Factors contributing to the successful reanimation of mice cooled to less than 1°C

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Abstract
Nearly all mice cooled to less than 1°C can be reanimated if first cooled slowly to less than 20°C in a sealed vessel of appropriate size. During this period the O2 content of the vessel falls to 6%, the CO2 increases to 11% and the humidity to saturation. An analysis shows that hypercapnia is chiefly responsible for the success of the method, though hypoxia and increased humidity also are beneficial. This holds whether the percentage of recoveries, time and temperature of recovery of postural reflexes, or incidence of hind leg weakness is used as an index of success. During resuscitation the animals received artificial respiration with 100% O2, 95% O2 + 5% CO2, 90% O2 + 10% CO2, or air. Comparisons of percentage of recoveries, long term survivals and uninjured hind legs show that 95% O2 + 5% CO2 gives the best results and 100% O2 the poorest. These findings demonstrate the benefits of CO2 in deep hypothermia.