If he could be woken up briefly by the nurse then probably he is not brain dead
. Apart from that it is difficult to comment really as to why he is in unconscious state
. The treating doctors who are aware of his blood reports, scans etc only may be able to tell it clearly. You may have to just wait and watch and pray at this moment. Heart valve replacement
is a major surgery with its own set of associated risks. We hope for the best.