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User question:

Leadless Pacemakers and Fibrosis

Could you please provide the percentage of leadless pacemakers that are associated with the development of fibrosis?

All leadless pacemakers (like Aveir VR, Micra VR, etc.) cause some fibrosis — but the clinical (problematic) fibrosis happens in only a small percentage of patients, estimated at about 1% to 2% in current studies.

Aspect	Explanation
Normal Reaction	Fibrosis (scar tissue formation) around the device tip is normal and expected. The body treats the implanted pacemaker like a "foreign body" and walls it off.
Problematic Fibrosis	Rarely, fibrosis becomes excessive and causes issues: higher pacing thresholds (requiring higher output voltage), battery drain, difficulty retrieving the device, or even device malfunction.
Incidence	Based on studies of Micra and early Aveir data, significant fibrosis causing clinical problems occurs in roughly 1%–2% of patients.
When It Happens	It typically appears after months to years, not right after implantation. The longer a device is inside, the more fibrosis can build up.
Risk Factors	Multiple implants (replacing old devices) Preexisting heart disease (especially with damaged or fibrotic ventricles) Higher pacing burdens (the more it paces, the more reaction)

Very important:

Even when fibrosis occurs, it's usually localized to the tiny area of the electrode, not the entire heart muscle.

Problems only arise if the fibrosis is so thick that the pacemaker has to use more voltage to stimulate the heart effectively — which shortens the battery life.

Summary

Mild fibrosis: Happens to everyone (normal healing).
Excessive fibrosis: Happens to about 1%–2%, might lead to higher output settings or harder device retrieval.