Introduction to Cardiac Anatomy

An understanding of normal and abnormal cardiac morphology is basic to the management of congenital heart defects. Problems exists in the nomenclature of cardiac morphology, suffice it to say that the most appropriate terminology is that which is functional, accurate, and to the best extent possible, accepted by the majority of workers in the field. In a typical example, a ventricle may be referred to as left-sided, systemic, pulmonary, arterial, etc., but the question remains, which ventricle is it - the morphologically left or the morphologically right? The diagnostic problem is that a positional left ventricle may be a morphologically right ventricle. Morphologic anatomic identification is therefore cornerstone of accurate diagnosis.

The segmental approach to the diagnosis of congenital heart defects is based upon an understanding of the morphologic features of the different segments of the heart. The cardiac segments are the building blocks out of which all hearts are built, and include the great veins, atria, ventricles, and great arteries. The ability to distinguish the different segments of the heart is the cornerstone of cardiac morphology, and distinctions are based on knowledge of the structure of the normal heart. However, what would seem to be the most diagnostic features of any particular cardiac segment in the normal heart may not always be the most reliable indicator of its true identity in the malformed heart. For example, the most distinguishing feature of the left atrium might well be considered its connections to the pulmonary veins. However, this feature is of little value in identifying the left atrium in the presence of total anomalous pulmonary venous connection. Hence, the morphological method, by which cardiac segments are identified according to their most constant features, is the most accurate way to identify structures in a malformed heart. The morphological features identifying the great veins are the organs that they drain; for the atria, it is their appendages; for the ventricles, it is the pattern of their apical trabeculations; and for the great arteries, it is the pattern of their branching.

The method by which the cardiac segments are joined one to another are referred to as the connections. These connections are termed the venoatrial, atrioventricular, and ventriculoarterial connections. The venoatrial connection defines the connection of the great veins to the atria. It is principally concerned with the situs of the heart when considering the systemic venoatrial connection, and with anomalous pulmonary veins when considering the pulmonary venoatrial connection. Unlike the remaining two connections, the venoatrial connection does not include any functional valves. The atrioventricular connection consists of the atrioventricular septum and the ventricular valves (mitral and tricuspid). The atrioventricular connection defines the morphology and function of the ventricular valves and the atrioventricular septum, along with the mode of connection of the atria to the ventricular mass. The ventriculoarterial connection consists of the subarterial infundibuli and the arterial valves (aortic and pulmonary). The ventriculoarterial connection defines the morphology and function of the arterial valves and the subarterial infundibuli, along with the mode of connection of the ventricular mass to the great arteries. It is important to note that these connections can be related one to another. In the normal heart, for example, there is aortic-mitral valvar continuity, while the pulmonary valve is separated from the ventricular valves. This relationship of the atrioventricular to the ventriculoarterial connection is due to the normal presence of a muscular infundibulum beneath the pulmonary valve which separates the pulmonary valve from the ventricular valves, and the absence of a subaortic infundibulum, which results in aortic-mitral fibrous continuity. Hence, the various connections may have important relationships to each other.

For the sake of surgical diagnosis, seven segments and connections need to be completely identified, these being the four cardiac segments (the great veins, atria, ventricles, and great arteries) and their connections (venoatrial, atrioventricular, and ventriculoarterial). The approach in the present communication is to consider first the cardiac segments then the connections by which they are joined together. Special consideration is then given to the septation of the human heart, cardiac malpositions, and the conduction system of the heart.