Compression ultrasound has been performed by emergency physicians to evaluate ED patients suspected of having lower-extremity DVT.13 Such studies detect venous thrombosis based on the fact that a vein filled with thrombus is not compressible whereas normal veins can be easily compressed. An examination of the lower extremity for DVTs, therefore, consists of sonographically identifying the common femoral vein and artery and proceeding distally to the trifurcation of the popliteal vein, imaging the veins at 3- to 5-cm intervals. The inability to compress a vein by transducer pressure is diagnostic of DVT in the portion of the vein being imaged. Echogenic thrombus is occasionally visualized within the vein, but slow-flowing blood may have a similar appearance, making this finding nonspecific. Compression ultrasound has been found to have both a sensitivity and a specificity of approximately 95 percent in venographically proven proximal leg DVTs, but the test is not accurate in identifying calf DVTs. Since approximately 20 percent of calf DVTs propagate to the proximal leg, repeat studies of the lower extremity are recommended in patients suspected of having a DVT found to have negative findings on compression ultrasound study. (See Chap.,55 for a detailed discussion.)
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