Cardiac Anatomy and Pathology

It has been 500 years since the birth of Andreas Vesalius, the father of anatomy. In his most important and influential work, “De Humani Corporis Fabrica Libri Septem,” published in seven volumes between 1543 and 1555, Vesalius demonstrated the importance of dissection of cadavers for detailed knowledge of human anatomy. Historically, anatomy is a fundamental part of the medical curriculum. These days, to greater or lesser degrees, traditional dissection has been replaced by computer-based interactive learning, problem-based learning, plastinated specimens, and so on, leading to concerns in some departments.

Nevertheless, the basic anatomy of the heart has remained unchanged over the centuries. It is as relevant to the understanding of cardiac diseases for diagnosis and therapy then as now. However, the perspectives from which clinicians can view and approach the heart have evolved, especially over the past few decades, alongside changes in therapeutic strategies and rapid advances in imaging and interventional technologies. The importance of revisiting cardiac anatomy is particularly relevant to operators who need to access the heart chambers and great vessels. This chapter on cardiac anatomy aims to provide an overview of the fundamentals of the normally structured heart with emphasis on components relevant to device implantation. Much of the information is basic revision for the experienced operator but, it is hoped, will help the beginner to build upon the modern day anatomic curriculum. Because the heart structures are not transparent, the figures accompanying the chapter are orientated mostly in anatomic fashion instead of attempting to simulate precisely the views as portrayed in fluoroscopic projections.

The Heart in the Chest

The topographic anatomy of the heart is crucial to practitioners, as highlighted by Walmsley in 1958 when he commented that descriptions of cardiac anatomy disregard the cardinal principle of using terms in relation to anatomic position. He noted that many textual descriptions and innumerable figures throughout the medical literature view the heart as if it would be held in the hand, with the atria above the ventricles and the left and right hearts lying alongside each other in a sagittal plane providing basic and false concepts that have caused untold confusion in the past. For instance, by standing the heart on its apex, it is easy to see how the anterior and posterior descending coronary arteries acquired their names. It was MacAlpine who, in emphasizing the importance of describing the heart in its anatomic location for appropriate clinical correlations, termed the orientation of the heart seen in its living condition as attitudinal. The names of the chambers, however, remain unaltered, although right heart chambers are not strictly to the right or left heart chambers strictly to the left.

Viewed from the anteroposterior perspective, the cardiac silhouette is generally taken to be trapezoidal in shape. The rib cage provides good markers for charting the cardiac silhouette. The normal position of the cardiac apex is generally taken to be in the fifth intercostal space in the midclavicular line. The lower border is a nearly horizontal line in the area of the left sixth rib to the right sixth costal cartilage. The upper border is hidden behind the sternum at the level of the second and third cartilages. The right margin of the heart peeps out behind the right border of the sternum between the right third and sixth cartilages. In the infant, the upper part of the cardiac shadow is broad due to the prominence of the overlying thymus gland.

Inferior to the thymus, a fibrous pericardial sac encloses the mass of the heart. The sac has cuff-like attachments around the adventitia of the great arteries and veins as they enter or emerge from the heart. The pericardial cavity is contained between the double-layered serous pericardium. The parietal pericardium is adherent to the fibrous pericardium, whereas the visceral layer is densely adherent to the cardiac surface forming the epicardium. Because of the contours of the heart and great arteries, there exist two recesses within the pericardial cavity. These are the transverse and oblique sinuses. The transverse sinus occupies the inner heart curvature and lies between the posterior surface of the great arteries and the anterior surface of the atrial chambers. The reflection of the serous pericardium around the four pulmonary veins and the inferior caval vein forms the oblique sinus.

It is important to note that the right phrenic nerve descends along the lateral aspect of the superior caval vein to pass in front of the hilum of the right lung and then along the fibrous pericardium lateral to the right atrium to reach the diaphragm ( Fig. 1-1A ). It is closely related to the anterior wall of the right upper pulmonary vein. The left phrenic nerve descends on the left side close to the aortic arch and onto the fibrous pericardium over the left atrial appendage and the left ventricle, taking variable courses along the anterior and lateral aspects in close relationship to the left coronary veins and great cardiac vein.

