September 04 , 2007
by Aschwin de Wolf
This article was written in 2006 for the second Immortality Institute anthology on life extension, which was never completed. A revised version of the article was published on the Alcor Life Extension Foundation website.
The goal of stabilization in human cryopreservation is to keep the brain viable by contemporary medical criteria after the patient has been pronounced legally dead. The importance of stabilization does not only reflect a desire to protect the patient from injury after pronouncement; successful stabilization may also improve the chance of successful cryoprotection. Stabilization of the patient includes cardiopulmonary support, combinational pharmacology, rapid induction of hypothermia, and substitution of the blood with an organ preservation solution. A basic introduction to the pathophysiology of cerebral ischemia is presented, and different modalities of intervention are reviewed. Conceptual issues surrounding the concept of brain viability are also discussed. The article concludes by identifying some weak links and unknowns in current stabilization procedures and proposing future research objectives.
The objective of human cryopreservation (or cryonics) is to use cryogenic temperatures to preserve patients that are pronounced dead by today’s medical criteria with the possibility that future medical technology may be able to treat them. Technical objections to the science of human cryopreservation fall broadly into two categories: (1) that freezing humans causes ice formation that destroys cells; and (2) that patients who present for cryonics are dead.
Historically, advocates of human cryopreservation have responded to the freezing damage objection by pointing out that the argument that freezing results in massive intracellular ice formation, which causes cells to rupture, is a gross oversimplification of what currently is known about freezing damage in biological systems . Cryoprotectants such as glycerol and DMSO, which are routinely used to preserve cells at cryogenic temperatures, also reduce ice formation in complex organs and cryonics patients. And even if some freezing damage remained unavoidable and irreparable by contemporary medical technology, it might be reparable with more advanced technology. As long as ice damage is not of such a nature that the structural biochemical basis of identity and memory can no longer be deduced, the only technical limitation to resuscitation is the feasibility of cellular repair by molecular machines .
One claim that advocates of human cryopreservation couldn’t make, however, is that resuscitation of patients in human cryopreservation would not require any kind of repair at all. A fair degree of technological optimism has therefore always been a fundamental element of cryonics. The situation improved again at the dawn of the 21st century when the Alcor Life Extension Foundation started offering cryopreservation by means of vitrification. Instead of reducing ice formation by perfusing a patient with high concentrations of glycerol, new cryoprotectants (B2C and M22) were introduced that prevent ice formation altogether. For the first time good ice-free structural preservation was shown to be possible in some tissues, including the brain.
The current challenge is to achieve this degree of preservation without compromising cellular viability or causing fracturing during cryogenic cool down. The paradigm shift from mitigating ice damage to designing vitrification agents that further reduce toxicity is of such a nature that the possibility of reversible vitrification of humans merits serious scientific debate.
The prospect of reversible vitrification eliminates the freezing damage objection to human cryopreservation, but still leaves the second objection – that cryonics patients are dead – unaddressed. Currently all patients who present for human cryopreservation have been pronounced legally dead. How can legal pronouncement of death be reconciled with the prospect of future resuscitation?
The answer to this question revolves around the fact that most people who are currently pronounced dead still possess the neural biochemistry that constitutes identity and memory. Even in the case of advanced Alzheimer’s disease, a person is ultimately not declared dead because of the loss of personhood, but as a consequence of secondary whole brain death or cardiac and respiratory arrest. In essence, today’s medicine routinely prolongs life while allowing destruction of the person but pronounces death, using cardiac criteria, without loss of personhood.
Advocates of human cryopreservation argue that long term preservation of a terminally ill person is possible if the patient is maintained at cryogenic temperatures (currently at -196 degrees Celsius). This low temperature form of critical care allows a terminally ill patient to reach a time when medical technologies may be available for effective treatment. As a consequence, minimizing brain injury during this transition from terminal illness to cryogenic temperatures is of the utmost importance.
Although many pathophysiological events occur within minutes of cessation of blood flow (see below), necrosis of brain cells is a much more time-consuming affair and research on global permanent ischemia indicates this process may take 9-24 hours at room temperature. The claim that the brain, or the most vulnerable parts of the brain, cannot tolerate interruptions of blood flow in excess of 5 minutes does not refer to immediate destruction of brain cells but to the clinical observation that even short interruptions of oxygen and glucose to the brain can produce irreversible death of some brain cells (such as the CA1 neurons of the hippocampus) after resuscitation. Because normothermic resuscitation of the patient is not the objective of cryonics procedures, this observation is largely irrelevant to typical cryonics patients. Long term care at cryogenic temperatures may even offer some patients a better chance of preserving personhood than contemporary medical technologies can offer after successful resuscitation.
