X-Message-Number: 33144 From: Date: Sun, 26 Dec 2010 21:20:03 EST Subject: Scoring Cases Content-Language: en >Any attempt to quantify damage is, I think, valuable, so long as we realize that it's partly >handwaving at this point. And of course those who have the greatest confidence in molecular >repair of the brain will feel that it's mostly irrelevant anyway. Careful reading of what I wrote should make it clear that the scoring system I generated is an approximation to the point of being almost arbitrary. And I suppose it is fair to call it hand waving; in precisely the same way the first attempts to catalog the stars, or explain the motion of heavenly bodies was hand waving. This will be the case with any attempts to score injury or project survival based on surrogates, absent the kind of feedback that is really required: recovery of a living system which exhibits functions that can be assessed. Even the most sophisticated imaging technology currently available can't show us a memory, or describe the structural determinants of personal identity - or if they can, we don't yet know it. This is, in fact, the critical Achilles heel of cryonics because it both enables its existence by providing hope, and generates the principal cause of its downfall: over-optimism to the point of self destruction. We should be mindful that there is no such thing as a quantum of hope. Hope, by its very nature, is infinite: and therein lies its great power to nourish and sustain, and its even greater power to delude and corrupt. >The frustrating part is that Steve Harris made the most sophisticated attempt to >quantify ischemic inujury, but never finished writing the paper, so far as I know. He >derived a number that he referred to as the "E-HIT" score, meaning Equivalent >Homeothermic Ischemic Time. See >http://www.depressedmetabolism.com/2008/07/10/critical-cooling-rate-to-prev ent->ischemic->brain-injury/ Steve Harris' E-HIT paper is indeed valuable, and it is unfortunate that he did not complete it. >The problem with Darwin's approach is that he is applying a linear formula to a >phenomenon which cannot be linear. That is, the damage caused by the tenth minute >of warm ischemia is >highly unlikely to be the same as the damage caused by the >first. Actually, this is the key flaw in Steve Harris' approach, as well as in Mike Perry's effort to quantify ischemic injury with his "Measure of Ischemic Exposure "(MIX). Both of these approaches to quantifying ischemic injury rely on the "Q10 rule" which posits that each 10oC decrement of temperature reduction (below 37oC) results in an approximate halving of metabolic rate (in man), or to be more precise, a reduction of metabolic activity by a factor of ~2.2 (where Q is oxygen consumption (O2 used per unit of time) which decreases by 1/2.2 with each 10oC drop in body temperature).[2] BTW, its important to keep in mind that Q10 is a unit-less quantity, as it is the factor by which a rate changes, and is thus a useful way to express the temperature dependence of a process. In point of fact, the Q10 rule (or more accurately, the Arrhenius equation from which it was derived) serves as one of the three pillars upon which human cryopreservation rests; [*] i.e., continued reduction in temperature eventually results in the slowing of metabolic and catabolic activity to the extent where, at approximately the boiling point of liquid nitrogen (-196 oC ), all biochemical change is arrested, more or less indefinitely.[3] [*] The other two pillars are the information theoretic criterion of death and the assumed continued advance of technology and medicine. The Q10 rule shows surprising constancy across species, with the value being typically between 1 and 3 and, under conditions of hypothermia, has been verified as operational in the brains of rats, dogs and men to ~5oC, at a value of ~2.2.[2] The decrease in metabolic rate predicted by the Q10 rule is exponential; thus, a decrease in body temperature from 37oC to 17oC results in a decrease in metabolic rate by a factor (1/2.2)2 = 1/4.8. If the Q10 rule is applied to the human brain, using the tolerable limit of cooling before ice formation occurs inflicting freezing damage (~0oC), the predicted slowing of catabolism during ischemia would be such that each hour spent at 0oC would be the equivalent of approximately three and a quarter minutes spent under conditions of normothermic ischemia ([60 min]* 2.2-3.7 = 3.24). [I would like to pause to note that the Q10 rule is much abused in biology, but that's another pot at another time.] The Q10 rule has important implications for surgery employing deep hypothermic circulatory arrest (DHCA) where there is the need to bound the safe period of cold ischemia with a high degree of confidence. In 1991 Greeley, et al.,[2] derived an equation for approximating the safe circulatory arrest time at any temperature; the Hypothermic Metabolic Index (HMI) Two important caveats accompany the HMI, and they are that the hematocrit (HCT) and pH be taken into consideration when making the calculation. HCT determines the hemoglobin decay curve that will take place during the period of hypothermic circulatory arrest (in essence the stored oxygen available in the blood at the time that circulation is interrupted). The pH strategy management strategy management employed during cardiopulmonary bypass (CPB) will affect cerebral blood flow and thus may impact brain metabolic housekeeping. Use of pH stat management[*] results in higher cerebral blood flow (CBF) during CPB and thus, typically, better brain oxygenation and overall metabolic status at the time circulatory arrest begins.