Duplex ultrasound examination of the peripheral arterial and venous systems has been refined to the point where it has become the initial modality of choice for vascular diagnosis. Technical advances have improved diagnostic accuracy such that treatment decisions previously based on angiographic studies can now be based solely on noninvasive studies. This is most evident in the noninvasive diagnosis of deep venous thrombosis (DVT),1 and is becoming more prevalent in the management of carotid occlusive disease and atherosclerotic peripheral vascular disease.
Ultrasound Examination of the Peripheral Venous System
Risk factors associated with the development of DVT are common in the critical care setting. Virchow’s triad of stasis, endothelial injury, and altered coagulation are readily seen in today’s intensive care unit (ICU). Clinical factors such as major trauma, which include neurological injury, pelvic and long bone fractures2,3; prolonged immobilization due to altered mental status, paralysis, morbid obesity; multiple sites for venous access and central monitoring; and advancing age, all contribute to this increased risk.4
The true prevalence of acute DVT in the ICU setting is unknown. Reported incidence varies widely (4–60%) due to patient population, detection methods, and the application of surveillance programs.5–7 Despite increased awareness and aggressive application of protocols for the prevention of DVT, postmortem studies indicate that subclinical, undetected DVT and pulmonary embolism (PE) continue to exist.8 In addition, many ICU patients are at risk for rebleeding and are not candidates for anticoagulation.
Invasive hemodynamic monitoring or prolonged central venous access is common in the critical care environment. Catheter-associated thrombosis occurs in response to endothelial injury and the alterations in normal venous flow patterns caused by the catheter. This may be more significant in children, where small diameter veins can be functionally occluded by catheterization.9
Although the most common sequelae of DVT are the late problems of venous insufficiency and stasis ulceration, PE is the primary concern in the acute care setting. The present emphasis on DVT prophylaxis arises from the recognition that PE is one of the most preventable causes of death and major morbidity in hospitalized patients. Because most clinically significant PE arises from deep veins of the lower extremities, some centers have advocated routine duplex ultrasound surveillance of patients during their ICU stay (Video 23-1).
Continuous-wave Doppler ultrasound technology was introduced to clinical practice in the 1970s. Although no images were possible, these devices allowed the examiner to assess venous flow patterns by auditory waveform analysis. The combination of ultrasound imaging and Doppler spectral analysis provided the basis for current duplex ultrasound technology. By the early 1990s, the venous duplex ultrasound examination replaced contrast venography as the gold standard for the diagnosis of DVT.
Standard practice requires a trained sonographer to transport the ultrasound machine (portable but bulky) to the ICU, where a full lower extremity examination is performed and recorded on videotape or digital media. The study is reviewed by the interpreting physician who generates a report that is transcribed and returned to the patient’s chart. This process, although very accurate, is time consuming and may not always serve the needs of the very dynamic and often unstable conditions in the critical care environment.
The recently developed portable, handheld, duplex scanners with multihertz transducers and color-flow Doppler capability bring the possibility of a focused venous examination to the bedside. The clinician is now able to obtain diagnostic information rapidly and interpret these results within the context of the patient’s overall clinical condition. Bedside investigation can eliminate the need for patient transport to the ultrasound department or the computerized tomography (CT) scanner, often an enormous task with critically ill patients that itself has inherent risks. Where results are uncertain or equivocal, a formal diagnostic study can be obtained to assist in a definitive diagnosis. With proper training and experience, a focused venous duplex examination at the bedside can be accomplished by any clinician familiar with venous anatomy, venous flow characteristics, and the basics of duplex ultrasonography.
The venous systems of both the upper and lower extremities are divided into deep and superficial components. The deep venous system is composed of those veins draining the muscle compartments and paired with named arteries. The superficial systems drain cutaneous structures and run in the subcutaneous space. These superficial veins are not associated with adjacent arteries.
In the lower extremity, the deep venous system includes the external iliac, common femoral, superficial and deep femoral, popliteal, anterior and posterior tibial, peroneal, and soleal and gastrocnemius veins (Figure 23-1). All of the deep veins are accompanied by named arteries, except for the soleal and gastrocnemius veins. The two main veins of the superficial venous system are the greater and lesser saphenous veins. The lesser saphenous is located in the lateral calf and drains into the deep system at the popliteal vein. The greater saphenous vein runs along the medial aspect of the leg from ankle to proximal thigh, where it traverses the fossa ovalis and drains into the common femoral vein. An unfortunate consequence of the traditional anatomic nomenclature is that the “superficial” femoral vein is actually a deep venous structure and is often the site of acute DVT leading to all the complications of thrombosis, including PE. Therefore, it is important to understand that thrombosis of any segment of the femoral vein (superficial, deep, or common) constitutes a DVT and should be so identified for documentation and treatment purposes.
In the upper extremity, the deep veins include the internal jugular, subclavian, axillary, brachial, radial, and ulnar veins (Figure 23-2). These veins each have a companion artery. The major components of the superficial venous system of the arm are the cephalic vein, running laterally from wrist to shoulder and draining into the subclavian vein, and the basilic vein, running medially from antecubital fossa to the axilla, where it drains into the axillary vein. When available, the superficial veins are the sites of choice for peripheral venous access.
Sonographic imaging is performed with B-mode scanning, often using color-flow imaging to add more information. The external iliac artery and vein exit the pelvis deep to the inguinal ligament to become common femoral vessels. The common femoral vein is medial to the artery and is slightly larger in diameter. These relationships are easily identified when scanning in a transverse plane at the groin crease (Figure 23-3). The transverse plane is used to assess the veins for compressibility (see below). Moving distally along the common femoral vein, the greater saphenous can be identified as it passes from the superficial plane to drain into the femoral vein. Again, no artery accompanies the greater saphenous vein. Moving slightly more distally, common femoral vessels divide into superficial and deep femoral vessels. At this level, four vessels are seen in cross-section (Figure 23-4). From this point distally, the superficial femoral artery (SFA) and vein continue to the adductor canal, where they enter the popliteal space and are designated as popliteal vessels (Figure 23-1).
