Atrial septal defect (ASD) with bidirectional shunt
Atrial septal defect (ASD) with bidirectional shunt: Subcostal four chamber view shows a large defect in the interatrial septum (ASD). Right atrium (RA) and right ventricle (RV) are dilated, while left atrium (LA) and left ventricle (LV) are not. Blue color on colour flow mapping indicates right to left shunt across the atrial septal defect (R>L Shunt) in the subcostal view as the flow is away from the transducer.
This view shows red coloured flow across the ASD indicating a left to right shunt (L>R shunt). In addition there is a mosaic (multi colored) jet spreading from the closed tricuspid valve into the right atrium, due to tricuspid regurgitation (TR jet).
Bidirectional shunt can occur in ASD even without pulmonary hypertension. This is because transient right to left shunt can occur in certain phases of respiration. The volume of that right to left shunt is so small that there won’t be any clinical cyanosis. Still there is a small risk of paradoxical embolism. But here there is severe pulmonary hypertension which has caused reversal of shunt, though some left to right shunting is also persisting. As the pulmonary hypertension progresses, the shunt may become fully right to left.
Bidirectional shunts in uncomplicated ASD has been described by Galve E et al . They noted this in older patients, unrelated to pulmonary hypertension and considered it to be a sequel of chronic right ventricular volume overload. Shunts were determined by measuring the pulmonary vein to systemic artery oxygen stepdown.
- Galve E, Angel J, Evangelista A, Anivarro I, Permanyer-Miralda G, Soler-Soler J. Bidirectional shunt in uncomplicated atrial septal defect. Br Heart J. 1984 May;51(5):480-4.