Physiologic responses to hypothermia include:

c. Pupillary dilatation and loss of cerebral autoregulation at temperatures below 26°C

d. A cardiac rhythm contraindicates cardiopulmonary resuscitation even in the absence of a palpable pulse 

The physiologic response to hypothermia is one of transitional changes, with few exact temperature-dependent responses. Broadly speaking, the transition from a “safe zone” of hypothermia (where physiologic adaptations to heat loss are working) to a “danger zone” of hypothermia occurs between 33°C and 30°C. The cardiovascular response includes tachycardia followed by progressive bradycardia which starts at about 34°C , and which results in a 50% heart rate decrease at 28°C. Asystole occurs below 25°. Due to difficulty in palpating weak, bradycardic pulses in cold, stiff hypothermic patients, the presence of an organized rhythm should be taken as a sign of life that contraindicates CPR, despite the absence of a palpable pulse. Respiratory drive is increased during the early stages of hypothermia, but below 30°C progressive respiratory depression occurs, resulting in a decrease in minute ventilation. The neurologic response to hypothermia is heralded by progressive loss of lucidity and deep tendon reflexes, and eventually flaccid muscular tone. Pupillary dilatation and loss of cerebral autoregulation occur at temperatures below 26°, and electroencephalography becomes silent at 19–20°. It is important, however, to remember that patients have been revived with core temperatures as low as 17°C, and therefore the saying “No one is dead until warm and dead”.