All those years ago... when Terri's heart stopped beating... it was the result of a dietary problem, involving to much potassium, that caused her heart to stop. Her brain was deprived of oxygen for 15 minutes or so... and that's what did all the damage.
Right now... Terri is dehydrating. As the fluid in her body is used up... there is massive kidney damage... and the electrolyte levels rise.
As they rise... the level of potassium in her system sky rockets.
When the level of potassium reaches a certain point... her heart will stop, and her brain will be deprived of oxygen.
The very same physiological process that started Terri's ordeal, is now ending it.
And I suppose this is a good time to point out that we use the same process... a leathal injection of potassium, to execute those sentenced to death.