Postural changes in venous pressure gradients in anesthetized monkeys.
We examined the hypothesis that head-up and head-down tilt produce a symmetrical hydrostatic load on the veins. Venous pressure was measured in anesthetized monkeys with a transducer-tipped catheter. Changes in venous pressure gradients during head-up tilt corresponded to changes in hydrostatic load. However, changes in venous pressure gradients during head-down tilt were not symmetrical to those during head-up tilt. During head-down tilt, venous pressure in the superior vena cava rose, venous pressure around the right atrium did not change, and venous pressure in the inferior vena cava on the caudal side of the diaphragm rose considerably. The venous pressure of the inferior vena cava caudal to the renal vein then gradually decreased. The inferior vena cava passes through the central tendon of the diaphragm. Thus, during head-down tilt, the gravitational shift of venous blood is impeded by this anatomic structure, and venous pressure around the hepatic vein increases significantly. These data disproved our hypothesis that head-up and head-down tilt induced symmetrical but opposite influences on vena caval pressures.