Traditionally, lower extremity deep venous thrombosis (DVT) is diagnosed by radiology-performed venous duplex ultrasound. Providers appropriately trained in focused point-of-care ultrasound (POCUS) can perform a rapid bedside examination with high sensitivity and specificity in critically ill patients. We describe the scanning technique for focused POCUS DVT lower extremity examination.
Acute lower extremity deep venous thrombosis (DVT) is a serious vascular disorder that requires accurate and early diagnosis to prevent life-threatening sequelae. While whole leg compression ultrasound with color and spectral Doppler is commonly performed in radiology and vascular labs, point-of-care ultrasound (POCUS) is becoming more common in the acute care setting. Providers appropriately trained in focused POCUS can perform a rapid bedside examination with high sensitivity and specificity in critically ill patients. This paper describes a simplified yet validated approach to POCUS by describing a three-zone protocol for lower extremity DVT POCUS image acquisition. The protocol explains the steps in obtaining vascular images at six compression points in the lower extremity. Beginning at the level of the proximal thigh and moving distally to the popliteal space, the protocol guides the user through each of the compression points in a stepwise manner: from the common femoral vein to the femoral and deep femoral vein bifurcation, and, finally, to the popliteal vein. Further, a visual aid is provided that may assist providers during real-time image acquisition. The goal in presenting this protocol is to help make proximal lower extremity DVT exams more accessible and efficient for POCUS users at the patient's bedside.
Deep venous thrombosis (DVT) is the formation of a thrombus in the deep peripheral veins of the extremities. It is a common and important finding, affecting about 300,000-600,000 people in the United States annually1. The propagation of DVT into a pulmonary embolism can occur in 10%-50% of patients and can be deadly, with a mortality rate of 10%-30%, which is higher than the in-hospital mortality for myocardial infarction1,2,3. The risk factors for thrombus formation include hypercoagulable states from genetic factors (family history of DVT, factor V Leiden, protein C or S deficiency), acquired factors (older age, malignancy, obesity, antiphospholipid antibodies, and others), and situational factors (pregnancy, oral contraceptives, recent surgery, travel, trauma, or prolonged immobilization, including from hospitalizations)1.
Early diagnosis of DVT in critically ill patients can expedite patient care and potentially prevent life-threatening complications such as pulmonary embolism, pulmonary infarct, and cardiac involvement1,2,3. A systematic review by Pomero et al. showed a pooled prevalence of 23.1% for DVT in critically ill patients4. Screening for lower extremity DVT has traditionally been performed by radiology ultrasound technicians conducting comprehensive whole-leg duplex exams including both grayscale compression ultrasound and color/spectral Doppler. However, several smaller or community clinical sites lack the direct availability of a sonographer during certain times of the day, such as on nights or weekends, thus creating a delay in patient care5. More recently, acute care providers have devised methods of screening for proximal lower extremity DVTs using point-of-care ultrasound (POCUS)-focused imaging protocols, which demonstrate similarly high sensitivity and specificity in critically ill patients3,4,6. Proximal lower extremity DVTs are defined as DVTs occurring anywhere in the groin, thigh, or knee within the femoral or popliteal venous system. Falling outside of this category are DVTs in the following locations: calf veins (where DVTs are of uncertain clinical significance) and pelvic veins (i.e., the common, external, and internal iliac veins), which are only detectable indirectly using the color and spectral Doppler portion of consultative lower extremity venous ultrasound exams2,3.
Understanding the typical anatomic distribution of DVTs makes performing these bedside exams rapid and easy. First, 70%-99% of proximal lower extremity DVTs involve the femoral or popliteal regions7,8,9. Second, grayscale compression ultrasound is a simple and accurate method for evaluating DVTs; when enough pressure is applied to indent an adjacent artery, normal veins should collapse fully, whereas veins harboring a DVT will not. Combining these principles, the two-zone or three-zone lower extremity DVT POCUS examinations center on compression ultrasound of veins in the inguinal, thigh, and popliteal areas. These techniques have been clinically validated in prior intensive care and emergency medicine studies, demonstrating high sensitivity (96.1%, with a 95% confidence interval (CI) of 90.6%-98.5%) and specificity (96.8%, with a 95% CI of 94.6%-98.1%), with high overall diagnostic accuracy (95%)3,4,6. However, in the experience of the authors, the DVT POCUS exam remains grossly underutilized in the care of critically ill patients, possibly because clinicians are not familiar with the image acquisition sequence. This narrative review with associated visual aids describes an image acquisition protocol for performing a POCUS exam to screen for proximal lower extremity DVTs to assist providers in proper expedited image acquisition during clinical care.
