The Effects of Gravity on the Fluid-Handling Properties of Wound Dressings Covered With Compression Bandages: Results of a Pilot Laboratory Investigation

Introduction. The management of exudate from leg ulcers remains a major challenge for health care practitioners. Many different dressings are available for this purpose, often used in conjunction with graduated external compression provided by extensible bandages applied in different configurations to achieve the required physiological effect. Although a significant amount of data has been published on the fluid-handling properties of some of these primary dressings, this has mainly been derived from laboratory studies using standard tests conducted in the horizontal plane, often without the addition of a secondary compression dressing. Materials and Methods. A review of the literature failed to identify any publications which examined either the effects of compression bandages or gravity upon dressings tested in the vertical plane, simulating normal conditions of use. The present study was therefore undertaken to investigate the effect of both of these factors. Results. While the application of multiple layers of bandages were found to have relatively little effect upon the loss of moisture vapor from a given dressing, compression and the effects of gravity were, in some instances, shown to significantly accelerate the rate of dressing failure, although important differences between products were noted in this regard. Conclusion. As a result of this work it is proposed that new test methods be devised which take account of both parameters to better inform potential users of the relative merits of various available dressing materials.

The significant volumes of fluid produced from wounds, such as leg ulcers and fungating cancerous wounds, can represent a serious practical problem to both the patient and those responsible for their care. These fluids soil clothing and bedding, as well as affect other aspects of patients’ lives by causing maceration or discomfort resulting from skin irritation, or pronounced malodor which may inhibit normal social interactions.¹

  The effective management of wound fluid is therefore one of the principal functions of products used in the management of wounds of all types and, as such, is a key requirement of the ideal dressing.² However, despite the well-recognized problems caused by excess exudate, and the enormous health care resources devoted to its management worldwide, limited information is available on the amount of exudate produced by various types of wounds.

  In one study³ that included 10 patients with leg ulcers, fluid was collected using pre-weighed dressings that incorporated an outer impermeable plastic film to prevent measurement errors caused by failure to capture fluid lost by evaporation. Multiple recordings for all 10 patients revealed that exudate rates appeared to fall into 2 groups. Seven were in the range 0.41-0.52 g/cm²/24 hours, and the remaining 3 (where infection was suspected) were in the range 0.87-1.2 g/cm²/24 hours.³

  These results suggest that a circumferential ulcer on a large leg could, in theory, produce in excess of 500 mL of exudate every day, representing a significant challenge for the dressings that are applied to deal with this problem.

Most dressings designed to manage exudate do so by means of 1 or more of the following mechanisms.

  Absorption. The simplest form of exudate management is absorption. Surgical absorbents made from bleached cotton have been used for well over a century to absorb blood and tissue fluid during and after surgery. Although gauze is still used extensively in the operating room, the development of alternative products, including those made from foam, which offer numerous advantages over the earlier fibrous cellulose products for this application, are used more routinely.

  Transpiration. If a low- to moderately exuding lesion is left exposed to the air, the aqueous component of the exudate will evaporate, resulting in a progressive increase in the concentration of dissolved solids such as proteins and electrolytes, which eventually come out of solution, coalesce, and dry out to form a scab – nature’s own dressing. Scab formation is characteristic of minor traumatic injuries; heavily exuding wounds typically do not form scabs in this way. Despite the valuable protective role of a scab, its presence delays healing as the newly formed epithelial cells are forced to burrow down beneath the dry tissue to migrate over the surface of the wound. Accordingly, a family of dressings were developed that could allow the evaporation of water, maintain a moist wound environment to prevent scab formation, and form an effective bacterial barrier. In their simplest form, these dressings consist of an adhesive film, commonly made from polyurethane, which is placed directly over the wound and fixed firmly to the surrounding skin. Although the moisture vapor permeability of the early versions of these dressings was too low to cope with exudate production for an extended period, more modern products are sufficiently permeable to cope with even the most heavily exuding wounds.⁴

