#conduction-system-pacing #his-bundle-pacing #cardiac-pacing

His Bundle Pacing

Definition

His bundle pacing (HBP) is a form of conduction system pacing that captures the atrioventricular bundle, activating all His bundle fibres directly. The pacing lead is positioned near the tricuspid valve summit (atrial or ventricular side of the tricuspid annulus) with a His potential-to-QRS interval ≥35 ms. sources/csp-ehra-2023

Key Concepts

Anatomy

His bundle: ~20 mm long, encapsulated in fibrous tissue insulating it from atrial and ventricular myocardium; therefore, recording a His potential does not guarantee capture sources/csp-ehra-2023
Proximal His (atrial aspect of tricuspid annulus): selective capture more common; sensing issues more frequent (low R-wave, atrial/His oversensing)
Distal His (ventricular aspect of tricuspid annulus): non-selective capture more common; superior electrical parameters

Implantation Technique

Mapping in 20–30° RAO view; target: near-field (sharp) His signal + absent/small atrial EGM + larger ventricular EGM
Pace mapping at 5 V@1 ms as alternative when His EGM not visible (useful in AV block)
Lead fixation: counter-clockwise rotation with forward force; torque build-up (not number of rotations) indicates adequate tissue penetration
Current of injury (COI) of His potential + negative deflection of His potential = adequate tissue contact and predictor of stable thresholds sources/csp-ehra-2023
Absent COI after fixation → repeat rotation with stronger torque
Lead stability test: withdraw sheath 5–8 cm, gently pull/push lead, two additional rotations

Confirming His Capture

Primary method: Output-dependent transitions in QRS morphology during threshold testing
- ns-HBP → s-HBP → loss of capture (or ns-HBP → myocardial capture → loss of capture)
- Paced QRS with LBBB-like pattern (notch/slur in I, V4–V6; stimulus-V6RWPT >110 ms) = non-physiological; not acceptable
Secondary method: Pacing stimulus-to-V6RWPT vs His-potential-to-V6RWPT difference <12 ms (accuracy 96.7%) sources/csp-ehra-2023
Testing at shorter pulse widths (0.2–0.3 ms) enhances threshold divergence between His and myocardium
Programmed stimulation useful when threshold testing inconclusive

Electrical Targets

Optimal threshold: ≤1.5 V/0.5 ms; acceptable up to 2.0–2.5 V@0.5 ms in HF/CRT scenarios
In patients with long HV or HV block: demonstrate 1:1 conduction at cycle lengths ≤400 ms at working output
Bipolar sensing: ideally >2.0 mV (or unipolar >4 mV)
Check for atrial and His bundle oversensing — fatal risk in complete heart block

Selective vs Non-Selective HBP

Both achieve equivalent LV synchrony and clinical outcomes sources/csp-ehra-2023
ns-HBP preferred: back-up ventricular capture present, superior sensing parameters
s-HBP: may be preferable if ns-HBP produces broad QRS (LBBB pattern); use shorter pulse widths to widen selective zone

Complications

Threshold rise / loss of capture: Most frequent HBP complication; occurs in up to 17% of patients; lead revision required in up to 11% sources/csp-ehra-2023
Sensing issues: Atrial/His oversensing, ventricular undersensing
Slitting precautions: Implant all other leads first; maintain adequate lead slack; check thresholds after slitting

Back-up Ventricular Lead Indications

Pacemaker dependency
High-grade AV block or infra-nodal block
High capture threshold
Poor sensing
Planned AV junction ablation (risk of HBP threshold rise with both RF and cryo ablation) sources/csp-ehra-2023

Hemodynamic Evidence

HBP vs RVP (Keene et al., n=18): HBP produced –56 ms shorter QRS (95% CI –67 to –46 ms; P<0.0001) and +5.0 mmHg acute systolic BP improvement (P<0.0001) sources/csp-jaccep-2023
Myocardial perfusion (Zanon et al.): HBP significantly better than RVP by Tc-MIBI scintigraphy at 3 months (perfusion score 0.44 vs 0.71; P=0.011) sources/csp-jaccep-2023
HBP-CRT vs BVP-CRT (n=18/23): HBP-CRT shortened LVAT by 26 ms more than BVP-CRT and produced ~60% greater improvement in acute systolic BP (+4.6 mmHg; P=0.04) when LBBB corrected sources/csp-jaccep-2023
HOPE-HF trial (n=167, crossover): AV-optimised HBP in long PR + LV impairment + narrow QRS/RBBB — no change in peak VO₂ or LVEF at 6 months; significant QoL improvement; HBP preferred by clear majority sources/csp-jaccep-2023

Antitachycardia Pacing from the His Bundle

First test of conduction-system ATP (canine ischaemia-reperfusion model, 6 dogs, 128 ATP trials): His ATP terminated VT as often as RV apical ATP (23 vs 25 trials) but caused fewer VT accelerations (6 vs 14, p = 0.05) and fewer degenerations into VF (1 vs 7, p = 0.02). (sources/his-atp-vt-jce-2023, rating: medium)
Mechanism: incomplete epicardial capture by the end of the ATP train was less frequent with His ATP (25% vs 43%, p = 0.05), attributed to Purkinje-mediated recruitment of a larger myocardial volume and a less curved wavefront that interacts with the VT circuit earlier.
Preclinical caveats: His capture thresholds were three times those of the RV lead (3.3 vs 1.1 mA) and ATP was delivered at twice diastolic threshold, so capture was non-selective; the model used induced, fast canine VT and acute endpoints only.
Contrast with LBBA ATP: the companion LBBA study found a higher termination rate than RV ATP (70.2% vs 47.3%), so the efficacy signal appears specific to the left-sided Purkinje engagement rather than to conduction-system pacing in general. (sources/lbbap-atp-jaccep-2026, rating: medium)

