Open wounds are a frequent reason for emergency and general medical consultations in our small animal patients. Managing wound healing can be a real challenge, particularly when primary wound closure is not possible. Many techniques and equipment are now available to accelerate wound healing. These include negative pressure wound therapy (NPWT), which has been developed in veterinary medicine over the last 10 years.
Why is negative pressure wound therapy useful in veterinary practice?
Negative pressure wound therapy involves applying sub-atmospheric pressure across a wound to improve and accelerate healing. This pressure modifies the interstitial fluid gradient, creates stress and tension in the extracellular matrix and establishes macro-deformation in the tissue, resulting in the release of growth factors (VEGF and FGF-2, in particular) (Erba et al., 2011; Morykwas et al., 1997; Saxena et al., 2004).
Negative pressure wound therapy involves applying sub-atmospheric pressure across a wound to improve and accelerate healing
The supposed advantages of this therapy are to (Erba et al., 2011; Morykwas et al., 1997; Saxena et al., 2004):
- Improve microvascular perfusion of the wound
- Reduce oedema and drain exudate and cellular debris from the wound
- Stimulate granulation tissue formation
- Stimulate cell proliferation
- Draw wound edges together
- Help protect the wound by creating a moist and closed environment
Improved patient comfort in humans has been demonstrated in a randomised controlled trial (Braakenburg et al., 2006).
The role of NPWT on the bacterial burden of the wound is unclear, and studies are contradictory (Braakenburg et al., 2006; Patmo et al., 2014). However, in acute infected wounds, NPWT could up-regulate some interleukins (IL-1β, IL-8) that may, in turn, trigger the accumulation of neutrophils and accelerate bacterial clearance (Liu et al., 2014).
How to install negative wound therapy systems
A system of negative pressure wound therapy must be implemented under sterile conditions and general anaesthesia (Figure 1). The main principles of wound management should always be applied regardless of the dressing used.
Preparing the patient for NPWT
Hair must be clipped with 3 to 5cm margins to allow adequate contact with the adhesive drape. The wound and peri-wound should be cleaned and copiously rinsed, and all necrotic tissue and debris should be removed. Peri-wound skin must be completely dry to allow adhesion of the drape; a liquid skin adhesive could be applied to the peri-wound skin.
Building and installing the system
The contact layer used is usually a polyurethane foam, with 400 to 600µm pores (Figure 1A). The foam should be cut to fit inside the wound, and practitioners should take care to avoid compressing the wound edges. A tulle gras dressing can be placed in contact with the wound to prevent the foam from adhering to the injury. An impermeable sterile drape (with topical adhesive) is then applied to seal the wound (Figure 1B) – folds should be avoided.
A 2 to 3cm round hole is cut in the drape and placed over the foam dressing. Then a second drape formed of an adhesive fenestrated disc with evacuation tubing is placed over the hole (Figure 1C). (In some systems, the evacuation tubing is already connected to a small adhesive drape.) This tubing connects the foam and wound to a vacuum pump with a collection chamber (Figure 1D).
Once the pump is activated, the dressing contracts and becomes firm. Activation of the pump drains exudate from the wound into the absorbent layer and the excess is retained in the collection chamber. If NPWT is used on a limb, the dressing can be re-covered by a soft padded bandage.
Many NPWT systems are available: the original V.A.C. system (Kinetic Concepts Inc), Venturi Compact (Talley Group), MoblVac (Ohio Medical Corporation) and Prospera (DeRoyal). Some work with a mechanical manual pump (3M Snap; Figure 1D) and not an electric one.
Management of the patient and NPWT system
The dressing should be checked every four hours and should be changed every two to five days for a total of two to three changes before surgical closure or second intention healing. If the wound is contaminated, a daily change is recommended. In young dogs and cats, NPWT dressings should be changed every two to three days because granulation tissue develops quickly. Each time the dressing is changed, the wound must be cleaned and necrotic tissue removed.
Negative pressures of 50 to 150mmHg can be used, but generally, a pressure of −125mmHg is used for open wounds (Morykwas et al., 1997, 2001). This pressure is used because the princeps study by Morykwas et al. (1997) showed that the application of −125mmHg to pig wounds increased microvascular blood flow to four times above the baseline. Lower negative pressures may be used in some situations: pressures of −50mmHg for wounds with excessive serous drainage and post-operative prevention of seroma and oedema, or pressures from −65 to −75mmHg for skin grafts.
