An atrial septal defect (ASD) — sometimes called a hole in the heart — is a type of congenital heart defect in which there is an abnormal opening in the dividing wall between the upper filling chambers of the heart (the atria).

In most cases, ASDs are diagnosed and treated successfully with few or no complications.

The size of an ASD and its location determine the symptoms it causes. Most kids who have ASDs seem healthy and appear to have no symptoms. Most feel well, and grow and gain weight normally.

An ASD that isn’t treated in childhood can lead to health problems later, including an abnormal heart rhythm (an atrial arrhythmia) and problems in how well the heart pumps blood. As kids with ASDs get older, they also might be at an increased risk for stroke because a blood clot could form, pass through the hole in the septum, and travel to the brain. Pulmonary hypertension (high blood pressure in the lungs) also may develop over time in older patients with larger untreated ASDs.

What Happens in an Atrial Septal Defect?

In an atrial septal defect, there’s an opening in the wall (septum) between the atria. As a result, some oxygenated blood from the left atrium flows through the hole in the septum into the right atrium, where it mixes with oxygen-poor blood and increases the total amount of blood that flows toward the lungs.

The increased blood flow to the lungs creates a swishing sound, known as a heart murmur. The murmur, along with other specific heart sounds, often is the first tip-off to a doctor that a child has an ASD. ASDs can be located in different places on the atrial septum and can vary in size.

What Causes Atrial Septal Defects?

Children with ASDs are born with the defect. ASDs happen during fetal development of the heart. The heart develops from a large tube, dividing into sections that will eventually become its walls and chambers. If there’s a problem during this process, a hole can form in the wall that divides the left atrium from the right.

In some cases, the tendency to develop an ASD might be inherited (genetic). Genetic syndromes can cause extra or missing pieces of chromosomes that can be associated with ASD. Most ASDs, though, have no clear cause. It’s also not clear why ASDs are more common in girls than in boys.