Spectral waveforms obtained in the superior vena cava show phasic changes due to back-pressure from the right atrium and changes in amplitude with respiration. The a, S, v, and D waves all are visible. In the superior vena cava, flow velocity increases with inspiration, as negative intrathoracic pressure increases the venous return, and decreases with expiration. Maximum flow velocity in the superior vena cava in children is 60–80 cm/sec . In young adults, the reported maximum value is 32–69 cm/sec .


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