24 Ultrasound Diagnosis of Venous Insufficiency
The term chronic venous insufficiency is associated with a form of venous dysfunction that has been widely researched and yet is poorly understood. Most often, the term refers to venous valvular incompetence in the superficial, deep, and/or perforating veins. Incompetence of the vein valves permits reversal of flow and promotes venous hypertension in distal segments. This form of venous dysfunction may be the result of recanalization of thrombosed venous segments, pathologic dilatation of the vein, or the congenital absence of competent valves. It is important to understand that venous valvular incompetence may occur alone or in association with venous obstruction. Venous insufficiency is associated with physical findings that are characteristic, yet these findings are nonspecific with respect to cause. They do not differentiate between obstruction and valvular incompetence, nor do they define the location or extent of valvular dysfunction.
Historically, chronic venous insufficiency was evaluated using methods that were inaccurate, nonspecific for incompetence or obstruction, or invasive and associated with patient discomfort and poor acceptance. For these reasons, investigators have pursued a variety of noninvasive vascular procedures that have defined lower extremity venous flow dynamics globally or segmentally. As a pathway to understanding these laboratory procedures, it is important to review the mechanism of venous valvular incompetence.
Normal venous anatomy and physiology were described in Chapters 20 and 21. It is necessary to appreciate that venous valves are present throughout the lower extremity venous system in the deep, superficial, and perforating veins. The concentration of valves is higher in the calf veins than in the deep veins of the thigh.
Ambulation results in activation of the calf muscle pump. With calf muscle contraction, venous blood is propelled, or augmented, toward the heart. The valves distal to the contracting muscles, and those in the perforating veins, close to prevent reflux. This reduces venous pressure in the foot from the pressure of a standing column of blood, approximately 90 mm Hg while standing at rest, to 20 to 30 mm Hg during walking. During muscle relaxation, there is slow filling of the venous system from arterial inflow, but venous pressure remains low if the valves are competent. In the limb with chronic venous insufficiency, incompetent valves allow blood to move from the deep to the superficial system during muscle contraction. During relaxation, incompetent valves in the deep, superficial, and perforating veins allow blood to flow back toward the foot. This results in an uninterrupted column of blood with gravity and hydrostatic pressure causing persistently elevated venous pressure, both at rest and during exercise. Venous hypertension may lead to leakage of protein-rich fluid and blood cells through the capillary walls into the intercellular space. The immediate result is soft tissue edema, but the long-term result is skin thickening and hyperpigmentation, and, ultimately, skin ulceration. The pathogenesis of stasis-related ulceration is not well understood, but the chronic debilitating effects of ulceration are easily appreciated.
Chronic venous insufficiency may affect only the superficial veins, or it may be related to deep venous thrombosis. The valves below the knee are most often implicated in the clinical sequelae of venous thrombosis. Patients who develop ulceration following an episode of deep venous thrombosis quite often exhibit both deep venous incompetence and incompetence of the great and small saphenous veins. It is of interest to note that patients without significant incompetence of the deep veins below the knee may not suffer from ulceration if there is normal valvular function in the superficial veins.
Patients with chronic venous insufficiency typically present with symptoms of leg pain and clinical signs of edema, dilated veins, and skin changes in the region of the ankle. Patients with incompetent venous valves involving the superficial, perforating, and deep venous systems may demonstrate the full spectrum of signs and symptoms, whereas those with only segmental incompetence of the superficial veins may experience lesser degrees of disability.
Mild swelling in the region of the ankle is usually the first sign seen in patients with valvular dysfunction. The edema usually resolves with bed rest or with elevation of the limb. In patients with severe venous insufficiency, the swelling may involve the lower limb to the midcalf level and may or may not be associated with pitting in response to moderate pressure applied to the skin.
The increased venous pressure that results from incompetence of the superficial vein valves causes dilatation of the superficial veins in the distal extremity (Figure 24-1). This is generally noted first on the medial aspect of the lower calf and around the ankle. With progressively worsening dysfunction, the veins become enlarged and tortuous.
