Historical Background

Of the 25 million Americans with venous disease, approximately 7 million exhibit serious symptoms such as edema, skin changes, and venous ulcers. One million seek formal medical advice annually. Of these, 80% of venous patients are managed conservatively with observation, leg elevation, and support stockings, while the remainder are treated surgically with vein stripping or endovenous ablation. With the development of safe, less traumatic, and effective endovenous techniques for venous insufficiency, most venous surgeons acknowledge that more patients will seek treatment, and physicians will be more inclined to move from conservative therapy to surgical therapy.

Physiologic testing is used to identify, grade, and follow venous insufficiency and to define deep venous thrombosis (DVT). Since more patients will be presenting for therapy because of improved outcomes with endovenous techniques over traditional surgery, physiologic testing will take on increasing importance. For purposes of this chapter, physiologic testing includes the various plethysmography devices and color flow duplex imaging. The goal of these studies is to provide accurate information describing the hemodynamic or anatomic characteristics of the patient with chronic venous insufficiency, precluding the need for invasive studies² (Fig. 2-1).¹

Fig 2–1 Flowchart of chronic venous insufficiency.

Etiology and Natural History of Disease

The venous system in the lower extremities is composed of three interconnected parts: the deep system, the perforating system, and the superficial system. In healthy veins, blood flows toward the right side of the heart (i.e., upward) and from the superficial system to the deep system (i.e., inward), driven by the venous muscular pump and unidirectional valves. Lower extremity muscle compartments contract during ambulation; this contraction compresses the deep veins, producing a pumping action, which propels blood upward toward the right side of the heart. Transient pressures in the deep system have been recorded as high as 5 atmospheres (atm) during strenuous lower extremity exertion. This pumping action secondary to ambulation has the effect of reducing pressure within the superficial system (Figs. 2-2 and 2-3).

Fig 2–2

Fig 2–3

All three venous systems of the lower extremity are subjected to hydrostatic pressure. A fluid column has weight and can produce a pressure gradient. In an individual who has a height of 6 feet, the distance from the level of the right atrium to the ankle is 120 cm and produces a hydrostatic pressure of approximately 90 mm Hg (Fig. 2-4). Deep veins can withstand elevated pressure because the fascia in which they exist limits dilation. In contrast, the superficial system, surrounded by fat and elastic skin, is constructed for low pressure; therefore, elevated pressure in the superficial system can produce dilation, elongation, and valve failure. Dilation increases the diameter of the veins and elongation causes them to be more tortuous.

Fig 2–4

Because of valve failure, supraphysiologic pressure develops in the superficial venous system and venous dilation ensues (other theories suggest that it is the vein wall that fails with subsequent loss of valvular coaptation). With dilation and multiple valve failure, venous blood will flow in the direction of the pressure gradient, which is downward and outward. This flow direction is directly opposite physiologic flow (i.e., upward and inward). The early result is varicose veins and telangiectasia, which are visible on the skin surface. Symptoms of early or mild superficial venous incompetence produce low-level pain, edema, burning, throbbing, and leg cramping. As the disease progresses, patients can develop venous stasis changes that can lead to debilitating severe soft tissue ulceration. On the basis of hemodynamics and clinical experience, symptoms can improve dramatically on elimination of high pressure or flow in diseased superficial venous channels.

Instrumentation

Plethysmography

To understand lower extremity venous hemodynamics, venous pressure measurements by dorsal foot vein cannulation can be instructive. The cannula tubing is connected to a fluid column. With the subject standing erect, the fluid column will rise to the level of the right atrium. This is due to the fact that right atrial pressure is near zero and, therefore, the dorsal foot vein pressure at the cannulation site is almost entirely based on the subject’s hydrostatic blood column (the subject’s blood and the fluid in the column have nearly the same specific weight). When the subject is asked to perform repeated ankle flexion, the fluid column drops to between 50% and 60% of its resting height. This simulates walking and the reduction in superficial venous pressure secondary to the ambulatory venous pump. In subjects with venous insufficiency, the fluid column will not drop to normal levels. If a subject’s fluid column falls to normal levels during occlusion of the superficial system, the observer knows the deep system is intact and the superficial system is incompetent. If the fluid column remains elevated with exclusion of the superficial system, the observer knows the deep system is incompetent. Physiologic venous testing is based on these principles (Fig. 2-5).