The objective of stabilization in human cryopreservation, therefore, is to keep the brain viable by contemporary medical criteria after the patient has been pronounced legally dead . Several reasons to strive for this goal have been proposed by cryonics authors.
The most fundamental reason to keep the brain viable by means of stabilization is that we do not completely understand to what extent a patient can sustain brain injury without destroying identity- and memory-relevant information, even allowing for sophisticated future molecular repair technologies. The idea of preserving viability by contemporary medical criteria is therefore perceived to reflect a prudent and conservative approach.
Assuming that the patient does not suffer from a serious identity- and memory-compromising disease, by preserving the brain in the state it was in immediately before cardiac arrest, the “only” obstacle to future resuscitation is a cure for the illness that interfered with the patient’s ability to function as an integrated biological system. For many patients this will also depend upon significant scientific progress in halting, or even reversing, the aging process itself to ensure that a patient will not go through endless futile cycles of pronouncement of legal death, cryopreservation, and resuscitation.
Another important reason to maintain the brain in a viable state through the cryopreservation process is that, even if long periods of warm cerebral ischemia and freezing induced cell damage are compatible with future resuscitation, resuscitation might be a more costly and time consuming effort in cases where damage is most severe. This will make the patient more vulnerable to social and political threats to his survival. All other things being equal, the expected resuscitation attempts of patients in cryostasis will likely have the character of “best in, first out; worst in, last out.”
Although one might expect a strong correlation between this principle and “first in, last out; last in, first out,” it should be kept in mind that the state of the art in stabilization and cryoprotection technologies is only one element that determines the quality of patient care. Indeed, though technological capability is paramount, it currently competes with more mundane issues such as rapid access to the patient, the risk of autopsy, hostile family members, and the quality of the personnel providing care.
Stabilization of the patient requires immediate intervention in order to meet the goal of brain viability. To this purpose providers of human cryopreservation deploy a standby team to ensure that knowledgeable and experienced cryonics technicians will be at the bedside of the patient at the moment of pronouncement of legal death.
To stabilize the patient, the team employs three kinds of medical interventions. First, blood circulation and oxygenation are restored to supply oxygen and nutrients to energy-deprived cells. Second, medications are administered to improve (cerebral) blood flow, prevent blood coagulation, and mitigate cerebral ischemia. Third, hypothermia is induced to reduce metabolic rate. If standby and stabilization are performed in a remote location, the patient’s blood is washed out and replaced with an organ preservation solution to prevent cold-induced agglutination (clumping) of red blood cells, blood coagulation during transport, and to provide metabolic support for the cells.
The challenge of securing brain viability after pronouncement of death demands a thorough understanding of the mechanisms of cerebral ischemia. If our understanding of the biochemical cascade that follows reduced or no blood flow enables us to maintain viability, let alone significantly extend the window of opportunity to secure this aim, the objection that human cryopreservation patients are irreversibly dead can be countered.
Cardiac arrest produces a state of global ischemia characterized by lack of blood flow to the brain and other vital organs. The length of time humans can withstand normothermic cardiac arrest without neurological injury after resuscitation is a complicated question; but in the absence of sophisticated resuscitation technologies, the limit is currently assumed to be around five minutes. A fundamental question in resuscitation science is whether this window of opportunity can be extended.
Although the adult human brain accounts for only 2% of total body mass it accounts for about 20-25% of total oxygen consumption. A significant amount of this energy is expended to maintain ion gradients across cell membranes. Because the amount of oxygen and glycogen that is stored in the brain is extremely limited, sudden loss of oxygen produces unconsciousness in less than 10 seconds, and a flat EEG in about 20 seconds. The first response of the brain is to switch from oxygen-based aerobic metabolism to glucose-based anaerobic metabolism, producing lactic acid in the process. Within about 4-6 minutes neurons completely run out of energy, setting in motion a complex multi-factorial biochemical cascade.
When brain cells run out of energy, membrane-bound ion pumps fail to maintain cellular homeostasis. Potassium leaves the cell and sodium and calcium enter the cell in unphysiological concentrations. Cytosolic calcium concentrations are further increased by release of calcium from intracellular calcium stores (e.g., the endoplasmic reticulum and mitochondria). This marked increase of cytosolic calcium overwhelms the ability of cells to buffer and sequence excess calcium, activating multiple biochemical pathways implicated in neuronal injury. The loss of physiological ion gradients across cell membranes also causes cytosolic edema as a result of a net influx of water.
