Introduction

Small animal veterinarians are frequently challenged with traumatic degloving, shear and bite wounds. We clean, debride and dress these wounds until either the wound is suitable for a reconstructive effort, or it has healed by second intention. This entails frequent (often daily) dressing changes over a prolonged period. Over the last decade, several mechanical adjuncts have been developed to enhance wound healing, and, of these, negative pressure wound therapy (NPWT) has shown the most clinical promise.

NPWT (also called vacuum-assisted closure (VAC), VAC therapy, topical negative pressure therapy, subatmospheric dressings) was originally developed to ameliorate the healing of chronic wounds in debilitated patients, but is now widely employed in both acute and chronic wounds as well as other surgical applications, such as free grafts, compromised flaps, incisional dehiscences, cytotoxic sloughs, abdominal drainage, orthopaedic trauma and burns. NPWT has become the first-line of therapy in the military arena to address high-energy, complex soft tissue trauma, and is able to drain, protect and immobilise wounds while wounded personnel are transported to a military hospital. There are many case reports and series in human medicine, and several randomised clinical trials. Results in veterinary medicine have been extremely promising and this modality may prove to be an invaluable adjunct to wound management for both large and small animal veterinarians.

Application and Mechanisms of Action

NPWT therapy involves the distribution of a subatmospheric pressure uniformly to all tissues within the wound. Following cleansing and meticulous debridement of the wound, a porous, open-cell polyurethane ether foam or saline-moistened open-weave gauze is packed on to the defect. Fenestrated tubing (or tubing with a fenestrated disc) is placed within the packing, and the whole wound area sealed with transparent, impermeable, adhesive sheets, which adhere to the periwound skin. The evacuation tubing is then connected to the reservoir canister of a programmable vacuum pump and either continuous or intermittent negative pressure applied to the sealed wound, between -80 and -125 mmHg. Once powered on, the dressing should contract noticeably and become hard and 'raisin-like'. NPWT dressings are changed every 48–72 hours in small animals.

NPWT has been shown to decrease interstitial oedema, thus increasing hydrostatic pressure within the capillaries, and increase perfusion to the wound and periwound. The mechanical strain on fibroblasts stimulates them to divide and produce collagen, and shear forces that deform the extracellular matrix are thought to result in a higher mitotic rate and thus increase the production of granulation tissue. A reduction in wound bacterial flora has been inconsistently documented. Early studies in swine, now also supported in dog studies, have consistently shown significantly earlier appearance of granulation tissue, with a smoother appearance.

The Veterinary Experience

In contrast to the publication numbers in the medical literature, the veterinary application of NPWT has been reported infrequently. There are a handful of case reports in several species ranging from a rhinoceros to a tortoise, a retrospective clinical case series in dogs and a recent controlled study on the effect of NPWT on the healing of open wounds in dogs. The clinical case series evaluated the outcome in 15 dogs with traumatic extremity wounds undergoing foam-based NPWT. All animals underwent successful reconstruction at an average of 4.6 days of NPWT, following rapid development of granulation tissue within the wounds. Reconstructive procedures included seven flaps and eight skin grafts, with NPWT being applied following grafting as well. Complications were considered minor and included dermatitis at the wound margin and loss of vacuum causing wound desiccation. It was concluded that VAC therapy is an effective ancilliary treatment for traumatic distal extremity wounds, and that VAC provides an effective method of securing skin grafts over the wound bed.

A randomised, controlled, experimental study compared NPWT with standard-of-care wound management in 20 forelimb wounds on 10 dogs. Granulation tissue appeared much earlier in the NPWT wounds compared to the control wounds (day 2 vs. day 7). The quality of the granulation tissue was smoother and more consistent in the NPWT wounds, and notably irregular and exuberant in the control wounds. The NPWT wounds did not contract as well as the control wounds after day 7; and NPWT wounds did not epithelialise as well after day 11. It was concluded that NPWT accelerated the appearance of smooth, non-exuberant granulation tissue. However, prolonged use of NPWT in the traumatic wound impairs wound contraction and epithelialisation.

