Plethysmography can be used to identify abnormal reflux in the lower extremity by examining volume changes when moving the limb from the supine to standing position. The test exploits the concept that the volume of the leg increases when placed in a dependent position. If venous valve function is good, this increase in volume occurs slowly compared to the rapid increase in volume that occurs with poor valve function. Again, plethysmography cannot identify the specific area of valve dysfunction but the overall sequelae of venous reflux. Protocols using calf muscle contraction can also be performed with plethysmography to examine calf muscle pump function as a further measure of venous hemodynamics. These measures can be useful in the separation of venous reflux from other potential causes of limb pain. A patient may have reflux in a venous segment, but if their plethysmographic test measures are normal, the refluxing vein may not be causing symptoms, and a search for other causes is warranted.

The technique of SGP employs a Silastic conductor tube connected to a plethysmograph via electrical contacts. As the gauge is stretched by a change in calf circumference, the resistance increases in the conductor and a voltage change is recorded. The strain gauge is calibrated such that a 1% increase in voltage corresponds to a 1% change in limb volume. The venous volume (VV) is defined as the difference between baseline volume and the volume at peak venous capacitance, while the maximal venous outflow (MVO) is measured from the steepest portion of the outflow curve [6].

SGP has primarily been utilized in the diagnosis of DVT, based on evaluation of the MVO. Normal values for venous volumes average 2–3% above baseline, while limbs with venous outflow pathology would record a VV of less than 2%. This, however, is an unreliable diagnostic criterion, with MVO being a more reliable diagnostic tool. Barnes and associates [7] reported the MVO to have a sensitivity of 90% for above the knee DVT, but only 66% for below the knee DVT, while the overall specificity was 81%. Rooke et al. [8] evaluated patients during exercise using strain-gauge plethysmography by plotting volume against time for each limb, calculating the volume of blood expelled and the time required for veins to refill following exercise. They observed that a shortened postexercise refilling time accurately identified limbs with incompetence, the clinical severity of incompetence was inversely related to refilling time, and the type of exercise performed had little effect on the study results.

Prolonged recumbency, postural changes, muscle wasting, arterial insufficiency, and cardiac failure may alter venous filling and lead to measurement errors with this technique. Thrombosis in tributary veins, partially occlusive clots, clots in one of a paired vein, or the presence of significant collaterals from previous episodes of thrombosis may not affect capacitance or outflow significantly and could result in normal or nearly normal VV and VO recordings. For these reasons SGP is rarely used for the measurement of venous obstruction or reflux at the current time.

This method measures changes in skin impedance which correlates with changes in volume. Changes in limb circumference produce a comparable change in the electrical resistance, which is proportional to relative volume change. Impedance plethysmographs consist of electrodes wrapped around the leg. An increase in blood volume decreases impedance and is amplified to reflect volume change.

Increased limb volume results in decreased resistance. Two electrodes are placed 10 cm apart on the calf to be evaluated. The voltage output is used to derive the resistance based on Ohm’s law (voltage = current × resistance) and is then displayed as a continuous tracing. A thigh occlusion cuff is inflated, and the tracing change across the electrodes is measured as it rises. After rapid cuff deflation, the tracing falls to baseline levels in 3–4 s.

The “rise” of the tracing from the baseline to the plateau measures venous capacitance, and the “fall” of the tracing from the peak value over a 3-s period measures venous outflow. These measurements are plotted on a graph with the vertical axis representing the “fall” and the horizontal axis representing the “rise” [9].

In general, high capacitance and outflow indicate normal venous function and a low probability of DVT; conversely, low capacitance and outflow are highly suggestive of abnormal venous function with a high probability of DVT. The sensitivity of IPG in diagnosing DVT ranges from 33 to 96%. The test is more sensitive in patients with symptomatic and proximal DVT. Asymptomatic patients or those with DVT below the knee have significantly lower test sensitivity. The difference can be attributed to non-occlusive thrombi and well-developed collaterals [9–11].