When the pericardium is removed, the major part of the heart visible from the front is the ventricular mass. Here, the morphologically right ventricle occupies the greater part ( Fig. 1-1B ). The left ventricle appears only as a narrow slip along the left cardiac border. The shape of the heart is generally likened to a pyramid with a base and an apex. The apex points downward, forward, and to the left, whereas the base faces posteriorly and to the right. The cardiac apex is usually represented by the vortex of the left ventricle, but the cardiac base is less well defined. The anatomic base is formed mainly by the left atrium receiving the pulmonary veins and to a small extent by the posterior part of the right atrium. The base in clinical practice, however, refers to the portion of the heart near the parasternal parts of the second intercostal spaces. The cardiac surfaces are described as the sternocostal, diaphragmatic, left, and right. The sternocostal surface is covered anteriorly by the sternum and pleurae. The diaphragmatic surface is horizontally orientated. The sharp angle formed mainly by the right ventricle and occupying the lower heart border is the acute margin of the heart. The rounded obtuse margin of the heart is formed mainly by the left ventricle to the left of the sternocostal surface.

Relationships of Cardiac Chambers

The relative positions of the cardiac chambers and great vessels are readily displayed with an endocast ( Fig. 1-2 ). The so-called right heart chambers are anterior and to the right (see Fig. 1-2A, B ). From the frontal aspect, the right border of the cardiac silhouette is formed exclusively by the right atrium. The superior and inferior caval veins join the upper and lower margins of the venous component of the right atrium. The inferior cardiac border lying nearly horizontally on the diaphragm is marked by the right ventricle. The sloping left border is made up of the left ventricle, but it merges with the pulmonary trunk near the upper border. Apart from the tip of its appendage curling around the edge of the pulmonary trunk, the left atrium is not visible from the frontal aspect. Being the most posterior of the cardiac chambers, the left atrium lies directly in front of the esophagus.

A key feature of the anatomy of the normally structured heart is the central location of the aortic root (see Fig. 1-2A, B ). It springs from the left ventricle to be located anterior to both atrial chambers and the atrial septum, and posterior to the right ventricular outflow tract. Any procedures carried out in the aortic root can potentially impact upon all the other cardiac chambers and valves, the major coronary arteries, and the atrioventricular conduction system.

Viewing the endocast from the right lateral aspect shows the location of the right atrium posterior and to the right of the right ventricle (see Fig. 1-2C ). The plane of the right atrioventricular junction containing the annular insertion of the tricuspid valve is orientated nearly vertically. The right ventricle sweeps from posterior to anterior and passes cephalad such that its outflow tract lies superior to that of the left ventricle (see Fig. 1-2A ). This crossover relationship between right and left ventricular outflow tracts is due to the left outflow tract being directed rightward and cephalad in a projection toward the right shoulder. The plane of the pulmonary valve is nearly horizontal and located well cephalad, making the pulmonary valve the most superiorly situated of the cardiac valves. The aortic valve is adjacent but located to the right and posterior in relation to the pulmonary valve. The two sets of semilunar valves are not at the same level. Instead, the plane of the aortic valve tilts inferiorly at an angle to the pulmonary valve, and its orifice is directed not only upward but also rightward at an angle of at least 45 degrees to the median plane.

From the left lateral aspect, the left ventricle can be seen projecting forward and leftward with the apex directed inferiorly (see Fig. 1-2D ). The finger-like left atrial appendage usually points anterosuperiorly from the left anterior side of the body of the left atrium. Usually, there are four pulmonary veins entering the posterior aspect of the left atrium, but variations are not uncommon. The great cardiac vein and its continuation into the coronary sinus pass along the epicardial side of the inferior atrial wall. This venous channel is a good guide to the left parietal border of the heart in left anterior oblique view.

Like the tricuspid valve, the plane of the mitral orifice is not horizontal. In right anterior oblique view the plane of the tricuspid orifice forms an angle of 25 degrees with that of the mitral orifice. Although the anterosuperior parts of these planes merge, they diverge and are separate inferoposteriorly. Viewed from the anteroposterior perspective, the orifices of the four cardiac valves are like a cascade of plates. The pulmonary valve is situated most superiorly in a nearly horizontal plane, whereas the aortic valve slopes rightward and posteriorly from the pulmonary valve. The aortic valve is sandwiched by the D-shaped mitral orifice, which lies leftward and posteriorly, and the tricuspid orifice, which is located rightward and anteriorly.

The Right Atrium

Anatomically, the right atrium can be considered as having four components: a venous component, an appendage, a vestibule, and the right atrial aspect of the atrial septum (see Fig. 1-2C ). However, other than the terminal crest (crista terminalis) clearly marking the border between the appendage and the venous component, the other borders are indistinct. The right atrium is situated more anteriorly relative to the left atrium. Hence in the transverse bodily plane, the plane of the atrial septum extends obliquely rightward, from anterior to posterior, normally at an angle of approximately 60 degrees to the sagittal plane.

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