For over 60 years, implantable cardiac pacemakers have been available for the treatment of bradycardia. Initial pacemakers required tethering to an external power source. The first fully implantable device was developed in 1958, and for the following 50 years, improvements have been incremental, with advances in battery longevity, reliability, and deliverability. However, in the last several years, there have been paradigmatic shifts in pacing, in the form of new approaches to cardiac synchronization and leadless pacing. In this article, we review our EP team’s early experience at Mission Health with the leadless pacemaker. (Figure 1)
The Micra Transcatheter Pacing System (Medtronic) is a single-chamber pacemaker (Figure 2) that occupies 0.8 cm3 and weighs 1.75 grams. It is delivered via a 27 French outer diameter venous access sheath (Figure 2). The device offers rate adaptive pacing and remote monitoring, with a projected battery life of 12 years. This longevity is made possible despite the small battery due to an optimized electrode – tissue interface.1 When the battery expires, a new device may be implanted adjacent to the original.
The leadless pacemaker was evaluated in a seminal study published in November 2015.2 The prospective, multicenter study evaluated patients with guideline-based indications for ventricular pacing, with a primary safety endpoint of freedom from system-related or procedure-related complications and a primary efficacy endpoint of percentage of patients with low and stable pacing capture thresholds at 6 months.2 Performance of the Micra was compared against historical transvenous pacing controls. The device was successfully implanted in 719 of 725 patients. The primary safety endpoint was achieved in 96.0%, and the primary efficacy endpoint achieved in 98.3%. Twenty-eight major complications occurred in 25 patients, though there were no dislodgments, and the rate of complication was lower than that noted amongst historical controls with transvenous implants (Figure 3).3 The Micra received FDA approval in 2016.
Review of Technique
We have been using the device for 4 months thus far, and have implanted approximately 15 devices. Appropriate patients for implant include those with compromised vascular access, elevated risk of infection, increased risk for lead fracture (usually due to young age), atrial fibrillation with preserved ejection fraction post AV junction ablation, and anyone else expected to pace infrequently. Independent risk factors for procedural complication include advanced age, female gender, and chronic steroid use.
Another category of patients particularly well served with the leadless pacemaker are those with preserved ejection fraction presenting for a combined pacemaker implantation and atrioventricular junction (AVJ) ablation. In these cases, it is our practice to deliver the Micra pacemaker through standard technique as described below, followed by delivery of an ablation catheter via the Micra delivery sheath to the AVJ. We insert an inner 16F short sheath through the external valve of the Micra delivery sheath to prevent back-bleeding before inserting the ablator (Figures 3 and 4). This approach is time efficient and minimally invasive for a population of patients that struggle to tolerate prolonged or complex procedures.
We perform the implant procedure in the EP lab under sterile conditions. Vascular access is obtained under direct ultrasound visualization. This step is critical, as the vascular access sheath, albeit exceptionally smooth, is stiff and large. We initially place a 6F short sheath by the modified Seldinger technique. Provided that step is straightforward, we then deliver an Amplatz Super Stiff Guidewire (Boston Scientific) to the high SVC under fluoroscopy. The 6F sheath is removed, and the Micra vascular access sheath is delivered over the wire to the mid right atrium. The wire and dilator are then removed, and the sheath is pulled back to just inferior to the diaphragm. A total of 3,000 units of heparin are provided, and a 500 mL/hr infusion of heparinized saline is delivered through the sheath. The Micra delivery catheter is then maneuvered to the right atrium under fluoroscopy. The fluoroscope is placed in the RAO projection. The tip of the catheter is deflected and rotated counterclockwise to cross the tricuspid valve. Next, in a single motion, flexion from the catheter is removed while advancing and rotating clockwise, in order to position the tip of the catheter in the mid to apical septum. We then inject contrast in the RAO projection to ensure the catheter is outside of the apex. The camera is moved to the LAO projection to ensure the tip of the catheter is septal. When contrast is injected in the LAO projection, an opaque stream should be seen coursing up and down, but not to the right. (Video 1 and Video 2)
