Point-Of-Care Ultrasound Screening for Proximal Lower Extremity Deep Venous Thrombosis

Summary

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.

Abstract

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.

Introduction

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 infarction^1,2,3. 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 involvement^1,2,3. 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 patients^3,4,6. 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 exams^2,3.

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 regions^7,8,9. 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.

Protocol

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 publications^3,10. 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

1. 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

1. Setting the depth, gain, and focus
   1. 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).
   2. 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).
   3. 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.
2. 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.
3. For optimal scanning ergonomics, place the machine with the ultrasound screen in direct line of sight with the ultrasound probe.

3. Patient position

1. Before scanning, expose the patient's entire leg from the groin to the knee.
2. 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

1. 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.
2. Knee: Apply gel to the transducer itself, as a generous application of gel will facilitate the scanning efficiency.
3. 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.
4. 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

1. 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 1^3,10.
   1. 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.
2. Click Save Clip and compress and then release at the common femoral vein (CFV) and femoral artery (FA) just below the inguinal ligament.
3. 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).
4. 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.
5. 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).
6. Click Save Clip, then compress, and release the probe at the CFV at the popliteal vein behind the knee.
7. 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.

Results

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 t...

Discussion

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 2...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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