During respiration there are phasic changes seen in the venous flow of the lower limb veins. This is best studied using Triplex imaging, and especially by spectral pattern of venous flow. Upper left image shows changes in flow (in superficial femoral vein), with increase during inspiration, caused by lower intra-thoracic pressure and vise versa during expiration. The Doppler image on right side shows similar changes in the popliteal vein of the same limb. This suggest continuity of flow with transmission of pressure all the way upto the vena cava and the right atrium.
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