  Gelation. Some fibrous dressings, such as those made from alginate or carboxymethylcellulose (CMC), change their physical state as they interact with exudate to form a semisolid gel. Gel formation has the advantage that it increases the viscosity of wound fluid, thereby immobilizing it on the wound surface or within the dressing to form a moist environment claimed to facilitate healing without causing maceration. Others, such as hydrogel sheets, may be applied to the wound in the hydrated form and some, but not all, of these materials, have the ability to take up additional fluid while still retaining their semisolid state. Anhydrous gel-forming agents can also be added to simple dressing pads to improve their performance, and CMC powder or superabsorbents made from acrylic polymers are sometimes used for this purpose.

  It is possible to construct individual dressings that make use of 2, or even all 3, of these exudate-handling mechanisms in combination, and many different commercial preparations are now available which vary in composition, construction, and fluid-handling capacity (FHC). The FHC is the sum of the weight of fluid absorbed or retained within the dressing and that lost as moisture vapor through the outer surface.

  Every year dressing manufacturers invest heavily in new product development. Most undertake laboratory studies designed to facilitate comparisons in an objective and quantifiable manner, and ensure their new products fulfill their intended role or have significant advantages over existing materials. The results of these studies are also often used in marketing or promotional literature to draw comparisons between different types of brands of products, both for clinical and commercial reasons.

  Various types of absorbency and permeability tests have been described in the literature for dressings where effective fluid handling is a primary requirement. Some of these tests form the basis of national or international standards or specifications.

  While it is generally recognized that the absorbency of a dressing may be adversely affected by the application of pressure, and that this in turn may impact a product’s clinical performance, changes in temperature and relative humidity of the local environment are likely to have a limited effect upon a dressing’s absorbent capacity. In contrast, products which rely heavily upon their permeability to moisture vapor to manage exudate can be affected significantly by changes in these parameters, and the effect of environmental changes, including the weather, on the performance of semipermeable dressings has been described previously.^5-6

  For laboratory test data to be as clinically relevant as possible, the methods employed sometimes include a requirement that the test samples be subjected to a degree of compression, commonly that used in the treatment of venous leg ulcers, approximately 40 mm Hg.

  In almost all instances, for practical reasons, these tests are conducted with the dressing in a horizontal plane with a known mass placed upon the outer surface of the test sample to provide the required level of compression. During such tests it is commonly observed that the fluid absorbed by a dressing spreads uniformly through dressings from the point of application forming a circular area that increases in size as the test progresses.

  This type of test is easy to undertake and control and provides an effective and reproducible way of comparing the performance of different types of products. It can also be argued it is entirely applicable to some clinical indications, for example, products applied to the back, sacral region, or any other horizontal surface on patients in a supine position.

  For products designed for the treatment of leg ulcers, particularly in ambulant subjects or those who sit with their legs in the dependent position for extended periods, such tests may be criticized as they fail to take account of the effects of gravity on exudate distribution. Under these conditions absorbed fluid might be expected to move preferentially downwards within the dressing rather than in a circular pattern, which could have a significant impact upon product performance.

  An online search of the literature was completed to determine if the effects of gravity and compression had been formally investigated or previously reported. The authors searched the National Library of Medicine’s PubMed database to identify any publications related specifically to the effects of bandaging upon exudate management. Details of the simple search strategy adopted, together with the results obtained, are summarized in Table 1.

  Surprisingly, despite the widespread use of compression bandages for the treatment of venous leg ulcers, no publications in English were identified that were specifically related to the effects of compression on the fluid handling characteristics of commonly used dressings. This, despite the fact that exudate and its management are universally regarded as a real clinical problem. Only 1 publication in German appeared to be of relevance.⁷

  One manufacturer of dressings has published some data on their website⁸ comprising a simple visual comparison of the results of testing 7 different dressings in a vertical plane. In that study, no attempt was made to control the environmental conditions during the test, and no compression was applied to the dressings during the procedure. Despite this, the results clearly showed marked differences between the products examined and, with one exception, all showed that the movement of absorbed fluid was greatly influenced by gravity, which in many instances resulted in pooling and/or leakage at the lower edge of the dressing.

