Initial Experience with the Micra Transcatheter Pacing System

Permanent transvenous pacemaker systems have been demonstrated to be safe and effective in the treatment of patients with bradyarrhythmias. The involved technologies of battery longevity, circuit miniaturization, and electrical consumption as well as implant techniques have improved notably over the years. In patients requiring permanent pacing, the traditional systems with intravascular leads and a subcutaneous generator may expose patients to inherent risks in both the short and long term. Serious risks include pneumothorax, cardiac perforation, effusion/tamponade plus pocket infection, erosion, lead fractures, venous obstruction, and tricuspid valvular insufficiency.¹ In addition, intravascular access may be challenging or impossible in those who have had previous surgery, vascular catheterization, or existing leads in place. The development of a completely intracardiac pacing system promises certain clear benefits by eliminating a portion of the risk profile associated with conventional systems. 

Despite potential advantages, a solely intracardiac device delivered through a transcatheter approach may be associated with unique risks, including those encountered during venous access with a large sheath, or device dislodgement. However, a recent review of 725 patients referred for leadless pacemaker placement showed that major complications, hospitalizations, and revisions were less frequent as compared to a historical cohort of 2667 patients who underwent traditional transvenous pacemaker implantation.² In this brief case report, we present our initial experience with the single-chamber, rate-responsive Micra Transcatheter Pacing System (TPS; Medtronic). (Figure 1)

Case Report

The patient is a 67-year-old male with hypertension, insulin-dependent diabetes, prior stroke, COPD, permanent atrial fibrillation, preserved cardiac ejection fraction, and tachy-brady syndrome, who underwent single-chamber pacemaker implantation in 2011. Two years later, he developed a pocket infection with coagulase-negative Staph aureus, and underwent extraction and reimplantation on the right side (Figure 2). In 2014, he developed Strep bovis bacteremia and was managed with a course of long-term antibiotics. In November 2016, he developed a skin blister with purulent drainage that led to device and lead erosion. Cultures from the pocket grew methicillin-resistant Staph aureus (MRSA), Proteus, and Corynebacterium, and he was treated with vancomycin and ceftriaxone. He underwent a second extraction with successful complete removal of the system. Due to his complete heart block, a temporary active fixation pacing wire was established through the left internal jugular vein. Following completion of one of the planned four-week courses of antibiotics, due to the high risk of recurring infection, erosion, as well as limited access and implant sites, implantation of a leadless pacemaker was planned.

A 6 French (Fr) short sheath was initially placed in the right femoral vein. Two Perclose ProGlide (Abbott Vascular) percutaneous closure devices were used for pre-closure. The entry site was progressively dilated up to an 18 Fr dilator. The Micra introducer sheath (27 Fr) was then inserted into the level of the right atrium. The delivery tool was inserted into the introducer and steered into the right ventricle. The ventricular septum was targeted, initially near the apex. Contrast dye was used to assess positioning of the device against the myocardium. However, secondary to the geometry and size of the right ventricular apex, the delivery tool could not be well-positioned against the apical septum. An alternate site at the mid-septal region of the RV was then selected. Device deployment in this area resulted in a relatively higher pacing threshold. Subsequent delivery at another site resulted in very high impedance (>2000 ohms), and the device was retrieved. Upon inspection, there was significant thrombus around the cathode electrode and in the insertion tool. The delivery system was retrieved, and the electrode was carefully cleared with saline flushes. The device was reintroduced and implanted onto the mid septum (Figure 3). A tug test performed with manual retraction on the tether resulted in splaying of 3 of 4 tines (at least 2 out 4 is recommended), with cardiac beating cycles indicating secure engagement to the myocardium. Initial interrogation of the device demonstrated excellent sensing and pacing parameters. The tether was cut and the device was released. Repeat interrogation again showed excellent pacing and sensing parameters. The delivery tool was retracted under fluoroscopy and removed. The introducer sheath was removed, and the vein was closed using the closure device and an additional figure-of-eight stitch around the skin puncture. Chest x-ray subsequently showed stable position of the device (Figure 4). 

At two-month follow-up, the device was interrogated and found to have a threshold of 0.75V with a 0.24 ms pulse width, R wave of 11.1V, and impedance of 490 ohms. With 95% right ventricular pacing, the device longevity was estimated at over 8 years. The patient reported doing well without subjective complaints, and the excision at the extraction site has healed completely. He has returned to his usual level of activities without any difficulties.

