Microvasculature perfusion failure in cryonics
Under ideal circumstances cryonics patients are stabilized immediately after pronouncement of legal death by restoring blood flow to the brain, lowering temperature, and administering medications. In most cryonics cases, however, there is a delay between pronouncement of legal death and start of cryonics procedures. In some cases there are no stabilization interventions at all. Provided that these periods of warm and cold ischemia are not too long, such patients can still be perfused with a vitrification agent. But how thorough cryoprotectant perfusion (and thus vitrification) in these cases can be remains an unresolved issue.
Since the late 1960s a number of studies have been published that document that cerebral blood flow cannot be completely restored after prolonged periods of cerebral ischemia. Brains that have been perfused with black ink after increasing periods of ischemia have shown progressive development of no-reflow areas in the brain (as evidenced by the absence of ink). In 2002 Liu et al. used a technique that allows direct visualization of trapped erythrocytes by treating fixed brain tissue with sodium borohydride (NaBH4), which renders trapped erythrocytes fluorescent. In a rat model of focal ischemia the authors found that a significant fraction of the capillary bed (10% to 15%) in the penumbra (the area surrounding the ischemic core) is blocked by trapped erythrocytes, even after 2 hours of reperfusion.
The authors discuss a number of clinically relevant issues. They propose that the lower density of trapped erythrocytes in the ischemic core of the brain reflects hypoxia-induced lysis (which releases cytoxic hemoglobin). They further speculate that the older ink methods may have underestimated the degree of no-reflow because areas that are not accessible to red blood cells may still be accessible to other molecules. This presents an opportunity to deliver oxygen to the brain by using small oxygen carrying molecules such as perfluorocarbons. The authors did not investigate variations in perfusion pressure or the efficacy of volume expanders to restore no-flow areas to circulation.
A focal ischemia model is not a good model for cryonics and one can only speculate what the effects of various periods of complete ischemia would be on cerebral blood flow and erythrocyte trapping. Older studies on ischemia and perfusion impairment, however, indicate that periods of 30 minutes of complete ischemia can produce substantial areas of no-flow in the brain. Unless these areas are opened to circulation during either stabilization or cryoprotectant perfusion, straight freezing of pockets of the brain is a real possibility. It remains to be investigated if areas that are obstructed by trapped red blood cells are accessible to cryoprotectant agents and how much of these areas can be opened by a combination of hemodilution and non-penetrating perfusate components (through dehydration). Although cryopreservation of ischemic brains is the norm in cryonics, our knowledge about the effects of ischemia on vitrification of the brain remains limited.