Anti-embolism hosiery testing: comparison of compression profiles

Gavin Hughes, Louise Barry, Joanna Ford and Pete Phillips (Surgical Materials Testing Laboratory, Bridgend)

May 2017

Introduction

In 2015, SMTL embarked on a test programme for NHS Wales Shared Services Partnership - Procurement Services (NWSSP PS) to test the performance of anti-embolism hosiery for an All-Wales Contract (our reference project 4753).  Part of the terms of contract stated that SMTL and NWSSP PS could publish the results of testing and assessments without restriction or censure.  This paper provides an overview of the laboratory testing results.

Upon entering hospital, many in-patients (including those undergoing surgery) will be given a pair of anti-embolism stockings (or hosiery) to wear. The purpose of this hosiery is to reduce the risk of clot formation in the legs (deep vein thrombosis). However, there is a wide range of anti-embolism stockings  on the market and it is known that performance can vary widely.

Blood pooling in the deep veins of bed-ridden patients during hospital stays is a common problem.  When the circulation of blood slows and fluid starts to pool in the deep veins of immobilised legs, clots can readily form which can lead to deep vein thrombosis (DVT).  If a clot breaks off and travels to the lungs, this can cause a pulmonary embolism (PE) which is potentially fatal.  Leg compression has been used to maintain blood flow for centuries (Meyerowitz and Nelson 1964).

It was Sigel in the mid Seventies who first reported that for legs in the supine position, graduated external compression was more effective at maintaining blood flow than uniform compression (Sigel et al., 1975).  Sigel also stipulated the range of compression required at different parts of the leg that led to optimal velocity of blood flow (known as the 'Sigel profile').  Further evidence for the value of the compression profile identified by Sigel has been provided in later studies by Lawrence and Katter (1980), Sparrow et al. (1995) and others.   Evidence suggests that calf pressures of 14-15 mmHg and slightly higher pressure at the ankle (i.e. a graduated compression) prevents pooling of blood in the deep veins (Sigel et al., 1975, Sparrow et al., 1995, Thomas et al., 2000).  These pressure ranges also comply with NICE guidance (NICE, 2010).  If the opposite is true and pressure is greater at the calf than the ankle (i.e. a negative compression gradient) pooling and potential DVT may result.

A British Standard (BS 7672)  describes tests for anti-embolism hosiery but the performance limits they quote are outdated, and the compression limits are considerably lower than those determined by clinical studies.  It is now accepted that the profile originally proposed by Sigel and supported experimentally should be used to assess performance.  As a result of this, the Sigel profile was used as the basis for the tests described in this paper.

Methods

A technical memo was circulated in advance to all companies participating in the contract process which detailed the sample requirements, testing methodology and the specification that devices would be assessed against.  SMTL are UKAS accredited to ISO 17025 for performing the tests described here.

NWSSP Procurement Services undertook a tender exercise and this resulted in two potential suppliers for the contract.  The anti-embolism stockings of 2 companies (Urgo Medical and Covidien) were tested in this study, and each was sent a 'testing requirement document' asking them to provide samples of thigh length stockings that were of an appropriate size for application to each of five specified legs of different lengths and circumferences  (dimension ranges provided).  These standard leg sizes were selected to represent patients of widely different height and width to whom hosiery may be applied in normal clinical practice, and corresponding to the following 5 types of leg profile:

1. Short with small circumference

2. Long with small circumference

3. Medium with medium circumference

4. Short with large circumference

5. Long with large circumference

Asking the companies to provide stockings that corresponded to 5 common sizes was considered more practical than testing the manufacturer's whole range, as sizes vary.  In addition, as these stockings tend to be used for short term hospital stays, repeated washing was not assessed in this pre-contract testing program. Therefore samples were tested in their unwashed states. Measurements were taken at the ankle, calf and lower thigh. Stockings were conditioned for at least 16 hours (20±2^oC and 65± 10% relative humidity) and then left to hang for 2 hours before being tested on the apparatus.

The 'Hatra hosiery pressure tester' was used to perform the test (Figures 1 and 2).

The Hatra consists of 2 metal bars (one above the other) attached to a hinge which can adjust the gradient between bars (simulating legs of different sizes). The distal end connects the upper and lower bars by a small piece of metal which simulates the foot. Curved pieces can be added to the lower bar to model the natural curves of the leg (calf and thigh).  A conditioned stocking is positioned onto the Hatra, the stocking is then stretched to the patients limb size and the tension measured along the length of the hosiery at different measurement points via the Hatra measuring head.  The tension readings are then used to determine the level of compression (in mmHg) that the stocking would exert on a patient with the same leg dimensions.

Figure 1. The Hatra apparatus showing upper and lower bar with foot form at one end. The measuring head can be seen at the bottom of the image.

Figure 2. The Hatra with stocking ready for testing

The Sigel profile was used to construct the pressure requirements at each position, using ±20% tolerance limits (to take into account method variability and product consistency) as shown in Table 1. To pass the test, pressures must fall within the limits shown in the final column.  In addition stiockings must not exhibit negative gradients, where there is the potential to cause pooling in the leg, leading to a higher risk of DVT and PE.

Table 1. Sigel pressure profile with 20% tolerance limits

Results

The pressure profiles of 15 samples of each stocking size were measured. Table 2 shows the results for Covidien stockings and Table 3, the results for Urgo stockings. They are presented as mean values with the standard deviations in brackets.  Results are presented graphically in Figure 3 below the tables.  Negative gradients are shown in red.

Table 2: Pressure profiles of unwashed Covidien T.E.D. Anti-embolism Hosiery.

Table 3: Pressure profiles of unwashed Urgos hosiery.

Figure 3: Pressure profiles of unwashed Covidien (above) and Urgo (below) hosiery, shown as graphs.

Results show that for Covidien (Table 2, Figure 3), 4 out of the 5 sizes failed to comply with the test limits (with pressure profiles falling outside of the limits stipulated in Table 1).  Furthermore, two sizes of stocking (size 1 and 3) exhibited negative pressure gradients (i.e. higher pressures at the calf position than the ankle, shown in red on Table 2).  In contrast, all 5 sizes of Urgo complied with the limits and no negative gradients were found. 

Discussion

The results presented in this paper clearly show that one of the brands tested did not meet the performance requirements stipulated by the NWSSP PS. Some stockings did not provide sufficient compression whilst others exhibited a negative compression gradient.

Bibliography

BS 7672:1993 Specification for compression, stiffness and labelling of anti-embolism hosiery

Meyerowitz B R and Nelson R. Measurement of the velocity of blood in the lower limb veins with and without compression. Surgery 1964;56(3):481-6

NICE (National Institute for Health and Care Excellence), Venous Thromboembolism: reducing the risk for patients in Hospital. Clinical Guideline. Jan. 2010

Sigel B, Edelstein A, Savitch L, Hasty J, Felix R. Type of compression for reducing venous stasis. A study of lower extremities during inactive recumbency. 1975 Arch Surg ;110:171-175

Lawrence D and Kakkar V. Graduated, static, external compression of the lower limb, a physiological assessment. British Journal of Surgery 1980;67:119-121

Sparrow R A, Hardy J G, Fentem A, Muller J M. Effect of 'antiembolism' compression hosiery on leg blood volume.  British Journal of Surgery 1995;82(1):53-59

Thomas S, Toyick N, Fisher B. Graduated external compression and the prevention of deep vein thrombosis. 2nd Ed. 2000. Surgical Materials Testing Laboratory, Bridgend.