Discussion

Our experience implanting the first leadless pacemaker was both challenging, due to reimbursement coverage limitations, and rewarding, due to achievement of the desired outcome. The contained unit of the leadless pacemaker eliminated the required surgical incision and device pocket, both of which were associated with recurring complications in our patient described here. Our patient has permanent atrial fibrillation with complete heart block. Being a single-chamber system, the Micra TPS did not result in hemodynamic impacts such as in patients who need atrial-based pacing for sick sinus syndrome or ventricular resynchronization therapy for congestive heart failure. During the procedure, the device was repositioned several times for thresholds between 1 and 2 Volts. A recent analysis suggests that acute thresholds below 2V may decrease over time. If this may be confirmed with further implant experience or registry data, repeated repositioning to achieve a threshold below 1V may not be necessary.³ 

The reimbursement for leadless pacemaker systems was uncertain at the time of implant. The decision reached in this case followed an involved discussion between the physicians and hospital administration at our institution. The leadless system was felt to provide distinct advantages for our patient over the traditional system. The existing and ongoing complication risks for the patient with another traditional pacemaker system involving a pacing lead and device pocket were thought to be significant enough to offer the patient a new leadless system that mitigated these risks, even though the financial issues were unsettled. The associated increase in the cost of leadless pacemaker systems must be balanced with the needs of the patient. 

While approved by the FDA in April 2016, at the time of this implantation, the Micra system was not covered by the Centers for Medicare and Medicaid Services (CMS). This remains an obstacle. As of January 18th, 2017, CMS announced through a final National Coverage Determination to begin coverage of Micra through Medicare’s policy of Coverage with Evidence Development. Coverage will initially apply only to patients enrolled in one of two FDA- and CMS-approved studies designed to register outcomes.⁴ As of February 9, 2017, CMS announced the review and approval of the Micra FDA Post-Approval Study (National Clinical Trial number 02536118), thus providing reimbursement for Medicare patients who will also be enrolled.⁵ The approval for the Micra prospective, longitudinal study is still being awaited as of the time of this report, though they are expected in the next few months. More widespread use of the leadless pacing system will certainly depend on these further decisions by the CMS and the private insurer reimbursement coverage. 

Based on our experience, we have thus far limited the use of the device for patients with recurring pocket infection/erosion, very limited upper extremity venous access issues, superior central venous obstruction concerns, or pre-existing pacing indications undergoing planned tricuspid valve procedures. Plus, we plan to undertake this in patients who can reasonably accommodate a large-sized femoral sheath for implant. 

Expected further development of leadless pacing will advance the technology towards multi-chamber systems such as atrial-ventricular dual-chamber pacing for bradycardia as well as interventricular resynchronization therapy for congestive heart failure. The expected next frontier for leadless pacing is to provide anti-tachycardia pacing in patients with subcutaneous ICDs. A proof of concept ovine model has already been described.⁶ As technology evolves to include device-device communication and interaction with subcutaneous ICDs in humans, we would expect progressively expanding indications for their use.

Disclosures: The authors have no conflicts of interest to report regarding the content herein.   

References 

1. Mulpuru SK, Madhavan M, McLeod CJ, et al. Cardiac Pacemakers: Function, Troubleshooting, and Management: Part 1 of a 2-Part Series. J Am Coll Cardiol. 2017;69(2):189-210.
2. Reynolds D, Duray GZ, Omar R, et al. A Leadless Intracardiac Transcatheter Pacing System. N Engl J Med. 2016;374:533-541.
3. Piccini JP, Stromberg K, Jackson KP, et al. Long-term Outcomes in Leadless Micra Transcatheter Pacemakers with Elevated Thresholds at Implantation: Results from the Micra TPS Global Clinical Trial. Heart Rhythm. 2017 Jan 19. doi: 10.1016/j.hrthm.2017.01.026. [Epub ahead of print]
4. Decision Memo for Leadless Pacemakers (CAG-00448N). CMS.gov. Published January 18, 2017. Available online at https://go.cms.gov/2l0PTLG. Accessed February 13, 2017. 
5. Coverage with Evidence Development. CMS.gov. Published April 10, 2015. Available online at https://www.cms.gov/Medicare/Coverage/Coverage-with-Evidence-Development/index.html. Accessed February 24, 2017.
6. Tjong FV, Brouwer TF, Kooiman KM, et al. Communicating Antitachycardia Pacing-Enabled Leadless Pacemaker and Subcutaneous Implantable Defibrillator. J Am Coll Cardiol. 2016;67(15):1865-1866.