Venous Diagnostic Tools

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. Diagnostic testing is used to identify, grade, and follow venous insufficiency and to define deep venous thrombosis (DVT). Because more patients will be presenting for therapy as a result of improved outcomes with endovenous techniques over traditional surgery, diagnostic testing will take on increasing importance. For the purpose of this chapter, diagnostic testing includes the various plethysmography devices, color flow duplex imaging, intravascular ultrasound, and cross-sectional imaging. The goal of these studies is to provide accurate information describing the hemodynamic or anatomic characteristics of the patient with chronic venous insufficiency ( Fig. 4.1 ).

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. 4.2 and 4.3 ).

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 (183 cm), the distance from the level of the right atrium to the ankle is 120 cm, and this produces a hydrostatic pressure of approximately 90 mm Hg ( Fig. 4.4 ). 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.

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. Based on 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 caused by 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 ( Figs. 4.4 and 4.5 ).

Plethysmographs are devices that measure volume change. During the past 50 years, plethysmographs have been developed and used clinically with 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 versus 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 the circumference of a limb segment, which is related to the segment cross-sectional area. The cross-sectional area multiplied by length equals the 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 electrical 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) can 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. 4.6 and 4.7 ).