Temperature Control
The management of hypothermic casualties in field and combat environments.
by Lieutenant Colonel Booker T. King, M.D.
and Major Kevin Chung, M.D.
Hypothermia is a common complication of trauma and burn injuries. Once these patients become hypothermic, rewarming becomes a difficult task. Managing these patients in austere settings is even more difficult. Hypothermia is defined as a core body temperature that is less than 35 degrees Centigrade. Hypothermia can be sub-divided into mild hypothermia (34-36 degrees C), moderate hypothermia (33-32 degrees C) and severe hypothermia (less than 32 degrees C).
The incidence of hypothermia in combat casualties is not known but a review of patients that presented to the 31st Combat Support Hospital in 2004 revealed that 18 percent of the patients were hypothermic upon arrival to the emergency room. Low core body temperatures in trauma and burn patients increases their morbidity and mortality significantly. Injured patients with core body temperatures less than 32 degrees C have mortality that approaches 100 percent. Hypothermia is a part of the triad of death (hypothermia, acidosis and coagulopathy) and will lead to irreversible injury.
Several factors contribute to the development of hypothermia in trauma and burn patients. Trauma patients experience a decline in core body temperature after infusion of cold crystalloid solutions and transfusion of cold blood products. The transfusion of 10 or more units of blood products is an independent risk factor for the development of hypothermia. General anesthesia will often result in hypothermia by removing the patient’s natural ability to maintain normal core body temperature. Under general anesthesia patients cannot shiver and many anesthetic agents cause vasodilatation of the skin leading to the loss of heat from the body’s core to the environment.
Casualties with burns greater than 40 percent of their total body surface area will almost certainly develop hypothermia at some point in their early care. Burn patients have difficulty with thermoregulation and are often unable to maintain their core body temperature due to evaporative heat loss from burned skin. Infants and children with large (greater than 40 percent total body surface area) burn are at significant risk. Burn patients are exposed to the environment during excision and grafting of burn wounds and their core body temperature can decline rapidly. Many burn surgeons will terminate the surgery once the patient’s temperature decreases below 35 degrees C, often subjecting the patient to multiple operations.
All these issues are magnified when treating trauma and burn patients in combat settings. The provider may be forced to rely on basic methods of warming the patient. Keeping the patient warm however is secondary to ensuring the safety and security of patients and medics during transport out of a hostile area. In these settings hypothermia will often not be addressed until the patient reaches the operating room. It is important that providers and medics are cognizant of the risks and consequences of hypothermia in trauma and burn patients.
In an attempt to prevent hypothermia it is essential to teach medics, physicians, nurses and other medical providers the techniques to keep patients warm. Medics and corpsmen are often first-responders and are on the frontline of combat, therefore they must be trained to keep their patients from developing hypothermia. PHTLS (pre-hospital trauma life support course) and combat-lifesavers courses are required for most medics and corpsmen and hypothermia prevention is included in the curricula. Other courses such as ATLS (advanced trauma life support course), TNCC (trauma nurse care course) and TCCC (trauma casualty care course) discuss hypothermia prevention and management to physicians and nurses. Emphasis is placed on performing simple maneuvers and reserving invasive techniques and procedures for patients who fail simple maneuvers or patients with severe hypothermia.
TRIED AND TRUE
Basic methods of re-warming battle casualties have been used since World War I where blankets and candles were used for casualties in trenches and foxholes. Patients should be wrapped in warm blankets and all fluid and blood products should be warmed if possible. Saline solution used to irrigate body cavities and extremity wounds should be heated to prevent decreasing core body temperature. Warm gastric lavage and bladder irrigation have been used in patients with moderate and severe hypothermia but warming by these methods is inefficient and may require several hours before any effect is seen. Since patients will lose a tremendous amount of heat from their head, warm hats, blankets, towels or insulated aluminum head covers should be used.
Several advances in temperature management have allowed better treatment of patients with hypothermia. Many of these devices are being used by level III combat hospitals in theater. Some products commonly used for temperature management in the field include: conductive warming devices, convective warming systems and fluid warmers.
Convective warming systems use a high flow of heated air that is blown into specially designed blankets. Heat is transferred from the blanket to the patient by convection. Examples of convective warming systems include: Bair Hugger models 505/750 warming units (Arizant Healthcare Inc. Eden Prairie, Minn.), WarmAir with FilteredFlo blankets (Cincinnati Subzero Products, Cincinnati, Ohio) and Thermacare TC 3003 Power Unit and full body blanket (Gaymar Industries Inc., Orchard Park, NY).
