We are sorry to hear about the frail nature of your mother. Many people assume that pacemakers prolong life and keep people's hearts beating after death. Pacemakers generally improve quality
of life in people with certain cardiac conditions but are not resuscitative devices(unless it is a pacemaker with a defibrillator). Also, many people are not dependant totally on the pacemaker, so it may not even be functioning most of the time.
Generally at the time of death the heart becomes so weakened that the electrical cells in the heart do not respond to the pacemaker. In addition, there are other signs such as lack of respirations or breathing, that can determine a patient has expired.
Pacemakers can be deactivated, but that may only worsen some symptoms in cardiac patients prior to death. The best thing to do would be to speak with your mother's cardiologist and detemine how dependant she is on the pacemaker and discuss your concerns with him/her.