A doctor mentions that there wasn't much research on the clinical and physiological aspects of how extreme cold affects the human body, but mentions that in the Army he learned that if a person had been hypothermic for a long period, it was important to warm them up slowly. He is referring to a condition now known as rewarming shock, by the turn of the century early modern medical science, and doctors, hadn't had a lot of clinical experience with treating hypothermia. That changed with World War I, several of the war's largest battles occurred during the winter at temperatures well below freezing, an estimated 115,000 British, 90,000 American, 80,000 French and 38,000 Italian troops suffered casualties from hypothermia. Army and Navy doctors discovered a pattern when it came to rewarming hypothermic patients who had been exposed to the cold for extended periods: if they were rewarmed too quickly they would experience sudden cardiac arrest and death, but were perplexed as to why this happened. It is now understood that the phenomenon is caused by the rapid influx of cold blood from the extremities. When someone is hypothermic, the blood vessels and capillaries in the extremities constrict to restrict blood flow to the abdominal cavity in an attempt to conserve heat and keep the body's core temperature up. If the hypothermic patient is immediately placed in a warm environment again the blood vessels in the extremities dilate, which causes a sudden influx of cold blood into the heart and lungs, this can cause a person's blood pressure to rapidly drop leading to reduced blood flow to organs, causing shock and cardiac arrest. Rapid influx of cold blood into the abdomen also causes core body temperature to rapidly drop as well, this can cause a sudden electrolyte imbalance, in particular it can elevate potassium levels leading to dangerous cardiac arrhythmias. Another phenomenon that can contribute to shock and death is hypoxia-reoxygenation injury, the tissues in the body slowly adapted to the gradual reduction of blood flow and oxygen as the temperature dropped, a rapid elevation in oxygen levels can actually lead to tissue damage if it isn't given time to gradually adjust to normal oxygen levels.
