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The clinical indications for performing a venous ultrasound examination are as follows:
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An examination of the deep veins of the lower extremity is indicated whenever a proximal DVT is suspected. Proximal DVT is loosely defined as a clot or thrombus in the popliteal vein or higher in the leg. Its significance is that it is associated with a greater risk of embolization than a calf vein DVT.18
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The complete duplex ultrasound examination performed by imaging specialists is somewhat lengthy, with one leg taking up to 37 minutes in one study.19 In addition, it may or may not include an evaluation of the veins below the trifurcation of the popliteal vein distal to the knee. The complete study involves a slow, painstaking evaluation of each vein roughly one-probe width at a time. Each small segment of vein is visualized and checked for complete collapse under pressure from the transducer. Multiple blood flow measurements are taken using color and pulse wave Doppler. A difficult examination of the entire leg can take as much as an hour in some vascular laboratories. The main culprit for this extended period of time is the calf and its veins.
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Several studies have shown that the traditional complete examination may not be necessary. An abbreviated approach may maintain high accuracy and patient safety while decreasing the time required for an examination. The abbreviated approach argues for spot checks of vein compressibility, usually at the junction of the common femoral, deep femoral, and superficial femoral veins and the popliteal vein. This allows for considerable time savings and increases patient comfort, and there is considerable evidence that this method is safe and effective.
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One study evaluated 204 consecutive patients who had undergone elective, major lower extremity surgery.20 These patients were part of a general screening for lower extremity DVT and were not specifically referred for testing. The investigators found that simply compressing the vein to assess its patency produced a sensitivity of 60% and a specificity of 96%. The addition of color Doppler did not improve the study results. In these patients, venous compression was performed over the entire leg, a time-consuming task. Perhaps the most important point here is that color Doppler did little to improve detection of proximal DVT.
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In 1989, a group of investigators published their findings on a head-to-head comparison of contrast venography with B-mode ultrasonography in 220 consecutive patients.21 The only criterion used to detect DVT with ultrasound was lack of compression in either the common femoral or popliteal veins. For proximal vein thrombosis, the study yielded a sensitivity of 100%. When patients with calf vein thrombi (located on venogram) were included, the sensitivity and specificity were 91% and 99%, respectively. The study showed that in comparison to the gold standard this simple method of DVT detection was highly accurate.
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A number of other studies also confirmed high sensitivity and specificity for the detection of proximal lower extremity DVT without the utilization of color Doppler.8, 9, 19, 20 The studies noted considerable time savings when segmental checks were performed, which raised the question for the necessity of compressing every inch of vein in the proximal venous system. Proponents for compressing every inch of vein in the proximal venous system point out the potential for occurrence of segmental DVTs. These are venous thrombi that are limited to only one section of the deep venous system. For example, the popliteal vein may be without thrombus as well as most of the superficial femoral vein; however, a 3-inch section of the superficial femoral vein in the mid-thigh could contain thrombus. One study examined the incidence of a small isolated thrombus segment, such as in the iliac and popliteal veins, and found an incidence of 0 out of 195 legs studied.22 Thus, the chance of missing evidence of segmental thrombi is minimal. Another study prospectively analyzed 72 patients for DVT using an abbreviated technique with only two compression sites per leg.19 The two sites used were the saphenofemoral junction and the lower popliteal vein. For proximal DVT, the sensitivity and specificity of simple compression in these two sites were 100% and 98%, respectively. The authors addressed potential criticism of their abbreviated technique by noting that in a study of 491 patients found to be without DVT by compression ultrasonography, only 1.5% had a proximal DVT during the 6-month follow-up evaluation.23
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In 1996, a study of 721 patients, on whom 755 examinations were performed, examined the safety of abbreviated studies for DVT.24 All patients were referred to a vascular laboratory for suspicion of DVT. A complete examination was performed on each patient and the authors attempted to make retrospective inferences from the results. They concluded that DVT limited to a single vein occurs with enough frequency that the ultrasound survey for thrombosis should not be limited. However, the investigators failed to note that even the isolated thrombus segments in their study were within the area interrogated by the abbreviated approach and would have been detected.