Newer convective warming systems have Hepa filters to filter out contaminants and help maintain the sterility of the surgical field. Conductive warming devices can actually be used for warming or cooling. These devices utilize a flow of water that circulates through a rubber mat. The thermistor (temperature regulating console) can be set at a variety of temperatures ranging from 85 to 107 degree Fahrenheit. Examples of conductive warming systems are the Medi-therm 7900 Hyper/Hypothermia system and the Rapr Round Hyper/Hypothermia wraps (Gaymar Industries). These systems are generally regarded as inferior to convective systems for the perioperative management of hypothermic trauma and burn patients.
Rapid infusers have become indispensable at echelon III facilities. Rapid infusers can deliver up to 750 milliliters of fluid per minute. The added benefit of rapid infusers is that they can warm crystalloid and blood products to body temperature. The Belmont FMS 2000 (Bellerica, Mass.) is currently at use at most echelon III facilities throughout Iraq and Afghanistan. Most rapid infusers are certified for use on most fixed wing aircraft but usually not rotary wing so there are limitations on the use of them for medical evacuation.
The hypothermia prevention and management kit (HPMK)(North American Rescue Inc.) was specifically designed to transport trauma and combat casualties. The kit consists of a heat reflexive shell and hood or separate skull cap. This design allows patient’s body heat to be maintained beneath the shell and the outer layer is waterproof. The entire kit is contained in a vacuum sealed package with easy to open tabs. Preliminary studies conducted at U.S. Army Institute for Surgical Research (USAISR) showed HPMK was superior to many other conventional methods of maintaining and preventing hypothermia during transport.
Recently new intravascular warming devices have been used to treat hypothermia in trauma and burn patients. Ironically these devices were initially designed to create a therapeutic mild hypothermia in patients who sustain traumatic brain or spinal cord injuries. Also it has been used as a method to stabilize patients after cardiac arrest. A special catheter is inserted in the inferior vena cava via the femoral vein in the groin or distal superior vena cava via internal jugular or subclavian routes. The catheter is a closed loop system where saline is circulated through small balloons located on the shaft of the catheter. The desired temperature is set on the main console and the rate of fluid flow and fluid temperature are adjusted until target temperature is reached. Core temperature can be measured from the esophagus or bladder and relayed to the main console. The catheter can also function as a standard central venous line and has multiple ports. The most commonly used intravascular warming device is Thermogard XP (Alsius, Irvine, Calif.). Another such device, SetPoint (Radiant Medical Inc, Redwood City, Calif.), is currently being investigated.
Intravascular temperature modulation is being used to warm hypothermic patients who have sustained thermal injury. This technology was first used in the United States at the University of Miami, Jackson Memorial Hospital, on a burn patient during excision and grafting. Intravascular warming is currently under investigation for use in burn patients at the University of Miami and the U.S. Army Institute of Surgical Research in San Antonio, Texas. It is also being studied for use in warming hypothermic massively injured trauma patients at several medical facilities worldwide. Results from these studies may determine the feasibility of using such innovative technology for treatment of hypothermic trauma and burn patients in the combat environment.
Hypothermia is commonly seen during management of severely injured patients. It significantly increases mortality and morbidity and makes treatment of the primary injury more difficult. Every attempt should be made to avoid hypothermia in casualties on the battlefield. The use of warm blankets and head covers, transfusion of warm blood products and crystalloid and the use of warm saline during the irrigation of extremity wounds and body cavities will mitigate the effects of hypothermia. New technological advances have improved our ability to prevent and treat hypothermia in trauma and burn patients. Intravascular warming is currently being studied for use in maintaining normothermia in patients with trauma and burn injuries.
The views expressed in this article are solely the author’s and do not reflect those of the Department of the Army, the Department of Defense or the United States government. Lieutenant Colonel Booker T. King, M.D., FACS, is the associate director of Burn Center, U.S. Army Institute of Surgical Research; clinical assistant professor of surgery, Brooke Army Medical Center. Major Kevin K. Chung, M.D., is the director of the burn intensive care unit, U.S. Army Institute of Surgical Research; clinical assistant professor, Brooke Army Medical Center. ♦