[4] The advantage that the HMI enjoys over the Q10 rule is that it has been empirically "proved" in humans via the Boston Circulatory Arrest Trial.[5],[6] [*] The Boston Circulatory Arrest Trial was carried out using alpha stat pH management which is no longer used by most centers for pediatric cardiac surgery involving DHCA. The Importance of Cold Scission: Applying the Q10 rule to cryopatients, or to dogs or rats for that matter, would suggest that 3 hours of cold ischemia is the limit beyond which recovery (absent reparative therapies) would be impossible, and this is indeed the case. Application of the Q10 rule to cryopatients who experience prolonged periods of cold ischemia during Transport, on the order of 24 to 72 hours, would suggest a grim situation pertains; indeed one where decomposition has begun. However, there are problems in extending the Q10 rule over long periods of time at temperatures close to 0oC; with apparent contradictions surfacing in the form of successful preservation of meat and other foodstuffs by simple refrigeration (~4-10oC) for prolonged periods of time,[7] and of even more relevance, the successful storage of human organs (which are comparably sensitive to the brain in terms of cold ischemic injury), for periods of 48 to 72 hours at 1-4oC[3].[8],[9],[10],[11] Preservation of ischemia-intolerant organs such as the liver and kidney is made possible not by any sophisticated interruption of metabolism, but by the use of intracellular organ preservation solutions which act primarily by inhibiting cellular edema and scavenging free radicals.[12] So, while the Q10 rule predicts the mammalian brain's response to ischemia (at least to ~5oC) reasonably well,[13],[14],[15],[16] it does not predict the behavior of other ischemic mammalian organs under the conditions of cold storage for transplantation. Similarly, preservation of foodstuffs by refrigeration and prolonged storage of organs near 0oC are possible because the Q10 rule does not take into account several important facts; the first, and probably most important of which, is that much of the metabolic and catabolic activity characteristic of biological systems is facilitated by the catalytic action of enzymes. In fact, biology as we know it is largely an artifact of the greatly accelerated speed of chemical reactions made possible by enzymes, as compared to the rate of reaction predicted on the basis of the Arrhenius equation.[17] Enzymes are proteins with complex shapes - shapes that are essential to their action as facilitators of chemical reactions - and these shapes are critically dependent upon the structure of the enzymes - in particular, their folding pattern. Profound and ultraprofound hypothermia can destabilize the folding of proteins resulting in a loss of stereospecificity in the case of many enzymes. This phenomenon was first described during cooling of enzymes to below 10oC by Irias and Olmstead in 1969, who referred to it as "cold scission," or "cold lability," and noted its effectiveness in halting their biochemical activity.[18] Additionally, phase changes in the non-aqueous lipid components of cells, brought on by deep cooling, can also relieve these molecules of their normal physical mobility and thus their availability for biochemical activity.[19] Additionally, enzymes embedded in lipds that undergo phase change upon cooling to below room temperature may be spatially inhibited by being confined in the solidified membrane.[20] Another factor that critically effects cell viability in both hypothermia and in ischemia is the Gibbs-Donnan Equilibrium; an unstable situation occurs in a solution if one side of a semi-permeable membrane contains a solution consisting of a permeable cation such as K+ with an impermeable anion (negatively charged protein), whilst the other side contains a solution of K+ and Cl-, both of which are permeable to the membrane with the K+ concentrations being equimolar on both sides of the membrane. The effectiveness of intracellular organ preservation solutions provides a clue that, at least near 0oC, it may be the case that much of the cold ischemic injury predicted by the Q10 rule (and which is in fact observed to occur) results from not from biochemical activity, per se, but rather from biophysical changes which proceed in the absence of metabolism or catabolism. Under normal metabolic conditions approximately 1/3rd of resting cellular energy expenditures are on ion homeostasis. The protein rich intracellular milieu is positively charged, and the sodium chloride (NaCl- ) rich extracellular milieu