Modern medical technology has made the determination and definition of death increasingly complex. There is lack of complete consensus on the definition of death which justifies extraction of organs of the body for other uses or withdrawal of medical care, although parts of the body can be kept alive by such care long after effective death. The traditional meaning of death was either the departure of the soul from the body or, in more recent secular thought, the irreversible stoppage of the flowing of bodily fluids associated with heart and lung function. The loss of vital signs, especially pulse, is a traditional indicator which still has application. Japanese research demonstrates that a fall in the blood pressure of 40 mm of mercury and persistent low blood pressure for six hours signal the imminence of death. Now, however, individuals may have totally destroyed brains with the irreversible loss of the ability to integrate bodily functions, while their respiration is supported mechanically and their hearts continue to beat. This has led many to argue that the irreversible loss of capacity for bodily integration is the necessary and sufficient condition for being dead. Others have opted for a concept of death that relates it to loss of brain function. There are two schools of thought in this matter. One is to equate death with the loss of all brain functions. The other, called the "higher brain" position, equates death with the loss of higher brain activities such as consciousness, the ability to communicate and to relate to others. In this case, those who are irreversibly in a permanent vegetative state are considered dead, even though lower brain activities such as breathing and certain reflexes may continue. This is especially difficult when, as is often the case, the presence of any amount of drugs in the blood introduces doubt as to the irreversibility of the vegetative state. Once one has chosen a concept of death, the next concern, of no lesser difficulty, is how to measure the irreversible loss of the chosen essential functions.
Many countries, but not Japan as of 1997, and many states within the USA have opted through statutes and case law for a concept of death that relates it to loss of brain function.