Measured orally, a healthy person’s body temperature can fluctuate between 36.1°C (97°F) and 37.8°C (100°F). Survival depends on maintaining temperature stability within this range by balancing the heat produced by metabolism with the heat lost to the environment. When environmental or other changes cause heat loss to outpace heat production, the brain triggers physiological and behavioral responses to restore the balance. If the cold stress is too great and the body’s defenses are overwhelmed, body temperature begins to fall.
In trauma and surgical patients even mild hypothermia is associated with many adverse events. The American College of Surgeons has redefined the classification scale of hypothermia in trauma patients to emphasize with clinicians the importance of preventing and responding to even slight changes in normal body temperature. Hypothermia is considered present in patients once core temperature falls below 36°C (96.8°F). Multiple studies and experience with trauma patients demonstrate the lethal effect of undesired hypothermia with 100% mortality when body temperature is allowed to fall to 32°C (90°F).
Accidental or undesired hypothermia is divided into two types; primary and secondary.
Primary hypothermia occurs when the body’s heat-balancing mechanisms are working properly but are subjected to extreme cold.
Secondary hypothermia affects people whose heat-balancing mechanisms are impaired in some way and cannot respond adequately to moderate or perhaps even mild conditions. This can be caused by the physiological response to trauma, or induced clinically through the introduction of anesthesia, medications, and other substances, including infusion of non-warmed or inadequately warmed blood products and IV solutions.
Even mild lowering of body temperature as little as 1°C (2°F) is associated with increased adverse event risks and costs in all patients undergoing surgery. There is a clear correlation between slight lowering of normal body temperature and reduced coagulability of platelets leading to increased bleeding, increased wound infections, impaired wound healing, adverse cardiac events, decreased medication metabolism, exacerbated postoperative pain, increased demand for oxygen, development of pressure sores, and venous stasis leading to development of deep vein thrombosis.
Accidental hypothermia can be difficult to reverse after it occurs, and thus early preventative measures need to be implemented. Intravenous fluids delivered even at room temperature contribute to accidental hypothermia as the gradient between the human body and room temperature is between 14°–16.6°C (26°–30°F). This issue is greatly exacerbated when refrigerated blood products are administered or in the austere and all climate conditions in which the military and pre-hospital Emergency Medical Services (EMS) operate. Thus, along with other measures, warming of intravenous infusions is a priority step in both the prevention and treatment of hypothermia.
It is vitally important to protect casualties from hypothermia in all climates. Ambient temperature IV resuscitation fluids are a contributor to hypothermia, and should be warmed starting at the point of injury.
The greater injury severity and progressive shock, the less volume of ambient temperature IV fluids it takes to induce the lethal triad of hypothermia, coagulopathy, and acidosis.
The dangerous consequences of hypothermia in a trauma patient is known as the Trauma Triad of Death. Intervention at the earliest opportunity, especially at the point of injury, is critical in order to prevent this cycle.
Although the ambient environment plays a role, the main cause of hypothermia in trauma patients is blood loss. This leads to lack of oxygen delivery to the tissues, and a shift from normal aerobic metabolism to anaerobic metabolism. Normal metabolism produces heat, anaerobic metabolism produces lactic acid, which in turn worsens coagulopathy and decreases cardiac output. Blood loss, hypothermia, acidosis, and coagulopathy occur in a vicious cycle, with each factor exacerbating the other.
Major clinical and healthcare societies and organizations responsible for patient care have supported and been involved with a multitude of studies citing the detrimental and costly effects of undesired hypothermia; many have developed their own guidelines or adopted the guidelines of similar organizations concerning hypothermia prevention. Some well known organizations include: