Taking the Chill Out of Trauma Victims

This article appeared in the EMS World special supplement Combating the Hidden Dangers of Shock in Trauma, developed by Cambridge Consulting Group and sponsored by North American Rescue, LifeFlow by 410 Medical, and QinFlow. Download the supplement here. 

Despite remarkable advances in developing community-based trauma systems and trauma care over the past 50 years, unintentional and violence-related injuries remain among the top 10 causes of death for all age groups in the United States.¹ In 2020 alone more than 180,000 deaths occurred due to traumatic injury.² Because so many of these deaths occur in young people, this results in nearly 3.5 million years of potential life lost.² 

About a third, or 60,000, of these traumatic injury deaths are a result of severe hemorrhage, second only to traumatic brain injury as a cause of injury-related death.³ Most deaths from hemorrhage occur within 2 hours after injury.³ Thus, managing severe hemorrhage is truly a time-sensitive, lifesaving task, and EMS is uniquely poised to improve outcomes for the millions of people injured every year.

Effective prehospital hemorrhage control in critically injured trauma patients requires EMS providers to have a working knowledge of, and strategy for, limiting the effects of 3 interrelated conditions: hypothermia, acidosis, and coagulopathy. 

Trauma’s lethal triad was originally described 40 years ago by physicians at Denver General Hospital after they found 89% of deaths in patients with major abdominal vascular trauma were due to bleeding, and half of them occurred after hemorrhage from major bleeding sites was controlled.⁴

They found hemorrhage and tissue injury precipitated what they called a “bloody vicious cycle,” and the components of the “lethal triad” are interrelated, complicating physiologic conditions that ultimately compound upon each other and will lead to death if left untreated (see Figure 1).

In their article, published in the Journal of Trauma in 1982, the physicians wrote, “These findings suggest that coagulopathy, hypothermia, and acidosis are complicating factors which demand as much attention by the surgeon as the initial resuscitation and operative control classically emphasized.”⁴

In recent years some experts in trauma care have questioned whether hypocalcemia, given its direct and indirect effects on each component of the lethal triad, should be considered as an additional component of a new “lethal diamond.”^5,6

Focus on Hypothermia

Understanding, preventing, and treating all components of the lethal triad—or lethal diamond—are important in maximizing patient outcomes, and all providers should understand all aspects of the lethal triad. However, this article will focus on hypothermia and its impact on critically injured trauma patients. 

To understand hypothermia and its role in trauma mortality, EMS providers first must distinguish primary hypothermia from trauma-induced hypothermia.

Primary hypothermia is defined as the unintentional fall in a healthy individual’s core body temperature (ie, of the heart, lungs, and brain) below 35°C (95°F) due to exposure to a cold environment.⁷ In other words, primary hypothermia occurs when a healthy individual’s ability to produce heat is overwhelmed by excessive cold exposure.

In contrast, hypothermia secondary to trauma occurs when hemorrhage, tissue injury, or cerebrospinal injury disrupts the body’s ability to produce or conserve heat.⁷ 

As an injured patient bleeds, they experience decreased perfusion to their body’s tissues. Anemia from acute blood loss, peripheral vasoconstriction, and overall decreased cardiac output impair delivery of oxygen to the tissues. This results in a switch from aerobic to anaerobic metabolism to produce cellular energy. Anaerobic metabolism results in significantly less heat-generating capacity, thus predisposing these injured patients to developing hypothermia.⁷ 

There are numerous risk factors for developing hypothermia after traumatic injury, and these commonly occur in combination. Patients at the extremes of age and those with certain medical conditions such as diabetes or hypothyroidism are at increased risk of developing hypothermia. Alcohol and drug intoxication, as well as the use of some cyclic antidepressants, sedatives, and antipsychotic medications, can affect the body’s ability to regulate its temperature.

