Dynamics of Venous Collapse in Superior Vena Cava System

Abstract

The emptying and filling of the extrathoracic veins with respiration were studied in closed-chest dogs. Natural inflow from the capillary bed was replaced by inflow from a constant pressure reservoir into the jugular vein and phasic inflow changes were recorded with a differential manometer. During inspiration venous collapse does not occur at one distinct point at the entry to the chest, but involves a more or less extensive segment of the extrathoracic vein. This segment forms a transition zone between the noncollapsed intrathoracic and the partially collapsed peripheral part of the extrathoracic vein. In states of hypovolemia, as, for example, in shock, the paucity of capillary return reduces the reserve supply in the collapsing segment, so that little, if any, augmentation of return flow can occur in response to respiratory effort. With greater venous filling the transition zone extends farther into the periphery. Thus the extrathoracic vein can accommodate a greater blood volume which is then available for emptying into the thoracic vein during inspiration. Rising right atrial pressure prolongs the duration of the depleting stage and postpones the onset of the collapsed stage with inspiration. In congestive heart failure, when right atrial pressure fails to descend below zero, collapse cannot occur and venous flow is mainly detd. by the pressure gradient between the periphery and the atrium which increases with inspiration. The question whether or not rhythmical depleting and refilling of the peripheral veins may increase net venous return was studied by measuring drainage from the cannulated peripheral end of the superior vena cava in the open-chest dog. Venous drainage is increased merely by alternating between 2 different degrees of suction instead of letting them act consecutively. The augmentation of flow is due to the fact that a reserve of fluid accumulates in the collapsible extrathoracic veins every time when the degree of suction is lessened. Thus the reservoir of the extrathoracic veins acts as a "collapse chamber" which provides a means whereby continuous venous flow is converted to a discontinuous one during inspiration and expiration. This provides a physical basis for a net increase of venous return to the heart due to respiration.