The oscillating vein

In 1824 D. Barry inserted a glass bulb into the jugular vein of a horse and observed that when the animal was standing the blood flowed intermittently in jets that were not synchronous with either respiratory or heart rates (Brecher). Holt in 1941 arranged a flow circuit where the fluid traversed an interposed segment of collapsible tube and where he could vary at will the ratio of internal pressures. He observed that when the ratio approached zero the tube collapsed and began to pulsate. Brecher in 1952 examined carefully this phenomenon by decreasing the downstream pressure in the vena cava of open chest dogs and concluded that it originated in a momentary complete collapse of a segment of the vein when the extravascular (atmospheric or tissular) pressure exceeded the intravascular pressure. The continuous inflow of blood from the capillaries would then elevate again the intravascular pressure forcing the closed segment of vein to reopen. With repetition of the cycle the vein oscillated at frequencies and amplitudes whose complex relationships he studied in physical analogues made of collapsible rubber tubes. Furthermore, he pointed out the clinical significance of this "chatter" during cardiopulmonary bypass surgery when the collecting reservoir for venous return is positioned at an excessively low level. In 1953 Robard and Saiki and in 1955 Robard reported detailed studies on flow through collapsible tubes and on the clinical implications of its instabilities in relation to apparently anomalous flow-pressure patterns in various vessels of the body. Our own studies on the effect of quiet respiration in a two-chamber-hepatic valve model of venous return indicate that in the intact (closed chest) dog and man the venae cavae operate at the limits of equilibrium and that instability may develop during vigorous respirations.