Initial Experience with the Medtronic Micra Transcatheter Pacing System

INTRODUCTION

On April 6, 2016, the U.S. Food and Drug Administration approved an entirely self-contained leadless pacemaker in the Medtronic Micra Transcatheter Pacing System™ (Figure 1). The Micra system is a VVIR pacemaker that is delivered directly to the right ventricular apical endocardium via a femoral venous sheath.¹ In more traditional systems, pocket- and lead-related complications can occur in up to 11% of patients at five years.² Because the Micra is self-contained, there is no need for a lead to connect the myocardium to a remote pulse generator. This report outlines the first use of this novel technology at our institution.

CASE PRESENTATION

A 79-year-old male with a history of severe aortic stenosis, persistent atrial fibrillation, and transient ischemic attack underwent successful transcatheter aortic valve replacement (TAVR) at our institution using a 26 mm Edwards Sapien XT™ heart valve (Edwards Lifesciences). There was no documented heart block following the procedure, but approximately four months later, he presented with syncope and intermittent symptomatic bradycardia (Figure 2). An Eluna ProMRI DR-T (BIOTRONIK, Inc.) dual-chamber pacemaker was subsequently inserted, with improvement in his symptoms. Ten months later, he developed fevers and was found to have enterococcal bacteremia. A transesophageal echo was not able to be performed due to a significant Schatzki ring, but a positron emission tomography (PET) scan was consistent with lead infection. He went on to have an uncomplicated complete system extraction with plans for a six-week course of intravenous antibiotics. Following a device-free period of at least 72 hours, a decision needed to be made with regard to what type of system to re-implant. In light of his persistent atrial fibrillation and a relatively low ventricular pacing burden (19%) on his previous device, the patient was given the option of either a single-chamber transvenous system on the contralateral (right) side or the Micra — he ultimately chose the latter. 

No changes were made to his anticoagulation strategy (warfarin) beforehand, and on the day of the procedure, his INR was 2.1. He was brought to the electrophysiology lab, and prepped and draped in the usual sterile fashion. Left femoral venous access was obtained under ultrasound guidance and an 0.035” Amplatz Super Stiff™ (Boston Scientific) J-tip wire was advanced to the right atrium. After a skin nick, serial dilations of the access site were performed using an 8 French (Fr) short sheath, followed by dedicated 16 Fr and 18 Fr dilators (Medtronic). The 27 Fr Micra introducer sheath (Figure 3) and dilator were then inserted over the wire and advanced to the right atrium. After the dilator and wire were removed, the introducer sheath was flushed with a 50 cc syringe and connected to a continuous heparinized saline infusion. Next, the delivery system containing the Micra was inserted to the mid-atrium and the sheath withdrawn to the inferior vena cava. The articulated delivery system was then manipulated across the tricuspid valve and the tip positioned at the apical septum (Figure 4). A 50% contrast and 50% saline solution were injected through the delivery system in order to verify opposition to the endocardial surface. Once adequate contact was confirmed, the cup containing the device was pulled back, exposing the four nitinol tines (Figure 3B) that attach it to the myocardium. At this point, a tether was still attached to the proximal end of the device so that it could be repositioned, by drawing back into the cup, if necessary. Once attached, a “Pull-and-Hold” test was performed by zooming in on the device under fluoroscopy, applying gentle tension to the tether for 2-3 cardiac cycles and watching for deflection of the tines associated with heart movement. The goal was to ensure that at least two of the four tines were engaged. A wand connected to the Medtronic analyzer was then placed on the surface of the chest using a sterile sleeve. Initial interrogation revealed R-waves of 14.0 mV (goal >5 mV), an impedance of 710 Ohms (400-1500 Ohms), and a capture threshold of 0.38 V at 0.24 msec (goal ≤1.0 V at 0.24 msec). Programming was set to VVI 50 bpm. With an appropriate position, adequate tine engagement, and acceptable electrical measurements, the implantation was completed by cutting the tether and removing it through the proximal end of the delivery system. A figure-of-8 stitch was placed prior to removal of the introducer sheath, and manual pressure was held until adequate hemostasis was achieved (approximately 5-10 minutes). The patient remained on bedrest for six hours after sheath removal, and the figure-of-8 stitch was removed the following morning. A chest x-ray performed on post-operative day 1 demonstrated stable position of the device (Figure 5). 

DISCUSSION

The patient described here was quite similar to those in the study by Reynolds et al,¹ where the population was 59% male with a mean age of 75.9 years, a normal left ventricular ejection fraction (mean 58.8%), hypertension (78.6%), and atrial fibrillation (72.6%). Among those 725 patients, there were 719 successful implantations and few complications: 11 (1.6%) perforations or effusions, 5 (0.7%) groin site complications, and zero dislodgements. Four of the six unsuccessful implantations were due to perforations or effusions, one patient had tortuous venous anatomy, and one had inadequate pacing thresholds. 

One obvious downside to leadless systems is the limitation to RV-only pacing and sensing. However, despite the recent declining trend in their implantation,³ there remains a population that still benefits from single-chamber devices. The potential advantages of this technology should warrant a reconsideration of who really requires an atrial lead. An additional potential concern is the size of the introducer sheath, given its 27 Fr external diameter. There were few access site complications in the study (0.7%), though real-world experience with a sheath of that size remains to be seen. Lastly, while the device is acutely retrievable and there have been reports of successful near-term removal,^4,5 its long-term extractability is not yet known.

At our patient’s one-week follow-up visit, the groin access site showed no evidence of hematoma or bruit. An interrogation revealed R-waves of >10 mV, an impedance of 710 Ohms, and a capture threshold of 0.25 V at 0.24 msec. Two months later, the R-waves had increased to >20 mV and the impedance and capture threshold were stable at 710 Ohms and 0.38 V at 0.24 msec, respectively.

The Medtronic Micra Transcatheter Pacing System is an exciting and safe, new option for VVI or VVIR pacing delivered via a minimally invasive approach. The self-contained nature of these systems eliminates the potential for pocket- or lead-related complications as well as any outward visual signs of the device.

Disclosure: The author has no conflicts of interest to report regarding the content herein.   

References

1. Reynolds D, Duray GZ, Omar R, et al. A Leadless Intracardiac Transcatheter Pacing System. N Engl J Med. 2016;374:533-541.
2. Udo EO, Zuithoff NP, van Hemel NM, et al. Incidence and predictors of short- and long-term complications in pacemaker therapy: The FOLLOWPACE study. Heart Rhythm. 2012;9:728-735.
3. Greenspon AJ, Patel JD, Lau E, et al. Trends in permanent pacemaker implantation in the United States from 1993 to 2009. J Am Coll Cardiol. 2012;60:1540-1545.
4. Karim S, et al. Extraction of a Micra Transcatheter Pacing System: First-in-human experience. HeartRhythm Case Reports. 2015;2:60-62.
5. Koay A, Khelae S, Wei KK, et al. Treating an infected transcatheter pacing system via percutaneous extraction. HeartRhythm Case Reports. 2016;2:360-362.