Mechanism of wide fixed split in Atrial septal defect (ASD)

• First you can review the Mechanism of normal split in second heart sound
• In case of atrial septal defect, the right atrium receives blood from the left atrium in addition to the normal venous return
• Due to the increased amount of blood reaching the right ventricle, it takes a longer time to eject the blood
• Hence the pulmonary valve takes longer to close and we appreciate a wide split (In both inspiration and expiration)

So, why does it cause a wide fixed split instead of a wide variable split?

• During inspiration, the blood flow through the vena cava increases, whereas that from the left atrium decreases
• Whereas during expiration, blood flow through the vena cava decreases, and that from the left atrium increases
• During the respiratory cycle, the blood flow through the vena cava and left atrium vary reciprocally
• Hence the wide fixed split

You can also read about : Split of second heart sound in Ventricular Septal Defect (VSD)