VOL: 97, ISSUE: 35, PAGE NO: 51
HELENA BAXTER, MSc, RGN, is clinical nurse, specialist tissue viability, Tissue Viability Unit, Guy’s Hospital, London
KATE BALLARD, BSc, RGN, is clinical nurse specialist, tissue viability, and clinical trials coordinator, Tissue Viability Unit, Guy’s Hospital, London
Vacuum-assisted closure (VAC) is a simple but effective method to promote rapid wound-healing. In recent years it has been shown to be an effective therapy for the management of large, complex, acute wounds as well as chronic wounds that have failed to heal by conventional methods (Joseph et al, 2000).
What is VAC?
Vacuum-assisted closure is an active wound therapy rather than a wound dressing. First described by Morykwas and Argenta (1997), the VAC system applies a subatmospheric, or negative, pressure to the wound bed via an open-cell polyurethane foam dressing.
The foam acts as the wound contact material and fills the wound. It is then connected via a suction tube to a canister, which fits on the side of the vacuum pump unit. The whole system is reliant on there being an effective seal around the dressing and this is achieved using the VAC semi-permeable film drape.
Once the seal has been obtained, the pump can be set to deliver continuous or intermittent pressures, ranging from 50mmHg to 200mmHg. The normal therapeutic level is 125mmHg, but this may be reduced if the wound is particularly painful. An intermittent regime normally follows a seven-minute cycle of two minutes off and five minutes on, with the negative pressure maintained for the whole time.
The foam dressing is usually changed between days two and five, depending on the speed of tissue growth.
If granulation is rapid the length of time between changes should be reduced to prevent adherence to the wound bed.
The open-cell polyurethane foam dressing enables equal distribution of the negative pressure over the whole wound bed. It also allows exudate to flow freely for collection and removal in the canister. The foam can be used to pack open cavity wounds and can also be cut to size to fill undermining areas.
The pore size of the VAC foam dressings differs from other foam dressings in that the pores are larger to maximise tissue growth (Morykwas et al, 1997). Therefore, it is important to use only the VAC foam dressing with the VAC pump.
Objectives of treatment
Applying negative pressure to the wound bed via the VAC pump achieves three main objectives:
- It removes excess exudate and promotes a moist, rather than wet, wound-healing environment. It also reduces oedema in the surrounding tissues that, if left, can impair wound-healing by reducing localised blood flow (Joseph et al, 2000);
- It promotes granulation tissue through increased angiogenesis (Morykwas et al, 1997);
- It reduces bacterial count at the wound bed.In animal studies, blood flow level to the wound bed increased four-fold when 125mmHg subatmospheric pressure was applied (Morykwas et al, 1997).
Granulation tissue formation was increased by approximately 64% on 125mmHg continuous pressure, and 103% on 125mmHg intermittent pressure when compared to standard wound management (Morykwas et al, 1997).
One of the main strengths of the VAC is its ability to promote healing in wounds with complex aetiology and in those that have failed to heal with conventional therapy (Morykwas and Argenta, 1997; Joseph et al, 2000).
Which wounds can be treated with VAC?
Almost any type of wound can be treated with VAC, provided that a seal can be obtained and maintained during therapy. VAC can be used successfully on the following wound types:
- Pressure ulcers;
- Diabetic foot ulcers;
- Acute/trauma wounds;
- Leg ulcers;
- Dehisced surgical wounds;
- Skin grafts;
- Rotational/free flaps;
- Postoperative mediastinitis (Catarino et al, 2000).
Contraindications and precautions
Because VAC therapy increases cell generation and applies suction to the wound bed, it is contraindicated in known or suspected malignant wounds, in wounds with a fistula present or in untreated osteomyelitis. Similarly, the exposure of large blood vessels at the wound site would negate its use owing to the increased risk of bleeding (KCI, 1999).
Presence of a thick, necrotic eschar is a contraindication for VAC therapy, as the wound ideally needs to be clean before application. The VAC system is not able to debride large amounts of devitalised tissue, although it can cope adequately with small amounts of soft slough at the wound base (KCI, 1999).
Although not a contraindication for VAC therapy, extra care should be taken with patients who are likely to experience bleeding problems. These include patients on long-term anticoagulant therapy or those with haemophilia or haemoglobinopathies, such as sickle cell disease. These patients require more careful observation and may require lower pressure settings.