  The present pilot semi-quantitative study was therefore undertaken to determine, possibly for the first time, if different bandaging techniques can impact on specific aspects of the performance of 3 different dressings maintained in the vertical plane, by comparing both the amount of fluid absorbed and lost by transmission while under compression in simulated in-use conditions.

  The dressing materials used during the course of the study are described in Table 2.

  Wound model. A rigid cylinder 34.5 cm in circumference was used as a leg model. A 5.5 mm hole was drilled centrally in the cylinder which allowed a 6 mm PVC tube to form an interference fit with the cylinder wall. A filter paper (Whatman 41, Whatman International Ltd, Maidstone, UK) 50 mm in diameter was placed centrally over the tube where it exited flush with the cylinder surface to facilitate initial spreading of the test fluid, and simulate a wound of a reasonable size.

  Test fluid was delivered at a constant rate by means of a syringe driver (Cole Parmer 78-9100C, Cole Parmer, Vernon Hills, IL) fitted with a Plastipak 60 ml syringe (BD, Drogheda, Ireland). A flow rate of 0.8 ml/hour (circa 1ml/cm²/24 hours) was selected to represent a highly exuding wound. The authors chose a test fluid described in a European Standard for wound dressings,⁹ Solution A, which consists of a solution of sodium/calcium chloride containing 142mmol/l of sodium ions and 2.5 mmol/1 of calcium ions, values typical of those found in serum. For the purpose of this investigation, bovine serum albumin (Sigma Aldrich, Gillingham, Dorset, UK) was added to this solution to give a final concentration of 3.3% w/w with a density of 1.04 g/ml.

  The leg model was placed in an incubator (Memmert ULE-500 incubator, Memmert GmbH + Co KG Schwabach, Germany) at 34°C (skin temperature) with either an appropriate salt solution to maintain a relative humidity of 56%-62%, or a tray of silica gel to reduce the relative humidity to around 33%-38%. Both temperature and humidity within the chamber were monitored using an Easy Log EL-USB-2 data logger (Lascar Electronics, Salisbury, UK).

  All dressing samples were weighed before application to the test rig and immediately following the completion of the test. From these values, combined with details of the flow rate of the syringe pump, it was possible to calculate the total weight of liquid applied to the dressing, the weight retained at the end of the test, and by calculating the difference, the weight lost by leakage or evaporation.

  Application of compression. The Profore Multi-Layer Compression Bandaging System (Smith and Nephew, London, UK) was used to apply compression as recommended by the manufacturer. Both compressive layers were applied with 50% extension, a process that was facilitated by pre-marking each bandage at specific intervals prior to application.

  To determine the tension within these compressive layers at this level of extension, samples were placed in a constant rate of traverse machine (Instron Mini 44, Instron, High Wycombe, UK) and extended at a rate of 200 mm/min while recording the extending force in newtons (N). This simple procedure indicated that both products required the application of an extending force equivalent to about 4 N/10 cm width to achieve this degree of extension.

  If the 2 compression bandages are both applied as a spiral with a tension of 4 N, this will result in a total of 4 layers of elasticated fabric. Using the modified Laplace formula previously described in the literature,¹⁰ it is possible to calculate that together these will produce circa 22 mm Hg on the model leg. Note that this estimated figure takes no account of any effect upon the total pressure caused by the absorbent padding and the inner conformable bandage that also form part of the multilayer compression system. The third bandage was applied either in the form of a spiral or as a figure eight, and tests were undertaken using both configurations.

