Hypothermia & Frostbite

"Hypothermia occurs when the body releases more heat than it absorbs or generates." CDC: "Lengthy exposures will eventually use up your body's stored energy, which leads to lower body temperature." Cold air (convection), cold water (conduction) and sweating (evaporation) drive it; the Yellow Book ranks conduction — water immersion and wet clothing — as the fastest route.

Defenses, then failure. "Early cold-defense mechanisms include peripheral vasoconstriction, increased sympathetic activity, and increased muscle tone. As cold stress intensifies, shivering thermogenesis substantially increases metabolic heat production." "Newborns lack the shivering mechanism due to incomplete development of the nervous system." Vasoconstriction also causes cold diuresis, which "may occur due to peripheral vasoconstriction, leading to increased diuresis and subsequent volume depletion."

Brain and heart. "Body temperature that is too low affects the brain, making the victim unable to think clearly or move well. This makes hypothermia especially dangerous, because a person may not know that it's happening" (CDC). "Shivering typically ceases when the core temperature reaches 30 to 32 °C, at which point, paradoxical undressing may be observed." "Atrial fibrillation is the most common dysrhythmia associated with hypothermia, while severe hypothermia increases bradycardia and susceptibility to ventricular arrhythmias, including ventricular fibrillation."

Resuscitate and protect. "Management of hypothermia focuses on preventing further heat loss, supporting the airway, breathing, and circulation, and initiating rewarming appropriate to the severity of illness. Wet clothing should be removed promptly." "Patients with moderate or severe hypothermia should be handled gently because rough movement may increase myocardial irritability and precipitate fatal dysrhythmias."

The ladder. Passive external rewarming (dry insulation, warm room) suits mild hypothermia in patients who are "alert, hemodynamically stable, and able to generate heat through shivering." Active external rewarming — "Forced-air warming systems and heat packs applied to the trunk, axillae, chest, and back" — is for moderate hypothermia and severe hypothermia with circulation. Active internal rewarming adds "warmed humidified oxygen, warmed intravenous fluids, and warm lavage of body cavities." "Cardiopulmonary bypass surgery and veno-arterial (VA) ECMO are the most effective but highly invasive rewarming methods," reserved for hypothermic cardiac arrest and instability.

Trunk before limbs. "Extremity warming may be useful in mild hypothermia, but rapid peripheral vasodilation in more severe hypothermia can contribute to hemodynamic instability." CDC: "Warm the center of the person's body—chest, neck, head, and groin—using an electric blanket, if available."

Do not give up early. CDC: "Perform CPR, even if the person appears dead." "Hypothermia-related dysrhythmias may be refractory to standard advanced cardiac life support until rewarming has progressed." "Prognostication in hypothermic cardiac arrest should not be based on core temperature alone." "Full neurological recovery has been reported in accidental hypothermia reaching 14 °C."

"Frostbite, also known as a freezing-cold injury, results from exposure to cold at temperatures below 0 °C (<32 °F), during which ice crystals form, causing tissue destruction." "Initial tissue cooling causes vasoconstriction and a progressive reduction in blood flow. Extracellular ice crystal formation increases osmotic gradients, promotes cellular dehydration, and disrupts membrane function. Continued exposure leads to endothelial injury, edema, hemoconcentration, microvascular stasis, thrombosis, and tissue ischemia."

Thaw–refreeze. "During tissue thawing, a reperfusion-associated inflammatory response mediated by proinflammatory cytokines may cause additional tissue damage. Moreover, repeated thaw-refreeze cycles can progressively worsen tissue ischemia and promote subsequent thrombosis." That is why "Do not thaw the affected tissue in the field if there is any possibility of refreezing."

Depth. "Superficial frostbite, involving the epidermis and superficial dermis, typically produces clear or milky cloudy blisters upon rewarming." "Deep, full-thickness frostbite causes hemorrhagic blistering with rewarming and may progress to gangrene." "Notably, the initial examination does not accurately reflect the extent of injury"; demarcation "can develop over 1 to 3 months."

[NFCI] Trench foot: "the body constricts blood vessels to shut down circulation in the feet. Skin tissue begins to die because of lack of oxygen and nutrients" (NIOSH); it "can occur at temperatures as high as 60°F if the feet are constantly wet."