To understand intracardiac shunts, it is first important to understand normal cardiac anatomy and physiology. The heart is divided into two sides, left and right. The right side receives low-oxygen venous (or "blue") blood returning from the rest of the body and in turn pumps the blood to the lungs. Blood flowing through the lungs is exposed to oxygen from breathing, subsequently turning venous blood into high-oxygen arterial blood. Arterial blood then returns to the left side of the heart where it is then pumped to the body.
Intracardiac shunting refers to the diversion of normal blood flow to alternate pathways within the heart as a result of a hole in structures that normally separate arterial from venous blood. These alternative pathways cause blood flow to bypass the normal circulation, resulting in the mixing of arterial and venous blood. This mixing of blood occasionally can lead to low oxygen levels, decompression illness, strokes, or more rarely, heart rhythm problems, heart failure, and pulmonary hypertension.
Most commonly, these holes exist within the innermost walls of the heart called the interatrial septum, and less frequently, the interventricular septum. Defects in the interatrial septum include patent foramen ovale (PFO) and atrial septal defect (ASD).
Everyone is Born with a Hole in the Heart
All of us have a PFO early in our development. In fact, PFO's are necessary to support our circulation in fetal life. However, very shortly after birth, PFOs close spontaneously in 70 percent of individuals. While 30 percent of the population has a residual PFO after birth, only a small percentage will ever require treatment. In contrast, ASDs are relatively rare, occurring in less than 1 percent of the population.