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THE BASIC PRINCIPLES OF WOUND HEALING
An understanding of the basic physiology of wound healing provides the clinician with the framework necessary to implement the basic principles of chronic wound care. Introduction:
Wound healing is a complex and dynamic process with the wound environment changing with the changing health status of the individual. Theknowledge of the physiology of the normal wound healing trajectory through thephases of hemostasis, inflammation, granulation and maturation provides aframework for an understanding of the basic principles of wound healing. Throughthis understanding the health care professional can develop the skills required to carefor a wound and the body can be assisted in the complex task of tissue repair.
A chronic wound should prompt the health care professional to begin a search for unresolved underlying causes. Healing a chronic wound requires care that ispatient centered, holistic, interdisciplinary, cost effective and evidence based.
This is one of five articles made available by the Canadian Association of WoundCare to assist the wound care clinician develop an increased understanding of woundhealing. This article explores: • Why wounds happen.
• How wounds heal.
• When is a wound considered chronic?• The nature of good chronic wound care It is hoped that these basic principles will provide a framework for further study and exploration into the complex area of wound care. For more articles in thisseries see www.cawc.net Why Do Wounds Happen?
In any natural disaster the damaging forces must be identified and stopped before repair work can begin. So too in wound care the basic underlying causes andfactors that affect healing must be identified and controlled as best we can beforewound healing will begin. Following are some of the common underlying causes orfactors, which may interfere with wound healing: • Trauma (initial or repetitive)• Scalds and burns both physical and chemical• Animal bites or insect stings• Pressure• Vascular compromise, arterial, venous or mixed• Immunodeficiency• Malignancy• Connective tissue disorders• Metabolic disease, including diabetes• Nutritional deficiencies• Psychosocial disorders• Adverse effects of medications In many cases the underlying causes and factors interfering with wound healing may be mutlifactorial.
Figure 1 illustrates an elderly patient who suffered trauma when she banged her legon a coffee table. She is on coumadin which contributed to the injury becoming alarge black hematoma of old blood. What is the safest way to heal this wound?In figure 2 we see a young spinal cord injured patient with a chronic pressure ulcersurrounded by erythema. Is the erythema caused by infection, irritation of woundfluid, incontinence or continual pressure to the area?In figure 3 we see chronic ulcers in a frail elderly woman that has lower leg edemarelated to decreased mobility. The ulcer drains copious amounts of chronic wounddrainage causing irritation to the surrounding skin. The patient sits most of the day ina dependent position which worsens the leg edema. How can the wound fluid becontrolled to enable healing?The clinician working in wound care needs to be a good detective and needs toconsider all possible factors influencing healing.
How Do Wounds Heal?
Research work on acute wounds in an animal model shows that wounds heal in four phases. It is believed that chronic wounds must also go through the samebasic phases1. Some authors combine the first two phases.
• Hemostasis• Inflammation• Proliferation or Granulation• Remodeling or Maturation Kane’s analogy to the repair of a damaged house provides a wonderful framework to explore the basic physiology of wound repair2 (See Table 1).
Hemostasis:
Once the source of damage to a house has been removed and before work can start, utility workers must come in and cap damaged gas or water lines. So tooin wound healing damaged blood vessels must be sealed. In wound healing theplatelet is the cell which acts as the utility worker sealing off the damaged bloodvessels. The blood vessels themselves constrict in response to injury but this spasmultimately relaxes. The platelets secrete vasoconstrictive substances to aid in this process but their prime role is to form a stable clot sealing the damaged vessel.
Under the influence of ADP (adenosine diphosphate) leaking from damaged tissuesthe platelets aggregate and adhere to the exposed collagen3. They also secretefactors which interact with and stimulate the intrinsic clotting cascade through theproduction of thrombin, which in turn initiates the formation of fibrin from fibrinogen.
The fibrin mesh strengthens the platelet aggregate into a stable hemostatic plug.
Finally platelets also secrete cytokines such as platelet-derived growth factor(PDGF), which is recognized as one of the first factors secreted in initiatingsubsequent steps. Hemostasis occurs within minutes of the initial injury unless thereare underlying clotting disorders.
Inflammation Phase:
Clinically inflammation, the second stage of wound healing presents as erythema, swelling and warmth often associated with pain, the classic “rubor et tumorcum calore et dolore”. This stage usually lasts up to 4 days post injury. In the woundhealing analogy the first job to be done once the utilities are capped is to clean up thedebris. This is a job for non-skilled laborers. These non-skilled laborers in a woundare the neutrophils or PMN’s (polymorphonucleocytes). The inflammatory responsecauses the blood vessels to become leaky releasing plasma and PMN’s into thesurrounding tissue4. The neutrophils phagocytize debris and microorganisms andprovide the first line of defense against infection. They are aided by local mast cells.
