Canine Delayed Wound Healing Study Guide

Overview and Clinical Importance

Delayed wound healing is a common clinical challenge in veterinary practice that occurs when wounds fail to progress through the normal phases of healing in a timely manner. Understanding the mechanisms behind impaired wound healing is essential for the NAVLE, as questions frequently test knowledge of wound physiology, factors affecting healing, and appropriate therapeutic interventions. Chronic wounds in dogs are defined as full-thickness defects that fail to heal despite appropriate antibiotic therapy and bandaging.

The normal wound healing process consists of four overlapping phases: hemostasis, inflammation, proliferation (repair), and remodeling (maturation). Any disruption to these phases can result in delayed or non-healing wounds. Recognition of the specific phase affected and identification of underlying causes are critical skills for successful wound management.

Normal Wound Healing: A Review

Understanding normal wound healing is essential for recognizing when healing is delayed. The process involves a complex, finely-tuned continuum of molecular and cellular events.

Phase 1: Hemostasis (Minutes to Hours)

Immediately following injury, vasoconstriction occurs for 5-10 minutes to limit blood loss, followed by vasodilation to allow entry of fluid and cells. A fibrin clot forms, serving as a scaffold for subsequent cellular migration.

Phase 2: Inflammation (1-5 Days)

Neutrophils are first to arrive (within 30 minutes), predominating for the first 24-48 hours. Monocytes/macrophages become predominant by days 2-3 and are considered essential for wound healing. Macrophages produce greater than 100 proteins active in healing. In a healthy wound, this phase completes in approximately 72 hours.

Phase 3: Proliferation/Repair (4-21 Days)

This phase involves three concurrent processes:

• Fibroplasia: Fibroblasts appear approximately 3 days post-injury, secrete collagen
• Angiogenesis: New capillary formation follows the oxygen gradient toward hypoxic wound center
• Epithelialization: Epithelial cells migrate from wound edges across granulation tissue

Granulation tissue appears around day 5 in dogs. It is characteristically bright pink-red, slightly lumpy, and glistening, friable but highly resistant to infection.

Phase 4: Remodeling/Maturation (21 Days to 2 Years)

Collagen fibers reorganize along lines of tension. Type III collagen is gradually replaced by stronger Type I collagen. Wound tensile strength: ~10% at 14 days, 25% at 4 weeks, maximum 80% at several months.

Normal Wound Healing Timeline in Dogs

Causes of Delayed Wound Healing

Factors affecting wound healing can be categorized as systemic (patient-related) or local (wound-related).

Systemic Factors

Endocrine Diseases

Hyperadrenocorticism (Cushing's Disease): Excess cortisol inhibits capillary budding, fibroblast proliferation, collagen synthesis, and epithelialization. Approximately 5-10% of dogs with HAC have concurrent diabetes mellitus.

Diabetes Mellitus: Hyperglycemia impairs neutrophil function, reduces angiogenesis, and causes microangiopathy via AGE, polyol, PKC, and hexosamine pathways.

Hypothyroidism: Reduced metabolic rate affects all phases of wound healing.

Nutritional Deficiencies

Hypoproteinemia (total protein less than 2 g/dL) significantly delays wound healing. Methionine and cysteine are particularly important amino acids. Vitamin C is required for collagen synthesis. Zinc is essential for epithelial and fibroblast proliferation.

Other Systemic Factors

• Anemia: Creates low tissue oxygen levels
• Uremia: Uremia induced in first 5 days causes wound disruption
• Age: Geriatric patients may have compromised healing capacity
• Immunosuppression: Chemotherapy or immunosuppressive medications

Medications Affecting Wound Healing

Local (Wound-Related) Factors

Infection

Wound infection prolongs the inflammatory phase. Bacterial counts greater than 105 organisms per gram generally prevent closure. Common pathogens: Staphylococcus pseudintermedius (most common), Pasteurella (bite wounds), Pseudomonas.