Fig 2–5

Plethysmographs are devices that measure volume change. During the past 50 years, plethysmographs have been developed and used clinically that employ completely different principles. Descriptions of four plethysmographs are given next.

Impedance Plethysmograph

The impedance plethysmograph (IPG) is based on a fundamental principle of electronics, which states that voltage (V) across a segment is equal to the impedance (Z) of the segment multiplied by the current (I) flowing through the segment (V = Z × I). It is possible to isolate a portion of a limb (e.g., thigh, calf) and subject the limb segment to a standard and known current while measuring the voltage across the segment. Blood, subcutaneous tissue, and even bone v impedance. In practice, the operator places circular electrodes around the segment of interest, generally the proximal calf, and connects the electrodes to the electrical console. The subject is asked to perform a series of maneuvers, and outputs from the device are recorded. This method has been used with success by some investigators in the assessment of DVT and venous insufficiency.

Straingauge Plethysmograph

A straingauge plethysmograph (SGP) measures circumference of a limb segment, which is related to segment cross-sectional area. Cross-sectional area multiplied by length equals volume. The device is constructed using a small hollow elastic tube filled with mercury and an electrical circuit capable of measuring voltage across the tubing length. The tube containing the mercury is carefully placed around the limb segment of interest and connected to the electric circuit. The subject is asked to perform a series of maneuvers and outputs from the device are recorded. As the limb segment circumference is changed secondary to venous blood volume, the length of the elastic tube changes. By measuring circumference as a function of time, venous blood volume as a function of time may be measured.

Photoplethysmograph

Photoplethysmographs (PPGs) are not true plethysmographs because they measure cutaneous microvasculature. PPG instrumentation includes a surface transducer, which is taped to the lower leg just above the medial malleolus and connected to an electrical circuit. The electrical circuit excites the transducer and records and interprets the returning signal. The PPG transducer is designed with an infrared light emitting diode and a photosensor. The transducer transmits light to the skin, which is both scattered and absorbed by the tissue in the illuminated field. Blood is more opaque than surrounding tissue and therefore attenuates the reflected signal more than other tissue in the field. The intensity of reflected light is reduced with more blood in the field. If the electrical circuit filters the higher frequency arterial pulsations, it is possible to register a signal, which qualitatively corresponds to venous volume in the segment of interest.

Air Plethysmograph

An air bladder (cuff) is connected to a console via a single rubber tube, and any change in limb volume is measured by a pressure change within the bladder. If limb volume increases, the bladder volume will decrease, but the bladder pressure will increase. An air plethysmograph (APG) is able to detect changes in venous limb volume secondary to various patient maneuvers. The APG is used in the clinical assessment of venous insufficiency and DVT. The use of the APG is based on the use of air bladders, which are devices similar to standard blood pressure cuffs. Physiologic parameters related to chronic venous disease such as chronic obstruction, valvular reflux, calf muscle pump function, and venous hypertension can be measured (Figs. 2-6 and 2-7).

Fig 2–6

Fig 2–7

Deep Venous Thrombosis by Plethysmography

The deep venous system is not only a conduit for returning blood to the right side of the heart but is also a storage or capacitant system. This means its volume changes rapidly relative to pressure. If one examines a vein at low pressure, the walls are nearly fully collapsed and only a small flow channel is present. It takes very little increase in internal fluid pressure to expand the flow channel of a vein. Finally, if there is obstruction in a segment of deep vein, despite rich venous collateral channels, venous pressure distal to the obstruction will increase. Examination by plethysmography makes use of these two principles (i.e., volume change with increased pressure and resistance).

Typically, a plethysmograph transducer is placed at the calf or distal thigh with the patient lying supine on a table. In the case of APG, the transducer is an air bladder inflated to 5 mm Hg; in the case of PPG, the transducer is a light-emitting diode. Proximal to the transducer, a method of rapidly occluding the deep system must be used. For all transducers, this can be a thigh cuff inflated rapidly by hand bulb or automatic inflator.

