VOL: 97, ISSUE: 09, PAGE NO: 11
Richard White, PhD, BSc, MIBiol, is a clinical research consultant and medical writer, website: www.medicalwriter.co.uk
Wound-healing results from a complex interaction between cells, fluids and biochemicals, which occurs in four overlapping phases: haemostasis and inflammation, granulation, epithelialisation and tissue remodelling (Davidson, 1992). Immediately on wounding the body responds with vasoconstriction to reduce blood loss, and clotting to seal the damaged area - haemostasis. Once this is achieved, the normal inflammatory phase of healing begins. This includes the production of wound exudate - fluid produced by the tissues surrounding a wound in response to the damage. Exudate is an essential component of the healing response in both acute and chronic wounds. It can, however, present a clinical management problem and be a sign of local infection (Cutting and Harding, 1994; Gilchrist, 1999).
In a healthy healing wound exudate is a normal feature. It arises from the vasodilation of blood vessels that occurs after haemostasis has been achieved. Pores between endothelial cells enlarge, allowing the contents of the blood vessel to leak into the surrounding tissues. This produces erythema and oedema, two of the four classical signs of inflammation. Wound exudate is composed of the fluid from the leaking blood vessels, growth factors from cells in the wound environment and debris from the damaged tissues. Later, exudate may contain contaminating micro-organisms and their components (Vickery, 1997).
Many different factors influence the production and nature of exudate: the type of dressing used, venous hypertension, posture, infection, pressure, wound type, depth and surface area are all examples. As a fluid component of the healing response, exudate provides the moisture that is essential for ‘moist wound-healing’. The biological nature of this fluid has been shown to be essential for local tissue remodelling, that is the removal of dead tissues and the promotion of new tissue growth.
Exudate from a variety of acute and chronic wounds has been characterised and differences identified. Exudate from acute wounds is rich in growth factors (Chen et al, 1992), while that from chronic wounds contains tissue-degrading enzymes (Rogers et al, 1995). In a leg ulcer study Trengrove et al (1996) found high lactate and low glucose levels in the exudate. As the wounds began to heal, glucose, bicarbonate and protein levels all increased. Donor sites have been found to contain elevated levels of the antimicrobial agent lysozyme (Buchan et al, 1980).
Wound exudate may be beneficial, but it can also be problematic. Exudate from the wound bed provides the moisture that is now known to enhance re-epithelialisation (Field and Kerstein, 1994). Exudate can, however, cause problems, especially in chronic wounds such as leg ulcers. Because it contains protease enzymes it can, on exposure to the skin around a wound, provoke excoriation and a ‘contact dermatitis’ reaction (Cameron and Powell, 1992).
Copious exudate can saturate the wound bed and peri-wound skin and cause maceration. This, in turn, can lead to enlargement of the wound. Indirectly, exudate can also increase the risk of infection as it can ‘strike through’ primary and secondary dressings and be a portal for the access of pathogens. Therefore the control of exudate is central to good wound management.
Which wounds exude
All types of wound will exude at some stage in their life cycle. The generation of exudate will depend on the nature of the wound, the mode of healing and the stage of healing. Complications such as infection will also influence the production of exudate. Typically all wounds will exude in the early, inflammatory stage of healing. As haemostasis is achieved and the repair process begins, exudate levels will fall, dropping off further as the granulation and re-epithelialisation stages progress to healing. The intrinsic factors that influence exudate production are:
- Biochemical changes - for example, those affecting capillary permeability;
- External pressure, such as might be applied by compression bandaging;
- Hydrostatic pressure, such as venous hypertension in leg ulceration;
- Temperature - this is associated with capillary dilation;
- Type of dressing and topical treatment;
- Wound depth and surface area;
- Wound infection;
- Wound type (Thomas, 1997).
Each type of wound poses its own specific exudate management problems. Chronic wounds are often the most difficult to manage in this respect, as they tend to be present in patients who have serious underlying pathologic problems, such as vascular disease, diabetes, neuropathy and cachexia. Consequently such wounds are slow to heal and at risk from infections and skin problems (Gilchrist, 1999).
The management of exuding leg ulcers is complicated by the need to compress those of venous aetiology and keep the patient ambulatory. The danger of dependent leakage and skin damage must also be considered (Figs 1 and 2). In leg ulcer patients, exudate levels may be high even from small ulcers (Fig 3) and must be kept away from the surrounding skin (Figs 4 and 5). The dressing itself has been shown to reduce the generation of exudate (Thomas et al, 1996). If graduated, sustained compression bandaging is to be applied, this too will reduce exudate, particularly if the patient elevates the affected limb. Foam-based dressings are generally not effective in absorbing and retaining high levels of exudate when used under compression (Hofman, 1997). Multilayer bandaging systems using a primary absorbent dressing and a secondary absorbent layer have been shown to be effective in extending wear time and healing venous ulcers (RCN Institute, 1998).
