Open Wounds

Chapter 76

Open Wounds


A wound represents a loss in integrity of the skin and underlying tissue. Wound healing is the physiologic process that restores continuity of the tissues after injury, ultimately resulting in a continuous epithelial surface. Wound healing consists of four steps: (1) formation of a fibrin–platelet clot at the site of injury, (2) recruitment of white blood cells to protect the site from infection, (3) neovascularization and cellular proliferation, and (4) tissue remodeling. Wound healing is often described by its different phases: inflammation, debridement, repair, and maturation (see Chapter 10). These phases overlap and are affected by multiple endogenous and exogenous factors.

In general, immediate response of the skin to injury is vasoconstriction, which lasts for 5 to 10 minutes. Subsequent vasodilation brings fluid and cells to the site and, within 30 to 60 minutes, the endothelium of vessels within the injured area is coated with leukocytes. Through gaps between venule endothelial cells, fluid and macromolecules leak into the injured area, with magnitude of transudation increasing over the next 3 days. Attracted by chemotactic factors, leukocytes migrate into the wound. Initially, polymorphonuclear cells predominate; however, they are short lived and, by day 5, mononuclear cells predominate in wounds. Early repair starts within 3 to 5 days after the initial injury, with proliferation of fibroblasts and endothelial cells forming granulation tissue. When a sufficient granulation bed is present (usually 4 to 5 days after wounding) in a full-thickness open wound, epithelialization begins. Healing is facilitated by transformation of some fibroblasts into specialized myofibroblasts about 1 week after wounding. Contraction of myofibroblasts decreases wound size. Eventually, collagen is deposited by fibroblasts while fibrin strands are removed. Collagen undergoes continuous remodeling to strengthen the wound.

Types of Wounds

The practices of wound management used to facilitate wound healing depend on the type of the wound and the active stage of wound healing.

Thermal Burn

A thermal burn is the result of close proximity or direct application of heat to the skin. Fire, cage dryers, heating pads, electrical cords, heat lamps, hot liquids, and malicious incidents are all causes. A thermal burn may have varying levels of tissue injury, depending on the severity. Deep and extensive injury can cause serious systemic compromise because of severe fluid, electrolyte, and protein loss; the risk for development of wound infection and sepsis is high. Burns have classically been described by the depth of injury as superficial partial (epithelial), deep partial (epithelial and partial dermal), and full thickness. The depth of injury is difficult to predict in the clinical situation, however, largely because of the phenomenon of delayed microvascular damage.46 Assessment of burn depth has been the subject of considerable investigation with the expectation that improved assessment will facilitate decision making in regards to treatment.41 Consequently, strict classification is no longer favored. Management of burns is described in Chapter 81.

Goals of Wound Management and Wound Classification

The primary goal of wound management is to facilitate wound healing without development of wound infection. A clear understanding of the differences among wound contamination, colonization, and infection is important. By definition, contamination is the presence of microbes on a surface. Contamination can lead to colonization in which surface microorganisms are replicating. Colonization can lead to infection in which there is invasion and replication of microbes within the tissue.4

Historically, as a guide to decision making, wounds have been classified according to duration, extent of tissue trauma, and degree of microbial contamination. Although these classifications gave strict time frames and considerations to the classification, the clinician’s consideration of the above factors, rather than strict classification, should dictate wound management. A burden of 105 colony-forming units (CFU) per gram of tissue has long been considered the threshold for development of infection, with the time frame required for this to occur being 6 hours or more.51 Investigation in people with burns showed the presence of more than 105 CFU/g in quantitative wound cultures correlated with histologic evidence of infection34,35,39 and was associated with a substantial mortality rate (i.e., 75%).49 Although 105 CFU/g may be a substantial burden and 6 hours sufficient to reach this number, the actual burden reached in a wound depends on other factors. These include the type and virulence of the contaminating microorganism; host factors, such as tissue trauma, systemic health, and immune compromise; and the presence of foreign material.