Use on colonised or infected wounds
Heavy colonisation or wound infection does not prevent VAC therapy from being used. On the contrary, VAC therapy has been shown to reduce bacterial count at the wound bed up to 1,000 times after four days of VAC treatment compared to non-VAC-treated wounds (Morykwas and Argenta, 1993).
The application of negative pressure creates a hypoxic environment at the wound bed/dressing interface and therefore discourages growth of aerobic bacteria. However, it is thought that anaerobes also find survival difficult, as the negative pressure encourages blood flow into the wound bed which supplies macrophages, neutrophils and oxygen to the area (Hampton, 1999).
Since its mode of action is not selective, VAC therapy is effective against difficult invaders/colonisers, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. Infected wounds should be treated with the appropriate systemic antibiotic therapy and, as with other conditions, extra care should be taken with VAC therapy in acute wound infection. The dressing should be changed, and the wound bed observed, every 24 hours.
To ensure constant drainage of contaminated material away from the wound bed, continuous therapy is recommended for the acute period.
Sample case study
Fifty-nine-year-old Maria Brown has had multiple sclerosis for 30 years. She lives alone with help from a part-time carer. Three to four days before her emergency admission her condition had begun to deteriorate, her mobility had reduced and she was having difficulty talking and swallowing. On arrival at the A&E department Ms Brown was hot, sweaty and tachycardic, and the primary diagnosis was given as an infected suprapublic catheter. Further examination revealed she had a large pressure ulcer on her left ischium, the size of a fist and 2cm deep. It was necrotic, malodorous and oozing moderate amounts of exudate.
A tissue viability assessment was requested and pressure-relieving equipment was immediately provided. The assessment documented an 8x9cm necrotic, grade four pressure ulcer with induration and cellulitis of the left hip. Although sharp debridement of the devitalised tissue was attempted, much still remained and it was difficult to determine whether the ulcer tracked into the hip joint. Bone scans and X-rays were organised to rule out osteomyelitis, and a plastic surgery referral was made for full debridement and exploration. Appropriate broad spectrum antibiotics were prescribed.
Over the next two days the ulcer increased in size, the cellulitis spread and the indurated area now extended down to the mid-thigh. Although the X-ray excluded bony changes, the infection was clearly spreading and Ms Brown’s general condition was not improving.
Surgical debridement of the affected area revealed a wound extending from the left ischium down to the mid-thigh (36cm long) down to the deep fascia, with two sinuses extending further down to the knee. With the tissue flaps lifted, the wound measured 17cm across, incorporating the whole lateral thigh. Immediately after surgery the wound was packed with betadine-soaked gauze, which was later changed to alginate dressings. The exudate was so heavy that the dressings needed to be changed three to four times a day, which was time-consuming and was causing Ms Brown a great deal of pain and distress.
Forty-eight hours after surgery, when the wound had settled, VAC therapy was started. This enabled dressing changes to be reduced to once every three to four days with excellent management of exudate. In order to achieve a better seal and facilitate a longer wear time the surrounding skin was prepared with an ostomy skin wipe. This cleared the greasy surface of the skin and helped the film dressing to stick.
Initially, the VAC was applied at 100mmHg, but was reduced to 75mmHg as Ms Brown found this too uncomfortable. After some adjustments to her analgesia, the pressure was increased to 100mmHg continuous pressure after the first dressing change.
Dressing changes were performed every three to four days when the foam dressing, drape, tube and canister were all replaced. Before removal of the dressing 1% lignocaine solution was inserted into the tube and left for two minutes to ensure painfree dressing changes.
After two weeks of VAC therapy, it was decided to give Ms Browne a rest from this treatment as she was still experiencing pain from her wound. At this time the wound bed showed healthy, highly vascularised granulation tissue and had reduced in size. Alginate packing was started. Although VAC therapy had been discontinued, the ensuing two weeks saw dramatic improvements in the wound bed. The 10cm sinuses healed within two days and the wound settled and contracted, with granulation tissue reaching the surface.
Although the wound-healing rate slowed after the VAC had been discontinued, therapy was not restarted as Ms Brown had begun intensive rehabilitation at the neuro-gym. Alginate dressings were then deemed a more suitable, less bulky choice to continue the healing process.
Although no formal studies have yet been undertaken on cost benefit analysis of the VAC versus standard wound management, a recent study by Catarino et al (2000) showed that the use of VAC as a first-line therapy in poststernotomy mediastinitis led to reduced length of hospitalisation, reduced pain and fewer treatment failures, which negated the need for further reconstructive surgery.