Pressure and stiffness of a new air-inflated compression wrap

Introduction

Compression therapy is one of the most effective treatment modalities in the conservative management of venous-lymphatic disease. ¹ Compression may be exerted by applying different kinds of devices to the affected leg such as medical compression stockings (MCSs), inelastic bandages (IBs), short stretch bandages (SSBs), multicomponent bandages (MCBs), and adjustable compression wraps (ACWs). In the clinical routine, MCSs are mainly used for symptomatic varicose veins, edema, and lipodermatosclerosis. Medical compression stockings are also indicated when acute events (deep venous thrombosis in the acute stage) and chronic events (leg ulcers, post-thrombotic syndrome, and lymphedema maintenance phase) have been solved, and it is necessary to maintain the results and prevent recurrences.^1,2

When a strong hemodynamic effect is required in the treatment of venous-lymphatic disease, compression by inelastic materials, such as IB, SSB, and MCB, has to be preferred due to their significantly higher hemodynamic effectiveness.^3–6

Unfortunately, all the abovementioned systems are burdened with the following issues. Elastic stockings have a very low stiffness and they are unable to overcome the intravenous pressure; thus, their hemodynamic effect will be inadequate and, as such, should not be used in the early treatment stages. ⁷ Inelastic bandages require skills to be correctly applied and thus the health care providers need to be instructed. As education is very often missing, they are usually poorly applied exerting a much lower pressure than recommended.^8,9 Furthermore, they lose pressure overtime due to leg volume reduction.^10–12 Adjustable compression wraps are easy to apply and able to maintain the pressure overtime when the patients feel a looseness sensation and readjust the device. ¹³ But, at least some of them are not so stiff as they are claimed to be. They seldom have a static stiffness index >10 mmHg, that defines the inelastic materials, in all the available publications.^14–17

This study aims to report the exerted pressure and stiffness of a new compression device, specifically designed for leg compression, based on a hook-and-loop closure with a built-in air bladder (Aero-Wrap^®, Sun Scientific, NY, USA), being inflated by an external pump (AeroGauge^®, Sun Scientific, NY, USA) at pressures ranging from 20 to 50 mmHg.

Materials and methods

Protocol

Thirty healthy volunteers of both sexes were enrolled in this study (15 males and 15 females; age 44.6 years ± 12, range 22–65 years) with a BMI of 24.76 ± 3.02; the calf circumference was 34.1 cm ± 2.8 and the ankle was 22.6 cm ± 1.9 with a calf/ankle circumference ratio of 1.51 ± 0.08.

Study approval was obtained through the Institutional Review Board. The volunteers were informed about the details of the examination and gave their consent.

The pressure exerted by the tested device was measured in both legs, and data were obtained from 60 legs. PicoPress^® (MicrolabItalia, Padua, Italy) was used to measure the interface pressure in different conditions. It was connected to a computer provided with the PicoPress^® software able to record the whole test. The measuring probe had a diameter of 5 cm and was inflated with 2 mL air. It was placed under the compression device, in the most bulging area of the leg during muscle contraction. This area, conventionally named B1, is located in the medial aspect of the leg where the gastrocnemius tendon turns in its muscular part. It is usually located 10–15 cm above the inner malleolus (Figure 2). This point was chosen according to a published consensus document, ¹⁷ which was widely accepted.OPEN IN VIEWER

Figure 2.

Left side: three probes were attached to a cylindric plastic tube involving all the length of the compression device. Right side: after applying the device, the compression pressure was measured by the three probes.

As the air-bladder is located all along the compression device and the inflated air evenly distributed, we had preliminarily to verify if the pressure measured in one area is exerted throughout the length of the compression wrap. We first applied the device to a perfectly cylindric plastic tube as, on the human legs, some pressure variations are possible due to different radii on different leg sections. Then we did the same on the leg of some volunteers. Three probes were applied along the cylindric plastic tube (Figure 3) and the human leg (Figure 4), enclosing the whole area covered by the compression device and compression pressure was measured. The compression pressure of the device was preset in the range 30–40 mmHg on the plastic tube and in the range of 40–50 mmHg on the human legs.OPEN IN VIEWER

Figure 3.

Left side: three probes were attached to the leg of four volunteers at lower, mid, and high levels of the lower leg, involving all the length of the compression device. Right side: after applying the device, the compression pressure was measured by the three probes.

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Figure 4.

Probe position on the leg to measure the compression pressure and calculate the stiffness indices. The probe is placed on the medial aspect of the leg in the B1 point, the point where gastrocnemius tendon turns in its muscle, about 10–15 cm above the inner malleolus.

