288 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



MEMBRANE 



+ 



+ 



75-100 mv.<; 



AXOPLASM 



Na K CI 

 49 410 40 



■-»►+ + + + + + + 

 440 22 560 

 TISSUE FLUID 



Fig. 1. — Distribution of ions between the extracellular and intracellular fluid. Squid giant axon. 

 (Redrawn from Eccles — The N europhysiological Basis of Mind, The Clarendon Press, 1953.) 



versal and depolarization. It is reasonably certain that the downswing of the action 

 potential and repolarization of the membrane involve an increase in membrane 

 resistance, a return of soditim impermeability, an outward flux of K* and activity 

 of an ion exchange pump. In the subsecjtient paper evidence will be presented 

 which indicates that repolarization process consists of more than one phase and is 

 far from a simple process. Additional information concerning excitatory processes 

 can be found elsewhere (Hodgkin, Huxley and Katz, 1952; Brooks et al., 1955). 



In conclusion it can be said that excitation of the heart involves numerous com- 

 plex processes which might be affected differently by cooling. Furthermore there 

 is an excitability cycle. Since excitation involves depolarization, the heart cannot 

 be reexcited following one stimulation or an intrinsically originating beat until 

 repolarization has again occurred. One might also expect excitability of the heart 

 to be subnormal or at least abnormal during the phase of progressive repolarization 

 and this is the case. 



The next questions which arise are : how stable is the cardiac cell membrane 

 normally ; how easily and by what process can excitation occur ; how does hypo- 

 thermia modify these reactions? 



THE INTRINSIC ORIGIN AND PROPAGATION OF EXCITATION 



The heart is comprised of potentially autonomic tniits which are dominated by 

 a pacemaker. This pacemaker initiates an excitatory process which drives other 

 units at a faster rate than that which would be established intrinsically. Two differ- 

 ent phenomena are involved in establishment of the cycle of excitability changes 

 and mechanical events which typify cardiac activity. These are first, the intrinsic 

 excitatory process in pacemakers and second, the excitation of other portions of 

 cardiac tissue by activity beginning in a pacemaker. 



Hypothermia and heart rate. Knowledge that cooling affects the intrinsic 

 excitatory process was obtained first from studies of heart rate. There have been 

 many determinations made of the effects on heart rate of cooling the body and the 

 heart (Badeer, 1955; Brooks et al., 1955). When the skin is cooled, when muscles 

 become active and when epinephrine is released from the adrenal medulla on cool- 

 ing of the body (Cannon, 1932) the heart is caused to accelerate by the resulting 



