Examining the Environments of Wound Healing
The systematic study of wound healing is a relatively young science. Careful characterization of the normal healing process is essential to understanding the potential deviations from normal healing which can result in chronic wounds. As we learn more about the healing process we are discovering that it is an intricate balance of a wide variety of local and systemic factors and that even minor disturbances in these factors may affect the rate or quality of healing. The many local and systemic factors that affect healing can be loosely grouped into several broad categories, which may also be viewed as environments in which the healing process is occurring. We will briefly review the normal healing process and introduce the environments of healing.
The healing process can be segregated into various phases characterized by the particular cells involved and the chemicals being produced. Three such phases are traditionally recognized.
The Inflammatory Phase
The inflammatory phase of healing occurs immediately after injury and after hemostasis is achieved, involves predominantly white blood cells such as neutrophils, monocytes, and macrophages. These cells mount an initial defense against microbial invasion and secrete a variety of proteolytic enzymes capable of liquefying nonviable tissue components and microorganisms in the wound area.
Proliferative or Tissue-Building Phase
As nonviable tissue is degraded and bacterial levels are controlled, the inflammatory process resolves and the proliferative or tissue -building phase of healing begins. This phase of healing is dominated cellularly by fibroblasts, which synthesize and deposit collagen proteins and other extracellular matrix components, endothelial cells which bud from existing vessels to revascularize the wound and epithelial cells which proliferate, migrate, and stratify to reestablish the epidermal barrier. These various cells must coordinate their different contributions to the healing process and do so via the secretion of and response to intercellular communication chemicals known generally as growth factors and cytokines.
Growth factors are small molecular weight proteins, which essentially transmit messages between cells that result in increased cell proliferation and migration as well as in the production and deposition of noncellular tissue components such as collagen, glycosaminoglycans, and proteoglycans. Cytokines are similar proteins, which tend to transmit messages between cells that result in the controlled proteolysis of tissue components or foreign materials in the wound.
Remodeling or Maturation Phase
The final phase of healing occurs in the dermis and is referred to as the remodeling or maturation phase. In this phase of healing, fibroblasts are again dominant as tissue tensile strength (that is the ability to withstand disruption) is increased through the gradual replacement of Type 3 collagen in the scar tissue by Type 1 collagen, thickening of the collagen fibers, and reorientation of those fibers along lines of tissue tension.
These three phases of wound healing are not distinctly separated from each other but overlap in time. When healing occurs without incident, no one phase stands out from the other clinically. However, not all wounds heal without incident. There are a wide variety of local and systemic factors that can interfere with the healing process, many of which appear to arrest the wound in a particular phase of healing or impair the functionality of the healed wound. These local and systemic factors will now be discussed in the context of constituting environments of the healing process.
The Environment of the Patient
Epidemiological studies have suggested that there are several types of patient factors that have an impact on their risk for developing wounds as well as their expected rate of wound healing.
Such factors may be:
- Characteristics of the patient’s physical constitution such as their age and ethnicity,
- Their body type or composition,
- The position of their body (e.g. do they have contractures, etc.),
- Their mental status (which may affect their mobility),
- Whether they have an existing amputation of a lower extremity,
- Characteristics of the patients lifestyle or habits such as the use of alcohol, smoking,
- The presence or absence of family or a social support system (which may affect compliance).
The Physical Environment of the Wound
There are key characteristics of the wound itself that have an impact on healing rates for the patient concerned.
These characteristics include:
- The overall shape and dimensions of the wound,
- The location of the wound on the body,
- Whether there is one or multiple wounds on the patient,
- The quality of the tissues in the wound bed (e.g. are the tissues healthy or dying),
- The temperature of the exposed tissues,
- The amount and quality of the wound exudates,
- The presence of maceration.
Additional characteristics of the wound may be indicative of an infective process, such as the presence of cellulitis, odor from the wound, pain in the wound, condition of the wound margins, continence status (if the wound is in the perineal area), and the presence of edema or lymphedema. Also, the duration of time that the wound has been present or if it has re-occurred seem to correlate with healing rates.
The Macroscopic and Microscopic Biochemical Environments of the Wound
There are also key macroscopic aspects of the overall biochemistry of the body, which can impact the biochemistry or cells involved in the healing process.
These aspects include the patient’s:
- Metabolic control,
- Nutritional status,
- Immune status,
- Presence of inflammation or infection,
- Circulatory status (perfusion),
- Presence of infection,
- Moisture balance in the exposed tissues of the wound.
Microscopic aspects of the wound which affect cells or biochemistry include:
- Levels of tissue proteases,
- Levels of key intercellular communication chemicals, such as cytokines and growth factors,
- Presence and duration of tissue hypoxia,
- Proliferative capacity of the cells, especially the fibroblast,
- Levels of bacteria and bacterial products in the wound.
It is the unique interplay of factors in these four areas or environments of healing, which create challenges in the management of patients with nonhealing wounds. Clinicians must have a clear understanding of these environments and how to assess and address the different aspects of each in order to achieve successful outcomes. Details of the environments of healing and advances in diagnosis and treatment will be the subject of future columns.
This article was written by Liza G. Ovington, PhD, CWS.