HUMAN HIBERNATION FOR SPACE FLIGHT: UTOPISTIC VISION OR REALISTIC POSSIBILITY?

摘要

In view of the huge distances from earth to potential destinations in the outer solar system and beyond, the idea of lowering the body temperatures of astronauts so as to favor future space missions by a kind of "artificial hibernation" was born. However, mammalian hibernation, though often regarded as an "experiment of nature" for hypothermic reduction of metabolic rate, is now known to reflect an endogenous hypometabolism with subsequent lowering of body temperature, the limits of cooling being due to a critical metabolic rate rather than to a lower temperature level. Moreover, hypothermia in human adults is accompanied by a number of adverse effects, imposing narrow limits on the depth and duration of cooling to be achieved without harm. Thus, hypothermia per se does not seem to be a promising way to reduce human metabolic rate for long-range expeditions into space. On the other hand, natural hibernation is characterized by a "switching-off' of metabolic size allometry, i.e., of the increase in specific metabolic rate with decreasing body mass to be found in all living beings. A similar metabolic reduction has been observed during intrauterine life, with the mammalian fetus behaving "like an organ of its mother", and the metabolic increase up to the regular size-related level taking place only after birth. Hence, the endogenous hypometabolism occurring in hibernators might be founded on a metabolic strategy that is common to all mammals at birth. Based on these considerations, it does not seem fully unreasonable to assume that a "hibernation-like" reduction in metabolic rate could be re-inducible in human adults, provided the resulting drop in body temperature would be limited by appropriate, "fetal-like" ambient conditions. Even though it remains doubtful, whether this kind of metabolic reduction would be achievable without a loss of mental consciousness or physical fitness incompatible with the requirements of space flight, it could also open new perspectives for the medical treatment of trauma victims, stroke patients, and asphyxiated neonates who would all profit from a protective reduction in metabolic rate.