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All procedures performed in studies involving human participants were in accordance with the ethical standards of the Duke University Health System institutional research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The protocol was performed using inputs from the following publications3,10. Images were performed on the authors themselves for normal images and as part of routine educational ultrasound scans done for teaching purposes for positive images, with preceding verbal consent as per institutional standards. The patients were selected based on the following criteria: inclusion criteria: any patient with lower extremity pain, swelling, or other clinical reason to suspect DVT; exclusion criteria: patients with a lower extremity amputation who may be missing the popliteal or distal femoral views.
1. Transducer selection
- Select the high-frequency linear transducer (5-13 MHz) for the DVT scan to ensure a high-resolution image of the veins.
NOTE: In obese patients or, occasionally, for the popliteal vein, DVT scanning with a low-frequency curvilinear (2-5 MHz) transducer is necessary to have adequate penetration and depth. However, patients challenging enough to require a low-frequency transducer generally fall outside the scope of point-of-care ultrasound training and usually warrant a comprehensive radiology ultrasound.
2. Machine settings
- Setting the depth, gain, and focus
- Set the depth so that the target vessels appear in the middle 1/3 of the ultrasound screen by pressing the up or down depth button (between 3-6 cm).
- Set the gain so that the vessels contain a few spots of grey but are otherwise black by pressing the right or left gain buttons (MIS between 0.6-0.8, TIS 0.1).
- If the machine has the capability, aim the focus beam at or slightly below the level of the target vessels by clicking the box on the screen and moving it while holding down the button, and then release.
- Set the machine mode. Click on B-Mode, which is a 2-dimensional (2D) greyscale ultrasound exam. Use B-mode to obtain both the non-compressed and compressed images.
NOTE: If the B-mode images are ambiguous, providers trained in color and spectral Doppler can consider adding these techniques, but they are not classically included in lower extremity POCUS DVT protocols.
- For optimal scanning ergonomics, place the machine with the ultrasound screen in direct line of sight with the ultrasound probe.
3. Patient position
- Before scanning, expose the patient's entire leg from the groin to the knee.
- Place the patient in the supine position as this is the ideal view for DVT examination of the common femoral and femoral veins. Place the patient in the frog-leg position (external rotation at the hip with knees slightly bent) to enable better visualization and scanning of the distal veins.
NOTE: The popliteal veins can also be assessed with either a frog-leg position or with slight flexion of the knee. Although lateral decubitus and prone positioning might improve visualization (particularly for patients with a higher body mass index), they may not be feasible in all situations (e.g., in critically ill patients or intra-operatively).
4. B-mode scanning
- Groin and thigh: Apply gel to the patient's skin in a linear path that traces out the expected path of the ultrasound transducer to increase the efficiency of motion for performing the examination of this part.
- Knee: Apply gel to the transducer itself, as a generous application of gel will facilitate the scanning efficiency.
- Transducer orientation: Place the transducer in a transverse position with the orientation marker directed toward the patient's right side to ensure that images captured during scanning correspond to the anatomical direction of the structures.
- Place the probe perpendicular to the path of the vein to visualize the vein in a short-axis view. Center the venous structure on the screen. Add long-axis imaging in ambiguous cases by rotating the probe 90° so that the marker points toward the patient's head.
5. Scanning and compression technique
- Compression sequence: Begin scanning immediately caudal to the inguinal crease, progress distally sequentially with downward compression, and then release at each point as shown in Figure 13,10.
- Compress such that the entire vein collapses, with the anterior wall touching the posterior wall, while the artery remains pulsatile. Do not apply resting surface pressure between each compression as it can obscure the visualization of the veins. The vein should collapse fully when enough pressure is applied with the transducer to indent an adjacent artery.
- Click Save Clip and compress and then release at the common femoral vein (CFV) and femoral artery (FA) just below the inguinal ligament.
- Slide the probe distally about 1 cm and record the same compression and release technique at the CFV and FA at the intake of the great saphenous vein (GSV).
