Bundle pacing

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"left bundle branch area pacing" OR "His bundle pacing"

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Conduction System (Bundle) Pacing

Conduction system pacing (CSP) is an umbrella term for pacing modalities that engage the His-Purkinje system to produce physiologic ventricular activation, avoiding the dyssynchrony caused by conventional right ventricular (RV) apex pacing. The two main forms are His bundle pacing (HBP) and left bundle branch area pacing (LBBAP/LBBP).

Conduction system disturbances and pacing options - Fuster and Hurst's The Heart, 15th Edition

Why It Matters: The Problem with RV Pacing

Conventional RV apical pacing activates the ventricles in a non-physiologic, myocyte-to-myocyte wavefront, creating interventricular and intraventricular dyssynchrony. With prolonged RV pacing burden (>40% of beats), this can lead to:

Pacing-induced cardiomyopathy (PICM)
Deterioration of LV systolic function
Increased risk of heart failure and AF

Conduction system pacing preserves or restores normal electrical activation by recruiting the His-Purkinje system. - Fuster and Hurst's The Heart, 15th Ed., p. 1237

Anatomy Relevant to CSP

The His bundle is a cylindrical fascicle extending inferiorly and leftward from the AV node. It has two segments: a penetrating portion (~5-10 mm, over the atrial membranous septum) and a branching portion (~5-10 mm, where the left bundle branch [LBB] begins to fan out).
The LBB fans out beneath the LV subendocardium - a wider target than the narrow His bundle. It divides into anterior and posterior fascicles, each heading toward the corresponding papillary muscle head.
Purkinje fibers arborize from the fascicles into the ventricular myocardium. - Fuster & Hurst, p. 1237

1. His Bundle Pacing (HBP)

First described for permanent pacing in 2000 (Deshmukh et al.), HBP directly stimulates the bundle of His, engaging the downstream His-Purkinje system.

Types of His Bundle Capture

Type	Mechanism	ECG Features
Selective HBP (S-HBP)	His bundle only; ventricular activation entirely via HPS	S-QRS = native H-QRS; paced QRS = native QRS morphology; isoelectric interval between spike and QRS; discrete local ventricular EGM on lead
Nonselective HBP (NS-HBP)	His bundle + adjacent RV myocardium fused	S-QRS = 0 (no isoelectric interval); wider QRS than S-HBP due to fusion; no discrete local ventricular EGM

In patients with His-Purkinje conduction disease (HPCD, e.g. BBB or HV block), HBP can correct the bundle branch block by capturing latent fascicular tissue distal to the site of block - in this setting, the paced QRS may be narrower than the native QRS, and two distinct capture thresholds may be observed (with and without BBB correction). - Fuster & Hurst, p. 1238

Limitations of HBP

Technically demanding (small target)
Pacing thresholds are sometimes high (~1-2 V) and may rise over time
Lead revisions required in ~6.7% of cases
Undersensing of ventricular signals and atrial oversensing can occur
In infranodal block, backup pacing safety is a concern if the lead is above the site of block - Fuster & Hurst, p. 1239

2. Left Bundle Branch Area Pacing (LBBAP / LBBP)

Introduced by Huang et al. (2017), LBBAP emerged to overcome the limitations of HBP. The lead is screwed deep into the proximal interventricular septum from the RV side until it crosses the septum and engages the LBB or its proximal fascicles.

Definition: Direct capture of the LBB or one of its fascicles along with the LV septal myocardium at low output (<1 V at 0.5 ms pulse width). - Fuster & Hurst, p. 1240

LBBAP vs. HBP - Key Advantages

Feature	HBP	LBBAP
Target size	Small (His bundle, ~1-2 mm)	Larger (LBB fan, ~10-15 mm wide)
Pacing threshold	Often higher, can rise	Lower and more stable
Sensing (R-wave)	Often lower	Higher
Lead stability	Less predictable	More reliable
BBB correction	Can correct supra-Hisian block	Corrects LBBB by pacing distal to the block
Safety backup	Concern in infranodal block	Lead position distal to AV node provides backup

ECG Criteria for LBBAP

Right bundle branch block (RBBB) morphology in V1 (current flowing away from RV lead toward LV)
Narrow or normal QRS duration (when LBB is captured)
Short R-wave peak time (RWPT) in V5-V6 (<80 ms) confirming fast LV activation
A "W" pattern may be seen in V6 if only septal myocardium is captured without true LBB capture

Indications for Conduction System Pacing

CSP is particularly preferred over RV pacing in:

Patients with LVEF <50% and AV block who are expected to require >40% ventricular pacing - to prevent heart failure. - Harrison's Principles of Internal Medicine, 22nd Ed., p. 1974
Patients with existing or anticipated pacing-induced cardiomyopathy
Patients with LBBB and heart failure who need cardiac resynchronization therapy (CRT) - as an alternative to biventricular pacing
AV node ablation for AF - to prevent RV pacing-induced dyssynchrony
Patients where biventricular pacing (CRT) has failed or coronary sinus lead placement is not feasible

Harrison's states: "The selection of pacing lead location should be individualized based on expected pacing requirements, underlying heart disease, and level of conduction block." - Harrison's 22nd Ed., p. 1974

Goldman-Cecil notes: "Cardiac resynchronization therapy requires synchronized pacing of both ventricles or physiologic pacing by means of His-bundle pacing or left bundle branch pacing." - Goldman-Cecil Medicine

Clinical Outcomes (from Fuster & Hurst Tables 38-9 and 38-10)

HBP outcomes (selected studies):

Deshmukh 2000: 18 patients with AF + AV node ablation; 66% success; improved LVEF, NYHA, LV dimensions at 36 months
Deshmukh 2004: 54 patients; 72% success; improved LVEF, NYHA, peak VO₂ at 42 months
Multicenter registry (Vijayaraman 2018): Success rates improved to >90% with dedicated tools; long-term thresholds generally stable

LBBAP outcomes (selected studies):

Implant success rates: 80-100% across studies
Paced QRS duration: 110-140 ms (near-normal)
Pacing thresholds: 0.5-0.8 V at 0.5 ms - stable
R-wave sensing: 9-18 mV - excellent
Lead revision rates: 0-3%

Recent Evidence (2024-2026)

Three recent meta-analyses reinforce the role of LBBAP:

El Iskandarani et al. (2024) - Systematic review & meta-analysis in heart failure found LBBAP achieves significant improvement in LVEF and NYHA class in heart failure patients. [PMID: 38812213]
Ellenbogen KA et al. (2024) - Meta-analysis of lumenless leads for LBBAP: confirmed favorable pacing parameters with lumenless (SelectSecure-type) leads. [PMID: 38664898]
Chen X & Dong J (2024) - Stylet-driven vs. lumenless leads for LBBAP: both lead types are feasible, each with distinct implant characteristics. [PMID: 39462327]

Practical Summary

Aspect	HBP	LBBAP
Physiologic?	Yes (most physiologic)	Yes (near-physiologic)
Technical difficulty	High	Moderate
Threshold stability	Variable (can rise)	Generally stable and low
BBB correction	Yes (for supra-Hisian block)	Yes (for LBBB)
Preferred when	Normal His-Purkinje, mild HPCD	LBBB, high HBP threshold, failed HBP
Backup safety	Concern in infranodal block	Better (distal to AV node)

Both modalities represent a major advance over conventional RV pacing. LBBAP has largely supplanted HBP at many centers due to its more favorable lead parameters, but HBP remains the reference standard for true physiologic (complete His-Purkinje system) activation.

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