138 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



Fig. 6.— a schematic classification of hypothermia. Ti = normal body temperature; Tj = 

 inversion temperature (inversion of temperature coefficient, peak of metabolism) ; T3 = critical 

 temperature (onset of the fall of metabolic rate below the prehypothermic level) ; T^ = limit- 

 temperature (starting the depression below the BMR level); Tg = suspension temperature 

 (respiratory, then cardiac arrest), Tg = freezing point; T^ = supercooling limit. Data based 

 upon the rat. 



The clinician should be aware of the physiological characteristics of the different 

 states in order to choose the most suitable level of hypothermia. If a substantial 

 decrease of the metabolic rate is desired, fig. 6 shows that this can be obtained in the 

 animal only in the range of cold narcosis or below. The same is true if "physical 

 anesthesia," induced by cold only, is wanted. Should a long reversible cardiac arrest 

 be desired, the range of suspension would be best. 



The experimental physiologist and biochemist can find many interesting problems 

 associated with hypothermia. For example, in the range of active defense, at tem- 

 peratures between 20 and 24° C, the glomerular filtration is still present in the rat's 

 kidney, while the processes of reabsorption of some ions (sodium) are inhibited 

 (Andjus, data presented to this Conference). The fixation of iodine and the syn- 

 thesis of iodinated organic compounds in the thyroid gland, though greatly slowed 

 down, are still present even in an animal maintained at a body temperature of 15 

 to 18° C. (Andjus ct al., 1954). 



Maintained hypothermia. Comparing the time limits compatible with survival 

 in the lowest ranges of hypothermia (from 25° C. downward), we found that the 

 maximal survival time in rats decreases from about 50 hours at 23-25° C. of body 

 temperature to two hours at 0° C. It is obvious, moreover, that the cause of death 

 cannot be the same in the animal maintained at 0° C. of body temperature, with 

 arrested circulation and respiration, as in the animal kept at 23° C, with a rela- 

 tively high metabolic rate and even the capability of locomotion. If survival at 0° C. 

 (determined by resuscitation) is compared to survival in other ranges of hypo- 

 thermia with artificially arrested circulation, then our data speak in lavor of the 

 lowest range of temperatures. 



