Effect of Respiratory Movements on Superior Cava Flow Under Normal and Abnormal Conditions

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Abstract
Blood flow was measured in the superior vena cava of anesthetized dogs with a low resistance differential flowmeter. Venous return to the heart increased significantly with inspirations due to the depleting of the extrathoracic veins into the chest veins. With Muller''s expt. venous return was augmented in the beginning of the inspiratory effort, but failed to increase further during the later part of the inspiration when the extrathoracic veins had become depleted and reached their collapsed stage. Even in the presence of the partial peripheral venous collapse, toward the end of inspiration, the flow rate was greater than during the expiratory pause. In spite of a slight flow reduction during expiration, the effect of respiration consisted in a significant net increase of venous return over that when respiratory efforts were absent. The respiratory flow augmentation was reduced by hypovolemia and enhanced by hypervolemia. During artificial respiration in the open as well as closed chest, positive pressure lung inflation consistently diminished venous return first; changes in arterial pressure followed several heart cycles later. Compared with normal respiration, superior vena cava return to the heart was significantly reduced by intermittent positive pressure respiration in the closed chest.

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