In addition, patients with prolonged exposure to cold, wet, or windy environments and those with severe burns are at risk for rapid heat loss exacerbating hypothermia after injury.⁸ 

It is important to note, however, that the prevalence of trauma-induced hypothermia remains high, even when air temperature at the point of injury is mild or even warm.⁸ In recent military conflicts trauma-induced hypothermia has been repeatedly found in combat casualties independent of ambient outside temperature (eg, in hot Middle Eastern deserts).⁹ 

Lastly, the temperature of infused resuscitative fluids, temperature inside the transport vehicle, presence of traumatic brain injury, and need for prehospital sedation and intubation have all been found to be risk factors for developing trauma-induced hypothermia.⁸ 

Hypothermia is all too common in trauma patients, with published studies demonstrating that 40%–50% of moderately to severely injured patients arrive at civilian trauma centers hypothermic, and more than 80% of nonsurviving trauma patients arrive with core temperatures less than 34°C (93.2°F).^10–12

The significant impact of prehospital hypothermia on mortality is clear. A recent meta-analysis of 7 studies found the likelihood of in-hospital death is 5.2 times higher if the patient arrives in the ED hypothermic.¹³ Furthermore, a 100% mortality has been reported when the injured patient’s arrival core temperature is less than 32°C (89.6°F).¹⁴

Recognizing the frequency and poor prognosis of trauma-induced hypothermia, a separate hypothermia classification system was created for trauma victims (see Table 1).

In this classification system, mild hypothermia begins at or below 36°C (96.8°F) as opposed to the 35°C (95°F) threshold used for primary hypothermia. This reflects the increased morbidity and mortality in trauma-induced hypothermia compared to patients with primary hypothermia.

Not only does trauma-induced hypothermia independently increase the likelihood of death in critically injured patients, it also causes additional physiologic effects that may result in patients having increased morbidity and requiring more complicated clinical care. 

Most notable is the effect of hypothermia on the coagulation system. Hypothermia decreases the effectiveness of clotting factors and reduces the number and function of platelets. This effect depends upon the degree of hypothermia and becomes clinically measurable when core temperature drops below 33°C (91.4°F).¹⁵ Moderate hypothermia directly reduces coagulation activity by approximately 10% for each degree Celsius decrease in temperature.⁷ 

Trauma-induced hypothermia, with its role in acute traumatic coagulopathy on arrival to the trauma center, is associated with increased mortality.⁷ In addition, studies have reported a higher blood transfusion requirement in hypothermic trauma patients related to hypothermia-induced coagulopathy.^16,17 

Furthermore, a review of the National Trauma Data Bank found ICU lengths of stay and number of days on a ventilator were significantly longer in patients who presented with hypothermia after injury versus patients who were normothermic.¹⁸

Given the incidence of trauma-induced hypothermia, the speed at which it develops in the prehospital setting, and its impact on mortality, EMS providers must aggressively prevent, recognize, and treat hypothermia in every patient with or at risk for critical injury. 

To identify best practices for prevention and treatment of hypothermia at the point of injury and during transport, civilian EMS agencies can look to the lessons learned and tactics developed over the past 2 decades of military conflict. 

The Department of Defense Committee on Tactical Combat Casualty Care (CoTCCC) places significant emphasis on prevention and treatment of hypothermia as the letter H in its MARCH framework for prioritized trauma care. As a result, hypothermia prevention is the second-most frequent lifesaving intervention performed by military medics on battlefield casualties after hemorrhage control (see Figure 2).¹⁹ 

Preventing Hypothermia

The easiest and most important treatment strategy for trauma-induced hypothermia is to prevent its occurrence altogether. Thus, EMS providers must make a focused effort to prevent heat loss starting from the point of injury.

Preventing the development of hypothermia includes: 

• Minimizing a patient’s exposure to wind, as well as cold ground and air temperatures. This is particularly important for patients at the extremes of age and those undergoing extrication.⁸
• Placing insulation material as soon as possible between the injured trauma patient and any cold surface, such as a transfer, lifting, or moving device.
• Removing wet or bloody clothing as soon as possible and replacing it with a dry body cover, such as a hypothermia wrap or warm blanket. The significant benefits of removing wet clothing, regardless of ambient outside temperature, have been demonstrated in both thermal manikin and human studies.^20,21 This simple intervention decreases metabolic rate, increases skin temperature, and decreases shivering thermogenesis, resulting in overall improvement in the patient’s thermal status.
• Exposing only those body parts that are being actively examined during patient assessment. Otherwise, ensure the body remains covered with a dry covering.
• Infusing only warmed resuscitation fluid (eg, warmed blood products if available, otherwise warmed crystalloid fluids) to the appropriate target mean arterial pressure or systolic blood pressure. 
• Preheating the transport vehicle prior to patient contact.

