The initial shock of quickly and unexpectedly being immersed in cold water lasts roughly two minutes and is followed by a period of increasing functional disability as vasoconstriction continues to shunt blood to the victim's core and the muscles in their extremities continue to cool. Muscle cells function best at 104º F (40º C). Cooling alters nerve and muscle function sharply decreasing both strength and endurance, limiting the victim's ability to self-rescue. As their core temperature reaches 96º F (35.5º C), the victim starts to shiver. Severe shivering begins with the true onset of mild hypothermia at core temperatures around 95º F (35º C). Similar to moderate exercise, severe shivering increases heat production up to five-fold; however, it also decreases muscular coordination and impairs physical performance, further reducing their ability to self-rescue. Given limited insulation and no life jacket, most immersion victims have roughly two to fifteen minutes to self-rescue before they can no longer swim or hold on to their boat (or other floating object); due the added insulation and the ability to rest, wearing a life-jacket can double an individual victim's time to self-rescue.
In most healthy people it takes 20-30 minutes to become mildly hypothermic. Shivering eventually stops as the victim's available calorie stores become depleted or the hypothalamus shuts down. Somewhere between 89º F (31.6º C) and 91º F ( 32.8º C) the victim's mental status drops, they are no longer able to support their head, and at this point they will drown if not wearing a life-jacket.
Below 90ºF (32º C) the victim's heart becomes electrically unstable and increasingly predisposed to ventricular fibrillation (spontaneous cardiac arrest). If the victim is wearing a life-jacket and their head is out of the water, death due to immersion hypothermia—rather than drowning—typically takes at least an hour, even in ice water.
Prevention—clearly related to the physiology discussed above—and rescue are discussed in detail in Part 2 of this article.
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