is negatively charged. Because NaCl- is osmotically active, movement of NaCl- from the extra- to the intracellular space across the cell membrane (to balance the charge difference represented by the positively charged intracellular protein; the Gibbs-Donan Equilibrium), the result is cellular edema. It is cellular edema, and the biophysics of the Gibbs-Donan Equilibrum, that appear to be a major driver of cold ischemic injury. This is antagonized by intracellular organ preservation solutions by removing most of the offending sodium from the extracellular spaces and replacing it on a roughly equimolar basis with cell membrane impermeable osmotically active species; typically sugars such as lactobionate and raffinose or the sugar-alcohol, mannitol. Because of enzymatic inhibition due to chilling, and especially if impermeant species have been used to replace the edema causing small extracellular ions in cold stored brains, simple metrics that employ the Arrhenius equation cannot be used to quantify warm or cold ischemic injury in cryonics (or in organ preservation). Much as is the case when biological or chemically reacting systems are rendered into the solid state by vitrification or desiccation, the Arrhenius equation ceases to be of direct use. Mike Darwin References 1. Hillyard Industries I: Vindicator+ Technical Data Sheet #168. In. St. Joseph, MO: Hillyard Industries, Inc; 2010. 2. Greeley W, Kern, FH, Ungerleider, RM. et al.: Cerebral metabolic suppression during hypothermic circulatory arrest in humans. The Annals of Thoracic Surgery 1999, 67(6):1895-1899. 3. Hixon H: The question column: How cold is cold enough? Cryonics 1985, 6(1):19-25. 4. Bellinger DC WD, duPlessis AJ, Rappaport LA, Jonas RA, Wernovsky G, Newburger JW.: Neurodevelopmental status at eight years in children with dextro-transposition of the great arteries: the Boston Circulatory Arrest Trial. J Thorac Cardiovasc Surg 2003, 126(5):1385-1396. 5. Wypij D, Newburger, JW, Rappaport, LA, duPlessis, AJ, Jonas RA, Wernovsky, G, Lin, M, Bellinger, DC.: The effect of duration of deep hypothermic circulatory arrest in infant heart surgery on late neurodevelopment: the Boston Circulatory Arrest Trial. J Thorac Cardiovasc Surg 2003, 126(5):1397-1403. 6. Ungerleider R, Gaynor, JW.: The Boston Circulatory Arrest Study: An analysis. J Thorac Cardiovasc Surg 2004, 127:1256-1261. 7. Lorentzen G: Food preservation by refrigeration, a general introduction. International Journal of Refrigeration 1978, 1(1):9-12. 8. Ross H MV, Escott ML.: 72-hr canine kidney preservation without continuous perfusion. Transplantation 1976, 21(6):498-501. 9. Sung D, Woods, JE.: Forty-Eight-Hour Preservation of the Canine Liver. Ann Surg 1974, 199(4):422-426. 10. Monden M, Fortner, JG.: Twenty-four- and 48-hour canine liver preservation by simple hypothermia with prostacyclin. Ann Surg 1982, 196(1):38-42. 11. Todo S, Hamada, N, Zhu, Y, Zhang, S, Subbotin, V, Nemoto, A, Takeyoshi, I, Starzl, TE.: Lazaroid U-74389G for 48-hour canine liver preservation. Transplantation 1996, 61(2):189-194. 12. Belzer F, Southard, JH.: Principles of solid-organ preservation by cold storage. Transplantation 1988, 45(4):673-676. 13. Michenfelder J, Milde, JH.: The effect of profound levels of hypothermia (below 14 degrees oC) on canine cerebral metabolism. J Cereb Blood Flow Metab 1992, 12(5):877-880. 14. Haneda K, Thomas, R, Sands, MP, Breazeale, DG, Dillard, DH.: Whole body protection during three hours of total circulatory arrest: an experimental study. Cryobiology 1986, 23(6):483-494. 15. Drabek T, Fisk, JA, Dixon, CE, Garman, RH, Stezoski, J, Wisnewski, SR, Wu, X, Tisherman,, SA K, PM.: Prolonged deep hypothermic circulatory arrest in rats can be achieved without cognitive deficits. Life Sci 2007, 8(7):543-552. 16. Goldzveig S, Smith, AU.: A simple method for reanimating rats and mice. J Physiol 1956, 132(2):406-413. 17. Benjamin-Cummings T: Chemical Kinetics, Third Edition. Weinheim: Wiley-VCH Verlag GmbH & Co. KGaA; 1997. 18. Irias J, Olmsted, MR.: Pyruvate carboxylase. Reversible inactivation by cold. Pyruvate carboxylase. Biochemistry 1969, 19:91-98. 19. Avery S, Lloyd, D, Harwood, JL.: Temperature-dependent changes in plasma-membrane lipid order and the phagocytotic activity of the amoeba Acanthamoeba castellanii are closely correlated. Biochem J 1995, 312((Pt 3)):811-816. 20. Zakim D, Kavecansky, J, Scarlata, S.: Are membrane enzymes regulated by the viscosity of the membrane environment? . Biochemistry 1992, 31(46):11589-11594. 21. Perry R: Towards a measure of ischemic injury. . Cryonics 1996, 17(2):21. 22. Harris S: Initial cooling in cryonics from body temperature to ice temperature: Physiologic and physics theory, quality control proposals, historical cryonics case analysis examples, lab experimental results, literature review, numerical recipe examples, and practical summaries and recommendations for the future. In. Rancho Cucamonga, CA: Critical Care Research; 2003. --- Content-Type: text/html; charset="UTF-8" [ AUTOMATICALLY SKIPPING HTML ENCODING! ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=33144