12. Conduction System Pacing (His Bundle and Left Bundle Branch Area Pacing)
Conduction System Pacing (CSP) represents the modern paradigm of physiological cardiac pacing, directly engaging the intrinsic His-Purkinje network to deliver narrow, physiological ventricular activation and eliminate pacing-induced dyssynchrony.
1. His Bundle Pacing (HBP) vs Left Bundle Branch Area Pacing (LBBAP)
| Parameter | His Bundle Pacing (HBP) | Left Bundle Branch Area Pacing (LBBAP / LBP) |
|---|---|---|
| Anatomical Lead Position | Fixation into the central fibrous body at the bundle of His. | Deep trans-septal screw deployment through the RV septum to the subendocardial left bundle branch. |
| Pacing Thresholds & Stability | Higher thresholds; risk of threshold rise over time; smaller sensing amplitudes. | Low, stable pacing thresholds; large R-wave sensing amplitudes ($>10-15\text{ mV}$). |
| Paced QRS Morphology | Selective HBP: Perfectly identical to native narrow QRS with isoelectric stimulus-QRS interval. Non-Selective HBP: Narrow QRS with pseudo-delta wave (local RV myocardial capture). | Incomplete RBBB pattern in $V_1$ ($qR$ or $rSR'$) with narrow QRS duration ($<120-130\text{ ms}$). |
2. Definitive Criteria for Left Bundle Branch Capture
Diagnostic Criteria for Validating Left Bundle Branch Capture during LBBAP:
- Paced QRS Morphology: $qR$ or $rSR'$ pattern in lead $V_1$.
- Short and Constant Left Ventricular Activation Time (LVAT / RWPT in $V_5-V_6$):
- Measured from pacing stimulus to the peak of the R wave in lead $V_5$ or $V_6$.
- $\text{LVAT} < 75-80\text{ ms}$ at high and low pacing outputs confirms direct left bundle capture.
- Demonstration of Left Bundle Potential ($V-LBB$): Recorded on pacing lead at target depth before active fixation.
- Programmed Electrical Stimulation: Abrupt transition from non-selective to selective LBB capture with output reduction.
3. Clinical Indications for Conduction System Pacing
- Heart Failure with Reduced Ejection Fraction (HFrEF) and LBBB: LBBAP directly corrects LBBB dyssynchrony, serving as a powerful alternative to traditional coronary sinus CRT.
- High Anticipated Ventricular Pacing Burden ($>40\%$): Prevents pacing-induced cardiomyopathy caused by traditional RV apical pacing.
- "Pace-and-Ablate" Strategy: For refractory Atrial Fibrillation with rapid ventricular rates.