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In 1998, a large study evaluated the abbreviated compression approach in 405 consecutive outpatients suspected of having a first-time lower extremity DVT.9 Each patient had his or her common femoral and popliteal veins assessed for compressibility. In those with normal results, testing was repeated in 5–7 days. Regardless of symptoms, patients with negative results on compression ultrasound did not receive anticoagulation. Follow-up was performed on all patients at 3 months after the initial negative study. Of the patients studied, 63 had DVT detected on initial ultrasound examination. Repeat ultrasound studies picked up 7 proximal DVTs that were not present on initial examination and may have propagated proximally from veins in the calf. None of the patients with normal results died of pulmonary embolism during the follow-up period.
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The patency of lower extremity veins should be checked in any patient in whom a clinician suspects of having a lower extremity DVT. Patients with symptoms suggestive of DVT typically have calf swelling and pain posteriorly.25 Redness of the posterior calf can also be suggestive. Although a number of studies have suggested that DVT is unlikely to be present without a minimum of a 2-cm difference in calf diameter between the affected and contralateral sides, in practice this is not always the case.26 Debilitated patients or ones who are unable to communicate may require a lower threshold to image because of their inability to relate common symptoms of DVT.
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The risk factors commonly described for the development of DVT include smoking, abdominal surgery, lower extremity injury, venous stasis, previous history of DVT, and congestive heart failure, among others.27–29 Thus, any person who has a risk factor and presents with calf pain and swelling is a candidate for undergoing a study to exclude DVT. Despite a number of studies that have suggested at least several risk factors must be present to warrant emergency evaluation for DVT, many physicians may feel obligated to exclude the presence of the disease for marginal indications.27–29
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Bedside emergency ultrasound examination of lower extremity veins is of greatest use in facilities that lack vascular laboratory access at night or on weekends. Emergency or primary physicians often find themselves having to empirically treat patients they suspect of having a DVT. This typically involves either admission for heparin therapy or outpatient treatment with low-molecular-weight heparin therapy. The latter can be made difficult if expeditious follow-up cannot be arranged or if the local medical culture frowns on outpatient treatment of uncomplicated DVTs. The high accuracy that can be achieved with a modified lower extremity duplex will allow discharge without anticoagulant therapy for those patients found to have a negative study. It is imperative to keep in mind that moderate- and high-risk patients who require a lower extremity ultrasound to exclude a DVT should have a repeat scan in 5 to 7 days. This will allow for diagnosis of DVT if a distal thrombus (one in the calf itself) progresses proximally. This is expected to occur in not more than 20% of distal DVT cases.9
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In general, bedside emergency ultrasonography is not recommended for diagnosing DVTs limited to the calf or ankle. Success rates can be as low as 40% even for experienced vascular technologists; if complicated by edema or large body habitus, success rates will be even lower.30 Many vascular laboratories no longer scan below the calf and prefer to reexamine patients or perform venography if high suspicion of a calf DVT exists.
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Prior to the 1970s, upper extremity DVT was thought to account for less than 2% of all DVT cases.31 Since that time, some studies have shown upper extremity DVTs to make up as much as 18% of all DVTs and 0.18% of adult hospital admissions.32 The most common causes of this disease process include malignancy, central venous access lines, and pacemaker wires. Studies have demonstrated that as many as 7 to 9% of upper extremity DVTs will lead to an acute pulmonary embolism.33, 34
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While the gold standard test of venous compression works well in the lower extremities, the subclavian vein does not lend itself to easy compression. Excluding upper extremity DVT depends on indirect confirmation of vein patency in a major venous segment. Although not well studied, this suggests that ruling out an upper extremity DVT may be more difficult for the novice sonologist than in the lower extremities. The solution for some sonologists has been to rely on positive findings for an upper extremity DVT, but not on their negative findings. Patients who have a negative ultrasound examination may be referred to vascular laboratories or radiology for a formal examination. Essentially, since most sonologists are familiar with the appearance of thrombosed deep and peripheral veins, diagnosis of an upper extremity DVT will be highly reliable.