Despite best efforts to prevent trauma-induced hypothermia, a significant number of severely injured patients still develop hypothermia after injury. Note it is often challenging to accurately assess core temperature in the prehospital setting. This can be especially true for logistically complex incidents or when a patient has critical injuries. Thus, EMS providers should assume all moderately to severely injured patients are rapidly becoming hypothermic and treat accordingly. 

Passive Rewarming 

Rewarming patients with trauma-induced hypothermia can be either passive or active. Passive rewarming involves retaining a patient’s own heat production inside a covering placed over or around them. Common coverings include the standard wool blanket, a synthetic-filled sleeping bag, a Mylar space blanket, a human-remains pouch (ie, body bag), and the improvised hypothermia wrap.²² 

Also known as the “burrito wrap,” a hypothermia wrap consists of an outer waterproof tarp, 1–3 ground insulation pads, and 1–3 sleeping bags (see Figure 3). This strategy is equipment- and time-intensive and thus not typically practical for first-response trauma care.²² 

However, certain commercially available passive rewarming coverings have been found to provide good heat-loss prevention with a simple and small design.²³ In a prospective, randomized study utilizing a simulated human torso model, the Blizzard blanket and Heat Reflective Shell both were found to prevent heat loss (see Figure 4). 

Specifically, the Blizzard blanket maintained temperature better than the classic military-issued wool blanket as early as 15 minutes after cooling began and at 2 hours performed the same as 2 of the 3 active heating methods.²³

It’s important to remember that passive rewarming may be ineffective in patients who are unable to generate heat via shivering. 

Patients with core temperatures below approximately 30–32°C (86–89.6°F) may no longer shiver and can continue to cool despite being well insulated from the environment.²² In addition, certain trauma patients secondary to multiple etiologies (eg, sedative medications, alcohol intoxication, severe TBI) may also have a blunted shiver response and require external heat.²²

Active Warming

Active warming involves providing external heat via a heating system in addition to a body cover. Numerous heat sources have been developed and tested, from body-to-body skin contact to hot water bottles or bags to charcoal-burning heat packs to electrical and chemical heating pads/blankets.

Regardless of the method used for generating heat, external heat sources are most effective if concentrated on the upper torso (ie, axillae, chest, and back), where the potential for heat transfer to the core is highest.²² 

Active warming is recommended by both Prehospital Trauma Life Support and Advanced Trauma Life Support guidelines. In the setting of moderate to severe traumatic injury, there is no absolute contraindication to initiating active warming.

Active warming with an insulated hypothermia wrap has performed better in preventing heat loss than most passive warming strategies and is recommended for all cold-stressed or hypothermic trauma victims.^22,23 

The CoTCCC guidelines have recommended the Hypothermia Prevention and Management Kit (HPMK) since 2006. The HPMK system is a low-cost, lightweight, vacuum-sealed commercial product that provides both passive and active rewarming.²⁴

The second-generation HPMK contains both a ruggedized outer shell called the Heat Reflective Shell (HRS) as well as the internal ReadyHeat Blanket (RHB; see Figure 5). 

The HRS is a strong but lightweight, wind- and rainproof encapsulating shell with a built-in hood and a protected nonconductive reflective layer that provides excellent thermal insulation.²³ The RHB is a 4-cell, oxygen-activated chemical heat pack that provides a maximum temperature of 52°C (125.6°F) for up to 10 hours.²⁴ It is intended to be placed on the patient’s torso but not directly on bare skin.

Both the HRS and RHB are commercially available individually or combined to form the HPMK.

