Cephalon Cooling Curves, Practice and Theory: A Brief Progress Report By R. Michael Perry
Introduction The initial stages of cryopreservation of a cryonics patient involve cooling from body temperature to the temperature of water ice (from approximately 37°C down to 0°C). Ideally, this cooling (and further cooling down to cryogenic temperatures, -100°C or lower) should happen as fast as possible to minimize damage from warm ischemia. Though the effects of warmer temperatures on tissue are complex, as a start in estimating or quantizing them we would like to know simply how the temperature varies as a function of time at different points within the cooling subject. Toward this end, temperatures are monitored using probes as cryopreservation proceeds, but opportunities are limited by the necessity of minimizing any possible harm to the patient while cryopreservation is in progress.
with nasopharyngeal and average temperature also plotted.1 (Further details, some involving other cooling protocols, will be found in the reference cited.) Results representing an average over 13 individual cases are shown in fig.1 for the different cooling curves. (Note: the curves for “Naso” and “Brain Surf” are nearly coincident.)
Recent work at Oregon Cryonics with cadaver cephalons has furnished additional data on temperatures within the brain during various cooling protocols. Mathematical modeling of such processes can furnish useful insights into problems that might be encountered in the Figure 1. Cephalon cooling data from Oregon Cryonics, used with kind permission. clinical (cryonics) setting, as well as serving as a low-cost, noninvasive adjunct and possible alternative to expensive and invasive laboratory procedures. For the mathematical work, software was created in Mathematica Here we compare experimental cephalon cooling curves as the for modeling heat flow in a sphere, based on the work of Art temperature is reduced from body temperature (37°) to near 0° Quaife in the 1980s2, to approximate the cooling of an isolated over an eight-hour period, with temperatures calculated from a cephalon. Quaife’s simplifying assumptions, adopted here, spherical model of the human head, based on work of Art Quaife are that initially the sphere is at a temperature that is uniform in the 1980s.2 The calculated cooling curves overall showed throughout and that the surrounding medium is likewise at a good, sometimes excellent, correspondence to the experimental uniform but lower, temperature. Further, the heat conductance curves, with some unexplained discrepancies perhaps pointing is uniform and independent of temperature within the sphere, to deeper insight to be gained from further study. which does not generate heat but only loses it as cooling Main Results
proceeds. Similarly, there is uniform, temperature-independent conductance out of the sphere into the surrounding medium.
For the Oregon Cryonics results, cephalons starting at body temperature (37°) were immersed in an unstirred ice water bath, and temperatures were tracked as cooling proceeded toward water ice temperature (0°). Depths within the brain at which temperatures were monitored consisted of core (approximate brain center, 8 cm. downward from calvarium vertex, exterior top of skull), mid-range (5 cm. down) to surface of the brain,
With these assumptions, plots were obtained from calculations of cooling curves for the temperature at different points (radii r) within the sphere as a function of time t, ranging from core (r = 0) to surface (r = 1), plus an average temperature for the whole sphere. (Due to symmetry, the temperature within the sphere depends only on the radius r and time t.) The temperature range
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