What is conduction system pacing and why is it better?

Conventional method of permanent pacing in complete heart block involves pacing from the right ventricular apex. Though it is easy to achieve, there is a potential risk of pacing induced cardiomyopathy in the long run. That is because of left ventricular dyssynchrony. Normal sequence of activation of the left ventricle is lost. Septum is activated first and left ventricular lateral wall later. Some of those with right ventricular apical pacing may need an upgrade to cardiac resynchronization therapy. There is also an enhanced chance for heart failure hospitalization and mortality. Conduction system pacing is a pacing strategy to avoid these potential downsides of right ventricular pacing. Conduction system pacing aims to capture the conduction system early in its course in the ventricles.

A recent randomized controlled trial published in the Journal of American College of Cardiology (CSPACE) compared these two modalities of pacing in 202 consecutive patients with atrioventricular block without indication for cardiac resynchronization therapy [1]. Please note that those with heart failure would have been considered for CRT rather than simple cardiac pacing. The composite endpoint of the study included pacing induced cardiomyopathy, upgrade to biventricular CRT, heart failure hospitalizations and all cause mortality.

Conduction system pacing was performed by cardiac electrophysiologists with experience of at least 20 previous successful implantations. Options were His bundle pacing, left bundle branch pacing and left ventricular septal pacing. Aim was to achieve an optimal paced ECG morphology with pacing threshold less than 2.0 V at 1.0 ms pulse width. Specialized pacing leads and delivery sheath was used for conduction system pacing. The exact site to be tried first was left to the operators decision. Left ventricular septal pacing was pacing the left bundle branch area without direct electrographic or electrocardiographic evidence of selective or nonselective left bundle branch capture. In the control group, the aim was to implant leads on the right ventricular aspect of the septum rather than at the apex. Active fixation pacing leads were used for this. Septal pacing has chance for earlier capture of the conduction system and is thought to be better than right ventricular apical pacing in terms of left ventricular synchrony.

CSPACE clinical trial demonstrated the superiority of conduction system pacing over right ventricular septal pacing in terms of improved clinical outcomes. Successful conduction system pacing could be achieved in 89 of the 101 patients in the study group. They had lower incidence of pacing induced cardiomyopathy and need for CRT upgrade. There was no statistically significant difference in heart failure hospitalizations or mortality between the two groups. Lead revision was needed in 8 patients in the conduction system pacing group while it was needed only in one patient in the control group. This study used lumenless leads for conduction system pacing while other leads with a stylet are also there.

Current guidelines of various professional societies published during 2018 to 2023 recommend conduction system pacing in patients with AV blocks expected to have higher ventricular pacing requirement. It is a Class 2B recommendation with level B nonrandomized evidence. The results of this randomized study may lead to upgradation of the recommendations in near future. It has been mentioned that when the trial was designed, His bundle pacing was the dominant method of conduction system pacing. Left bundle branch area pacing became more popular during the course of the study. Though heart failure hospitalizations did not achieve statistical significance in the study, six of the seven heart failure hospitalizations occurred in the right ventricular septal pacing group.

Procedure time was longer by about 22 minutes in the conduction system pacing group, with about twice the fluoroscopy time and radiation dose. This might coming later as the procedure becomes more common. Though other reports have noted lower incidence of atrial fibrillation with conduction system pacing, this study did not document that benefit. Implant thresholds were higher in His bundle pacing compared to left bundle branch pacing in previous studies as well as in this study.

Reference

1. Chow CLD, Wong C, Sutherland N, Obeyesekere MN, Wong GR, Eastwood CM, Abduloska J, Davey CM, Bhutani AS, Tran V, Asari F, Patel AD, Zahin MR, Singh AK, Tacey MA, van Gaal WJ, Vijayaraman P, Lim HS, Mohamed U. Clinical Outcomes of Conduction System Pacing vs Right Ventricular Septal Pacing in Atrioventricular Block: The CSPACE Randomized Controlled Trial. J Am Coll Cardiol. 2025 Aug 26;86(8):563-573. doi: 10.1016/j.jacc.2025.06.043. PMID: 40835365.