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Exudate theory and the clinical management of exuding wounds

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Jacqui Fletcher, BSc (Hons), PG Cert, ILT, RN.

Senior Lecturer, University of Hertfordshire, Hatfield, Hertfordshire

Exudate, also known as wound fluid or wound drainage, plays a normal part in the healing process in all wound types and aetiologies. In general, the quantity of exudate produced decreases as the wound progresses towards healing (Thomas, 1997a). Exudate is particularly noticeable during the inflammatory and proliferative phases of healing because it provides nutrients as an energy source for metabolising cells and plays a role in regulating the moisture level in the local wound environment.

Managing a wound with a high level of exudate is one of the most commonly cited clinical problem areas, with high levels of exudate causing problems in terms of selecting dressing products capable of handling the fluid levels and also in preventing damage to the surrounding skin. Leakage of exudate through the dressing not only increases the risk of cross-infection, it may also be distressing for the patient because of soiling of clothing and bedding and possible malodour.

There are differences in the composition of acute and chronic wound fluid and even between the fluids of different types of acute wounds, which have differing profiles of proteinases, proteinase inhibitors and cytokines (Baker and Leaper, 2000). A considerable amount of research has been directed towards determining the composition of both acute and chronic wound exudate because it is thought that, by manipulating exudate, it may be possible to speed up or improve the healing process.

However, due to the many different wound types and underlying patient conditions, and because the phases of wound healing overlap, it is difficult to determine the ‘optimum balance’ of cytokines, growth factors and proteolytic mediators that should be present at any particular point during the healing process (van Rijswijk and Harding, 2000). There are also important variations in the constituents and volume of exudate produced during healing, so it is difficult to define ‘normal’ levels of exudate production (Harding, 1997).

The constituents of exudate

Exudate is known to be similar to serum, having similar concentrations of potassium and sodium ions (Vickery, 1997). Exudate has fewer red blood cells but approximately six times as many white cells as does blood, supporting the role of exudate in wound cleansing and debridement (Thomas, 1997a). Perhaps the most important constituents of the exudate are the growth factors, which stimulate various parts of the wound-healing process, the matrix metalloproteinases (MMPs), which break down the wound matrix and, finally, the inhibitors of MMPs known as TIMPs (tissue inhibitors of matrix metalloproteinases).

Metalloproteinases are proteolytic enzymes, that is enzymes that degrade protein. A subgroup of metalloproteinases, the matrix metalloproteinases (MMPs) specifically degrade or cleave at least one of the extracellular matrix proteins, for example collagen or elastin. MMPs include collagenase and elastase.

In chronic wounds there is an imbalance in the amount of MMPs and TIMPs, with higher levels of the destructive MMPs and lower levels of their inhibitors the TIMPs (Krieg and Eming, 1997; Vickery, 1997). This is thought to be related to the persistence of the inflammatory phase in these types of wounds. The damage frequently seen and attributed to wound exudate is therefore due not only to the amount of fluid on the skin but also to the high level of proteolytic activity by the MMPs.

MMPs not only degrade proteins but also cause degradation of key functional molecules such as growth factors (Trengrove et al, 1999).

Comparable volumes of exudate may cause differing levels of tissue damage because of the impact of this proteolytic activity on the skin adjacent to the wound (Krieg and Eming, 1997).

Over-hydration and skin damage

Over-hydration occurs when fluid is trapped against the skin. The outer layer of the skin - the stratum corneum - is a physical barrier but does allow passage of water through the skin by a process known as trans-epidermal water loss (TEWL).

When liquid is trapped on the skin surface or the skin is occluded, preventing TEWL, the keratinocytes swell and are capable of absorbing several times their own weight in liquid. This can be seen as the plump, wrinkling of the skin that commonly occurs after swimming or prolonged bathing. The process of TEWL usually resolves this once the skin is no longer immersed. When the peri-wound skin is exposed to fluid in the form of exudate, however, the same process also occurs and the moisture remains in contact with the skin for much longer periods. It is suggested that this over-hydration of the epidermis may stimulate the release of pro-inflammatory cytokines that initiate a further inflammatory response and the associated release of MMPs (Klingman, 1995).

Once over-hydration has occurred, the skin is less able to withstand physical trauma because the stratum corneum becomes weaker and less elastic and is therefore more susceptible to physical damage, such as that caused by the removal of adhesive dressings or the chemical assault from the wound exudate.

