Understanding Cardiac Anatomy and Imaging to Improve Safety of Procedures

Figures

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  Figure 1. Inferior Pyramidal Space When the (A) medial free wall of the right atrium located at the floor of the triangle of Koch between the coronary sinus and septal tricuspid valve annulus is (B) carefully removed, the thin epicardial adipose tissue of the inferior pyramidal space filling between the right atrium and left ventricle is exposed. The atrioventricular node artery is buried within this space. When the box in B is magnified in C, the intact atrioventricular node and the penetrating portion of the bundle of His can be macroscopically observed at the apex of the inferior pyramidal space,10,11 (C) the right atrial side of the central fibrous body at the inferoposterior part of the membranous septum. (D) The inferior pyramidal space (dotted line) can be readily observed using cardiac computed tomography as the triangular fat plane wedging between the tricuspid and mitral valve orifices.8,9

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  Figure 2. Inferior Crux of the Heart (A) The diaphragmatic surface of the heart is viewed from the inferior direction after removal of the epicardial adipose tissue of the atrioventricular groove.2 The inset clearly shows the inferior crux of the heart (black circle), where the left ventricle faces the right atrium with the intervening inferior pyramidal space, anterior to the coronary sinus. When the region enclosed by the white circle in A is magnified in B, the thin atrioventricular node artery (B, arrow) can be observed, ascending within the inferior pyramidal space toward the atrioventricular node. Because the atrioventricular node artery is known to provide multiple branches perfusing the adjacent part of the basal left ventricle, referred to as the inferoseptal process10, 11, 12 and the atrioventricular conduction axis, its mean external diameter decreases from 1.48 ± 0.52 mm at the origin to 0.76 ± 0.37 mm at the compact node.6

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  Figure 3. Progressive Dissection of the Triangle of Koch (A) The floor of the triangle of Koch is viewed from the right lateral direction. (B) A light source placed in the left ventricular outflow tract transilluminates the membranous septum. (C) A small window is created by removing the medial free wall of the right atrium, corresponding to the apical part of the triangle of Koch. In this fashion, progressive dissection exposes the inferior pyramidal space sandwiched between the right atrium and the left ventricle (Figure 1).9 (D) Further careful removal of the epicardial adipose tissue filling the inferior pyramidal space shows the atrioventricular node and the atrioventricular node artery (arrow). Behind them, the epicardial surface of the basal left ventricle facing the right atrium can be observed as the inferoseptal process of the left ventricle,10, 11, 12 originally referred to as the posterior superior process,2,13 whereas the attitudinally appropriate term is the inferoseptal process.10, 11, 12

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  Figure 4. Right and Left Atrioventricular Node Artery and the Inferoseptal Process The atrioventricular node artery (arrows) can originate from either (A and C) right (82%-87%) or (B and D) left coronary arteries (7%-17%), or even both (0%-10%),1,4,5,7 as shown in (A and B) dissection performed by McAlpine2 and (C and D) images obtained from cardiac computed tomography. (A and B) The atrioventricular node artery ascends on the epicardial side of the inferoseptal process toward the atrioventricular node, as indicated by a red bead placed at the top of the atrioventricular node artery. (C and D) Dotted lines demarcate the inferior pyramidal space.

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  Figure 5. Atrioventricular Node Artery in Cardiac Computed Tomography (A to C) Cardiac computed tomography can also clearly visualize the atrioventricular node artery (arrows) ascending within the inferior pyramidal space (Figure 1). (B and C) The inferior pyramidal space is sandwiched between the coronary sinus and the inferoseptal process, containing the atrioventricular node artery. Note the close proximity with the anterior wall of the coronary sinus. (D) A dissection cut through the right atrium to the left ventricle shows the atrioventricular sandwich composed of the coronary sinus, inferior pyramidal space, and inferoseptal process. L = left coronary aortic sinus; N = noncoronary aortic sinus; R = right coronary aortic sinus.

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  Figure 6. Subvalvular Membrane and the Atrioventricular Node Artery (A and B) The subvalvular membrane,2 commonly referred to as the mitral annular disjunction, is visualized with transillumination. Thus, the subvalvular membrane of the medial scallop of the posterior mitral leaflet represents the septal side wall to the epicardial side of the inferoseptal process. Given this proximity, in case the atrioventricular node artery (arrows) runs closer to the mitral valve annulus (Figure 7), invasive procedures around the medial mitral valve annulus can damage the atrioventricular node artery.

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  Figure 7. Location of the Atrioventricular Node Artery Relative to the Mitral and Tricuspid Valve Annuli Examination of the location of the atrioventricular node artery (arrows) in 60 heart samples2 showed that (A) the artery ran in contact with the mitral valve annulus in 28 hearts, thus indicating the potential for damage during mitral valve replacement. (B) In 20 cases, the atrioventricular node artery ran through the middle of the inferior pyramidal space. (C) In 12 cases, the atrioventricular node artery ran adjacent to the tricuspid valve annulus. As shown in Figures 4 and 5, preprocedural cardiac computed tomography can readily identify these variations.

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  Figure 8. Reverse U-Turn of the Distal Right Coronary Artery Because the inferior pyramidal space is filled with loose adipose tissue, in the case with an elongated and eventually tortuous right coronary artery, (D to G) its distal part can show reverse U-turn morphology within the inferior pyramidal space.1 (A to C) Compared with the control case, (E and F) in the case with a reverse U-turn, the right coronary artery (yellow arrows) has a higher position within the inferior pyramidal space and is located in front of the coronary sinus (stars). Thus, in such a case with a reverse U-turn, an invasive approach performed on the septal isthmus and the anterior wall of the coronary sinus, including slow pathway modification, has the potential to damage not only the atrioventricular node artery (red arrows), but also the right coronary artery. White arrows indicate the Eustachian valve.

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  Figure 9. Slow Pathway Modification (A and B) Biplane fluoroscopic images and (C) a dissection image show the slow pathway modification with the tip of the ablation catheter (yellow arrowheads) placed at the septal isthmus between the coronary sinus orifice and the tricuspid valve annulus, also referred to as the inferior paraseptal region.11 (C) Note the close proximity with the atrioventricular node artery (red arrow). In 20% of examined hearts, the atrioventricular node artery lay just beneath (1 mm) the endocardium near the coronary sinus orifice, a location suggesting a potential risk of atrioventricular node artery damage during radiofrequency ablation in the slow pathway region.3 In another study, the average minimal depth of the artery relative to the endocardial surface of the right atrium was reported to be 3.5 ± 1.5 mm (range, 1.5-9.0 mm).8 Despite this proximity, the induction of atrioventricular block during slow pathway modification is rare, possibly because of the cooling (heat sink) effect of the normal blood flow together with the fibrous adventitia protecting the arteries in these regions.8

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  Figure 10. Nerve Fascicles Around the Atrioventricular Node Artery (A to C) Horizontal section made at the level of the coronary sinus orifice shows the atrioventricular node artery (red arrows) within (C) the inferior pyramidal space sandwiched between the coronary sinus orifice and the inferoseptal process of the left ventricle. The white box in B is magnified in C. Refer to Figure 5C. (D to F) Histologic examination revealed multiple nerve fascicles (black arrowheads) of the intrinsic autonomic nervous system running along the atrioventricular node artery. Thus, slow pathway modification can also affect these nerve fascicles. The black boxes in D and E are magnified in E and F, respectively. PGP = protein gene product; SMA = smooth muscle actin.