Background and Indications for Examination
Deep venous thrombosis (DVT) is typically caused by one or more of Virchow’s triad: stasis, hypercoagulability, and/or endothelial damage. DVT may occur in ambulatory patients presenting to the emergency department (ED) with leg pain and/or swelling, and is also a frequent complication of critical illness due to multiple and often coexisting risk factors, including immobility, surgery, trauma, indwelling devices, malignancy, and inflammatory states. A vexing problem is the unreliability of the symptoms and signs of DVT in the critical care setting, which are often limited by obesity, edema, and surgical dressings. In the intensive care unit (ICU), 10%–100% of DVTs are clinically unsuspected, and pulmonary embolism is the most frequent incidental autopsy finding, directly contributing to death in approximately 5% of all cases.
The true incidence and prevalence of lower extremity DVT (LEDVT) in the critical care setting is unknown, and patients may often be asymptomatic. Based on screening studies using ultrasonography for the diagnosis, incidence rates vary considerably as a result of differences in patient population, adequacy of prophylaxis, sonographic technique, and sonographer skills. Reported incidences range from as low as 8% to as high as 18% for proximal LEDVT, with the majority of cases occurring within the first week of an ICU stay. In the ambulatory and ED setting a DVT is typically symptomatic, and ultrasound is a reliable way to exclude one.
Clinically, LEDVTs are classified according to their embolic risk. Proximal (popliteal and higher veins) DVTs present a significant embolic risk, while isolated calf vein thrombosis is unlikely to embolize. Given that only 20% of calf DVTs will extend proximally, anticoagulation may be held in the ICU setting to avoid unnecessary complications. Therefore, calf veins are not routinely examined by ultrasound in the ICU setting. In the ambulatory and ED setting, if calf veins are not imaged it is recommended that patients return in 5–7 days to be reimaged in case a calf vein thrombus has propagated proximally.
Bedside ultrasonography can play a pivotal role in the diagnostic algorithms of venous thromboembolic disease. In order for the clinician to incorporate ultrasound into the timely diagnosis of DVT, a thorough understanding of the strength and limitations, clinical applications, and technical performance of lower extremity sonography is necessary.
Given the rapid growth in availability of portable ultrasound units in many EDs and ICUs, there are an increasing number of clinicians performing bedside diagnostic venous sonograms. While the examination is the same, whether it is performed in the ED or ICU, the ability of ultrasound to reliably exclude a DVT depends on the prevalence and/or pretest probability of a DVT. Ultrasound tends to perform much better in patients who are symptomatic in either setting.
Accuracy studies of ED and hospitalist physicians with variable amounts of ultrasound training (2–30 hours) reveal sensitivities between 70% and 100%, and specificities between 76% and 100% when compared to the “official” radiology study. Although these results lend support to the accuracy of bedside compression ultrasound (CUS) examinations performed by clinician sonographers, the experience of the sonographer plays an important role in the accuracy of diagnostic vascular sonography.
Probe Selection and Technical Considerations
A 7.5-MHz probe is usually sufficient for most lower extremity vessels. To image deeper vessels, probes utilizing lower frequencies may be helpful. Likewise, to image more superficial vessels, higher frequencies may be beneficial.
The focal zone should be adjusted and placed at the depth of the vessel being imaged. This improves the lateral resolution of the image.
Color-flow Doppler (CFD) detects blood flow and therefore is helpful in identifying a blood vessel and distinguishing it from other structures present in the lower extremity. CFD can be especially helpful in obese patients where imaging is more difficult and with smaller vessels, such as the popliteals.
Time-gain compensation (TGC) can be adjusted in order to increase the far gain of the image. Gain should be adjusted so that the image results in a uniform brightness and resolution in both the near and far fields. Adjustment of this control may be useful in obese patients with difficult anatomy.
Normal Ultrasound Anatomy
The LE veins are divided into deep and superficial vessels. The deep veins are paired with and accompany the arteries. The external iliac vein originates from the IVC and crosses the inguinal ligament to become the common femoral vein (CFV). The CFV is joined medially by the great saphenous vein (GSV), and approximately 1–2 cm beyond this point, the CFV divides into the deep femoral vein (DFV) and the superficial femoral vein (SFV). Recently, many have begun calling the SFV simply the “femoral vein” to avoid confusion—it is important to note that the “superficial” femoral vein is part of the deep venous system, just more superficial than the DFV. The DFV can usually be tracked for a short distance before it dives deeper and away from the acoustic window. The SFV runs anteromedially down the leg and through the adductor canal, where it then courses posteriorly into the popliteal fossa becoming the popliteal vein (PV). The PV then trifurcates into the anterior tibial, peroneal, and posterior tibial veins in the proximal calf (Fig. 15-1).
Depiction of normal lower extremity venous anatomy. A “two-site” or limited compression study involves interrogation of the upper and lower venous confluences alone (sites 1–5). A “complete” study includes areas 6, 7, and 8, which are performed only if thrombus is not found at areas 1–5.
The vessels are imaged in a transverse position, starting just below the level of the inguinal ligament. At this level, the CFV should be visualized medial and adjacent to the common femoral artery (CFA) (Fig. 15-2). The GSV will be seen joining the CFV from the medial side, high in the leg. When the probe is slid down about 1–2 cm distally, the CFA can be seen dividing into the superficial femoral artery (SFA) and the deep femoral artery (DFA). As the probe is moved distally and medially down the leg, the SFV should be visualized until it enters the adductor canal.
Normal common femoral vein (CFV) and common femoral artery (CFA). The CFV will appear medial to the CFA in the lower extremity. This image illustrates normal B-mode imaging of lower extremity vein and artery. The vessels appear anechoic signifying fluid (blood). Note the absence of echogenic material suggesting thrombus within the lumen of the vein.
The popliteal region should also be examined with the probe in a transverse orientation behind the patient’s knee. In this view, the PV will be seen on top of the popliteal artery on the screen. Keep in mind that the PV is still deep to the artery (ie, closer to the bed in a supine patient). Classically, the popliteal vein and artery will form a “figure eight,” with the vein being the upper part of the eight (Fig. 15-3). While the PV should be compressed to the trifurcation, calf veins are not routinely examined in most cases, unless there is a focal area of symptoms.