Richard M. Jerram, BVSc, DACVS
One of the most common emergency conditions faced in general veterinary practice is the presentation of an animal with a wound. Generally, this is accompanied by a distraught owner who wants to see the wound closed. Unfortunately, the desire by both the owner and the pressured veterinarian to close the wound immediately can result in later wound complications such as deep infection, dehiscence, seroma formation, and wound contracture.
Following initial triage and stabilization of the animal, some time should be taken to accurately assess and classify the wound before reaching for the suture material. It is important to inform the animal's owner that wound management can be time-consuming and expensive.
In general, wound healing is a complex process but for simplicity it can be broken down into three phases, inflammatory, proliferative and remodelling.
The inflammatory phase is characterized by increased vascular permeability, cell activation, release of cytokines and chemotaxis. Chemotactic substances (complement, interleukin, TNF-α, prostaglandins) released from platelets and damaged endothelial cells attract polymorphonuclear (PMN) cells and macrophages. PMN are responsible for removal of microorganisms and neutralizing irritants but are only short-lived. Macrophages help in wound debridement and phagocytosis. Activated macrophages release cytokines that promote fibroplasia and angiogenesis.
The proliferative phase is characterized by the proliferation of fibroblasts and endothelial cells. Platelets and macrophages regulate fibroblast activation. The fibroblasts, using fibrin as a scaffold, begin synthesizing collagen, proteoglycans and elastin by 4–5 days after injury. The capillary buds originate from existing venules and are supported by the developing collagen network with the newer collagen synthesis moving ahead of the capillary bud.
The remodelling phase is characterized by the deposition of collagen in the wound. This process can go on for up to 1 year and is the clinically critical phase of wound healing. A wound without collagen lacks the strength to withstand the forces of normal activity and will dehisce.
The other two important concepts in wound healing are epithelialization and wound contraction. The cells at the margin of the wound proliferate and migrate under the scab or clot secreting proteolytic enzymes to cleave a path in front of them. This can occur by 48 hours in sutured clean wounds. Specialized fibroblasts (myofibroblasts) attach to the underlying dermis and contract pulling the skin with them. Counter tension from limited available skin in areas like distal limbs can reduce the effect of wound contraction.
Approach to Wound Management
Although an open wound can be a distressing sight in an animal following trauma it may not be the most life-threatening condition present. It is critical to establish an overall "Animal Plan" before managing the wound. The animal should be stabilized with appropriate respiratory, analgesic, and circulatory aids such as oxygen therapy, intravenous fluid therapy and opioid medication. Thoracic radiographs are strongly recommended in patients following road traffic trauma and complete assessment of other injuries (orthopedic, neurologic, thoracic and abdominal) should be carried out.
Only once these serious considerations have been addressed should a "Wound Plan" be instituted. The "Wound Plan' can include wound classification, infection control, lavage, debridement, drainage, closure, bandaging and reconstructive surgery.
By definition, wound infection occurs when more than 105 organisms per gram of tissue are present. Local wound factors such as necrotic tissue, foreign bodies, tension at the wound edges, dead space, seroma formation, and excessive suture material can lower the number of bacteria needed to establish an infection. In addition, underlying systemic disease conditions such as anemia, hypoproteinemia, diabetes mellitus, hyperadrenocorticism can also impair the patient's resistance to infection.
The decision on if and when to close a traumatic wound is not always clear. Probably, the most common cause of wound dehiscence is inappropriate wound management and primary closure of a contaminated wound. Delayed primary closure is the preferred method of treating contaminated wounds. There is a temptation amongst us to deal with the wound there and then rather than recommend wound management and delayed closure. The contaminated wound is initially treated with daily lavage, debridement and bandaging followed by closure 2 to 5 days after the injury. The additional time allows for definitive judgment of tissue viability and therefore appropriate selection of debridement margins, as well as decreasing the incidence of wound infection.
Basic surgical techniques such as gentle tissue handling and aseptic technique apply to the closure of even small wounds. Adequate hemostasis, closure of dead space or the use of drains will decrease the opportunity for hematoma or seroma formation under the wound. A wide area of hair around the wound edges should be clipped prior to closure; a water-based lubricant jelly can be applied into the wound to reduce hair contamination. Tension at the wound edges will delay healing and increase the chance of dehiscence. Contemplate tension-relieving techniques such as walking sutures, patient positioning, undermining the adjacent skin, suture patterns (vertical mattress, stent, and far-near-near-far sutures) and local skin flaps. Monofilament nonabsorbable (nylon, polypropylene) and absorbable (polydioxanone, poliglecaprone) suture materials are preferred.
