Understanding Cardiac Anatomy and Imaging to Improve Safety of Procedures

Figures

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  Figure 1. Frontal View of the Right Ventricular Outflow Tract of the Human Heart When viewed from the frontal direction, the anterior wall of the right ventricular outflow tract is seen en face behind the left parasternal area of the anterior chest wall (Figure 3). The anterior wall of the right ventricular outflow tract is generally devoid of thick epicardial fat. In contrast, the region lateral and medial to the anterior wall of the right ventricular outflow tract is covered by thick epicardial fat of the right atrioventricular and anterior interventricular grooves, respectively. Understanding individual variation in the extent and thickness of epicardial fat around the right ventricular outflow tract is important for mapping and ablation of the epicardial substrate of patients with Brugada syndrome.4 The right ventricular outflow tract is adjacent to both atrial appendages, aortic and pulmonary roots, left ventricular outflow tract, proximal right coronary artery, proximal left anterior descending artery, anterior interventricular vein, left ventricular summit, and the anterior chest wall.

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  Figure 2. Segmentation of the Right Ventricle The heart is viewed from the frontal direction after removal of the right ventricular free wall. The right ventricle can be divided into 3 segments: the inflow, outflow, and apical trabeculated regions. The 3 outflow structures, composed of the supraventricular crest, septomarginal trabeculation, and moderator band, encircle the inferior border of the outflow above the inflow. The moderator band is located at the superior margin between the inflow and apical trabeculated region. The endocardial surface of the supraventricular crest covering the right coronary aortic sinus is smooth, and the pulmonary valve is at an elevated plane away from the tricuspid valve by this supraventricular crest. The apical trabeculated region contains multiple coarse and thick trabeculations that traverse the chamber. While these trabeculations can hinder catheter and lead manipulation within the region, they may also serve as anchoring structures for leadless pacemakers, helping to prevent dislodgement and migration. In contrast, the lack of traversing trabeculations in the inflow and outflow regions allows straightforward catheter manipulation, whereas these…

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  Figure 3. The Right Ventricular Outflow Tract in the Thorax When viewed from the frontal direction (A and B), the right ventricular outflow tract (RVOT) resides behind the left sternal border (A), where the precordial V2 electrode at the fourth intercostal space is placed with significant individual variation.21,22 Generally, the RVOT is not covered by the left lung, which is the reason why a left parasternal scan is feasible during transthoracic echocardiography. V4 is located around the cardiac apex (B). The RVOT and left ventricular outflow tract (LVOT) demonstrate a 3-dimensional crisscross relationship (dotted arrows), reflecting the embryological development of the arterial trunk. The RVOT follows a left posterosuperior axis, whereas the LVOT follows a right superior axis. When viewed from the superior direction (C), the pulmonary root is located left to the ascending aorta and left superior to the aortic root. The RVOT is located left anterior to the aortic root and ranges from anterior to anterosuperior relative to the LVOT. Note the sagittal relationship among the V2 lead, the RVOT, and the LVOT at the level of the low RVOT. L = left coronary aortic sinus; n = nonadjacent…

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  Figure 4. Segmentation of the Right Ventricular Outflow Tract Viewed From the Superior Direction The right ventricular outflow tract, at its higher subpulmonary level, consists of an encircling free wall divided into 5 segments: anterior, lateral, posterolateral, posterior, and medial. The sixth segment, the septal wall of the right ventricular outflow tract, is located beneath the medial and posterior free walls at the lower level. Thus, not all these 6 segments are located at the same horizontal level. Because the right ventricular outflow tract is located anterosuperior to the left ventricular outflow tract and overlies it, the septum between them is positioned posteroinferior to the right ventricular outflow tract, as in this image. The anterior, lateral, posterolateral, posterior, medial, and septal segments of the right ventricular outflow tract face to the anterior chest wall, right atrioventricular groove, right coronary aortic sinus, left ventricular summit, anterior interventricular groove, and left ventricular outflow tract, respectively. L = left coronary aortic leaflet; N = noncoronary aortic leaflet; R = right coronary aortic leaflet.

