Abstract Body (Do not enter title and authors here): Background: Premature ventricular contractions (PVCs) are one of the most common arrhythmias and can be associated with increased risk of mortality and major cardiovascular events including PVC-induced cardiomyopathy, ventricular fibrillation, and conduction system diseases.

Hypothesis: This study seeks to identify electro-anatomical association of PVC origin in patients with
elevated PVC burden who eventually develop Complete Heart Block (CHB).

Methods: This retrospective case series included patients with cardiac magnetic resonance (CMR) imaging
with elevated PVC burden (>1% burden, identified by Holter or implanted device). We also evaluated
patients who had electrophysiology study confirmed location of PVCs to correlate morphological
characteristics and electroanatomic localization. Records from 4217 patients between 2009 and 2023 were
screened.

Results: A total of 4 patients were identified with elevated PVC burden prior to the development of CHB,
without reported septal scar on CMR. Each had a single PVC morphology which was left bundle branch
morphology, V3/V4 transition, with left inferior axis with R or Rs morphology in lead 1 (Figure 1A), and all
developed infrahisian CHB. PVC characteristics most closely matched PVCs originating from the right
coronary cusp (RCC) and basal septum. Careful review of the CMR images in 2/4 (50%) patients found an
area of focal fibrosis at the basal septum at the junction RCC and medial papillary muscle of the right
ventricle (Figure 1B).In the other 2 patients, this area was not well imaged.

Conclusion: We identified a consistent PVC morphology in patients who developed infrahisian CHB with
imaging correlate of focal fibrosis at the site where the His bundle and proximal left and right bundles
traverse (Figure 1C). This focal area of scar is in a site that is difficult to image by CMR, and often only
visualized in one view and hence easily missed. This study demonstrates the need for careful electro-
anatomic-imaging correlation in patients with frequent PVCs of this morphology.