CHAPTER 4 Andrew J. Dart, BVSc, PhD, Diplomate ACVS, Diplomate ECVS, Albert Sole‐Guitart, DVM, Diplomate ACVS, Ted S. Stashak, DVM, MS, Diplomate ACVS, and Christine Theoret, DMV, PhD, Diplomate ACVS The aim of wound management is to optimize healing in an effort to return the horse to its previous function as rapidly as possible, while respecting the financial constraints of the owner. Infection, with its associated excessive or inappropriate inflammation, is the single most likely cause of delayed healing of wounds healing by second intention. Effective wound preparation, exploration, cleansing via irrigation, and debridement are central to reducing the incidence of infection and promoting uncomplicated healing of accidental wounds. Antimicrobial drugs should be used only when there is clinical evidence of infection or when development of an infection would be life‐threatening or career‐ending for the patient. Although empiric broad‐spectrum antimicrobial therapy may be used initially, definitive selection of antimicrobial drugs should be based on the results of bacterial culture and sensitivity testing of isolates. The use of antimicrobial drugs in surgical wounds should never replace aseptic and atraumatic surgical technique. Where there is evidence of formation of biofilm, repeat debridement and topical administration of antimicrobial drugs should be central to any treatment plan. The aim of wound management is to optimize healing in an effort to return the horse to its previous function, as rapidly as possible, while respecting the financial constraints of the owner. Infection, with its associated excessive or inappropriate inflammation, is the single most likely cause of delayed healing of wounds healing by second intention. This chapter reviews selected management practices that influence wound infection and healing. The subject is approached in the order in which a wounded horse would be evaluated and managed. When attending a horse that has suffered an accidental wound, excessive hemorrhage should be addressed immediately. Thereafter, a thorough history is obtained, and a general physical examination is performed to identify any important systemic problems prior to addressing the wound. Shock, injury to the central nervous system, peripheral nerve damage, fracture, systemic disease, or malnutrition may all negatively affect healing. If a tetanus toxoid booster has not been administered to the horse within the past year, it should be given at this time, even if the wound shows no signs of infection. The time between the occurrence of a wound and the onset of signs of tetanus can range from 2 days to 2 months.1 Tetanus antitoxin should be administered only if the horse has no history of receiving tetanus vaccination or if it exhibits clinical signs of tetanus. In these cases, tetanus toxoid should be administered concurrently, but at a site remote from where the antitoxin was injected. The horse usually should be tranquilized or sedated before the wound is examined, to diminish the horse’s pain and anxiety and thereby facilitate a careful examination. Nevertheless, substantial blood loss caused by wounding should be considered when selecting the drug. Some tranquilizers, such as acepromazine maleate, may cause vasodilation, which in turn causes hypotension by blocking alpha‐adrenergic receptors,2 thereby exacerbating pre‐existing low blood pressure. Furthermore, acepromazine has been shown to neutralize reactive oxygen species produced by stimulated equine neutrophils, at least in vitro.3 This may have important implications on the acute inflammatory response to wounding in horses, which is known to be deficient relative to that of ponies and other species.4 Special attention should be paid to the manner in which the wound was incurred, the time elapsed since wounding, the location, shape and depth of the wound, the extent of tissue damage, especially the state of the local vascular supply, potential involvement of underlying structures, such as synovial or body cavities, bone or tendon, and the extent of gross contamination of the wound. The reader is referred to Chapter 3 for more information about the factors that impair wound healing. In particular, the clinician must take special care to identify signs of inflammation and/or acute infection, such as swelling, heat, and pain. The cause of the infection can be confirmed later by the results of microbiologic testing. The overall aim of visually inspecting the wound is to make informed decisions on the subsequent steps required to manage factors that negatively influence healing. Of particular importance is the management of contamination by using appropriate measures, such as wound irrigation and debridement and the institution of antimicrobial therapy, when indicated. Local or regional anesthesia to desensitize the wound may be used, often in conjunction with tranquilization or sedation, to facilitate examination, exploration, and treatment of a wound while the horse is standing. It can also be used to provide pain relief of variable duration. Local anesthetic solutions have been shown, in humans, to exhibit antimicrobial effects against a wide spectrum of pathogens.5 Studies in laboratory animals have inconsistently found that common anesthetic solutions, including 2% lidocaine and 0.5% bupivacaine, may increase edema and inflammation, and inhibit synthesis of collagen. Local anesthetic solutions, however, have no long‐term effects on the rate or quality of healing, when infiltrated postoperatively into surgically created cutaneous wounds to manage pain.6,7 Until recently, it was thought that local anesthetic solutions