Exudate management

Despite the valuable role played by exudate in the wound-healing process, its potential to cause over-hydration and chemical damage means that thorough assessment and management of exudating wounds has considerable clinical benefits. Factors that may influence exudate production are listed in Box 1.

In order to manage exudate effectively, its quantity and type should be assessed and described. It is important to interpret correctly any changes in the level of exudate produced. At certain points in the healing process, clinicians will identify an ‘average’ or ‘normal’ level of exudate as being indicative of normal progression - and a sudden increase in the level of exudate is frequently taken to indicate the presence of infection. However, other practical factors, such as the presence of oedema, the rehydration of necrotic tissue or connection to another drainage site, such as a stoma, may also increase the level of exudate, and these factors should also be considered (Thomas, 1997a).

Measuring levels of exudate

The most obvious reason for estimating the level of exudate is to assist in the selection of an appropriate dressing or wound-management product. In this instance, an absolutely accurate measurement is not necessary - a simple estimation of the volume is all that is required. It may be beneficial to accurately measure exudate when there is copious exudate that may affect the patient’s general hydration or nutritional status or if the measurement is part of a research study.

There are practical difficulties associated with the objective measurement of the quantity of exudate.

Two possible methods of measuring exudate production are: collecting fluid in, for example, a drainage bag; and collecting and weighing dressings. Realistically, neither of these are practical options in most instances and therefore a more subjective measure is usually used in clinical practice. Most wound-assessment forms ask for the amount of exudate, which is often recorded using the following symbols: +, ++, +++. Alternatively, the amount may be described using adjectives such as ‘small’, ‘moderate’ and ‘heavy’. However, perceptions of what these descriptions mean vary considerably between people and Thomas et al (1996) demonstrated clearly that even very experienced nurses were unable to estimate correctly the amount of exudate.

Many practitioners estimate exudate volume in terms of dressing usage, equating frequency of dressing change to the volume of exudate. A simple version of this method has been described by Mulder (1994), who suggests using 10x10cm gauze as the base comparison. He defines the levels as absent, minimal, moderate and high (Figure 2).

Obviously the volumes of fluid concerned are relatively low and generally appropriate to only uncomplicated surgical wounds. In terms of chronic wound management, more appropriate definitions could be defined locally using a more absorbent dressing material as the base comparison - the important point is to standardise the descriptors used.

Colour and consistency of exudate

It is usual to describe the colour and consistency of the exudate as well as its volume. Mulder (1994) suggests descriptors based on the visual components of the exudate: serous, sanguineous, serosanguineous and purulent.

Pseudomonas infections produce thick, malodorous, sweet-smelling, green exudate, while proteus infections produce an ammonia-like odour. Difficulties also occur in distinguishing between thick exudate and liquefying necrotic tissue secondary to the debridement process. Necrotic tissue is usually connected to or adherent to the wound bed, but as it softens and liquefies it will increase the amount of fluid in the wound.

In addition to the visual descriptors, Sussman and Bates-Jensen (1998) also suggest the characteristics of the exudate and the clinical significance of these (Table 1).

Several other factors may also increase the amount of exudate produced and these should be identified as part of the holistic assessment of the patient (Box 1) and, where possible, addressed as part of the overall management plan.

Management plan

Once an accurate assessment has been made, an appropriate plan of management should be formulated. Contributory factors such as those listed in Box 1 should be addressed by appropriate measures. For example, compression bandaging may be used in venous hypertension and diuretics may be prescribed if heart failure is causing gross leg oedema. If possible, this assessment should also identify potential factors that may increase the level of fluid - if, for example, a necrotic wound is being rehydrated, it is obvious that what was initially a dry wound will become a very wet wound. The liquefying materials will mix with and add to the volume of exudate.

When a high level of exudate is expected, measures to protect the surrounding skin from damage should be employed. These include the use of simple barrier creams and the now widely available skin-protectant wipes or sprays. If there are likely to be very frequent changes of dressing, which may lead to tape damage to the surrounding skin, a keyhole dressing using a film or thin hydrocolloid may be more appropriate because it allows the tape to be secured to the dressing rather than to the vulnerable peri-wound skin.

Dressings may manage exudate in a variety of ways, including absorption, transmission (by allowing free drainage through the dressing or by transpiration), interaction or by a combination of these ways. There are several generic types of product, which may be classified within these categories.

