A study in this weeks Nature (Vrselja et al. ) has created an immediate media frenzy. Nature puts it like this: ‘Pig brains kept alive outside body for hours after death’ and ‘Revival of disembodied organs raises slew of ethical and legal questions about the nature of death and consciousness.’ The New York Times: ‘In a study that raises profound questions about the line between life and death, researchers have restored some cellular activity to brains removed from slaughtered pigs.; STAT: ‘The pigs were dead. But four hours later, scientists restored cellular functions in their brains’ etc.
That sounds spectacular. But if one reads the study (and the commentaries) is easy to spot that there are two main deficiencies: 1) The study lacks novelty, and 2) The assertion that it presents a relevant step towards restoring brain function after a prolonged interruption of cerebral blood flow is not only exaggerated, but simply wrong.
1) KA Hossmann and K Sato showed in 1970 (Vrselja had the courtesy to quote it!) that cat brains recover an EEG and evoked potentials after up to 1 hour complete arrest of brain blood circulation. The 1970 article was followed by an exchange of letters in the same year in Science on ‘Criteria of brain death’, and Hossmann concluded :‘ This suggests that even prolonged electrocerebral silence does not prove the irreversible loss of neuronal function’ . Nothing new here…
2) That an EEG can recover after a flatline we know for sure (btw: this did not even occur in the Nature study!). It happens daily all around the world: Patients who are operated in deep hypothermia (18oC) with extracorporal circulation have no measurable brain electric activity or evoked potentials, many of these patients recover fully after restoration of the patients circulation and slow rewarming. The same applies to patients after cardiac arrest. Circulatory arrests of up to 20 minutes are documented (silent EEG!) in which patients recovered after resuscitation, possibly with slight neuropsychological deficits (mostly memory deficits, see below, as a result of delayed neuronal cell death, this was shown by neuropathologists in the 80s and 90s, see Petito et al. ).
3) There is a large body of literature on ‘head transplantation’ dating back to 1900, even Wikipedia has an entry on this subject. This literature is relevant here insofar as these brains were ‘artificially’ perfused (after all, not through a pump, but another organism) and the brain was by definition cut off blood supply for some time (during ‘transplantation’).
4) There is an extensive literature especially from the 80s and 90s of the last century on ‘isolated brain perfusion’ (‘isolated brain’, different species, including pig and rat), but also more recent work, e.g. Breschi et al. . As in the present article, the focus of that older literature was on different forms of perfusion (e.g. pulsatile vs. non-pulsatile), synthetic oxygen carriers, and the addition of brain-protective substances. This technique did not become established in the neurosciences, possibly because confocal microscopic methods were developed at the same time, with which one could look into the brain of experimental animals with microscopic resolution, but with normal blood perfusion and without the artefacts of isolating the brain.
5) Most importantly, however, the authors do not show any restoration of brain function! They show temporary morphological preservation of vessels (which were reactive to pharmacological stimulation to superfusing vasoactive substance), and of glial cells (which still reacted to LPS, i.e. cell wall components of bacteria). This is unsurprising, for decades we have known (especially from cell culture) that these cells are very resistant to oxygen deficiency. Endothelial and glial cells can survive more than 24 hours in cell culture without oxygen and glucose. Unfortunately, we can’t think with the vasculature, and glial cells alone won’t help either. It is also not surprising that individual neurons still showed electrical activity, as this has been known for a long time (see also above). What the authors (and the reviewers) obviously did not know, however, is that most of these cells will begin to die after about 24 hours after the ischemic episode, and are dead after 72 hours. After 6h (as in the study) they look ultrastructurally normal, which has been demonstrated with electron microscopy already in the 1980ties. This phenomenon is called ‘delayed neuronal vulnerability’ (and by older neuropathologists ‘maturation phenomenon’, e.g. Ito, Klatzo and others . This was discovered in the second half of the last century and a major research focus of cerebral ischemia research of that time worldwide. We still do not know the exact mechanisms of selective neuronal vulnerability, but especially cells of the hippocampus (especially the CA1 region investigated in the study, but also cells of the cerebral cortex, etc.) have this property. This is one of the factors that prevensplantation’ dating back to 1900, even Wikipedia has an entry on this subject. This literature is relevant insofar as these brains were ‘artificially’ perfused (after all, not through a pump, but another organism) and the ‘donor brain’ was by definition cut off blood supply for some time (during ‘transplantation’).
4) There is an extensive literature especially from nt such a ‘reperfused brain’ from functioning again. Simply put: These neurons die with a delay. If you wait only a few hours, you will miss it. Vrselja et al. missed this because they studied the brains only for 6 hours because of technical limitations. But they could have consulted the literature.
The Nature article is flanked by two ethical comments relating the results to organ transplantation, resuscitation, animal welfare etc. However, for all the reasons given above, there is no new evidence that could provide a rational basis for the ethical implications advocated in these comments. Everything was known decades before!
Conclusion: This paper was published because because it was clear that it would create a lot of attention. For some reason the review process didn’t work. I consider the study methodologically sound, but it doesn’t solve a problem and doesn’t advance science. It is unfortunate that such a study can attract worldwide attention and trigger profound but unfounded discussions.
Regardless of what the study shows, the question of organ removal after induced cardiocirculatory arrest (due to termination of life support measures) is highly relevant. In Germany, organ removal under such conditions is not allowed. In other countries, e.g. the USA and Spain, it is practiced. If organ removal takes place immediately after the ventilation is stopped and the EEG flatlines, no one can rule out the possibility that consciousness- (or pain-) like states may still occur in the patient’s brain. The current study in Nature rekindles the question of when we consider a brain to be ‘dead’. But without making a novel contribution. So let’s discuss it, but without focus on the study by Vrselja et al., but rather the huge body of evidence that has accumulated over the last 50 years in animal studies and medical treatment of humans.