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The Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update provide an excellent evidence-based summary of the management of hypothermia. This clinical practice guideline recommends giving high-carbohydrate liquids and foods to any patient who is alert and shivering and is not at risk for aspiration. This is because vigorous shivering has been shown to increase thermogenesis by 5 to 6 times the resting metabolic rate and can raise core temperature by 3 to 4°C per hour. In order to retain heat from shivering thermogenesis, the patient needs to be dry and adequately insulated.

As all patients with moderate and severe hypothermia must progress through cold stress and mild hypothermia and this progression can be interrupted, it can be assumed that cold stress and mild hypothermia represent the greatest proportion of patients in the hypothermia continuum. The oral treatment of cold stress and mild hypothermia is an important way in which the progression of hypothermia may be avoided. This is done in two ways, providing hydration and calories to fuel shivering thermogenesis.

The human body appears to prefer carbohydrate to fuel shivering thermogenesis, but can also use lipid and protein. Aside from fueling shivering, ingestion of calories also temporarily increases the basal metabolic rate during digestion (specific dynamic action or SDA). The onset of the SDA is much faster with carbohydrates than with protein or fat. Studies have shown that muscle glycogen reserves provide about 75-80%, and plasma glucose about 20-25%, of the total carbohydrate oxidized. There is individual variation in shivering patterns. Individuals with higher burst-type shivering rates, as opposed to continuous shivering, will oxidize more glucose and deplete muscle glycogen sooner at the same shivering intensity. Shivering fatigue has been estimated to occur anywhere from 20-42 hours from the onset of shivering. Simulated military operations consisting of exertional fatigue, negative caloric balance, and sleep deprivation have been shown to blunt shivering thermogenesis. Although research has not determined the absolute need for carbohydrates, either for shivering intensity or duration, they are a readily available and an easily administered source of energy that can be converted by the shivering patient into heat.

In the hypothermic patient in an austere environment, cold-induced dehydration comes from sweating when well-insulated, blunted thirst, cold-induced diuresis, increased respiratory water losses, intentional limited drinking, and lack of water availability. Although hydration is needed to facilitate heat redistribution and reverse cold-induced blood viscosity, water provides no calories for thermogenesis. A cup of black coffee has only 2 kilocalories to fuel shivering. Although hot drinks feel good and boost morale, a patient cannot drink enough to warm their core. For an individual that weighs 75 kilograms with mild hypothermia (34°C), the following calculation shows theoretically how much 60°C water this individual would have to drink to raise their temperature to 37°C.

By the First Law of Thermodynamics, energy can neither be created nor destroyed (conservation of energy). Thus, energy is transferred from one part of the system to another and the total energy of a system (Q) is unchanged:

ΔQsystem = ΔQbody + ΔQwater = 0

Because Q = (mass)(specific heat)(change in temperature), then:

[(mass)(specific heat)(ΔT)]body + [(mass)(specific heat)(ΔT)]water = 0

Now plugging in the variables:

[(75.0 kg)(3470 J/kg°C)(37°C - 34°C)] + [(kg of water)(4186 J/kg°C)(37°C - 60°C)] = 0

Rearranging:

(kg of water)(4186 J/kg°C)(37°C - 60°C) = -(75.0 kg)(3470 J/kg°C)(37°C - 34°C)

And:

kg of water = -(75.0 kg)(3470 J/kg°C)(3°C) / (4186 J/kg°C)(-23°C) = 8.1 kg of water

Converting to liters of water (density of water = 1.0 kg/liter):

(8.1 kg) ÷ (1 kg/liter) = 8.1 liters of water

In order to raise core temperature to normal, this individual would theoretically have to drink 8.1 liters (2.1 gallons) in a short period of time. This assumes that there is no transfer of heat by radiation, conduction, evaporation, respiration, or convection, and no significant thermogenesis from shivering. One liter of a hot drink would be ambitious for most people, and would raise this individual’s temperature about 0.4°C. The following table shows the amount of 60°C water theoretically needed to normalize the body temperature in individuals of varying weights and degrees of hypothermia.

In order to raise core temperature to normal, this individual would theoretically have to drink 8.1 liters (2.1 gallons) in a short period of time. This assumes that there is no transfer of heat by radiation, conduction, evaporation, respiration, or convection, and no significant thermogenesis from shivering. One liter of a hot drink would be ambitious for most people, and would raise this individual’s temperature about 0.4°C. The following table shows the amount of 60°C water theoretically needed to normalize the body temperature in individuals of varying weights and degrees of hypothermia.

The bottom line for the alert hypothermia patient able to protect their airway: (1) let them shiver, (2) remove wet clothes and provide insulation so they can keep this heat, (3) provide hydration, and (4) the caloric content, particularly those from carbohydrates, is more important than the temperature of the drink. Hot drinks are not likely to significantly raise a patient’s core temperature. The WMS clinical practice guideline mentions that food and drink can be warmed, but should not be hot enough to burn the esophagus.

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