As fibrin is broken down as part of this clean-up the degradation products attract thenext cell involved.
The task of rebuilding a house is complex and requires someone to direct this activity or a contractor. The cell which acts as “contractor” in wound healing is themacrophage. Macrophages are able to phagocytize bacteria and provide a secondline of defense. They also secrete a variety of chemotactic and growth factors suchas fibroblast growth factor (FGF), epidermal growth factor (EGF), transforming growthfactor beta (TGF-__ and interleukin-1 (IL-1) which appears to direct the next stage5.
Proliferative Phase ( Proliferation, Granulation and Contraction):
The granulation stage starts approximately four days after wounding and usually lasts until day 21 in acute wounds depending on the size of the wound. It ischaracterized clinically by the presence of pebbled red tissue in the wound base andinvolves replacement of dermal tissues and sometimes subdermal tissues in deeperwounds as well as contraction of the wound. In the wound healing analogy once thesite has been cleared of debris, under the direction of the contractor, the framersmove in to build the framework of the new house. Sub-contractors can now installnew plumbing and wiring on the framework and siders and roofers can finish theexterior of the house.
The “framer” cells are the fibroblasts which secrete the collagen framework on which further dermal regeneration occurs. Specialized fibroblasts are responsible forwound contraction. The “plumber” cells are the pericytes which regenerate the outerlayers of capillaries and the endothelial cells which produce the lining. This processis called angiogenesis. The “roofer” and “sider” cells are the keratinocytes whichare responsible for epithelialization. In the final stage of epithelializtion, contracture occurs as the keratinocytes differentiate to form the protective outer layer or stratumcorneum.
Remodeling or Maturation Phase:
Once the basic structure of the house is completed interior finishing may begin. So too in wound repair the healing process involves remodeling the dermaltissues to produce greater tensile strength. The principle cell involved in this processis the fibroblast. Remodeling can take up to 2 years after wounding and explains whyapparently healed wounds can break down so dramatically and quickly if attention isnot paid to the initial causative factors.
Table 1 Phases of Healing
Phase of
Hemostasis
Inflammation
Proliferation
Granulation
Contracture
Remodeling
When Does a Wound Become Chronic?
In healthy individuals with no underlying factors an acute wound should heal within three weeks with remodeling occurring over the next year or so. If a wounddoes not follow the normal trajectory it may become stuck in one of the stages andthe wound becomes chronic. Chronic wounds are thus defined as wounds, whichhave “failed to proceed through an orderly and timely process to produce anatomicand functional integrity, or proceeded through the repair process without establishinga sustained anatomic and functional result.”6 Once a wound is considered chronic itshould trigger the wound care clinician to search for underlying causes, which maynot have been addressed. Better yet, an understanding of the causative factorsshould lead us to be proactive in addressing these factors in at risk populations sothat chronic wounds are prevented.
Basic Principles of Wound Care
There are three basic principles which underlie wound healing.
1. Identify and control as best as possible the underlying causes.
2. Support patient centered concerns3. Optimize local wound care. The CAWC Best Practice Recommendations for wound care (wound bed prep,venous ulcer management, pressure ulcer management and diabetic ulcermanagement) extensively covers all three principles (www.cawc.net). Figure 4, excerpted from Wound Bed Preparation, outlines an algorithm that provides aframework for chronic wound management.
Optimize Local Wound Care
In 1962 George Winter described improved wound healing under moist conditions7. Despite that seminal work it is only in the last decade that theadvantages of moist interactive wound healing have become more widely recognizedand applied in clinical practice. Some of the advantages include the following: • Decreased dehydration and cell death. As described earlier, the task of
wound repair requires the activity of a host of cells from neutrophils andmacrophages to fibroblasts and pericytes. These cells cannot function in adry environment.
• Increased angiogenesis. Not only do the cells required for angiogenesis
require a moist environment but also angiogenesis occurs towards regionsof low oxygen tension such that occlusive dressings may act as a stimulusin the process8.
• Enhanced autolytic debridement. By maintaining a moist environment
neutrophil cell life is enhanced and proteolytic enzymes are carried to thewound bed allowing for painless debridement9. Further as discussed earlierthese fibrin degradation products are a factor in stimulating macrophages torelease growth factors into the wound bed.
• Increased re-epithelialization. In larger, deeper wounds epidermal cells
must spread over the wound surface from the edges. They must have asupply of blood and nutrients. Dry crusted wounds reduced this supply andprovide a barrier to migration thus slowing rates of epithelialization10.