Thus, a more relevant conceptualization of the effect of the microbial burden on the wound is:


Types of Wound Management

Primary Wound Closure (First Intention Healing)

With primary wound closure, wound edges are apposed and allowed to heal by first intention. First intention healing is appositional healing (minimal gap), which is achieved by fixing the edges in contact (e.g., with suture, tissue glue, staples, and bandages) or by application of a graft soon after injury. Primary closure occurs in most surgical wounds and is indicated in clean, sharply incised wounds that have minimal trauma and contamination and are seen within hours of injury. Historically, class 1 wounds were considered amenable to primary closure. Class 2 wounds can be closed primarily if the amount of wound contamination and tissue trauma is minimal and can be managed at the time of closure. Note that any wound that can be completely excised (en bloc debridement) can be converted to a surgical wound and managed by primary closure.

Healing by Contraction and Epithelialization (Second Intention Healing)

Second intention healing occurs when a wound is left to heal by contraction and epithelialization, eventually producing a continuous epithelial surface. Although any wound can be left to heal by second intention, the process may be inefficient and fail to produce a functional outcome in some cases. New epithelium is fragile and easily abraded, and wound contraction, sometimes excessive, may impede normal function. Some wounds may fail to completely reepithelialize, leaving exposed, sometimes proliferative, granulation tissue in the center of the wound.

Open wound management that relies on second intention healing is indicated for dirty, contaminated, traumatized wounds in which cleansing and debridement is necessary but primary or delayed closure is prohibited. Opportunities to close the wound during open wound management (secondary closure) should be considered to expedite the wound closure process, preserve function, and provide a durable epithelial surface.

Decision Making

The decision to close a wound or continue to treat it as open depends on several factors and is one that should be revisited during the course of open wound management. To expedite treatment and provide a functional outcome, the opportunity to close a wound should always be considered. Although many factors are considered, how effectively the clinician can minimize the impact of each factor will affect the decision making. No one factor prevails. Factors to consider include the time lapse since injury (i.e., wound classification), degree of contamination and extent of tissue damage, thoroughness of debridement or ability to excise wound, status of blood supply, animal’s systemic condition, possibility of closure without tension or unacceptable dead space, possibility of closure based on wound location, and likelihood of undesirable consequences of open wound management (e.g., contracture).

Species differences should also be considered in determining wound management strategies. Compared with dogs, the strength of primarily closed wounds is poorer in cats, and healing of open wounds is slower.10,11 In one experimental study, open wounds in cats contained significantly less granulation tissue than dogs, and the granulation tissue was more likely to have a peripheral, rather than central, distribution.11 Total wound healing was significantly lower at 7, 14, and 21 days after wounding in cats, and the percentage of epithelialization was also significantly less at 14 and 21 days. At 21 days after wounding, epithelialization and total healing were 34% and 84%, respectively, for cats and 89% and 98%, respectively, for dogs.11 Causes of these differences are unknown.

Animals with open wounds that fail to progress toward healing should be evaluated for factors that may delay or interfere with the process. Potential causes include systemic disease, malnutrition, local tissue hypoxia and ischemia, bacterial colonization, altered cellular and stress response, repetitive trauma, presence of necrotic tissue, and tension. Blood work should be evaluated to rule out systemic illnesses such as uremia, hepatic disease, diabetes mellitus, hyperadrenocorticism, and feline immunodeficiency virus. Malnourished animals with severe hypoproteinemia or vitamin or mineral deficiencies may require supplemental nutritional support through use of feeding tubes until they are capable of sustaining an adequate balanced diet through normal alimentation. Normovolemia should be maintained to improve perfusion and oxygen delivery, and animals with severe anemia should receive red cell transfusions. Chronic wounds should be biopsied for histologic and microbiologic evaluation to rule out underlying local disease (e.g., sterile panniculitis or neoplasia) and persistent or resistant infections (Figure 76-1). Contraction will be delayed in wounds that are under tension or cavitated (indolent “pocket” wounds); surgical intervention may be required to reduce these effects (Figure 76-2). Necrotic or dense fibrotic tissue within the wound bed should be resected to stimulate formation of healthy, vascular granulation tissue. For wounds that are incapable of developing healthy granulation tissue, the blood supply can be improved with omental or muscular flaps or by closure with a vascular skin flap (e.g., axial pattern flap).

