Determination of atrial situs

Atrial situs refers to the relative position of the cardiac atria in relation to abdominal viscera and the midline.

Pathology

Identification of atrial situs is an important initial step in the antenatal and postnatal diagnosis of cardiac structural and situs anomalies.

Radiographic features

Basic determination of situs relies on the ability to discriminate right and left atria, which depends on the morphologic characteristics of the atria and other features including the:

• internal and external anatomy of the atrial appendages
• connections of systemic and pulmonary vascular systems to the heart
• anatomy of the interatrial septum

Out of these features, the most reliable method is the morphologic characteristic differences between the right and left atria.

Right atrium

• connections: superior and inferior vena cavae
• crista terminalis: it is a smooth thickened muscular structure which represents junction between sinus venosus and developing heart
• right atrial appendages: triangular or pyramidal shaped and broad-based; it contains pectinate muscles extending towards the atrioventricular valve
• see: normal anatomy of the left atrium

Left atrium

• connections: pulmonary veins
• crista terminalis: absent in left atrium
• left atrial appendages: long slender finger-like; contains pectinate muscles
• coronary sinus always runs inferoposteriorly to the left atrium
• see: normal anatomy of the right atrium