nonadjacent aortic sinus and looking toward the pulmonary trunk (Fig 37.5) Irrespective of the relationships of the arterial trunks, the sinuses supporting the coronary arteries are always located to the right and left hands of the observer The sinus as seen to the right is universally known as sinus 1, while that to the left is sinus 2.15 All patterns are then accounted for on the basis of whether the right, circumflex, and anterior interventricular arteries arise from sinus 1 or from sinus 2 Most usually, the arteries arise within the aortic sinuses or at the level of the sinutubular junction, albeit usually eccentrically placed within the sinus In some instances, the arteries can take a high origin above the sinutubular junction Of more significance is the arrangement when the arteries take a tangential course through the aortic wall, crossing the attachments of the valvar leaflets at the sinutubular junction This is the so-called intramural origin.16 In addition to sinusal origin (Fig 37.5), epicardial course is also important, in particular retropulmonary or anteroaortic location of any of the three major coronary arteries.17 The artery to the sinus node is of further significance It can arise from the initial course of either the right or the circumflex coronary arteries, or it can take a direct origin from one or other of the facing aortic sinuses However, its most important variation is when it crosses the lateral margin of the right atrial appendage In such a lateral position, it is at surgical risk during a standard atriotomy FIG 37.5 Potential variability of the aorta, when positioned anteriorly, relative to the pulmonary trunk With this degree of variability, it is not possible to account for the location of the aortic sinuses in terms of right or left coordinates, or anterior or posterior coordinates, without describing each heart separately Because the coronary arteries always arise from the sinuses closest to the pulmonary trunk, these sinuses can always be distinguished as being to the right hand of the observer standing in the nonadjacent sinus and looking toward the pulmonary trunk, noted as sinus 1, or to the left hand, noted as sinus 2 Ventricular Septal Defect The most significant, and frequently occurring, associated lesion in transposition is a ventricular septal defect.15 As with such defects found in the setting of concordant ventriculoarterial connections, these may be small, large, or multiple, and they can be located within any part of the ventricular septum (Fig 37.6) The most characteristic defects are those that open beneath the ventricular outlets, with the muscular outlet septum being malaligned relative to the rest of the ventricular septum and located within the right ventricle Such defects, which occupy a subpulmonary position when seen from the left ventricle, may have a muscular posteroinferior rim (Fig 37.7B) or may extend to become perimembranous FIG 37.6 As is the case with interventricular communications in the otherwise normal heart, those found in patients with transposition can be characterized as being muscular, perimembranous, or doubly committed The muscular defects can be positioned to open to the inlet, the apical component, or the outlet of the right ventricle Perimembranous communications are central, but they, too, can extend to open to the right ventricular inlet or outlet, the latter in the setting of malalignment between the apical septum and the outlet septum