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  Figure 5. Segmentation of the Right Ventricular Outflow Tract Viewed From the External Aspect The right ventricular outflow tract is viewed from the right lateral (A), frontal (B), left anterior oblique (C), and left posterior oblique (D) directions. The pulmonary root is lifted by the encircling free wall of the right ventricular outflow tract, composed of anterior, lateral, posterolateral, posterior, and medial free walls. The right ventricular outflow tract exhibits a conical shape when viewed from right lateral and anterior directions (A and B). The posterolateral free wall faces the right coronary aortic sinus (R), and the right coronary artery emerges lateral to the posterolateral free wall (A). The anterior free wall extends broadly beneath the right (r) and non (n) adjacent pulmonary sinuses (B). The medial free wall stands up from the epicardial surface of the anterior interventricular groove (C and D). The height of the posterior free wall is short, standing up from the septal margin of the left ventricular summit and close to the interleaflet triangle between the right (R) and left (L) coronary aortic sinuses (D).23 L = left adjacent pulmonary sinus; N = noncoronary aortic sinus.

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  Figure 6. Segmentation of the Right Ventricular Outflow Tract Viewed From the Internal Aspect Two different hearts demonstrate bilateral surfaces of the short-axis section made at the level of the basal left ventricles. They are viewed from the apical (A, left anterior oblique view) and basal (B, right posterior oblique view) directions. When viewed from the apical direction (A), the basal endocardial surface of the right ventricular outflow tract (RVOT) is smooth, mainly composed of the posterolateral free wall, identical to the supraventricular crest. Thus, the endocardial surface of this segment could be mapped without a gap, but it is difficult to fix the ablation catheters. In contrast, when viewed from the basal direction (B), the apical endocardial surface of the RVOT is filled with trabeculations. Thus, in this region, ablation catheters can become trapped within the recesses/pouches between the trabeculations, increasing the risk of steam pop and perforation, and gapless mapping/ablation can be challenging. Note the thin medial free wall of the RVOT, adjacent to the left anterior descending artery, indicating the risk of perforation and thermal/mechanical injury to the artery. The…

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  Figure 7. The Right Ventricular Outflow Tract in Multiple Vertical Sections The heart is viewed from the superior direction (A) to display the vertical sectional planes (dotted lines) of each panel (B to D). The right ventricular outflow tract (RVOT) is sectioned in the medial-lateral (B), anterior-posterolateral (C), and anterior-posterior (D) directions. Medial (C), posterolateral (C), and posterior (D) free walls face the anterior interventricular groove, right coronary aortic sinus (R), and left ventricular summit (asterisk), respectively. Thus, the septal margin of the left ventricular summit can be approached from the posterior free wall of the RVOT (Figure 11). A thin fibroadipose tissue plane intervenes between the RVOT and the aortic root (C) and the left ventricular summit (D). The endocardial surface of the right posterior RVOT is smooth (B), whereas that of the left anterior RVOT exhibits multiple trabeculations (C and D). The septal segment of the RVOT is located beneath the medial (B) and posterior (D) free walls of the RVOT. When viewed from the left lateral direction (D), the RVOT extends anterosuperior to the left ventricular outflow tract (LVOT). l = left adjacent…

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  Figure 8. The Right Ventricular Outflow Tract in Multiple Horizontal Sections The heart is viewed from the frontal direction (A) to display the horizontal sectional planes (dotted lines) at the level of high right ventricular outflow tract (RVOT) (B) and low RVOT (C). Five segments comprising the encircling free wall of the RVOT (Figure 4) are observed in the high RVOT section (B). The septal segment cannot be seen on the sectional plane at this level. In the low RVOT section (C), only 4 segments comprising the septal wall and medial, anterior, and lateral free walls can be observed, along with the interventricular portion of the membranous septum (arrowhead). The septal wall or the RVOT, corresponding to the septomarginal trabeculation, gives rise to the moderator band (MB) distally and the medial papillary muscle proximally. In addition to the membranous septum, these structures indicate close proximity to the conduction system. When viewed from the superior direction (B and C), the RVOT extends broadly anterior (B) to anterosuperior (C) to the left ventricular outflow tract (LVOT). L = left coronary aortic sinus; n = nonadjacent pulmonary sinus; N = noncoronary aortic sinus; r = right…

