Congenital Heart Defects in the Context of Typical Development and Circulation

Abstract

Solid foundational knowledge of the normal embryological development of the heart and great vessels is essential for understanding the etiology of various congenital heart defects (CHDs). As an increasingly diagnosed category of pathologies and overall the leading cause of neonatal morbidity, CHDs are often well tolerated when the developing human is in utero. Clinical sequelae, the signs and symptoms of patients with CHDs, become apparent after birth when the fetus no longer has access to a placenta and must use solely its own anatomy to provide and circulate the appropriate blood gases. CHDs can be divided into two major categories: critical, those which are life‐threatening and require surgery in infancy, and non‐critical, those which are not immediately life‐threatening and do not necessarily require surgery immediately or at all. One fundamental piece of information for the diagnosis of any CHD is whether or not the individual is cyanotic, and furthermore which specific tissues are subjected to this oxygen deficiency. Taussig’s classification of congenital heart defects includes those with no shunting of blood, those with a left to right shunt and acyanosis, and those with a right to left shunt and subsequent cyanosis. In the typical fetus, blood enters the right atrium of the heart at fairly high pressure. Some of this blood is entering via the inferior vena cava, which in the fetus contains some highly oxygenated blood via the ductus venosus and some blood of a low oxygenation rate that flowed through the developing liver tissues. Due to higher pressures on the right side of the fetal heart than the left, most blood passes through the foramen ovale and into the left atrium. This is a normal right to left shunt only in the fetus. When the fetus is born and air is introduced to the respiratory system the pressures change. Absent placental circulation leads to a decrease in pressure on the right side of the heart and increased pulmonary blood flow leads to an increase in pressure on the left side of the heart, specifically between the atria. This ex utero pressure differential closes the foramen ovale to form a fossa ovalis and no patent communication between the atria. The normal fetal right to left shunt should no longer be present. Since pulmonary vascular resistance is now lower than systemic vascular resistance, the blood flow in the ductus arteriosus between the pulmonary trunk and arch of the aorta reverses, the shunt constricts, and then forms into the ligamentum arteriosum. When these processes or those of cardiac septation occur incorrectly then the result may be a CHD. Proper identification of the pathological anatomy and etiology is relevant for understanding both normal and abnormal developmental processes in cardiac and great vessel morphology.