With the transducer recording a stable venous signal at 5 mm/s chart speed, the pressure in the proximal occluding cuff is rapidly elevated to 50 mm Hg. The transducer is measuring absolute levels of volume. With the increased pressure in the proximal cuff, venous blood in the deep system cannot pass under the cuff until the venous pressure reaches approximately occluding cuff pressure. This increase in venous pressure (i.e., pooling) develops because the proximal cuff does not obstruct the arterial inflow. After about 20 to 40 seconds, pressure in the distal venous system reaches the pressure in the occluding cuff and venous volume reaches a plateau. Once the plateau has been reached, the operator rapidly releases the pressure in the occluding cuff. The pooled venous blood can then return to the right side of the heart via the larger veins upstream. Two measures of venous hemodynamics are taken during this test. First, there is the volume increase from the baseline to the plateau. This is known as segmental venous capacitance and represents the blood storage capacity of the segment vein. This is generally quoted in millimeters of deflection or milliliters if the system is calibrated to volume. The second measurement is the slope of the volume-time curve immediately after the pressure in the occluding cuff is released. This is known as maximum venous outflow (MVO) and represents resistance to blood flow in the deep system. This may be quoted in millimeters of deflection per second or milliliters per second if the system is calibrated to volume.

This technique has been largely replaced by duplex ultrasound, which is much more sensitive and specific for the detection of DVT.

Continuous-Wave Doppler

Continuous-wave (CW) Doppler instruments are widely available, relatively inexpensive, and used extensively to rapidly investigate the peripheral vascular system. CW Doppler measurements can be used independently or, as mentioned earlier, combined with measurements from a plethysmograph.

Duplex Ultrasound

Duplex ultrasound has become the gold standard in the diagnosis of both DVTs and venous insufficiency and has replaced the use of venous plethysmographs, CW Dopplers signals, and contrast venography. Power color pulsed-wave Doppler signals with high-resolution B-mode imaging characterize state-of-the-art duplex ultrasound.

Deep Venous System

The examination should be performed on a flat examining table in which the patient’s lower extremities are placed in the dependent position at approximately 15 degrees. This slight angle dilates the deep system, which makes the identification of veins easier and improves the velocity signals. Deep vein interrogation from the level of the inguinal ligament to the ankle should include the common femoral, femoral, popliteal, and tibial veins. The deep femoral vein should also be included, especially in cases where femoral vein thrombosis has been identified.

The evaluation begins at the groin using gray-scale imaging. Usually seen are the common femoral vein, common femoral artery, and great saphenous vein (GSV), forming a “Mickey Mouse image.” We have found keeping the lateral (arterial) structures on the left side of the screen for both the right and left leg to be helpful. This requires that the technologist rotate the linear array probe 180 degrees when moving from the right to the left leg. The marker on the probe should be oriented to the lateral aspect of the leg. With this orientation, Mickey’s face is the common femoral vein and is the larger and lower of the three structures. The common femoral artery forms Mickey’s right ear, and the GSV forms Mickey’s left ear. As the probe is moved distally, the GSV will disappear, and the common femoral artery will divide into the superficial femoral artery and the deep femoral artery. As the probe continues distally, the technologist should focus on keeping the superficial femoral artery and the femoral vein in clear view. The popliteal artery and the popliteal vein are difficult to visualize in the adductor canal; therefore, these structures are identified from behind by placing the probe in the popliteal crease. In the calf, the duplicated posterior tibial and peroneal veins with their associated single arteries can be viewed from a medial approach as they travel between the muscle bellies. Similarly, the gastrocnemius and soleus veins are identified; however, they are located within the muscular bellies. In general, the anterior tibial veins are not interrogated because they are rarely pathologic (Fig 2-8).

Fig 2–8 Mickey Mouse image.

With the probe, the technologist can compress the vein in the short axis view. The ability to fully compress the vein walls—and obliterate the venous lumen momentarily—confirms vein patency and absence of thrombus formation (Fig. 2-9). If the technologist identifies thrombus, the next step is to determine its age. Acute thrombi are characterized by vein dilatation and noncompressible echolucent material, while chronic thrombi take on a speckled, hyperechoic ultrasonic appearance.

