The Wound Healing Process
Wound healing consists of a series of physiologic events that occur in response to tissue damage. Many of the vital processes of wound healing are oxygen dependent. Wounds vary greatly, and the healing of a wound may proceed with different tempo and quality due to local and systemic factors and due to variation among individuals. Variable healing is present within a specific wound and within different anatomic locations in the same patient. However, there are common themes to the healing of all wounds. The burn literature has classically described three zones of injury: necrosis, stasis, and hyperemia. This construct serves well in considering any wound. Tissue in the zone of necrosis is already lost, while tissue in the zone of stasis may be saved with good local and systemic care; otherwise, it is lost to infection, trauma, or dehydration. Tissue in the zone of hyperemia may become damaged or lost if problems arise in the zone of stasis. Recognizing that the key determinant of tissue survival is oxygenation, one can rephrase the burn zone paradigm as follows: zone of anoxia, zone of hypoxia, and zone of normoxia. In this oxygen-based paradigm, the wound healer must focus on treating tissue in the zone of hypoxia in order to preserve the maximum amount of viable tissue and encourage the healing process.
Oxygen levels in wounded tissue result from a balance of supply and demand. Supply may be low due to problems such as vascular disease, radiation (impaired delivery), or edema (increased diffusion distance). Demand may be high as a result of metabolic needs of specific cells at various points within the wound healing sequence. White blood cells consume oxygen during the respiratory burst necessary for killing ingested bacteria. Fibroblasts require a critical level of oxygen in order to secrete collagen and other extracellular matrix molecules. This allows angiogenesis and granulation tissue formation to fill the wound. Hypoxic wounds deposit collagen poorly and become infected easily. Epithelialization is a parallel process of resurfacing the wound that also proceeds optimally at high oxygen levels. Although the processes described may proceed optimally at elevated pO2 levels, they may still require some hypoxia as an intermittent signal.