Depolarization and calcium overload of brain cells triggers the release of massive amounts of the excitatory amino acid glutamate in the extracellular space. Without energy-dependent presynaptic glutamate reuptake, excitatory amino acid receptors are continually activated, leading to a sustained increase in cytosolic calcium. This again releases more glutamate into the extracellular space, triggering a positive feedback loop of excitotoxity-induced calcium overload. The role of calcium overload and excitotoxic amino acid neurotransmitter release in neural death is consistent with the fact that the brain cells that are most vulnerable to cerebral ischemia (such as the hippocampal CA1 neurons) have a relatively large number of excitatory amino acid receptors and calcium channels.
Intracellular calcium overload also activates calcium-dependent enzymes that break down membrane lipids and proteins, activate apoptosis (controlled cell death), and synthesize nitric oxide. Nitric oxide can be either beneficial or deleterious in ischemia, depending on its specific isoform. During ischemia and reperfusion, nitric oxide combines with superoxide to produce the harmful nitrogen radical peroxynitrite. Peroxynitrite and oxygen-derived radicals damage DNA, overactivating the nuclear DNA repair enzyme poly(ADP-Ribose) polymerase (PARP). Upon reperfusion, PARP will rapidly deplete vital energy sources for cell metabolism. Cardiac arrest-induced hypercoagulability and inflammation further contribute to neural injury by compromising microcirculation after reperfusion, resulting in the so called “no-reflow” phenomenon.
In the context of human cryopreservation it is important to understand the importance of optimum (micro) circulation. Securing brain viability is not the only reason for maintaining circulation during stabilization; long periods of warm ischemia with associated clotting and compromised cerebral circulation may frustrate subsequent attempts to adequately circulate the vitrification agent in the brain. As a consequence, cryoprotective perfusion may have to be abandoned early and parts of the brain will suffer ice formation. This relationship between mitigating cerebral ischemia and achieving optimum perfusion of the brain is another important reason for securing brain viability by contemporary medical criteria.
Although our understanding of the cellular pathophysiology of cerebral ischemia is still incomplete, it is clear that after the window of opportunity for securing brain viability by conventional resuscitation has closed, only combination treatment may achieve this goal. This conclusion can be derived from the fact that, further downstream in the ischemic cascade, just restoring adequate blood flow will not automatically reverse all the biochemical pathways that are activated by energy depletion of the cells. For this reason restoring adequate cerebral blood flow by cardiopulmonary support is only one form of treatment employed in human cryopreservation.
Unless access to the patient is significantly delayed, the first priority is to intervene in the ischemic cascade as early as possible. Because the most fundamental and upstream event is lack of blood circulation and oxygen, the importance of restoring blood circulation and oxygenation cannot be understated. In human cryopreservation this is called cardiopulmonary support because the goal is not to resuscitate but to stabilize the patient. Cardiopulmonary support is not only important to provide energy to the patient, it is also necessary to distribute medications and enhance surface cooling.
The ideal form of cardiopulmonary support would be to restart blood circulation using extracorporeal bypass to ensure adequate blood flow. Because this requires invasive surgery to obtain direct access to the circulatory system, this is not an option for immediate intervention. The second best option would be to perform open heart cardiac massage. This is not only an invasive procedure, but also would create a serious public relations issue in a hospital or hospice setting. Therefore, the preferred method of initial cardiopulmonary support in human cryopreservation is external cardiac massage using a mechanical device.
There are many advantages to using a mechanical device to do chest compressions and ventilations. Once set up, it frees team members to attend to other important tasks such as administering medications, drawing blood samples, and data acquisition. This is especially beneficial in situations where the number of team members is limited. It also prevents fatigue and the associated decline in performance and consistency of manual compressions. In cryonics, cardiopulmonary support for more than an hour is not uncommon. Another advantage is that if the device is used in combination with a portable ice bath, chest compressions can continue during transport of the patient from one location to the other. But the most important advantage of mechanical cardiopulmonary support is that adequate cerebral perfusion may require CPS techniques that are beyond the motor skills of even the finest paramedics.
