From the PBS Newshour on Friday, here’s a neurologist named Dr. Russell Portenoy on the question:

JEFFREY BROWN: Dr. Portenoy, a final medical question. When Miss Schiavo does die, what will she die of?
DR. RUSSELL PORTENOY: Well, patients who have hydration and nutrition develop biochemical changes in the blood. These biochemical changes progress and at a certain level of abnormality they are associated with abnormal heart beat, arrhythmias of the heart. And so ultimately she will die when her heart stops.
JEFFREY BROWN: All right. Dr. Russell Portenoy and Professor Robert George, thank you very much