Patients with valvular dysfunction frequently complain of a feeling of heaviness and aching in the legs after prolonged standing or sitting with the legs dependent. In patients with valvular incompetence in the absence of venous obstruction, the feeling may subside with walking or with elevation of the limb, actions that relieve venous congestion. In contrast, if the deep veins are obstructed, exercise results in venous claudication, consisting of severe cramping, burning pain that persists as long as the veins remain congested. Several investigators have shown that venous claudication is caused by a rapid increase in pressure in both the superficial and deep venous systems.1,2 This is usually the result of obstruction of the iliofemoral venous segment with inadequate collateral compensatory flow.
The goals of the noninvasive evaluation of patients with symptoms of venous insufficiency are to define which venous systems are involved (superficial and/or deep), the anatomic level of dysfunction, and whether the pathologic process includes both incompetence and obstruction.
Historically, investigators relied on the invasive procedures (namely, ascending and descending venography and ambulatory venous pressure measurements) to evaluate chronic venous insufficiency. Venography was considered to be the gold standard for visualization of anatomy, confirmation of the presence of venous obstruction and collateralization, and definition of the location and extent of valvular reflux. Ambulatory venous pressure measurements were used as a hemodynamic complement to the anatomic information obtained from venography.3 Pressures could be measured with the patient at rest in a supine position, while standing, and during exercise. This procedure had value as a means for recording venous pressure recovery time, which has been used as the basis for more recent plethysmographic studies.
The modern vascular laboratory evaluation of venous insufficiency has evolved steadily from continuous-wave Doppler velocimetry to indirect, plethysmographic procedures and finally, to quantification of venous reflux using duplex ultrasound imaging. Although duplex sonography is currently the most accurate method for assessing venous incompetence, continuous-wave Doppler remains in use as a convenient, “low-tech” method for diagnosing venous reflux. This technique is therefore included in this chapter. Plethysmographic methods also remain in clinical use as a means of assessing venous hemodynamics, and for that reason, they too are discussed.
Bidirectional, continuous-wave Doppler uses separate transmitting and receiving crystals that operate continuously to detect flow at all depths along the emitted sound beam (Figure 24-2). Because of this, the signal that is received may contain echoes from more than one vessel lying within the beam path. The depth of penetration of a sound beam is inversely proportional to the carrier Doppler frequency. For this reason, lower-frequency transducers (3-5 MHz) are required when studying the deeper veins of the thigh, whereas higher frequencies (8-10 MHz) may be used for evaluating the superficial and calf veins of patients with a normal body habitus.
FIGURE 24-2 Diagram of a continuous-wave (CW) Doppler demonstrating transmitting and receiving crystals with the ultrasound beam intersecting both an artery (red) and a vein (blue). Rx, receiving crystal, Tx, transmitting crystal.
Quadrature phase separation is used to detect the direction of flow. The analog waveform of the Doppler signal is displayed using a frequency-to-voltage converter and a zero-crossing detector. The voltage output is proportional to the number of zero crossings (Figure 24-3). This display method is highly dependent on the signal-to-noise ratio and on the amplitude of the return signal.4,5
Patients are examined in a warm room while lying supine in reverse Trendelenburg’s (10-15 degrees, feet down) position or standing to promote venous filling. In the supine reverse Trendelenburg’s position, the patient’s head is slightly elevated and the legs are externally rotated at the hip with the knees comfortably flexed. In the upright position, the patient should initially face the examiner with the body weight supported mainly on the contralateral leg. The limb must remain immobile throughout the examination to prevent muscle contraction and inadvertent augmentation of venous flow. The examination is facilitated if the patient stands on a platform that is approximately 2 feet high with a support railing on three sides.