Following the assessment of the pressure distribution, the test started with the volunteers in the supine position. After measuring the ankle circumference, the compression device of the proper size was applied consecutively to both legs. A small support, 10 cm high, was placed under the thigh immediately distally from the knee to have the leg slightly elevated, bent, and externally rotated to ensure the best comfort and muscle relaxation for the examined subject. This leg position is necessary to avoid the contact between the calf and the bed. Indeed, the calf muscle distortion resulting from the contact between calf and bed could interfere with the supine pressure measurement. The volunteer was then asked to perform 10 foot-dorsiflexions. Minimal and maximal pressures during dorsiflexions were recorded. Afterward, the volunteer was asked to stand up equally distributing his weight on both legs. The interface pressure rises until it reaches its plateau and the standing pressure was recorded. The volunteer was then asked to perform 10 steps on spot and minimal and maximal pressures during walking were recorded. The whole test was performed three times, with one test for each one of the preset pressures of 20–30, 30–40, and 40–50 mmHg provided by the AeroGauge^®.

Results

In the preliminary assessment of the pressure distribution along the device, the pressure was consistent in the three different points of the plastic tube in three sequential measurements (Figure 3). On the human leg, the pressure was slightly different in the three tested areas due to different area radii but in the same range (pressure average 43 ± 4, 47 ± 5, and 44 ± 3 mm Hg in the lower, mid, and higher leg segments, respectively) (Figure 4).

Regarding pressure measurement in B1 point (and stiffness assessment), the average supine pressure was 32 mmHg, 41 mmHg, and 50 mmHg when the pressure provided by the AeroGauge^® was set at 20–30, 30–40, and 40–50 mmHg, respectively. The corresponding standing pressure was significantly higher than supine pressure at every pressure range (p < 0.0001): 50 mmHg, 60 mmHg, and 72 mmHg when supine pressure was preset at 20–30, 30–40, and 40–50 mmHg, respectively (Figure 5). The scatter plot with the Spearman’ diagram shows a good correlation between the preset and the actually exerted pressure (Figure 6). At the pressure ranges of 20–30, 30–40, and 40–50, SSI was 17 mmHg, 19 mmHg, and 22 mmHg, respectively; DSI was 27 mmHg, 30 mmHg, and 33 mmHg, respectively; WPA was 20 mmHg, 20 mmHg, and 24 mmHg, respectively (Figure 7).OPEN IN VIEWER

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Figure 6.

Scatter plot diagram and the Spearman's correlation coefficient between preset pressure and exerted for each pressure range. (a) Preset pressure of 20–30 mmHg; (b) preset pressure of 30–40 mmHg; (c) preset pressure of 40–50 mmHg.

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Figure 7.

Static stiffness index (SSI), dynamic stiffness index (DSI), and walking pressure amplitudes (WPASs) of the new compression device when inflated at different preset pressures. Preset pressure ranges are on the x-axis. SSI, DSI, and WPA are shown in the box and whiskers plot. The table under the figure reports median values and interquartile ranges and mean and standard deviation.

Discussion

Applied pressure, its maintenance overtime, and material stiffness are the most important parameters of compression therapy. Compression pressure must be known as it is the dosage of compression and has to be adapted to the patient’s clinical condition. Actually, it can be low (<20 mmHg) in early venous disease stages, for example, in occupational edema or mild venous edema,^27,28 but it must be high or very high (at least >40 mmHg) in patients with skin damage or venous leg ulcers.^29–31

Unfortunately, we can have an approximate idea only about the pressure exerted by MCS as it is guaranteed by the manufacturer. Nevertheless, even with MCS, the exerted pressure does not correspond to the theoretical pressure in all cases.^32,33 With other compression devices (IB and ACW), the pressure only depends on the stretch applied to the material by the health care professional or the patient and cannot be known if it is not measured. This leads to an improper use of compression therapy. It has been highlighted by some studies that the great majority of expert health care professionals actually exerted a very low pressure by applying IB due to a poor application technique.^8,34–37 In addition, the applied pressure shows an amazing variability from a few mmHg to more than 140 mmHg, ⁸ which does not improve even after proper education programs. ³⁶ Unfortunately, compression pressure is almost never measured in published studies often making conclusions hard to trust. To make an example, compression by MCS exerting a low pressure has often been considered more effective than IB, theoretically exerting a much higher pressure.^38,39 This most likely depends on the poor application of IB, as compression pressure has seldom been measured in studies. ⁸ In the only study comparing IB with MCS where the compression pressure was measured, a higher effectiveness of MCS was claimed. However, this was because IB was so poorly applied that their pressure was lower than that of the elastic stockings. ⁴⁰ The tested compression device exerted an evenly distributed pressure along the compressed leg segment. The exerted pressure is only slightly and not significantly higher than planned when AeroGauge^® is set at the pressure range of 20–30 or 30–40 mmHg. When the pump was set at the higher range of 40–50 mmHg, the correspondence with the exerted pressure was better. It is also worthwhile to point out the relatively narrow range of variability of the exerted pressure, which is not the case when using IB. ⁸ The ability of this device to exert different pressure ranges makes it applicable in several clinical conditions when a low pressure is adequate or when a high to very high pressure is required.