Fluoroless and 3D Mapping Techniques

HBP feasible without fluoroscopy in 31/39 patients (79%); guided purely by intracardiac electrograms (Zanon et al.) sources/csp-jaccep-2023
3D electroanatomical mapping (Sharma et al., Richter et al.): safe, feasible with dramatically reduced radiation exposure; precise localisation of His bundle region
Particularly useful in challenging anatomy: congenital heart disease, enlarged RA, or pregnancy

Special Considerations

In atrial fibrillation: avoid His mapping site with large atrial EGM (>0.5 mV); begin mapping from ventricular side of membranous septum
3D electroanatomical mapping useful in challenging anatomy, congenital heart disease, or pregnancy
Follow-up: in-person every 6 months (ESC 2021 Class IIa, LOE C) due to high incidence of late threshold rises; automatic capture detection limited reliability

Right Bundle Branch Pacing — the Distal Right-Sided Target

The RBB, not only the His bundle, is an accessible right-sided target. RBBP is distinguished from HBP by a conduction-system potential–QRS interval <35 ms and, definitively, by a stimulus–V6RWPT ≥10 ms longer than the native potential–V6RWPT — because the paced wavefront must travel RBB → His → LBB, whereas in HBP the paced and native routes are identical. (sources/right-bundle-branch-pacing-hrs-2023, rating: high)

RBBP accounted for 19.6% of patients in whom HBP was intended (12.2% at working output), and 90.6% of those with a potential–QRS <35 ms
Versus HBP: sensing much better (5.6 vs 3.2 mV), thresholds equivalent, QRS 10.5 ms longer and V6RWPT 11.6 ms longer, with no difference in echocardiographic outcomes or 3-year mortality
RBBP cannot correct LBBB, so it is not a CRT strategy; its proposed roles are dual-branch pacing with LBBP and avoidance of transseptal access

Contradictions / Open Questions

The His target has a defined fluoroscopic address, and it is narrow. In a 45-patient series using a 9-partition method of the RAO 30° projection, all 11 HB pacing leads fell within the second partition, while 94.3% of LBB leads sat at the junction of the second and fifth — an anatomically adjacent but distinct location. The separation has a measured basis: lead-tip distance to the noncoronary/right coronary cusp junction was 1.9 ± 0.2 cm for HB versus 3.8 ± 0.6 cm for LBB. The caveat is the small number of HB leads (11) behind the partition-2 claim. (sources/lbbap-9-partition-hrs-2020, rating: medium)
HBP fails in a substantial fraction of the patients who most need resynchronisation. The His-SYNC study showed QRS duration cannot be normalised in almost half of patients with LBBB, indicating that where a discrete block is present it cannot be overcome by His pacing in that subset. This is the specific failure that motivated left bundle branch area pacing — pacing distal to the block rather than proximal to it. Other structural limitations: the His bundle is only ~1–2 mm in diameter and difficult to locate, thresholds are high and unstable in 5–10% of patients, sensed R-waves are small relative to atrial signals, and the His bundle can be damaged during implantation. See concepts/Left-Bundle-Branch-Area-Pacing. (sources/lbbap-jacc-2019, rating: high)
AV junction ablation and HBP: Both RF and cryo ablation can cause rise in HBP thresholds. LBBAP preferred when AVJ ablation is planned. sources/csp-ehra-2023
Long-term performance: Medium-term safety and efficacy data available; large RCTs with long-term follow-up still lacking.
HOPE-HF paradox: AV-optimised HBP in long PR + LV impairment improves QoL and is preferred by patients, but did not improve peak VO₂ or LVEF in RCT. Mechanism of symptomatic benefit without objective haemodynamic improvement is unclear. sources/csp-jaccep-2023
nsHBP vs sHBP synchrony: UHF-ECG confirms equivalent ventricular e-DYS; both equivalent to intrinsic narrow QRS. However, nsHBP produces wider surface QRS — whether this matters in specific subgroups is unresolved. sources/csp-jaccep-2023
His ATP is safer but not more effective than RV ATP — in dogs only. The single supporting study used six animals, induced VT, a homogeneous ischaemic lesion near the RV lead, and non-selective His capture at twice threshold; whether selective His ATP, a different substrate or human VT would show the same safety margin is unknown. (sources/his-atp-vt-jce-2023, rating: medium)

Connections

Related to concepts/Conduction-System-Pacing
Related to concepts/Left-Bundle-Branch-Area-Pacing
Related to concepts/Right-Bundle-Branch-Pacing — the distal right-sided alternative, distinguished by a potential–QRS <35 ms and ΔV6RWPT ≥10 ms
Related to entities/Heart-Failure (HOT-CRT)
Related to entities/Atrial-Fibrillation (AVJ ablation planning)