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Multiple determination of size, shape, and diameter of the left atrium were made during the control state and under conditions of varied ventricular outflow resistance in intact anesthetized dogs with markers chronically attached to the mitral annulus and the valve cusps. Measurements were made from left atrial angiograms (120 frames/s) obtained after injections of contrast medium into the pulmonary artery. Changes in atrial size were characterized by symmetrical circumferential diameter changes and eccentric variations of anteroposterior axis due almost entirely to valve ring displacement. With normal aortic pressure, maximal atrial area averaged 11.0+/-3.1 cm2. When aortic pressure was increased maximal area increased to 12.9+/-3.7 cm2 and percent emptying decreased, whereas maximal and minimal atrial areas were similar to control values during decreased aortic pressure. These findings suggest the possibility that atrial emptying may be near maximal at the resting control state.

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