- Click Save Clip, then compress, and release the probe at the CFV at the bifurcation of the FA into the superficial femoral artery (SFA) and deep femoral artery (DFA). Between the SFA and DFA, there will typically be a lateral perforator vein draining from lateral-to-medial into the CFV. Ensure that both the lateral perforator vein and GSV are compressible.
NOTE: Though these are superficial veins, clots in these locations have the potential to embolize, resulting in the same life-threatening complications as caused by DVTs.
- Click Save Clip, then compress, and release the probe at the CFV at the bifurcation of the CFV into the femoral vein (FV) and deep femoral vein (DFV).
- Click Save Clip, then compress, and release the probe at the CFV at the popliteal vein behind the knee.
- Once points 1-5 (as in Figure 1) are all fully compressible, scan the FV along the thigh from proximal to distal until the vein disappears in the adductor canal. During this scanning process, attempt to compress the vein approximately every 1-2 cm.
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We describe the interpretation of proximal lower extremity DVT POCUS in patients with an initial suspected DVT.
The attached Figure 2 demonstrates negative POCUS ultrasound images for DVT in the left and right lower extremities, with multi-point compression from the proximal to distal veins as demonstrated in Figure 1 (from thigh to knee). In a negative DVT study, the veins are completely collapsible, with the anterior wall touching the posterior wall during compression ultrasound in all the zones. In patients with a low clinical pretest probability for a clot, this study has a high sensitivity (>95%) and high negative predictive value for ruling out proximal DVT, with 3 month follow-up rates of venous thromboembolism of 0.5%11,12. For moderate to high-risk patients for DVT with a negative DVT POCUS exam, about 2% have a positive proximal DVT when retested 1 week later, which is similar compared to whole leg compression ultrasound (rate of 1%-2% for both POCUS and whole leg compression ultrasound)11,12.
In a positive DVT study, proximal vein POCUS can both directly visualize a thrombus or detect non-compressibility of the proximal veins (CFV, FV, or PV), which is diagnostic of a nonocclusive thrombus (Figure 3). Additional techniques such as augmentation using color flow at the FV can help detect thrombus in a calf vein, which is difficult to directly visualize11,12. Finally, proximal DVT POCUS is faster to perform than whole leg compression ultrasound (about 5 min compared to 15-20 min) and is less operator-dependent based on prior studies11,12.
Patients with a positive DVT exam should be initiated with anticoagulation treatment after determining the risks and benefits via discussion with the patient and their primary doctor. The anticoagulation choice depends on the extent of the thrombus, co-morbidities, and other factors that cannot be discussed in depth in this protocol11,12. Nondiagnostic exams can be managed based on the pretest probability and anticoagulation therapy deferred until serial ultrasound testing is complete11,12.
Figure 1: Left leg vascular anatomy. Point-of-care ultrasound compression points from proximal (1) to distal (5-6) in the femoral and popliteal veins. Legend: 1. Common femoral vein (CFV), common femoral artery (CFA), 2. Great saphenous vein (GSV), common femoral vein and artery (CFV, CFA), 3. Common femoral vein (CFV), superficial and deep femoral artery (SFA, DFA), 4. Femoral vein (FV), deep femoral vein (DFV), superficial and deep femoral arteries (SFA, DFA), popliteal vein and artery (PV, PA), 6. Femoral artery and vein (FA, FV). Figure reprinted with author’s (DC) permission from www.countbackwardsfrom10.com. Please click here to view a larger version of this figure.
Figure 2: Negative lower extremity DVT examples in transverse views. (A) Compression point 1 at the left common femoral vessels. The CFV lies medial to the FA at the level of the proximal thigh, immediately inferior to the inguinal crease (see Video 1). (B) Compression point 2 at the left great saphenous vein. The great saphenous vein (GSV) is a superficial vein that drains into the medial aspect of the CFV (see Video 2). (C) Compression point 3 at the left common femoral artery bifurcation. The CFV continues medially, while the CFA bifurcates into the SFA and DFA (see Video 3). (D) Compression point 4 at the left femoral and deep veins. The CFV bifurcates into the FV and DFV. This can be followed in the medial thigh until the adductor canal, just proximal to the medial knee (see Video 4). (E) Compression point 5 at the left popliteal vessels. After the femoral vessels traverse deep and underneath the femur to the posterior thigh and knee, they reappear as the PV, which is superficial to the popliteal artery (PA; see Video 5). (F) Compression point 6 at the left femoral vein. Moving the probe back up to the medial distal thigh just proximal to the knee, the FV now lies deep to the FA (see Video 6). (G-L) Transverse views of compression points 1-6 in the right leg. The analogous negative POCUS ultrasound images for DVT in the right lower extremity (see Video 7-12). Please click here to view a larger version of this figure.