Absorbency

Perhaps the most common method of management is the use of absorbent dressing products such as foams or alginates. These vary considerably in their levels of absorbency and comparative information both within the literature and from the manufacturers is difficult to interpret objectively due to lack of a standard methodology. For example, some products quote their absorbency by weight of product and others by area - neither is incorrect. Similarly, absorbency may have been measured using a variety of test solutions of differing concentrations or under differing conditions in terms of humidity or rate of delivery of the solution. The data produced by these tests are of limited clinical value and, although they may show statistical significance, they may not demonstrate clinically significant differences.

Furthermore, the tests used are usually specific to an individual product group and therefore do not allow comparison between different generic groups. A standard test methodology has been proposed (Thomas and Fram, 2001), which, if followed, should assist with this problem.

Different products absorb the same amount of fluid in different ways. For example, different types of alginate may be described as slow- or fast-gelling - the former absorbs fluid much more slowly but the level of absorption is sustained, while the latter takes up fluid rapidly and becomes fully absorbed within a short space of time. The type of absorption required depends on the characteristics of the wound being managed.

Some products claim selective absorbency of the liquid component of the exudate, concentrating the beneficial wound proteins in the wound space (Achterberg et al, 1996). However, in the light of the high concentrations of other factors, such as MMPs, which may have a detrimental effect, this issue requires further investigation, with some authors suggesting that, especially in chronic wounds, the fluid (exudate) may be so toxic that it may be beneficial to cleanse the wounds regularly (Wysocki, 1996).

A further consideration when selecting absorbent dressings may be their ability to retain the exudate when under pressure, which may be caused, for example, by compression bandaging or at a body site that is subjected to regular pressure, such as the buttocks or heel.

The amount of exudate a product of a given size may hold may also be affected by the way the fluid is handled within the dressing. Some products take up the fluid and retain it in the region of contact with the wound; others take the exudate into the dressing and spread it across the whole internal surface (known as lateral wicking), which results in a greater absorbency.

Transmission

Where there is a ‘very high’ level of exudate, absorbent products may not be sufficient and the exudate may be better handled by being allowed to drain freely, either into a drainage-type system similar to a stoma bag or by passing through a porous wound-contact layer and having less expensive outer padding layers, which are changed more frequently. More sophisticated systems of drainage allow the collection of fluid via a negative-pressure system.

Where there is minimal exudate, some products are able to handle the fluid by having a ‘breathable’ outer covering, which functions in a similar way to the stratum corneum. This breathability in a dressing product is described as the moisture vapour transpiration rate (MVTR). The higher the MVTR, the greater the dressing’s ability to allow fluid to evaporate (Bolton et al, 2000). This method of managing fluid is suitable only for low amounts of exudate, but is often used as part of a combined approach.

Interaction

Several categories of dressing undergo change while in contact with exudate. For example, hydrocolloids form a soft viscous gel into which the liquid binds. Alternative interactions include the management of fluid via an ion exchange, as occurs in alginate dressings.

Combinations

Several newer dressing products combine these approaches, having, for example, an interactive perforated contact layer allowing transmission of exudate through to a central wicking layer and a backing layer with high MVTR. This allows the product to optimally manage a range of exudate levels.

Patient-centred considerations

When selecting products to manage exudate, additional patient-focused factors must not be forgotten. Several studies addressing quality-of-life issues in patients with wounds suggest that many of them suffer embarrassment and social isolation due to dressing leakage or malodour (Hamer et al, 1994; Hyland and Thomson, 1994).

Many patients view exudate as ‘unclean’, and, while current recommendations suggest leaving dressings in situ for as long as possible and that wounds should be cleaned only when necessary, it should not be forgotten that there is a real need for people to ‘feel clean’. This is not to suggest that research-based practice should be ignored, simply that patients’ wishes and needs must be addressed. Some very absorbent dressings can become quite heavy when almost ready to change, especially in areas where they may be affected by gravity, such as on breast wounds, where the dressings may pull considerably on the surrounding skin, causing discomfort.

Conclusion

Management of exuding wounds remains a real clinical challenge. A basic understanding of the role and function of exudate should assist the practitioner in appropriate assessment of the patient and his or her wound. Defining terminology in a standardised and objective way will facilitate communication and evaluation of planned care. Knowledge of the way different categories of dressing products perform and how their usage may be optimised will allow a range of realistic treatment options to be considered and a tailored approach to the patient’s needs designed.

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