Leadless Pacemaker Overview

The leadless pacemaker family has matured from a single-chamber niche product into a credible alternative to transvenous pacing in well-selected patients. The current FDA-approved US lineup is Medtronic Micra (VR and AV) and Abbott Aveir (VR and DR). Both are delivered via the femoral vein, anchored in the right ventricle (and right atrium for Aveir DR), and contain a complete pacemaker — battery, electronics, electrodes — in a single self-contained capsule.

How the systems work

Medtronic Micra

The Micra capsule is approximately 25.9 mm long, 6.7 mm in diameter, weighing 1.75 g. It is fixed to the right ventricular myocardium by four self-expanding nitinol tines. The device is delivered through a 23-Fr femoral venous sheath via a steerable delivery catheter.

• Micra VR: single-chamber ventricular pacing (VVI/VVIR). Indicated for atrial fibrillation with slow ventricular response, AV-junctional ablation patients, and patients with chronotropic incompetence who don’t need atrial sensing.
• Micra AV: identical hardware to Micra VR with firmware changes. Uses a built-in 3-axis accelerometer to mechanically sense atrial contraction, then paces the ventricle in synchrony (VDD-like behavior). Best AV synchrony achievement is roughly 70–90% in selected patients; performance declines with patient activity and posture changes.

The device communicates with the standard Medtronic CareLink programmer and CareLink Express remote monitoring infrastructure.

Abbott Aveir

The Aveir capsule is slightly larger (38 mm long, 6.5 mm diameter) and has a screw-helix fixation rather than tines. The screw-helix design is the basis of the device’s retrievability.

• Aveir VR: single-chamber ventricular pacing.
• Aveir DR: dual-chamber system using two leadless devices. The atrial device sits in the right atrial appendage; the ventricular device sits in the right ventricular septum. They communicate via i2i (implant-to-implant) conducted electrical signals through the bloodstream — no Bluetooth or magnetic coupling. Latency is in the millisecond range and supports true DDD-like pacing.

Aveir uses the Abbott Merlin programmer.

Comparison: Leadless vs Transvenous

Indications and candidate selection

Strong leadless candidates:

• Permanent atrial fibrillation with slow ventricular response — no need for atrial sensing because the atria don’t have an organized rhythm to track. Micra VR or Aveir VR are ideal.
• AV-junctional ablation patients — ventricular pacing required after AVN ablation for refractory AF with rapid ventricular response. Micra VR or AV.
• Patients with venous access limitations — chronic dialysis with arteriovenous fistula, prior superior vena cava occlusion, congenital absence of vessels.
• Patients with prior CIED pocket infection or erosion — no pocket means no future pocket-related complications.
• Patients who place high value on cosmesis — no visible bulge, no chest scar.
• Selected younger patients with infrequent pacing need who want to preserve venous and pocket real estate for decades of future devices.

Patients best served by transvenous:

• Need for true dual-chamber pacing with reliable AV synchrony — Aveir DR is now an option, but the system is newer with shorter follow-up data; Micra AV synchrony is good but not perfect.
• Need for chronic high pacing-output programming — leadless devices have a smaller battery; output requirements above 2.5 V deplete the battery noticeably faster.
• CRT requirement — leadless cannot do this.
• Concomitant defibrillator need — separate ICD must be implanted; leadless cannot deliver defibrillation.
• Severe tricuspid regurgitation that would worsen with a device crossing the valve.
• Mechanical tricuspid valve — leadless devices cannot cross.
• Right ventricular structural abnormality that limits fixation site options.

Implant technique

Vascular access

• Right or left femoral vein; right is most common for Micra delivery sheath ergonomics.
• 23–25 Fr sheath required depending on system.
• Pre-procedure hemoglobin, renal function, and coagulation labs.
• Anticoagulation strategy: usually continued through the procedure for AF patients; reversal not routine.

Fluoroscopic deployment

• Pre-shape the delivery catheter so the device tip apposes the desired RV target site (typically apex-septum or mid-septum).
• Confirm position in two fluoroscopic views (AP and LAO 30°) before deploying.
• For Micra, deploy the tines under fluoroscopy; the capsule attaches immediately upon release.
• For Aveir, advance the screw-helix into the myocardium under torque indicator feedback (typically 1.5–2.5 turns).
• Confirm electrical contact: capture threshold, R-wave amplitude, impedance — all measured before final release.
• Pull test (Micra) or tug test (Aveir): apply gentle traction to confirm anchored fixation. Inadequate anchoring is an indication to recapture and reposition.

Acceptable acute values (at deployment)

• Capture threshold: <1.0 V at 0.24–0.4 ms pulse width (aim for <0.5 V at 0.24 ms when achievable)
• R-wave amplitude: >5 mV (acceptable down to 4 mV with reservation)
• Impedance: 400–1200 Ω
• Pull/tug test: stable apposition without detachment

If acute values are inadequate, recapture and redeploy at an alternative site. Both systems allow multiple redeployment attempts before final release.

Aveir DR specifics

• Atrial unit is implanted second, in the right atrial appendage.
• i2i communication must be confirmed during implant: paced atrial event followed by sensed/paced ventricular event with appropriate latency.
• Atrial device positioning is more variable than ventricular and may require multiple attempts.

Post-implant

• Hemostasis at the femoral access site with manual compression or a vascular closure device.
• Bed rest 2–4 hours.
• Same-day discharge in most cases; overnight observation for select patients (renal disease, anticoagulation concerns).
• Echocardiogram before discharge if perforation is a concern.
• Standard chest X-ray to document device position is optional but commonly performed.

Programming considerations

Mode selection

• Micra VR: VVI(R) — programmed to lower-rate limit with optional rate response from the accelerometer.
• Micra AV: VDD-like (programmed as VDD with internal mechanical atrial sensing). Lower-rate limit, AV delay, upper-tracking rate all programmable.
• Aveir VR: VVI(R).
• Aveir DR: DDD(R) — full dual-chamber programmability via the i2i communication.

Output and sensitivity

• Set output to 2× threshold at minimum acceptable pulse width.
• Sensitivity at 0.5–1.0 mV initially; adjust based on sensed amplitude.
• Rate response (accelerometer) calibration: standard 6-minute walk or step test at the 3-month visit.

AV synchrony (Micra AV)

• Programmable atrial sense levels — typically start at maximum sensitivity and titrate down if oversensing of motion artifact occurs.
• Activity threshold: programmable to define when the device is in “high-rate / activity” mode versus rest.
• Real-world AV synchrony performance is best at rest and during predictable activity; declines during sleep position changes, vigorous activity, or postural transitions.

Aveir DR programming

• Atrial-ventricular delay (PVARP, TARP) work as in conventional DDD.
• i2i communication parameters are factory-set but verified at every check.
• Mode-switching algorithm responds to atrial high-rate episodes.

Battery life and end-of-life management

• Micra VR: 12–13 years typical, longer with infrequent pacing.
• Micra AV: 9–11 years (mechanical sensing draws more current).
• Aveir VR: 10–12 years typical.
• Aveir DR atrial unit: 6–8 years typical (smaller battery; high atrial pacing burden).
• Aveir DR ventricular unit: 10–12 years.

When battery approaches end-of-service:

• Aveir: design supports retrieval via the screw-helix. A retrieval catheter recaptures the device, removes it, and a new device is implanted (often in a different location). This is the routine plan.
• Micra: not designed for routine retrieval. Standard practice is to leave the original capsule in place and implant a second leadless device at a different RV site. The first device is left inactive. Two or three Micra capsules can be accommodated within the right ventricle without functional impact. Long-term data beyond two devices remain limited.

If conversion to a transvenous system becomes necessary at any point, the existing leadless device is left in place and the transvenous system is implanted alongside.

Extraction and revision

• Aveir: routine retrieval at end-of-service is supported. Acute removal (intraoperative repositioning) is straightforward.
• Micra: acute removal is possible during implant; chronic removal is not routine and is generally avoided.
• For infection requiring removal: consider careful retrieval by an experienced operator; many programs prefer leaving the device in place and treating with prolonged antibiotics, given the low surface area of the leadless device compared to a transvenous system.

Common issues and troubleshooting

• Loss of AV synchrony (Micra AV): review accelerometer calibration; check activity threshold; ensure the patient hasn’t gained or lost significant weight. If suboptimal, accept VVI(R) mode or reprogram thresholds.
• Failure to capture chronic: check threshold trend; if rising, consider micro-dislodgement or local fibrosis. Increase output. Imaging if dislodgement suspected.
• R-wave amplitude decline: trend over visits matters more than any single value. Persistent drop below 3 mV warrants vector and amplifier review.
• i2i communication failure (Aveir DR): check ventricular device sensing — far-field atrial signals may be lost if amplitude is low. Reprogram or schedule revision.
• Femoral access complications: standard post-procedural assessment — pseudoaneurysm, AV fistula, hematoma. Low rate but watch for.
• Tricuspid valve impact: monitor for new or worsening TR on follow-up echo. Most patients tolerate the device-induced TR without clinical impact.
• Battery alert: schedule companion device implant 3–6 months before depletion.

Manufacturer references

• Medtronic Micra AV / VR
• Abbott Aveir DR / VR