The most basic form of mechanical cardiopulmonary support is a pneumatic piston-driven device that delivers chest compressions and ventilations at a consistent depth and rate. The most popular device in human cryopreservation has been the Michigan Instruments “Thumper.” The Thumper allows for a wide range of body sizes and compression depth and, in some models, has the option to do either continuous compression or compression and ventilations. The base of the Thumper slides onto a backboard under the patient and conversion from manual to mechanical CPS can be achieved within 30 seconds without interruption of chest compressions. A custom-made version of the Thumper has been produced that can be mounted on both sides of a portable ice bath, so that it can be used conveniently while the patient is partially immersed in melting ice.
In the mid 1980s a modality of delivering chest compressions was introduced that delivered chest compressions with high acceleration on the downstroke with the goal of improving forward blood flow. This technique, called high impulse CPR (HI-CPR), has received mixed reviews in peer reviewed literature to date . A more widely adopted refinement of conventional CPR has been active compression-decompression CPR (ACD-CPR). During ACD-CPR a suction cup is used to actively compress and decompress the chest. Venous return to the heart (cardiac preload) is enhanced by pulling up the chest during the recoil phase. This technique can be performed either using a manual suction cup or by modifying an existing mechanical device. In the mid 1990s, a custom designed Michigan Instruments Thumper combining high impulse and active compression and decompression was introduced to the field of human cryopreservation by cryonics researcher Michael Darwin. Like HI-CPR, ACD-CPR has received mixed reviews in peer reviewed literature . To date no ACD-CPR devices have been FDA-approved in the United States for use in resuscitation. Because human cryopreservation procedures can only start after pronouncement of legal death, this is an example where cryonics patients can benefit from a technology that is not available to the general public.
Another enhancement of CPS that optimizes cardiac preload is the use of an inspiratory impedance threshold valve. The valve is placed between the airway of the patient and the oxygen source and prevents air from entering the patient during the recoil phase of chest compressions. As a result, intrathoracic pressure is reduced, improving venous return to the heart. In the late 1990s Michael Darwin recognized the potential value of this mechanism, and built the first prototype for use in human cryopreservation. In 2005, two leading human cryopreservation service providers adopted the first commercially available impedance valve as a part of their stabilization protocols. Positive findings using this airway adjunct in conventional CPR and in conjunction with ACD-CPR culminated in a recommendation of the impedance valve in the American Heart Association’s 2005 guidelines for cardiopulmonary resuscitation.
The desirability and optimum rate of ventilations during CPR has been a much debated topic in emergency medicine. There is a growing consensus that frequent interruption of chest compressions for ventilations may do more harm than good. Especially in one-person CPR, interruption of chest compressions to give ventilations significantly reduces coronary perfusion pressure and reduces overall efficiency of cardiopulmonary resuscitation. Charles Babbs et al., assuming realistic parameters of lay person performance, evaluated a wide range of different compression to ventilation ratios using Monte Carlo simulations, and found that a ratio of 60:2 produces maximum oxygen delivery . In 2005 the American Heart Association announced a change from 2 ventilations for 15 chest compressions to 2 ventilations for 30 chest compressions for adult CPR.
As in conventional cardiopulmonary resuscitation, the patient can be ventilated by either manual or mechanical means. Airway access can be established by conventional emergency medicine techniques ranging from endotracheal intubation, in which a tube is placed directly in the trachea, to more invasive techniques like a tracheotomy, in which a surgical incision is made to access the trachea. Some cryonics organizations prefer the use of a dual-lumen intubation device called the Combitube because it is easier to place, and the other lumen can be used to administer Maalox to neutralize gastric hydrochloric acid.
Because of pre-mortem hypoxia and post-arrest ischemia, the question whether to oxygenate the patient or not is a complicated issue. During the early stages of the ischemic insult, oxygenation of the patient provides metabolic support. However, if rapid intervention does not take place, positive pressure ventilations with 100% oxygen further downstream in the cascade may produce more harm than good by generating oxidative stress. On the other hand, human cryopreservation medications generally include a variety of antioxidants and free radical scavengers, raising the issue whether administration of these medications can offset longer durations of cerebral ischemia. At any rate, it is undesirable to ventilate more than necessary. Ventilation volumes and ratios need to be adjusted for induction of hypothermia. Circulating free radical scavengers (and anti-inflammatory medications) prior to initiating ventilations or just using room air may be other modalities worth investigation.
