Leadless pacemakers

Leadless pacemakers

Weakest link in the pacemaker system is considered to be its lead, sometimes called it Achilles heel. This is because the lead system is prone for infection, fracture, failure and dislodgement. The more the number of leads the more the complications, as in sophisticated systems like dual chamber pacemakers and biventricular pacing devices. The incidence is compounded over time with longevity of the device and the recipient. The risk of lead extraction is also significantly high, in case it is needed. Hence the need for a leadless pacemaker system.

Micra Transcatheter Pacemaker System (Medtronic) can be delivered percutaneously using a delivery system via the femoral vein. It provides right ventricular sensing, pacing and rate responsiveness (VVIR). It uses an accelerometer sensor for rate responsiveness [1]. The device is delivered in the right ventricle.

Micra Transcatheter Pacing Study Group successfully implanted the device in 719 of 725 patients enrolled for a multicenter study [2]. Primary safety end point was freedom from system related or procedure related major complications. Primary efficacy end point was percentage of patients with low and stable pacing capture thresholds at 6 months. Cut off was ≤2.0 V at a pulse width of 0.24 ms and an increase of ≤1.5 V from the time of implantation. Major complications were compared with those in a control cohort of 2667 patients with transvenous pacemakers from six studies published earlier. Rate of primary safety end point was 96% and primary efficacy end point 98.3%. Complication rates were significantly lesser than the historical control cohort.

A feasibility study of accelerometer based atrioventricular synchronous pacing with a ventricular leadless pacemaker was published in 2018 [3]. Micra has a 3-axis accelerometer. They developed a custom software to detect atrial contraction using the accelerometer to enable AV synchronous pacing. The Micra Atrial TRacking Using A Ventricular AccELerometer (MARVEL) study tested the algorithm downloaded into previously implanted Micra devices. AV synchrony was defined as visible P wave on surface ECG followed by a ventricular event in less than 300 ms. A total of 64 patients in 12 centers across 9 countries were evaluated. Patients were implanted with a Micra for a median period of 6 months. 33 had high grade AV block while 31 had intrinsic AV conduction during the study. Average duration of AV algorithm pacing was 87% in those with high grade AV block. 94.4% AV synchrony was noted in those with intrinsic conduction. Here the pacemaker senses the mechanical activity of the atrium rather than the electrical activity which is sensed by conventional AV sequential pacemakers.

In an innovative case report, a new leadless pacemaker was synchronized with an existing transvenous atrial pacemaker [4]. Patient had sinus node dysfunction and intermittent Mobitz type II AV block for which a dual chamber pacemaker was implanted earlier. Pacemaker interrogation after syncopal episode showed right ventricular lead threshold of 5.5 V at a pulse width of 1 ms. There was only <1% right ventricular pacing and there was a previously documented chronic malfunction of right ventricular lead with impedance >3000 ohms. Atrial lead was stable with intact parameters. Patient was on hemodialysis using a left arm arteriovenous fistula for end stage renal disease. Hence risk of bleeding and infection for lead extraction and implantation of a new lead on the left side was deemed high. Considering all aspects, they decided to go for a leadless pacemaker and a Micra AV was implanted. Previous pacemaker was programmed to atrial pacing, atrial sensing, inhibited mode (AAI). Micra AV was synchronized to the atrial contraction resulting from atrial pacing of the previous transvenous pacemaker. Thus the patient had two pacemakers interacting to produce AV synchrony after careful programming.

A systematic review and meta-analysis on safety and efficacy of leadless pacemaker has been published [5]. They identified 36 observational studies of Nanostim and Micra leadless pacemakers, of which 69.4% reported outcomes for Micra. In 5 studies with 1 year follow up, Micra was associated with 51% lower odds of complications compared with transvenous pacemakers. 98.96% of the 1376 patients implanted with Micra had good pacing capture thresholds. They concluded that studies report outcomes for Micra associated with a low risk of complications and good electrical performance up to 1 year after implantation. They opined that further randomized controlled trials are needed to support the widespread adoption of these devices in clinical practice.

References

1. Neal Bhatia, Mikhael El-Chami. Leadless Pacemakers: A Contemporary Review. J Geriatr Cardiol. 2018 Apr;15(4):249-253.
2. Reynolds D, Duray GZ, Omar R, Soejima K, Neuzil P, Zhang S, Narasimhan C, Steinwender C, Brugada J, Lloyd M, Roberts PR, Sagi V, Hummel J, Bongiorni MG, Knops RE, Ellis CR, Gornick CC, Bernabei MA, Laager V, Stromberg K, Williams ER, Hudnall JH, Ritter P; Micra Transcatheter Pacing Study Group. A Leadless Intracardiac Transcatheter Pacing System. N Engl J Med. 2016 Feb 11;374(6):533-41.
3. Chinitz L, Ritter P, Khelae SK, Iacopino S, Garweg C, Grazia-Bongiorni M, Neuzil P, Johansen JB, Mont L, Gonzalez E, Sagi V, Duray GZ, Clementy N, Sheldon T, Splett V, Stromberg K, Wood N, Steinwender C. Accelerometer-based atrioventricular synchronous pacing with a ventricular leadless pacemaker: Results from the Micra atrioventricular feasibility studies. Heart Rhythm. 2018 Sep;15(9):1363-1371.
4. Siroky GP, Bisht D, Huynh H, Mohammad A, Mehta D, Lam P. Synchronization of the new leadless transcatheter pacing system with a transvenous atrial pacemaker: A case report. HeartRhythm Case Rep. 2020 Aug 25;6(12):899-902. 
5. Ngo L, Nour D, Denman RA, Walters TE, Haqqani HM, Woodman RJ, Ranasinghe I. Safety and Efficacy of Leadless Pacemakers: A Systematic Review and Meta-Analysis. J Am Heart Assoc. 2021 Jun 25:e019212.