Purpose: To provide a detailed description of the His bundle (HB) arrangement within the left ventricular outflow tract.
Methods: We examined by dissection techniques and histological sections the course of the AV conduction axis (penetrating, non-branching and branching HB) in relation with the membranous and muscular interventricular septum in 57 structurally normal human heart specimens (48 males, 77±7 years)
Results: The AV conduction axis is located along the inferior edge of the membranous septum (MS). The MS is divided into AV and interventricular components and is located at the base of the interleaflet triangle between the right and non-coronary leaflets of the aortic valve. The conduction axis enters the AV component of the MS and is encircled by the fibrous tissue of the central fibrous body to reach the left ventricular outflow tract. The MS showed in cadaveric hearts variable dimensions in length (4.6±1.5 mm) with a range bewteen 1 to 9mm. In 17.5% of specimens the MS length was =2 mm. After penetrating the AV membranous septum it 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 left bundle (LB) on the septal surface (Type A). The most anterior fibers of the LB originate at the end of the branching portion located underneath the inferior edge of the MS. In 5 hearts (9%) the HB division was found before it reached the interventricular MS (Type B) and in 3 cases (5.5%)2-3.5 mm distal to the crest (Type C). 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 HB measures were (mm, mean±SD(range)): length: 3±0.6(2-4.5), width: 3.7±1.4(2.5-5.6), thickness: 1.4±0.5(0.5-2.2), HB to endocardium: 0.7±0.3(0.2-1.5).
Conclusions: A shorter membranous septum length, the variable HB dimensions and the left-sided deviation of the AV conduction axis are extremely relevant anatomic features that are linked to the frequent injury to the HB branch or complete AV block following TAVI.