Asian Cardiovasc Thorac Ann 1999;7:349-352
© 1999 Asia Publishing EXchange Pte Ltd
Invited Commentary
Ho Siew Yen, PhD, FRCPath,
Robert H Anderson, MD, FRCPath
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Department of Paediatrics National Heart & Lung Institute Imperial College School of Science, Technology and Medicine Dovehouse Street London SW3 6LY, UK
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In Bhargava and Kornowski's review of a novel system that reconstructs anatomic and electromechanical maps of the cardiac chambers, the authors focus on the left ventricle. The distinct advantage of this system is its ability to provide anatomic information in a fashion that is readily comprehensible. For the operator to make best use of this information, it is desirable to have already a thorough understanding of cardiac structure. To com-plement Bhargava and Kornowski's review, we provide a description of the left ventricle and the coronary arteries as might be seen by cardiologists working in the catheterization laboratory.
The prime consideration when describing anatomy for clinicians is to provide an account of the cardiac structures as they would be seen with the heart lying in its normal position in the body. Although standing the isolated heart on its apex in the "Valentine" fashion has long been considered conventional for anatomists, this is no longer acceptable practice in the clinical arena.1 Clinical anatomy should help rather than hinder understanding, especially for the novice. With improvements in technology such as is now provided by computer-based mapping systems, clinical anatomy has become even more relevant.2,3 Although the cardiac structures remain the same, we are now able to visualize them in the living patient from virtually any vantage point. Accordingly, we must have a more logical way of describing their locations. For this, the body coordinates must be used as points of reference.
In the anatomical orientation, the head is superior and the feet or the diaphragm in the chest are inferior. The sternocostal surface of the chest is then anterior, while the spine is posterior. Self-evidently, right and left relate to the arms. For users of the NOGA system (Biosense-Webster, Tirat-HaCarmel, Israel), these coordinates are provided by the icons of the face and the right and left arms seen on the screen of the monitor.4 The orientation of the chamber during its reconstruction then aids in the navigation of the catheter around the chamber. However, the advent of this facility has now shown that some aspects of long-established terminology are no longer appropriate. An obvious example is the terminology for describing the walls of the left ventricle as seen in the left anterior oblique projection, a standard view that is familiar to cardiologists (Figure 1A
). Analyzed conventionally, the margin of the left ventricle related to the diaphragm is presently considered as being posterior, while its opposite side, nearer to the head, is described as anterior. This is obviously illogical and needs to be corrected. Attitudinally, the diaphragmatic margin seen on the lower part of the projection screen is inferior.5,6 Its diametrically opposite margin is, therefore, superior.
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Figure 1. (A) The coordinates of the body are shown in simulated left anterior oblique view. (B) The long axis of the heart is at an angle to the long axis of the body. The conical shape of the left ventricle is superimposed on the cardiac outline.
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The left ventricular chamber is shaped more-or-less like a cone, with its apex pointing leftward and inferiorly, whereas the base is located toward the midline of the body. As there is an obliquity between the long axis of the body relative to that of the heart, the apex is located anteriorly, adjacent to the sternocostal surface, whilst the base is positioned posteriorly, toward the spine (Figure 1B
). In short-axis sections, the ventricle has a circular profile. Occupying one quadrant of this circle, the septum is an integral part of the left ventricular wall and it is to the front or anterior (Figure 2). In sections toward the base of the heart, the walls of the left ventricle are then described most accurately as being septal or anterior, posterolateral, superior, and diaphragmatic or inferior (Figure 3A). Since the superior margin in basal sections is related to the outflow tract of the aorta, it would be also justifiable to consider this part as being aortic. When describing apical sections, it is logical to consider the walls as being septal or anterior, superior or anterosuperior in sections nearest the apex, posterolateral, and dia-phragmatic or inferior (Figure 3B).
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Figure 2. This representation of a lateral view shows a short-axis section through the ventricular mass. The ventricular walls are described attitudinally. The coronary arteries in the interventricular grooves are depicted as small circles. Note the inferosuperior relationships of the papillary muscles (cross-hatched) that support the mitral valve.
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Figure 3. (A) A short-axis section near the base of the heart. The outflow tract to the aorta interposes between the ventricular septum and the mitral valve. The aortic and mitral valves are in fibrous continuity (between small arrows). (B) This section at the level of the papillary muscles (dotted outlines) shows their oblique arrangement in the circular profile of the left ventricle. The coronary arteries in the interventricular grooves are diametrically opposite each other.
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Structurally, however, analysis of the left ventricle is simplified by considering it in terms of its inlet, outlet, and apical trabecular components (Figure 4). The inlet extends from the atrioventricular junction to the myocardial insertions of the papillary muscles of the mitral valve. The outlet surrounds and supports the aortic valve, incorporating the bases of the aortic valvar sinuses. At the cardiac base, these inlet and outlet components are adjacent to each other and accommodated within the circular cross section of the left ventricle (Figure 3A). Because of the area of fibrous continuity between the leaflets of the aortic and mitral valves, the outlet is surrounded in part by fibrous tissue and in part by muscle. The third component of the left ventricle is the apical component of the muscular cone. Its endocardial surface is characterized by fine criss-crossing muscular trabeculations (Figure 4). While the walls of the left ventricle are usually thicker than those of the right ventricle, it is worth noting that the apical part at the tip is remarkably thin, being no more than 1.5 to 2 mm in most hearts (Figure 4).