Pressure ulcers often require filling as well as exudate control. This, combined with the fact that they usually occur in difficult areas to dress, makes management a challenge (Berry and Jones, 1993). Absorbent fibrous dressings, such as alginate rope and hydrofibre ribbon, are effective in filling as well as absorbing exudate and maintaining a moist environment. Cavity foam dressings have also demonstrated efficacy (Harding et al, 1986). Non-healing exuding cavity wounds may benefit from vacuum-assisted closure (VAC). This system delivers negative pressure to the wound, causing arteriole dilatation locally. Preliminary reports suggest that healing is promoted even when exudate levels are high (Collier, 1997).
Diabetic foot ulcers
Diabetic foot ulcers, whether of neuropathic or neuro-ischaemic aetiology, frequently present as shallow cavity wounds that may exude heavily. Failure to control exudate will lead to maceration of the plantar callus tissue. The position of these ulcers on the foot margins and plantar surface makes management particularly challenging, as dressings are difficult to retain in position. Dressing choice is controversial, as there is very little published evidence and occasional catastrophic treatment failures are all too often blamed on the dressing. No single dressing can be recommended for these wounds. The choice depends on the condition of the wound, the patient’s circumstances and the risk of infection.
Fungating carcinomas characteristically exude heavily and may involve a large body surface area. Little clinical evidence exists as to which dressings are best. Healing is rarely a clinical objective; therefore patient comfort and avoidance of leakage and odour become paramount (Grocott, 1993). Dressings with good absorptive capacity, such as alginates and hydrofibre, are useful, as are those containing activated charcoal to reduce wound odour (Williams, 1994). Where exudate levels are particularly high, dressings with high moisture vapour transfer rates are required. These, as their description suggests, absorb fluid and then lose it to the air by evaporation (Grocott, 1997). At present there is only one widely available dressing with this property and no clinical data to support its use in this way.
Minor burns have been described as ‘a partial thickness burn involving less than 5% body surface area in adults… any burn not anticipated to require a skin graft’ (Gower, 1996). The management of exudate from minor burns requires the selection of absorbent dressings that will not adhere to the wound to avoid causing trauma and pain on removal.
The use of effective, non-adherent wound contact layers, such as perforated silicone sheet dressings with absorbent secondary dressings, has been successful (Williams, 1995). Hydrocolloids are also effective in out-patient burn management, owing to their protective barrier properties and their capacity to control moderate levels of exudate and provide long wear times (Wyatt, 1990).
Healing of donor sites is generally uncomplicated and rapid. Often total re-epithelialisation occurs in 10-14 days. Frequent dressing changes should be avoided where possible, as they are not conducive to fast healing. Exudate levels can be high, even from small areas.
Dressing selection must be aimed at keeping a moist environment without causing leakage and maceration. The current standard is paraffin gauze covered with absorbent padding. This is bulky, restricts bathing and offers no inherent pain relief. Modern dressings have been shown to be particularly beneficial in this respect, relieving pain and ‘accelerating’ healing.
Surgical wounds, such as pilonidal sinus excisions, will often exude heavily in the early stages of healing. This, combined with the difficult-to-dress location, presents a management challenge. Foster and Moore (1997) showed that these wounds are frequently dressed with ribbon gauze which causes pain and trauma on removal and does not maintain a moist environment. Further studies by Moore and Foster (1998a, 1998b) illustrated how the use of a modern hydrofibre dressing can control exudate and not cause pain and trauma.
Surgical excision wounds in the perineum present similar management difficulties. Filling to help volume reduction and exudate control is essential, as is patient comfort. Silastic foam has been used to good effect (Harding et al, 1986), as more recently has hydrofibre (Maxwell, 1998).
Acute traumatic wounds left to heal by secondary intention, such as pre-tibial lacerations, are frequent, particularly in elderly people. Hydrocolloids (Wijetunge, 1992) and hydrofibre (Premechandran, 1995) have both been shown to be useful dressings in this situation.
Recent studies illustrate the value of using modern dressings to control exudate, avoid pain to the patient and still be cost-effective. Part two will explain how to assess exudate levels, prevent complications and make the correct choice of dressing.