Review Question - QID 106454

This patient has transposition of the great arteries (TGA), for which the most common cause is failure of fetal aorticopulmonary septum to spiral normally during septation of truncus arteriosus.

TGA can take many distinct morphologies, but often appears as described above: the aorta arises from the right ventricle and the pulmonary artery originates from the left ventricle, resulting in parallel circulation in which oxygenated blood is not distributed to systemic circulation. In cardiac development, the bulbus cordis and truncus arteriosus represent the arterial ends of the primitive heart tube. A septum develops that divides the truncus arteriosus and bulbus cordis, and it further spirals to the distal end of the truncus arteriosus to create the aorta and pulmonary artery. It is thought that defects in this spiraling process result in TGA.

Warnes indicates that the above malformation is the most common TGA morphology and is one of the most common cyanotic heart defects seen in the newborn period. Because of the parallel circulation, mixing of the pulmonary and systemic circulation is required for the newborn to survive. Sometimes this is naturally present in the form of an atrial septal defect (ASD), ventricular septal defect (VSD), or a patent ductus arteriosus (PDA). If the newborn's heart does not have any of these defects, an artificial ASD may be created via a balloon procedure to maintain mixing.

Martins et al. state that while the diagnosis of TGA can be suggested by certain physical exam features and pulse oximetry, the official diagnosis is made through echocardiography. A short-axis view will reveal a pulmonary trunk in the central position with the aorta displaced anterior and to the right. Furthermore, TGA can be diagnosed prenatally and often leads to improved neonatal management of the condition.

Illustration A demonstrates typical transposition anatomy with the aorta arising from the right ventricle and pulmonary artery arising from the left ventricle. Illustration B shows a transverse section of the developing aorticopulmonary septum during fetal development. Illustration C shows a coronal section of the developing aorticopulmonary septum during fetal development.

Incorrect Answers:
Answer 1: Incomplete formation of septum secundum is the most common cause of an atrial septal defect.
Answer 3: Incomplete formation of ostium primum is the second most common cause of an atrial septal defect.
Answer 4: Failure of interventricular septation leads to development of a ventricular septal defect.
Answer 5: Failure of ductus arteriosus closure leads to patent ductus arteriosus. It presents with bounding pulses, respiratory distress, and in later stages a continuous machine-like murmur.