• Bacterial barrier and decreased infection rates. Occlusive dressings
with good edge seals can provide a barrier to migration of microorganismsinto the wound. Bacteria have been shown to pass through 64 layers ofmoist gauze11. Wounds covered with occlusive dressings have been shown to have lower rates of infection than those with conventional gauzedressings12.
• Decreased pain. It is believed that the moist wound bed insulates and
protects the nerve endings thereby reducing pain. Furthermore occlusivedressings often require fewer dressing changes, which may beuncomfortable for patients.
• Decreased costs. While occlusive dressings have a higher per unit cost
than conventional gauze, the reduced frequency of dressing changes andincreased healing rates may proved to be cost effective in the long term.
While moist wound healing has clear advantages, debate continues on how moist is moist. Dressings should retain enough moisture to stimulate good healingand yet should not cause maceration or irritation to the surrounding tissues.
The Ideal Dressing
So how do we provide for good moist interactive wound healing? In 1979 Turner described the ideal dressing as having the following characteristics13: • High humidity at the dressing wound interface • Free from particulate and toxic components • No trauma with removalOver the past 15 years an ever-expanding list of dressing products has come onto the market in an attempt to meet these conditions. Among these are thetransparent film dressings, hydrogels, hydrophilic foams, alginates, hydrocolloids andthe new antibacterials and biologic dressings or devices. There is however no magic“one-size-fits-all” dressing. The clinician needs to become familiar with thecharacteristics of the different classes of dressings and to tailor the dressing used tothe phase of healing, characteristics of the wound, the needs (and risk factors) of thepatient and the availability and skill of the caregiver.
In summary wound healing requires an approach that is: • Patient centered: It is always wise to remember that we are dealing with a
person who happens to have a chronic wound. We can develop awonderful management plan but if we do not have patient buy-in the plan isdoomed to failure.
• Holistic: Best practice requires the assessment of the whole patient, not
just the “hole in the patient”. All possible contributing factors must beexplored.
• Interdisciplinary: Wound care is a complex business requiring the skills of
many disciplines. Skilled nurses, physiotherapists, occupational therapists,dietitians and physicians both generalists and specialists (dermatologists,plastic surgeons and vascular surgeons depending on need) are central members of the team. In addition in some settings social work involvementmay be important.
• Evidence based: In today’s healthcare environment treatment must be
based on best available evidence and be cost effective.
1Kerstein MD: The scientific basis of healing. Adv Wound Care 1997; 10(3):30-36 2Kane D: Chronic wound healing and chronic wound management, in Krasner D, RodeheaverGT, Sibbald RG. (eds): Chronic Wound Care: A Clinical Source Book for HealthcareProfessionals, Third Edition. Wayne, PA, Health Management Publications, 2001,pp 7-17.
3MacLeod J (ed): Davidson’s Principles and Practice of Medicine, Thirteenth Edition.
Edinburgh UK, 1981, pp 590-5924Wahl LM, Wahl SM: Inflammation, in Cohen IK, Diegelman RF, Lindblad WJ (eds): WoundHealing: Biochemical and Clinical Aspects. Philadelphia, PA, W.B. Saunders, 1992, pp 40-625Kerstein MD: Introduction: moist wound healing. American Journal of Surgery 1994;167(1A Suppl): 1S-6S6Lazarus G, Cooper D, Knighton D, Margolis D, Pecoraro R, Rodeheaver G, Robson.
Definitions and guidelines for assessment of wounds and evaluation of healing. Archives ofDermatology 1994;130:489-4937Winter GD: Formation of scab and rate of epithelialization of superficial wounds in the skinof the young domestic pig. Nature 1962;193:293-2948Knighton DR, Silver JA, Hunt TK. Regulation of wound-healing angiogenesis: effect ofoxygen gradients and inspired oxygen concentration. Surgery 1981;90:262-2709Baxter CR. Immunologic reactions in chronic wounds. American Journal of Surgery1994;167(1A Suppl):12S-14S10Haimowitz JE, Margolis DJ: Moist wound healing, in Krasner D, Kane D (eds): ChronicWound Care: A Clinical Source Book for Healthcare Professionals. Wayne, PA, HealthManagement Publications, 1997, 49-5511Mertz PM, Marshall DA, Eaglestein WH. Occlusive dressings to prevent bacterial invasionand wound infection. J Am Acad Dermatol 1985;12:662-66812Hutchinson JJ, McGuckin M. Occlusive dressings: A microbiologic and clinical review. AmJ Infect Control 1990;18:257-26813Turner TD. Hospital usage of absorbent dressings. Pharma J 1979;222:421-426

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