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Determinations of pressure, blood flows, and calculated pulmonary vascular resistance were made prior to and during acute individual occlusion of the right and left main branches of the pulmonary artery in random sequence in eight anesthetized, closed-chest dogs witn chronic atrial septal defects. Similar observations were made with occlusion of only the left pulmonary artery in three additional dogs. Obstruction of the pulmonary artery to one lung was immediately followed by a rise in systolic pressure in the right ventricle and pulmonary artery proximal to the obstruction, from an average normal value of 33.1 mm. Hg to an average value of 40.9 mm. Hg (range 32 to 47). This change was not associated with demonstrable systematic effects on right atrial or systemic arterial pressure or on the systemic blood flow. As would be expected, the average calculated ratio of pressure to flow (vascular resistance) across the unoccluded lung was elevated during the obstruction. The resistance value during the occlusion, however, was less than the estimated pressure/flow ratio for this same lung, based on the measurements made prior to the occlusion. This apparent effective decrease in vascular resistance of the unoccluded lung is probably related to dilation of the pulmonary vessels as a result of an increase in intravascular pressure caused by the sudden removal of approximately half of the pulmonary vasculature from the circulation. In spite of the elevation in right ventricular systolic pressure, occlusion of the left pulmonary artery was associated with a slight increase in pulmonary blood flow and a definite increase in the magnitude of the left-to-right shunt across the atrial septal defect, whereas no systematic effects were obtained during obstruction of the right branch in association with the same approximate levels of right ventricular systolic pressure. These data are regarded as further evidence that the preferential shunting of blood from the right lung in the presence of an atrial septal defect of the secundum type is primarily due to the anatomical relation between the opening of these vessels into the left atrium and the position of the defect in the atrial septum. Furthermore, they emphasize the importance of dynamic factors, related to the manner, site, and direction in which the pulmonary venous blood drains into the left atrium, as important determinants of the left-to-right shunt via such defects.

Effect of Unilateral Pulmonary Artery Obstruction on Circulatory Dynamics in Dogs with Chronic Atrial Septal Defects