J. Dean, M.D. and Bon S. Ku, M.D.
I. Introduction and Indications
The presentation of a swollen or tender lower extremity is common in emergency medicine, and often mandates a work-up to rule out the presence of deep venous thrombosis (DVT). Venous thromboembolism has been shown to have an incidence of more than 1 per 1000 annually in the United States.(1) Patients with a DVT are at risk for morbidity and mortality since a fragment of the thrombus can embolize to the lungs. It has been suggested that about one half of patients with an untreated proximal DVT will develop a pulmonary embolism (PE) within 3 months.(2)
A variety of diagnostic techniques have been used to identify DVT. These include impedance plethysmography, contrast venography, ultrasonography, computed tomography, and magnetic resonance imaging. In the past, contrast venography was considered to be the gold standard. However, due to its associated expenditure of manpower resources and time, the need for specialized personnel, space and equipment, and its limited availability and associated morbidity (including iatrogenic DVT) (3), contrast venography has been replaced with other tests with more favorable risk / benefit profiles. Among these, ultrasonography is as accurate as any, with many advantages over CT, MRI and plethysmography, including low cost, portability, non-invasiveness, and simplicity. It has become the standard initial test to diagnose a DVT and has proven to be highly accurate and cost-effective.
Two distinct protocols have been developed for the sonographic evaluation of the lower extremities: limited compression ultrasonography and duplex ultrasonography. The latter is a comprehensive examination usually requiring 45 minutes or more. It involves a variety of maneuvers as well as color flow and Doppler techniques, and is performed by specialist technicians. The former, employed at the bedside, is a simpler more focused technique readily mastered by practicing clinicians with basic understanding of ultrasonography. Limited compression ultrasonography is directed to identifying clot in the common femoral and/or the popliteal veins. The rationale for this approach is based on specific pathological and anatomic features of DVT. First, thrombi distal to the popliteal vein almost never embolize. This leads to the important clinical distinction between distal and proximal DVTs. While distal DVTs are of limited clinical consequence per se, they may propagate proximally, at which time they are at risk for embolization. For this reason, patients suspected of having distal DVT require repeat ultrasound evaluation in 3-5 days to rule out proximal extension. Second, studies of proximal DVT have demonstrated that isolated superficial femoral vein thrombosis is extremely rare. Clot is almost always present in either the common femoral and/or popliteal veins, or all three.(4) In the technique of compression ultrasonography, the clot is identified by the absence of normal compressibility of the vein. Limited compression ultrasonography has been shown to be as accurate as Duplex ultrasonography and superior to plethysmography, in the detection of a proximal DVT. (5) Its use by emergency physicians permits quick and accurate diagnosis with reduced time to disposition of proximal DVT.(6,7)
For the purposes of the emergency ultrasound (EUS) evaluation of the lower extremity, the veins posing a significant risk of PE include the common femoral, superficial femoral, and popliteal veins. It is important to note that the superficial femoral vein is part of the deep system, not the superficial system as the name suggests. Conversely the deep femoral (profunda femoris) vein is not considered to be a source of embolizing thrombi, and is therefore not included in the evaluation for DVT.
The popliteal vein is formed by the confluence of the anterior tibial,
posterior tibia, and peroneal veins approximately 4-8 cm distal to the
popliteal crease. The popliteal vein becomes the superficial femoral
vein as it passes through the adductor canal approximately 8-12 cm proximal
to the popliteal crease. The superficial femoral vein joins the deep
femoral vein to form the common femoral vein approximately 5-7 cm below
the inguinal ligament. Prior to passing under the inguinal ligament
to form the external iliac vein, the common femoral is joined by the greater
saphenous vein (a superficial vein) merging from the medial thigh. In
relation to the companion arteries, the common femoral vein lies medial
to the artery only in the region immediately inferior to the inguinal ligament. The
vein quickly takes a location posterior to the artery within 2 - 4 centimeter
of the inguinal crease, and remains posterior to the arteries into the
III. Scanning Technique and Normal Findings
Transducer. A linear array vascular probe with a frequency of 6 – 10 MHz and width of 6 – 8 cm is often ideal. Narrower transducers may make it harder to localize the veins and to apply uniform compression. For larger patients, a lower frequency or even an abdominal probe will facilitate greater tissue penetration.
Compression. The sonographic evaluation is performed by compressing the vein directly under the transducer while watching for complete apposition of the anterior and posterior walls. If complete venous compression is not attained with pressures sufficient to deform the artery, obstructing venous thrombus is likely to be present.
Patient positioning. To facilitate the identification of the veins and test for compression, they need to be distended. This is accomplished by placing the lower extremities in a position of dependency preferably by placing the patient on a flat stretcher in reverse trendelenberg. If the patient is on a gurney where this is not possible, the pt should be placed semi-sitting with 30 degrees of hip flexion
Real-Time Scanning Technique:
The common femoral vein. (Figure 1, video clips 1, 2, 6, and 7) Gel is applied to the groin and medial thigh for a distance about 10 centimeters distal to the inguinal crease. Filling of the common femoral vein might be augmented by placing a small bolster under the knee resulting in slight (about 10 degrees) hip flexion. Mild external rotation (30 degrees) may also be helpful. Many patients do not have the classic anatomic relationship between the vein and artery described above. Distinction of the two vessels may therefore depend on size (the vein is usually larger), shape (the vein is more ovoid) and compressibility, unless color-flow or Doppler is available. In this case the characteristic arterial waveform can help with differentiation.