  Prior to commencing the main part of the study, the bandages were applied over 1 dressing with the leg model held in a horizontal configuration to determine if the presence of multiple layers of fabric has a significant impact upon the moisture vapour transmission rate (MVTR) of a primary dressing as has sometimes been suggested in the course of the authors’ clinical practice. Only 1 dressing (Allevyn Non-Adhesive Hydrocellular Polyurethane Dressing, Smith & Nephew, London, UK) was examined in this way, as this preliminary study was actually a test of the performance of the bandages rather than the primary dressing. For the main part of the investigation, the leg was placed in a vertical configuration to investigate the influence of gravity on fluid handling. In all these tests an attempt was made to ensure the various bandaging components were applied and retained under a degree of extension (ie, 50% for the compression layers), which would approximate that used clinically.

The effect of compression bandaging upon the fluid-handling properties of a hydrocellular polyurethane dressing in the horizontal plane. The effect of the bandages upon the loss of moisture from the nonadhesive hydrocellular polyurethane dressing tested in the horizontal plane are summarized in Figure 1. No leakage of fluid from the dressing was observed at any time during the course of the test which means it is reasonable to assume that all of the solution applied was either retained in the dressing or lost as moisture vapor by evaporation through the outer membrane applied to the foam pad into or through the overlaying bandages.

  As the values quoted in Figure 1 represent the mean of only 2 determinations, they should be regarded simply as illustrative, rather than providing a definitive statement of product performance. However, the results clearly suggest that the presence of the bandages has a limited effect (ie, approximately 10%) upon the loss of moisture vapor from the dressing and it can be assumed, therefore, that they have no more than a minor effect upon this aspect of a dressing’s performance.

  The effect of compression bandaging upon the fluid-handling properties of dressings in the vertical plane. The various test samples were applied to the leg models as previously described and maintained under the specified test conditions for about 21 hours, after which they were examined for evidence of leakage and photographed.

 When tested under relatively low humidity (relative humidity [RH] 33%-38%) the resultant images, reproduced in Figure 2, show no evidence of leakage through to the outer compression bandage from any of the test samples, although some leakage into the innermost bandage was detected with one of the absorbent foam dressings (Mepilex, Molnlycke Health Care Inc, Norcross, GA).

  However when tested under more clinically relevant conditions (RH 55%-62%), the nonadhesive hydrocellular polyurethane dressing and the absorbent foam dressing showed evidence of leakage from the bottom edge of the dressing through to the outer layers with either spiral or figure eight bandaging applied. The third product examined, a superabsorbent foam dressing (Xtrasorb, Derma Sciences, Princeton, NJ), showed no evidence of leakage under these conditions (Figure 3). The effect of varying relative humidity upon the performance of the primary dressing under the conditions of this test may be clearly seen in Figure 4. When the hydrocellular polyurethane and absorbent foam dressings were tested at a relatively low humidity (RH 33%-38%), the test solution was visible behind the semipermeable film outer layer of both dressings, an observation that was recorded on numerous occasions. When tested under more humid conditions (RH 55%-62%), fluid was not readily visible within the dressing. These observations were somewhat unexpected. It might be thought that under conditions which favor the passage of moisture through the outer film backing layer, the more permeable products would not show the apparent accumulation of fluid in this way, and that under conditions which do not facilitate the loss of moisture vapor, fluid might be expected to become trapped beneath the membrane.

  However, a possible explanation for this observed effect has been offered previously, following a clinical study in which the migration of absorbed fluid in foam dressings was monitored using ultrasound.⁴ It was proposed that under conditions which facilitate the loss of moisture vapor through the outer film fluid, the nonaqueous (ie, dissolved) components of the test solution, particularly the serum albumin, remain behind at the inner surface of the membrane. As this process progresses and fluid continues to evaporate, there develops a concentration gradient from the inner to the outer surfaces of the absorbent core. This in turn produces an osmotic effect, effectively drawing more fluid into this part of the dressing producing the observed effect. Careful examination of both the nonadhesive hydrocellular polyurethane and absorbent foam dressings at the conclusion of the tests in the current study showed these proteins are largely dried out along the bottom edge of the wet area. In this way, they effectively formed a partial barrier to further downward movement of the test solution.

