Atrial septal defect (ASD) is a congenital heart condition in which the interatrial septum that separates the left and right atria allows blood to seep or flow from one atrium into the other. In a healthy person the interatrial septum is water tight and does not allow the blood flow between the two atria. If there is an atrial septal defect, it is possible for blood to travel from the left atrium of the heart to the right atrium, or vice versa. This results in the mixing of arterial and venous blood, which may or may not be clinically significant. The heart right atrium receives deoxygenated venous blood (with a low oxygen content) from the superior and inferior vena cava, and the left atrium receives oxygenated arterial blood (with a high oxygen content) from the pulmonary vein. A normal heart has an interatrial septum that prevents venous blood and arterial blood from mixing together.
During human embryological development, there is a normal aperture and communication between the two atria, allowing blood to bypass the lungs. But this opening usually closes tight around the time the baby is born. If the atrial septal defect is persistent, blood continues to flow from the left to the right atria. This is called a shunt. If too much blood moves to the right side of the heart, pressures in the lungs build up. The shunt can be reversed so that blood flows from right to left. Small atrial septal defects often cause very few problems and may be found much later in life. Many problems can occur if the shunt is large, however. In advanced and severe cases with large shunts the increased pressure on the right side of the heart would result in reversal of blood flow (now from right to left). This usually results in significant shortness of breath and difficulty breathing.
Methods of closure of an atrial septal defect include surgical closure and percutaneous closure. Once someone is found to have an atrial septal defect, a determination of whether it should be corrected has to be made. Mortality due to a surgical procedure to close an atrial septal defect is lowest when the procedure is performed prior to the development of significant pulmonary hypertension. The lowest mortality rates are achieved in individuals with a pulmonary artery systolic pressure of less than 40 mmHg. Closure of an ASD in individuals under age 25 has been shown to have a low risk of complications, and individuals have a normal lifespan (comparable to a healthy age-matched population). Closure of an ASD in individuals between the ages of 25 and 40 who are asymptomatic but have a clinically significant shunt is controversial. Those that perform the procedure believe that they are preventing long-term deterioration in cardiac function and preventing the progression of pulmonary hypertension.