Figure 3: Positive lower extremity DVT examples. The positive POCUS ultrasound images for DVT in the lower extremities, with multi-point compression from proximal to distal as demonstrated in Figure 1 (thigh to knee). (A) Transverse view of a positive thrombus in the right common femoral vein (see Video 13). (B) Transverse view of a positive thrombus in the left common femoral vein (see Video 14). (C) Transverse view of a positive thrombus in the left common femoral vein at the arterial bifurcation (see Video 15). (D) Longitudinal view of a positive thrombus at the common femoral vein bifurcation. This long-axis still image of the CFV bifurcation shows a DVT (the area between the plus signs): an echo density partly occluding the junction between the FV and CFV. The common femoral vein (CFV) branches into the femoral vein (FV) and the deep femoral vein (DFV). This can be followed in the medial thigh until the adductor canal, just proximal to the medial knee. (E) Transverse view of a positive thrombus in the popliteal vein (see Video 16). (F) Transverse view of a positive thrombus in the left femoral vein (see Video 17). Please click here to view a larger version of this figure.
Figure 4: Troubleshooting for accurate identification of DVT. (A) Transverse view with an inguinal lymph node. The superficial portion of this still image shows something that could be confused with a DVT: lymphadenopathy. Lymph nodes can be distinguished from a DVT because they have a short cranial and caudal extent, whereas DVTs are contiguous in both those directions with the rest of the venous system3. (B) Longitudinal view of the common femoral vein bifurcation. The still image shows the bifurcation of the right CFV into the FV and DFV. The image shows two findings that could potentially be mistaken for a DVT: spontaneous echo contrast (SEC) and a venous valve. SEC is a smoke-like phenomenon with a swirling pattern that can be seen in the lumen of any blood-containing vessel during a state of slow flow (Video 18). Although SEC indicates stasis and possibly a higher risk of DVT, if the examined vein is fully compressible, then there is no DVT present at the time and location of the exam. Venous valves (like the one seen in the FV in these images) are thin filaments seen in veins that open and close cyclically, whereas DVTs are much thicker and more globular, as seen in Video 183. (C) Transverse and longitudinal views of a Baker's cyst superficial to the popliteal vessels. The two paired images show sagittal (SAG) and transverse (TRV) views of something that can be confused for a popliteal DVT: a Baker's cyst. A Baker's cyst is a fluid-filled cyst that develops behind the knee. It is distinguished from a DVT sonographically by the following constellation of features: (i) it is not contiguous with the venous system cranially or caudally; (ii) it is contiguous with the back of the knee joint, sometimes creating the visual appearance of a comma punctuation mark3. (D) Longitudinal view of a gastrocnemius muscle tear. This still image demonstrates a muscle tear deep in the calf vein. Differentiating a muscle tear adjacent to a vein from a partially occlusive DVT may be challenging, especially if the muscle swelling impinges on the vein. If the POCUS findings are ambiguous for DVT, a referral for consultative ultrasonography should be considered. Please click here to view a larger version of this figure.
Video 1: Compression point 1 at the left common femoral vessels. Please click here to download this Video.
Video 2: Compression point 2 at the left great saphenous vein. Please click here to download this Video.
Video 3: Compression point 3 at the left common femoral artery bifurcation. Please click here to download this Video.
Video 4: Compression point 4 at the left femoral and deep veins. Please click here to download this Video.
Video 5: Compression point 5 at the left popliteal vessels. Please click here to download this Video.
Video 6: Compression point 6 at the left femoral vein. Please click here to download this Video.
Video 7-12: Transverse views of compression points 1-6 in the right leg. The analogous negative POCUS ultrasound images for DVT in the right lower extremity. Please click here to download this Video.
Video 13: Transverse view of a positive thrombus in the right common femoral vein. The right CFV (right side of the screen) shows two features of a DVT: (1) echo density visible in the vein lumen and (2) the vein being non-compressible with enough pressure to indent the adjacent femoral artery. Please click here to download this Video.
