Mechanism of wide fixed split in atrial septal defect

Mechanism of wide fixed split in atrial septal defect

The pulmonary component of second heart sound does not move with respiration in atrial septal defect because the right ventricular output and pulmonary hang out interval does not change with respiration. Pulmonary hang out interval does not change because the pulmonary circulation is already overloaded by the left to right shunt. Right ventricular output does not change because the change in left to right shunt with respiration balances the change in right ventricular inflow due to change in venous return.

In inspiration, when the venous return increases, left to right shunt across the atrial septal defect decreases and vice versa. This is the mechanism of wide fixed split in ASD. The split is wide in ASD because the right ventricular emptying is more prolonged due to the volume overload.

Hang out interval is the time taken for the actual valve closure after the pulmonary artery and right ventricular pressure tracing crossover. Cross over occurs when the right ventricle relaxes enough for the pressure to fall below the pulmonary artery pressure. Hang out interval is more on the right side than on the left so that aortic hang out interval is shorter than the pulmonary hang out interval. This is because of the higher systemic vascular resistance. Pulmonary hang out interval will shorten when there is pulmonary hypertension.

Read more on hang out interval in the article by Joel M. Felner [1].

Reference

1. Joel M. Felner. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Walker HK, Hall WD, Hurst JW, editors.
   Boston: Butterworths; 1990.