During fetal development PFO allows the necessary shunting of oxygenated blood from the right atrium to the left atrium; thus, oxygenated blood flows into the sys¬temic circulation to organs and tissues. This right-to-left shunting of blood is crucial for fetal development, but it is also vital that no shunt should occur after birth, because the right side of the heart is no longer filled with oxygenated blood. A right-to-left shunt in individuals with congenital heart disease (CHD) causes cyanotic congenital heart disease which is life-threatening (see chapter entitled Congenital Heart Disease).
The foramen is formedby overlapping flaps of the septum primum and septum secundum (See Figure 1). These septa finally fuse to form the atrial septum in the newborn (see the figures in the chapter Embryology and the figures provided in the chapter Anatomy of the Heart and Circu¬lation). The foramen usually closes at the time of birth because of the acute decrease in pulmonary vascular resistance and increased pressures on the left side of the heart. It normally fuses completely within 2 years. A PFO has been noted in up to 25% of structurally normal hearts at autopsy, and TEE studies report similar figures. A PFO is not similar to an atrial septal defect (ASD), where there is a failure of part of the atrial septum to form rather than fuse, which results in a hole in the septum that separates the right and left atrium (see Figure 2 and the chapter Atrial Septal Defect). A PFO is not a hole in the heart. Instead, it is a failure of fusion of the two flaps described above which causes a less than 5-mm, slit-like valvular opening in the interatrial septum (Figure 1). Mechanisms that lead to a marked increase in pressures in the right atrium, such as occur during a Valsalva maneuver or in patients with pulmonary hypertension, can increase the degree of shunting of blood from the right atrium through the slit-like opening of the PFO into the left atrium. This spurt of blood from the right side of the heart through the PFO gains access to the left ventricle, the aorta, and arterial supply to organs of the body. Thus, thrombi that form in the right side of the circulation, from veins in the legs and other areas that drain into the venous circulation to the heart (see the chapter Anatomy of the Heart and Circulation, Figs. 5 and 10), may gain access to the vital organs.