Anatomic Insights for Catheter Ablation of Ventricular Tachycardia

Home , Aortic sinus, Moderator band (heart), Pulmonary valve

Anatomic insights for catheter ablation of ventricular tachycardia

Siew Yen Ho, PhD, FRCPath

From the Cardiac Morphology Unit, National Heart & Lung Institute, Imperial College London, London, United Kingdom.

Safe maneuvering of catheters in and around the ventricles, and ventricular outlets. Importantly, the central location of the aortic ablations for ventricular tachycardia, require knowledge of the valve in the heart puts it in the vicinity of all four cardiac anatomy of the cardiac and extracardiac structures. This review chambers, both for access and for reducing risk of damage to vital emphasises the three-dimensional spatial relationships between structures such as the atrioventricular conduction bundle. The the right and left ventricles and highlights the important extra- locations of the cardiac chambers are not as simple as implied by cardiac structures in the vicinity of the ventricles. The spatial the commonly used terms ‘right heart’ and ‘left heart’. relationships between the right and left ventricles are intricate owing to the curvature of the ventricular septum. The crossover KEYWORDS Anatomy; Conduction system; Ventricles; Catheter ab- relationship between the right and left ventricular outﬂow tracts lation and different planes and angulations between the aortic and the (Heart Rhythm 2009;6:S77–S80) © 2009 Heart Rhythm Society. All pulmonary valves is particularly relevant to intervening in the rights reserved.

Introduction “wrapped” by the crescentic right ventricle. Particularly Whatever the substrate for ventricular tachycardia, knowl- relevant to ventricular tachycardia ablation is understand- edge of cardiac anatomy is important when maneuvering ing the “crossover” relationship between the right and catheters in and around the ventricular chambers in order to left ventricular outflow tracts. The right ventricular outlet reduce the potential for procedural complications. This re- passes cephalad in a posterior and slightly leftward di- view highlights some relevant spatial relationships between rection. In some hearts, the tip of the left atrial appendage chambers in the structurally normal heart, the morphology may overlie the left anterior wall of the pulmonary trunk of the right and left ventricles, and the anatomy relevant or outflow tract. The left ventricular outlet passes under- to epicardial approach. No attempt is made to review all neath the right ventricular outlet in a rightward and ceph- aspects of cardiac anatomy or distortions resulting from alad direction pointing toward the right shoulder (Figure pathologic changes. 1a). Furthermore, the pulmonary and aortic valves are not at the same level. The pulmonary valve, the most supe- Disposition of the cardiac chambers riorly situated of the cardiac valves, lies at the level The terms right heart and left heart commonly used in corresponding to the third left costal cartilage at its junc- clinical practice obfuscate the true disposition of the tion with the sternum. The transverse plane of the aortic cardiac chambers.1 The availability of imaging tools to valve slopes inferiorly, away from the plane of the pul- visualize the heart in situ together with three-dimensional monary valve, such that the orifice of the aortic valve reconstructions helps greatly in demystifying some of the faces rightward at an angle of at least 45° from the intricacies of gross cardiac anatomy, allowing better ap- median plane.4 Not only that, the difference in levels preciation of the spatial relationships between chambers, between the two sets of arterial valves may be exagger- valves, and the orientations of the atrial and ventricular ated by the length of the freestanding cone of muscle septal structures.2 The plane of the cardiac septum at an supporting the pulmonary valve known as the subpulmo- angle of approximately 45° to the median plane places nary infundibulum (see below). In between the aortic the right atrium and right ventricle anterior to their re- valve and the infundibulum is an epicardial tissue plane, spective left counterparts rather than side by side, and the not a part of the ventricular septum. atria are posterior relative to their ventricles.3 In addition, The key to cardiac anatomy is the central location of the ventricular septum is not straight but curves such that the aortic root (Figure 1). Anterosuperior to the root lies the left ventricle appears circular in cross-section and the pulmonary valve and subpulmonary infundibulum. Owing to the offset in levels between aortic and pulmonary valves, the aortic sinuses closest to the pulmonary Dr. Ho has no conflicts of interest. Address reprint requests and valve in fact lie behind the subpulmonary infundibulum. correspondence: Prof. Siew Yen Ho, Cardiac Morphology Unit, National Heart & Lung Institute, Imperial College London, Guy Scadding Building, These are the aortic sinuses that give origin to the main Dovehouse Street, London SW3 6LY, United Kingdom.E-mail address: right and left coronary arteries. As these main coronary [email protected]. arteries descend toward their respective atrioventricular