Compressive interrogation of the vessel commences at the highest view obtainable at the inguinal ligament with the probe held transverse to the vein. Angling superiorly, a short section of the distal common iliac vein might be scanned. Systematic scanning, applying compression every centimeter, should be continued to the bifurcation of the common femoral vein into its superficial and deep branches and 1 – 2 cm beyond, since branch points are particularly susceptible to thrombosis. If difficulty is encountered in following the common femoral vein to the bifurcation, or in clearly identifying the two branching vessels, techniques to optimize the angle of insonation should be used. (The clearest signal will be obtained from the vessel walls when the incident ultrasound beam is at right-angles to the vein. This angle can be identified by rocking the probe in real time.) Other techniques of image adjustment that may be needed include decreasing the dynamic range and optimizing the gain. In obese patients it may be necessary to use a widely curved array general abdominal probe with lower frequencies than the vascular probe. In equivocal cases, comparison with the contralateral side may be helpful. If the clinical suspicion is high, the superficial femoral vein can also be evaluated. As noted above, DVT is identified by the absence of normal compressibility of the vein (Video clips 6 and 7).
Video clip 1: This is a normal study showing compressibility of the femoral vein. The bifurcation of the common femoral artery and vein into the superficial and deep vessels is seen in this clip. In addition, at the beginning of the clip, the saphenous vein can be seen entering more superficially at the left side of the screen.
Figure 1: A still image of the left common femoral vein (V) and artery (A) shows the saphenous vein (SV) merging from medially.
Video clip 2: This video clip shows the absence of a DVT. There is full compressibility of the femoral vein.
The popliteal vein. (Video clips 3, 8, 9, and 10) The patient can be placed in either a prone or decubitus position, or seated on the edge of the gurney with the knee flexed and the foot supported. In the decubitus and prone positions, the knee is flexed 10 – 30 degrees, and the leg being examined should be down. If the patient is prone, placing a bolster under the ankle to flex the knee to about 15 degrees facilitates filling of the popliteal vein. Again reverse trendelenberg positioning is required to ensure venous filling. Gel is applied from about 12 centimeters superior, to 5 centimeters inferior to the popliteal crease. The vein usually lies superficial to the artery. Both vessels lie superficial to the bony structures, which can be used as landmarks to anticipate the depth of the vessels. If difficulty is encountered in identifying the terminal branches of the popliteal vein, it is possible that the patient has one of the common variants of venous anatomy. In the absence of clear anatomic identification of the termination of the popliteal vein, the major venous structures should be imaged to approximately 10 centimeters below the popliteal crease (Video clip 3). In equivocal cases, comparison with the contralateral side may be helpful. As with the thigh, optimizing the angle of insonation may also help in identifying and defining the veins.
Video clip 3: Shows the trifurcation of popliteal vein. This may not be identified until the veins are compressed as far as 10 cm below the popliteal crease.
Video clip 4: Demonstrates the use of color flow to identify vessels. In this case the popliteal artery is seen pulsating while the popliteal vein has complete absence of flow. Color flow can be helpful when the popliteal tissues are very thick or echogenic. By convention, color flow Doppler shows flow towards the probe as red and away from the probe as blue; therefore, both arteries and veins can appear both red and blue depending upon the orientation of the probe.
Additional Components of the Exam:
The superficial femoral vein: As noted previously, this vein is not a primary focus of the standard lower extremity EUS evaluation. In cases where there is a high suspicion of DVT and an otherwise normal exam of the common femoral and popliteal veins, the superficial femoral vein may also be evaluated.
Color flow and Doppler: Color flow and Doppler assessment (Video clip 4) may be used to localize the vessels, although the use of this technology is beyond the scope of the standard EUS exam.
Gray scale identification of clot: While thrombus may be hyperechoic (Video clip 5), and thus directly visualized on exam, it is also frequently isoechoic to unclotted blood. Consequently, failure to see echogenic clot should not be used to exclude the diagnosis of DVT.
Video clip 5: Shows an example of an echogenic clot within the lumen of the vessel. Excessive compressive force is being applied causing collapse of the artery. This can lead to misidentification of the artery as vein; as well as discomfort for the patient.
As noted previously, DVT is identified by the absence of normal compressibility of the vein, although occasionally clot can be directly visualized on gray scale. Since this is a dynamic test, it can most clearly be demonstrated in video clips rather than still images. Video clips 6-10 show examples of non-compressible DVT.
Video clip 6: This large right DVT shows the femoral vein in its characteristic position deep to the artery. At the beginning of the clip, the saphenous vein can be seen entering from the medial or right side.
Video clip 7: A large common femoral DVT is evidenced by a lack of compressibility and an echogenic clot.
Video clip 8: This very large popliteal DVT shows the vein in its characteristic position superficial to the artery. At the beginning of the clip, the femoral condyles can be seen deep to the popliteal space. Towards the end, the straight transverse line of the tibial plateau is apparent.
Video clip 9: An echogenic clot can be seen in this popliteal DVT that lies in a 2 o’clock position relative to the artery. Also note the presence of the tibial plateau at the bottom of the image. When scanning the popliteal vessels, it is often useful to identify the bony structures at the bottom of the popliteal space for the purposes of orientation and depth adjustment. In this clip the gain settings are too high.
Video clip 10: Adequate compression has been applied
in this popliteal DVT as demonstrated by the deformation of the underlying
artery with compression.
V. Pearls and Pitfalls
Video clip 11: An inguinal lymph node is seen at the top of the image.
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