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Figure 4. This heart specimen has been bisected longitudinally along its long axis to display the components of the left ventricle. The right ventricular outflow tract is superior to the left ventricle. The left ventricular outlet is in part muscular and part fibrous. The aortic leaflet (between small arrows) of the mitral valve is like a curtain between the inflow and outflow tracts. Note the smooth endocardial surface of the septum toward the aortic valve. In contrast, the apical part is lined by a network of muscular trabeculations.
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Hinged at the atrioventricular junction, the orifice of the mitral valve is kidney-shaped. It is guarded by two leaflets of dissimilar proportions that close along a solitary line of apposition. One leaflet, being adjacent to and in fibrous continuity with the aortic valve, is termed the aortic leaflet (Figure 5). It is also conventionally described as the anterior leaflet. This leaflet is deep and hangs like a curtain between the inflow and outflow tracts (Figure 4). The second leaflet, the mural leaflet, is hinged to the posteroinferior margin of the atrioventricular junction. It occupies two-thirds of the valvar perimeter, it is not as deep as the aortic (anterior) leaflet, and frequently has a scalloped appearance. It is also called the posterior leaflet. The tension apparatus of both leaflets insert exclusively to two groups of papillary muscles that are arranged obliquely. The inferosuperior relationship of the papillary muscles is best appreciated in crosssectional view (Figure 3).
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Figure 5. The atrial aspect of the mitral valve shows the arrangement of its leaflets. The asterisks mark the two ends, the commissures, of the line of apposition between the two leaflets.
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The left ventricular outlet is guarded by the semilunar leaflets of the aortic valve (Figure 6A). The basal parts of the semilunar lines of attachment are to the ventricular wall anterosuperiorly but to the leaflet of the mitral valve posteroinferiorly (Figure 3A). With this arrangement, crescents of ventricular tissue are incorporated into the valvar sinuses, whereas triangular areas of the arterial wall between the semilunar lines of attachment extend the ventricular outlet to the sinu-tubular junction of the aorta (Figures 6B and 6C). The fibrous triangle between the noncoronary and right coronary sinuses is continuous at its base with the membranous part of the septum. The specialized atrioventricular conduction axis penetrates through this membranous septum to emerge on the crest of the ventricular septum. The left bundle branch then descends subepicardially along the smooth upper part of the ventricular septum (Figure 6C).
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Figure 6. (A) The two leaflets of the aortic valve adjacent to the pulmonary valve are conveniently described as the facing leaflets. Their corresponding aortic sinuses give rise to the main coronary arteries. (B) The aortic outflow tract opened to display the arrangement of the valvar leaflets. The peaks of the semilunar lines of attachment of the valvar leaflets can be traced along the sinu-tubular junction. (C) Removal of the valvar leaflets along their lines of attachment (dotted lines) shows the basal parts crossing the ventriculoarterial junction (broken line). This arrangement results in small crescents of ventricular musculature incorporated into the valvar sinuses. Between adjacent leaflets are small triangular areas of fibrous tissue marked by dotted lines at the sides and a broken line at the base.
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The normal aortic valve has three sinuses with the main coronary arteries arising from those sinuses that are adjacent to or facing the pulmonary valve (Figure 6A). Since the third aortic sinus is furthest from the pulmonary valve and hardly ever gives rise to a coronary artery, it can be described as the noncoronary, nonfacing, or nonadjacent sinus. The coronary arterial orifices are usually located eccentrically in the sinuses, close to the sinu-tubular junction.7 The right coronary artery encircles the right atrioventricular groove, giving off branches to supply the musculature of the right atrium and ventricle. In nine-tenths of individuals, it continues beyond the cardiac crux to supply part of the diaphragmatic or inferior surface of the left ventricle. The left coronary artery emerges from its aortic sinus to enter the space between the subpulmonary infundibulum and the left atrial appendage. Within a centimeter of its origin, the main stem usually bifurcates into two major arteries. The first artery, the circumflex artery, turns laterally to the anterosuperior aspect of the left atrioventricular groove, whilst the second artery runs anteriorly to enter the superior interventricular groove. The second artery is commonly called the left anterior descending artery (LAD) because of the previous anatomical practice of standing the heart on its apex. Likewise, the artery running in the inferior interventricular groove is known as the posterior descending artery (PDA). It is interesting to note that in the earlier English literature the term "interventricular" is used in preference to "descending".8 Perhaps, "interventricular" highlights the fact that these arteries give rise to branches that perforate the ventricular septum as well as side branches that supply the musculature of both right and left ventricles. Using the same attitudinal principles for describing the cardiac mass, it is readily apparent that the major coronary arteries that occupy the interventricular grooves are superiorly and inferiorly located, a fact that is clearly demonstrated in crosssectional views (Figure 3). The shorthand terms of LAD and PDA are perhaps too firmly etched in the mind of clinicians to change them to superior and inferior interventricular arteries, respectively. Similarly, the convention for right and left chambers of the heart remains convenient. However, such a change would make it much easier to correlate anatomy and pathology properly with the features shown so clearly by the NOGA system.
As with nearly everything, there is resistance to change, particularly when existing terms and phrases have become well-established. In the United Kingdom for instance, purists lament about the English language being adulterated by foreign, mainly North American, imports, but the beauty of "Queen's English" is that it continually evolves like many other languages. Similarly, if we are to match technological advances, nomenclature in cardiac anatomy also needs to change so as to be more clinically relevant.
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References
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