Chapter 76 Open Wounds PDF

C H AP T E R 7 6 Open Wounds Giselle Hosgood Pathophysiology 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 heali...

C H AP T E R 7 6 Open Wounds Giselle Hosgood Pathophysiology 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 described by its different phases: inflammation, debridement, repair, and maturation (see Chapter 9). 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 in a full-thickness open wound (usually 4 to 5 days after wounding), epithelialization begins. Healing is facilitated by transformation of some fibroblasts into specialized myofibroblasts approximately 1 week after wounding. Contraction of myofibroblasts decreases wound size. Eventually, collagen is deposited by fibroblasts, whereas 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. Abrasion An abrasion is a partial-thickness epithelial injury typically resulting from blunt trauma or a shearing force. Abrasions can be painful but are associated with minimal bleeding and heal rapidly by reepithelialization. Puncture Wound A puncture wound is caused by penetration of an object into the tissues and is characterized by a small skin opening with deep tissue contamination and damage. Puncture wounds include bite wounds, gunshot injuries, and wounds caused by penetration of foreign objects such as sticks, wire, or bone. Laceration A laceration is characterized by sharply incised skin edges and may extend into deep tissue, such as muscle and tendons. Lacerations are typically associated with minimal peripheral trauma to the wound edges; however, if the incised tissue is elevated or avulsed, large areas of tissue may become devitalized from concurrent avulsion of the blood supply. Degloving Injury A degloving injury is characterized by extensive loss of skin and underlying tissue and immediate or delayed exposure of the wound bed. Degloving injuries are most common on the distal limbs as a result of roller or wringer type injury or scraping across a hard surface (e.g., being hit by a car). With an anatomic degloving injury, the skin and various levels of underlying tissue are torn off the body. With a physiologic degloving injury, the skin surface is intact but separated or avulsed from the underlying subcutaneous tissues and blood supply, resulting in delayed necrosis of the skin. 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 largely because of the phenomenon of delayed microvascular damage.59 Assessment of burn depth has been the subject of considerable investigation with the expectation that improved assessment will facilitate decision making in regard to treatment.53 Consequently, strict classification is no longer favored. Management of burns is described in Chapter 81. Decubital Ulcer A decubital ulcer is the result of compression of the skin and soft tissues between the bony prominence and a hard surface, resulting in skin loss over a bony protuberance. Tissue damage may extend into deeper soft tissues and bone. Decubital ulcers are typically seen in thin, recumbent animals. Ulcers over the greater trochanter, lateral elbow, and lateral hock are common. 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.3 A burden of 105 colony-forming units (CFUs) 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.67 Investigation in people with burns showed the presence of more than 105 CFU/g in quantitative wound cultures correlated with histologic evidence of infection46,47,51 and was associated with a substantial mortality rate (i.e., 75%).65 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 (Number of microorganisms × Virulence)/Host resistance.4 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. Any wound that can be completely excised (en bloc debridement) can be converted to a surgical wound and managed by primary closure. Delayed Primary Wound Closure Delayed primary wound closure describes appositional closure within 3 to 5 days after wounding but before granulation tissue is evident in the wound bed. This is indicated for mildly contaminated, minimally traumatized wounds that require some cleansing, debridement, and open wound management before closure. This management practice allows manipulation of the wound bed environment over several days to ensure microbial contamination is effectively reduced and tissue health is maximized before closure is performed. Secondary Wound Closure Secondary wound closure describes appositional closure of a wound more than 3 to 5 days after wounding, by which time granulation tissue has formed in the wound bed. In some texts, this is called third intention healing, although there are inconsistencies in that term's definition. Secondary closure is indicated for severely contaminated, traumatized wounds in which ongoing open wound management allows manipulation of the wound environment to ensure reduction of microbial contamination, treatment of infection if present, and improvement in tissue health before closure. Development of granulation tissue in the wound bed provides a microbial-resistant, vascular substrate that facilitates healing. Closure is performed over the granulation tissue, although some debridement may be necessary to allow 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. Open wound management can be used for surgical wounds after tumor excision, usually restricted to the lower limbs where primary closure is difficult.64 Decision Making The decision to close a wound or continue to manage it open depends on several factors and is a decision 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 manipulate the impact of each factor will affect the decision making. Factors to consider include the time lapse since injury, 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, 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 lower in cats, and healing of open wounds is slower.12,13 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.12 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.12 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. Results of 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 via 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 such as Mycobacterium sp. infection (Figure 76.1). Contraction will be delayed in wounds that are under tension or cavitated (indolent “pocket” wounds), and surgical intervention may be required to reduce these effects. Necrotic or dense fibrotic tissue within the wound bed should be resected to stimulate formation of healthy, vascular granulation tissue. Some wounds are incapable of developing healthy granulation tissue due to poor vascularity, and healing can be improved with omental or muscular flaps or by closure with a vascular skin flap (e.g., axial pattern flap). FIGURE 76.1 A, Nonhealing open wound in a Dalmatian with sterile panniculitis. The wound bed was covered by an avascular sheet of fibrous tissue that was freely moveable from the underlying subcutaneous tissues. B, The affected area was surgically debrided and managed open with topical dressings and a tie-over bandage (note the suture loops in the skin). An indolent “pocket” wound that developed along the cranial margin of the largest wound required surgical correction. C, Wound healing was delayed because of tension along the wound margins, which limited contraction. D, Appearance of the wound 3 months after debridement. The epithelium over the region is thin and easily traumatized. (Photos courtesy Dr. Karen M. Tobias, University of Tennessee Teaching Material.) 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, and 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 people showed no difference in occurrence of wound infection after tap water or sterile saline irrigation.81 This was verified by a later randomized clinical trial in 715 people that showed no difference in risk for infection (relative risk, 1.21; 95% confidence interval, 0.5 to 2.7).56 Some studies suggest that tap water reduces the risk for infection. A meta-analysis of six clinical studies in people showed the odds of infection for wounds irrigated by tap water compared with saline was 0.72 (95% confidence interval, 0.71 to 0.74).56 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.57 No difference in development of wound infection is evident with use of tap water, distilled water, or boiled water.25 Low-pressure (