Military Winter Operations and Cold-Weather Injuries
There is a long history of describing how cold and wet weather have affected military operations and tactics (eg, in 218 BC, General Hannibal’s troops passing over the Alps, American Revolutionary War, Civil War, World War I, World War II, Korean War, Vietnam War, and, more recently, US Special Operations Forces in Afghanistan mountain warfare). In addition to the direct impact of cold on infantry personnel and combat casualties, the challenges of operating medical transport and equipment in cold environments further degrades medical care. For example, ambulances break down, intravenous solutions and other medications freeze, plastic components become brittle, batteries may not work, etc. Moreover, military personnel have experienced a significant loss of fighting force caused by trench (immersion) foot, frostbite, hypothermia, coagulopathy, and death.
12- Cannon W.B.
- Fraser J.
- Cowell E.M.
The preventive treatment of wound shock.
, 13Hypothermia: threat to military operations.
, 14Cold, casualties, and conquests: the effects of cold on warfare.
, 15- Penn-Barwell J.
- Fries C.A.
- Bennett P.M.
- Midwinter M.J.
- Baker A.
Mortality, survival and residual injury burden of Royal Navy and Royal Marine combat casualties sustained in 11-years of operations in Iraq and Afghanistan.
When combat casualties incur hemorrhage and shock, the effects of TIH create a cascade of physiological effects. This results in poor patient outcomes and accelerated mortality, even with minor decreases in core temperature (see further discussion in Trauma-Induced Hypothermia section).
Trauma-induced Hypothermia
Accidental or primary hypothermia is defined as the unintentional fall in core body temperature below 35°C (95°F), caused by the impact of the cold environment in otherwise healthy individuals who did not sustain injury.
In contrast, hypothermia secondary to trauma is a metabolic derangement in which a shift to anaerobic metabolism results in significantly less heat-generating capacity, making these patients particularly vulnerable to hypothermia, and they have a very poor prognosis as core temperature decreases.
17Advances in the management of hypothermia.
, 18Clinical and translational aspects of hypothermia in major trauma patients: from pathophysiology to prevention, prognosis and potential preservation.
Consequently, a separate hypothermia classification was created for trauma victims, which begins at ≤36°C (≤96.8°F). It is appropriate to use a higher temperature threshold to define TIH because there is increased morbidity and mortality starting at this core temperature when compared to accidental hypothermia patients. In these injured patients, mild hypothermia is defined as a temperature between 34 to 36°C (93–96.8°F), whereas moderate hypothermia occurs between 32 to 34°C (90–93°F), and severe hypothermia is <32°C (<90°F).
17Advances in the management of hypothermia.
Both civilian
18Clinical and translational aspects of hypothermia in major trauma patients: from pathophysiology to prevention, prognosis and potential preservation.
, , 20Hypothermia in trauma patients.
, 21- Waibel B.H.
- Schlitzkus L.L.
- Newell M.A.
- Durham C.A.
- Sagraces S.G.
- Rotondo M.F.
Impact of hypothermia (below 36 degrees C) in the rural trauma patient.
and military
22The 2004 Fitts Lecture: current perspective on combat casualty care.
, 23- Arthurs Z.
- Cuadrado D.
- Beekley A.
- et al.
The impact of hypothermia on trauma care at the 31st combat support hospital.
, 24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
, 25Damage control resuscitation: a sensible approach to the exsanguinating surgical patient.
, 26- Nesbitt M.
- Allen P.
- Beekley A.
- Butler F.
- Eastridge B.
- Blackbourne L.
Current practice of thermoregulation during the transport of combat wounded.
, 27Resuscitation of trauma-induced coagulopathy.
, 28United States military surgical response to modern large-scale conflicts: the ongoing evolution of a trauma system.
trauma studies report that an >15 ISS is associated with progressive increase in the frequency and severity of TIH. These studies clearly indicate that no matter the mechanism of injury, traumatized patients are more likely to become hypothermic resulting in greater mortality. It is important to note that TIH was recorded in combat casualties independent of the ambient temperature (eg, hot Middle Eastern deserts).
23- Arthurs Z.
- Cuadrado D.
- Beekley A.
- et al.
The impact of hypothermia on trauma care at the 31st combat support hospital.
, 25Damage control resuscitation: a sensible approach to the exsanguinating surgical patient.
, 28United States military surgical response to modern large-scale conflicts: the ongoing evolution of a trauma system.
