The use of hypothermia following cardiac arrest shows increased likelihood of survival. It is the re-warming period that, if not controlled properly, can have detrimental effects. Hyperthermia during the re-warming period shows unfavorable neurologic outcomes. For each degree the body is warmed above 37 °C, there is an increased association with severe disability, coma, or vegetative states. Excessive rewarming with temperatures above 37 °C can increase the risk of cerebral ischemia secondary to the increased oxygen demand that occurs with rapid rewarming. Several theories have been proposed, with one being during re-warming, the body releases increasing catecholamines which increase heat production leading to a loss of thermoregulation. Hyperthermia in the preperfusion period can also be caused by an increase in the production of oxygen radicals, which influences brain metabolism. These oxygen radicals attack cell membranes, leading to a disruption of intracellular organelles and subsequent cellular death.