Permanent His‐Bundle pacing in a patient with advanced conduction system disease: What is the mechanism of QRS narrowing?

Figures

• 

  Figure 1. A, Baseline 12‐lead electrocardiogram (ECG) and intracardiac electrograms. The patient's baseline 12‐lead ECG on admission for syncope showed atrial fibrillation with a slow ventricular response. The baseline QRS duration was 156 ms and a right bundle branch block (RBBB) with left anterior fascicular block (LAFB). The baseline intracardiac electrograms during electrophysiology study and permanent his bundle (PHB) lead implantation are shown. The octapolar His (H) catheter placed via the right femoral vein is displayed. These show that infra‐Hisian block results in the slow ventricular rate. Pacing with the octapolar catheter resulted in QRS narrowing so a PHB lead was implanted. The electrogram from the PHB lead is displayed on the EP laboratory recording system as shown. B, 12‐Lead ECG during high‐output unipolar pacing from permanent His‐bundle lead. High‐output unipolar pacing from the PHB lead results in resolution of the conduction disease with QRS normalization with a duration of 104 ms. On the intracardiac electrograms recorded at this time, the His potentials that were recorded on both the octapolar His catheter and PHB lead were not captured although the QRS normalized

• 

  Figure 2. A, Octapolar catheter position relative to permanent His‐bundle lead. Fluoroscopy of the relative positions of the octapolar His catheter and permanent His‐bundle lead is shown after repositioning of the octapolar catheter to record a His or proximal right bundle signal distant to the PHB lead. B, Intracardiac electrograms during permanent His‐bundle lead pacing with a narrow QRS. These intracardiac electrograms were recorded during PHB lead pacing at an output that resulted in QRS normalization. The His signal recorded on the PHB lead and the signal distal to the lead recorded on the octapolar His (H) catheter are both not captured despite QRS narrowing (solid arrow). A closer look at the octapolar His catheter recordings shows antegrade conduction (dotted arrow). These tracings indicate that recruitment of the recorded His bundle and more distal right bundle electrograms are not necessary for QRS normalization in this case, a finding that is not consistent with longitudinal dissociation of the His bundle

• 

  Figure 3. Mechanism for QRS narrowing via a septal fascicle. This figure shows the purported mechanism of QRS narrowing in this case. Early branching, an anatomic variant of the right bundle (RB), functioning as a septal fascicle (SF) allows for conduction (lightning bolt) from the permanent His‐bundle (PHB) lead to bypass the distal his/RB potential recorded on the His (H) catheter and the blocked left anterior fascicle, insert more distally, and overcome the baseline right bundle branch block (red lines) to allow for QRS narrowing and normalization of the QRS axis