Thus, it is now recommended in both civilian and military trauma courses to initiate early and aggressive use of active rewarming as the standard of care in traumatized patients.
, Even though mortality in combat casualties with hypothermia is double that of normothermic casualties with similar injuries,
24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
TIH is a topic not solely unique to combat casualties.
18Clinical and translational aspects of hypothermia in major trauma patients: from pathophysiology to prevention, prognosis and potential preservation.
The majority of these TIH peer-reviewed studies emanate from civilian trauma surgeons primarily beginning the 1980s.
, 20Hypothermia in trauma patients.
, 21- Waibel B.H.
- Schlitzkus L.L.
- Newell M.A.
- Durham C.A.
- Sagraces S.G.
- Rotondo M.F.
Impact of hypothermia (below 36 degrees C) in the rural trauma patient.
, 31- Gentilello L.M.
- Jurkovich G.J.
- Stark M.S.
- Hassantash S.A.
- O’Keefe G.E.
Is hypothermia in the victim of major trauma protective or harmful? A randomized, prospective study.
These retrospective and prospective studies all report the relationship between trauma, hypothermia, coagulopathy (known as trauma triad), and greater mortality. This is principally caused by a thermoregulatory dysfunction resulting in decreased metabolic heat production particularly in patients with hemorrhagic shock, that typically uncouples normal metabolic pathways. Hypothermia is further exacerbated by secondary causes (eg, based on prolonged ground exposure with conductive heat loss or accelerated convective heat loss in cold and windy environments, and/or the administration of cool intravenous fluids during patient management). One documented clinical challenge is that 40 to 50% of moderate to severely traumatized patients arrived hypothermic at civilian hospitals, and >80% of nonsurviving patients had a core temperature <34°C (<93°F).
32- Luna G.K.
- Maier R.V.
- Pavlin E.G.
- Anardi D.
- Copass M.K.
- Oreskovich M.R.
Incidence and effect of hypothermia in seriously injured patients.
, 33- Beilman G.J.
- Blondet J.J.
- Nelson T.R.
- et al.
Early hypothermia in severely injured trauma patients is a significant risk factor for multiple organ dysfunction syndrome but not mortality.
, 34- Jurkovich G.J.
- Greiser W.B.
- Luterman A.
- Curreri P.W.
Hypothermia in trauma victims: an ominous predictor of survival.
In both civilian and military trauma studies, it is reported that 100% mortality occurred when core temperature is ≤32°C (≤90°F).
23- Arthurs Z.
- Cuadrado D.
- Beekley A.
- et al.
The impact of hypothermia on trauma care at the 31st combat support hospital.
, 31- Gentilello L.M.
- Jurkovich G.J.
- Stark M.S.
- Hassantash S.A.
- O’Keefe G.E.
Is hypothermia in the victim of major trauma protective or harmful? A randomized, prospective study.
The effect of hypothermia on the coagulation system is multifactorial.
35The pathogenesis of traumatic coagulopathy.
, 36- Maegele M.
- Scho H.
- Cohen M.J.
An update on the coagulopathy of trauma.
, 37- Farkash U.
- Lynn M.
- Scope A.
- et al.
Does prehospital fluid administration impact core body temperature and coagulation functions in combat casualties?.
Moderate hypothermia directly reduces coagulation activity approximately 10% for each degree Celsius decrease in temperature.
25Damage control resuscitation: a sensible approach to the exsanguinating surgical patient.
The acquired coagulopathy accounts for a large percentage of early and preventable trauma deaths when not managed early.
37- Farkash U.
- Lynn M.
- Scope A.
- et al.
Does prehospital fluid administration impact core body temperature and coagulation functions in combat casualties?.
, 38- MacLeod J.B.
- Lynn M.
- McKenney M.G.
- Cohn S.M.
- Murtha M.
Early coagulopathy predicts mortality in trauma.
, 39- Dobson G.P.
- Letson H.L.
- Sharma R.
- Sheppard F.R.
- Cap A.P.
Mechanisms of early trauma-induced coagulopathy: the clot thickens or not?.
, 40- Chang R.
- Cardenas J.C.
- Wade C.E.
- Holcomb J.B.
Advances in the understanding of trauma-induced coagulopathy.
Both civilian and military trauma centers have linked TIH and coagulopathy on arrival with increased mortality.
22The 2004 Fitts Lecture: current perspective on combat casualty care.
, 23- Arthurs Z.
- Cuadrado D.
- Beekley A.
- et al.
