Overview

Topical negative pressure therapy assists wound closure by applying localized negative pressure to a wound to promote wound contraction, angiogenesis and removal of excess fluid. Foam or gauze is inserted into the wound and covered with a film. Negative pressure of between 80 mmHg and 150 mmHg is then applied. Examples: V.A.C. Therapy, Renasys Go and VENTURI™ AVANTI. Indication: Acute and chronic wounds. Pros: Promotes localised blood flow, reduces localised oedema, promotes granulation and epithelisation, supports moist wound healing, allows gas exchange, protects the wound base from environmental contaminants. Cons: Requires a power source and specialised dressings. Notes: should be avoided in presence of necrotic eschar, untreated osteomyelitis and malignant wounds (49).

 

Description of System

The concept of using negative pressure is to create a suction force enabling the drainage of surgical wounds to promote wound healing. If excess fluid is not adequately removed, its components may be deterrents to wound healing. The TNP therapy equipment consists of a reticulated foam dressing that is inserted in the wound and sealed in place with an adhesive dressing. A suction force is then applied by a VAC (Vacuum Assisted Closure) machine across the wound surface. The polyurethane (black and easily deformable) or polyvinyl-alcohol foam (White, stiffer, requires higher pressures) is cut to fit the wound cavity exactly. Black foam is used more in the UK. The foam is covered with an adhesive drape into which a small hole is cut and a TRAC (therapeutic regulated accurate care) pad applied over the hole. The adhesive dressing creates a sealed environment for moist healing and the TRAC pad is connected to the VAC machine: source for suction and drainage (49).

 

Once in position the TNP dressing provides a stable environment, enabling the wound to heal even in a mobile patient without shear forces affecting the wound. Most clinicians use TNP therapy at a sub-atmospheric pressure of 125 mm Hg using PU (black) foam. TNP dressings should be changed every 48–56 hours except in exceptional circumstances (e.g. over a skin graft). Some manufacturers recommend that dressings are also changed more frequently in the presence of aerobic infection. Dressings routinely left for >56 hours can lead to increased discomfort during dressing changes because of ingrowth of granulation tissue into the foam (49).

 

Mechanisms Involved

Despite clinical success and widespread introduction into clinical practice, it is not known exactly how TNP therapy exerts effects on the wound but several mechanisms have been proposed. TNP is said to increase local blood flow and reduce oedema and bacterial colonisation rates. It is thought to promote closure of the wound by promoting the rapid formation of granulation tissue as well as by mechanical effects on the wound. It concurrently provides a moist wound environment and removes excess wound exudates thus helping to create the “ideal wound healing environment” (49).

 

Accelerated cell cycling and DNA synthesis have been seen. Clinically, TNP removes large amounts of fluid from wounds especially acute burns. The resulting reduction in oedema may aid in the enhancement of blood and nutrient flow into the wound. However, this removal of exudate and oedema from the surrounding tissues encourages nutrient movement into the wound area even if blood flow is not increased. Removal of fluid prevents a build-up of inflammatory mediators and encourages diffusion of further nutrients into the wound. This is all beneficial to the healing process especially in the case of chronic wounds where it has been hypothesised that an imbalance of metalloproteinases can inhibit healing (49).

 

Costs and Evidence

The daily rental charges for a VAC machine and consumables are significant and has discouraged many from using the system. However, there have been some reports showing that the increased healing times and downgrading of required operations correlates to decreased overall costs of care. The dressing should also enable larger wounds to be treated in the community with minimal nursing care impact. This would free up hospital beds permitting faster throughput of operative patients and preventing waiting list build up (49).

 

Oedema reduction and bacterial clearance, mechanisms that were attributed to TNP therapy, were not proven in basic research. Several RCTs reported a positive effect of TNP therapy on wound healing in diabetic ulcers. In total, 3 RCTs and 2 large retrospective studies reported favourable results using TNP therapy in grade 3 and 4 pressure ulcers. It was observed in several studies that wound treatment using TNP therapy was more comfortable for patients and could very well be used in outpatient care. Cost efficiency was in favour of TNP-treated wounds in both pressure ulcers and venous leg ulcers. In abdominal wall wounds, Closure rates between 78% and 93% were achieved with VAC therapy however, should be used with caution. It is especially beneficial for chronic wounds like diabetic foot ulcers, venous and pressure ulcers as they seemed to do well after the adjunctive therapy (50).