containing epinephrine/adrenaline should be avoided because of their vasoconstrictive effect, but it appears that the effects of local anesthetic solutions on wound healing are not influenced by the presence of epinephrine/adrenaline in the solution.8 Some older studies do suggest, however, that local anesthetic solutions may be toxic to leukocytes,9 which might further hamper the already weak inflammatory response to wounding when administered intralesionally on the limbs of horses. Instilling local anesthetic solution perineurally (i.e., using regional anesthesia) reduces the volume of local anesthetic solution required to desensitize the wound. The solution is deposited around the nerve or nerves innervating the site of the wound but remote from it. This technique can be used to desensitize wounds to the head or to the distal aspect of the limb, in particular, those wounds located in the foot or pastern. In these areas, because the nerves are easily palpated, the technique provides a simple, precise, and reliable approach to desensitizing the wound. Alternatively, regional anesthesia, in the form of a “U” or an “L” block can achieve the desired desensitization, while avoiding the potential side‐effects associated with intralesional anesthesia. Intralesional anesthesia, although more invasive, is effective and provides good compliance by the patient, and is, therefore, commonly used in practice, especially for wounds in regions that are difficult to desensitize by using perineural anesthesia. Ideally, the anesthetic solution should be infiltrated intralesionally after cleansing the wound and clipping the surrounding hair to minimize seeding contaminants into deeper tissues. The anesthetic agent should be distributed evenly along the wound’s edge to minimize the total dose administered. To facilitate this, the entire length of the needle (20–22 gauge; 1–1.5 in) can be inserted subcutaneously from within the wound, parallel to the wound’s edge, to the needle’s hub. A small volume of local anesthetic solution is injected as the needle is withdrawn; the volume of anesthetic solution injected should be less than that that would markedly deform the wound’s edge. This technique is usually well tolerated by the horse. Generally, 2% lidocaine or mepivacaine hydrochloride is used, and either provides approximately 2 hours of clinically effective and safe anesthesia.10 Bupivacaine hydrochloride is longer acting, providing 4–8 hours of anesthesia, and therefore, may contribute to postoperative analgesia.10 Some horses may experience postanesthetic pain as a consequence of either the anesthetic solution used or the anesthetic technique. Lidocaine has been reported to produce a less intense inflammatory reaction than mepivacaine or bupivacaine when injected subcutaneously in rats,11 but no studies have specifically compared the horse’s inflammatory response to various local anesthetic solutions injected subcutaneously. The intensity of the inflammatory response does not appear to be related to the concentration of the drug.11 Although subcutaneous infiltration of a local anesthetic solution is the most common approach to desensitizing a wound, evidence suggests that a topical anesthetic cream (e.g., lidocaine/prilocaine) provides sufficient analgesia to allow debridement of an open surgical wound, at least in human patients.12 In a recent study, applying an anesthetic cream to the labia of mares was as effective as subcutaneous infiltration of a solution of lidocaine in providing local anesthesia when performing episioplasty.13 If a topical anesthetic cream/gel is used to provide analgesia, the veterinarian should keep in mind that the onset of blockade may take 45–60 minutes, which is much slower than that achieved by subcutaneous infiltration of local anesthetic solution.12,13 Rarely must the horse be anesthetized to initially examine and cleanse the wound, but for foals or horses unaccustomed to being handled, general anesthesia may be necessary to ensure the safety of the patient and the attending veterinarian. Similarly, examination, cleansing, and initial treatment of the wound may be easier with the horse anesthetized if the wound is large and/or heavily contaminated or if it involves a structure other than skin, such as a bone, synovial cavity, tendon, or body cavity. The depth and duration of anesthesia and the duration of surgery14 have been shown to independently increase the risk of postoperative infection in clean and clean–contaminated wounds of dogs.15–17 The rates of wound infection were shown to increase by 0.5% per minute after 60 minutes of anesthesia.17 This translates to a 30% greater risk of postoperative infection for each additional hour of anesthesia. The rate of infection was shown to double after 90 minutes of surgery and nearly triple when surgery exceeded 120 minutes.15 Incisional complications of horses undergoing ventral midline celiotomy were twice as likely when surgery took longer than 2 hours,18 and patients undergoing orthopedic procedures were 3.6 times more likely to develop a surgical site infection (SSI) when the time of surgery exceeded 90 minutes.19 Excessive depth of anesthesia reduces perfusion and oxygenation of tissue, whereas prolonged anesthesia impairs the function of alveolar macrophages and depresses systemic migration and function of leukocytes.20,21 Decreased tissue oxygenation, associated with excessive depth of anesthesia, is a critical factor in the development of SSI because it hinders neutrophil‐mediated oxidative killing, a mechanism essential to defense against surgical pathogens.22 Human patients who suffer from decreased tissue