Surgical technique being applied to wounds
By Liza G. Ovington, PhD, CWS
President, Ovington & Associates
Most of the advanced wound dressings currently available share a fundamental approach to enhancing the rate of wound healing: They create an optimal local environment by creating and maintaining physiological moisture levels. A new dressing/device called Warm-Up Active Wound Therapy takes a slightly different and novel approach to optimizing the local wound environment by providing physiological levels of heat to the wound.
Unlike the infrared heat lamps of old that were used to dry out wounds, the new device generates moist heat, creating a humid environment around the wound. The device has been described as a "mini-greenhouse" consisting in part of a foam "collar" that is adhered to the periwound skin. A transparent film covers the top of the foam and is raised above the surface of the wound by the foam's thickness so there is no contact with the wound surface. There is a pocket or sleeve built into the film covering into which an infrared warming card is inserted. The warming card is powered by a battery pack that can be worn in a pouch for mobility (alternatively, the power unit can be plugged into a wall outlet).
When turned on, the warming card raises the temperature inside the bandage to 38 degrees Centigrade. Normal body temperature is 37 degrees Centigrade (equivalent to 98.6 degrees Fahrenheit). Tissue temperature beneath the bandage has been measured at 36 degrees to 37.5 degrees C.1
Hypothermia common during surgery
The idea of using heat to aid wound healing has its origins in surgery. It was here that the importance of temperature was made apparent by its absence. Mild hypothermia (35 degrees C or 95 degrees F) is common during major surgical procedures due to impairment of thermoregulation by anesthesia. Hypothermia can lead to increased risk of infection. The mechanism for this effect involves tissue oxygen levels.
Hypothermia causes vasoconstriction and increases hemoglobin's affinity for oxygen, making it less available to tissues and cells. Reduced oxygen availability leads in turn to impaired bactericidal activity by neutrophils. Neutrophils fight infection by migrating to an area of inflammation, engulfing (phagocytosing) bacteria, and killing them. Part of their killing mechanism involves generating reactive chemical intermediates from oxygen. In vitro and in vivo studies of neutrophils have shown that reduced temperatures2,3 decrease both their phagocytic activity and their production of reactive oxygen species. In vitro studies also have shown that reduced temperatures impair neutrophils' ability to migrate in response to a chemical stimulus.2 It is also known that under conditions of decreased oxygen availability (such as those induced by hypothermia), phagocytic cells can engulf bacteria, but later eject them unharmed, whereas under conditions of adequate oxygen they are able to destroy the phagocytosed bacteria.4
Hypothermia increases LOS, infection rate
Such findings were instrumental in the development of patient warming systems for use during surgical procedures to maintain normal body temperature. A recent study of 200 surgery patients demonstrated that 19% of those who experienced hypothermia developed infection, while only 6% of those who received additional warming to maintain normothermia developed infection.5 The length of hospital stay for the hypothermic group was 2.6 days longer than the normothermic group. (For more study results, see story, at right.)
The idea of supplying mild local heating to chronic wounds to optimize healing was a natural progression of the surgical application. In fact, the company that manufactures the wound warming device [Augustine Medical, Eden Prairie, MN; telephone: (800) 733-7775] is a leading manufacturer of patient warming systems for surgery.
While a surgical patient develops systemic hypothermia, temperature loss in chronic wounds is a local effect due to loss of moisture vapor. Studies of wounds in swine have measured the tissue temperatures of wounds covered with different materials.6 Wounds left uncovered measured 21 degrees C; those covered with gauze were measured at 25 to 27 degrees C; those covered with foam measured 33 to 35 degrees C. The described warming device supplies moist, local heat and raises tissue temperature to normal body core levels. By maintaining normal tissue temperature, oxygen is made more available for neutrophil function and cellular metabolism. The wound can be observed through the film covering by removing the warming card, and the foam "collar" is capable of absorbing excess exudate.
1. Ikeda T, Tayefeh F. Sessler DI, et al. Local radiant heating increases subcutaneous oxygen tension. Am J Surg 1998; 175:33-37.
2. Akriotis V, Biggar WD. The effects of hypothermia on neutrophil function in vitro. J Leukoc Biol 1985; 371:51-61.
3. Wenisch C, Narzt E, Sessler DI, et al. Mild intraoperative hypothermia reduces production of reactive oxygen intermediates by polymorphonuclear leukocytes. Anesth Analg 1996; 82:810-816.
4. Silver IA. The physiology of wound healing. In: Hunt TK, ed. Wound Healing and Infection. New York: Appleton-Century- Crofts; 1980, pp. 11-32.
5. Kurz A, Sessler DI, Lenhardt R. Perioperative normothermia to reduce the incidence of surgical wound infection and shorten hospitalization. Study of Wound Infection and Temperature Group. N Engl J Med 1996; 334:1,209-1,215.
6. Lock PM. The effect of temperature on mitotic activity at the edge of experimental wounds. In: Lundgren A, Soner B, eds. Symposia on Wound Healing; Plastic, Surgical and Dermatologic Aspects. Molndal, Sweden: 1998, pp. 103-109.