What Are The Symptoms Of Dead Brain?

my mother is 49 yuears old and she is in ccu she's unresponsive and she's on a ventilator. she's had asthma copd for 10 years now. they discharged her from the hospital yesyerday and said she was fine she had a cold and they gave her breathing treatments and 12 hiurs later she was un responsive no pulse she was down for 15 min and they performed cpr and got a pulse and they also shocked her. i don't understand is she brain dead since she was down for 15min? i can't get any answers from them besides wait