Tip pressure is then applied by advancing the catheter until a swan neck deformity of the sheath can be seen across the tricuspid valve. The Micra is unsheathed from the cup that contains it, allowing its 4 nitinol tines to engage the trabeculae of the right ventricle. The catheter is then pulled back as the cup retraction is completed to relieve the tip pressure. The Micra is now connected to the sheath by only a looped tether. That tether is gently pulled under magnified high-resolution fluoroscopy to ensure that at least two tines of the Micra are engaged. (Video 3)
Provided that at least two tines are engaged (preferably not adjacent to one another), the catheter is flushed with heparinized saline. The tether emerging from the handle of the catheter is then “flossed” back and forth, to determine the direction with the least resistance. The side with the greatest resistance is cut, and the tether is then very slowly removed over approximately three minutes. We intermittently check the location of the Micra as well as the relationship of the catheter with the device on fluoroscopy. If resistance builds, the catheter will bend toward the Micra. We also pace at threshold. If capture is lost, it is suggestive of tension on the device. If resistance is met, it is best to flush the catheter, apply gentle traction, and allow the motion of the heart to slowly work back the tether.
If recapture is required, it is best to do so before the tether is cut. If the device is free, a snare may be delivered through the Micra delivery catheter or through a large deflectable sheath. (Video 4)
The EP service was asked to see an 88-year-old woman for evaluation of symptomatic sick sinus syndrome/tachy-brady syndrome along with paroxysmal atrial fibrillation. She had multiple comorbid conditions, including obstructive sleep apnea, anemia, poorly controlled diabetes, chronic kidney disease, and recurrent urinary tract infections. The primary team raised the possibility of dual-chamber pacemaker implantation followed by AV junction ablation. The EP team felt she could be managed more conservatively with adjustments in her medical therapy coupled with backup pacing for her symptomatic intermittent sinus bradycardia, with heart rates occasionally in the 40 bpm range. We elected to proceed with Micra implantation, which was a relatively simple and short procedure for this elderly woman. The physician procedure time was less than 30 minutes, and intraoperative measurements were superb (capture threshold 0.5 volts at 0.24 ms, impedance 580 ohms, and R waves 8.9 mv).
Pacing has become exciting again — the last several years have brought paradigmatic shifts in pacing, including interventional cardiac resynchronization therapy, His bundle pacing, and leadless pacing. Interventional CRT will require implanters to undergo additional training should they decide that while their current approaches are sufficient, an alternative approach may be better. His bundle pacing will achieve greater uptake should large, multicenter, prospective, randomized control trials demonstrate benefits relative to dual-chamber pacing and biventricular pacing, lead delivery tools improve, and payment mechanisms recognizing the increased effort that accompany these cases become available. Leadless pacing has already found a niche, and wider scale adoption of this approach will follow smaller delivery sheaths, better extraction tools, and multi-chamber pacing.
Disclosures: The authors have no conflicts of interest to report regarding the content herein. Outside the submitted work, Dr. Souza reports he is a consultant for Medtronic. Dr. Rhyner reports personal fees for fellows education honoraria from Medtronic, advisory board and staff education from Boston Scientific, and staff education from Abbott, outside the submitted work.
- Bonner M, Eggen M, Haddad T, Sheldon T, Williams E. Early performance and safety of the Micra transcatheter pacemaker in pigs. Pacing Clin Electrophysiol. 2015;38:1248-1259.
- Reynolds DW, Duray GZ, Omar R, et al. A leadless intracardiac transcatheter pacing system. N Engl J Med. 2016;374(6):533-541.
- Duray GZ, Ritter P, El-Chami M, et al. Long-term performance of a transcatheter pacing system: 12-month results from the Micra Transcatheter Pacing Study. Heart Rhythm. 2017;14(5):702-709.