Normal Doppler spectrum from the intrahepatic portion of the inferior vena cava in a 4-day-old boy shows the waves that result from right atrial back-pressure.

The waveform of the inferior vena cava varies according to the specific segment sampled. The flow in the proximal inferior vena cava is influenced by the activity of the right atrium and shows back-pressure changes identical to those seen in hepatic venous flow (18) (Fig 15). Distally, the cardiac activity has a lesser effect on flow velocities, and variations in thoracic or abdominal pressure cause greater variability in forward flow (18). In the common iliac veins, the flow pattern is more phasic, like that seen in the proximal lower limb veins (17).

http://radiographics.rsna.org/content/28/3/691.full