Legally dead traditionally has meant a human being is dead when her heart and lungs have irreversibly ceased to function. In some cases, permanent loss of consciousness may precede cardiopulmonary failure. Today however, with modern medical technology, a patient may lose consciousness a decade or more before his heart and lungs fail.
All fifty states and the District of Columbia have adopted the Uniform Determination of Death Act (UDDA). The UDDA also recognizes whole-brain death -- irreversible cessation of all functions of the entire brain -- as a legal standard of death. A person can be legally dead even if her cardiopulmonary system continues to function. If a patient's entire brain is nonfunctioning, so that breathing and heartbeat are maintained only by artificial means, that patient meets the whole-brain standard of death.