Strandness and Baker introduced CW Doppler in the 1960s.19 Its initial application was peripheral arterial assessment. With the development of additional maneuvers the instrumentation was applied first to the diagnosis of DVT and later to deep vein insufficiency.
We recommend that the subject be studied on a flat examining table in which the lower extremities may be placed in the dependent position at approximately 15 degrees. This slight angle dilates the deep system, which makes the identification of veins easier and improves the velocity signals. We recommend that target veins include the common femoral vein at the inguinal ligament, popliteal vein at the popliteal fossa, and the posterior tibial vein just behind the medial malleolus.
With the pencil-like probe positioned toward the venous flow and at 60 degrees to the flow streamline, the target velocity is optimized. The fact that a velocity is identified means the vein is patent at the target level; this is the first of three major diagnostic criteria. The second diagnostic criterion is associated with the spontaneous and phasic nature of the signal. When veins are not obstructed proximal to the target vein, the local pressure is low and local velocity changes as a function of respiration. Low-pressure veins collapse and local velocity often is reduced to zero shortly after inspiration. This is due to the fact that when the diaphragm moves down on inspiration, pressure in the closed abdominal cavity increases and collapses veins at low pressure. With proximal obstruction this phasic velocity is disturbed in the sense that velocity is no longer phasic with respiration and in fact may be continuous. The third criterion is associated with velocity response secondary to distal compression. When veins are unobstructed proximal to the target and compression is performed distally, the local velocity will increase in response to compression. In a high resistance proximal venous system, distal compression will not evoke increased velocity.
If a subject demonstrates at the femoral, popliteal, and posterior tibial veins good velocity signals that are phasic with respiration and augment with distal compression, the chance of DVT involving the iliac, common femoral, femoral, or popliteal veins is very low. DVT limited to the calf veins is more problematic due to vein duplication at this level. As mentioned earlier, when CW Doppler is combined with venous plethysmography (SVC and MVO) the sensitivity and specificity of the combined package is 85%, respectively.11
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