Equipment

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

FIGURE 24-3 Analog waveform from a continuous-wave Doppler using a zero-crossing detector and quadrature phase separation to show direction of flow.

(Modified from Scissons R: Physiological testing techniques and interpretation, North Kingstown, RI, 2003, Unetixs Educational Publishing.)

Patient Positioning

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.

Examination Technique and Diagnostic Criteria

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.

FIGURE 24-4 Lateral view of a lower extremity showing placement of an upper-thigh tourniquet used to compress and stop blood flow through the great 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.

Advantages

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

Limitations

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.

Equipment

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.

FIGURE 24-5 Medial view of the lower calf demonstrating correct placement of a photoplethysmographic sensor used for the evaluation of venous refill time.

Patient Positioning

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.

Examination Technique and Diagnostic Criteria

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.

Advantages

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.

Limitations

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.

Equipment

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).

FIGURE 24-7 Air-filled calf cuff and instrumentation required for air plethysmographic evaluation. A, Correct placement of the air-filled cuff on the patient’s calf. B, Air-calibrated pressure transducer, amplifier, recorder, and display system.

Patient Positioning

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.)

Examination Technique and Diagnostic Criteria

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