Atrial Septal Defect Murmur: Understanding Its Origin

An atrial septal defect (ASD) occurs when the septum separating the two upper chambers of the heart — the atria — does not develop properly, resulting in a hole or gap. In many cases, small ASDs do not produce any noticeable heart murmur, as the volume of blood flowing through the defect is minimal and does not create significant turbulence.

How Does a Heart Murmur Develop in Larger ASDs?

However, in cases where the defect is larger, a significant amount of blood from the left atrium flows into the right atrium. This increased blood volume passes through the right ventricle and into the pulmonary artery, leading to higher flow across the pulmonary valve. As a result, this excessive blood flow can generate a loud systolic murmur, often graded as 3-4 out of 6 in intensity.

Where Is the Murmur Typically Heard?

This type of murmur is usually best heard at the second or third intercostal space along the left sternal border. It is often described as a "machinery-like" sound due to the continuous turbulence of blood flow through the pulmonary valve area.

Factors Influencing the Presence and Intensity of the Murmur

The presence and loudness of the murmur can vary significantly among individuals. Factors such as the size of the ASD, the degree of left-to-right shunting, and changes in pulmonary artery pressure — especially in later stages of the condition — all play a role in determining whether a murmur is present and how noticeable it is during a physical examination.

In some cases, particularly when pulmonary hypertension develops, the typical murmur may become softer or even disappear. This is because the pressure difference between the left and right sides of the heart decreases, reducing the volume of shunted blood and the associated turbulence.