Describe the Physiology of atrial septal defect?
In ASD the magnitude of left to right shunt is determined by the size of the defect and the relative compliance of the right ventricle and the left ventricle. Because the compliance of the right ventricle is greater that1 the left ventricle, left to right shunt is present. The heart murmur in ASD is not because of the shunt at the atrial level. Since he pressure gradient between the ataria is so small and the shunt occurs throughout the cardiac cycle, the L>R shunt is silent. The heart murmur in ASD originates from the pulmonary valve because of he increased blood flow through this normal sized valve; therefore the murmur is systolic in timing. An increased flow through the tricuspid valve results in a diastolic rumble at the lower left sternal border. The dilated RV cavity prolongs the time required for depolarization of RV resulting in delayed P2. In Addition the large ASD tends to abolish respiration related fluctuations in systemic venous return to the right side of the heart; thereby the fixed S2.Children rarely experience CHF. However CHF and pulmonary hypertension develop in the third or fourth decade of life.