Medications and fluids are administered in human cryopreservation to accomplish different objectives. Vasoactive medications and fluids are given to enhance cardiopulmonary support by supporting blood pressure and hydrating the patient. Anticoagulants, fibrinolytics, and antiplatelet agents are given to dissolve existing blood clots and keep the circulatory system open. Neuroprotective agents that intervene in different parts of the ischemic-reperfusion cascade are given as well. Some of the core medications of stabilization pharmacology are reviewed here.
The first medication administered is the general anaesthetic propofol. Propofol is a rapidly acting lipid soluble intravenous anaesthetic that enhances inhibitory synaptic transmission in the brain. Propofol is given for two reasons. The main reason is to prevent the patient regaining consciousness after legal death has been pronounced. Second, propofol reduces metabolic demand in the brain. Propofol has antioxidant and free radical scavenging properties, and there is also evidence that propofol delays the onset of excitotoxic neuronal death and reduces peroxynitrite mediated apoptosis . The major disadvantage of propofol, however, is a reduction in blood pressure.
Ideally, vasopressors such as epinephrine and vasopressin are immediately administered to selectively increase blood flow to the core of the body and the brain. Another vasoactive medication is S-methylthiourea (SMT), an inducible nitric oxide synthase (iNOS) inhibitor. SMT is primarily used to mitigate the production of inducible nitric oxide and associated formation of peroxynitrite. SMT increases mean arterial pressure (MAP).
Premortem pathology, post-ischemic inflammation, and cardiac arrest-induced blood stasis combine to produce hypercoagulability, which frustrates attempts to circulate medications and subsequent cryoprotection of the brain. A number of medications are administered to prevent and reverse this situation such as the anticoagulant heparin, the fibrinolytic streptokinase, and the antiplatelet agent aspirin. A volume expander like Dextran-40 is given to improve cerebral microcirculation by hemodilution. Dextran-40 has also been reported to mitigate cold-induced clumping of red blood cells, a problem that is more serious in human cryopreservation than conventional medicine because of the central importance of inducing deep hypothermia during stabilization procedures.
Medications to mitigate cerebral ischemia have always been a cornerstone of human cryopreservation stabilization pharmacology, ranging from interventions to support cell metabolism to molecules that specifically inhibit specific parts of the ischemic cascade. Current stabilization protocol includes a cocktail of antioxidants and free radical scavengers including vitamin E, melatonin, and alpha-phenyl-tert-butyl nitrone (PBN), as well as chemicals that inhibit excitotoxicity, inducible nitric oxide synthase, and PARP. PARP inhibition is a relatively recent addition to the cryonics stabilization protocol and intervenes further downstream in the ischemic cascade, providing a larger window of opportunity for intervention. The oncotic agent mannitol, primarily given to prevent and reverse cerebral edema, is also reported to have free radical scavenging properties. Another fluid that has been a mainstay in stabilization is tris-hydroxymethyl aminomethane (THAM), used to maintain pH balance. Although buffer therapy is a controversial topic in resuscitation medicine, maintaining physiological pH in a cryonics patient is important because some of the stabilization medications are pH sensitive.
Gaining access to the circulatory system of a patient to deliver these medications and fluids is not a trivial matter. Many patients have advanced arteriosclerosis and are severely dehydrated at the time of pronouncement, presenting a time-consuming challenge for achieving rapid intravenous access. Unless medical staff can be persuaded to leave an IV line in place, attempts to place an IV have often interfered with the goal of rapid stabilization. Following a renewed interest in emergency medicine to administer medications and fluids through the bone marrow, two cryonics organizations have recently adopted a technology to access the circulatory system through the sternum. Sternal intraosseous infusion is expected to reduce the time to establish vascular access (even by relatively inexperienced team members), in addition to other advantages such as the ability to infuse larger volumes.
The third, and most fundamental, intervention in stabilization is induction of hypothermia. Induction of hypothermia is the most fundamental intervention because it confers potent neuroprotection and is the first step in cooling down the patient to cryogenic temperature. Unlike cardiopulmonary support or pharmacologic treatment, induction of hypothermia is implied in the concept of human cryopreservation itself. This does not necessarily mean that induction of hypothermia is necessarily the first priority in stabilization. If immediate access to the patient after pronouncement is possible, cardiopulmonary support in conjuncture with vasoactive medications is the most time-efficient intervention to mitigate cerebral ischemia. A scenario where the cryonics team needs to choose between CPS, medications, or cooling generally indicates inadequate preparation, an uncooperative hospital, or sudden death. Immediately starting all three types of interventions is a core objective of stabilization procedures.