oxygenation in the perioperative period are at risk of SSI.23 A low intraoperative arterial partial pressure of oxygen has been shown to contribute to the development of SSI following colic surgery in horses.24 In this latter study, a nine‐fold increase in the risk of SSI was observed when the period of general anesthesia exceeded 2 hours and the PaO2 was <80 mmHg, indicating an additive effect of these two factors. Although anesthetic agents and monitoring of anesthetized horses to optimize oxygenation and tissue perfusion continue to improve, surgeons must nevertheless carefully plan and execute surgical procedures to minimize the time a patient is anesthetized. The benefits of using general anesthesia in a well‐equipped hospital environment to allow for a systematic and controlled surgical approach to manage complex wounds should not be underestimated. The advantages of treating wounds with the horse anesthetized are often lost, however, if the procedure is performed in the field. The objective of wound preparation is to reduce contamination and produce a clean surgical site. A clear benefit of removing hair preoperatively from human patients on reducing the rates of SSI has not been established.25 One study showed that effective aseptic preparation of the carpal and distal interphalangeal joints of horses (for arthrocentesis) can be achieved without removing hair.26 Nevertheless, in veterinary patients, hair is usually removed from the surgical site and from the periphery of an accidental wound because it can interfere with the surgeon’s ability to clean and suture the edges of the wound. Methods of removing hair include clipping, shaving, and chemical depilation, though the latter is seldom used in veterinary medicine and contraindicated in the case of an open (accidental) wound. If the area around the wound on a horse is to be shaved, the hair is usually clipped before shaving commences. Clipping or shaving may “nip” the skin at its creases thereby causing abrasions and microscopic cuts that may subsequently become contaminated by opportunistic pathogens.27 Of these two approaches, clipping appears to be the less traumatic and was shown, in humans, to be associated with a lower incidence of SSI.25 Good‐quality, sharp, clean equipment suited to the purpose reduces the risk of damaging and contaminating skin. Using a guarded razor (i.e., a safety or disposable razor) instead of an open‐bladed system (e.g., a scalpel or Wick blade) may reduce injury to the skin. The clipper head should be disinfected between patients to minimize the risk of cross‐contamination. Removing the hair surrounding an accidental wound may contaminate the open wound with hair clippings and dander. To ensure against this risk, a sterile, water‐based lubricating gel should be placed onto the wound prior to clipping. The gel prevents tiny hairs from accumulating on the wound and can be easily rinsed from the wound after clipping. Alternatively, sterile gauze sponges soaked in sterile isotonic fluid or a water‐based lubricating gel can be packed into the wound prior to clipping. Sponges are removed after each stage of the preparation, the wound rinsed, and packing replaced. A wide margin of hair should be removed at the wound’s periphery, and the loose hair vacuumed away from the clipped site. Scissors often work better than clippers for removing long hair at the wound’s edge. In the case of an accidental wound, the skin adjacent to the open wound should be prepared as if for aseptic surgery. It is common practice to don sterile gloves for the final stage of skin preparation. Skin is usually prepared immediately prior to irrigating and debriding the wound. If the wound is heavily contaminated or infected, it may be necessary to perform skin preparation again after surface debridement of the wound is complete and before further surgical manipulation. The skin surrounding the wound is prepared by scrubbing it with an antiseptic detergent to remove oils, dirt, and debris and then rinsing it with sterile isotonic saline solution or 70% isopropyl alcohol. Antiseptic scrubs and isopropyl alcohol are cytotoxic and should not contact the wound itself.28 Povidone–iodine (PI) and chlorhexidine (CH) are the two most commonly used surgical detergents. A common PI‐based detergent is Betadine®, which is a complex of iodine and polyvinylpyrrolidone (povidone), a synthetic polymer. Betadine is available as a 7.5% w/w surgical detergent equivalent to 0.75% available iodine. A study showed that the skin of horses can be prepared for aseptic surgery using a 10‐minute scrub with PI, a 5‐minute scrub with PI, three 30‐second scrubs with PI, or by blotting the surgical site with a commercial, one‐step iodophor surgical solution.29 Chlorhexidine is available as a 4% CH gluconate detergent (Hibiclens, Mölnlycke Health Care). A recent study in horses showed that skin preparation using a 4% CH gluconate detergent without repeated mechanical scrubbing (i.e., back and forth motion of CH gluconate‐soaked gauze sponges for less than 15 seconds, just long enough to produce a lather, then leaving the detergent in contact with the skin for 255 seconds) was as effective in reducing bacterial counts on the skin as the standard 5‐minute mechanical preparation.30 Both PI and CH have broad‐spectrum antimicrobial activity, but PI is reported by the manufacturer to be active against viruses, fungi, protozoa, and yeast. As an antiseptic skin detergent, CH is suggested to have superior antibacterial activity and a greater residual effect due to its ability to bind to the stratum corneum. Clinically, however, PI and CH appear to be equally effective as