REVIEW AND APPRAISAL OF PART III 



CHANDLER AIcC. BROOKS and BRIAX F. HOFFMAN 



Tliroughiiut the S\ini)()sium, statements have been made which relate to the effect 

 of coohng on tlie l)eha\ior of the heart and its responsiveness to stimuh. The major 

 contributions to this subject, however, are to be found in the papers deahng with 

 the action of hypothermia on the myocardium. 



This brief summary has been written to call attention to and correlate statements 

 made throughout the Symposium which are pertinent to the study of Cardiac Ir- 

 ritability, to consider and if possible resolve contradictions and to indicate various 

 problems needing additional study. These three considerations will not be handled 

 independently but will be treated under specific headings chosen for discussion from 

 among the many presented. Tt is the opinion of the reviewers that these are the 

 subjects most extensively dealt with in considerations of the action of hypothermia 

 on heart function. 



SPECIFIC THEMES AND CONCEPTS DISCUSSED 



The oxygen supply and cardiac metabolistn. One conclusion was definite and 

 that is that in hyi)othermia the blood carries sufficient oxygen to the heart to meet 

 its calculated recpiirement (Adolph. Friedman, D'Amato, Kao, Horvath and 

 Spurr ) as long as that organ continues to beat in a coordinated fashion and the 

 oxygen can be released to the tissues (Fuhrman). Although there is an increase in 

 anatomic dead space there is no evidence of difficulty in elimination of CO2 due to 

 a failure of gas transport (Severinghaus). Gross coronary flow is adecjuate (Berne, 

 D'Amato) until fibrillation or asystole occur. 



A second conclusion can also be accepted. Despite the adequacy of the oxygen 

 supply metabolism cannot be said to proceed normally. It is of course reduced but 

 more significant is the fact that imlialances develop. Whether or not these al)- 

 normalities are due to perfusion difficulties and transport between capillaries and 

 cells cannot be decided on the basis of evidence presented but the reported changes 

 in H ion, potassium, sodium and calcium levels, as well as in the efficiency of con- 

 tractile proces.ses, indicate that hypothermia has had a selective action. Normal 

 balances and exchanges are not maintained. 



Analysis of functional reactions. Subdivision of the reactions of the heart 

 into individual, but interrelated processes, was stressed in several papers presented. 

 The differential effect of temperature change has been employed in this type of anal- 

 ysis. Brown pointed out a number of examples of information thus obtained. Low 

 temperatures slow the rates of all processes and modify the action of metabolites 

 but this is not necessarily harmful until anoxia and chemical imbalances begin to 

 develop. Circulatory failure is the limiting condition in hypothermia which initiates 

 such anoxia and imbalance. The development of imbalances depends upon the tem- 

 perature coefficients of the several processes involved in cellular function. Metabolic 

 and rhythmical processes exhibit a Oio of 3, contraction has a Qio of 2, and physical 

 processes such as diffusion of metabolites a On, of 1 ; thus. metal)olic and rhythmical 



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