The Heart

Mon, 12 Mar 2012 16:51:15 +0100

The operations of the heart are a vita area of human anatomy
"Aortic sinuses (of Valsalva)

The aortic sinuses are more prominent than those in the pulmonary trunk. The upper limit of each sinus reaches considerably beyond the level of the free border of the cusp and forms a well-defined complete circumferential sinotubular ridge when viewed from the aortic aspect (Fig. 56.11C). Coronary arteries usually open near this ridge within the upper part of the sinus, but are markedly variable in their origin. The walls of the sinuses are largely collagenous near the attachment of the cusps, but the amount of lamellated elastic tissue increases with distance from the zone of attachment. Strands of myocardium may enter this fibroelastic wall. At the mid level of each sinus, its wall is about half the thickness of the supravalvular aortic wall and less than one-quarter of the thickness of the sinutubular ridge. At this level, the mean luminal diameter of the beginning of the aortic root is almost double that of the ascending aorta. These details are functionally significant in the mechanism of valvular motion.

Opening of the aortic valve

During diastole, the closed aortic valve supports an aortic column of blood at high but slowly diminishing pressure (Fig. 56.7). Each sinus and its cusp form a hemispherical chamber. The three nodules are apposed and the margins and lunular parts of adjacent cusps are tightly apposed on their ventricular aspects. From the aortic aspect, the closed valve is triradiate, three pairs of closely compressed lunules radiating from their nodules to their peripheral commissural attachments at the sinutubular junction (Fig. 56.9). As ventricular systolic pressure increases, it exceeds aortic pressure and the valve is passively opened. The fibrous wall of the sinuses nearest the aortic vestibule is almost inextensible but, in the upper parts of sinuses, the wall is fibroelastic. Under left ventricular ejection pressure, the radius here increases 16% in systole. Hence the commissures move apart, making the orifice triangular when fully open. The free margins of the cusps then become almost straight lines between peripheral attachments. However, they do not flatten against the sinus walls, even at maximal systolic pressure, which is probably an important factor in subsequent closure. During ejection, most blood enters the ascending aorta, but some enters the sinuses, forming vortices that help to maintain the triangular ‘mid position' of the cusp during ventricular systole and probably initiate their approximation with the end of systole. Tight and full closure ensues, with the rapid decrease in ventricular pressure in diastole. Commissures narrow, nodules aggregate and the valve reassumes its triradiate form. Experiments indicate that 4% of ejected blood regurgitates through a valve with normal sinuses, whereas 23% regurgitates through a valve without them. The normal structure of the aortic sinuses also promotes non-turbulent flow into the coronary arteries.

Echocardiography

The gross anatomy of the heart can be evaluated by two-dimensional echocardiography in the para-sternal, apical, suprasternal and subcostal positions (Fig. 56.14). The standardized planes used are long axis, short axis and four-chamber. Echocardiography allows a detailed assessment of the functional anatomy of the heart. The long-axis view is obtained by placing the ultrasound transducer in the left apicosternal position and provides detailed images of the left ventricle, aorta, left atrium, and mitral and aortic valves (Fig. 56.14C). Angling the beam towards the right also allows assessment of the right atrium, right ventricle and tricuspid valves. Rotating the transducer by 90° in the clockwise direction produces the short-axis view, which allows assessment of the left ventricle, papillary muscles, chordae tendineae and mitral valves (Fig. 56.14B). The four-chamber view demonstrates the ventricles, atria, and mitral and tricuspid valves (Fig. 56.14A). Rotation of the transducer allows two-chamber views of the heart and more detailed assessment of the aorta and aortic valves. Cardiac magnetic resonance and computer tomography provide similar information on cardiac structure and function (Fig. 56.14D–F), together with complementary information on great vessels and other extracardiac intrathoracic stuctures."