What Causes Severe Asthma Attacks In Child?

My 3-year-old had a severe asthma attack and has been in a induced coma with intubation for 4 days. They removed the ventilator and stopped the barbiturates this morning, but she has not woken up. I am hearing talk of brain damage. She was talking and coherent when they induced the coma. Can it in itself cause brain damage? Is it unusual for her to not be waking up?