surgical scrub detergents.31,32 One disadvantage of CH detergent is that short exposure of the detergent to the eye, even in small concentrations, can lead to conjunctivitis and corneal opacification.33,34 Either 70% isopropyl alcohol or sterile, 0.9% saline solution are commonly used to rinse between applications of antiseptic detergents. Rinsing with 70% isopropyl alcohol has been reported to decrease the residual antimicrobial activity of 4% CH gluconate detergent.35 The authors of that study were unable to provide a basis for this finding, and clinically, this loss of antimicrobial activity may not translate into an increased risk of infection, particularly if the procedure is short. With this in mind, however, rinsing with sterile isotonic saline solution may be more appropriate than rinsing with isopropyl alcohol when using a CH‐based detergent. In contrast, 70% isopropyl alcohol potentiates the antimicrobial activity of PI by increasing the release of free iodine.31,36 Isopropyl alcohol also has considerable antibacterial activity when used alone at 70% or 99%.36 Consequently, the combination of PI surgical detergent and 70% isopropyl alcohol rinse, used commonly in equine practice, is effective for preparing skin for surgery. Systematic reviews of the literature in human medicine suggest there is little evidence to favor one skin antiseptic detergent over another, at least for clean surgery. Extrapolating the findings of these studies to animals must nevertheless be done with caution because cleansing the skin of animals may be more difficult than cleansing the skin of people. Moreover, the reader should be aware that PI, as a 10% solution, can occasionally cause a transient, acute contact dermatitis in people;37 in the authors’ experience, it can also cause the same in horses, particularly when applied to the throatlatch or inguinal regions. Wounds to the hoof require special preparation that involves removing the superficial layer of hoof wall and exfoliating the sole with a rasp and hoof knife before scrubbing with PI detergent. This approach was shown to be more effective in reducing bacterial concentration than a 6‐minute scrub with PI followed by 24‐hour submersion in PI solution‐soaked cotton.38 Moreover, a recent study found that a prolonged soak (12 hours) with either iodine tincture or PI damages the skin and favors bacterial recolonization.39 Nevertheless, although bacterial concentration is significantly reduced by removal of the superficial layer of the hoof, prior to scrubbing, final bacterial counts were shown to exceed 105 bacteria per gram of tissue, sufficient to induce infection if the local environment and the host’s immune response are compromised. The inadvertent transfer of microorganisms from the surgeon to the patient can result in SSI. Surgeons usually wear sterile gloves to prevent transferring bacteria from their hands to the patient, but gloves may be perforated during surgery.40 Hands, therefore, should be as bacteria‐free as possible prior to gloving, and to achieve this, surgical hand antisepsis should be performed immediately before donning sterile gloves when preparing for a surgical procedure.41 The following three types of antiseptic solutions are available for surgical hand antisepsis: aqueous scrubs, alcohol rubs, and alcohol rubs containing additional active ingredients. The most commonly used products for surgical hand antisepsis in the human healthcare field are aqueous scrubs containing CH or PI. According to a recent survey, this also seems to be the case for veterinary surgeons, 80% of whom used water‐based hand scrubbing with CH gluconate or PI detergents for hand antisepsis in 2011.42 Scrubbing the dirty hands of large‐animal veterinarians with CH or PI detergent prior to gloving initially reduces colony forming units (CFU) by ~80% on those hands for up to 120 minutes.43 However, prolonged residual effect is an important feature of presurgical hand antisepsis because hair follicles, as well as sebaceous and sweat glands, are sources of recolonization of the surgeon’s skin by bacteria, which may be a problem in the case of a perforated glove. Bacterial regrowth occurs faster after scrubbing with PI than with CH.44 Several alcohol‐based handrubs have been licensed for the commercial market and include long‐acting compounds (e.g., CH gluconate and quaternary ammonium compounds) that limit regrowth of bacteria on the gloved hand. According to the World Health Organization, the antimicrobial efficacy of alcohol‐based formulations, when applied a minimum of three times, for a period of 3–5 minutes, is superior to that of all other currently available methods of preoperative surgical hand preparation in the human healthcare field.44 These alcohol‐based rubs offer the advantage of rapid and immediate action, as well as reduced skin damage after repeated use, thereby improving compliance to the protocol for surgical hygiene, and their use saves large quantities of water. Not every alcohol rub solution sold over the counter, however, is appropriate for use in surgical hand preparation, because some may contain an ineffective concentration of alcohol. The alcohol‐based rub used for preoperative surgical hand preparation should meet the standards required by the American or European regulatory agencies. The body surface of veterinary patients is likely to have higher bacterial counts than that of human patients, potentially resulting in greater contamination on the hands of veterinary surgeons. Consequently, products for surgical hand antisepsis of veterinary surgeons, specifically, should be tested to verify their effectiveness. One study compared