Dehydration is by far the biggest killer with fever, and fluid loss can be counteracted by monitoring the patient and using IV to push more fluids
in. With loss of fluids,
cells will start to die, blood pressure will drop and this is not a good situation to be in. From reports I've read with brain cells, they may start to die at 42c.
From the patients I've observed in intensive care, they keep the heart rate low, 70-80 to keep it from overworking over long periods of time.