Video 14: Transverse view of a positive thrombus in the left common femoral vein. The left CFV (left side of the screen) at the level of the GSV shows two features of a DVT: (1) echo density visible in the vein lumen and (2) the vein being non-compressible with enough pressure to indent the adjacent femoral artery. The DVT appears to be extending into the GSV. Please click here to download this Video.
Video 15: Transverse view of a positive thrombus in the left common femoral vein at the arterial bifurcation. The left CFV (non-pulsatile large circle in the middle of the screen) at the level of the CFA bifurcation shows no compressibility, while the adjacent arteries are being indented with probe pressure. The CFV continues medially, while the CFA branches into the SFA and the DFA. Please click here to download this Video.
Video 16: Transverse view of a positive thrombus in the popliteal vein. The right PV (middle of the screen) shows two features of a DVT: (1) echo density visible in the vein lumen and (2) the vein being non-compressible with enough pressure to indent the adjacent popliteal artery. Please click here to download this Video.
Video 17: Transverse view of a positive thrombus in the left femoral vein. The left FV (middle of the screen) shows two features of a DVT: (1) echo density visible in the vein lumen and (2) the vein being non-compressible with enough pressure to indent the adjacent femoral artery. Please click here to download this Video.
Video 18: Venous valve and SEC as seen in the lumen. Please click here to download this Video.
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Venous thromboembolism is a common disease, affecting approximately 300,000-600,000 people in the United States annually, with serious complications including pulmonary embolism. Mortality rates in these patients range from 10%-30%2,3,4. Studies have consistently found significant delays in the diagnosis of DVT, with one prospective study of 1,152 patients across 70 medical centers identifying a delay of greater than 1 week in 21% of patients diagnosed with acute DVT13. In another prospective observational study of 76 consecutive patients with proximal DVT, the mean delay from symptom onset to the initiation of anticoagulation was 12.9 days and correlated with increased time to complete recanalization and increased rates of incomplete recanalization14. Thus, protocols leading to expedited diagnosis and treatment can have significant clinical implications4,15.
Critical steps in the protocol
Since 70%-99% of proximal lower extremity DVTs have some manifestation in the femoral and/or popliteal zones of the leg, the core of any lower extremity DVT POCUS exam centers on these two scanning zones7,8,9. The simplest two-zone lower extremity DVT POCUS examination evaluates compressibility at just two points: the common femoral vein (CFV) and the popliteal vein. Traditionally, it performs very well in critically ill patients in the ICU, ED, and inpatient wards, with a sensitivity of 100% for proximal DVT in prior studies16,17. Thus, it has been the recommended exam as per the 2015 College of Critical Care Medicine guidelines for the evaluation of lower extremity DVT16,17,18. However, the distribution of thrombi in outpatients may be different, with lower sensitivity rates as noted in prior studies; thus, this exam should be performed with caution in this patient population16,19.
In contrast, a more comprehensive version of the two-zone POCUS DVT examination expands on the CFV and popliteal vein views, with three additional compression points (Figure 1): the CFV just caudal to the inguinal ligament but cranial to the great saphenous intake; the CFV at the bifurcation of the CFA into the SFA and DFA; and the bifurcation of the CFV into the FV and DFV.
If the initial two-zone exam involving the groin and knee is negative, a third zone can be added: the thigh. Specifically, the ultrasound probe can be used to scan the femoral vein along the thigh in 1-2 cm advancements as distally as possible in the thigh, searching for noncompressible veins or a visible thrombus. The three-zone POCUS exam, thus, increases the sensitivity of the two-zone exam by being capable of detecting DVTs isolated to the femoral veins. For instance, one study in emergency department patients showed a rate of 6.3% of isolated DVT in veins other than the CFV and PV7. Thus, the authors recommend this three-zone method3,7,16. Finally, additional adjunctive techniques may be helpful, such as any of the following:
1. Ambiguous findings: If any ambiguous findings are seen in the short-axis view, prior to compression, one can consider doing either or both of the following, obtaining a long-axis view of the vein to see if the short-axis abnormality occupies space in all three spatial dimensions (consistent with an actual intra-luminal structure) or exists only in the short-axis views (suggestive of an artifact); adding color Doppler to see if the echogenic structure disrupts the Doppler flow.