the efficacy of a commercial alcohol‐based handrub containing 45% 2‐propanol, 30% 1‐propanol, and 0.2% mecetronium ethylsulfate (Sterillium, Bode‐Chemie) with that of two common detergent scrubs, 4% CH digluconate (Hibiscrub, Regent Medical) and 7.5% PI (Vetclean, Ecuphar), in reducing the number of CFU found on hands of equine and small‐animal surgeons.45 The alcohol‐based rub and CH digluconate detergent scrub had a similar immediate effect, although the sustained effect was significantly better for the alcohol rub.45 The PI detergent scrub had a significantly lower immediate and sustained effect.45 A recent study comparing the efficacy of a 5‐minute scrub with 4% CH gluconate water‐based detergent scrub to that of a 90‐second application of a commercial alcohol‐based hand rub containing 1% CH gluconate and 61% ethyl alcohol in an emollient (AvagardTM, 3M Healthcare), for hand antisepsis for sterile, equine elective surgical procedures, found no statistically significant difference in mean bacterial CFUs between the two techniques immediately after hand antisepsis or after surgery.46 A similar study conducted at Massey University likewise showed no difference in the mean bacterial CFUs either immediately or 120 minutes after preparation with either CH scrub or with AvagardTM, for clean, contaminated hands or for dirty hands (simulated by soaking hands for 5 minutes in a 20% w/v solution of horse feces).47 Nevertheless, more studies are needed to verify the impact of hand antisepsis on rates of SSI rather than on CFU. The only such study performed in the human healthcare field found that hand rubbing with an alcohol solution was as effective as hand scrubbing with an antiseptic soap, either PI or CH, for preventing SSI.48 Wound irrigation/cleansing remains an important component in managing accidental wounds. The objective of irrigation is to gently remove loosely adhered contaminants, bacteria, and devitalized tissue from the wound’s surface, using a continuous or pulsatile flow of fluid. More specific techniques of cleansing and debriding should be used if contaminants cannot be removed by using gentle irrigation. Although irrigation with tap water or a sterile isotonic saline solution may be all that is needed to adequately cleanse a lightly contaminated, acute wound (<3 hours’ duration) in preparation for primary closure, more chronically contaminated wounds (>3 hours’ duration), particularly those involving the distal aspect of the limb, often require debridement, in addition to irrigation, before they can be sutured. Wounds healing by second intention require repeat irrigation as long as they contain superficial slough, excess exudate, visible debris, or any residue from topically applied wound‐care products or from dressings, that might delay healing. Irrigation may be used before, after, or as an alternative to debridement, depending on the amount of contamination or tissue damage.49 For example, briefly soaking the desiccated surface of a wound or the surface of a wound containing dry, imbedded debris with moist gauze (a form of dressing debridement) hydrates the wound, softening the underlying tissue, and thereby enhancing irrigation. In equine practice, the delay between the time of wounding and the time the patient is presented is often substantial. Unfortunately, the effectiveness of irrigation decreases as the wound ages. For example, one study using a complex musculoskeletal wound model in goats found that irrigation resulted in a 70%, 52%, and 37% reduction in bacterial counts at 3, 6, and 12 hours, respectively, after the wounds were inoculated with Pseudomonas aeruginosa.50 This is because bacteria invade tissue and aggregate to form a biofilm that becomes more resistant to irrigation over time. A biofilm‐based wound‐care management plan, with debridement as its central component, should be instituted after a biofilm forms (more information on the management of biofilms is available later in this chapter and also in Chapter 19). The three major variables of irrigation are method of delivery, volume delivered, and solution delivered. Pressurized wound irrigation is more effective in cleansing the wound than is swabbing, and consequently more cost effective because it shortens the time required for wounds to heal by second intention and is better tolerated by the patient (Figure 4.1).51 The overall conclusion of a critical review conducted on wound irrigation is that optimal irrigation pressures have yet to be determined for different degrees of wound contamination in humans.52 Likewise, the relationship between irrigation pressure and the ability to reduce bacterial counts and eliminate debris from contaminated wounds of horses has not been studied. Nevertheless, in the authors’ experience, pressures ranging from 8 to 15 psi are beneficial in achieving the objectives of irrigation when managing wounds in horses. Our recommendations to use irrigation pressures between 8 and 15 psi echo those recently made by wound‐care experts in the field of human healthcare who claim that this range of pressure is strong enough to overcome adhesive forces of bacteria.53 Although higher pressures are more effective in removing necrotic tissue and foreign particles, high pressures have been shown to drive contaminants deeper into the wound or surrounding tissues, including into cancellous bone.54 High irrigation pressures may also damage granulation or epithelial tissues,55,56 thereby increasing the rate of wound infection and delaying healing. Furthermore, the likelihood of causing aerosolization of bacterial particles and subsequent environmental dispersal of airborne