The examination is initiated with the continuous-wave Doppler probe placed over the femoral vein pointing toward the head (cephalad) at an angle approximating 45 degrees to the skin. The identification of the vein is confirmed by first insonating the common femoral artery (noting the pulsatile, multiphasic, caudad flow signal) and then moving the probe in a medial direction to locate the common femoral vein. Care must be taken to avoid pressure on the probe, because the veins are quite easily compressed.
Normal venous blood flow is spontaneous and phasic during respiration, yielding a wind-like audible Doppler signal. Manual compression of the limb below the probe should augment forward flow, with resultant increased amplitude of the audible Doppler signal. When the limb is compressed above the probe, the Doppler signal will normally cease, because competent valves restrict retrograde venous flow. When compression above the probe is released, an augmented, forward flow signal should be noted. Blood flow signals will also decrease when the patient coughs or performs a Valsalva maneuver. Both actions cause an increase in intra-abdominal pressure, which restricts escape of blood from the lower limb. The same compression and/or respiration procedure is repeated over the femoral, popliteal, and posterior tibial veins and in the regions of the saphenofemoral and saphenopopliteal junctions. Spontaneous Doppler signals are most often present in the larger-diameter thigh and popliteal veins. If the patient is examined in a cool room, however, vasoconstriction may reduce extremity blood flow, and augmentation of flow signals may be required to confirm patency of the small-diameter tibial veins.
If the valve immediately distal to the site of probe placement is incompetent, reversed blood flow signals will be noted with compression of the limb above the probe. Given this, a retrograde Doppler flow signal over the common femoral vein following an appropriately performed Valsalva maneuver should suggest an incompetent valve immediately distal to that site. Moreover, the presence of a single competent valve at any site proximal to the probe will prevent reflux and may lead to a false-negative result.
The anatomic location of valvular incompetence can be inferred by simple compression maneuvers that exclude the superficial venous system during the examination. The continuous-wave Doppler probe is placed over the region of the saphenofemoral junction, and the presence of retrograde flow is confirmed with release of compression of the limb below the level of the probe. A tourniquet is placed around the limb approximately 10 cm distal to the expected location of the saphenofemoral junction and tightened sufficiently to compress the great saphenous vein (Figure 24-4). Compression of the limb below the level of the probe is repeated. The continued presence of reversed blood flow signals suggests incompetence of the common femoral and/or proximal femoral vein(s). If retrograde blood flow is abolished by tourniquet application, incompetence of the great saphenous vein is suggested. The saphenopopliteal junction should be examined in a similar manner to distinguish popliteal/gastrocnemius reflux from incompetency of the small saphenous vein.
Absence of a Doppler signal along the anatomic course of a vein suggests occlusion of the vessel. Remembering that the major arteries and veins course together through the lower extremity, a venous signal found at a distance of more than 1 cm from the corresponding artery suggests the presence of a large collateral and occlusion of the primary vein. Low-amplitude Doppler signals may imply partial thrombosis, a collateralized venous occlusion, or a recanalized vein.
In the hands of experienced examiners, bidirectional, continuous-wave Doppler velocimetry has been shown to have excellent sensitivity (92%) with acceptable specificity (73%) for the assessment of venous incompetence.6 While some applaud this method as a valuable portable tool for detection of valvular incompetence or obstruction of the deep and superficial veins,7–9 others note that the continuous-wave Doppler test is extremely operator dependent and subjective.10
It is important to be aware of the considerable limitations associated with continuous-wave Doppler examination of the extremity veins. Because this is a nonimaging modality, there is no way to be certain which veins are being insonated. Duplication of the deep and superficial veins is common, and a Doppler signal may be elicited from a patent vein that lies adjacent to a thrombosed venous segment or from a large collateral vein. It is often quite difficult to differentiate reflux in the deep venous system from reflux in a superficial vein or major tributary at the saphenofemoral and saphenopopliteal junctions. Similarly, incompetence of large perforating veins may be confused with reflux in the saphenous or deep veins. Finally, standardization of the testing protocol is not possible because of the variability associated with tourniquet application. There is no assurance that the superficial veins are adequately compressed or that the compression does not obliterate flow in the deep venous system or perforating veins.