Once the distal external iliac vein is identified at the inguinal ligament, turning the transducer 90° provides a longitudinal image of the vessel. This view is best suited for a rapid survey of the veins and allows for assessment of the extent and nature of a thrombus when seen (Figure 23-5). Vein compression is not reliable while scanning in the longitudinal plane. Compressibility must be confirmed in the transverse view.
Diagnostic criteria for identifying DVT can be divided into vessel characteristics and flow characteristics. The primary vessel characteristic is compressibility; the ability to demonstrate wall-to-wall apposition of the vein when adequate pressure is applied using the ultrasound transducer in the transverse plane. Adequate pressure is determined by noting mild deformation of the adjacent artery (Figure 23-6). Noncompressibility indicates that intraluminal thrombus is preventing the vessel walls from collapsing. It is important to remember that fresh, immature thrombus may not be echogenic because newly formed clot has an acoustic impedance similar to blood. The second vessel characteristic is identification of intraluminal echogenic material. This can often be seen on the initial survey of the venous system and should alert the clinician to the presence of thrombus. Intraluminal echogenic material should be confirmed in both imaging planes, and when thrombus is identified, compression should be limited due to the possibility of dislodging the clot. Longitudinal imaging provides the best view for determining the extent or length of thrombus and whether it is adherent to the vessel wall or may have a free -floating tip (Figure 23-7 and Video 23-2). A third vessel characteristic that may be helpful is the assessment of valve function. Occasionally, venous valves are visible in situations where higher frequency transducers can be used (thin patients, children). Normal valves open and close in conjunction with venous flow. However, because valve cusps are often the site of thrombogenesis, an immobile valve cusp may be a clue to the presence of thrombus.
Common femoral artery and compressed common femoral vein (compare with Figure 23-3).
Venous blood flow characteristics are also important in assessing the presence of acute DVT. Normal venous flow patterns show a phasicity that varies with respiration. Normal inspirations decrease intrathoracic pressures and cause associated increase in venous flow. Similarly, expiration increases intrathoracic pressure and is reflected in a decrease in venous flow. A Valsalva maneuver increases intrathoracic pressure sufficiently to completely interrupt venous flow that is associated with an augmentation in venous flow when the Valsalva maneuver is released. These changes are easily identified by observing the Doppler waveform or by listening to flow patterns with a continuous-wave Doppler unit. Any obstructive process between the thoracic cavity and the site of insonation of the veins of the lower extremity can alter the normal phasic changes associated with respiration. Absence of phasic changes with a continuous-flow pattern or loss of augmentation with deep inspiration suggests obstruction of the venous system. Additional maneuvers to augment flow include compression of calf muscles and the distal thigh to demonstrate increased flow at the site of insonation. Loss of normal augmentation with compressions also suggests the presence of obstruction in the venous system. The assessment of both vessel characteristics and venous blood flow characteristics lead to highly accurate and reliable detection of DVT. Normal vessel and flow characteristics also provide a very high negative predictive value for DVT.
Equipment requirements for venous duplex examinations include high-resolution gray-scale imaging, color Doppler capability, and spectral analysis directional Doppler. The transducer selection should allow for optimal imaging and Doppler analysis, and generally means using the linear-array, high-frequency (7–10 MHz) transducer for most patients. Occasionally, a low-frequency (3–5 MHz), curved-array transducer will be necessary if the patient is obese or there is considerable edema.
Patient positioning is very important, as proper positioning enhances the demonstration of abnormalities, eases the strain on the sonographer, and reduces time to complete the study. For lower extremity examinations, the patient is placed supine with the head elevated approximately 30°, if possible. The leg to be scanned is externally rotated with the knee flexed (Figure 23-8). For upper extremity studies, the patient is also placed supine, with the head turned away from the side being scanned. The chin should be raised (neck extended) slightly, if possible (Figure 23-9).
The first component of the lower extremity protocol involves transverse compressions in gray-scale imaging mode. Beginning at the femoral crease, with the transducer in a transverse (cross-section) orientation, identify the common femoral vein. There should be one artery and the saphenofemoral junction should be visible. Using gentle-to-moderate probe pressure applied toward the femur, observe the femoral vein compression with wall-to-wall contact. Identification of intraluminal echogenic material mandates gentle pressure only. Visualization of a mobile thrombus within the vein precludes further compression to avoid dislodging an embolus. If compressions are easy and complete, the probe can be moved a few centimeters distally, and the division of the common femoral vein into the deep and superficial femoral veins can be observed. Repeat the compression maneuver every 5 cm, proceeding distally along the superficial femoral vein. At the distal thigh, the vein traverses the adductor canal and enters the popliteal space. At this point, position the transducer in the popliteal space, identify the popliteal artery and vein, and compress the vein to assure wall-to-wall contact.
The second phase of the lower extremity venous examination evaluates flow characteristics using color Doppler imaging and Doppler spectral analysis. The transducer is returned to the groin and the common femoral vein is again identified in the transverse view. The transducer is then rotated 90° to obtain a longitudinal (sagittal) image of the vein (Figure 23-10). Observe the Doppler signal for flow that is spontaneous and phasic with respiration. A Valsalva maneuver will halt flow at end-inspiration and will demonstrate augmented flow when respiration resumes. Flow can also be augmented by squeezing the calf muscles. This should produce a spike in the spectral signal. These observations are made in the superficial femoral and popliteal veins to complete the study (Videos 23-3 and 23-4).