Cardiac Segmental Anatomy
The anatomy of the heart can be divided three separate segments; the position of the atria, the looping of the ventricles, and the relationship between the ventricles and great vessels which allows for the standardization of describing cardiac anatomy. Each segment (atria, ventricles, great vessels) is given a letter that describes how the patient’s anatomy compares to normal, creating a 3-letter system. Each segment is described individually, not in comparison with other structures.[1]
Atria classification
The atria classification represents the first letter in the segmental anatomy system. The position of the atria is compared to normal cardiac anatomy. Situs solitus means that the atria are in normal configuration. It is documented by the letter S in segmental anatomy. Atria may also be considered situs inversus meaning the atria are in mirror image of normal anatomy. This is represented by the letter I. Finally, atria can be situs ambiguous, meaning the atria are indistinguishable from each other. It is represented by the letter A.
Ventricular looping
The next letter in a segmental anatomy approach describes the looping of the ventricles. The ventricles are described as either D- looped or L-looped, represented by the D or L respectively, as the second letter in the anatomy description. The first step is to identify the right and left ventricles based on morphology, then describe their position. Normal position of the ventricles is D-looping, meaning the right ventricle is rightward and anterior to the left ventricle. If the right ventricle is any other position than rightward and anterior, then the ventricles are considered L-looped.
Great vessel relationship
The final letter in segmental anatomy describes the relationship between the ventricles and the great arteries (pulmonary artery and aorta). First, the great arteries can be situs solitus (S), meaning that the arteries are normally related to the ventricles. Conversely, they can be situs inversus (I) where the arterial position is inverted. When the vessels are malposed, they can either be dextro-positioned (D) or levo-positioned (L). In dextroposition, the aortic valve is rightward of the pulmonary valve whereas, in levoposition, the aortic valve is leftward of the pulmonary valve
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[1] Geva, T. (2009). Nomenclature and segmental approach to congenital heart disease. In Cohen, M. (Ed.) Echocardiography in Pediatric and Congenital Heart Disease.