Defining human life is hard, but defining death used to be easy: no heartbeat or respiration. That changed with modern technology--e.g. a patient in the midst of a heart transplant may have no heart, but may have a long life ahead.
And of course, if we are who we think we are, it is brain death that matters most: loss of heartbeat and respiration kills our brains, our selves. But defining brain death is not so easy, especially for a patient on a ventilator who may have a pulse and be visibly breathing.
A recent discussion in JAMA seeks to standardize definitions of brain death, which are of more than academic interest, because deceased donor organ transplantation mostly goes on after brain death but while the organs are still receiving oxygen from (artificially maintained) heartbeat and respiration.
Greer DM, Shemie SD, Lewis A, et al. Determination of Brain Death/Death by Neurologic Criteria: The World Brain Death Project. JAMA. Published online August 03, 2020. doi:10.1001/jama.2020.11586
"The concepts of life and death have always been complicated, but ever more so as medical and technological advances continue to extend the limits to saving life and prolonging physiological function. For previous generations, cardiorespiratory death was the sole clinical definition of death, often without any standard criteria, leading to the risk of misdiagnosis. As resuscitation techniques and mechanical ventilation developed, a new definition of death was needed.
"The idea of brain death/death by neurologic criteria (BD/DNC) was first recognized in 1959 as “coma depassé”1 and subsequently described as “brain death” with the first published clinical definition in 1968, commonly known as the Harvard Brain Death Criteria.2 Since then, many other guidelines and protocols have been published, adopted, and revised throughout the world with general acceptance of the concept of BD/DNC among medical groups, major religions, and governments.3
'However, there continues to be confusion and dilemmas that arise regarding BD/DNC. The wide variance in practice reflects this confusion and numerous other challenges. Inconsistencies in concept, criteria, practice, and documentation exist internationally and within countries.3,4 Difficulties in conducting randomized clinical trials and large-scale studies on BD/DNC have resulted in a lack of robust data from which to develop evidence-based recommendations. Challenges to the validity of BD/DNC continue to promote controversy. These factors initiated this project to harmonize practice and improve the rigor of BD/DNC determination."
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Here's an earlier take, with a more philosophical slant:
Sarbey, Ben. “Definitions of death: brain death and what matters in a person.” Journal of law and the biosciences vol. 3,3 743-752. 20 Nov. 2016, doi:10.1093/jlb/lsw054
Here are the concluding sentences:
"The neurological criteria for death represent a remarkable advance in our ways of responding to changes in death and dying. However, as medical technology and life extension techniques develop, we must also develop increasingly precise notions of what aspects of our neurological lives are the most important. While the current total brain death standard currently suffices in the vast majority of cases, the standard does not fully line up with what we value in persons. Should we retain the current brain death standard despite its mismatch with our values and despite negative consequences in determining death and in organ donation? Technological advances seem as if they will inevitably make this question inescapable."
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