A similar study was undertaken to determine the effects of NPWT on the acceptance of free cutaneous grafts in dogs. NPWT or standard bolster dressings were placed over 10 full-thickness, meshed sheet skin grafts. Outcomes of graft acceptance were compared between groups. First appearance of granulation tissue was significantly earlier in the NPWT grafts (day 2 vs. day 7). Superficial necrosis and epidermal sloughing were observed in 12.1% of the grafts with standard bolster dressings, compared to none in the NPWT grafts. Percentage of open meshed area was significantly smaller in NPWT compared to grafts treated with standard bolster dressings. It was concluded that NPWT can be used to optimise graft survival, and may be especially valuable for large grafting procedures where immobilisation is challenging. All dogs were adopted at the end of these studies.

We have been using NPWT since 2006 at Michigan State University, mostly in dogs, but also a few cats, horses and a rhinoceros. Just over half of these cases had acute or subacute traumatic wounds, but the rest were usually several months old, with a few over 6 months old. Most wounds were on the limbs, with some animals having various concurrent injuries, largely fractures or other underlying orthopaedic trauma (such as shear injuries, joint exposure, hardware exposure). Cause of injury was vehicular trauma in most cases, but bite wounds were also seen, and also incisional dehiscences. All wounds underwent initial management before NPWT, which included wound culture, periwound cleansing, surgical wound debridement, copious wound lavage with a pulsed irrigation system, intravenous fluids and the administration of antibiotics and painkillers. Patients had NPWT applied within 2 days of admission on average, and the mean length of time on NPWT was 5.3 days (range 1–22). Dressing changes were performed every 2–3 days with the majority of patients remaining hospitalised; however, three patients were managed at home with NPWT and had dressing changes on an outpatient basis every 72 hours. Final closure method in most of the wounds was following the appearance of granulation tissue, using a tension-relieving technique, skin flap or free graft. When free skin grafts were applied, NPWT was reapplied post grafting.

Complications associated with NPWT were minor, especially as clinicians and staff gained experience in applying and utilising this modality. Early loss of vacuum due to inadequate periwound adhesion was seen, especially in areas such as the digits, where crevices and movement can cause drape detachment and leakage. Meticulous attention to clipping and cleansing the periwound skin, using a spray adhesive and building up of uneven surfaces such as interdigital areas with stoma paste enabled us to overcome this issue. Chewing or kinking of the tubing also caused failure of the device, but only in a couple of cases (surprisingly). Once technical issues were resolved, management became much easier. Many cases developed mild skin irritation with prolonged NPWT, but our patients tolerated NPWT well, and using the modality became much easier once we overcame initial technical hurdles and learnt tricks of ensuring adequate periwound adhesion. This modality is most useful in preparing the wound bed for a reconstructive procedure. It appears to 'kick-start' the wound bed into the reparative phase, optimising the wound environment for epithelialisation and contraction. One of the biggest advantages is the avoidance of daily anaesthesia or sedation to change wet-to-dry dressings. The patient can recover between sedations to eat, drink and ambulate. Strike-through is also eliminated in NPWT as all wound fluids are evacuated.

In summary, it appears that NPWT will play a beneficial role in veterinary medicine, certainly in the early management of traumatic open wounds and to increase the acceptance of free skin grafts. It also seems to be of benefit in dehiscences, flaps and possibly even closed wounds under tension. Further clinical trials in small animals are warranted.

References

1.  Ben-Amotz R, Lanz OI, et al. The use of vacuum-assisted closure therapy for the treatment of distal extremity wounds in 15 dogs. Veterinary Surgery 2007;36:684–690.

2.  Demaria MG, Stanley BJ, et al. Effects of negative pressure wound therapy on the healing of open wounds in dogs. Veterinary Surgery 2011;40:658–669.

3.  Harrison TM, Stanley BJ, et al. Surgical amputation of a digit and vacuum-assisted closure (V.A.C.) management in a case of osteomyelitis and wound care in an Eastern Black Rhinoceros (Diceros bicornis michaeli). Journal of Zoo and Wildlife Medicine 2011;42:317–321.

4.  Morykwas MJ, Argenta LC, et al. Vacuum-assisted closure: a new method for wound control and treatment: animal studies and basic foundation. Annals of Plastic Surgery 1997;38:553–562.

5.  Owen L, Hotston-Moore A, et al. Vacuum-assisted wound closure following urine-induced skin and thigh muscle necrosis in a cat. Veterinary and Comparative Orthopaedics and Traumatology 2009;22:417–421.