2. Pulse-wave Doppler of the CFV: complete absence of respiratory variation is suggestive of more proximal DVTs such as in pelvic/iliac veins that cannot be directly seen on bedside ultrasound.
3. Augmentation: one can squeeze the calf while holding the probe over the CFV and looking for increased pulse-wave Doppler velocities to evaluate for a thrombus between the calf and the CFV.
Limitations and troubleshooting
First, patient positioning is key. Ideal patient positioning while supine involves bending the leg into a frog-leg position with external rotation at the hip and the knee bent at less than 90° for improved vein visualization. Other scanning options include placing the patient in lateral decubitus with the knee slightly bent or prone with a towel roll underneath the ankle to maintain a bent knee. Placing the patient in slight reverse Trendelenburg increases venous pressure and, thus, improves visualization of the veins but makes compression more difficult3. Next, the ultrasound user should optimize the machine settings with appropriate depth and gain to obtain the best quality images, with the vein in the center of the screen. The user should ensure that the probe is perpendicular to the skin so that the ultrasound beams are not traversing the vein at an angle, which could create a false positive scan. The user should be careful not to apply resting pressure, which causes the vein to collapse and not be visible. Also, when assessing for compressibility, the user should ensure that the vein completely compresses with the anterior and posterior walls touching, as a thrombus could result in only partial compression of the vein, and that the adjacent artery is pulsatile and indents with pressure3. The exam should begin at the most proximal aspect at the inguinal crease, with the common femoral vein positioned medially and the femoral artery lateral on the screen to ensure that the veins are being viewed in full. Furthermore, the user should be careful to not confuse nerves (non-collapsible, cylindrical, hyperechoic structures with anisotropy) or lymph nodes (non-collapsible elliptical structures with a central hilum; Figure 4A) for veins (hypoechoic, collapsible; Figure 4). They should be aware of another potential common finding on popliteal views, which is a superficial fluid-filled anechoic cystic structure known as a popliteal or Baker's cyst located superficial to the popliteal vessels (Figure 4C)3. Finally, the user should note that deep veins run adjacent to arteries, whereas superficial veins do not, which can be particularly tricky in obese patients with veins found deeper than expected given the increased overlying fatty soft tissue3,16.
Recent studies demonstrate that a POCUS DVT exam still achieves high sensitivity and specificity when compared to radiology-performed comprehensive DVT lower extremity ultrasound1,6. A systematic review by Burnside et al. described focused emergency physician-performed ultrasonography of lower extremity DVT compared to traditional radiology-performed examinations, identifying six studies including 936 patients that demonstrated a significant overall sensitivity of 0.95 (95% CI = 0.87 to 0.99) and a specificity of 0.96 (95% CI = 0.87 to 0.99)6. However, study weaknesses are the inclusion of academic sites with ultrasound fellowship-trained physicians, lack of details regarding patient enrollment methods or clinical characteristics, and lack of information about the specific anatomic location of the DVT, which may affect results. Furthermore, calf veins along with iliac vein thrombus are less easily compressed, and these DVTs are not typically included in the analysis6. Future prospective studies using this method can further validate the sensitivity and specificity of DVT diagnosis and compare inter-rater reliability between expert and novice users.
The strengths of the described method in performing focused POCUS for proximal lower extremity DVT are that providers can expedite the diagnosis of DVT and thereby initiate earlier treatment, with potentially decreased complication rates and improved patient clinical outcomes. Furthermore, the exam can be done at the bedside in patients who are critically ill and cannot travel unmonitored to radiology for a lengthy examination. By performing a two-zone or three-zone focused POCUS exam, providers can perform the exam rapidly in minutes, and it is easy to learn, thus making it a much more accessible clinical examination. Finally, this can improve patient disposition times, especially in emergency department settings, with a reduction of up to 2 hours for patients awaiting a radiology study5. In conclusion, based on the high sensitivity and specificity of DVT detection by critical care and emergency medicine physicians in prior studies, we recommend focused-POCUS lower extremity DVT examination as a rapid and simple bedside tool in the diagnosis and management of thromboembolism in critically ill patients.
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Robert Jones is an educational material writer for www.emsono.com. All the other authors have nothing to disclose.
The authors have no acknowledgments.
|Edge 1 ultrasound machine||SonoSite||n/a||Used to obtain normal images/clips|
|SPARQ ultrasound machine||Philips||n/a||Used to obtain abnormal images/clips|
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