Figure 1 a: Endocast of a normal heart viewed from the front showing the crossover relationship between right and left ventricular outflow tracts (arrows). Dotted ovals represent the orifices of the pulmonary and aortic valves. b: Atrial chamber transected and pulmonary and aortic valves cut at the level of the sinutubular junctions. The heart viewed from the right and posterior shows the central location of the aortic valve. Note the near alignment between a closure line of the aortic valve with that of the tricuspid valve. Dotted line represents plane of the atrial septum. c: Epicardial fat removed to show the ventriculoarterial junction (dotted line) between the pulmonary sinuses and the infundibulum and the relationship between the infundibulum and the aortic sinuses. Open arrow indicates the aortic mound in the right atrium. Ao ϭ aorta; L ϭ left coronary aortic sinus; LAA ϭ left atrial appendage; LCA ϭ left coronary artery; LAD ϭ left anterior descending artery; LV ϭ left ventricle; MV ϭ mitral valve; N ϭ noncoronary aortic sinus; PT ϭ pulmonary trunk; R ϭ right coronary aortic sinus; RA ϭ right atrium; RAA ϭ right atrial appendage; RCA ϭ right coronary artery; RV ϭ right ventricle; TV ϭ tricuspid valve. grooves, they pass within millimeters of the epicardial Right ventricle and outflow tract 5 aspect of the infundibulum (Figure 1). The posteroinfe- The orifice of the tricuspid valve marking the inlet to the rior wall of the infundibulum is related to the bulging right ventricle is oriented nearer to the vertical plane than coronary sinuses of the aortic root. The third aortic sinus, the horizontal plane. A muscular fold representing the inner the noncoronary sinus, bulges toward the plane of the curvature of the primitive heart tube separates the tricuspid atrial septum. Thus, the posteroinferior margin of the from pulmonary valves (Figure 2a). This fold, the ventri- aortic root is flanked by atrial walls. In turn, the bulge of culo-infundibular fold, inserts into the top of the ventricular the sinus causes an eminence in the right atrial wall septum forming the supraventricular crest. It continues into known as the “aortic mound” or “torus aorticus,” not to the subpulmonary infundibulum without any anatomic de- be mistaken for the atrial septum, which lies more pos- marcation between the two structures. The infundibulum is teroinferiorly (Figures 1b and 1c). Another important the term given to the conical-shaped right ventricular out- relationship is the near alignment of the closure line between the right and the noncoronary aortic leaflets with flow tract that adjoins the arterial wall of the pulmonary the closure line between the septal and anterosuperior trunk at the ventriculoarterial junction (Figures 2a and 2b). leaflets of the tricuspid valve (Figure 1b). The two valves Thus, its wall thickness tapers from the regular 3 to 5mm at are separated by the membranous septum at the central the ventricular body to 1 to 2 mm at the junction. The fibrous body. From the atrioventricular node, the His pulmonary sinuses are not as prominent as the aortic si- bundle penetrates through the central fibrous body to nuses. Nevertheless, owing to the semilunar configuration emerge as the atrioventricular conduction bundle on the of the valvar leaflets, the hinge line of each leaflet crosses left side, sandwiched between the membranous septum the ventriculoarterial junction at two points. Consequently, and the crest of the ventricular septum. As a result of this there are always small segments of myocardium of the close relationship, focal atrial tachycardia near the His infundibulum at the nadirs of the three sinuses (Figure 2b). bundle may be targeted from the aortic sinuses.6 Between adjacent sinuses, the wall comprises small trian- Ho Anatomic Insights for Catheter Ablation of Ventricular Tachycardia S79