The inaccuracies of this technique are due to the same issues that affect SGP. For these reasons, given the reliability of duplex ultrasound techniques, SGP and IPG are now rarely used to diagnose DVT.

PPG uses light absorbance by hemoglobin as a reflection of blood volume in the venous and capillary networks in the skin to estimate the degree of venous stasis.

PPG uses a transducer that emits infrared light from a light-emitting diode into the dermis. The amount of light absorbed from the transducer is a good estimation of blood volume in the venous and capillary networks of the skin. The backscattered light is measured by an adjacent photo detector, and net absorption is displayed as a line tracing. Absorption of light is high when skin venous and capillary blood is increased during sitting or standing; conversely, it is decreased during exertion when venous blood is expelled from the limb by the action of the calf muscles.

The patient is asked to sit comfortably with the legs hanging freely. The transducer is applied to the leg, and a baseline recording is obtained. The patient is then asked to perform five consecutive ankle flexion/extension maneuvers. The tracing drops as the calf muscles empty the veins. The time taken to recover to 90% of the original baseline tracing after the exertion is completed is recorded as the venous refill time (VRT, Fig. 39.4) [12, 13].

In normal individuals, VRT tends to be longer than 20 s and can extend to as long as 60 s. Significant venous reflux results in a VRT of less than 20 s, and reducing times generally reflects increasing severity of reflux. In the presence of abnormal findings, the test can be repeated with a tourniquet inflated to 50 mmHg, thus eliminating reflux in the superficial system to identify the source of reflux (Fig. 39.5).

Although PPG can provide an assessment of the overall physiologic function of the venous system, it is most useful as a relatively simple measure to detect the presence of venous reflux. Quantitative measurements are not obtained. PPG measurements have not been proved to be a strong discriminator of the severity of CVI.

In addition to being inversely proportional to the degree of reflux, refill time is also affected by arterial inflow. In the presence of occlusive arterial disease, VRT may be reduced despite an absence of venous reflux. This test also requires full patient cooperation, and inability to perform the contraction maneuvers or inability to access the calf for proper placement of the transducer will preclude accurate results. VRT can vary depending on the site of photosensor placement, the small sample area obtained, and the type and amount of exercise performed during the recording. Placement near the site of a varicose or perforating vein may also affect the results. The test has limited reliability in differentiating deep from superficial reflux despite the addition of tourniquet testing. Because of its inability to reliably grade the severity of CVI, PPG has limited utility for assessing the results of corrective venous surgical procedures. Therefore, PPG is a reasonable measure of the presence or absence of CVI that is best used when no further information concerning the venous hemodynamic situation is desired. If information on the severity of CVI or evaluation of improvement after venous surgery is required, a quantitative test will be more useful [14].

This equipment has been used to assess for the absence of DVT where a refill time of greater than 20 s would suggest that a DVT is highly unlikely [15, 16]. Thomas et al. describe a prospective study over a 5-month period where 131 legs of 119 patients were assessed. The findings identified a negative predictive value (NPV) of 92% and therefore provided a sensitivity of 92%. The researchers also reported a high specificity of 84%, which suggests that the majority of those with a positive result did in fact have this condition. This high sensitivity was also found in a further study, but these researchers reported a lower specificity. In this study 103 limbs of 100 patients referred to the X-ray department with suspected DVT were assessed, each of the patients had ascending contrast venography (ACV) and color flow duplex imaging (CFDI) . In addition to these investigations, digital PPG (D-PPG) was performed [15]. Of the 103 limbs, 37 were found to have a DVT. All the patients with a venous refill time of greater than 20 s had normal ACV and CFDI demonstrating 100% sensitivity. Therefore, the D-PPG provided a negative predictive value of 100% validating it as a screening tool in the diagnosis of DVT. Of the patients identified as positive, i.e., those with a venous refill time of less than 20 s, only 51% went on to have a DVT confirmed by ACV and CFDI, thereby gave a positive predictive value and specificity of 51%. The researchers concluded by stating that a negative D-PPG effectively excludes a DVT, and a positive test requires further confirmation.