Tolerance To The Combined Effects Of Cold And Of Abnormal Atmosphere

Abstract

The following types of relationship between the effects of cold (external and internal) and different forms of anoxia will be discussed on the basis of data from animal experiments, especially from the point of view of tolerance limits and with some reference to the underlying mechanisms.

1. Resistance to external (environmental) cold is impaired by hypoxia (and hypercapnia) which interferes with thermoregulation and renders difficult the maintenance of thermal homeostasis.

The critical tension of oxygen (below which oxygen consumption starts to decrease) can be taken as a measure of the resistance of the body thermostat to the hypoxic load. It is shown that this parameter is not necessarily related, as often assumed, to the overall rate of O consumption (thermogenesis), but only to complementary heat production, i. e. that facultative part of total thermogenesis which is under the control of thermoregulatory centres.

Hypoxia may act as a hypothermia-inducing agent in a cold environment which by itself can be tolerated without change of body temperature. On the other hand, even such changes of ambient atmosphere, which at higher environmental temperatures can be compensated by physiological regulatory mechanisms, may induce in the cold serious disturbances of thermal homeostasis. From the point of view of homeostatic resistance, therefore, a mutual potentiation of the effects of cold and anoxia may be described.

2. From the survival point of view, however, internal cold (hypothermia), induced by anoxia in a cold environment, may have a protective value; the fall of body temperature renders the homeotherm capable of surviving under anoxic conditions which would be lethal at normal body temperature. In other words, failure of the body thermostat to resist anoxia may be of survival value under severe anoxic conditions. This will be illustrated by quantitative data on the relationship between critical and lethal oxygen tensions in different thermal environments; and conditions will be described under which a decreased resistance to anoxia, as far as maintenance of thermal homeostasis is concerned, causes an increased tolerance to anoxia evaluated by survival criteria. The relationship between body temperature and tolerance to hypoxia will be analysed with special emphasis on the relative independence of the protective effects of hypothermia from its effects on the rate of oxygen uptake.

3. Finally, although it can protect against anoxia, internal cold (hypothermia), below a given level of body temperature, causes anoxia at the tissue level in spite of a normal or even increased oxygen tension in the ambient air. In the extreme, hypothermia through its basic inhibitory effect on life processes, causes the cessation of oxygen supply and transport (respiratory and circulatory arrest). At the same time, however, through its protective effect, it renders the organism capable of tolerating relatively long periods of such “suspended animation” (or “clinical death”).

Time and temperature limits of suspended animation will be defined and correlated with data on brain metabolism.