When the CoTCCC recommended the HPMK device for deployment across the US military, it was supported by a single study performed on a nonhuman fluid torso model at normal room temperature.²³ Although the data from this simulation study supported the HPMK’s ability to prevent heat loss, the data was not validated on human volunteers, and the study was not repeated in cold or wet conditions.²² 

A more recent study compared 5 heated hypothermia wrap systems including the HPMK. Four of these were insulated; only the HPMK was not. Physiologic and subjective responses were assessed in 5 normothermic volunteers during 1 hour of exposure to -22°C (-7.6°F) in a cold chamber. The subjects were coldest, had the highest level of shivering, and reported the most “whole body cold discomfort” in the noninsulated HPMK.²⁵ 

The heated enclosure systems that performed better than the HPMK varied greatly in weight, size, and cost. These additional factors are important to consider logistically and operationally and make some more feasible to use than others, depending on the mission. Only the HPMK and 1 other enclosure system in this study were of suitable weight and size to be carried in a backpack.²⁵

These findings and feedback from operational forces led the CoTCCC to update its hypothermia-prevention guidelines in 2020 to state the following:²⁶

• Place an active heating blanket on the patient’s anterior torso and under the arms in the axilla.
• Enclose the patient with an exterior impermeable enclosure bag.
• As soon as possible, upgrade the hypothermia enclosure system to a well-insulated enclosure system using a hooded sleeping bag or other readily available insultation inside the enclosure bag/external vapor barrier shell.
• Prestage an insulated hypothermia enclosure system with external active heating for transition from the noninsulated hypothermic enclosure system. 

Fluid Temperature

An important consideration in prevention and management of trauma-induced hypothermia is the temperature of infused resuscitation fluids. A growing body of literature based on recent military and civilian experience suggests whole blood is the ideal resuscitation fluid for patients in hemorrhagic shock.^27–29

Whole blood is natural, unseparated blood collected from a healthy donor. Because it is unseparated, whole blood contains all the normal blood components needed to directly treat the lethal triad (ie, red blood cells, plasma, platelets, and clotting factors) in a single infusion bag. As a result the CoTCCC now recommends cold-stored low-titer type-O whole blood as the preferred resuscitation fluid in hemorrhagic shock.²⁶ 

In addition, an increasing number of EMS agencies across the US are deploying low-titer type-O whole blood for treatment of hemorrhagic shock.³⁰ An in-depth discussion of prehospital whole blood transfusion is beyond the scope of this article, but a few key points pertaining to hypothermia should be mentioned.

Prior to transfusion blood products must be stored at 2–6°C (35.6–42.8°F). However, rapid infusion of blood at this temperature will lead to a significant drop in the patient’s core temperature. With as little as 500 ml of cold blood, a patient’s core temperature may drop by about 1°C (1.8°F).³¹ This 1-degree drop in core temperature reduces coagulation factor activity approximately 10%–15%.⁷ 

Even relatively small amounts of room-temperature fluid contribute to worsening hypothermia.^31,32 Thus it’s recommended that all infused resuscitation fluid be warmed to 38–42°C (100.4–107.6°F) with the primary goal of preventing further heat loss rather than to actively warm the patient.²² 

The CoTCCC guidelines recommend the use of a battery-powered warming device capable of infusing fluids at a flow rate up to 150 ml/min with a 38°C (100.4°F) output temperature.²⁶ Multiple portable fluid-warming devices are commercially available, and several studies have evaluated and compared their performance characteristics.^34,35 

In addition to cost, EMS agencies must also consider the device’s size, weight, durability, battery duration, time to reach maximum temperature, and final fluid delivery temperature when selecting the appropriate warmer for their agency.

Conclusion

In recent decades EMS providers and practitioners of prehospital medicine have made great strides in understanding, preventing, and treating the lethal triad and specifically trauma-induced hypothermia. 

Military conflicts around the world and advances in civilian EMS care here at home have taught us all important lessons about how to improve survival after trauma, from a better understanding of hypothermia physiology and the development of evidence-based guidelines to enhanced warming equipment and aggressive resuscitation strategies such as prehospital whole blood transfusions.

