During hypothermia, vasoconstriction causes shutdown of blood flow to the extremities, leading to cold and acidotic blood in the periphery due to hypoxia.
Warming the extremities leads to vasodilation, and blood in the core returns to the extremities so blood pressure decreases suddenly and the heart cannot compensate. Also, cold, acidotic blood enters the core circulation, leading to an even greater decrease in temperature, and acidosis makes the heart even more stressed.
This is called the afterdrop phenomenon and it is often the cause of death in hypothermic human patients after they have been rescued from the cold, due to cardiac arrest.
