Tracking Down the Anatomy of the Left Bundle Branch to Optimize Left Bundle Branch Pacing

Figures

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  Figure 1. Macroscopic View From a Human Postmortem Heart in Simulated RAO and LPO View When viewed from the right atrial cavity (A), the fibrous membranous septum forms the apex of the triangle of Koch (transillumination). The hinge of the septal leaflet of the tricuspid valve (STV) provides the dividing line between the atrioventricular and interventricular components of the membranous septum. (B) The LPO view shows the transilluminated membranous septum located inferior to the interleaflet triangle between the right (R) and noncoronary (N) sinus of the aortic valve. Note that we have highlighted in dark color the limits of the endocardial position of the left bundle branch (LBB) of His and its 3 fascicles, the left anterior (1), the left septal or middle (2), and the left posterior (3). LPO = left posterior oblique; RAO = right anterior oblique.

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  Figure 2. Anatomy and Fluoroscopy of the Left Bundle Branch (A) A gross dissection of the human heart to reveal in simulated right anterior oblique view (RAO) the location of the compact atrioventricular (AV) node and penetrating AV bundle relative to the septal leaflet of the tricuspid valve (STV) anteriorly. In this case, the transition from the compact AV node to the penetrating AV bundle occurs inferiorly, and the length of the AV bundle implies a large His bundle recording zone. A dashedblue line marks the position of the His bundle as a reference to identify the initial site for left bundle (LB) pacing about 0 to 15 mm below the STV tricuspid valve annulus distal (ventricular) and apical to the His position. Red dashed lines indicate the schematics of the left bundle branch (LBB) on the left side of the interventricular septum. (B) An RAO 50º fluoroscopic view of a right atrial angiography taken during an EP study to display the tricuspid valvar annulus (TVA) and the relationship between the site of recording of the largest His bundle potential (His) and the landmarks of the triangle of Koch. In this case, the His bundle was recorded superiorly at the vertex of the triangle. Pink…

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  Figure 3. Histological Findings of the His Bundle Area and the LBB Area (A and B) Histological sections (stained with Masson's trichrome) through the division of the bundle of His and its right and left branches at the level of the interventricular component of the membranous septum, being attached inferiorly to the crest of the muscular interventricular septum. Note the relationship of the left branch (always subendocardial) with the nadir of the noncoronary leaflet of the aortic valve. (B) The right branch is running intramyocardially 1 or 2 mm from the endocardium of the right ventricle. (C1, C2, C3) Histological sections of 3 specimens stained with Masson's trichrome of the left branch 12 to 20 mm distal from its origin. The left branch is located between the muscular septum and the endocardium of the left ventricle. The thickness of the muscle fibers of the left branch is very variable, small in C1 and abundant in C3; the endocardium has smooth muscle cells in its thickness; and surrounding the left branch there is connective tissue of variable thickness. C1: Green star indicates fibrous tissue; black arrow indicates septal artery. ∗Left bundle branch myocytes separated by…