Axial pressure gradient in the canine superior vena cava

In the superior vena cava of anaesthetised open chest dogs the axial pressure gradient was measured simultaneously with the blood flow velocity under different preload conditions. Both pressure gradient and velocity curves showed distinct systolic and diastolic waves. Peak pressure gradient ranged between 26 and 93 Pa X cm-1 (0.2-0.7 mm Hg X cm-1) and velocity varied between 0.095 and 0.19 m X s-1. Peak systolic pressure gradient, but not peak diastolic pressure gradient, was significantly linearly correlated to peak systolic velocity and peak diastolic velocity respectively. The shape of the two curves corresponded fairly well, but variations in pressure gradient preceded the variations in velocity. Both the correspondence in shape and the phase lag between pressure gradient and velocity waves were evaluated by the normalised cross correlation technique. During volume expansion the shape correspondence improved and the phase lag decreased. It is concluded that the transient vena caval blood velocity variations are directly related to the pulsatile axial pressure gradient.