Rapid advances have been made in electrophysiologic mapping, imaging, use of catheter ablations for drug refractory arrhythmias, and transcatheter implantation of pacing leads. A better understanding of cardiac anatomy is essential to make further progress, especially in fine-tuning interventional techniques and in developing computer models of arrhythmias. To this end, describing cardiac structures in attitudinal perspective1 is much more useful to the clinical electrophysiologist than the conventional approach2 of describing the heart as if it is standing on its apex and rotated with right and left heart chambers side by side. This chapter on cardiac anatomy highlights features of particular relevance to electrophysiologists.
For the clinician the heart must be viewed in the context of its location and relationship to surrounding structures. The frontal silhouette of the heart is nearly trapezoidal. The right border of the heart is more or less a vertical line just to the right of the sternum. It is formed exclusively by the right atrium, with the superior and inferior caval veins joining at its upper and lower margins. The inferior border lying horizontally on the diaphragm is marked by the right ventricle. The sloping left border is made up of the left ventricle and, as it merges with the upper border, the silhouette is formed by the pulmonary trunk. The upper border of the silhouette is made by the arterial trunks with the pulmonary valve leftward and superiorly situated relative to the aortic valve. On the frontal silhouette, the left atrium is barely seen; only its appendage curling round the edge of the pulmonary trunk is visible. Thus, the left atrium is the most posteriorly situated cardiac chamber. When viewed in situ, the proximity of the esophagus to the posterior wall of the left atrium is clear (Fig. 37–1). This spatial relationship is crucial to ablationists in order to reduce the risk of the post-procedural complication of atrioesophageal fistula.3,4
Two halves of the same specimen bisected longitudinally to show the relationship between the posterior wall of the left atrium and the esophagus. Ao, aorta; Es, esophagus; LA, left atrium; LAA, left atrial appendage; LI, left inferior pulmonary vein; LS, left superior pulmonary vein; MV, mitral valve; RI, right inferior pulmonary vein; RS, right superior pulmonary vein.
The heart is enclosed in a fibrous sac, the pericardium, which separates the surface of the heart from adjacent structures. The mediastinal pleura is the outermost lining of the fibrous pericardium. Within the fibrous pericardium there is a thinner double-layered membrane, the serous pericardium. One layer of the serous pericardium is fused to the inner surface of the fibrous pericardium while the other layer lines the outer surface of the heart as the epicardium. The pericardial cavity then is the space between the layers of the serous pericardium. Two recesses are found within the pericardial cavity. One is the transverse sinus lying between the back of the arterial trunks and the front of the atrial chambers. Another is the oblique sinus lying behind the left atrium and is limited by the right pulmonary veins and the inferior caval vein to the right side and by the left pulmonary veins to the left side.5,6 Understanding the topography of surrounding structures is important also for those using intra-pericardial catheter techniques. For instance, the descending aorta close to the left inferior pulmonary vein is at risk of damage.
On the surface of the pericardial sac descends the phrenic nerves and their accompanying pericardiophrenic arteries which are branches from the internal mammary artery. The right phrenic nerve descends vertically along the right anterolateral surface of the superior caval vein to be related to the right aspect of the intercaval atrial wall, and in front of the root of the lung, to reach the diaphragm adjacent to the lateral border of the entrance of the inferior caval vein.7 Along the way, the right phrenic nerve can be less than 2 mm from the anterior wall of the right superior pulmonary vein (Fig. 37–2A).7 The left phrenic nerve descends on the left side close to the aortic arch and onto the pericardium over the left atrial appendage and the left ventricle. It takes one of two courses over the left ventricle that takes it either over the anterior surface or leftward over the obtuse margin. The latter course is close to the lateral vein or left obtuse marginal vein (Fig. 37–2B).7,8
A. The right phrenic nerve (arrows) descends along the pericardium, in close proximity to the right pulmonary veins as they enter the left atrium. B. The descent of the left phrenic nerve is either related to the great cardiac vein and anterior descending coronary artery or, as in this specimen, over the course of the obtuse marginal vein. Abbreviations as before, and OV, obtuse vein; RA, right atrium; SCV, superior caval vein.
Viewed from the front, the cavity of the right atrium is right and anterior, while the left atrium is situated to the left and mainly posteriorly. The anterior wall of the left atrium lies just behind the transverse pericardial sinus, while its posterior wall is just in front of the tracheal bifurcation and the esophagus. In man, the right superior pulmonary vein passes posterior to the superior caval vein, with the right inferior vein posterior to the venous sinus of the right atrium (Fig. 37–3A).
A. A view of the endocast from the right shows the characteristically extensive pectinate muscles of the right atrial wall contrasting with the smooth vestibule. The right superior pulmonary vein is ...
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