Hypothermia: A Risk in Summer, Too by Michael Metzler, M.D.

Peripheral vasoconstriction caused by cooling shifts intravascular volume centrally and causes a cold diuresis which depletes circulating volume. Cold is a negative inotrope and also causes arrhythmias. Ventricular automaticity increases with decreasing temperature below 26 oC. Fibrillation occurring in this temperature range does not respond to drug therapy, requiring rewarming before abatement. Cold causes measurable dysfunction in the soluble components of the coagulation system: APTT increased to 57.2 seconds and PT increased to 16.6 seconds (therapeutic anticoagulation values) at 28oC. Platelets also aggregate and are dysfunctional when cooled. Patients usually have altered mental status below 35oC and are comatose at 26 oC. Blood gasses cannot be interpreted at lower temperatures where gasses increase in solubility and pH changes result.

Hypothermia may result from exposure to cold in the pre-hospital setting, but may also be an undesirable byproduct of in-hospital evaluation and care. Large volume fluid resuscitation, removal of clothing, muscle paralysis, mechanical ventilation, and diagnostic studies all contribute to cooling. Most Emergency Departments and CT scanners are air conditioned for staff comfort and technical requirements. The naked, intubated trauma patient would require a room temperature of 85 o F in order not to loose heat. Postoperative and intensive care patients are also at risk.

Hypothermia is easier to prevent than treat. Attention to rapid transport, cold protection and external warming should be part of all scene responders. In the Emergency Department, the patient's wet clothing should be removed as soon as possible, body parts should be exposed for examination only, warmed fluids should be routinely used, intubated patients should have cascade heaters set at 40-45oC, forced air heaters should be considered for routine year-round use. Operating rooms should be kept at 85oF and cooled only after draping and stabilization of patient body temperature. There is mounting evidence that operative strategies aimed at rapid control of mechanical hemorrhage and packing of other bleeding, followed by warming and normalization of coagulation and hemodynamic parameters before returning to the operating room, increase survival.

1993 ATLS Algorithm Change
A = airway B = breathing C = circulation D = disability E = expose and environment

The American College of Surgeons Advanced Trauma Life Support (ATLS) course has recognized the importance of hypothermia in its 1993 revision. The "E" of the Primary Survey, formerly denoting only "expose," now also stands for "Environment," a cryptic reminder to avoid hypothermia.

References:

1. Jurkovich GJ, Greiser WB, Luterman A, et al. Hypothermia in trauma victims: An ominous predictor of survival. J Trauma 27:1019, 1987.
2. Rohrer MJ, Natale AM. Effect of hypothermia on coagulation cascade. Crit Care Med 20:1402, 1992.
3. Fischer RP, Souba WW, Ford EG. Temperature-associated injuries and syndromes. In: Mattox KL, Moore EE, Feliciano DV, eds. Trauma , 2nd ed. Appleton and Lange, Conn., 1991, pp 765-776.

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