Effect of inspiration on inferior vena caval blood flow in dogs

Inferior vena cava flow of anesthetized open-chest dogs was drained to a reservoir from a cannula above the diaphragm and returned to the atrium at constant rate. At selected base-line caval pressures, the caval flow and pressures in the abdomen (Pab), iliac vein (Piv), and downstream cavae (Pvc) were recorded during spontaneous breathing, cyclic phrenic nerve stimulation, and cyclic lowering of caval drain pressure. Each augmented flow unless Pab exceeded Pvc by at least ca. 5 cmH2O. In other dogs a cannulating flow probe was placed in the thoracic inferior cava and the chest was reclosed. Flow was augmented throughout most or all of spontaneous inspiration and was never depressed even though Pab exceeded right atrial pressure and Piv. I conclude that the collapse of hepatic veins and proximate cava does not occur at most normal pressures and a Starling resistor analog of abdominal veins based solely on abdominal and venous pressures is inappropriate. Both falling atrial pressure and rising Pab probably augment inspiratory abdominal venous return.