Review Question - QID 108835

The most likely diagnosis in this child is a persistent truncus arteriosus. Partial development of the aorticopulmonary septum will result in mixing of oxygenated and deoxygenated blood, leading to diffuse cyanosis.

The imaging finding of blood from the left ventricle entering both the aortic and pulmonary circulations suggests that the aorticopulmonary septum – the wall that divides the aorticopulmonary trunk into the ascending aorta and pulmonary trunk – failed to form. This would allow for deoxygenated blood from the right ventricle to pass into the aortic outflow tracts, leading to cyanosis. This is consistent with a diagnosis of a persistent truncus arteriosus. The truncus arteriosus is an embryologic cardiac structure that gives rise to the ascending aorta and pulmonary trunk following formation of the aorticopulmonary septum. In this condition, a loud single S2 is heard due to increased flow through the single truncal valve.

Incorrect Answers:
Answer 1: The ductus arteriosus is an aorticopulmonary shunt that exists in the embryonic heart to allow oxygenated blood from the right heart to bypass the immature pulmonary vasculature and directly enter the systemic circulation. Failure of this shunt to close will lead to a patent ductus arteriosus which presents with a machine-like murmur, lower extremity cyanosis, bounding pulses, and mild dyspnea.

Answer 2: Tricuspid atresia is a congenital heart defect that causes early cyanosis. Failure of the atrioventricular valve to form can lead to tricuspid atresia, which presents with immediate severe cyanosis. Tricuspid atresia requires the presence of a right-to-left shunt (typically via a patent foramen ovale) and a left-to-right shunt (typically a ventricular septal defect) to overcome the blockage at the tricuspid valve.

Answer 3: The interatrial septum forms from the septum primum and septum secundum between the left and right atria. Excessive resorption of the septum primum and/or septum secundum may lead to a patent foramen ovale.

Answer 4: The truncal and bulbar ridges of the embryonic heart twist, spiral, and fuse to the form the aorticopulmonary septum. If the ridges fail to spiral, transposition of the great vessels may occur. This condition results in two separate circulatory pathways with the aorta arising from the right ventricle and the pulmonary artery arising from the left ventricle.

Bullet Summary:
The truncus arteriosus is results from the failure of the truncus arteriosus to septate leading to a mixing of oxygenated and deoxygenated blood causing cyanosis.