Rewarming Shock (also known as rewarming collapse) has been described as a drop in blood pressure following the warming of a person who is very cold. The real cause of this rewarming shock is unknown.
There was a theoretical concern that external rewarming rather than internal rewarming may increase the risk. These concerns were partly believed to be due to afterdrop, a situation detected during laboratory experiments where there is a continued decrease in core temperature after rewarming has been started. Recent studies have not supported these concerns, and problems are not found with active external rewarming.
References
Rewarming shock Wikipedia(Text) CC BY-SA