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  Figure 9. Histology of the Right Ventricular Outflow Tract The heart is viewed from the frontal and cranial direction (A) to exhibit the sectional plane (dotted line) of the right ventricular outflow tract (RVOT) (B to D). The dotted box (C) indicates the area submitted for histological analysis (D). As this section was made just beneath the level at which the RVOT begins to detach from the left ventricular mass, virtual dissection (B), real dissection (C), and histology (Masson's trichrome staining) (D) demonstrate all 6 segments except the posterior free wall, which arises from the left ventricular summit (Figures 5 and 7). The RVOT is located anterosuperior to the left ventricular outflow tract (LVOT), separated by the crest of the ventricular septum supporting the right coronary aortic sinus. A thin fibroadipose tissue plane (asterisk) is observed between the posterolateral free wall of the RVOT and the right coronary aortic sinus. Thus, ventricular arrhythmias originating from the posterolateral free wall of the RVOT can be technically ablated from the right coronary aortic sinus. The first septal branch enters at the epicardial border between the medial free wall and septal wall of…

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  Figure 10. Variations in the Posterior Free Wall of the Right Ventricular Outflow Tract The left heart (A to C) demonstrates that the thin fibroadipose tissue (red asterisk) intervenes between the posterior free wall (black asterisk) of the right ventricular outflow tract (RVOT) and the left ventricular summit. In contrast, the right heart (B to D) does not demonstrate such an intervening fibroadipose tissue plane. Therefore, the posterior free wall is absent, and the corresponding segment can be referred to as the septal wall (white asterisk) of the RVOT. This variation alters the extent of the septal margin of the left ventricular summit and its relationship to the left adjacent pulmonary sinus (L).23 In the left heart (A to C), the septal margin of the left ventricular summit can be ablated from the posterior free wall of the RVOT intervened by the fibroadipose tissue plane, whereas in the right heart (B to D), the region can be ablated from the bottom of the left adjacent pulmonary sinus using a candy cane–shaped approach.26 LVOT = left ventricular outflow tract; n = nonadjacent pulmonary sinus.

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  Figure 11. Cases With an Elevated Infundibulum/Posterior Free Wall of the Right Ventricular Outflow Tract Each heart is viewed from the posterior (A and B) and superior (C) directions. Beneath the left adjacent pulmonary sinus (L) and adjacent to the aortic root, the posterior free wall (asterisk), which is also referred to as the “elevated infundibulum” of the right ventricular outflow tract, is elevated with respect to the left ventricular summit (A and B).27 This is a region of particular risk for cardiac perforation during catheter ablation in the right ventricular outflow tract, in addition to the medial free wall. When the pulmonary root is virtually dissected out, and epicardial fat is virtually removed, the encircling free wall of the right ventricular outflow tract (Figure 4) exhibits a short but intact posterior free wall of the right ventricular outflow tract, “elevated” from the left ventricular summit. In these cases, the septal margin of the left ventricular summit can be approached from the posterior free wall of the right ventricular outflow tract. L = left coronary aortic sinus; MB = moderator band; N = noncoronary aortic sinus; R = right coronary aortic sinus.

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  Figure 12. Cases Without an Elevated Infundibulum/Posterior Free Wall of the Right Ventricular Outflow Tract Each heart is viewed from the posterior (A and B) and superior (C) directions. The posterior free wall of the right ventricular outflow tract (RVOT) is absent in these cases. The ventriculoarterial junction of the pulmonary root is in direct continuity with the left ventricular summit. Thus, the myocardial crescent (asterisk) supporting the left adjacent pulmonary sinus (L) attaches to the left ventricular summit.28 In these cases, the septal margin of the left ventricular summit can be approached from the left adjacent pulmonary sinus.23,26 Because of the absence of an intervening fibroadipose tissue plane (Figure 10), energy application can be more effective to create lesions on the septal margin of the left ventricle summit. In this type of arrangement, the use of the pulmonary root as an autograft for the Ross procedure29 can be challenging without cutting deep into the septum. L = left coronary aortic sinus; LVOT = left ventricular outflow tract; n = nonadjacent pulmonary sinus; r = right adjacent pulmonary sinus; R = right coronary aortic sinus.