Atrial septal defectAtrial septal defect (ASD) is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria. Without this septum, or if there is a defect in this septum, it is possible for blood to travel from the left side of the heart to the right side of the heart, or vice versa. This results in the mixing of arterial and venous blood, which may or may not be clinically significant. This mixture of blood may or may not result in what is known as a "shunt". The amount of shunting present, if any, dictates hemodynamic significance (see Pathophysiology below). A "right-to-left-shunt" typically poses the more dangerous scenario (see Pathophysiology below).
The right side of the heart contains venous blood with a low oxygen content, and the left side of the heart contains arterial blood with a high oxygen content. A normal heart has an interatrial septum that prevents oxygen-rich blood and oxygen-deficient blood from mixing together.
During development of the fetus, the interatrial septum develops to separate the left and right atrium. However, the foramen ovale (pronounced /fɒˈreɪmən oʊˈvɑːli/) allows blood from the right atrium to the left atrium during fetal development. This opening allows blood to bypass the nonfunctional fetal lungs when the fetus obtains its oxygen from the placenta. A layer of tissue called the septum primum acts as a valve over the foramen ovale during fetal development. After birth, the pressure in the pulmonary circulatory system drops, thus causing the foramen ovale to close entirely. In approximately 25% of adults, the foramen ovale does not entirely seal. In this case, elevation of pressure in the pulmonary circulatory system (i.e.: pulmonary hypertension due to various causes, or transiently during a cough) can cause the foramen ovale to remain open. This is known as a patent foramen ovale (PFO).