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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



coronary flow is decreased at low temperature. 

 However, the per cent reduction in cardiac output is 

 greater than that in coronary flow, which results in an 

 increase in the coronary fraction of cardiac output at 

 temperatures of 25 to 26 C (103, 322). There is little 

 change, a decrease, or an increase in peripheral 

 resistance in the coronary bed, whereas in the systemic 

 bed an increase in peripheral resistance invariably 

 occurs (1 77, 322). A constant or increasing mechanical 

 efficiency is usually observed in the open- or closed- 

 chest dog (128, 175) although it has been reported to 

 fall (103, 198). Similarly, in the heart-lung prepara- 

 tion or isolated heart, the mechanical efficiency is 

 fairly constant when cardiac work per beat (same 

 stroke volume and arterial blood pressure) and 

 heart rate are constant (15). Myocardial function 

 appears to be adequate and myocardial hypoxia not 

 to exist (198). However, many hearts are apparently 

 not too far from failure because if total venous inflow 

 occlusion (which decreases coronary flow close to 

 zero) is now added to permit open cardiotomy, 

 myocardial failure supervenes, as evidenced by- 

 elevation in mean right atrial pressure and post- 

 mortem findings. This trend can be reversed by 

 perfusion of the coronary system with small volumes 

 of oxygenated blood (239). 



Hyperthermia 



The systemic dynamic changes resulting from 

 elevation of body temperature by fever or external 

 application of heat (hot baths, diathermy) are quite 

 similar in man and dog, are well documented, and 

 include considerable elevation of heart rate, blood 

 pressure, cardiac output, right and left ventricular 

 work, a decreased peripheral resistance and a con- 

 stant stroke volume and stroke work. The little 

 information available on the associated coronary 

 changes indicates that a large elevation of body 

 temperature (up to 105 F) by means of diathermy, 

 in the closed-chest dog, increases considerably 

 coronary blood flow, myocardial oxygen usage, 

 coronary A-V oxygen difference, increases mildly the 

 stroke coronary oxygen usage, decreases external 

 efficiency, and leaves unchanged the stroke coronary 

 flow and coronary resistance (257). In open-chest 

 dogs with an initial hypotensive systemic blood 

 pressure, diathermy has no effect on coronary flow 

 (253). In the heart-lung preparation, when the 

 myocardium is warmed, coronary flow and oxygen 

 usage are increased; but they are not when the 

 coronary blood is warmed (10). 



Summary 



Over the years, the basic mechanisms affecting 

 coronary flow and oxygen usage have been related 

 experimentally to various parameters, and the state- 

 ment is often made that there is one controlling or 

 unifying influence for coronary flow per heart beat 

 and also one for oxygen usage of the left myocardium 

 per cardiac cycle, a) Directional changes in stroke 

 coronary flow correlate with stroke coronary oxygen 

 usage. This is so, however, because normally most 

 oxygen is removed from the coronary blood and the 

 level of coronary sinus oxygen is usually fairly con- 

 stant under stress, i.e., it does not change more than 

 10 to 15 per cent. In those instances in which the 

 coronary arteriovenous oxygen difference increases 

 or decreases by this amount, it does not greatly 

 affect the relation of coronary flow to oxygen usage 

 since the change is very small relative to the magni- 

 tude of the coronary flow change, but this does not 

 document a functional correlation between these 

 parameters, b) The stroke coronary flow and stroke 

 coronary oxygen correlate fairly well with the stroke 

 work under a variety of conditions of changing 

 systemic stress, but it is possible to so regulate experi- 

 ments that the response of the coronary circulation is 

 dissociated from stroke work, c) The exceptions to 

 the usual correlation of stroke work and coronary 

 oxygen usage constitute a group of conditions in 

 which the outflow channels of the two ventricles have 

 been restricted in some manner. In these, one can 

 show excellent correlation of the stroke coronary flow 

 and oxygen usage with the mean systolic arterial 

 blood pressure alone, or with the product of the 

 systolic blood pressure and the duration of systole, 

 the so-called "tension-time index." However, experi- 

 ments in the unanesthetized dog during exercise and 

 excitement do not always support this view. In 

 addition, there is quite a list of determinants that 

 have been thought to be fundamental. Attempts 

 have been made to relate coronary flow and oxygen 

 usage to the mean arterial blood pressure, ventricular 

 filling pressure or mean atrial pressure, ventricular 

 diastolic volume or fiber length, tension within the 

 ventricular wall, oxygen tension of the arterial blood, 

 oxygen tension within the myocardium, action of 

 local metabolites or vasodilating substances. Possibly, 

 the best correlation of all should be with the reduction 

 of cytochrome oxidase and the needs of the hydrogen 

 transport system. Probably no final answer is avail- 

 able. Final decision as to whether any of these 

 determinants of coronarv flow or oxygen usage are 



