Hemodynamic alterations caused by passive changes in body position in dogs with chronic atrial septal defects.

• Changes in posture are almost instantaneously associated with cardiovascular alterations, the nature of many of which has not yet been entirely elucidated. Several studies have demonstrated that the assumption of the erect from the recumbent posture is associated with changes in the position of the heart and diaphragm and a decline in cardiac output. Experimental investigations have shown that tilting dogs to 30 degrees, in either the head-up or the head-down position, causes a decrease in heart volume. Likewise, the distribution of blood flow through the different segments of the lungs is affected by gravitational forces and, hence, by changes in posture. It was suggested by Lagerlof and co-workers that with the body in the erect posture the blood flow would decrease to a great extent in the apical segments of the lungs as compared with the blood flow at the bases. Bjorkman' demonstrated an increase in oxygen uptake in the dependent lung with the body in the lateral deeubitus position, and he suggested that these changes are due to an increase in blood flow through the lowermost portions of the lung, as a result of the effects of gravitational forces on the distribution of the blood stream across the pulmonary circulation. Since previous investigations have demonstrated that a preferential shunting of blood from the right lung and from the inferior vena cava occurs in the presence of atrial septal defects in the region of the fossa ovalis,"' investigations were carried out to