Aortic valve, aortic root, left ventricular outflow tract and pulmonary valve

Embryology

Septation of the initially single outflow tract begins in the fourth week of gestation (26–32 days post fertilization), occurring in the conotruncal region of the primitive heart tube, which extends from the primitive right ventricle to the boundaries of the pericardial cavity. Development of the arterial valve orifices occurs in the region of the truncus arteriosus, the most distal point of the outflow tract, between the conus arteriosus (precursor of the subpulmonary infundibulum on the right, which regresses on the left) and the aortic sac. Formation of the arterial septum and arterial valves occurs largely through the appearance and migration of two main truncal cushions, the dextrosuperior and sinistro-inferior endocardial cushions, which merge to form the truncal septum and spiral caudally in an anticlockwise fashion to separate the future aorta and pulmonary artery. The spiralling of the truncal septum is responsible for the relationship the great vessels share and for the position of the valve orifices. These main truncal cushions also form the right and left cusps of both the aortic and pulmonary valves. Two other intercalated truncal cushions appear parietally within the truncus arteriosus, the right going on to form the non-coronary aortic cusp, and the left forming the anterior pulmonary valve cusp.

Two endocardial cushions appear in the conus arteriosus, named the dextrodorsal and the sinistroventral cushions, which persist as the infundibulum of the right ventricle. On the left side, the conal tissue resorbs, with the fibrous aortomitral curtain instead forming part of the left ventricular outflow tract.

Surgical surface anatomy

The pulmonary valve has the most superior surface projection, lying horizontally behind the intersection of the left third costal cartilage with the sternum. The aortic valve is located inferomedial to the pulmonary valve, projecting inferiorly and to the right from the most medial end of the left third intercostal space ( Fig. 52.1 ). The aortic valve is directed upwards and to the right. The optimal site of auscultation of heart valve sounds depends on the direction of blood flow, rather than the surface projection of the heart valve. The aortic and pulmonary valves are best auscultated in the second intercostal space, just to the right and left of the sternum, respectively. The murmur of aortic stenosis may radiate to the carotid arteries in the neck, while the murmur of aortic regurgitation is usually best heard in the left parasternal region between the second and fourth intercostal spaces, which marks the location of the regurgitant stream of blood as it courses back into the left ventricle.

Clinical anatomy

When viewing the heart from the base, with the atria removed (see Fig. 51.1 ), it is apparent that the aortic valve occupies a central position within the fibrous skeleton of the heart and is intimately related to all of the cardiac chambers. A thorough understanding of the structures which surround the root of the aorta is essential.

The aortic semilunar valve forms both the anatomical and haemodynamic boundary between the left ventricle and the aorta, relying on the surrounding structures of the aortic root to maintain correct apposition during diastole and thus preventing regurgitation of ejected blood into the ventricle. The nomenclature of the aortic valve and the surrounding structures is variable and often confusing. The aortic valve proper is best considered as consisting of the leaflets and their basal attachments. The aortic root is best considered as the area surrounding the proximal and most distal attachments of the aortic valve leaflets. The limits of the aortic root are the sinutubular junction (commissural ring) distally and the basal ring proximally. Within this continuity are the sinuses of Valsalva, the coronary artery orifices, the commissures of the valve and the interleaflet triangles ( Fig. 52.2 ).

Aortic valve

The normal aortic root consists of three sinuses of Valsalva, giving rise to two coronary arteries. The latter arise from the two sinuses which face the pulmonary trunk, the so-named left- and right-facing sinuses, with the reference point being the third (non-pulmonary-facing) sinus; in most cases, this is conveniently the position from which a cardiac surgeon examines the aortic valve ( Fig. 52.3 ; see Fig. 51.1 ).

The aortic valve leaflets are named left, right and non-coronary leaflets. Each is described as consisting of a hinge, a belly, a coaptation surface and a lunule/nodule of Arantius. The nodule of Arantius marks the middle of the free edge, and is continued laterally along the free edge as a thin supporting layer (the lunule), to the attachment at the wall of the aortic root. The bulk of each leaflet is referred to as the belly of the leaflet and consists of three distinct histological layers ( Fig. 52.4 ).

The inflow or ventricular layer, the ventricularis, consists of a dense sheet of elastic fibres which provide flexibility to the leaflet. The outflow or arterial layer, the fibrosa, features a high density of collagen fibres, which provide structure and strength. The middle spongiosa layer consists of proteoglycans and glycosaminoglycans, which provide durability to the valve and resistance to compression. The average thickness of the leaflet is 1.5–1.75 µm, though there is variability in the thickness of each section of the valve leaflet. The hinge describes the point at which the leaflet attaches to the anulus in a semilunar fashion.