Immune system
The effect of hypothermia on the immune system is rarely considered in reviews. In a real-life scenario, hypothermia is usually associated with infections that might compromise the tolerance of the victim. In controlled cold stress or hypothermic studies in either Department of Defense laboratory or military field experiments in which the subjects were previously screened for illness-the hypothermic subjects rarely became sick. In an extensive number of hypothermic studies conducted at the University of Minnesota in which more than 250 medical students were made mildly hypothermic none became ill following a 3-week period of evaluation. However, in both hospitals and field operations, in which various stressors interact to compromise the immune system, hypothermia and infection go hand in hand.
Everyday experiences demonstrate that decreased ambient temperature inhibits immune function. When a soldier injures a joint, for instance, ice is used to prevent the infiltration of immune cells and the subsequent release of inflammatory cytokines. Conversely, heat can be applied to abscesses to speed healing. A more dramatic example would be the high propensity of leukopenia and bacterial infections in children kept hypothermic for clinical reasons Although advances in immunoogy have not yet been integrated with existing knowledge of hypothermic sequelae, unanticipated nonthermal positive effects may be seen
Fever augments immune function because hyperthermia of two Centigrade degrees above normal core temperature temporarily raises the mononuclear cell count in patients with cancer and increases the mitogenic response. Thus, an increase in body temperature, whether induced or spontaneous, can confer an advantage to the immune response
On the other hand, decreases in core temperature are detrimental to immune function, as opposed to having merely a neutral effect Sessler and coleagues demonstrated that wounds are larger in guinea pigs that are infected under hypothermic conditions than in those infected under control conditions Because more than half of the body volume i 1 inch from the surface and significantly cooler than the core body temperature of 37 C, local skin temperatures may influence the growth of infections. Vasoconstriction lowers resistance to infection by decreasing the partial pressure of en in tissues This decrease in oxygen pressure decreases oxygen- and nitrogencontaining free radicals, both of which play major roles in microbial killing.
One explanation for cold-induced immunosuppression is that the immune cells are specificall inhibited by decreased temperature. In cases of secondary hypothermia, when thermal compensatory mechanisms become inadequate, certain observations can be made about the effect of cold on specific populations of immune cells. Histamine release from type I mast cells is decreased at low temperatures, and Biggar and colleagues showed that neutrophils were impaired in their migration, both in vivo and in vitro, at reduced temperature. When the cooled cells were rewarmed, they exhibited optimal activity. In a clinical study, hypothermic patients (as assessed by tympanic temperature)who had undergone colorectal surgery had more surgical wound infections and their sutures were removed 1 day later than patients who were given additional warming. Peripheral vasoconstriction was seen in 78% of the hypothermic patients versus 22% for the normothermic group. From a military perspective, there was another interesting finding three times as many infections were found among smokers in both groups. Minimizing smoking among troops might do as much to minimize infections postoperatively as efforts to rewarm patients who are mildly hypothermic
Wang-Yang and colleagues reported that some in vitro responses of helper T cells in mice are inhibited by cold, but that B cells were not similar suppressed. Cold interfered with interleukin (IL) production in virgin helper T cells, implying an early block in the activation of these cells. However the responses of these cells to IL-2 and IL-4 were not affected by cold