricle, which may cause difficulties at the time of removal.
Both systems have different fixation mechanisms and therefore may be more or less amenable to chronic removal. The Micra device uses four electrically inactive, curved tines for attachment. The Nanostim pacemaker is fixated to the myocardium with a screw-in helix. Early pre-clinical animal experience with the Micra showed successful retrieval up to 28 months after implant.3 In-human experience with attempted removal of the Micra device showed success in one out of three attempts (33%).2 A poster at the 37th Heart Rhythm Society Scientific Sessions (HRS; 4–7 May, San Francisco, USA) showed an unsuccessful retrieval of a device implanted for 228 days using off-the-shelf snares (Cook Medical).4 So the early experience is mixed and more research needs to be performed to assess the safety and effectiveness of chronic retrieval with the Micra pacemaker.
Like Micra, Nanostim incorporates a docking mechanism on the proximal end of the pacemaker. Unlike Micra, Nanostim has developed a proprietary Nanostim Retrieval System, which uses a steerable, deflectable catheter with a single loop and tri-loop version that is delivered via the femoral vein. It is designed to navigate the snare to the right ventricle, position it behind the docking feature, grasp the docking feature and mate it to the catheter and finally unscrew the leadless pacemaker from the endocardium.
Early animal experience using this retrieval system has been positive. An ovine study by Koruth et al5 has demonstrated the safety and effectiveness of early and mid-term retrieval of Nanostim. Retrieval was attempted in a cohort of eight and 10 sheep at a mean of 5.3 months and 2.3 years, respectively. Device retrieval was successful in 18 of 18 subjects (100%) with no complications and no evidence of pericardial effusion after assessment with intra-cardiac echocardiography. The catheter time needed to retrieve the leadless pacemaker was 2:35 and 3:05 minutes in the short- and mid-term groups, respectively. There were small amounts of adherent tissue on the docking button and helix.
Recently, Vivek Reddy (Mount Sinai Hospital, New York, USA)6 and myself presented the worldwide clinical experience of Nanostim retrieval at HRS 2016 and CARDIOSTIM-EHRA EUROPACE (8‒11 June, Nice, France), respectively. As of 26 April 2016, 1,185 devices had been implanted in the LEADLESS, LEADLESS II and European Registry. Fifteen patients underwent an attempted retrieval at 12 centres with 12 different operators. Fourteen of fifteen devices were successfully retrieved (91%) with no complications. The primary reasons for retrieval were worsening heart failure and elevated thresholds. The time to retrieval ranged from one to 1,188 days. These data look promising but additional longer-term retrieval data and enhanced tools to deal with more chronically implanted leadless pacemakers may be required.
With the advent of leadless pacemakers, strategies to deal with devices reaching end of service or requiring upgrades need to be developed. One strategy could be that after battery depletion the device is not retrieved but a second leadless pacemaker is implanted next to it since both leadless pacemakers use only a very limited volume of the right ventricle (about 1cc). We are familiar with the concept of leaving an abandoned lead in place when a lead fracture occurs and we may consider this strategy also for leadless devices.
However, I propose that the preferred strategy would be to limit the amount of non-functioning hardware one leaves in the human heart. The best way to avoid the need for retrieval is to strive for economic battery use to increase longevity. Care should be taken during implantation avoiding relatively high pacing thresholds because they are inversely related to the life expectancy of the leadless pacemaker. Also, economic programming can increase longevity, especially in non-pacemaker dependent patients where the output of the pacemaker can be programmed close to the pacing threshold. Although, longevity with these leadless pacemakers at actual use conditions looks promising—12 and 15 years for Micra and Nanostim, respectively,1,2 retrievability is an important consideration when making decisions about which patient populations should receive these promising new devices. For many of these patients, these devices could be the last they receive, but the inability to retrieve chronically implanted devices may limit the application of this revolutionary new technology.
References
1) Reddy et al, Percutaneous implantation of an entirely intracardiac leadless pacemaker, N Engl J Med 2015;373(12):1125‒1135
2) Reynolds et al, A Leadless Intracardiac transcather Pacing System, N Engl J Med 2016;374(6):533-541
3) Bonner et al, Extraction of the Micra Transcatheter Pacemaker System, HRS Poster Session 2015
4) Giocondo et al, Unsuccessful extraction of a Medtronic Micra Leadless Pacemaker, HRS Poster Session 2016
5) Koruth et al, Percutaneous retrieval of implanted leadless pacemakers: Feasibility at 2.5 years post-implantation in an in vivo ovine model, JACC: Clinical Electrophysiology 2015;1(6):563-570
6) Reddy et al, Worldwide clinical experience of the retrieval of leadless cardiac pacemakers, HRS Poster Session, HRS 2016
Reinoud Knops is with the Academic Medical Center, Amsterdam, The Netherlands