Wound management begins with lavage to actively flush debris from the wound. The wound edges should be widely clipped and cleaned. Appropriate lavage requires sufficient pressure to reduce bacterial numbers. Lactated Ringer's solution has been shown to be the least damaging lavage solution to fibroblasts. Chlorhexidine diacetate (0.05% solution) can be added to the lavage solution as it can reduce bacterial contamination, has a residual activity, and has activity in the presence of organic debris. Povidone-iodine (1% solution) has good antimicrobial activity but is inactivated by organic matter and has no residual effects.
Hydrogen peroxide and Dakin's solution (bleach) have no benefit as antiseptics and may injure cells and delay healing.
Debridement is the removal of obviously devitalized tissue from a wound. This is generally accomplished surgically using a scalpel or sharp tissue scissors. Debridement may need to be performed on several occasions during management of a wound. Surgical debridement is best performed under general anesthesia or heavy sedation but local analgesia may be sufficient in some animals. Demarcation of vital tissue is often difficult in an acute wound due to local vasospasm and edema. In these cases, debridement can be delayed several days to fully distinguish devitalized from healthy tissue. Muscle and fat can be aggressively debrided but nerves, supporting structures (tendons, ligaments) are preserved. Mechanical wound debridement can be performed using wet-to-dry bandaging techniques (see later).
The use of systemic antibiotics has not been shown to decrease infection in dogs and cats with bite wounds. As antibiotics should reach a therapeutic level within three hours of the wound occurring, intravenous antibiotics would seem appropriate early in wound management. Topical antibiotics are reserved for superficial wounds or wounds that are being prepared for skin grafting. Topical antibiotics are not a substitute for poor wound lavage and debridement and they may delay epithelialization.
Wound Medications and Dressings
Over the last few years many new or "rediscovered" topical medications have become available that can stimulate wound healing at a cellular level using natural or synthetic growth factors. Not all of these products have been clinically evaluated in veterinary medicine but the beneficial qualities of the medications means that their use may help in healing, particularly of large or complicated wounds.
Honey has been used for centuries to treat wounds and its use has come back into "vogue" for large, contaminated wounds. Honey enhances healing, decreases inflammatory edema, and has a bacteriocidal effect making it an extremely cost-effective wound medicant.
Moist Wound Healing
Although wet-to-dry bandaging is probably still the most commonly used method of wound debridement in veterinary cases it has disadvantages. Removal of the bandaging is painful and non-selective in that new epithelial cells, white blood cells, and fibroblasts are also removed. Recently, Moist Wound Healing (MWH) has become the standard of care for human wounds. Basically, the principles of MWH are to maximise the body's wound healing abilities by maintaining the cells, growth factors and matrix in a warm, moist environment. Granulation tissue formation and wound contraction are enhanced by MWH as white blood cells and proteases provide debridement selectively to necrotic tissue only. Infection rates are not increased with MWH despite the moist wound environment as the white blood cells and proteases are maintained in the wound. Dressings can be maintained for longer periods and as the dressing is occlusive the risk of introduced infection is reduced.
Despite the obvious advantages of MWH, careful control of the animal and the wound is still paramount and although there are less bandage changes, overall costs may not be much different from traditional wet-to-dry debridement dressings.
I have treated several wounds with calcium alginate as the initial debridement material. Calcium alginate is derived from seaweed that looks like a piece of felt when dry. When placed in a wound, the calcium ions are exchanged with the sodium ions in the wound exudate, this turns the dressing into a gel conforming to the wound. Calcium alginate stimulates granulation tissue formation and hemostasis. Dressings are changed every 1–3 days depending on strike through. The wound surface looks yellow and smells a bit but this is normal for calcium alginate.
Once debridement is complete, polyurethane foam can then be used to support epithelialization across the developing granulation bed. This product absorbs fluid while maintaining a moist wound environment. Bandages can be changed every 3–7 days and the foam can be lifted out of the wound without causing pain.
As these products become more commonly used in veterinary practice I believe that they will ultimately replace the traditional wet-to-dry form of wound management.
