Clinical Context: Why This Scenario Is High-Stakes
A ventricular pacing burden of 98% is the clinical definition of true pacemaker dependency. The patient's intrinsic rhythm — if present at all — is insufficient to sustain hemodynamic stability. This is most commonly seen in patients with:
- Complete (third-degree) atrioventricular block
- Advanced second-degree AV block (high-grade Mobitz II or 2:1)
- Sick sinus syndrome with unreliable junctional escape
- Post-ablation AV nodal block (intentional or inadvertent)
The Abbott Aveir VR single-chamber leadless pacemaker, implanted in the right ventricular apex or septum via transcatheter delivery, provides reliable ventricular pacing. However, chronic right ventricular pacing at high burden produces iatrogenic left bundle branch block morphology, dyssynchronous LV activation, and — in vulnerable patients — pacing-induced cardiomyopathy (PICM).
Left Bundle Branch Area Pacing (LBBAP) has emerged as the physiologic alternative: it delivers septal capture at or beyond the left bundle branch, producing narrow QRS and synchronous biventricular activation. The question of how to safely transition from the Aveir VR to LBBAP is therefore not academic — it is an urgent procedural safety question.
The Pacing Gap: Understanding the Core Danger
At the heart of this risk is the concept of the pacing gap — the interval between retrieval of the Aveir VR and confirmed stable LBBAP capture. During this window, the patient's rhythm is entirely dependent on their intrinsic conduction system.
In a patient with 98% pacing burden, the native escape rhythm is likely a slow ventricular or junctional escape at 20–45 bpm — insufficient for hemodynamic stability and potentially absent entirely. Any unprotected pacing gap creates immediate risk of life-threatening arrhythmia or circulatory collapse.
LBBAP implantation is technically demanding. The electrophysiologist must:
- Position the sheath at the mid-septum under fluoroscopy
- Screw the lumenless lead (e.g., Medtronic 3830) into the septum with sufficient depth to cross the right bundle and reach the left bundle branch or its fascicles
- Confirm LBBAP capture via QRS morphology, paced QRS duration, LVAT measurement, and unipolar morphology
- Verify thresholds, sensing amplitude, and lead impedance before anchoring
This process can take 15 to 60 minutes depending on anatomy, operator experience, and sheath stability. A pacing-dependent patient cannot safely tolerate this duration without an active pacing backup.
Specific Cardiac Arrest Risks
The following table enumerates the principal mechanisms by which cardiac arrest can occur during an unprotected pacing gap in a 98%-burden patient:
| Arrest Mechanism | Pathophysiology | Severity |
|---|---|---|
| Asystole | Absence of intrinsic escape rhythm after pacemaker removal; no electrical or mechanical cardiac output | Critical |
| Ventricular Fibrillation (VF) | Profound bradycardia → myocardial ischemia → QT prolongation → R-on-T phenomenon → VF | Critical |
| Pulseless Electrical Activity (PEA) | Inadequate escape rate (20–35 bpm) with no mechanical cardiac output despite electrical activity | Critical |
| Severe Bradycardia + Hypotension | Escape rhythm at 40–50 bpm may maintain electrical activity but not perfusion pressure, especially in patients with LV dysfunction | High |
| Bradycardia-Induced Polymorphic VT / TdP | Slow rate → QTc prolongation → torsades de pointes, particularly in patients on QT-prolonging drugs or with electrolyte abnormalities | High |
| Cerebral Hypoperfusion | Even non-arrested but hemodynamically inadequate rhythm may cause syncope, seizure, or hypoxic brain injury | Moderate–High |
The Optimal Strategy: Implant First, Explant Second
The universally preferred approach in pacing-dependent patients undergoing Aveir VR–to–LBBAP upgrade is the implant-before-explant strategy. This eliminates the unprotected gap entirely by ensuring that a functional pacing system is active at all times.
Left subclavian or axillary venous access for LBBAP lead delivery system (deflectable sheath + Medtronic 3830 or equivalent lumenless lead). The Aveir VR remains active throughout.
