Conduction System Pacing

Conventional right ventricular apical (RVA) pacing produces a dyssynchronous left ventricular contraction pattern resembling left bundle branch block (LBBB). The paced wavefront originates at the RV apex and spreads cell-to-cell through the working myocardium rather than the Purkinje network, resulting in delayed lateral LV activation, interventricular dyssynchrony, and reduced cardiac efficiency. In patients with high pacing burden (>40%), this dyssynchronous activation leads to pacing-induced cardiomyopathy (PICM) in approximately 10–20% of patients, manifesting as progressive LV dilation and systolic dysfunction.

Conduction system pacing (CSP) was developed to overcome this limitation by delivering pacing stimuli through the native His–Purkinje conduction system, preserving physiological ventricular synchrony. There are two primary CSP approaches:

The anatomical basis of CSP is the conduction system architecture. The His bundle emerges from the compact AV node at the apex of the triangle of Koch, penetrating through the central fibrous body to reach the crest of the interventricular septum. It is a compact, insulated structure approximately 20 mm long and 2–4 mm in diameter. The left bundle branch fans out from the His bundle on the left septal endocardial surface, approximately 15–20 mm distal to the His recording site. Unlike the compact right bundle branch, the LBB is a broad, fan-shaped structure with anterior, septal, and posterior fascicles, making it a wider target for pacing.

Both HBP and LBBAP aim to produce a narrow, synchronized QRS by engaging the Purkinje network distal to any conduction disease. HBP can correct intra-Hisian block but not infra-Hisian disease. LBBAP, by targeting the LBB directly, can correct LBBB by pre-exciting the left conduction system — a critical advantage in patients requiring cardiac resynchronization therapy (CRT).

Recognizing paced QRS morphology is essential for confirming conduction system capture and distinguishing it from myocardial-only pacing. Each CSP modality produces a characteristic 12-lead ECG pattern.

The key discriminating measurement for LBBAP is the left ventricular activation time (LVAT) — the interval from the pacing stimulus to the peak of the R-wave in leads V5 or V6. An LVAT <80 ms indicates rapid LV activation via the Purkinje system (LBB capture), while LVAT >80 ms suggests myocardial-only capture (septal pacing without LBB engagement). A W-shaped pattern in V6 (notched R-wave) suggests non-physiological septal pacing without true LBB capture.

Confirming LBB Capture

Several criteria help confirm true LBB capture during LBBAP implant and follow-up:

• Morphology: rS or Qr pattern in V1 (RBBB-like) with tall R in V5–V6
• LVAT <80 ms in V5–V6, abruptly shortening at LBB capture threshold
• Output-dependent morphology change: At lower output, loss of LBB capture produces widened QRS and prolonged LVAT (unipolar septal pacing only); at higher output, LBB recaptured with narrow QRS and short LVAT — a characteristic threshold discordance
• Programmed stimulation: S1S2 pacing demonstrating refractoriness characteristic of the His–Purkinje system rather than myocardium

His Bundle Pacing Implant Technique

HBP implantation uses a specialized delivery system, most commonly the Medtronic SelectSecure 3830 lead with a fixed helix and a deflectable delivery sheath (C315 His or similar). The target is the His bundle region at the apex of the triangle of Koch. The procedural steps are:

1. His mapping: Advance the lead to the tricuspid annular region and record a His bundle electrogram (HBE). The His signal should be clearly visible between the atrial and ventricular electrograms on the unipolar or bipolar recording
2. Lead fixation: With the sheath deflected to the His position, rotate the lead clockwise (4–5 turns) to screw the helix into the fibrous tissue surrounding the His bundle
3. Threshold testing: Confirm His capture — selective (narrow QRS with isoelectric spike-to-QRS interval) or non-selective (narrow QRS without isoelectric interval). Typical capture thresholds are 1.5–3.0 V at 1.0 ms
4. Sensing assessment: His bundle R-wave amplitude is often modest (2–6 mV), which can create sensing challenges

LBBAP Implant Technique

LBBAP targets the left bundle branch on the left septal endocardial surface, accessed by screwing the lead deep through the interventricular septum from the RV side. The same 3830 lead is used but advanced 15–25 mm into the septum.

