116 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



it is apparent that absolute refractoriness should increase in duration much more 

 than the phase of relative refractoriness when the heart temperature is lowered. 

 In auricular muscle, since the action potential does not normally reveal a definite 

 plateau, the change in repolarization consists mainly of a progressive increase in 

 the duration of the final limb of the action potential. In some cases, however, cool- 

 ing gives rise to a definite inflection in the auricular action potential which resem- 

 bles the plateau of ventricular records.-'-' Similarly, in some cases the time-course of 

 repolarization of the papillary muscle is altered so that a definite plateau is no 

 longer apparent. ^^ 



It is difficult to compare the degree of prolongation of the action potentials 

 recorded from auricle, ventricle, and conducting tissues unless all preparations are 

 driven at the same rate and alterations in frequency of contraction are prevented 

 during the decrease in temperature. Comparison of available data^^- -^' ■*°' '*''' *^ in- 

 dicates that the relative prolongation is similar in all three cases within a tempera- 

 ture range from 38-25° C. However, at lower temperatures there is often a marked 

 increase in the action potential duration of certain preparations for a slight de- 

 crease in temperature and thus parallelism between different tissues is lost. 



At 38° C. full excitability is restored at the same time as, or very shortly after, 

 the completion of repolarization. This temporal coincidence of the recovery of 

 membrane potential and excitability is not necessarily maintained at all times. ^°' ^^ 

 In the case of the isolated papillary muscle stimulated through surface electrodes, 

 it has been shown that cooling delays the recovery of excitability considerably after 

 the end of membrane repolarization. At temperatures between 13-14° C. (fig. 11) 

 the duration of the absolute refractory period may exceed twice the duration of the 

 action potential. It is certain that this change in duration of refractoriness is not a 

 result of slowed conduction; the mechanisms responsible, however, remain un- 

 certain. 



(d) Conduction velocity. Low temperature is known to decrease conduction 

 velocity in the auricle and ventricle of the mammalian heart'"''- and to slow propaga- 



FlG. 



iB-M -100 mv 

 11. — The effect of temperature on the recovery of cxcitahility in ventricular muscle. 



At left, action potential and early extrasystole at 38° C. Shows decrease in amplitude and in- 

 crease in duration. Earliest possible extrasystole at 13° C. shown at right. Time and voltage 

 calibrations shown in figure. 