Vacuum-Assisted Wound Closure
Vacuum-assisted (VAC) closure of wounds has recently been promoted for use in degloving wounds, large bite wounds, and in chronic, non-healing wounds. VAC is an active wound management system that creates subatmospheric pressure within the wound to improve circulation, remove exudate and promote granulation tissue formation. Under a VAC dressing, capillary blood flow is enhanced, endothelial spaces are narrowed and the capillary basement membrane integrity is restored. These factors result in reduced edema formation and restoration of blood flow to collapsed blood vessels. VAC therapy also increases granulation tissue development by mechanically stimulating cells and by removal of degradative enzymes.
All open wounds require the application of a bandage to optimize wound healing. Unfortunately, careful early wound management can be undone by application of a carelessly constructed bandage. Bandages should be composed of three layers.
The primary or contact layer can be used to debride tissue, to provide an occlusive seal over a wound or to draw wound exudate away from the wound. Adherent primary layers are used in the debridement stage of wound healing and nonadherent layers are used during the reparative stage.
The wet-to-dry bandage is the most widely used adherent dressing for large open wounds containing foreign material or necrotic tissue. The primary layer is constructed by wetting sterile surgical gauze swabs with sterile saline, LRS, or a 0.05% chlorhexidine solution. A layer of dry sterile gauze is placed over the wet layer to increase the absorptive capacity. Removal of the gauze from the wound along with the attached necrotic material can be painful so sedation or analgesia should be considered.
Nonadherent primary layers can be semiocclusive or occlusive. The semiocclusive dressing allows excess exudate to be absorbed from the wound. Occlusive dressings speed up epithelialization and are indicated for wounds that have a good granulation bed and have begun to epithelialize.
The secondary layer is designed to absorb wound exudate, blood, and serum. This layer must be soft and thick enough to absorb fluid, to prevent excessive movement of the wound, and to protect the wound from further external trauma. Generally, the secondary layer consists of rolled cotton wool or cast padding.
The tertiary layer of a bandage holds the underlying bandage material in place. Extreme care must be taken to avoid applying the tertiary layer too tightly as absorption is restricted and constriction of venous or lymphatic drainage may occur around the wound.
The area of most neglect with bandaged wounds is aftercare. Regular scheduled appointments should be made with the owner and owners should be instructed on bandage care. Sedation and analgesia make regular bandage changing easier. Typically, a wet-to-dry bandage should be changed 1–2 times daily. Nonadherent dressings are initially changed daily and the progress of the wound healing dictates when to lengthen the time between bandage changes. No bandage should be left more than one week without removal or assessment.
The addition of a drain to a wound can help prevent seroma or abscess formation by eliminating dead space and removing accumulations of fluid. A drain is not a substitute for poor wound cleaning, lavage, and debridement. Where possible the drain should be covered with a sterile bandage to avoid nosocomial bacteria gaining access to the wound via the drain. A drain should not exit the wound through a suture line, as dehiscence is likely. Penrose drains are the most common drain used in veterinary practice. Active, closed-suction drains allow continuous drainage and can be placed independent of gravity. Closed-suction drains allow the bandage to be kept dry, reduce the risk of ascending infection, allow calculation of drainage amount, and can be placed distant from the wound. Drains act as foreign bodies and produce fluid themselves; therefore, a drain is usually removed when only a small volume of serosanguineous is retrieved.
Mild hypoxia in a wound is a stimulus to white blood cell adherence and activation but subsequent phagocytosis requires normal oxygen tension. In chronic wounds, the oxygen tension can be low contributing to increased infection and reduced angiogenesis. Hyperbaric oxygenation improves the distance oxygen can diffuse by a factor of four compared to normobaric oxygenation. Tissue oxygen tension remains elevated at greater than 10% of normal for up to 3 hours following a one hour HBOT session. HBOT also improves blood flow by reducing platelet aggregation and improving pliability of the red blood cells.
Hyperbaric oxygen has been advocated for treatment of chronic diabetic ulcers in humans and while veterinary research is limited, hyperbaric oxygen has been used as an aid in the management of large wounds in dogs and cats.
Reconstructive Surgical Techniques
Many large wounds are unable to heal by wound contraction and epithelialization or this second intention may result in debilitating wound contracture especially around joints or thin easily damaged epithelium. In these instances, skin reconstructive techniques should be considered. Numerous pedicle flaps, free skin flaps and skin grafting techniques have been described for reconstruction of wounds even on the distal extremities. Complete description of these techniques is beyond the scope of this discussion; however, early wound management can be directed towards a specific technique. Therefore, the veterinarian should have knowledge of appropriate skin reconstruction techniques.
References are available upon request.