Photoplethysmography is a relatively simple tool used to screen for valvular incompetence. This technique employs an infrared light-emitting diode, with a second diode used to sense light reflected from subdermal venous flow. The photoplethysmographic probe is most commonly affixed to the skin in the supramalleolar region, using double-stick tape (Figure 24-5). The plethysmograph is coupled to a direct current recorder (DC mode) to track the average changes in reflected light that occur over time in association with alterations in blood flow volume. In the normal limb, the volume of blood in the skin decreases in response to manual compression of the calf or dorsiflexion of the foot and ankle. In the absence of obstruction to arterial inflow, the venous microcirculation refills slowly. If venous valves are incompetent, however, reflux occurs and the microcirculation refills rapidly. The quality of venous emptying with calf muscle compression can be assessed subjectively, and the length of time required for venous refill can be calculated from the calibrations on the strip chart recording.
The patient is seated forward on a bed or examination table with the legs unsupported. The photoplethysmographic sensors are affixed to the medial aspect of the leg above the malleolus. Care is taken to avoid positioning the sensor over regions of inflammation or ulceration.
The patient is initially requested to relax the limb while a baseline tracing is recorded on the plethysmographic strip chart recorder. The stylus for the recorder is positioned near the top of the tracing.
The patient is then requested to dorsiflex the foot four or five times. This causes the calf muscles to contract, simulating ambulation, and empties the calf veins in normal individuals. Manual calf compression can be used for patients who are unable to achieve adequate emptying of the venous pool with dorsiflexion. When the leg is relaxed and immobile, the calf veins refill. The venous refilling time is defined as the number of seconds required for the photoplethysmographic tracing to reach a stable endpoint for at least 5 seconds. The refill time is measured from the time exercise ceases to the stable endpoint (Figure 24-6, A). As noted previously, normally there is a rapid reduction of venous volume (and venous pressure) with limb exercise. Capillary refilling is primarily a function of arterial inflow when vein valves are competent and venous refilling is relatively slow. In patients with competent deep and superficial veins, the venous refill time is lengthened and usually exceeds 20 seconds.
FIGURE 24-6 Strip chart recordings of photoplethysmographic measurement venous refill time (VRT). Note the placement of calipers at completion of exercise and at a stable endpoint. A, Normal venous refill time, exceeding 20 seconds. B, Abnormal response consistent with venous reflux. Venous refill time is only 6.8 seconds.
(From Scissons R: Physiological testing techniques and interpretation, North Kingstown, RI, 2003, Unetixs Educational Publishing.)
A venous refill time less than 20 seconds suggests venous insufficiency (Figure 24-6, B). Superficial venous reflux can be differentiated from deep venous reflux by application of tourniquets to compress the great and small saphenous veins. A tourniquet (latex tubing or blood pressure cuff inflated to 45 mm Hg) is initially placed above the knee. The test is repeated as described previously. If the venous refill time normalizes to longer than 20 seconds, the superficial venous system is implicated as the source of incompetence. If the refill time improves but does not normalize, the data imply that both the deep and superficial systems are incompetent. The tourniquet is then moved below the knee. If the refill time normalizes, this is diagnostic of superficial venous incompetence alone. If the refill time remains less than 20 seconds with tourniquet compression of the superficial veins, this suggests deep venous insufficiency.
Photoplethysmographic determination of venous refill time correlates with ambulatory venous pressure measurements.11 The application is technically simple, and the equipment is inexpensive and portable. This modality serves as a useful screening tool for evaluation of patients in whom venous insufficiency is suspected on the basis of history or physical findings.