The impact of hypothermia on trauma care at the 31st combat support hospital.
, 39- Dobson G.P.
- Letson H.L.
- Sharma R.
- Sheppard F.R.
- Cap A.P.
Mechanisms of early trauma-induced coagulopathy: the clot thickens or not?.
, 40- Chang R.
- Cardenas J.C.
- Wade C.E.
- Holcomb J.B.
Advances in the understanding of trauma-induced coagulopathy.
The primary mechanism of acute traumatic coagulopathy (ATC) is complex, but this pathophysiological cascade is beginning to be unveiled.
27Resuscitation of trauma-induced coagulopathy.
, 35The pathogenesis of traumatic coagulopathy.
, 36- Maegele M.
- Scho H.
- Cohen M.J.
An update on the coagulopathy of trauma.
, 40- Chang R.
- Cardenas J.C.
- Wade C.E.
- Holcomb J.B.
Advances in the understanding of trauma-induced coagulopathy.
, 41- Martin R.S.
- Kilgo P.D.
- Miller P.R.
- Hoth J.J.
- Meredith J.W.
- Chang M.C.
Injury-associated hypothermia: an analysis of the 2004 National Trauma Data Bank.
, 42Cause of trauma-induced coagulopathy.
, 43- Darlington D.N.
- Gonzales M.D.
- Craig T.
- Dubick M.A.
- Cap A.P.
- Schwacha M.G.
Trauma-induced coagulopathy is associated with a complex inflammatory response in the rat.
There are 6 primary mechanisms for ATC: tissue trauma, shock, hemodilution, hypothermia, acidemia, and inflammation.
27Resuscitation of trauma-induced coagulopathy.
The poor outcome of traumatized patients presenting with hypothermia is well documented.
24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
Eastridge and colleagues retrospectively reviewed over 1100 combat casualties presenting to surgical support hospitals during Operation Iraqi Freedom from January to July 2004. They reported that in normothermic versus hypothermic patients, the survival rate was 98% and 75% (
P < .05), respectively. They also observed that 88% of hypothermic patients versus 65% of normothermic patients (
P < .05) required surgery on arrival.
During Operation Iraqi Freedom, military trauma teams noted that many combat casualties were hypothermic after arriving from the battlefield to combat medical treatment facilities (MTF).
24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
, 27Resuscitation of trauma-induced coagulopathy.
They viewed this as a trauma system-related problem that needed a rapid intervention starting at the point of injury, during medical evacuation, and after arrival at military MTFs. Subsequently, Beekely describes the enhanced measures that addressed hypothermia prevention and management.
25Damage control resuscitation: a sensible approach to the exsanguinating surgical patient.
See
Table 1 for hypothermia-related DoD policies. Since the implementation of methods to standardized hypothermia prevention within the combat theater trauma system, the rate of casualties arriving at military MTFs with TIH dropped from 7 to <1%.
24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
Table 1Initial steps and long-term policies for theater-wide hypothermia prevention and management24- Eastridge B.J.
- Jenkins D.
- Flaherty S.
- Schiller H.
- Holcomb J.B.
Trauma system development in a theater of war: experiences from Operation Iraqi Freedom and Operation Enduring Freedom.
, 25Damage control resuscitation: a sensible approach to the exsanguinating surgical patient.
, 26- Nesbitt M.
- Allen P.
- Beekley A.
- Butler F.
- Eastridge B.
- Blackbourne L.
Current practice of thermoregulation during the transport of combat wounded.
Hypothermia Rewarming Research and Development
Seeking a solution for the DoD hypothermia management strategy, Allen and colleagues
44- Allen P.B.
- Salyer S.W.
- Dubick M.A.
- Holcomb J.B.
- Blackbourne L.H.
Preventing hypothermia: comparison of current devices used by the US Army in an in vitro warmed fluid model.
evaluated rewarming systems on the core temperature of a fluid torso model for hypothermic combat casualties. They tested 3 active warming systems for prehospital use (HPMK, Ready-Heat thermal blanket, and Bair Hugger Forced Air System) and 5 passive hypothermia prevention products (wool blanket, mylar space blanket, Blizzard blankets, Human Remains Pouch (body bag), as well as a hot pocket system (combination of 2 wool blankets, one-space blanket inside a Human Remains Pouch). Active warming devices included products with chemically or electrically heated systems (See
Table 2).
Table 2Passive and active warming products evaluated by Allen and colleagues42Cause of trauma-induced coagulopathy.
