Atrial Septal Defect Murmur Characteristics and Understanding the Underlying Mechanism
One of the key clinical signs in patients with atrial septal defect (ASD) is a systolic murmur typically heard along the left sternal border, specifically at the second to third intercostal spaces. This murmur is usually graded as less than or equal to grade 3, and it is often described as soft and blowing in nature.
Understanding the Second Heart Sound in ASD
In most cases, the second heart sound (S2) at the pulmonary area remains normal. However, in some patients, it may be accentuated. A highly characteristic feature of atrial septal defect is the presence of a fixed splitting of the second heart sound at the base of the heart. This phenomenon is commonly observed in individuals with ASD and is considered a hallmark of the condition.
Origin of the Systolic Murmur in Atrial Septal Defect
It is important to clarify that the systolic murmur associated with ASD does not result directly from the turbulence caused by blood flowing through the atrial septal opening. Instead, it arises due to increased right ventricular volume load. As the right ventricle handles a larger than normal blood volume, the pulmonary outflow tract may become relatively obstructed, leading to vibratory phenomena that produce the audible murmur.
Explanation of Fixed S2 Splitting
The fixed splitting of the second heart sound at the heart's base is primarily due to the increased capacity load on the right ventricle. Because of this overload, the right ventricle takes longer to complete systole compared to the left ventricle. This delayed right ventricular contraction results in a persistent and unchanging interval between the aortic and pulmonary components of S2, creating the characteristic fixed splitting sound.
Key Takeaways
- Systolic murmur: Soft, blowing, and located at the left sternal border (2nd–3rd intercostal spaces).
- Second heart sound: Often fixed splitting at the heart base, a key diagnostic sign.
- Mechanism of murmur: Due to relative pulmonary stenosis from right ventricular overload, not direct flow through the defect.