Inlet and Apical Trabecular Portions
The left ventricle can be subdivided into three components, similar to the right ventricle. The inlet component surrounds and is limited by the mitral valve and its tension apparatus. The two papillary muscles occupy anterolateral and posteromedial positions and are positioned rather close to each other. The leaflets of the mitral valve have no direct septal attachments because the deep posterior diverticulum of the left ventricular outflow tract displaces the aortic leaflet away from the inlet septum. The apical trabecular component of the left ventricle extends to the apex, where the myocardium is surprisingly thin. The trabeculations of the left ventricle are quite fine compared with those of the right ventricle (Fig. 2-28). This characteristic is useful for defining ventricular morphology on diagnostic ventriculograms.
This dissection of the left ventricle shows its component parts and characteristically fine apical trabeculations (anatomical orientation).
The outlet component supports the aortic valve and consists of both muscular and fibrous portions. This is in contrast to the infundibulum of the right ventricle, which consists entirely of muscle. The septal portion of the left ventricular outflow tract, although primarily muscular, also includes the membranous portion of the ventricular septum. The posterior quadrant of the outflow tract consists of an extensive fibrous curtain that extends from the fibrous skeleton of the heart across the aortic leaflet of the mitral valve and supports the leaflets of the aortic valve in the area of aortomitral continuity (see Fig. 2-5). The lateral quadrant of the outflow tract again is muscular and consists of the lateral margin of the inner curvature of the heart, delineated externally by the transverse sinus. The left bundle of the cardiac conduction system enters the left ventricular outflow tract posterior to the membranous septum and immediately beneath the commissure between the right and noncoronary leaflets of the aortic valve. After traveling a short distance down the septum, the left bundle divides into anterior, septal, and posterior divisions.
The aortic valve is a semilunar valve that is quite similar morphologically to the pulmonary valve. Likewise, it does not have a discrete annulus. Because of its central location, the aortic valve is related to each of the cardiac chambers and valves (see Fig. 2-4). A thorough knowledge of these relationships is essential to understanding aortic valve pathology and many congenital cardiac malformations.
The aortic valve consists primarily of three semilunar leaflets. As with the pulmonary valve, attachments of the leaflets extend across the ventriculoarterial junction in a curvilinear fashion. Each leaflet therefore has attachments to the aorta and within the left ventricle (Fig. 2-29). Behind each leaflet, the aortic wall bulges outward to form the sinuses of Valsalva. The leaflets themselves meet centrally along a line of coaptation, at the center of which is a thickened nodule called the nodule of Arantius. Peripherally, adjacent to the commissures, the line of coaptation is thinner and normally may contain small perforations. During systole the leaflets are thrust upward and away from the center of the aortic lumen, whereas during diastole they fall passively into the center of the aorta. With normal valvar morphology, all three leaflets meet along lines of coaptation and support the column of blood within the aorta to prevent regurgitation into the ventricle. Two of the three aortic sinuses give rise to coronary arteries, from which arise their designations as right, left, and noncoronary sinuses.
This dissection in anatomic orientation, made by removing the aortic valvar leaflets, emphasizes the semilunar nature of the hinge points (see Figs. 2-22 and 2-23). Note the relationship to the mitral valve (see Fig. 2-5).
By sequentially following the line of attachment of each leaflet, the relationship of the aortic valve to its surrounding structures can be clearly understood. Beginning posteriorly, the commissure between the noncoronary and left coronary leaflets is positioned along the area of aortomitral valvar continuity. The fibrous subaortic curtain is beneath this commissure (see Fig. 2-29). To the right of this commissure, the noncoronary leaflet is attached above the posterior diverticulum of the left ventricular outflow tract. Here, the valve is related to the right atrial wall. As the attachment of the noncoronary leaflet ascends from its nadir toward the commissure between the noncoronary and right coronary leaflets, the line of attachment is directly above the portion of the atrial septum containing the atrioventricular node. The commissure between the noncoronary and right coronary leaflets is located directly above the penetrating atrioventricular bundle and the membranous ventricular septum (Fig. 2-30). The attachment of the right coronary leaflet then descends across the central fibrous body before ascending to the commissure between the right and left coronary leaflets. Immediately beneath this commissure, the wall of the aorta forms the uppermost part of the subaortic outflow. An incision through this area passes into the space between the facing surfaces of the aorta and pulmonary trunk (see Fig. 2-30). As the facing left and right leaflets descend from this commissure, they are attached to the outlet muscular component of the left ventricle. Only a small part of this area in the normal heart is a true outlet septum because both pulmonary and aortic valves are supported on their own sleeves of myocardium. Thus, although the outlet components of the right and left ventricles face each other, an incision below the aortic valve enters low into the infundibulum of the right ventricle. As the lateral part of the left coronary leaflet descends from the facing commissure to the base of the sinus, it becomes the only part of the aortic valve that is not intimately related to another cardiac chamber.
Dissection made by removing the right and part of the left aortic sinuses to show the relations of the fibrous triangle between the right and noncoronary aortic leaflets (anatomical orientation).
Knowledge of the anatomy of the aortic valve and its relationship to surrounding structures is important to successful replacement of the aortic valve, particularly when enlargement of the aortic root is required. The Konno-Rastan aortoventriculoplasty involves opening and enlarging the anterior portion of the subaortic region.4,5 The incisions for this procedure begin with an anterior longitudinal aortotomy that extends through the commissure between the right and left coronary leaflets. Anteriorly, the incision is extended across the base of the infundibulum. The differential level of attachment of the aortic and pulmonary valve leaflets permits this incision without damage to the pulmonary valve (Fig. 2-31). Posteriorly, the incision extends through the most medial portion of the supraventricular crest into the left ventricular outflow tract. By closing the resulting ventricular septal defect with a patch, the aortic outflow tract is widened to allow implantation of a larger valve prosthesis. A second patch is used to close the defect in the right ventricular outflow tract.
This incision, made in a normal heart, simulates the Konno-Rastan procedure for enlargement of the aortic root.
Alternative methods to enlarge the aortic outflow tract involve incisions in the region of aortomitral continuity. In the Manouguian procedure (see Fig. 2-19), a curvilinear aortotomy is extended posteriorly through the commissure between the left and noncoronary leaflets down to and occasionally into the aortic leaflet of the mitral valve.6 A patch is used to augment the incision posteriorly. When the posterior diverticulum of the outflow tract is fully developed, this incision can be made without entering other cardiac chambers, although not uncommonly the roof of the left atrium is opened. The Nicks procedure for enlargement of the aortic root involves an aortotomy that passes through the middle of the noncoronary leaflet into the fibrous subaortic curtain and may be extended into the aortic leaflet of the mitral valve.7 This incision also may open the roof of the left atrium. When these techniques are used, any resulting defect in the left atrium must be closed carefully.
As discussed previously, the differential level of attachment of aortic and pulmonary valves, as well as the muscular nature of their support, allows the pulmonary valve to be harvested and used as a replacement for the aortic valve in the Ross procedure.8,9 This procedure can be combined with the incisions of the Konno-Rastan aortoventriculoplasty to repair left ventricular outflow tract obstructions in young children with a viable autograft that has potential for growth and avoids the need for anticoagulation.
Accurate understanding of left ventricular outflow tract anatomy is also important in the treatment of aortic valvar endocarditis.10,11 Because of the central position of the aortic valve relative to the other valves and cardiac chambers (see Fig. 2-4), abscess formation can produce fistulas between the aorta and any of the four chambers of the heart. Therefore, patients may present with findings of left-sided heart failure, left-to-right shunting, and/or complete heart block in addition to the usual signs of sepsis and systemic embolization.