microorganisms, may increase as irrigation pressure increases.57 In summary, the benefits of higher pressures in reducing bacterial count, dirt, and tissue debris in heavily contaminated wounds may outweigh the risk of tissue injury. Conversely, in relatively clean wounds, the potential damage of tissue resulting from high‐pressure irrigation may outweigh the benefits.52 Figure 4.1 (a) Example of a wound on the dorsal surface of the proximal metatarsus, close to the hock. The tarsometatarsal and distal intertarsal joints were injected with sterile isotonic saline solution to investigate a potential communication with the wound. These tarsal joints were not affected, however, exposure of the third metatarsal bone created a risk for sequestrum formation. (b) A Waterpik® Water Flosser was used for wound cleansing and irrigation and the limb was radiographed 7 days later to determine whether a sequestrum had developed. Pressures of 8–15 psi cannot be attained with gravity flow but can be obtained by using a 35‐ or 60‐mL syringe and an 18‐ or 19‐gauge needle,58 a spray bottle, or some of the commercially available irrigation devices. A syringe with an attached 19‐gauge needle typically delivers an output pressure range of 11–31 psi; however, the end pressure that reaches the wound could be as low as 8 psi.58 Although no studies have documented the ranges of psi delivered by a spray bottle with an adjustable nozzle, the experience of the third author supports the effectiveness of this method of delivery. Commercially available, battery‐powered, irrigation devices (e.g., Stryker Interpulse System; Equine Hydro‐T, info@hydrot.com) are capable of delivering very high pressures (70 psi), but their pressure output can be adjusted to deliver fluid at 10–15 psi. Fluid volume is another important component of wound irrigation; as volume increases, so does cleansing, up to a point, but the optimal volume with which to irrigate is unknown.59 The volume of fluid should reflect the degree of contamination and the size of the wound. At minimum, the gross contaminants should be removed, and irrigation should be discontinued before tissues become waterlogged. The ideal irrigant is sterile, isotonic, normothermic, non‐toxic, hypoallergenic, and compatible with any additives (antibiotics or antiseptics). Furthermore, it should be readily available, and its use cost effective. Several different products can be used for irrigating wounds, such as potable tap water, isotonic saline solution, Ringer’s solution, and solutions containing active ingredients, such as antimicrobial agents. Isotonic crystalloids, such as sterile isotonic saline solution or lactated Ringer’s solution, are commonly used but are expensive when large volumes are used on multiple occasions. A systematic review of the effects of irrigating wounds in human patients with isotonic saline solution or potable tap water concluded that using tap water is more economical and as effective as using isotonic saline solution in cleansing acute and chronic wounds, and that using tap water does not seem to increase the risk of infection or delay healing over that which occur when the irrigant is isotonic saline solution.60 Where potable tap water is not available, boiled and cooled water or distilled water are acceptable alternatives. Although irrigation with potable tap water, isotonic saline solution or Ringer’s solution may be sufficient to cleanse a fresh accidental wound, many wounds encountered in equine practice are not fresh and/or are heavily contaminated. No diagnostic test exists to allow practitioners to identify whether the bacterial load in the wound is capable of causing infection. For this reason, chronic accidental wounds and those exhibiting signs of infection or in which a biofilm is suspected, might benefit from irrigation with a solution containing an additive intended to decrease the bacterial inoculum in the wound to a concentraton that can be managed by the host’s defences. Antiseptics, defined as “agents that destroy or hinder the growth of microorganisms in or on living tissue,”61 are often added to irrigants. In the absence of evidence‐based guidelines for clinical practice, the use of antiseptics continues to provoke a great deal of debate among clinicians. Past in vitro experiments demonstrated that some antiseptics caused a variable amount of disruption to collagen synthesis, were toxic to fibroblasts, impaired migration of epithelial cells and/or inhibited the microcirculation.62 Other studies demonstrated the toxic effects of antiseptics on keratinocytes and leukocytes.63,64 Although possibly not a true reflection of the in vivo situation, these in vitro studies are at the source of the debate about whether antiseptics used for wound management do more harm than good.65,66 A definite advantage of antiseptics over antibiotics, if either is to be added to the irrigant, is that antiseptics have a broad antimicrobial spectrum coupled with a reduced propensity for inducing bacterial resistance. With the emergence of multi‐resistant strains of organisms, there is a renewed interest in using antiseptics in wound management. The cytotoxicity of antiseptics appears to be concentration dependent, with some antiseptics (e.g., PI and CH) maintaining antimicrobial efficacy at a low, non‐cytotoxic concentration.28 An “all or none” approach to using an antiseptic for irrigation would simplify decision making, but this approach fails to address the complexities of wounds encountered in clinical practice. Some wounds are seen early and are minimally contaminated, whereas others may be hours old and heavily contaminated, colonized, or infected (the reader is referred to Chapter 3 for more information about the bacterial impact continuum). Consequently, practitioners should be familiar with the properties of the commonly used commercial antiseptics to make the most informed choice of which antiseptic to use on a particular wound. Iodine and CH are discussed here because they are the two antiseptics most commonly added to irrigants by veterinary practitioners. The reader is referred to Chapter 5 for a comprehensive coverage of commercial wound cleansers and antiseptics. Iodine has a broad antimicrobial spectrum and is reported to be effective against Gram‐positive and Gram‐negative bacteria, viruses, fungi, yeasts, molds, and protozoans.67 None of these organisms are known to resist it, but iodine can be inactivated by blood, serum proteins, and other organic matter.67 Active, “free” iodine is usually coupled to a carrier (an iodophor) that acts as a slow‐releasing reservoir. This serves to reduce the toxicity of iodine (characteristic of aqueous solutions) and to overcome its inactivation by organic material. Two iodophor‐formulations of iodine are available: povidone–iodine (PI) and cadexomer–iodine (CI). Because CI is supplied as a hydrophilic modified‐starch polymer bead that gradually releases incorporated iodine into the wound, it is best used as dressing and not for irrigating wounds. PI or polyvinylpyrrolidone–iodine complex (Betadine®) is a combination of elemental iodine bound to the synthetic, water‐soluble polymer, polyvinylpyrrolidone surfactant. Betadine is available as a 10% solution in water.68 The bactericidal component of Betadine is free iodine, approximately one part per million (1 ppm).68 Diluting the PI solution actually increases the liberation of free iodine, and consequently increases the bactericidal effects of the solution.69 The existing evidence regarding the efficacy of PI as an antiseptic is complicated by conflicting results from laboratory, human, and animal studies using different experimental designs.67 Iodine does appear to have a place in wound management, however, and may be particularly useful when irrigating heavily contaminated or infected wounds. There are no studies to support the use of PI in non‐infected wounds of either humans67 or horses, but open wounds, of dogs, at risk of infection, were found to heal more rapidly when irrigated with diluted PI than were similar wounds irrigated with isotonic saline solution.70 More studies using horses are needed to determine the bactericidal concentration and the cytotoxic effects of different concentrations of PI, particularly on neutrophils and macrophages, which are important to the initial inflammatory phase of wound healing. The degree of contamination and the stage of bacterial colonization likely affect the efficacy of PI. On the balance of evidence, PI solutions for irrigation are probably safe and effective at reducing the likelihood of infection developing in heavily contaminated wounds of horses when delivered at concentrations of 0.1–0.2% (10–20 mL/L). Both chlorhexidine digluconate and chlorhexidine diacetate (2%) are also commonly used as additives for wound irrigation. CH exerts rapid antimicrobial activity against a wide spectrum of non‐sporing bacteria, including Staphylococcus aureus, Pseudomonas aeruginosa, and a wide range of clinical isolates,36 but resistance of methicillin‐resistant S. aureus (MRSA) to CH diacetate has been observed.71 Proteus spp. and Pseudomonas spp. appear to also have developed or possess a resistance to this antiseptic. CH diacetate has no effect against fungi or Candida,72 and a recent study showed that microorganisms in biofilms have greater resistance to CH than do bacteria not contained within a biofilm.73 Irrigation with a dilute solution of CH diacetate may improve healing of open wounds at risk of infection.74–76 CH should be diluted to a 0.05% solution (1:40 = 25 mL of the 2% concentrate diluted with 975 mL of sterile isotonic crystalloid solution) for use as an antiseptic for irrigating wounds of horses, as is recommended in humans.53 The emergence of antimicrobial‐resistant strains of bacteria commonly found on the skin has become a concern, even in equine practice.77–79 Consequently, the indiscriminate addition of an antibiotic to a wound irrigant is not in line with current knowledge of antibiotic stewardship. More information regarding topically administered antibiotics is available later in this chapter and in Chapter 5. A commercial wound cleanser may be used when enhanced cleansing is required. Most ionic and many non‐ionic surfactants present in wound cleansers, however, have been shown to be toxic to cells, to delay healing, and to inhibit the wound’s defense against infection.28 For a more complete discussion of wound cleansers, the reader is referred to Chapter 5. To cleanse a heavily contaminated wound, the wound can be sprayed with a wound cleanser, gently scrubbed with sterile gauze sponges, and then thoroughly irrigated with sterile isotonic saline solution. The coarseness of the scrubbing device should be minimal because wounds scrubbed with coarse sponges have been shown to be significantly more susceptible to infection.80 An advantage of commercial wound cleansers is that they contain a surfactant that substantially reduces the coefficient of friction between the scrubbing device and the wound.80 After the wound has been cleansed, it should be explored digitally to determine its boundaries, to identify involvement of structures deep to the skin, including bone, tendon, ligament, and synovial structures, or to confirm the presence of a foreign body. The examiner should