When is using negative wound therapy indicated and advantageous?
There are several indications that suggest that negative pressure wound therapy shows promising results. These are cases of acute open wounds secondary to trauma, burns or abscesses; non-tumoral chronic wounds; wound dehiscences; necrotising fasciitis; skin grafts and flaps; and high-risk, closed surgical incisions or open peritoneal drainage.
Traumatic wounds
Above all others, the major advantage of NPWT for acute traumatic wounds lies in the faster appearance of granulation tissue (Figure 2) compared with conventional dressings in which granulation appears during the epithelialisation and wound contraction phases (Ben-Amotz et al., 2007; Demaria et al., 2011). The granulation tissue is also smoother in wounds treated with NPWT.
The major advantage of negative pressure wound therapy for acute traumatic wounds lies in the faster appearance of granulation tissue compared with conventional dressings
Several studies have also shown an earlier surgical closure time than various other dressings (non-adherent gauzes, silver dressings) in dogs and cats (Nolff et al., 2017, 2018).
Chronic wounds
In chronic wounds such as decubital ulcers, the interest of NPWT would be to stimulate the wound to a proliferative phase. However, the underlying cause (pressure, chronic infection, foreign body, etc) should be addressed first.
Skin grafts
Another major indication is to protect skin grafts and facilitate their stabilisation. NPWT also helps to maintain graft–wound-bed contact by reducing fluid accumulation under the graft and limiting desiccation and bacterial contamination (Nolff and Meyer-Lindenberg, 2015; Stanley et al., 2013). In an experimental study comparing NPWT with conventional graft bandages, the authors observed a faster appearance of granulation tissue, better contraction of mesh graft holes, faster adhesion to the recipient bed and less graft necrosis (Stanley et al., 2013).
Negative pressure wound therapy […] helps to maintain graft–wound-bed contact by reducing fluid accumulation under the graft and limiting desiccation and bacterial contamination
Dehiscence and infections
NPWT has been used for closed wounds at risk of dehiscence or infection, such as wounds under tension or in a mobile area, arthrodesis wounds or wounds secondary to high-intensity trauma and skin flaps. Studies have shown that this technique can reduce the incidence of post-operative haematomas and seromas, as well as tension and shear forces on the incision (Or et al., 2015; Perry et al., 2015).
Contaminated open wounds
NPWT can also be used in large, open, contaminated abdominal wounds or in septic peritonitis treated with open abdominal drainage. In these cases, the negative pressure system is applied after initial surgery to explore the wound and/or celiotomy to eliminate the primary disease process.
For open abdominal drainage, the cranial third of the linea is loosely sutured with a simple continuous pattern (1 to 6cm gaps between skin edges). Sterile foam is placed over the gap, and NPWT is set up as previously described. A second procedure is then performed to close the wound between 24 and 72 hours after the installation of NPWT depending on the amount of fluid collected and its cellularity. This technique has proved to be an alternative in the management of septic peritonitis (Buote and Havig, 2012; Spillebeen et al., 2017). However, there is a significant risk of nosocomial infection; further comparative studies are needed to assess its value and influence on survival.
Contraindications and complications of NPWT
Contraindications to the use of NPWT include tumoral wounds, uncontrolled infection or the presence of a fistula, necrotic wounds requiring surgical debridement, open joints and underlying untreated osteomyelitis, active bleeding and coagulopathies.
The most frequent complications are negative pressure losses, adhesions of granulation tissue to the contact layer (foam or compress) and skin irritation. To prevent adhesions, a non-adherent dressing can be placed over the wound and under the foam.
Conclusion
Negative pressure wound therapy is a wound treatment tool with a wide potential field of application that is likely to offer benefits in terms of wound healing. However, NPWT comes at a significant cost due to the equipment used, but this can be offset by the time saved on healing and the absence of daily dressing changes.
Negative pressure wound therapy is a wound treatment tool with a wide potential field of application that is likely to offer benefits in terms of wound healing
