Why might flow from the foot vessels be absent in a resting patient when examining tibial veins?

• A. Flow velocity is typically very high and exceeds wall filters
• B. Flow velocity is typically low and below wall filter cutoff
• C. Vein collapses due to external compression
• D. The patient must be standing to see flow

Answer: (B)

In resting legs, venous flow in the distal vessels is usually very slow. Doppler ultrasound uses a wall filter to suppress low-velocity signals from tissue motion, so when the flow velocity is below that cutoff, the scan won’t display a flow signal. That’s why flow from the foot vessels can appear absent in a resting patient when examining the tibial veins—the velocity is simply too low to be detected by the system.

If you want to visualize it, you can lower the wall filter or use more sensitive methods (like power Doppler) and/or have the patient augment flow with movement or calf/foot maneuvers to raise velocity. High flow would be detected; external compression or standing aren’t the primary explanations for typical resting tibial venous flow.