desaturation However, as highlighted, a small amount of right-to-left shunting may go undetected if there is minimal reduction in oxygen saturation This is particularly the case if only one systemic vein is anomalously connected Even in patients with totally anomalous systemic venous connections, the cyanosis may be minimal if a large interatrial communication permits the mixing of blood In patients with coexisting heart disease, greater than anticipated cyanosis should always raise the suspicion of anomalous drainage of the systemic veins Abnormalities of the P-wave axis on the electrocardiogram may indicate anomalies of the right superior caval vein.28 In contrast, the chest radiograph is normal in patients only having anomalous drainage of the systemic veins Nonetheless, in patients with associated cardiac malformations, cardiomegaly, or pulmonary blood flow better than expected for the cyanosis should once again raise the suspicion of anomalous drainage of one or more systemic veins On echocardiography, anomalous drainage of one or more systemic veins should be suspected if there is unexplained dilation of the left atrium and left ventricle.27,28 In addition, unexplained hypoplasia of the right atrium and right ventricle is an important clue However, the change in the size of the right-sided chambers is variable; it depends on the amount of anomalous blood flow and the extent of flow from the left to the right side of the heart.28 Anomalies of Course These anomalies pertain to an anomalous course of systemic vein, which has no effect on the connection or the drainage of the involved venous channel Such patients will remain asymptomatic, despite the most unusual course that might be taken by the systemic vein Nonetheless, the ever-increasing need for placement of diagnostic catheters and devices highlights the importance of optimal identification of these clinically silent anomalies Superior Caval Vein During embryonic life, multiple vascular channels join the left- and the rightsided superior cardinal veins Similar to the extensive remolding that occurs in other areas of the vascular tree, these channels undergo a process of development and regression Based on the location of the connecting channels, the brachiocephalic vein can take an anomalous course despite its normal connection and drainage into the right atrium The retro-aortic course, also known as the subaortic or postaortic course, is possibly the most common variant The vein can also be found in the retroesophageal position, with this variant thought to result from patency of the retrotracheal venous plexus instead of the anterior plexus.57,58 There can be double brachiocephalic veins, possibly reflecting the preserved patency of both anterior and posterior venous plexuses.59–61 Rarely, the brachiocephalic vein can even run within the thymus gland.62 With the advances made in the field of cardiac imaging, it is expected that many more such anomalies will be encountered, even if they are clinically silent Apart from the problems they produce in vascular access, implantation of pacemakers, and construction of the cavopulmonary connection, these anomalous channels also may be a source of inadvertent trauma and bleeding during surgery Retro-Aortic Brachiocephalic Vein This variant is characterized by the brachiocephalic vein coursing behind the ascending aorta and inferior to the aortic arch, instead of occupying its usual position anterior and superior to the aortic arch The malposition is thought to result from failure of formation of the transverse capillary plexus, which usually joins the left and right superior cardinal veins The connecting vein in such instances develops from the lower venous plexus Hence, it lies inferior to its normal position.63 On occasion, such a channel may coexist with the normally positioned brachiocephalic vein, producing a double brachiocephalic vein.5 The retro-aortic brachiocephalic vein is a relatively common anatomic variant with hardly any clinical implications Therefore the published literature is likely to underestimate its true incidence Most affected patients reported thus far have associated cardiac malformations, although the retro-aortic brachiocephalic vein does exist as an isolated anomaly The majority of patients have right aortic arch and subpulmonary obstruction This may be related to the right aortic arch interfering with the superior transverse plexus, and underdeveloped pulmonary arteries permitting retention of inferior transverse plexus.64,65 Although it has no clinical significance in itself, its presence can pose technical difficulties in the identification of adjacent structures, such as the right pulmonary artery and the right upper pulmonary vein It may also interfere with the exposure of pulmonary arteries during surgery, particularly when the surgical approach is through a thoracotomy rather than a midline sternotomy.64 Retro-Esophageal Brachiocephalic Vein Reported only a few times in the world literature, the brachiocephalic vein in this interesting setting runs posterior to the trachea and esophagus (Fig 27.18).57,66 The lesion possibly results from patency of the retrotracheal venous plexus, along with regression of both the superior and the inferior anterior venous plexuses.66 FIG 27.18 Three-dimensional volume rendered images taken from a computed tomographic dataset of a child with tetralogy of Fallot, who incidentally had a retroesophageal brachiocephalic vein (A) View from the anterior aspect (B) Appearance as viewed from behind BCV, Brachiocephalic vein; SCV, superior caval vein (Courtesy Dr Sanjeev Kumar, Department of Cardiovascular Radiology and Endovascular Interventions, All India Institute of Medical Sciences, New Delhi.) Intra-Atrial Left Superior Caval Vein In only a single reported instance, the left superior caval vein was found to enter the left atrium anomalously However, instead of connecting to the cavity of the left atrium, it traversed within the left atrial wall and drained to the right atrium This intraatrial course was initially mistaken as a left atrial mass The diagnosis was clinched by saline contrast injection during transesophageal echocardiography.28,67