CRT and Conduction System Pacing

In left bundle branch block (LBBB), the right ventricle activates normally through the intact right bundle branch. The left ventricle receives no signal through its blocked bundle. The electrical wavefront must travel slowly through working myocardium, cell by cell, to reach the LV lateral wall.

This detour adds 80 to 120 milliseconds of delay. The interventricular septum contracts first, pushing against a relaxed, empty left ventricle. By the time the lateral wall finally activates, the septum is already relaxing. The lateral wall contracts against rising intraventricular pressure, performing more work for less forward output.

The net result: roughly 15 to 20% of the left ventricle's mechanical work is wasted on internal shuffling of blood between early-activating and late-activating segments. Over months and years, this inefficiency drives progressive dilation, worsening mitral regurgitation, and declining ejection fraction.

CRT adds a second ventricular pacing lead. A standard right ventricular lead sits at the RV apex or septum. A second lead is advanced through the coronary sinus and positioned in a lateral or posterolateral cardiac vein, overlying the LV free wall.

When both leads fire simultaneously (or with a small programmable offset), the device pre-excites the late-activating lateral wall at the same time the right ventricle is paced. The two activation wavefronts converge in the middle, producing a narrower QRS and a more coordinated contraction.

Response is measured by improvement in left ventricular ejection fraction, reduction in LV end-systolic volume, and symptomatic improvement. A strong response often produces reverse remodeling: the dilated, failing ventricle shrinks over months as it pumps more efficiently.

The evidence for CRT is strongest in a specific population. The landmark trials (COMPANION, MADIT-CRT, RAFT) enrolled patients with LVEF of 35% or below, NYHA class II through IV symptoms despite guideline-directed medical therapy, and a wide QRS.

Among these patients, those with true LBBB morphology and QRS duration above 150ms derive the greatest benefit. The wider the QRS, the more dyssynchrony is present, and the more room CRT has to improve coordination.

Patients with non-LBBB morphologies (RBBB or nonspecific intraventricular conduction delay) show consistently lower response rates. In RBBB, the left ventricle already activates normally. It is the right ventricle that is delayed. A left ventricular pacing lead addresses the wrong chamber.

QRS durations between 120 and 150ms represent a gray zone. Some of these patients respond well; others do not. The degree of true mechanical dyssynchrony at these intermediate widths is variable, and echocardiographic dyssynchrony measures have not proven reliable enough to guide selection.

Approximately 30% of patients who receive CRT do not improve. Understanding the reasons helps guide both patient selection and implant strategy.

Suboptimal lead position is the most common modifiable cause. If the LV lead lands on a segment that is already activating early, pacing there adds little. Imaging-guided lead placement, targeting the segment of latest mechanical activation, improves response rates.

Scar at the pacing site prevents CRT from working. Pacing a region of dense fibrosis produces no meaningful contraction, because there is no viable myocardium to recruit. Pre-implant cardiac MRI can identify scar distribution and guide the operator away from non-viable tissue.

Some patients have a wide QRS with minimal true mechanical dyssynchrony. In non-LBBB patterns especially, the wide QRS may reflect diffuse conduction disease rather than a correctable septal-to-lateral delay.

Finally, suboptimal AV and VV timing can limit CRT benefit. Fine-tuning the atrioventricular delay and the offset between RV and LV pacing can improve hemodynamics in borderline responders.

His-bundle pacing (HBP) engages the heart's native conduction system directly. A pacing lead is positioned on the His bundle, and the stimulus travels down both bundle branches through the Purkinje network, producing a narrow, physiological QRS.

In some patients with proximal LBBB (a block located within the His bundle itself), HBP can bypass the block and recruit the left bundle fibers, correcting the LBBB entirely. When this succeeds, the paced QRS narrows dramatically, and the hemodynamic benefit mirrors or exceeds that of biventricular pacing.

The limitations are practical. His-bundle capture thresholds tend to be higher and less stable than standard myocardial pacing. The R-wave signal sensed at the His position is small, creating sensing challenges. The implant is technically demanding, with a narrow target zone. Battery longevity may suffer from the higher output required to maintain capture.

Left bundle branch area pacing (LBBAP) has emerged as a practical alternative that retains the physiological advantage of conduction system capture while solving many of HBP's limitations.

A pacing lead is advanced through the interventricular septum from the right ventricular side until its tip reaches the region of the left bundle branch or its proximal fibers. The paced impulse captures the left bundle directly, activating the left ventricle through its native Purkinje network.

The resulting paced QRS is narrow and shows a characteristic RBBB morphology in lead V1. This pattern confirms that the left ventricle is activated via the fast conduction system while the right ventricle depolarizes slightly later through the myocardium. Compared to HBP, LBBAP offers lower and more stable capture thresholds, better R-wave sensing, and a technically easier implant.