A Y-shaped broad muscle band, the septomarginal trabeculation, buttresses the ventricular septum. It clasps the ventriculo-infundibular fold between its limbs to form the supraventricular crest. The medial papillary muscle inserts into its inferior limb and is a good landmark for the emer- gence of the right bundle branch from its transseptal course into the subendocardium (Figure 2a). From there, the right bundle branch encased within its fibrous sheath sometimes can be seen as a white line descending down the body of the septomarginal trabeculation toward the ventricular apex. The apical portion of the right ventricle is characterized by a meshwork of coarse muscular trabeculations including the moderator band. Left ventricle and outlet Unlike the right ventricle, the inflow and outflow tracts of the left ventricle are at an acute angle to one another (Figure 2c). This is because the central location of the aortic valve places the outflow tract between the mitral valve and the ventricular septum. In turn, approximately half of the aortic outlet is muscular and the other half, being the area of valvar continuity between mitral and aortic valves, is fibrous. The curvature of the ventricular septum continuing into the free wall forms the anterosuperior wall of the left ventricular outflow tract. It is the deep anterior (aortic) leaflet of the mitral valve that forms the aortic–mitral curtain. The ex- tremities of the fibrous continuity are the left and right Figure 2 a: Parietal wall of the right ventricle removed from this heart fibrous trigones, the right trigone forming the central fibrous viewed from the front. There is overlap between the ventriculo-infundib- body. In similar fashion to the pulmonary valve, the semi- ular fold (VIF) and the freestanding subpulmonary infundibulum (inf). lunar hinge lines of the aortic leaflets enclose a segment of These structures are closely related to the aortic sinuses. Green triangle ventricular myocardium in the nadirs of the right coronary indicates the approximate location of the His bundle. The right bundle branch emerges to become subendocardial (blue spot) and continues as a aortic sinus (Figure 2d) and part of the left coronary aortic thin cord (green spots) down the septomarginal trabeculation (SMT). Open sinus. The coronary orifices usually are located in the si- arrow indicates the moderator band. b: Right ventricular outflow tract nuses immediately below the level of the sinutubular junc- splayed open and leaflets of the pulmonary valve removed. The ventricu- tion rather than toward the nadirs.9,10 Because the right loarterial junction, interleaflet fibrous triangles, and segments of myocar- coronary aortic sinus abuts the crest of the ventricular sep- dium at the nadirs of the sinuses (s) are visible. c: This longitudinal section shows the inflow and outflow tracts of the left ventricle (broken line) and tum, separated from right ventricular cavity by the ventri- the right ventricular outflow tract (RVOT) passing over the aortic outflow culo-infundibular fold, it is easy to mistake musculature of tract. d: Magnified view showing the wall of the subpulmonary infundib- the fold as myocardial extensions on the adventitia aspect of ulum (white arrows) lying anterior to the aortic sinus and tubular portion the aortic root (Figure 2a). Similarly, the posteroinferior of the ascending aorta, separated by epicardial tissues (open arrow). At the wall of the subpulmonary infundibulum overlies the an- nadir of the right coronary aortic sinus (R) is the muscular crest of the ventricular septum (black arrow). The atrioventricular conduction bundle terior walls of the left and right coronary sinuses and may emerges from the central fibrous body to run between the muscular septum give the impression of myocardial sleeves covering these and the membranous septum (asterisk). The left bundle branch descends sinuses (Figure 2d). Rarely, there is ventricular myocar- from the atrioventricular conduction bundle (pale green shape). Other dium in between the aortic and mitral valves, akin to the abbreviations as in Figure 1. usual arrangement in equines. Each leaflet of the aortic valve is slightly thickened gles of fibrous tissue that become incorporated into the toward the free margin, with the nodule of Arantius meeting ventricle when the valve closes.7 On the epicardial aspect, at the middle of the closure lines when the valve is in the the ventriculoarterial junction is not always a sharply de- closed position. In the elderly, the leaflets often have small fined line. Extensions of ventricular myocardium into the fenestrations above the closure lines. Like the pulmonary adventitia occur in approximately 20% of individuals and valve, there is an interleaflet fibrous triangle between adja- have been traced to a maximal distance of 6 mm beyond the cent sinuses.11 Although functionally they form part of the junction.8 Ablations at so-called supravalvar locations for ventricular outflow, they project like the prongs of a coronet some cases of ventricular arrhythmia may be at the level of above the ventricular chamber. Thus, perforating the trian- the valve rather than much beyond the level of the valvar gle between right and noncoronary leaflets leads to epicar- sinuses into the tubular portion of the pulmonary trunk. dial space toward the right atrium while the triangle be- S80 Heart Rhythm, Vol 6, No 8S, August Supplement 2009 tween the noncoronary and left coronary leaflets points courses of the right and left phrenic nerves that descend toward the left atrium. The third triangle leads to the tissue along the fibrous pericardium.13 The left phrenic nerve may plane between subpulmonary infundibulum and aortic si- be vulnerable when ablating in the vicinity of the obtuse nuses. margin or the sternocostal surface of the left ventricle. The ventricular septum immediately proximal to the aor- Utilizing the coronary veins via the coronary sinus is an- tic valve is smooth. As discussed above, the atrioventricular other option, but bear in mind the left phrenic nerve may be conduction bundle emerges on the left side, between the close to the obtuse marginal vein or the great cardiac vein, membranous septum and muscular ventricular septum (Fig- depending on the course it takes on its descent. In addition ure 2d). The left bundle branch descends in the subendo- to the course of the left phrenic nerve, potential hazards cardium of the muscular septum in fanlike fashion, giving include the presence of venous valves and the thin, unpro- three main fascicles that interconnect.12 The distal ramifi- tected outer surface of the vein. The relationship between cations branch into fine strands that may be carried within vein and artery is variable; the vein may cross over an artery so-called false tendons that traverse the cavity to the parietal and vice versa. wall and papillary muscles of the mitral valve. 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