Cerebral ischemia and impairment of circulation
Cryopreservation of the brain depends on the removal of blood from the brain’s vasculature and its replacement with cryoprotective solutions in order to prevent ice crystal formation (freezing) during cooling (i.e., facilitate vitrification). Ultimately, the success of a good cryoprotectant is limited by perfusability of the brain, or the ability of cryoprotective solutions to penetrate all areas and cells of the brain via the cerebral vessels. Long periods of global cerebral ischemia detrimentally affect reperfusion of the cerebral vessels, thereby significantly degrading perfusability of the brain. Many possible causes for post-ischemic impaired perfusion have been hypothesized in the past, including swelling of the endothelial cells that make up the inside lining of blood vessels. However, a landmark 1972 paper by Fischer & Ames provided evidence implicating changes in the blood itself as the probable reason for post-ischemic reductions in perfusability of the brain.
Ames, et al. had already demonstrated impaired reperfusion in rabbits after cerebral circulatory arrest followed by infusion of a suspension of carbon black and examination of coronal brain sections. Brains undergoing less than five minutes of arrest perfused well and were evenly stained by carbon black ink. Ischemia in excess of five minutes resulted in patchy white areas where perfusion was impaired and blood vessels did not fill with ink. In an extension of this work, Fischer and Ames investigated the effects of perfusion pressure, anticoagulation, and hemodilution on post-ischemic perfusability.
In their experiment, the researchers induced cerebral circulatory arrest in rabbits and then perfused the head and neck with carbon black immediately following various ischemic periods. The perfusion solution was introduced to the cerebral circulation from a reservoir placed above the rabbit and through tubing that was inserted into and secured in the ascending aorta. Perfusion pressure may thus be modulated by varying the height of the reservoir above the animal. In some animals, acute hemodilution was achieved by rapid infusion (50 ml/kg) of saline into the femoral vein for moderate hemodilution or by a combination of saline administration and blood removal for extreme hemodilution prior to ischemia and carbon black infusion. A series of animals were also anticoagulated by giving heparin (500 units/kg) intravenously 15 minutes prior to ischemia. Brains were then removed and diffusion fixed in 10% formalin, allowing coronal sections of the brain to be examined macroscopically.
Brains from 5 groups of animals were studied:
Group 1: 4.5 minutes’ ischemia, reservoir at 28 cm above the heart (two animals), 40 cm (two animals), and 110 cm (two animals).
Group 2: 15 minutes’ ischemia, reservoir at 70 cm (six animals), 110 cm (eight animals), and 170 cm (seven animals).
Group 3: 30 minutes’ ischemia, reservoir at 170 cm (six animals).
Group 4: Hemodilution, 15 minutes’ ischemia, reservoir at 70 cm, hematocrit (HCT) 21 to 32 (five animals); HCT 4 to 13 (six animals).
Group 5: Heparin, 15 minutes’ ischemia, reservoir at 70 cm (six animals).
As previously observed, “animals with 4.5 minutes of ischemia failed to demonstrate impaired cerebral reperfusion even with perfusion pressures as low as 28 cm of water, all brains being completely and evenly black.” However, significant impairment of perfusion was observed after 15 minutes of ischemia. The effect was greatest in the thalamus and brain stem, while cerebral cortex remained adequately perfused after 15 minutes of ischemia at all levels of perfusion pressure. White (non-perfused) areas were significantly greater after 30 minutes of ischemia than after 15 minutes at comparable pressures and in all areas examined (including cortex). Hemodilution greatly improved reperfusion after 15 minutes of ischemia, but no difference between anticoagulated animals and controls was observed.
The authors speculate that blood viscosity, rather than clotting or cellular swelling, was the most probable cause of impaired reperfusion following ischemia. They further noted that “improved postischemic cerebral circulation has also been noted to follow the infusion of hyperosmolar agents,” which they speculated was also due to reducing blood viscosity. Red cell aggregation during blood stasis was assumed to be a major contributing factor to the increase in viscosity, and differences in vascular resistance throughout the brain manifesting as differences in circulatory impairment were thought to underlie the observation of impairment of discrete, macroscopic regions of the brain — particularly subcortical regions.
The results of Fisher et al. support rapid restoration of perfusion pressure (mechanical cardiopulmonary support) after cardiac arrest in cryonics patients, and hemodilution with agents like Dextran-40. It is harder to reconcile with the emphasis in cryonics to administer fibrinolytics and anticoagulants to eliminate and prevent blood clotting. The results of Fisher et al. are also at odds with those of Böttiger et al., who found that activation of blood coagulation after cardiac arrest is not balanced adequately by activation of endogenous fibrinolysis. Perhaps these findings can be reconciled if we allow for the possibility that cardiac arrest and cerebral ischemia induce formation of (micro)thrombi, but that these are not clinically significant, or at least do not affect reperfusion as greatly as other blood abnormalities such as red cell aggregation. And perhaps the formation of thrombi in small cerebral vessels can adversely affect cryoprotectant perfusion without being visible by gross examination. Formation of large clots may still be a problem after longer periods of circulatory arrest. Tisherman et al. have observed large vessel blood clots in rats and dogs after normothermic cardiac arrest of more than 20 minutes. Finally, cryonics patients may present with existing blood clotting problems as a result of (septic) shock.
Although the emphasis on antithrombotic therapy to maintain circulatory patency in cryonics patients seems to be warranted, more emphasis on other factors that affect cerebral (micro) circulation in cryonics patients seems desirable. As the work of Fisher et al. indicates, hypertension and hemodilution during cardiopulmonary support may be just as, if not more, important. The relationship between in vivo stasis of blood circulation and coagulation remains elusive and could benefit from more research.