356 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



activity decrease in hypothermia three times as fast as diffusion and twice as fast 

 as contractiHty. 



The selectivity of actions of B strophanthine and pressure on certain periods of 

 the reaction cycle of the heart cells suggests that they amplify the alpha process of 

 the contractile reaction and thus affect tensions developed. Temperature change can 

 contribute to a much needed study of the specific physicochemical changes going on 

 in the membrane which influence its polarization, repolarization and the associated 

 reactions of the contractile process. 



Similarly, hypothermia enhances the action of certain drugs and hyperthermia 

 enhances that of others. This permits a degree of analysis of the specific action of 

 these substances on cardiac tissue components and reaction processes (see Brown, 

 Horvath, and Spurr). 



The effects of hypothermia on the depolarization, repolarization and recovery of 

 excitability described by Brooks for the whole heart and by Hoffman for cardiac 

 cells provide other examples of subdivision of functional processes and the selective 

 action of cooling. The analysis of the hypothermia-produced changes in the various 

 phases of the cardiac cells' resting and action potential presented by Hoffman ex- 

 plains the modification of many of the grosser cardiac reactions. It should be 

 pointed out that the temporary augmentation of the action potential at certain 

 phases of cooling in both heart and nerve (Hoffman ; Brooks and Koizumi) indicates 

 that description of changes produced by hypothermia in simple Qio and unidirec- 

 tional relationships cannot be correct for all ranges of cooling (see Fuhrman). 



A great deal of attention was paid to ion flux and partition. The ultimate bal- 

 ances maintaining at various temperatures were measured rather than the cyclical 

 fluxes associated with the rising and falling phases of the action potential. Con- 

 clusions were somewhat contradictory but the majority of participants (e.g. 

 Horvath, Spurr) claimed a slight loss of K from the cell in hypothermia. Shifts in 

 Ca (an increased intracellular level) appeared to be more significant to the develop- 

 ment of fibrillation (Hegnauer and Covino ; McMillan, etc.). Such studies should 

 be continued because our knowledge is inadequate at present. 



Species and tissue peculiarities. In considering hypothermia and cardiac effects 

 it is stated that in some species, hibernating animals for example, "the reaction 

 rates of the various cellular processes have a better relative setting" although the 

 temperature coefficients are the same. This hypothesis was employed by Brown to 

 explain the cessation of cardiac rhythm at 13° C. in some species but not in hiber- 

 nators. Such species differences may explain in part the discrepancies in results of 

 tests of tolerance of the heart to cooling. 



There are optimum temperatures for the development of tension by the cardiac 

 tissues of different species (Brown, Lyman and Chatfield). Critical temperatures 

 for conduction in nerves of different species differ (McQueen). Effect of tempera- 

 ture on various diametered nerve fibers differs. 



Since it has been shown that cardiac cells differ (Hoffman, Brooks) specific 

 studies of cellular susceptibilities to cold would add to tiie understanding of changes 

 in behavior of the heart in hypothermia. 



It is possible that protection of the ventricle from fibrillation in hypothermia by 



