314 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



accumulation of K in the extracellular space, or a relatively greater decrease in the 

 outward Na transport than in inward K transport, h^ither of these effects would de- 

 crease the resting potential. The marked decrease in the amphtude of the action po- 

 tential at low temperatures (10^20° C.) certainly results in part from a direct effect 

 of the decreased resting potential on the amplitude of the action potential. •- 



(b) Rate and rhythniicity. Records obtained from the pacemaker of a single iso- 

 lated Purkinje fiber during changes in temperature clearly reveal the nature of the 

 rate changes associated with heating and cooling.^" The records reproduced in 

 figure 9 show that, as temperature is lowered from 38 to 25° C, the slope of the 

 slow diastolic depolarization of the pacemaker is decreased. Because this loss of 

 resting potential proceeds at a decreased velocity the critical threshold level of 

 membrane potential is attained only after longer intervals of time and frecjuency of 

 action potentials is diminished. It should be noted, however, that the absolute value 

 of the membrane potential at which self sustaining activity occurs (the critical 

 threshold potential) is not altered; the threshold of the fiber, therefore, is not 

 changed between temperatures of 38-25° C. Rate changes thus result from tem- 

 perature effects on slow diastolic depolarization rather than from alterations in 

 excitability. 



At this time little is know'u about the mechanism responsible for slow diastolic 

 depolarization of pacemaker tissue. It has been observed that a decrease in the 

 concentration of extracellular sodium slows the rate of spontaneous activity^^ and 

 that this slowing is associated with a decrease in the slope of diastolic depolariza- 

 tion.^° It is possible, therefore, that the depolarization characteristic of pacemakers 

 results from a high sodiuin permeability of the membrane and greater than normal 

 inward Na current during diastole. On the other hand, the high temperature coeffi- 

 cient of the slope of diastolic depolarization — Qio = 6.2 between 40-25° C.*" — sug- 



FiG. 9. — The effect of temperature on pacemaker activity in a single Purkinje fiber. Tem- 

 perature in degrees centigrade for each action potential indicated in figure. Note that cooling 

 decreases the rate of slow diastolic depolarization hut docs ni)t alter the level at whicli the 

 action potential upstroke ensues. Also note changes in actinn jiotential duration. Adapted 

 from Corahoeuf and Weidmann (40). 



