Factors that Interfere with Wound Healing in Animals
Factors that interfere with wound healing may be divided by source into physical, endogenous, and exogenous categories.
Physical factors that affect wound healing include temperature and mechanical forces. Temperature affects the tensile strength of wounds. Ideal conditions to allow wound healing occur at 30°C. Decreasing the temperature to 12°C results in a 20% loss of tensile wound strength. Mechanical forces include pressure and sheer force. Pressure can compromise blood flow in the region, decreasing oxygen levels in the tissue. Sheer forces result in tearing of the vessels. Because adequate oxygen levels are required for appropriate wound healing, anything that interferes with blood flow will slow wound healing. Low levels of oxygen interfere with protein synthesis and fibroblast activity, causing a delay in wound healing. Oxygen levels may also be compromised by many other factors, including hypovolemia, the presence of devitalized tissue, hematomas, seromas, and excessively tight bandages.
Endogenous factors typically reflect the overall condition of the animal. Anemia may interfere with wound healing by creating low tissue oxygen levels. Nutrition has a significant overall effect on the body. Although the ideal nutritional level for wound healing is unknown, hypoproteinemia delays wound healing when the total serum protein content is <2 g/dL, based on some studies. Because wound healing is a function of protein synthesis, malnutrition can alter the healing process. The addition of dl-methionine or cysteine (an important amino acid in wound repair) can reverse some delayed wound healing. Uremia can interfere with wound healing by slowing granulation tissue formation and inducing the synthesis of poor quality collagen. Obesity contributes to poor wound healing through decreased blood supply and with poor suture holding in the subcutaneous fat layers.
Exogenous factors include any external chemical that alters wound healing. Cortisone is commonly implicated in wound complications. Corticosteroids markedly inhibit capillary budding, fibroblast proliferation, and rate of epithelialization. Similar to cortisone, vitamin E adversely affects wound healing by slowing collagen production. Vitamin C is required for hydroxylation of two amino acids, proline and lysine. Zinc is required for epithelial and fibroblastic proliferation; however, excessive zinc delays wound healing by inhibiting macrophage function. Other factors that inhibit wound healing include radiation, alkylating agents (eg, cyclophosphamide, melphalan), and inappropriate concentrations of antiseptics. In human medicine, NSAIDs have been shown to slow wound healing.