Immediate Wound Care

Immediate wound management is aimed at reducing the microbial burden and preventing further contamination. In unstable animals at initial presentation, this may include copious irrigation with a readily available solution such as tap water, coverage of the wound with an antimicrobial agent, and ongoing protection with a bandage. This bandage can be left in place until the animal is stable and more definitive wound management can take place.

Irrigation Solutions

In acute traumatic wounds, the microbial burden may be substantial. In these cases the role of irrigation is to physically remove the gross contamination. The sterility of the solution and any antimicrobial properties are not as important as its availability in copious volumes. Systematic review of clinical studies evaluating tap water and sterile saline for wound irrigation in humans showed no difference in occurrence of wound infection after tap water or sterile saline irrigation.61 This was verified by a later randomized clinical trial in 715 humans that showed no difference in risk of infection (relative risk, 1.21; 95% confidence interval, 0.5 to 2.7).44 In fact, some studies suggest that tap water reduces the risk for infection. A meta-analysis of six clinical studies in humans showed the odds of infection for wounds irrigated by tap water compared with saline was 0.722 (95% confidence interval, 0.709 to 0.735).43 Thus, initial copious irrigation with tap water is a useful technique to rapidly reduce the contamination burden. Although tap water is hypotonic, it is less so than distilled water. No difference in development of wound infection is evident with use of tap water, distilled water, or boiled water.17

Low-pressure (<5 psi), high-volume irrigation can be generated by devices such as a single-lumen port-type device spiked into a fluid bag. High-pressure irrigation is above 5 to 8 psi; an 18- to 19-gauge needle on a 35-mL syringe can generate 7 to 8 psi. Jet pressure (70 psi) is provided by commercial pulsatile or continuous irrigation devices with a single- or multi-jet tip. Level of pressure that is appropriate for irrigation is unclear. Although irrigation appears beneficial, irrigation pressure is a balance of removal of bacteria and particulate matter versus local tissue damage, lateral fluid and contaminant dissemination, and edema.62 A critical review of irrigation techniques in human and animal models using low-pressure, high-volume irrigation; high pressure; and jet pressure concluded that irrigation was useful but did not find any technique superior.16 Clinicians should balance the effectiveness of high pressure with the consequence of tissue damage when considering the pressure needed for heavily versus mildly contaminated wounds.

Antimicrobial Treatment

The role of antimicrobial agents in wound management is based on the route of agent delivery. In general, topical agents are indicated early in wound management to control contamination and reduce microbial burden. Systemic agents are indicated for treatment of wound infection. Topical agents are generally broad spectrum, which is a desirable property because a contaminating microbial population is often quite mixed. Systemic antimicrobial treatment should be narrower in spectrum and is best based on results of culture and sensitivity testing of tissue from the wound. Detection of a positive tissue culture result is consistent with the definition of wound infection. Cultures from swabs of the exudate or surface of a wound are representative of contaminants. Acute wound infections are usually the result of one dominating microorganism. Chronic wound infections are often polymicrobial, and prolonged administration of broad-spectrum antimicrobials usually favors development of resistant microorganisms. In some wound infections, topical and local antimicrobial delivery are used as adjuncts to systemic therapy. Topical antimicrobial agents suitable for immediate wound management include antimicrobial ointments, silver-based dressings, and hyperosmotic dressings (20% hypertonic saline, sugar, honey) (Table 76-1).

Wound Protection

After application of a topical antimicrobial agent with a contact layer, protecting the wound with a clean, dry bandage (see Chapter 21) is indicated to prevent further contamination and reduce hemorrhage. This also stabilizes the wound area, which improves comfort for the animal and may reduce further trauma. The covering remains in place until definitive treatment can be performed. Application of gauze impregnated with 0.2% polyhexamethylene biguanide into the secondary layer may be indicated to reduce existing microbial burden and further contamination.33,45

Bandages may be difficult to place over some wounds because of motion or location (e.g., proximity to the anus or external genitalia). In these cases, dressings can be secured with a tie-over bandage or adhesive drape or stapled directly to the skin around the wound. Tie-over bandages have the advantage of relieving tension on the wound, and adhesive drapes form a waterproof barrier that reduces external contamination while keeping the wound moist. Lycra body suits are also useful for keeping bandages in place and preventing self-trauma.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Open Wounds
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