wear sterile gloves, from which talcum powder has been rinsed away with sterile isotonic saline solution, prior to touching the wound. Talcum powder may act as a foreign body and has been shown to elicit signs of toxicity by every tissue in the body.81 If digital exploration is inhibited by the size of the wound, a sterile probe or surgical instrument can be used to measure the depth of the wound, to determine if a foreign body is present, or to verify if bone has been contacted. If the wound is deep or involves bone, the wounded region may be examined radiographically, using plain and contrast radiographs obtained with and without a probe, to identify a fracture, a foreign body, or the relationship of the wound to bone or to a synovial cavity (Figure 4.2). One must keep in mind that some foreign bodies may not appear on radiographs and that contrast dyes do not always define the limits of the wound accurately. The effectiveness of the contrast study can be improved by slowly moving the affected body part through a range of motion while the contrast medium is being injected, thereby ensuring better distribution of the contrast medium to all areas involved in the wound (Figure 4.3). Figure 4.2 Example of a heel bulb laceration (a) where the foreign body (barbed wire) was not identified during visual appraisal and digital exploration of the wound. However, it was clearly identified on radiographs of the hoof (b, c). None of the adjacent synovial cavities (navicular bursa, coffin joint, digital flexor tendon sheath) had been breached by the wound. The barbed wire was localized and removed under general anesthesia. Figure 4.3 This horse suffered a penetrating wound to the lateral surface of the distal antebrachium 3 weeks earlier. Initial exploration revealed several pieces of wood in the wound, which were subsequently removed. Following debridement and irrigation, the wound was sutured and the horse received antimicrobial therapy. The horse became non‐weight‐bearing within 7 days and the wound broke open and began to drain 10 days postoperatively. (a) Craniocaudal contrast radiographic study revealing multiple filling defects (a result of pieces of wood) in the carpal canal. (b) Lateral radiographic view identifying multiple filling defects in the carpal canal. Particular attention should be paid to the presence of fluid discharging from a wound located near a synovial structure (joint, tendon sheath, bursa), especially if the wound is located at the distal aspect of the limb, because fluid discharging from a wound is suggestive of synovial penetration. Normal synovial fluid, because of its high viscosity, forms a “string” between the thumb and forefinger. This sign of normality is eliminated by the activity of degradative enzymes produced during inflammation. Unless the wound is fresh or unless the synovial cavity is able to drain freely, communication of the wound with a synovial structure usually causes a moderate to severe lameness due to the pressure exerted by the increased volume of fluid within the closed synovial space, and trapping of inflammatory mediators. If the limb is flexed and extended, the accumulated fluid is often forced to exit the wound, under pressure, in a stream. Obtaining synovial fluid for gross and cytologic examinations and perhaps for microbial culture and antimicrobial sensitivity testing of isolates, is one of the first steps taken to determine if a synovial structure is infected. If synovial penetration is suspected, but no synovial fluid is present at the site of the wound, an area overlying the synovial structure, remote from the wound, should be aseptically prepared for sampling (Figure 4.4). Figure 4.4 A sterile needle has been placed in the distal interphalangeal (coffin) joint at a site remote from the wound. Source: Stashak 2006.82 Reproduced with permission of American Association of Equine Practitioners. After collecting a sample, sterile, isotonic saline solution should be injected, under pressure, into the synovial structure; this solution flows from the wound if the synovial capsule has been breached. If breached, the synovial structure should be lavaged with 3–5 L of a sterile isotonic solution. Intrasynovial damage is best determined by endoscopic examination of the synovial structure. The synovial structure is lavaged most effectively by using endoscopic instrumentation. For more information regarding the management of wounds involving synovial structures, see Chapter 16. Radiography (with or without the use of a positive contrast medium) may be useful in documenting a fracture, joint subluxation or luxation, and the presence of a radiodense foreign body (Figure 4.2). Ultrasound may be useful in defining the extent of soft‐tissue injury, in identifying the presence of gas, fluid, or a foreign body not visible radiographically, and in identifying synovial involvement. Ultrasound can be useful in determining if a wound to the chest or abdomen has penetrated the pleural or peritoneal cavity and can assist in retrieving pleural or peritoneal fluid samples for cytologic examination and microbial culture and antimicrobial sensitivity testing of bacterial isolates.
Management Practices that Influence Wound Infection and Healing
Summary
Introduction
Initial assessment
Visual appraisal of the wound
Local anesthesia
General anesthesia and duration of surgery
Wound preparation
Hair removal
Skin preparation
Patient
Surgeon
Irrigation/cleansing of accidental wounds
Irrigation technique/delivery method
Irrigation solutions
Additives
Antiseptics
Antibiotics
Commercial wound cleansers
Wound exploration
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