While attractive as a screening tool because of its technical simplicity, photoplethysmographic assessment of venous refill time has significant limitations. Most notable is the fact that it is a subjective and nonquantitative modality. It also is not capable of anatomically localizing the site of incompetence. As with bidirectional continuous-wave Doppler, the technique cannot be standardized because of variability in sensor placement and tourniquet pressure. The sensor may be placed over incompetent perforators or a region of localized inflammation or ischemia. There is no assurance that the superficial veins are compressed or that the deep veins remain patent with tourniquet application. In addition, it must be recognized that the results of photoplethysmographic studies may be influenced by body temperature, with alterations of blood flow and filling time occurring in response to vasodilatation and/or vasoconstriction.
Air plethysmography (APG) was first introduced in the 1960s to study lower extremity volume changes that occur in response to alterations in posture and muscular exercise. Once it became possible to calibrate the system, interest was renewed in this noninvasive modality that could replace the older diagnostic devices such as strain gauge, segmental volume, and water plethysmographs. Christopoulos and colleagues12 introduced APG as a diagnostic tool in 1987 to detect global limb volume changes that occur with exercise and gravity.
With respect to venous incompetence, APG measures the following: (1) calf venous volume, (2) the rate at which calf venous volume is restored normally or as a result of reflux, (3) the effectiveness of the calf muscle pump, and (4) ambulatory venous pressure (indirectly).
APG uses an air-filled, polyvinyl cuff, which surrounds the calf and functions as a sensing device to detect calf volume changes. The cuff is connected to an air-calibrated pressure transducer, amplifier, and recorder (Figure 24-7).
The patient initially reclines in the supine position with the heel slightly elevated on a support and the limb externally rotated and flexed to allow application of the cuff. Volume changes in the limb are recorded during limb elevation (which empties the veins), venous refilling, and a series of maneuvers with the patient upright, as shown in Figure 24-8 and described in the following section.
FIGURE 24-8 Top row: Typical positions of the lower limb during an air plethysmographic study. Upward arrows indicate transient elevation of the leg. Bottom row: Resulting air plethysmographic tracings and measurement of the venous filling index (VFI), ejection fraction (EF), and residual volume fraction (RVF). EV, ejection volume; RV, residual volume; VFT, venous filling time; VV, venous volume.
(From Christopoulos DG, Nicolaides AN, Szendro G, et al: Air plethysmography and the effects of elastic compression on venous hemodynamics of the leg, J Vasc Surg 5:148-159, 1987.)
With the patient’s heel supported and the limb properly positioned, the air-filled cuff is adjusted over the calf so that it encloses the calf from the knee to the ankle. The patient’s limb is elevated 45 degrees to empty the calf veins (see Figure 24-8). Maximal venous emptying is indicated when the baseline recording stabilizes. The patient is then quickly brought to a standing position with the weight supported on the opposite limb. Filling of the calf veins is recorded continuously until a steady baseline is again obtained. This indicates that functional venous volume (VV) has been reached. Venous filling should result in an increase in leg VV of 100 to 150 mL in limbs with competent vein valves and 100 to 350 mL in limbs with venous insufficiency.
The venous filling index (VFI) is the ratio of 90% of the VV divided by the time required to achieve 90% venous filling (venous filling time, or VFT90%). The VFI, which evaluates overall valvular competence, is calculated from the equation VFI = 90% VV/VFT90%. This measurement of average filling rate is expressed in milliliters per second. A VFI of 2 mL/sec or less indicates normal valvular function, while a VFI greater than 7 mL/sec is consistent with deep and/or superficial incompetence and is associated clinically with symptoms of chronic venous insufficiency. Application of a narrow below-knee tourniquet to occlude the small and great saphenous veins may reduce the VFI to less than 5 mL/sec in limbs with incompetent common femoral vein valves but competent popliteal valves.13 Christopoulos and associates12 found that a VFI between 2 and 30 mL/sec was associated with superficial venous incompetence, while patients with a VFI between 7 and 28 mL/sec had evidence of deep venous insufficiency.
After the measurements cited previously are obtained, the patient is asked to rise up once on the toes and return to normal position. This maneuver activates the calf muscle pump, which decreases venous volume. The ejection volume