atrium has also been encountered.12 These instances suggest that the lumen of the left superior caval vein obliterates, but does not disappear, remaining as a potential vascular channel.10,12 Although persistence of the left superior caval vein has no direct effect on the systemic venous drainage, its anomalous connection to the coronary sinus has been shown, on occasion, to produce obstruction to adjacent cardiac structures (Fig 27.8) There are reports of the vein impinging on the vestibule of the mitral valve and causing obstruction to the inflow of the left ventricle, sometimes requiring surgical plication.13 Left-sided obstructive lesions themselves also occur at an increased frequency in patients having persistence of the left superior caval vein.14 However, the effect of such persistence is much more pronounced during placement of permanent pacemakers through left axillary venous puncture The incidence of abnormalities of the sinus node and arrhythmia is also known to be higher in these patients, especially when there is absence of the right superior caval vein.15 FIG 27.8 Dissection showing the course of a persistent left superior caval vein (dotted lines) along the vestibule of the left atrioventricular junction There is an obvious bulge into the cavity of the morphologically left atrium Absence of the Right Superior Caval Vein On occasion, the right superior caval vein can be absent, with the left superior caval vein providing the only source of drainage of blood from head and arms Over 200 cases have been reported in the world literature, although it is suspected that, being clinically silent, this variant may remain largely undetected In the absence of the right-sided channel, the left-sided vein receives the brachiocephalic vein and the azygos vein However, the left-sided venous channel is not the mirror image of the right superior caval vein Instead, this vascular channel has an origin and course exactly the same as that of the left superior caval vein (see Fig 27.6) Hence, it drains via the coronary sinus in the majority of cases but may connect directly to the left atrium in some In some autopsy specimens, a vestigial cord has been identified, which represents the remnant of the right superior caval vein.16 The anomaly results from involution of the right, instead of the left, superior cardinal vein during embryonic life It is known to exist both as an isolated anomaly and in association with other cardiac malformations Abnormalities of rhythm are common, more so with coexisting cardiac malformations, and are more pronounced in older patients.17 Once again, since drainage is normal, the anomaly is clinically silent Obviously, the effect on cardiac catheterization, implantation of pacemakers, and surgical repairs is much greater than when the superior caval veins are present bilaterally If there is simultaneous unroofing of the coronary sinus, arterial desaturation will dominate the clinical presentation Absence Bilaterally of the Superior Caval Veins On rare occasions, both superior caval veins can be absent.18,19 The blood from the head and arms drains to the inferior caval vein via the azygos or hemiazygos systems Although the exact morphogenesis remains unclear, we presume that this anomaly results from failure of development of the superior cardinal venous system Since the drainage of blood remains unobstructed, it remains clinically silent, as has been the case in most reported examples, coming to attention only during pacemaker implantation or central line insertion Inferior Caval Vein Absence of the Hepatic Segment of Inferior Caval Vein With Azygos Continuation This is the most common anomaly involving the inferior caval vein It exists because of the absence of the infrahepatic portion of the inferior caval vein— hence the term interrupted inferior caval vein The blood from the lower part of the body reaches either the left or right superior caval vein via the dilated azygos venous system (Figs 27.9 and 27.10) Therefore, with this arrangement, only the hepatic veins continue to drain blood from the liver and the splanchnic circulation to the morphologically right atrium The lesion is the consequence of abnormal development of the right subcardinal vein, which fails to anastomose with the right vitelline vein The supracardinal venous system, which normally forms the azygos system, provides continuation of systemic venous return to the developing superior caval vein The malformation is said to occur in 0.6% of all patients with congenital cardiac disease.20 When found, its presence should always raise the suspicion of isomerism of the left atrial appendages, although it can be found in patients with usual or mirror-imaged atrial arrangement (see Fig 27.9).21,22 Although commonly seen in the setting of malformed hearts, it can occur as an isolated anomaly.23 Like the other anomalies of connection discussed thus far, interruption of the inferior caval vein does not manifest clinically Accurate identification, however, is needed because of its frequent association with left isomerism Its presence can also complicate diagnostic and therapeutic cardiac catheterization when venous vascular access is needed to reach the heart It is most relevant in the setting of patients having functionally univentricular hearts who are undergoing construction of a cavopulmonary connection The mere connection of the superior caval vein to the pulmonary arteries in this setting is equivalent to a near total cavopulmonary connection This, of course, is the so-called Kawashima repair.24 Although nearly nine-tenths of the venous return from the lower body does reach the lungs, the blood has to traverse against gravity We suspect that it is this less energy-efficient flow of blood that partly contributes to the higher incidence of venovenous collaterals in these patients.25,26