1. Initial positioning: Using a deflectable sheath, position the lead tip on the RV septum approximately 1.5–2 cm apical to the His bundle position (in RAO 30° view) and just below the tricuspid annulus
2. Septal penetration: Rotate the lead clockwise aggressively (8–12 turns typically) to advance the helix deep into the septum. Monitor fluoroscopy in LAO 40–45° to confirm progressive leftward movement of the lead tip
3. LBB capture confirmation: Monitor V1 morphology during advancement — transition from LBBB to RBBB pattern indicates the lead has crossed from RV myocardium through the septum to capture the LBB or left septal endocardium. Confirm with LVAT <80 ms in V5–V6
4. Depth optimization: Fine-tune lead depth to achieve the lowest LBB capture threshold while maintaining adequate safety margin. Typical LBB capture thresholds are 0.5–1.5 V at 0.4 ms

Stylet-driven vs. deflectable sheath systems: The original LBBAP approach used a stylet-shaped to target the septum through a fixed or deflectable sheath. Newer purpose-built delivery systems offer improved torque control and more precise depth management. Some operators use a stylet-only technique (shaping the stylet to direct the lead, without a deflectable sheath), which simplifies the procedure but requires more experience.

HBP Outcomes and Limitations

His bundle pacing was the first CSP approach to enter clinical practice and has been studied extensively. Its primary advantages are true physiological activation and potential to correct conduction disease at or above the His level. However, several limitations have curtailed widespread adoption:

• Higher capture thresholds: HBP thresholds are typically 2–3 V @ 1.0 ms at implant and tend to rise over time, leading to increased battery consumption and programming challenges
• Lead dislodgement: Approximately 5% dislodgement rate, higher than conventional RV leads, due to the anatomically challenging fixation in the fibrous His region
• Sensing difficulties: Low R-wave amplitude at the His position can lead to undersensing, particularly problematic for ICD sensing
• Implant success rate: Reported at 80–90% in experienced centers, lower in less experienced hands

LBBAP Outcomes and Advantages

LBBAP has rapidly gained adoption since Huang's first description in 2017, largely because it addresses many of HBP's limitations:

• Lower, more stable thresholds: LBB capture thresholds are typically 0.5–1.5 V @ 0.4 ms at implant and remain stable over follow-up, providing better battery longevity
• Superior sensing: R-wave amplitude of 8–15 mV allows reliable sensing without dedicated high-output programming
• Higher implant success rate: >95% in most series, reflecting the wider anatomic target of the LBB fan
• Easier to perform: The learning curve for LBBAP is generally shorter than for HBP

CRT Equivalence

A critical clinical question is whether CSP — particularly LBBAP — can replace traditional biventricular (BiV) CRT for cardiac resynchronization. Multiple observational studies have demonstrated that LBBAP produces significant LVEF improvement (8–15 percentage points) and QRS narrowing in patients with LBBB and reduced EF. The LBBP-RESYNC concept trials have shown comparable or superior echocardiographic response rates compared to BiV CRT. LBBAP is particularly valuable as a rescue strategy for BiV CRT failures — patients in whom CS lead placement fails or who are BiV non-responders can be upgraded to LBBAP with improved outcomes.

Complications

• Septal perforation: Over-advancement of the lead through the full thickness of the septum into the LV cavity — monitored by fluoroscopy and impedance drop. Managed by partial withdrawal
• LBBB injury: The act of screwing through the septum can transiently (rarely permanently) injure the LBB, paradoxically creating LBBB. Usually resolves in days to weeks
• Lead dislodgement: Less common with LBBAP than HBP (<2%) due to deep septal fixation
• Interventricular septal hematoma: Rare, usually self-limiting