Most traditional mythologies have equated life to breath. Breath was considered to be something more than respiration, linked -in a tradition that goes back to myths of Anima Mundi- to some sort of universal life. In this context, death was the departure of such transcendental airy entity towards fabulous scenarios of different kinds. The link of breathing and the capacity for speech contributed for a further metaphysical development of the physiological act of oxygenation. The Parmenidean tradition in Greece, but especially the Aristotelian philosophy, linked life more to intelligence than to any other physiological action. In fact, in the Metaphysics, Aristotle declares life to be some sort of intelligence. This double link, to breathe and to intelligence, in their transcendental or materialistic interpretations, gave for centuries a cornerstone for the definitions of life. Even, in the practice of modern medicine up to the 1960’s, it was the loss of the capacity to breathe (together with the loss of blood flow) the key property for the definition of death. The definition of death elaborated by the Academy of Medical Royal Colleges in 2008, declares death as the irreversible loss of the capacity for consciousness, combined with irreversible loss of the capacity to breathe. Here, the concept consciousness merely describes a physiological process that would be better described as a state of vigilance, opposed to sleep, coma or anesthesia, but in any case, implies the expression of intelligence in its more basic terms. Thus, according to this definition death is a combination of an irreversible loss of any state (process) of vigilance and the capacity to breathe.
Irreversibility of functional performance seems at first a good indicator for death, especially irreversible loss of critical systems which lead to a general collapse of the body, like it used to be the loss of capacity to breathe or to pump blood. However, the invention of mechanical ventilators during the 1960’s changed the conceptual scene and patients with massive brain destruction could maintain with the help of machines a relatively healthy functioning of other systems. David Cole has convincingly argued that irreversibility is not an absolute concept in relation to the medical definition of death, for it is conditioned both by the state of the medical technology and by the scenarios where the death situation may occur (it is not the same a massive heart attack in a hospital that in a camping in the mountains). If we consider the changes in medical science, we may induce that what today is not irreversible may be reversed tomorrow, so it seems futile to try to give final definitions and limits for such a dynamical field of knowledge in which biomachines may alter radically the way we look today to the functioning of human organs.
All that is needed is a working definition which can be used in the context of the main legal and ethical frames (Human Rights Declaration, etc.) to avoid abuses and violations of human rights. The common ground for ethics and medicine is not the performance of a particular organ or a system of them, but the social persona. Therefore, irreversibility has to be measured in relation to that persona, and in many cases it means that there are irreversible loses even in situations when no major organs have been affected but a general impairment ends with the life of the patient as s/he knew it. This problem would need to be addressed and complemented in relation to the questions of assisted suicide and the property which the individual has over his or her body. Since human beings are symbolical, the symbolical emergent dimension has to be added to the physiological one. The symbolical approach can be so different as the materialistic one of physiology in relation to the transcendental one which speaks about resurrections. In fact, the discrepancies affect to the social persona, for while science speaks of the persona in terms of the political legal system in which it is developed, religion speaks of a transcendental persona beyond the political one. How to harmonize such a complex symbolical scenario? One possible solution is a plain appeal to consistency of action: medicine treats the socio-political persona according to the general legal frames, while leaves the metaphysical person as a private question of the patient. But this implies that religion also does not interfere with the clinical practice (beyond its conditionings in the general beliefs which are gathered in the ethical codes of the group). A person would be clinically dead when its body functions where so impaired as to impede any psycho-social communication. Such an assessment needs a careful and thorough examination of each individual case and has to be independent of considerations in relation to organ donation, unless previously specified by the patient in actual statements and reports. We would need databases with patient decisions in relation to different health scenarios which could affect them.
Academy of Medical Royal Colleges  A Code of Practice for the Diagnosis and Confirmation of Death. http://www.aomrc.org.uk/doc_view/42-a-code-of-practice-for-the-diagnosis-and-confirmation-of-death.
Cole, D. J. (1993), Statutory Definitions of Death and the Management of Terminally
Ill Patients Who May Become Organ Donors After Death, Kennedy Institute of Ethics
Journal, 3: 145–55.
Youngner, Stuart J. The Definition of Death.  In The Oxford Handbook of Bioethics. p.p. 285-303. Oxford University Press. 2007.