Background: The instability and high output needed for direct His-bundle (HB) pacing have led to increasing use of left bundle branch (LBB) pacing via a transeptal deployment of the lead. The anatomical basis for it is poorly known.
Objective: To describe the HB arrangement within the left ventricular outflow tract that is critical to allow for a correct deployment of a transeptal lead.
Methods: We examined by dissection techniques and histology the course of the atrio-ventricular (AV) conduction axis in 57 structurally normal human hearts.
Results: In 17.5% of specimens the MS length was ≤2 mm. After penetrating the AV membranous septum the conduction axis has a non-branching component that in 85.5% of cases runs only for a short distance (1-3 mm) along the septal crest before giving rise to the fascicles of the LBB on the septal surface. In 5 hearts (9%) the HB division was found before it reached the interventricular MS and in 3 cases (5.5%) 2-3.5 mm distal to the crest within the muscular interventricular septum. 22 hearts (49%) were shown to have a relatively left-sided deviation of the AV bundle with the anterior part of the bundle closely related to the nadir of attachment of the right coronary leaflet of the aortic valve. In the remaining 51% of hearts, the bundle coursed centrally or with a right-sided deviation. The thickness of the interventricular muscular septum at 10 below the muscular crest was 10±2.5 mm (7.3-15.3 mm).
Conclusion: The a) variable anatomic arrangement of the branching portion of the HB in relation with the muscular and interventricular membranous septum and b) the left-sided deviation of the AV conduction axis are extremely relevant anatomic features for successful LBB pacing.