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Introduction Permanent and irreversible damage to the conduction tissue is one of the most common complications of TAVI. Detailed knowledge of the anatomy of the atrioventricular (AV) bundle is crucial to minimize the potential for injury to the His bundle branch block or complete AV block. Purpose Preexisting anatomic location and damage of the right bundle branch (RBB) may have important unrecognized clinical implications. Methods The myocardial arrangement and the presence of fibrous and fatty tissue infiltration of the AV conduction axis and right bundle branch were examined by dissection techniques and histological sections in 57 structurally normal human heart specimens (48 males, 77±7 years). Results The AV conduction axis enters the AV component of the membranous septum and is encircled by the fibrous tissue of the central fibrous body. The AV bundle is divided in a non-branching portion and a branching portion. After a short distance of the non-branching component along the septal crest, the RBB arises at the end of the branching portion of the conduction axis and is located superficially in the muscular ventricular septum. The RBB takes off from the bundle at the level of origin of the superior fascicle of the left bundle, passing then through the thickness of the ventricular septum to emerge beneath the medial papillary muscle of the tricuspid valve. In 22 hearts (49%) in which we found to have a relatively left-sided deviation of the AV bundle in relation to the interventricular membranous septum the RBB runs intramyocardial along the muscular interventricular septum. In the remaining 51% of the hearts the RBB runs subendocardially in the crest of the interventricular muscular septum. We found connective tissue and fatty infiltration along the right bundle branch in 23 hearts (40%) from its origin to its distal part. A significant correlation was found between age and the presence of RBB fibrosis (85% of hearts from individuals with age >80 years). These 2 conditions may make the RBB very vulnerable to self-expanding aortic valves during or after TAVI (figure). RBB and its critical region Conclusions The presence of an intramyocardial location of the RBB with a lef-sided deviation and the fibro-fatty infiltration found in the mayority of senescent hearts are relevant anatomic determinants that may increase the risk of complete AV block following a TAVI procedure.

Background/Introduction The deleterious effects of long-term RV apical pacing have been well recognized. Permament His bundle (HB) pacing has emerged as a promising technique for patients who need ventricular pacing. Purpose To describe the anatomy of the HB region and its variations for successfully approaching HB pacing in a safer and more efficient way and to understand selective vs non-selective HB pacing and to avoid permanent damage to it. Methods In 57 structurally normal human heart specimens (48 males, 77±7 years) we examined by dissection techniques and histological sections the course of the penetrating and non-branching His bundle in relation with the membranous septum and the tricuspid valve (TV) annulus. We correlated these anatomic findings with a series of angiographic studies in 60 patients (47% males, 45±16 years old) by recording the largest His electrogram sites within the limits of the triangle of Koch (TK) and the plane of the TV. Results The membranous septum is divided by the attachment on its right side of the septal leaflet of the tricuspid valve into atrioventricular (AV) and interventricular components. The AV component of the membranous septum forms the anterosuperior apex of the TK and showed in cadaveric hearts variable dimensions in length (4.6±1.5 mm, range 1–9 mm). The AV node becomes the His bundle as the AV conduction axis enters the AV component of the membranous septum and is encircled by the fibrous tissue of the central fibrous body in 100% of hearts with none of them having a “naked” entrance to it. In 30 hearts (53%) the HB penetrates the AV membranous septum at the apex of the TK at the hinge point of the septal leaflet of the TV near its commissure with the anterosuperior leaflet. However in 47% of cases the HB crossed the fibrous tissue of the central fibrous body in a lower position in the medial area of the paraseptal right atrial region of the TK with a mean distance to the AV membranous septum of 4±1.4 mm (range 1.5–8 mm) above and behind the TV annulus. These findings correlate with those obtained in patients in which the site of recording of the largest His bundle deflection does not always coincide with the anterosuperior vertex of the triangle as judged angiographically. In 51% of patients the site of the largest His bundle electrograms was found in the medial area of the triangle above (posterior-inferior) the TV annulus while in 49% of patients the His was recorded at the same level (15%) or below (anterior-inferior) the TV annulus (34% of patients) (figure). Histology of HB and catheter locations Conclusions Knowledge of the marked variability in the location of the AV conduction bundle within the paraseptal right atrial region is crucial for approaching permanent HB pacing. An anterior-inferior and ventricular HB location could yield to restriction of the septal leaflet of the tricuspid valve if a lead is to be deployed in the area.