Fig 2–9 Compression of femoral vein.

If the evaluated system from the common femoral vein through the tibial veins is compressible and no evidence of thrombus formation is seen, the study is considered negative for DVT. The technologist may use the Doppler portion of the duplex system in the long-axis view to verify artery versus vein and determine flow direction. Color Doppler signals, power Doppler signals, compression maneuvers, and respiratory maneuvers can be used to supplement this procedure if necessary. Normal veins have spontaneous flow, which is phasic with respiration.

Superficial Venous System

For superficial venous studies, patients are examined in the erect position. The patient is asked to rotate the leg of interest to expose the medial surface of the lower extremity from the groin to the ankle. To the extent possible, weight should be shifted from the leg of interest to relax the musculature. A standing stool with arm support may be necessary.

Once positioned, the technician begins at the groin and produces the Mickey Mouse landmark described earlier. Starting from the three-vessel image in the transverse view, the probe descends down the leg following the course of the GSV. The normal GSV extends from the saphenofemoral junction to the ankle and is enveloped by superficial fascia above and muscular fascia below. Diameter measurements are recorded in millimeters, and the presence of reflux (positive or negative) is documented at the saphenofemoral junction, midthigh, and below knee. If reflux is present, the duration of retrograde flow in seconds is also documented.

Reflux is determined at locations of interest using the following technique.² The technologist adjusts the color box of the Duplex system in the measurement location. The velocity scale is adjusted (maximum 25 cm/s). While a signal is being obtained, the technologist compresses the calf (below the probe) in a brisk manner. The vein highlighted in the color box should demonstrate an increase in velocity toward the heart with compression. On release, the vein should demonstrate no velocity or minimal velocity away from the heart. We have found that reflux (venous flow away from the heart after release) lasting between 0.5 to 2.0 seconds is mild. Reflux is severe if present longer than 2.0 seconds.

The same evaluation is repeated posteriorly for the small saphenous vein (SSV). This vein originates in the distal calf and can terminate in the upper thigh. We access this vessel with ultrasound by rotating the subject to expose the back of the legs. We identify the SSV at the distal calf and advance over the course of the SSV. Multiple levels may be assessed; however, we generally record a characteristic SSV diameter (in millimeters) and assess reflux in the most diseased location (Fig. 2-10).

Fig 2–10 Small saphenous vein.

It is important to note that there are variations in superficial venous anatomy. For example, the GSV may be quite small and complemented by an anterior accessory saphenous vein, which may be competent or incompetent. Further, the GSV may be duplicated in portions of its course. It is worth repeating that these variations are common and must be known and anticipated by the technologist if a comprehensive report is to be generated.

The lower extremity has some common perforators that play significant roles in venous insufficiency. Hunterian perforating veins are located in the midthigh. Dodd perforating veins are located at the distal thigh. The Boyd perforating vein is located below the level of the popliteal fossa. Finally, Cockett No. 1, 2, and 3 perforating veins are located between the ankle and the lower calf, respectively. This assessment must also be part of this work-up. If present, perforators should be assessed regarding diameter, degree of reflux, and extension to other superficial structures.

Duplex ultrasound is not only diagnostic but also plays crucial roles in endovenous ablation, ultrasound-guided sclerotherapy, and monitoring the success of vein closure procedures (Figs. 2-11 through 2-17).

Fig 2–11 Perivenous tumescent anesthesia.

Fig 2–12 Position of radiofrequency catheter at saphenofemoral junction.

Fig 2–13 Steam bubble formation during endovenous laser.

Fig 2–14 Thick vein walls after radiofrequency ablation of great saphenous vein (GSV).

Fig 2–15 No flow in saphenous vein by color flow duplex imaging (CFDI).

Fig 2–16 Ultrasound image of left common iliac vein compression from overlying right common iliac artery (RCIA). LCIA, Left common iliac artery; LCIV, left common iliac vein; RCIV, right common iliac vein.

Fig 2–17 A, Documentation of deep veins. B, Documentation of superficial vein mapping.