The importance of rapid induction of hypothermia in cryonics is routinely conveyed by referring to the rule that for every 10 degrees Celsius reduction in temperature metabolic rate is reduced by 50 percent. Although this Q10 = 2 rule is useful to communicate the effectiveness of hypothermia in reducing metabolic rate to the general public, it cannot explain the potent neuroprotective effects of hypothermia that are observed when the temperature is dropped by only a few degrees. It is possible that different parts of the ischemic cascade, such as excitatory amino acid release, are reduced by a rate vastly exceeding that expected by applying this Q10 value . Although the exact mechanisms of hypothermic protection need further elucidation, this phenomenon provides strong support for rapid cooling as a means to keep the brain viable after pronouncement of legal death.
Different degrees of hypothermia have been distinguished based on the core body temperature of the patient:
Mild hypothermia: 32-36 degrees Celsius
Moderate hypothermia: 28-32 degrees Celsius
Deep hypothermia: 18-28 degrees Celsius
Profound hypothermia: 5-18 degrees Celsius
Ultra profound hypothermia: 0-5 degrees Celsius
The objective in stabilization is to achieve ultra profound hypothermia as fast as possible. Under no circumstance is the patient frozen below 0 degrees Celsius during stabilization procedures and transport to the facility. This will cause freezing damage prior to cryoprotection and eliminates the ability to perfuse the patient with a cryoprotective agent.
The reason aggressive cardiopulmonary support is more effective as an immediate intervention is because induction of hypothermia by external cooling is relatively inefficient. The effectiveness of external cooling of the patient is further hampered by cold- and vasopressor-induced vasoconstriction. With limited peripheral blood flow, cooling efficiency is reduced.
Three different methods of external cooling of the patient are possible: cooling by evaporation, cooling by conduction, and cooling by convection. At high body temperatures, evaporating water is generally more effective in reducing body temperature than melting ice. The major limitation of this method of cooling is that it is impractical to continuously spray water on the patient and use a fan for evaporation. Its limitations during actual transport of the patient between locations are even more serious.
Packing the whole body in ice is the minimum that should be achieved during stabilization. In the case the cryonics team has a portable ice bath, the ice does not need to be bagged because this adds a layer of insulation between the patient and the ice. Emphasis is given to the areas that have large vessels close to the surface such as the anterior neck, the axilla, and the groin.
Convective cooling can be achieved by circulating ice water around the patient using a submersible pump. Circulating ice water around the patient in conjunction with cardiopulmonary support is the closest approximation of immersing a patient in streaming ice cold water that is compatible with other stabilization tasks such as cardiopulmonary support, medications administration, and monitoring.
A slight drop in temperature can also be achieved by introducing the stabilization medications and fluids at low temperatures. The advantage of this technique should be weighed against increased viscosity, and the risk that some medications and fluids that should not be chilled (such as Mannitol) will be included accidentally. Additional chilled fluids, or ice slurries, can be introduced intravenously to the patient, but care should be taken to ensure that the fluids are not hypotonic and (extreme) hypervolemia is prevented.
More effective methods to cool down the patient are generally more invasive. In gastric lavage a large bore gastric tube is used to introduce a chilled fluid by gravity and return it by suction. Because of the risk of pulmonary aspiration, this technique requires that the patient is intubated. In colonic lavage a chilled fluid is introduced through the rectum. In peritoneal lavage, a surgically placed catheter is used to introduce chilled fluids in the abdominal cavity. In a landmark cryonics case in 1995, the cryonics service provider BioPreservation used external circulating ice water cooling, colonic and peritoneal lavage and achieved cooling rates over 1.0 degrees Celsius per minute, by far the fastest cooling rates ever recorded during initial stabilization .
In the late 1990s Michael Darwin et al. developed a minimally invasive form of cyclic lung lavage to induce hypothermia. Although the feasibility of ventilating the lungs using fluids has been demonstrated since the 1960s, this liquid ventilation-derived technology represents a significant breakthrough for inducing rapid hypothermia in cryonics and emergency medicine.
Because all circulating blood must pass through the enormous surface area of the lungs, the lungs basically work as an endogenous heat exchanger. The fluid of choice in cold lung lavage is a perfluocarbon, a hydrocarbon in which the hydrogen atoms are replaced by fluorine atoms. Perfluocarbons are stable, inert, and have good oxygen and carbon dioxide carrying ability. Cyclic lung lavage only requires endotracheal intubation and can be started promptly after pronouncement of death. In conjunction with other internal and external cooling methods, cooling rates approaching those achieved by cardiopulmonary bypass are possible .
