294 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



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INTERVAL OF CYCLE — MSEC 



Fig. 4. — Supcrnnnnality. Insert shows total ventricular strength-interval curve from which 

 enlarged segment was taken. (From Brooks ct a!., 1955.) 



tance from rather than adjacent to the honndarv oi the depolarized area, thus 

 touching oft ectopic foci of excitation and disorganizing heart action (hg. 5, A 

 and B). Localized injury or depression such as might be caused by local applica- 

 tion of cold also creates a condition favorable to saltatory conduction and dis- 

 organized progression of the excitatory process (fig. 5C). 



The effects of hypothermia on the processes identifiable by the methods 

 described. In studying the effects of drugs, cooling and heating on organs such 

 as the heart it is necessary to differentiate between direct effects and those of a 

 secondary order. 



(a) Indirect effects. (1) Changes in heart rate result in modification of the 

 duration of certain phases of the action potential and the refractory period 

 (Siebens et al., 1951). Changes in rate modify the Q-T interval, the duration of 

 the absolute refractory period and the "plateau phase" of the transmembrane ac- 

 tion potential but have little effect on the late or (jtiick pliase of repolarization and 

 consequently do not modify the duration of the relatively refractory period 

 (fig. 6). If the heart is permitted to accelerate or slow during testing, the effects of 

 temperature per se on the duration of phases of refractoriness, latency of response, 

 etc., must be corrected for effects resulting merely from changes in heart rate. 



(2j Apparent effects of hypothermia on the heart might be merel\- the result of 

 slowing ion diffusion rates. Action of cold on ion partition and ionic flux across 

 cell membranes must be considered as an effect of hyu(jtherrnia on the heart since 



