362 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



since both local anoxia and local failure to perfuse away K^ will result in a lower- 

 ing of resting potential. This hypothesis gains support from the observation that 

 not all animals develop the lowering of thresholds; some (possibly those with 

 better perfusion) go on to show a progressive elevation of thresholds and terminal 

 asystole. 



The attempt to do balance studies of ions, based on A-V differences and per- 

 formed in the absence of any study of water shifts is not necessarily meaningful. 

 However, if the threshold studies indicate a decrease in resting potential, we cer- 

 tainly are entitled to a loss of K" from the fiber, and possibly also a loss of H*. 

 Our feeling about the positive Ca*"^ balance in particular, and all ion studies in 

 general is that, as long as the methods used cannot explain the finding of a con- 

 stant negative Na* balance in all animals under all conditions, the other small differ- 

 ences noted are not worth worrying about. 



The finding of an increased sensitivity of the heart to Ca*"^ at low temperatures, 

 regardless of pH, is probably accurate. However, this may be related to the finding 

 (see Lepeschkin) that frogs in winter have low serum Ca, in summer high Ca, and 

 addition of Ca to winter frogs converts them to summer condition. In other words, 

 we have other evidence that Ca sensitivity of the heart is related to body tempera- 

 ture and perhaps the nonfibrillators, like the frogs, have an adequate mechanism 

 to lower the serum Ca during cooling. 



CONCLUDINCx SUGGESTIONS 



It should be stated, without implied criticism of those w'ho have made contribu- 

 tion to this field, that fuller studies are needed. Additional multiple recordings and 

 analyses are required before conclusions can be drawn about many matters dis- 

 cussed in this symposium. 



Despite the vast amount of work already done, little is as yet known concerning 

 the action of hypothermia. It is abundantly evident, however, that unless future 

 experiments are well conceived, carefully done, and more extensive, they will add 

 little to our present knowledge. 



It is felt that the individual cell and specific reactions must be studied more 

 thoroughly; that the action of hypothermia on membrane function and on the 

 contractile process should be determined ; and that a more extensive study than has 

 yet been attempted of the functional changes in the intact heart during hypothermia 

 must be undertaken by groups qualified and equipped to study many aspects of 

 physiology. 