Mechanical Factors

• Tension: Excessive wound tension compromises blood supply - 'tension is the enemy of wounds'
• Motion: High-motion areas (axilla, inguinal, joints) subject to shearing forces
• Self-trauma: Licking and chewing at wounds is major cause of complications
• Foreign material: Debris, hair, suture, necrotic tissue prolongs inflammation

Moisture Imbalance

• Desiccation: Damages fibroblasts, epithelial cells, capillary buds
• Maceration: Excessive moisture causes tissue breakdown

Clinical Presentation and Diagnosis

Recognition of Delayed Healing

Chronic non-healing wounds are defined as full-thickness defects that fail to heal despite appropriate antibiotic therapy and bandaging. Clinical indicators include:

• Wound remaining in inflammatory phase beyond 5-7 days
• Absence of granulation tissue by day 5-7
• Persistent exudate or purulent discharge
• No reduction in wound size over 2-week period
• Wound expansion rather than contraction

Wound Assessment: The TIME Principle

Exam Focus: The TIME acronym is commonly tested. Remember T = Tissue, I = Inflammation/Infection, M = Moisture, E = Edge.

Diagnostic Workup

• Wound cytology: Identify intracellular bacteria, cell types
• Deep tissue culture: Preferable to surface swabs for chronic wounds
• Radiography/Ultrasound: Evaluate for sequestra, foreign bodies, bone involvement
• CBC/Chemistry: Assess for anemia, hypoproteinemia, glucose abnormalities
• Endocrine testing: LDDS, ACTH stim, thyroid panel when indicated

Treatment of Delayed Wound Healing

Wound Bed Preparation

Debridement

Sharp surgical debridement is preferred for large amounts of necrotic tissue - continue until tissue bleeds. Mechanical debridement (wet-to-dry bandages) provides nonselective removal.

Lavage

Use 7-8 PSI pressure (30-35 mL syringe with 18-gauge needle). Isotonic saline is preferred. If antiseptics used: 0.05% chlorhexidine or 0.5-1% povidone-iodine. Higher concentrations are cytotoxic.

Wound Dressing Selection

Moist wound healing is the current standard. Dressing selection depends on wound phase and exudate level.

Critical Point: Once granulation tissue is present (day 5+), switch to non-adherent dressings to protect fragile new tissue.

Negative Pressure Wound Therapy (NPWT)

NPWT (VAC therapy) applies sub-atmospheric pressure to wounds through a sealed dressing system.

Mechanism and Benefits

• Increases blood flow to wound (up to 4x baseline at -125 mmHg)
• Reduces interstitial edema
• Removes inhibitory cytokines and exudate
• Stimulates FASTER granulation tissue formation
• Draws wound edges together

Application Parameters

• Pressure: -125 mmHg standard for open wounds; -50 mmHg for seroma prevention
• Dressing changes: Every 2-5 days; daily for contaminated wounds
• Contraindications: Malignant wounds, exposed vessels, untreated osteomyelitis

Antimicrobial Therapy

Antibiotics are NOT routinely needed for all wounds. Indications: systemically ill patients, extensive/deep wounds, bite wounds, wounds over bone/tendon.

Important: Fluoroquinolones are second-line drugs - do NOT use empirically. Reserve for culture-confirmed resistant infections.

Wound Complications

Seroma

Accumulation of serous fluid in dead space. Small seromas resorb in 1-2 weeks. Large ones may need aspiration or drain placement.

Wound Dehiscence

Wound dehiscence (separation of wound edges) is potentially the most serious complication. If tissue protrudes, it is a surgical emergency. Causes: tension, infection, poor nutrition, self-trauma, seroma/hematoma.

Memory Aids

"HEALER" - Factors That DELAY Wound Healing

• H - Hyperadrenocorticism/Hypothyroidism
• E - Exogenous steroids/Edema
• A - Anemia/Age
• L - Low protein/Low oxygen
• E - Excessive tension/Exposed bone
• R - Radiation/Resistant infection