An Atrial Septal Defect (ASD) is a heart condition that is present at birth (congenital). It's a defect in the septum between the heart's two upper chambers (atria). The septum is a wall that separates the heart's left and right sides. In ASD, there is an abnormal opening in this septal wall.

This opening allows oxygen-rich blood from the left atrium to mix with oxygen-poor blood in the right atrium. Normally, oxygen-rich blood from the lungs enters the left atrium, passes into the left ventricle, and then is pumped out to the body. In contrast, oxygen-poor blood from the body enters the right atrium, goes into the right ventricle, and is sent back to the lungs to be oxygenated.

In cases of ASD, because of the abnormal opening, some oxygen-rich blood gets shunted from the left atrium back to the right atrium and then to the lungs, instead of going to the rest of the body. This can lead to an increase in the blood volume and pressure in the right atrium and the lungs.

The symptoms and treatment of ASD depend on the size of the defect and how much blood flow it allows between the atria. Small ASDs might close on their own during childhood and may not require any treatment. Larger ASDs, however, can cause significant problems and may require surgical repair. Symptoms of a significant ASD might include shortness of breath, fatigue, and heart palpitations, among others.

Diagnosis of ASD often involves echocardiography, which is an ultrasound of the heart. This imaging technique allows doctors to see the heart's structure and function, and to identify any abnormal openings in the septum. Treatment options can range from medical management to monitor the condition to surgical interventions to close the defect.