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The relationship between the longitudinal pressure gradient and the blood flow velocity in the canine superior vena cava.

In the superior vena cava of anaesthetized open chest dogs the axial pressure gradient (delta P) was measured simultaneously with the blood flow velocity (V) under a variety of preload conditions. Both delta P and V curves showed distinct systolic and diastolic waves. Peak delta P ranged between 26 and 93 P/cm (0.2-0.7 mm Hg/cm) and V varied between 0.095 and 0.19 m/s. Peak systolic delta P, but not peak diastolic delta P was significantly linearly correlated to respectively peak systolic V and peak diastolic V. The shape of delta P and V curves corresponded fairly well but variations of delta P preceded the variations of V. Both the shape correspondence and the phase lag between delta P and V were evaluated by means of the normalized 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.