Active external rewarming is recommended for moderate hypothermia. Extracorporeal rewarming is the fastest method for those with severe hypothermia. Passive external rewarming is recommended for those with mild hypothermia. Afterdrop is not observed in the rewarming of all hypothermic patients. However, outcomes varied greatly depth and duration of hypothermia as well as rewarming procedures. Controlled rewarming has been cited in the literature as beneficial in preventing reperfusion injury. Blisters and spasm of blood vessels (vasospasm) can develop after rewarming. Insulating the first or second stage inhibits rewarming from the surrounding water and accelerates freezing. This heat gradient does not reverse immediately upon initiation of rewarming. Active external rewarming involves applying warming devices externally, such as a heating blanket. Eutherian arousal relies on a heat-producing brown adipose tissue as a mechanism to accelerate rewarming. After subjects were frozen, they then underwent different methods for rewarming. Active rewarming seeks to warm the injured tissue as quickly as possible without burning. This is followed by gradual rewarming over the next 12 to 24 hrs. Attempts to recover frozen mammals by simply rewarming them were abandoned by 1957.