The products with active heating performed better to prevent heat loss than most passive prevention methods. The original HPMK achieved and maintained significantly higher temperatures than all other methods at 120 minutes (P < .05). None of the devices with an actively heated element achieved the sustained 44°C (111°F) that could damage human tissue. The best passive methods of heat loss prevention were the hot pocket system and Blizzard blanket, which performed the same as 2 of the 3 active heating methods. All active and most passive treatments were better than wool blankets used alone. Under these conditions, passive treatment methods (Blizzard blanket or the hot pocket system) were as effective as active warming devices other than the original HPMK.
Extrapolating results from this study to efficacy in humans may be questionable, because the fluid model (37°C/98.6°F) had no ability to generate heat intrinsically. Another limitation of this study is that they only studied these products at one environmental temperature (37.8°C/100°F). Consequently, the results may be much different when evaluated at colder environmental temperatures. These data do not support the continued use of wool blankets solely; they should be used together as insulation along with an impermeable outer layer, such as a Human Remains Pouch, Blizzard blanket, or the Heat Reflective Shell.
Research and Development Future Direction
The study by Allen and colleagues
44- Allen P.B.
- Salyer S.W.
- Dubick M.A.
- Holcomb J.B.
- Blackbourne L.H.
Preventing hypothermia: comparison of current devices used by the US Army in an in vitro warmed fluid model.
was the best evidence supporting a lightweight, nonconsumable heating source for prehospital rewarming of hypothermic patients. Although there were no published clinical studies with the use of the HPMK, their study was the basis for selecting this kit by DoD personnel. The HPMK and enhanced hypothermia management procedures were added to the TCCC Guidelines as a means to provide a rapid active rewarming in the prehospital for severely traumatized combat casualties. See
Table 3 for the current hypothermia management recommendations in the TCCC Guidelines. Additional research is needed to confirm the findings from Allen et al
44- Allen P.B.
- Salyer S.W.
- Dubick M.A.
- Holcomb J.B.
- Blackbourne L.H.
Preventing hypothermia: comparison of current devices used by the US Army in an in vitro warmed fluid model.
., ideally using human volunteers and varying cold air exposures. An ongoing study is currently evaluating 5 commercial warming systems, including the HPMK, using human volunteers (personal communication, Gordon Giesbrecht, PhD on June 15, 2016). This study may provide evidence for more effective rewarming devices for hypothermic patients in the prehospital environment.
Table 3TCCC Guideline recommendations for hypothermia prevention and management (2016)*For full TCCC Guidelines, see http://www.naemt.org/education/TCCC/guidelines_curriculum.
Transition of the Hypothermia Prevention Management Kit
A core skill taught in wilderness and prehospital medicine is how to manage an accidental hypothermic patient, with an emphasis on preventing further heat loss with an improvised passive heat retention system.
45- Johnson D.E.
- Schimelpfenig T.
- Hubbell F.
- et al.
Minimum guidelines and scope of practice for wilderness first aid.
, 46Hubbell F. Environmental emergencies. In: Hubbell F, ed. Wild Care. Conway, NH: Stonehearth Open Learning Opportunities; 2014.
, 47- Giesbrecht G.G.
- Steinman A.M.
Immersion into cold water.
, This system, traditionally known as a hypothermia wrap, basically consists of a vapor-tight waterproof tarp, 1 to 3 ground insulation pads, and 1 to 3 sleeping bags. The patient is placed in the middle sleeping bag, and the outer layer then is folded up systematically to prevent cold air entry and to maximize heat retention. This approach is equipment intensive, bulky, and generally limited to those personnel responding to manage a hypothermic patient. The advantages of the HPMK are that it is lightweight, 1.6 kg (3 lb. 8 oz), packaged in a small dimension, 17 cm x 27 cm x 14 cm (6.75 in. x 10.5 in. x 5.5 in.), and vacuum sealed as a kit. The HPMK can also be used with or without the Ready-Heat thermal blanket. It is available in either smaller torso or full-body sizes, both with 10 hours of continuous dry heat by an oxygen-activated (15 to 20 minutes to 40°C [104°]), self-heating liner that has no external power supply requirement. As compared to the traditional hypothermia wrap, the HPMK provides an option of active rewarming, most ideal for moderate to severe hypothermia, particularly in trauma patients in austere environments. Transitioning the HPMK to wilderness medicine and other prehospital programs should be investigated, and is likely to be highly successful for managing accidental and TIH patients.