"heart"

*(PRO): “He nodded”. Action – nodded. Case acknowledges his knowing of both Flatline himself and his cause of death.
**(PRO): An answer – “heart” can be taken for the cause of death. This dead topic of conversation, Dixie Flatline, died because of his heart. This also reveals some reason for his/her nickname – flatline.
***(SYM): Is Flatline’s heart death a symbol of his natural death. Certainly the construct that Case works with isn’t McCoy Pauley, Case’s old mentor. The death of the heart conjures its association with the soul. Gibson is reaffirming that Pauley is dead, and just a ‘construct’ or ‘recording’ of his memory remains. As human as the conversations between Case and the construct may seem, they are just the digital representation of his memory, nothing more. Nothing more is possible – his heart gave out.