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PHYSIOLOGY OF INDUCED HYPOTHERMIA 



vated calcium. In the case of the isolated Purkinje fiber elevated Ca increases the 

 amount of depolarization required to stimulate and lowered Ca has the opposite 

 effect. Decreases in rate in high Ca solutions are explained by this mechanism.** 

 Although the configuration of the Purkinje fiber action potential is not altered by 

 calcium concentrations between 0.7-10.8 niM, in the case of auricular and ventric- 

 ular fibers marked changes in duration of repolarization are seen when the level of 

 this ion in the extracellular fluid is altered (figs. 14 and 15). 



Concluding remarks. It is apparent from the brief summary presented in the 

 preceding pages that certain of the phenomena associated with cooling of the intact 

 heart can be explained on the basis of information obtained from records of the 

 transmembrane potentials of cardiac fibers. The decrease in heart rate which occurs 

 during cooling results from a change in the slope of diastolic depolarization in 

 pacemaker tissues. The occurrence of A-V block and idioventricular rhythms results 

 from the great temperature sensitivity of auricular muscle, wherein conduction fails 

 between 25-23° C, and the persistence of spontaneous activity in Purkinje fibers 

 down to temperatures of 20-15° C. Changes in conduction velocity in the cooled 

 heart are seen to result from the effect of temperature on the upstroke of the action 

 potential ; at temperatures below 25° C. conduction is also slowed by the low resting 

 potential and resulting decrease in action potential amplitude. In addition, the 

 lowered stimulating efficacy of these small and slowly rising action potentials may 

 contribute to local conduction failure and disorganization of activity. The increased 



Fig. 14. — Effect of high and low calcium concentration on transmembrane potentials of 

 auricular fibers. (A) Control, (B) 4 times normal (10.8 mM), (C) \ normal (0.65 mM), 

 (D) Vio normal (0.27 mM). At the high concentration (B) the fiber responds only to alter- 

 nate stimuli. The response in (D) is not propagated. Time in 10 and 50 msec. Stimulus arte- 

 fact before each potential. 



