Effects of acute occlusion of superior vena cava were studied in dogs. Occlusion of superior vena cave was made by catheter with inflatable baloon inserted into superior vena cava via jugular vein. Following results were obtained. 1) Azygos vein played the most important role as collateral circulation when superior vena cava was occluded. 2) A steady state of circulatory hemodynamics was reached about 30 minutes after acute occlusion of superior vena cava at the point distal to azygos vein and then gradually retnrned to normal state in accordance with complete development of collateral circulation. 3) Marked changes in circulatory hemodynamics developed when superior vena cava and azygos vein were concomitantly occluded. Dogs, whose azygos vein had been divided one week ago, died whthin 70 to 90 minutes on acute occlusion of superior vena cava.
On the contrary, dogs, whose azygos vein had been divided two weeks ago, survived two hours' occlusion of superior vena cava. However, cardiac output did not return to preocclusion level even 20 minutes after release of occlusion. 4) The cause of death in the case of acute occlusion of superior vena cava including azygos vein was supposed to be central nature due to functional loss of central nervous system.