The bottom line was that the Micra Transcatheter Pacing System was used in people that mostly did not have a functioning atrium. Most of the time that was in patients with AF and either intermittent or persistent need for pacing and it didn't matter what the atrium was doing. That's a small fraction of the patients that need pacemakers.
Now, with the ability to coordinate the upper and lower chamber, use of the device is opened to all the patients that have a functioning atrium and have some form of atrioventricular block. A leadless pacemaker can now be used in the majority of patients who need pacemakers.
The atrium provides a few things. It produces a sort of prefilling of the ventricles, allowing for a larger and more efficient stroke volume with each heartbeat. The atrium also contributes the appropriate rate because it is driven by the sinus node, which gives the body the appropriate rate for the amount of activity the patient is doing at that time.
Most pacemakers in the United States are dual-chamber pacemakers and the Micra AV provides most of, but not all of, the function of a dual-chamber pacemaker. The one thing that it's lacking is atrial stimulation, which would be for people who the atrium doesn't beat fast enough, such as in sick sinus syndrome or chronotropic incompetence. In that case, you would still need a leaded pacemaker in order to address these problems.
The Micra AV benefits a portion of the people that have various forms of pacemaker syndrome, which is poor synchronization between the atrium and the ventricle. These patients often feel short of breath and have palpitations. If people who have heart block and a functioning atrium receive a single-chamber ventricular pacemaker, their atrium and ventricles would become unsynchronized. They would feel much better with a Micra AV. However, for the portion of patients who have sinus node dysfunction where the atrium doesn't go fast enough, the Micra AV would not help and would actually facilitate the unsynchronized situation and promote pacemaker syndrome.
In the U.S., less than one-quarter of patients get a single-chamber pacemaker. Maybe as low as 10%, in some practices. These are mostly in patients with AF.
This approval makes this pacemaker a reasonable alternative for as much as one-third to one-half of the patients that need a transvenous pacemaker.
There are other advantages to using a leadless pacemaker. Complications from infection or clots that developed and closed off the veins are likely to be much less common if you don't have any leads. So while the device may not represent 100% of what a leaded pacemaker can do with two leads, there are some advantages and there will be times when the slight disadvantage of not being able to stimulate the atrium would be overwhelmed by the advantage of having a lower risk for infection or lead failure or an occlusion to one of the subclavian veins.
This device doesn't provide atrial stimulation and also doesn't provide for ventricular resynchronization such as with biventricular pacing, and there are still important things to be done with leadless pacing that are not addressed here. But this device is simple and very effective and expands the applications of the overall migration to removing transvenous leads from the pacing and ICD systems.
Removability is a question still not addressed with the Micra AV. It's not necessary to remove these pacemakers most of the time but ultimately, removability is a question that still has to be answered. To be honest, the problem is neither better nor worse than it was with the regular Micra Transcatheter Pacing System. It should be exactly the same for the Micra AV.
Bruce L. Wilkoff, MD
Cardiology Today Editorial Board Member
Cleveland Clinic Lerner College of Medicine
Case Western Reserve University
Disclosures: Wilkoff reports he is an advisor for Medtronic but has no involvement other than as an implanter of the Micra AV or Micra Transcatheter Pacing Systems.