Holding the patient close to zero degrees Celsius during (remote) transport significantly reduces metabolic rate, but long stabilization and transport times may still risk loss of viability of the brain. Induction of (ultra) profound hypothermia can also produce adverse effects such as cold-induced red cell clumping and edema. To keep the brain viable during transport, the patient’s blood is washed out and replaced with an organ preservation solution similar to the organ preservation solutions used in conventional organ transplantation.
In an ideal remote stabilization case, surgery to gain access to the circulatory system is started immediately after pronouncement of legal death. A more typical scenario is to transport the patient to a cooperating funeral home where surgical access and blood washout is performed. In either scenario stabilization procedures should not be halted for most parts of the surgery. Vascular access is generally secured by cannulating the femoral artery and vein using sterile technique. Following femoral cannulation, the patient’s blood is substituted with the organ preservation solution using an air transportable perfusion device (ATP). The ATP also contains a heat exchanger and oxygenator to cool down and oxygenate the patient.
The organ preservation solutions that have been used for more than 20 years in cryonics are variants of a solution called MHP, which stands for Mannitol HEPES Perfusate. MHP is designed as an “intracellular” organ preservation solution to provide metabolic support and reduce hypothermia-induced passive movement of ions across cell membranes. Ischemia- and hypothermia-induced edema is minimized by the sugar-alcohol mannitol and the synthetic colloid hydroxyethyl starch. Other components include adenine and d-ribose to support cellular metabolism, HEPES and sodium bicarbonate to stabilize pH, and glutathione to mitigate oxidative stress. Some versions of MHP also include insulin to improve glucose uptake in non-neural tissue.
In a series of seminal experiments, cryonics researchers Mike Darwin, Jerry Leaf, and Hugh Hixon used MHP to recover dogs from three hours of ultra profound hypothermic asanguineous circulatory arrest, and five hours of low flow perfusion . These experiments support the theory that continuous physiological metabolic activity is not necessary for life. The experiments also indicate that the stabilization phase in human cryopreservation is potentially reversible by contemporary medical technology. The increased clinical interest in asanguineous hypothermia as an intervention for sudden cardiac arrest and hemorrhagic shock provides further validation of the research that was done by these early cryonics pioneers.
Evaluating Brain Viability
If securing brain viability by contemporary medical criteria is the goal of stabilization, how can we know if this objective has been achieved when the patient is not being resuscitated? Given that the patient’s brain is viable at the time of pronouncement of death, vital signs and blood chemistries during stabilization can be collected and compared to baseline values obtained prior to, or immediately after, the start of stabilization. Although this may provide a rough idea of the efficacy of stabilization, brain injury can be consistent with adequate cerebral blood flow and physiological blood chemistries. As our understanding of the pathophysiology of cerebral ischemia grows, more selective biomarkers of neural injury may be considered.
The central question of what exactly constitutes viability by contemporary medical criteria may not be as simple as it appears. It requires that we take several other related questions into consideration. What constitutes contemporary medical technology? How do we classify a patient who suffers brain injury that can be reversed by contemporary medical technology?
A good example of the complications surrounding the concept of cerebral viability is apoptosis of brain cells. Because the execution of apoptosis requires energy, DNA transcription, and protein synthesis, ischemia induced apoptotic cell death is only possible by restoration of blood flow. A somewhat troubling implication of this fact is that, during typical stabilization procedures, energy is supplied to assist cell death by apoptosis. On the other hand, because apoptosis is an organized and time-consuming process, the alternative – cell death by necrosis – is likely to be worse in cryonics.
Because cells undergoing (very) early apoptosis may still be considered viable, it is an interesting question if a patient in the early stages of neural apoptosis should be considered viable by contemporary medical criteria. This may depend on the question whether the activation of apoptosis can be reversed by contemporary medical technology. If this is not the case, the argument that the patient’s brain can be considered viable by contemporary medical criteria is problematic.
The science of human cryopreservation raises some important ethical issues. Although our understanding of the molecular mechanisms of cerebral ischemia and potential treatments is growing, irreversible neurological injury is still a common outcome in many cardiac arrest and stroke cases. The time separating onset of the insult and completion of cell death is generally sufficient to stabilize and cool down such patients to cryogenic temperatures. Human cryopreservation may not only offer a treatment option for patients that are considered dead by contemporary medical criteria, it should also be investigated as a treatment option for patients who have been resuscitated after serious cardiovascular and cerebral insults but who will lose all dignity if kept alive at normothermic temperatures after the insult.
