Traumatic injury is the third leading cause of death for all age groups in the United States and the leading cause of death for those under age 44 years.
More Americans under the age of 34 years die from unintentional injury than from all other diseases combined. More than half of all deaths occur before the patient reaches the hospital. About three-fourths of the patients who arrive at the hospital will die within the first four hours. About one-fourth of those have survivable injuries but quickly succumb due to complications.
One of those complications is hypothermia, occurring in about 10% of patients.
Non-trauma related mild hypothermia, defined as a body core temperature between 32 degrees Celsius and 35 degrees Celsius, is generally well tolerated. Researchers found a 21% mortality rate in patients with a core temperature less than 32° C originating from environmental exposure. However, that same degree of hypothermia following traumatic injury resulted in 100% mortality, independent of the presence of shock, injury severity score, or fluid resuscitation.
Both civilian and military patients suffering traumatic injury have significantly increased mortality if they arrive at the hospital with lowered body temperatures compared to normal. Unintentional hypothermia occurring in the prehospital environment is associated with a three-fold mortality increase in isolated, blunt, moderate to severe traumatic brain injury, even when transport times are short.
Prehospital researchers in Pennsylvania analyzing a state-wide trauma database found a similar mortality association for all hypothermic trauma patients over the age of 16. A 12-year retrospective review of burn patients in New York State found that hypothermia was more common in patients with large body-surface burns and was associated with higher mortality.
One major criticism of many of the studies reporting the effects of hypothermia on mortality in trauma patients is that patients who develop hypothermia are often more severely injured than patients who remain. In those cases, the observed increase in mortality may result from more severe injuries and not from the hypothermia. In fact, one researcher found that after matching trauma patients for anatomical and physiological indicators of severe injury, mortality was not different between hypothermic and normothermic groups. However, two other investigations found admission hypothermia to be an independent risk factor for mortality in trauma patients.
Intentional versus unintentional hypothermia
One important distinction that EMS providers must make is the difference between intentional and unintentional hypothermia. Clinicians induce hypothermia under controlled and monitored conditions for the purpose of protecting the brain from ischemic harm. Intentional hypothermia appears beneficial in a select subset of traumatic brain injury patients and during the post-cardiac arrest period. Animal studies suggest the theoretical benefits to humans likely arise from altering the inflammatory response to ischemia, decreasing oxygen consumption in the brain, and reducing free radical formation in ischemic tissue.
Unintentional hypothermia following traumatic injury, on the other hand, represents a failure of the body’s compensatory mechanisms for thermoregulation. The body responds to heat loss by shivering, which increases oxygen consumption in skeletal muscles by 40% to 400%. This high metabolic demand places some organs at risk of developing ischemia. By the time hypothermia develops, the energy reserves of the body are depleted and the system is showing signs of exhaustion. Hypothermia interferes with the clotting mechanisms of the blood by disrupting platelet function, slowing the chemical reactions that ultimately produce the protein strands necessary to build blood clots and by suppressing the immune system. A drop in body temperature of 1 degree Celsius results in a 6% to 7% decrease in cerebral blood flow, which could be dangerous for patients with traumatic brain injury.
It is difficult to determine whether admission hypothermia occurs because of prolonged patient exposure to ambient temperatures, heat loss from open body cavities, or because of aggressive fluid resuscitation with room temperature IV. Medics often disrobe trauma patients early in the assessment, thereby enhancing radiant heat loss that occurs when there is a significant temperature gradient between the environment and the patient’s skin. Reversing hypothermia is an effective strategy for improving outcome. However, once hypothermia develops, it is not resolved quickly or easily.
One of the easiest ways to begin combating the harmful effects of hypothermia is to undertake passive warming strategies such as removing the trauma patient from the cold environment as quickly as possible, raising the temperature in the back of the ambulance, and covering the patient with a blanket. Although passive interventions are effective for mild hypothermia if the patient’s thermoregulatory mechanisms are intact, this strategy will still allow trauma patients to cool.
Most invasive means of rewarming patients require specialized equipment and training and are not practical for the prehospital environment. Placing activated and insulated heat packs at the patient’s head, lower back, and under each axilla is an effective method of maintaining thermostasis.
Other suggested prehospital strategies for treating unintentional hypothermia include multi-layering of conventional blankets, specialized warming blankets, and warm intravenous infusion. Although warmed IV fluids efficiently transfer heat to the patient through conduction, wrapping standard bags of crystalloid IV solution in commercially available heat packs achieves only small increases in fluid temperature. On the other hand, commercially available fluid warmers are effective at maintaining the temperature of warmed IV solution or raising the temperature of room-temperature solution, but vary in their ability to warm cold IV solutions. In animal models under simulated battlefield condition, a combination of reflective and wool blankets with an IV fluid warmer is effective at preventing unintentional hypothermia.
Summary
Unintentional hypothermia is a frequent prehospital complication of traumatic injury and significantly increases morbidity and mortality in these patients. Although medics may initially find normal temperatures, removal of the patient’s clothing, heat radiation through large open wounds, and thermodilution with room temperature IV solutions all contribute to rapid heat loss.
Passive re-warming strategies and an effective first step on slowing the rate of deterioration, although these measures used in isolation are often insufficient to prevent hypothermia in severely injured patients. Medics must employ a combination of passive and active thermostatic measures to provide the patient with the best opportunity for survival.
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