Advance to mid-septum, screw in with progressive depth until left bundle branch capture is confirmed. Criteria: QRS ≤130 ms, RBBB morphology in V1, LVAT ≤75–80 ms, deep S-wave in V6, notched R in lateral leads.
Threshold ≤1.0V@0.4ms, R-wave sensing ≥5 mV, impedance 400–1200 Ω. If parameters are acceptable and capture is stable, proceed. The patient now has a functioning backup pacing system.
Connect to a dual-chamber or single-chamber device capable of driving the LBBAP lead. Program appropriate rate, output, and safety margins. Confirm pacing output on 12-lead ECG.
Only now, with the LBBAP system fully active and confirmed, proceed with Aveir VR retrieval using the dedicated retrieval catheter and steerable sheath via femoral venous access. The patient is paced throughout the retrieval.
With this sequence, the patient is continuously paced at all times. There is no moment during which their rhythm relies on an unreliable escape mechanism. This is the standard of care endorsed by implanting electrophysiologists experienced with Aveir VR and LBBAP.
Contingency: When Retrieval Must Precede Implant
Rare scenarios may require Aveir VR removal before LBBAP implantation — for example, device infection, lead positioning conflict, or fluoroscopic interference. In these cases, the procedural team must implement a robust safety protocol:
A transvenous temporary pacing lead must be placed and confirmed active before the Aveir VR retrieval begins. Only after confirming adequate temporary pacing capture, rate, and output should retrieval proceed.
Mandatory Safety Elements
The following infrastructure must be confirmed active before the Aveir VR is retrieved in this high-risk scenario:
- Active transvenous temporary pacing — RV apex via 6Fr bipolar lead through femoral or jugular access; confirmed threshold and sensing before retrieval begins
- Continuous invasive hemodynamic monitoring — arterial line for real-time blood pressure; SpO₂ monitoring
- Immediate external defibrillation readiness — pads on, defibrillator charged, ready to deliver shock within seconds
- Anesthesia or conscious sedation with airway-ready team — for rapid intubation if hemodynamics deteriorate
- Emergency ACLS medications immediately available — epinephrine, atropine, dopamine, amiodarone drawn and labeled
- Minimize retrieval dwell time — all retrieval equipment (steerable sheath, retrieval catheter) pre-positioned before Aveir is disabled
Advanced Consideration: Mechanical Circulatory Support
In patients who combine extreme pacing dependence with severely reduced LV ejection fraction (EF <35%), known coronary artery disease, or hemodynamic instability at baseline, some experienced centers elect to perform Aveir VR retrieval with prophylactic mechanical circulatory support on standby or active:
- Intra-aortic balloon pump (IABP) — diastolic augmentation; provides modest hemodynamic support during bradycardia
- Impella CP or 5.5 — provides active LV unloading; maintains perfusion pressure independent of heart rate
- Venoarterial ECMO — for the highest-risk patients with prior cardiac arrest or known inability to tolerate any pacing gap
The decision to use MCS is individualized and should involve the electrophysiologist, cardiac surgeon, and advanced heart failure team.
Frequently Asked Questions
Can you remove the Aveir VR before implanting the LBBAP lead if the patient has 98% pacing burden? ▼
What escape rhythm can be expected after Aveir VR removal in complete heart block? ▼
How long does LBBAP implantation take, and what does this mean for the pacing gap? ▼
Is external transcutaneous pacing an adequate backup during Aveir VR retrieval? ▼
Does the Aveir VR need to be deactivated before retrieval, and when does that happen? ▼
Clinical Summary & Takeaways
A 98% ventricular pacing burden = functional pacemaker dependence. The native escape rhythm is unreliable and potentially absent.
Unprotected retrieval of the Aveir VR before LBBAP capture is secured carries real risk of cardiac arrest from asystole, VF, or PEA.
The implant-first, explant-second sequence is the standard of care — it eliminates the pacing gap entirely.
If retrieval must precede implant, active transvenous temporary pacing is mandatory before the Aveir VR is removed.
In very high-risk patients (low EF, prior arrest, hemodynamic instability), prophylactic MCS should be considered.
Any upgrade plan for a pacing-dependent patient should be explicitly discussed with the managing electrophysiologist, including the retrieval sequencing strategy.