  In a final part of the study, samples of nonadhesive hydrocellular polyurethane dressing and the superabsorbent foam dressing were tested in the same way for an extended period of 69-72 hours (Figure 5). Under conditions of high humidity, the superabsorbent foam dressing showed evidence of slight leakage, which was limited to the innermost layer, but the nonadhesive hydrocellular polyurethane dressing leaked significantly from the bottom of the dressing. Under conditions of low humidity, no leakage was noted with the superabsorbent dressing, but some leakage into the compression bandage was noted from the side of the nonadhesive hydrocellular polyurethane dressing.

This study was not designed to produce quantitative comparative data on the relative performance of different types of dressings. Rather, it was undertaken to investigate—perhaps for the first time—the combined effect of compression and gravity upon the fluid handling of 3 different types of dressings in situations designed to reflect normal clinical practice. The authors took this approach instead of the more stringently controlled conditions of laboratory tests in which products are tested for compliance with official monographs or national or international standards.

  It was recognized that the bandaging technique employed could impact the pressure applied to the primary dressings beneath. Two principle techniques for application of compression bandaging exist, spiral and figure eight. With 4-layer bandaging, it is recommended that a figure eight technique be adopted for the first compressive layer and a spiral wrap for the second. However, it has also been suggested that when lighter compression is required, the first compressive layer should be applied in a spiral wrap.¹¹

  If the third bandage layer is applied as a figure eight, as recommended, this will result in 4 layers for this bandage alone. The fact that the bandage is applied at an angle, not perpendicular to the principal axis of the cylinder or leg, makes predicting sub-bandage pressure extremely problematic. The additional numbers of layers applied would clearly suggest that pressure will be elevated compared to the standard spiral wrap but the pressure developed by each layer will be lower than that achieved by the individual layers of the spiral wrap. The net effect of these 2 opposing variables is hard to determine—as in the case of the figure eight application technique, sub-bandage pressure will be determined in part by the angle at which the bandage is applied.

  According to results of a laboratory study reported by Melhuish et al¹² in which they compared the 2 systems on a solid cylinder, measured values for sub-bandage pressures were similar at each application tension although at higher application tensions there was a trend for the bandages applied in a spiral fashion to achieve higher sub-bandage pressures. In contrast, Coull et al¹³ found that in a cross-over experimental study involving 26 nurses responsible for applying bandages, the figure eight technique provided statistically significant higher compression at lower areas of the leg than the spiral technique.¹³ For the purpose of this study, therefore, no estimate of the pressures developed beneath the figure eight bandage has been offered, although all bandages were applied by 1 operator using a standard technique to ensure consistency in the test conditions for all examined dressing samples.

  Three principal conclusions may be drawn from the results of this work. First, there is persuasive evidence that despite a widely held belief among wound care practitioners that compression bandages greatly impair the loss of moisture from permeable dressings, this does not appear to be the case. While bandaging systems may have an effect on the MVTR of a dressing, the results of the preliminary study suggest that this is relatively minor and can probably be ignored in most circumstances. It is likely, although not investigated in this study, that compression will affect the ability of a dressing to absorb and retain fluid within a foam or fibrous absorbent core. Second, the effects of gravity on dressing performance can be significant, although the magnitude of this effect appears to vary from product to product and is probably greatly influenced by the construction of the dressing and/or the inclusion of gel-forming or superabsorbent components. Third, RH can have a major effect upon dressing performance as previously predicted. The authors propose efforts should be directed towards developing a nationally or internationally accepted standard method that may be used to characterize dressing performance in a vertical plane.

Affiliations: Stephen Thomas, BPharm, PhD is from Medetec, Cardiff, South Wales, United Kingdom. Hugh Munro, BSc, PhD; and William Twigger, BEng are from First Water Ltd, Wiltshire, United Kingdom.

Address correspondence to:
Stephen Thomas, BPharm, PhD
steve@medetec.co.uk

Disclosure: Dr. Munro and Mr. Twigger are employed by First Water Ltd, which manufactures advanced wound dressings. Dr. Thomas disclosed receipt of a fee from First Water Ltd as compensation for test method development and co-authorship of this manuscript.