The Venoatrial Connection

Van Praagh and associates have advocated a shorthand method to designate the organizational pattern of the venoatrial connection, the pattern of ventricular looping, and the ventriculoarterial connection. By this method, the situs of each of these segments is recorded in the sequence {venoatrial connection, ventricular looping, ventriculoarterial connection}. Situs solitus indicates the usual or normal pattern, situs inversus the mirror-image of the normal pattern, and situs ambiguus denotes an ambiguous or indeterminate pattern. the The situs of the venoatrial connection can either be solitus, inversus, or ambiguus:

Venoatrial situs solitus is the normal arrangement, in which the morphologically right atrium is right-sided, and the morphologically left atrium is left-sided. Situs solitus is present when, on a plain chest film, the liver shadow is right-sided and the stomach bubble is left-sided. Under these circumstances, the morphologically right atrium is almost always right-sided and the morphologically left atrium is left-sided.

Venoatrial situs inversus is the mirror-image or inverted type of venoatrial situs in which the right atrium is left-sided, and the left atrium is right-sided. Situs inversus is almost certainly present when the liver shadow is left-sided and the stomach bubble is right-sided, and under these circumstances, the right atrium is almost always left-sided and the left atrium is almost always right-sided. In venoatrial situs solitus or situs inversus, the inferior vena cava, or the suprahepatic inferior vena cava in the presence of an interrupted inferior vena cava, is a highly reliable diagnostic marker of the location of the morphologically right atrium. The suprahepatic cava is therefore a useful diagnostic marker with important clinical ramifications. In contrast, the superior vena cava is not at all a reliable diagnostic marker of the right atrium, since the superior vena cava may return to the left atrium, where it referred to as the levoatrial cardinal vein.

Venoatrial situs ambiguus designates anatomic uncertainty or an indeterminate type, in which the liver shadow is bilaterally symmetric, the stomach bubble can be left-sided, right-sided or midline and changes from chest film to chest film. Under these conditions, venoatrial situs is anatomically uncertain and is termed situs ambiguus. Situs ambiguus suggests the presence of the atrial isomerism syndromes. In right atrial isomerism, there is often bilateral right-sidedness, hence, the associated anomalies would include the asplenia syndrome, bilateral superior vena cavae, bilateral trilobed (morphologically right) lungs, and bilateral morphologically right atrial appendages. Conversely, left atrial isomerism is associated with bilateral left-sidedness, and hence, the associated lesions include polysplenia, an interrupted inferior vena cava with azygous continuation, bilateral bilobed (morphologically left) lungs, and bilateral morphologically left atrial appendages. Right or left isomerism are helpful mnemonics that assist one to remember the associated lesions, however, there is considerable variation within each syndrome, and features of one syndrome can be found in the other. Hence, both right and left isomerism are regarded are having situs ambiguus, since these syndromes are neither rigidly separated nor mutually exclusive.