Thanks for posting your query.
History of heart attack
with brain dead
could be sue to decreased blood pressure with hypoxic / hypoglycemic encephalopathy
Brain dead means non-functioning of cerebral cortex
While respiratory centre and heart centre are present in brain stem (made up of pons, medulla and mid brain), persistence of life with maintained blood pressure and respiration indicates well maintained brain stem.
Although it indicated bad prognosis but you shouldn't loss your hope and keep your fingers cross.
Dr. Mayank Bhargava