CARDIAC EXCITABI I. ITY— BROOKS 291 



reduction of membrane potential to the critical value by electrotonic current flow 

 and a slowing of the regenerative process which completes the reversal of mem- 

 brane polaritv explain slowing of conductiim in h\j)othermia. 



The same processes are involved in ])ropagation of excitation whether the im- 

 pulse originates in the activity of a pacemaker or in an a])plied stimulus. In discuss- 

 ing the initiation of activity by cxternrd stinnili as excitabilty is tested, no further 

 mention need l)e made of how the excitatory ])rocess spreads throughout the 

 heart. 



DETERMINATION OF THE EFFECT OF HYPOTHERMIA ON 

 CARDIAC EXCITABILITY BY THE USE OF TESTING STIMULI 



Methods. Relatively simple methods can be employed to study changes in ex- 

 citability which occur during the cardiac cycle and to determine the effect of hypo- 

 thermia on excital)ility. The major difficulties involved are: (a) the accurate place- 

 ment of testing stimuli at specific intervals of the cycle, and (bj the estimation of 

 the parameters of the effective stimuli so that the quantitation of work per- 

 formance required to elicit a response is known. 



Stimulation of the heart requires use of electrodes. Chronically implanted elec- 

 trodes and thermocouples presumably provide the most ideal conditions for such 

 work. In most instances, however, acute experiments have been performed, and 

 the results from these are similar to those obtained from chronic preparations, 

 provided the chest is closed and the heart has a relatively normal surround. 



Accurate placement of test stimuli has been accomplished by triggering the test- 

 ing stimulator from the R wave of the electrocardiogram or electrogram and place- 

 ment of stimuli at specific times after the peak or the beginning of the recorded 

 R wave. An easier and in some ways a more satisfactory method is to supplant 

 action of the intrinsic pacemaker with a driving stimulator. The testing stimuli can 

 then be placed with reference to the arrival of the driving stimulus. 



Phases of the excitability cycle identified by testing stimuli. Study of the 

 cyclic changes in cardiac excitability by this method has revealed many interesting 

 facts. If the cycle is considered to begin with the initiation of propagated activity 

 (the Q wave of the electrogram) it can be seen that: 



(a) There is a refractory period which in total duration is approximately identi- 

 cal with the phases of depolarization and repolarization or the Q-T interval of the 

 electrogram. This refractory period can be subdivided into (1) an early or ab- 

 solutely refractory period associated with depolarization and the early or slow 

 phase of repolarization and (2) a relative refractory period during which the ter- 

 minal or quickly occurring repolarization occurs. The boundary between the ab- 

 solute and relative refractory period is not well defined because it is determined 

 principally by the efficacy of the testing stimuli (fig. 2-A). 



(bj The refractory period is likewise an irresponsive period in that a normally 

 propagated response cannot occur until some time after completion of full repolari- 

 zation. The latency between application of testing stimuli and appearance of prop- 

 agated excitation increases as a stimulus is applied earlier and earlier in the 

 refractory period. Consequently, an effective stimulus, to act on the heart during 



