TEMPERATURE EFFECTS ON CARDIAC TRANSMEMBRANE 



POTENTIALS 



BRIAN F. HOFFMAN 



One of the major problems encountered in the use of hypothermia as an ad- 

 junct to cardiac surgery is the occurrence of either ventricular fibrillation or, some- 

 what less frequently, cardiac arrest. It seems likely, in light of recent investiga- 

 tions,^- -' ^ that in most cases a direct effect of temperature on the cardiac muscle is 

 not primarily responsible for the onset of fibrillation. To the contrary, considerable 

 evidence indicates that respiratory depression and resulting changes in Pco^ and 

 pH of the blood are more directly linked to the production of arrhythmias.^' * In 

 support of this proposition is the demonstration that fibrillation does not eventuate 

 in dogs at the usual critical temperature range if ventilation is controlled and de- 

 velopment of acidosis prevented.- Similarly, it has been shown that changes in the 

 ventricular excitability cycle similar to those found prior to the onset of hypo- 

 thermic fibrillation can be reproduced at normal body temperature by alterations 

 in the pH of the arterial blood. ^ The possible role of hypoxia in the production of 

 arrhythmias under hypothermic conditions is thought to be of minor impor- 

 tance;"'" adequate studies of the state of tissue oxygenation, however, have not 

 been performed. 



On the other hand, the importance of a direct effect of low temperature on the 

 myocardium should not be neglected. For example, even under conditions of con- 

 trolled ventilation and arterial pH there are major changes in the duration of 

 refractoriness, the time-course of the recovery of excitability and the resting or 

 diastolic threshold.-- ^ Conduction velocity is decreased by cooling as is spontaneous 

 rhythmicity ; moreover, cardiac arrest occurs during hypothermia even in the 

 absence of major alteration of arterial pH.- Low temperature has a profound effect 

 on the delivery of oxygen to the tissue cells by myoglobin" and may contribute to 

 the development of a metabolic acidosis in spite of artificial hyperventilation. 

 Finally, older investigations have shown that cooling may result in a temporal dis- 

 sociation between the recoverv of excitability and the repolarization of the fiber 

 membrane.^" 



It is difficult to dissociate any study of the effects of temperature or pH from a 

 consideration of what ma\- happen with respect to the common inorganic ions. In 

 both nerve' ^ and skeletal muscle'- a decrease in temperature results in a loss of K 

 from within the cell. Furthermore, it has been noted that ]:)rior to the onset of 

 fibrillation in hvpotbermic dogs there are significant changes in the net fiuxes of 

 K, Ca, and II across the cardiac cell membrane. - 



In view of these considerations, it seems likel}- that a review of the efi'ects of 

 temperature, ions, pi I and l*c,, (in the nicni])rane acti\'it\- ol single cardiac libers 

 is ])ertinent to this ])art of the .Symposium. The information presented has been ob- 

 tained by means of studies of the transmembrane i)otentials of single fibers. ^'Iie 

 advantages of this ex])eriniental ai)iiroacli art' se\H'ral. In the first i)lace, the activity 

 of a single unit in a nniltiliber preparation can be in\estigated without alteration ol 



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