130 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



to body temperatures never reached before with subsequent recovery (about five 

 degrees below the freezing point), and (2) to study the physiology of hypothermia 

 unmasked by the effect of anesthetics or other drugs. 



The routine technique as applied to rats consists of enclosing the animal in a jar 

 of two liters capacity, placed in a refrigerator at 0°-5° C. By the end of two hours 

 the concentration of oxygen in the residual air decreases to 2 to 4 per cent, CO2 to 

 about 15 to 16 per cent, and the body temperature falls to between 15° and 20° C. 

 (Andjus and Smith, 1955). A summary of some of the results of our investigations 

 along these lines follows : 



The critical and the lethal fall of oxygen tension. The heat production of an 

 animal exposed to a gradual fall of oxygen tension does not begin to diminish until 

 the oxygen tension has reached a critical level. The value of the "critical fall of 

 oxygen tension" (CFOT) may amount to as much as 70 mm. Hg in different ex- 

 perimental conditions. It is influenced, for example, by the ambient temperature and 

 the temperature of the environment to which the animal was adapted prior to the 

 experiment (Giaja, 1953). 



In order to answer the fundamental question whether the value of the critical fall 

 of oxygen tension depends on the rate of oxygen consumption, we have done a 

 series of experiments which will be described now. Four different groups of rats 

 with different metabolic rates were exposed to a gradual decrease of oxygen tension 

 and the values of their CFOT determined. Each experimental group had a group of 

 control animals. In three of the experimental groups the oxygen consumption rate 

 was very different from that of the corresponding control groups, due to thyroid 

 feeding, hypophysectomy or different age (T, II, and IV, respectively, in fig. 1). 

 It can be seen from fig. 1 that a highly significant difference in oxygen consumption 

 rates between the experimental group and the corresponding group of controls was 

 not necessarily accompanied by a significant difference in the values of CFOT 

 (I and II in fig. 1). On the other hand, a highly significant difference in the values 

 of CFOT was found between one of the experimental groups and its control group 

 whose oxygen consumption rates were almost identical (III in fig. 1). It may be 

 concluded that the value of the CFOT is not necessarily influenced by the rate of 

 oxygen consumption. In other words, the critical fall of oxygen tension necessary 

 to decrease the production of heat cannot be predicted on the basis of a simple oxy- 

 gen uptake measurement. 



The results in fig. 1 indicate, however, that although the value of CFOT is not 

 necessarily determined by the rate of the total oxygen consumption, it may be in- 

 fluenced by the value of the "complementary" oxygen consumption. This comple- 

 mentary oxygen consumption, a result of the chemical thermoregulation, is calcu- 

 lated as total oxygen consumption (measured at a given ambient temperature) 

 minus basal oxygen consumption (measured at 30-32° C). In the experimental 

 animals a greater complementary oxygen uptake was always accompanied by a 

 smaller CFOT, and was independent of the value of the total oxygen uptake which 

 might equal or differ from that of the controls (III and TV in fig. 1). 



The results represented in fig. 2 show that the /('///(// fall of oxygen tension at a 

 given ambient temperature is likewise not necessarily related to the rate of the 

 total oxygen consumption. The values of the lethal fall of oxygen tension found 



