Effect of unilateral pulmonary artery obstruction on circulatory dynamics in dogs with chronic atrial septal defects.

IT IS generally accepted that the left-toright shunt across an atrial septal defect is the result of a pressure gradient in this direction between the two atrial chambers during most of the atrial cycle. The maintenance of this pressure gradient and, hence, the direction of the shunt in the left-to-right direction has been attributed to a difference in the volume-elastic properties of the two atria and to a difference in the distensibility of the two ventricles." In addition, previous investigators have demonstrated by means of indicator-dilution techniques, that both in patients' and in dogs with atrial septal defects located in the region of the fossa ovalis, a higher percentage of blood from the right lung shunts in the left-to-right direction across the defect than from the left lung. This preferential shunting of blood from the right lung has been interpreted as being due primarily to the anatomical proximity of the openings of the right pulmonary veins into the left atrium and the position of the atrial septal defect in the atrial septum. Fleming found, on the basis of bronehospirometry of separate lungs with the subject in the supine position, that an abnormally high proportion of oxygen is taken up from the right lung in patients with atrial septal defects of the secunduni type. These results suggest that a greater than normal portion of pulmonary blood flow traverses the right lung