The other important parameter of compression therapy is the stiffness and the device we tested is quite stiff, even at low pressure range, as shown by SSI, DSI, and WPA. When the pressure increases, the stiffness also increases (Figure 7). The three stiffness parameters are significantly higher than the cutoff distinguishing elastic from inelastic materials placing the Aero-Wrap^® in the class of stiff material. Stiffness is defined as the rigidity of an object, the extent to which it resists deformation in response to an applied force. Dealing with compression therapy, in the document of the European Committee for Standardization (CEN) for medical compression hosiery (MCH), stiffness is defined as the pressure increase produced by MCS per 1 cm of increase in leg circumference. ⁴¹ A stiff compression material, able to resist the muscle volume increase during its contraction, will exert a much higher pressure during standing and much higher pressure peaks during physical activity starting from a lower and comfortable supine pressure. We can say that a stiff material is, in some way, able to adapt the exerted pressure to the body position. In addition, the high-pressure peaks during working will intermittently occlude the venous system, so restoring a kind of valve mechanism. With the tested device, when the supine pressure ranges of 30–40 and 40–50 mmHg were preset, the standing pressure was ≥60 mmHg. In previous papers from our group, compared to elastic bandages, inelastic bandages showed a significant improvement of ejection fraction ⁴ and a significant reduction of venous reflux. When stiff devices completely different from IB and completely different from each other were compared with elastic material,^22,23 we always got the same results as with IB. As the stiffness of this new device is in the same range as the inelastic bandages and the other stiff compression devices we previously tested, we can speculate that we could get the same hemodynamic result even if we need to prove this statement in future studies. Actually, stiff materials are significantly more effective than elastic devices in reducing venous reflux and increasing venous pumping function, so reducing ambulatory venous hypertension.^4–7,42 Finally, this device can provide reproducible results in terms of exerted pressure and stiffness, as shown by the similar data collected in three different centers by different investigators.

Weak points of our work: the device was applied by expert personnel rather than by the volunteers. Although the application was rated as easy by the expert personnel, data from patients’ side are missing. However, assessing the self-application ability is necessary when using compression devices that can be removed and re-applied by the patients themselves. Approaching a new device for the first time, we wanted preliminarily to be sure about its physical properties. This is why we limited our first study to the assessment of exerted pressure and correspondence of the theoretical pressure provided by the pump with the pressure actually exerted and the device stiffness. We found that the supine pressure exerted by the device can occasionally be significantly higher than the preset pressure. In addition, on some occasions, we found a compression pressure completely out of range of the preset pressure. In the device development process, we recommend that the pressure profile will be refined, ensuring greater precision within the range of the pump’s preset pressure. Also, an indication of the pressure loss and the need to readjust the pressure would be desirable. We do not have data, so far, on the hemodynamic effect of this new device, on its effect overtime, and on its effectiveness in the management of venous leg ulcers or other clinical aspects of chronic venous disease. Even if the new device was reported as comfortable by the volunteer after a short wearing time, data on comfort in the long term are missing. Also, risks and warnings have to be defined. All these data are mandatory and require to be assessed in future studies.

Conclusions

Aero-Wrap^® in combination with AeroGauge^® is able to exert the pressure ensured by the Manufacturer. At the lower-pressure ranges, the median exerted pressure is slightly higher than the preset pressure, which is not the case at the preset pressure range of 40–50 mmHg. Even if, with some exceptions, the range of exerted pressure at any preset pressure is narrow ensuring that we are providing a compression pressure close to the selected pressure. The chance to exert low, medium, and high pressures makes this compression device effective in many clinical conditions. High stiffness is another characteristic of this material, even at low pressure, as can be shown by the stiffness indices. All of them are in the inelastic range, making this device comfortable. We can also speculate that, as any other stiff device, this device could be hemodynamically effective even if its effectiveness in this respect must be confirmed in future studies. Additional data on self-management, comfort, and clinical effectiveness are necessary and will be provided by future studies.