322 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



transport of Na or K. Regardless of the mechanisms concerned, potassium loss 

 similar to that observed in intact hearts prior to fibrillation- is also encountered in 

 the isolated preparation. Moreover, the decrease in intracellular K and increase in 

 extracellular K may be associated with changes in a similar direction of the H ion 

 concentration-^ and may explain in part the pH changes encountered in the intact 

 animal. Finally, it has been demonstrated that changes in the concentration of 

 extracellular Ca have a marked effect on the sensitivity of the membrane to changes 

 in the level of K. This interrelationship may be of value in understanding the im- 

 portance of the net changes in K and Ca fluxes across the membrane which have 

 been associated with an increased likelihood of fibrillation in intact animals. - 



In conclusion, certain major deficiencies exist in present day knowledge of the 

 effects of temperature on the membrane and activity of cardiac fibers. Of primary 

 importance are (a) information concerning the effects of additional variables 

 (pH, K, Ca. Pen.,) during cooling of cardiac muscle, and (b) studies of the 

 metabolic activity and physico-chemical changes responsible for the phenomena 

 observed. 



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