Page 28 - JSOM Fall 2020

forced air warming (Table 2). Each heat source has limita- Currently, the HPMK configuration consists of the outer HRS
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tions based on (1) amount and duration of heat generated; and the RHB. The RHB has been enhanced and is now rated
(2) amount of heat transferred; (3) quantity of heat source re- with a maximum temperature of 52°C (125°F) for 10 hours of
quired; (4) placement location; (5) logistics for replenishing continuous dry heat; this is an increase from a maximum tem-
the heat source; and (6) failure of heat source. Recommenda- perature of 40°C (104°F) for 8 hours used during the Allen et
tions for specific external heat sources are made by Dow et al. study. It is important to follow the manufacturer’s recom-
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al. and Haverkamp et al. and are summarized in Table 2. mended guidelines for use. The RHB should preferentially be
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placed on the casualty’s torso to provide active warming but
Providing an external heating source against the torso inside not directly on bare skin, to prevent possible burns (discussed
a hypothermia-wrap enclosure system is recommended for under Safety of External Heat Sources). The HRS allows easy
treatment of hypothermic victims (injured and noninjured) by access to the casualty for reassessment and possible interven-
the Wilderness Medical Society, International Commission for tions (e.g., IV access or tourniquets) with the use of Velcro
Mountain Emergency Medicine, and the State of Alaska. 17,75,76 strips down each side of the HRS. The HRS also uses a spa-
cious mummy-shaped shell configuration that covers the head,
Placing a healthy, normothermic person inside a sleeping bag reducing heat loss from the neck and head. Both the RHB and
with close skin contact with a hypothermic patient does not the HRS are commercially available individually or combined
provide significant heat transfer to the core (i.e., heart, lungs, to form the HPMK.
and brain). There are various reports of cold-water immer-
sion accidents in which this technique was used to resuscitate Another study of hypothermia treatment devices, by Dutta et
hypothermic victims and, in some situations, may be the only al., compared enclosure systems containing a vapor-barrier
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heat source available; however, a significant benefit has not enclosure and heat source to improve total-body heat balance
been reported. In mildly hypothermic volunteers, body-to- in human volunteers exposed to cold air. All enclosure sys-
body rewarming blunted shivering and resulted in rewarming tems evaluated are designed for use in the prehospital envi-
rates no greater than shivering alone. There was also no ad- ronment, although some are too bulky and heavy to be carried
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vantage to body-to-body warming when shivering was phar- to the victim. This study compared the effectiveness of five
macologically inhibited. Alternatively, the authors of these heated hypothermia wrap systems (four heated systems were
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studies stated that there is some benefit to consider with body- insulated and one system [the HPMK] had no insulation): (1)
to-body warming to make the patient more comfortable and a user-assembled system (three-season, hooded sleeping bag
decrease shivering intensity. However, there are drawbacks by with an internal vapor barrier and three gel heat packs); (2)
a potential delay of evacuation and the loss of manpower in a Doctor Down (an insulated system with two gel heat packs;
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resource-deprived environment. http://www.doctordown.com/); (3); Wiggy’s Victims Casualty
Hypothermia Bag (an insulated system with dry chemical
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There are two comparative studies of hypothermia wraps that heat pad; Wiggy’s, https://www.wiggys.com/); (4) MARSARS
are most relevant to the military. 22,61 The study by Allen et Hypothermia Stabilizer Bag (an insulated system with three
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al. supported the decision in 2006 to implement the HPMK gel heat packs; MARSARS Water Rescue Systems, https://
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kit in the US military. These authors tested three active ex- marsars.com/); and (5) the HPMK (a noninsulated system
ternal warming hypothermia wraps: HPMK, Ready-Heat with a four-cell chemical heating blanket; Figures 1 and 2).
Blanket (RHB; TechTrade, https://www.ready-heat.com/),
and Bair Hugger (electric powered; not relevant for field use Physiologic and subjective responses were assessed in five
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and not discussed further; 3M Corp, https://www.bairhugger normothermic volunteers during 60 minutes of exposure to
.com/3M/en_US/bair-hugger-us/) and five passive hypother- −22°C (−8°F) in a laboratory cold chamber. The user-assem-
mia wraps (Table 2). The US military adopted the first- and bled enclosure system and Doctor Down system were most
second-generation HPMK, containing the Blizzard Blanket effective physiologically and subjectively, with higher skin
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(first generation) or the Heat Reflective Shell (HRS) (second temperatures, lower metabolic heat production, and less heat
generation; both North American Rescue), both containing loss, resulting in higher net heat gain. The subjects were cold-
the RHB (a four-cell, oxygen-activated chemical heat pack). est and had the highest level of shivering in the noninsulated
All enclosure systems were tested on a fluid torso (nonhuman) HPMK and subjectively rated greater “whole body cold dis-
model (nine 5,000-mL bags of a dialysate solution) warmed comfort” and lower “overall temperature” ratings than the
to 37°C (98.6°F) versus a control with no warming device other four systems tested.
applied, in a room in which the temperature was maintained
between 22.3°C and 22.7°C. The first- and second-generation The authors commented that the enclosure systems had signif-
bags were not statistically different from each other and both icant variability in weight, size, and heat-pack characteristics.
maintained significantly higher bladder temperatures than all The user-assembled system (estimated cost $170) and Doctor
other enclosure systems tested. One limitation of this study Down system (estimated cost $900) had the best physio-
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is that these data were never validated on human volunteers logic performances. However, only the user-assembled enclo-
as recommended by the authors. Another limitation is that sure system and HPMK system were suitable to be carried in
this study was not conducted with cold air temperatures. backpacks, because of their weight and volume. Larger, insu-
The HPMK manufacturer subsequently redesigned the HRS lated, commercial systems could be prepositioned in vehicles
so it is now water- and windproof; this is an important up- for convoys, or ground and air medical evacuation platforms,
date because the RHB will not generate heat effectively if the and in medical treatment facilities. The authors also noted
chemical heat packs get wet. Also, there are observations that that all the enclosure systems could be effective; however,
some of the four cells in the RHB will fail to generate heat clinically important differences between the systems evaluated
when exposed to air and the RHB will need to be replaced might be seen with extended cold exposures, particularly in
with another. 77 severe-trauma patients who might be already cold stressed.

26 | JSOM Volume 20, Edition 3 / Fall 2020