Because patients continue to experience preventable deaths from hemorrhage, the prevention and management of the lethal triad remains as important today as it was when first described in 1982. Through education, innovation, and research, EMS providers on the front lines of our nation’s trauma system have the very real opportunity to improve the outcomes for the millions of people injured every year.

References

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20. Lairet JR, Bebarta VS, Burns CJ, et al. Prehospital interventions performed in a combat zone: A prospective multicenter study of 1,003 combat wounded. J Trauma Acute Care Surg. 2012; 73(2 Suppl 1): S38–S42. 

21. Henriksson O, Lundgren P, Kuklane K, et al. Protection against cold in prehospital care: Evaporative heat loss reduction by wet clothing removal or the addition of a vapor barrier—thermal mannikin study. Prehosp Disaster Med. 2012; 27(1): 53–8. 

22. Henriksson O, Lundren PJ, Kuklane K, et al. Protection against cold in prehospital care: Wet clothing removal or addition of a vapor barrier. Wilderness Environ Med. 2015; 26(1): 11–20. 

23. Bennett BL, Giesbrect G, Zafren K, et al. Management of hypothermia in tactical combat casualty care: TCCC Guideline Proposed Change 20-01 (June 2020). J Spec Oper Med. 2020; 20(3): 21–35. 

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25. North American Rescue. Hypothermia Prevention and Management. Accessed March 31, 2022. www.narescue.com/nar-hypothermia-prevention-and-management-kit-hpmk.html

26. Dutta R, Kulkarni K, Steinman AM, et al. Human responses to 5 heated hypothermia wrap systems in a cold environment. Wilderness Environ Med. 2019; 30(2): 163–76. 

27. Drew B, Montgomery HR, Butler FK Jr. Tactical Combat Casualty Care (TCCC) Guidelines for Medical Personnel: 05 November 2020. J Spec Oper Med. 2020; 20(4): 144–51. 

28. Weymouth W, Long B, Koyfman A, et al. Whole blood in trauma: A review for emergency clinicians. J Emerg Med. 2019; 56(5): 491–8. 

29. Braverman MA, Smith A, Pokorny D, et al. Prehospital whole blood reduces early mortality in patients with hemorrhagic shock. Transfusion. 2021; 61(Suppl 1): S15–S21. 

30. Malkin M, Nevo A, Brundage SI, et al. Effectiveness and safety of whole blood compared to balanced blood components in resuscitation of hemorrhaging trauma patients—A systematic review. Injury. 2021; 52(2): 182–8. 

31. Sayre MR, Yang BY, Murphy DL, et al. Providing whole blood for an urban paramedical ambulance system. Transfusion. 2022; 62(1): 82–6. 

32. Boyan CP, Howland WS. Blood temperature: A critical factor in massive transfusion. Anesthesiology. 1961; 22(4): 559–63. 

33. Sessler DI. Mild perioperative hypothermia. N Engl J Med. 1997; 336(24): 1730–7. 

34. Søreide K. Clinical and translational aspects of hypothermia in major trauma patients: From pathophysiology to prevention, prognosis and potential preservation. Injury. 2014; 45(4): 647–54. 

35. Lehavi A, Yitzhak A, Jarassy R, et al. Comparison of the performance of battery-operated fluid warmers. Emerg Med J. 2018; 35(9): 564–70. 

36. Weatherall A, Gill M, Milligan J, et al. Comparison of portable blood-warming devices under simulated pre-hospital conditions: A randomized in-vitro blood circuit study. Anaesthesia. 2019; 74(8): 1026–32. 

Ryan Gerecht, MD, FACEP, began his career in EMS as an EMT in 2002. Today he’s a board-certified EMS and emergency medicine physician. He is assistant medical director for Washington, DC Fire and EMS and an attending physician at MedStar Washington Hospital Center. He previously served as EMS medical director for the Tacoma Fire Department in Washington.

Kaytlin Hack, MD, began her career in EMS in Chicago in 2008, specializing in mass-gathering medicine. She graduated as chief resident in emergency medicine at MedStar Washington Hospital Center and Georgetown University and is now an EMS fellow at Johns Hopkins University. She also serves as an officer and flight surgeon in the District of Columbia Army National Guard. The views in this article are her own.