When core body temperature drops below 90 degrees F, profound hypothermia develops.
The person becomes weak and lethargic.
The person has an altered mental state (is disoriented, confused, combative or irrational, or in a coma).
The person is uncoordinated.
When core body temperature drops below 88 degrees F, person stops shivering.
When core body temperature drops below 86 degrees F, person's heart pumps less than two thirds the normal amount of blood. Pulse and breathing are half their normal rates.
When core body temperature drops below 83 degrees F, the heart is very irritable and unstable, and likely to develop abnormalities. The person is in danger of cardiac arrest. Rough handling of the person increases the potential for cardiac arrest.
Sobriety Test for Hypothermia: An excellent test to determine if someone is developing profound hypothermia is to have the person try to walk a straight line, heel-to-toe, as in a sobriety test. If the person cannot perform this task and is not intoxicated, it indicates the progression from mild to profound hypothermia.
Profound Hypothermia Treatment
Treatment in the backcountry is aimed at stabilizing the person and preventing any further cooling.
Handle the person very gently. Rough handling may cause his heart to fail.
Place the person in a sleeping bag, or place blankets or clothing underneath and on top of him. Any heat that you can provide will probably not rewarm the person but will help prevent further cooling.
A person with significantly altered mental state should not be allowed to eat or drink because of the potential for choking or vomiting.
Rewarming is best done in a hospital, because of the potential complications associated with profound hypothermia. Professional assistance is usually needed to evacuate a profoundly hypothermic person.