Below 35°C, shivering first helps, then dangerously stops
Hypothermia means a human core colder than 35.0°C (95.0°F). Mild cases bring shivers and confusion; in moderate ones the shivering stops while confusion deepens; severe cases risk the heart stopping. Cold air and icy water are the classic causes, yet alcohol, low blood sugar and old age can tip the same balance.
The body normally holds its core between about 36.5 and 37.5°C by thermoregulation, fighting cold with shivering, extra movement and warmer clothes. Hypothermia sets in when heat loss outruns production, either from exposure to cold weather and water or from conditions that sap heat generation, including drunkenness, anorexia and advanced age. The name joins Greek words for under and heat, and its opposite, hyperthermia, is overheating caused by the same control system failing.
Doctors grade it by measured core temperature or, when no reliable thermometer reading is possible, by the Swiss staging system, which relies on visible symptoms. The mind suffers early. Victims may show poor judgement, lose track of time and place, turn unusually aggressive, deny anything is wrong or refuse help, and some become euphoric. In the severe stage come hallucinations and paradoxical undressing, in which a freezing person strips off clothing. Chilled infants may look bright red and oddly listless rather than pale.
Treatment scales with severity. Warm drinks, dry clothing and movement handle mild cases; heated blankets and warmed intravenous fluids suit moderate ones. Anyone moderately or severely affected should be moved gently. The worst cases may need ECMO or cardiopulmonary bypass, with CPR for those without a pulse. Rewarming usually continues past 32°C, and efforts may stop if there is still no improvement at that point or if blood potassium ever exceeds 12 millimoles per litre.
The cold claims at least 1,500 American lives a year, more often among men and older people, and it has shaped the outcome of many wars. Yet recovery can be astonishing. A two-year-old Polish boy named Adam survived a core temperature of 12.7°C, patients have lived after more than six hours of CPR, and about half survive when ECMO or bypass is used. Related cold injuries include chilblains from repeated near-freezing exposure, frostnip, frostbite and trench foot.
Source: Hypothermia
