102 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



Mares using the European ground squirrel and Dubois''- using the marmot studied 

 the circulation by injecting indigo carmine into the blood stream. Although they 

 intended to demonstrate the circulatory conditions that existed during deep hil)er- 

 nation, it is evident that they were actually observing the distribution of blood dur- 

 ing the wakening process, for the procedure could not have failed to arouse the 

 animals. Mares showed that the dye first appeared in the cranial portion of the body 

 and Dubois noted that the dye seemed to be held back at the capillaries. Thus a 

 differential vasoconstriction during arousal has been reported for three species of 

 hil)ernators, and it seems reasonable that it occurs in the other species. Lyman and 

 ChatfiekP-^ have also noted that the fore and hind feet of hamsters, which are quite 

 pink during hibernation, turn pale soon after arousal begins, but the pigmented skin 

 of most other species does not permit this observation. 



The heart. As soon as a hibernating hamster is disturbed, any A-V dissociation 

 which may be present immediately disappears, although the heart rate may not 

 change"" (fig. 6). Before any increase in temperature may be recorded, however, 

 the heart rate increases and continues to do so until it reaches as much as 550 beats 

 per minute as the hamster attains the 37° C. temperature.il II 



There has been much concern over the problem of whether the relationship of 

 heart rate to temperature is a linear function. Knowlton and Starling^-"' and Taylor^-'' 

 found a linear relationship in both cold and warm-blooded animals. Other investi- 

 gators have obtained a nonlinear curve in the isolated and ])erfused hearts of frogs, 

 rabbits, cats and dogs.^-"' ^^^' ^"^' ^^° Some workers have reported an exponential 

 curve in winter and a linear curve in summer in isolated frog hearts/^^- ^''- Endres, 

 ct al.,'^* obtained exponential curves from a marmot awakening from hibernation. 

 Their experiment is open to criticism, however, on the ground that they placed 

 thermocouples in the heart itself, which might have disturbed conduction, and the 

 temperature range covered was small. Chatfield and Eyman,'" in their studies of 

 arousing hamsters, found that as body temperature rose the heart rate also increased, 

 at first slowly, and then more rapidly, and eventually linearly. They ascribed the 

 change in the slope of the curve to an increased effectiveness of sympatheti co-adrenal 

 activity, since Gellhorn^'° had shown that epinephrine increased the temperature 

 coefficient of the heart (fig. 15). 



When the logarithm of the heart rate of waking hamsters was plotted against the 

 reciprocal of the absolute temperature a straight line was not ol)tained. showing that 

 the phenomenon did not fit the Arrhenius equation which describes simple physico- 

 chemical processes (fig. 16). This slowing was not due to vagal action, for a plot of 

 the heart rate of completely atropinized animals was precisely similar to that of 

 normal waking hamsters (fig. 17 ). The lack of vagal action was not due to inability 

 of these nerves to function at low temperatures, ior stimulation of the cut end of 

 the right vagus at temperatures as low as 10" C. caused slowing of the heart. These 

 findings may be compared with those of Badeer'"'' who studied the influence of tem- 

 perature on the denervated heart-lung preparation of the dog and found a linear 

 relationship within the range of 25 to v38° C. He felt that the nonlinear relationships 



III! A. R. Dawe and P. R. Morrison (Am. Heart J. 49: 367, 1955), have confirmed the obser- 

 vation that the heart rate increases before an increase in temperature. 



