questionable if angiography is now required for diagnosis Interpretation of the images, considered difficult in the past, should now be very much easier Even in potentially suitable cases, survivors of attempted separation have thus far been rare As with exteriorization of the heart, the situation must be anticipated rapidly to improve, depending of course on the degree of fusion of the cardiac structures, and the severity of the intracardiac lesions in each twin Abnormal Positioning of the Heart The heart is normally located in the mediastinum, with one-third of its bulk to the right, and two-thirds to the left of the midline With this arrangement, the apex usually points inferiorly and to the left This combined pattern is traditionally described as “levocardia.” The heart can be deviated from this position, or its apex be pointed in an unexpected direction, for various reasons, and not always because of a congenital cardiac malformation For example, the heart can be pushed to the right by a space-occupying lesion in the left lung or left pleural cavity, such as hyperinflation of the left lung, or pleural effusion and pneumothorax involving the left pleural cavity The heart can also be pulled to the right when the right lung is underdeveloped or collapsed An abnormal rightward position of the heart is also typically seen in association with right pulmonary hypoplasia in scimitar syndrome, ligamentous or ductal origin of right pulmonary artery, or atresia of stenosis of the right pulmonary vein Nonetheless, the abnormal position in itself does not necessarily imply an abnormality of the heart Normal individuals with mirror-imaged arrangement of the organs usually have a right-sided heart Therefore when assessing the significance of a right-sided heart or a heart with its apex pointing to the right, it is necessary to take account of all these various features Several questions should be asked What is the overall arrangement of the organs? Is there an abnormality of the lungs or the thoracic contents? If present, is it of congenital or acquired etiology? Is the heart itself abnormally structured, or are its chambers grotesquely enlarged? Only when these questions have been posed, and answered, can the significance of an abnormally positioned heart be fully appreciated Attempts to compress all this information into short phrases or single words have led to complex and confusing terms, such as dextrocardia, levocardia, dextroversion, levoversion, and dextroposition.21 Our preference is to avoid using these cryptic conventions A much simpler approach when considering the abnormally positioned heart, is first to account for its overall location This can be left-sided, central, or rightsided It is necessary to account for the orientation of its apex, which can similarly be left-sided, central, or right-sided All locations can be categorized solely in terms of the position of the heart, and the orientation of its apex This information, of course, must be placed in the context of the overall arrangement of the thoracic and abdominal organs, and the presence of acquired or congenital disease of either the heart or the lungs Therefore abnormal positioning of the heart should no longer be regarded as a diagnosis in its own right Finding a right-sided heart, for example, gives no clue as to what is happening inside the organ The heart itself may be entirely normal The description of an abnormal location is but one part of full sequential segmental analysis (see Chapter 1) Nonetheless certain well-recognized lesions, or combinations of lesions, are associated with right-sided hearts, or left-sided hearts in those with mirror imagery As already emphasized, the heart is typically right-sided in association with hypoplasia of the right lung in the scimitar syndrome and its variants, which is also known as the bronchopulmonary foregut malformation.22 Rightward displacement of the heart is also seen in association with ligamental ductal origin of the right pulmonary artery, with unilateral absence of the right pulmonary artery, and with unilateral atresia or stenosis of the right pulmonary veins The heart is also right-sided in well over a third of individuals who have an isomeric arrangement of their organs (see Chapter 22) The single lesion most associated with a right-sided heart, or left-sided heart with mirror-imaged arrangement, is congenitally corrected transposition (see Chapter 38) At best, these are clues to the final diagnosis The variety of lesions that can exist when the heart is abnormally positioned is protean