The science of human cryopreservation has made significant advances since the concept was first popularized by Evan Cooper and Robert Ettinger during the 1960s. To most observers, cryonics is mainly a practical cryobiology problem that needs to be solved. This perspective neglects that even in a society where suspended animation has become routine, medical emergencies requiring rapid stabilization may still exist. It should also not be assumed that the technical possibility of reversible vitrification of humans will lead to widespread acceptance of the practice of cryonics. Stabilization of cryonics patients will remain a core component of cryonics procedures in the foreseeable future.
One consequence of this “cryobiology bias,” however, is that more generous resources have been allocated for research to achieve reversible vitrification than for research into stabilization technologies. This trend has been further reinforced by statements within the cryonics community questioning the value of stabilization. As a result, it should not be surprising that progress and quality of care in stabilization in cryonics has been variable.
Evidence-Based Cryonics (EBC) represents an approach to human cryopreservation in which all procedures should be supported by research and evidence-based medical practice. An important element of evidence- based cryonics is the awareness that findings in mainstream research and conventional medicine may need further investigation in a realistic cryonics context. An example of such an approach would be to validate neuroprotective agents in a model that includes cryoprotectant perfusion. Meticulous data acquisition during cases and meta-analysis of case reports should be used to generate a general clinical theory of human cryopreservation.
The concept of evidence-based cryonics is basically a restatement of the research driven approach to cryonics that was pioneered by Jerry Leaf and Michael Darwin in the late 1970s and 1980s. A comprehensive review of desirable research objectives is beyond this article, but some general directions in the area of stabilization are discussed.
Cardiopulmonary support may be further optimized by a detailed study of the physiology of forward blood flow during chest compressions. Modalities of CPS that better reflect the typical pathophysiology of cryonics patients before and during stabilization may be beneficial as well. A systematic review of reperfusion injury in cryonics should give more specific guidelines to determine when restarting circulation and/or ventilating the patient is beneficial. Progress on practical issues such as the noise and ease of use of mechanical chest compression devices is desirable as well.
Human cryopreservation stabilization pharmacology reflects the increasing understanding that the complex multifactorial nature of cerebral ischemia requires combination therapy. Current cryonics protocol may benefit from an up-to-date exposition of the pathophysiology of global cerebral ischemia, emphasizing the temporal aspects of combination treatment. The relationship between hypothermia and the pharmacodynamics and pharmacokinetics of various drugs is a topic that has not yet been addressed in much detail. Sternal intraosseous infusion is a promising technique to gain access to the circulatory system, but more remains to be discovered about its efficacy after administration of high-dose vasoactive agents and its interaction with active compression decompression CPS.
In addition, the specific mechanisms of cerebral protection by induction of hypothermia require further elucidation. Awareness of the possibility that induction of ultra profound hypothermia may also produce detrimental effects in terms of brain viability is important. Induction of hypothermia by packing the patient in ice should be complemented by a variety of other cooling modalities, including routine use of liquid ventilation. It is possible that lung lavage and CPS techniques have to be modified to optimize the synergetic performance of both interventions. Monitoring techniques may be modified for liquid ventilation as well. In a hospice or rescue vehicle, some of the other internal cooling techniques discussed above should become routine.
Although the cold organ preservation solutions in cryonics give extended brain viability for asanguineous low flow perfusion, the current practice to use the solution as a static blood substitute during (air) transport of the patient to a cryonics facility means that the goal of stabilization cannot be secured for the majority of remote cases. The quality of care in cryonics may benefit from more emphasis on the element of perfusion during washout. A better understanding of how blood washout may contribute to reperfusion injury is desirable. New cold organ preservation solutions should be validated in models of warm and cold ischemia.
Without good scribe work and comprehensive data acquisition, rapid progress in the science of human cryopreservation will be limited. A renewed interest in physiological monitoring in cryonics is evidenced by the introduction of new technologies such as bedside blood gas analyzers (such as the i-STAT) and equipment that can produce a complete respiratory profile (such as the CO2SMO). This opens opportunities for real-time intervention that were not available in the earlier days of cryonics. It may also provide more specific answers to the question of how well the goal of stabilization is achieved for a particular patient.
Impressive progress in cryobiology and a growing understanding of the molecular mechanisms of cerebral ischemia have made the prospect for human cryopreservation to become a part of critical care medicine look better than ever.
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