BLOOD SUIM'LY TO THE HEART 



r 547 



tion can be used to explain happenings in the coro- 

 nary circulation of the normal heart exposed to the 

 stresses of everyday life. At least, in the latter instance 

 the systemic dynamics are quite different from those 

 listed above as occurring with transfusion. For ex- 

 ample, in exercise and excitement, while the heart 

 rate increases greatly and the duration of systole 

 decreases, cardiac size may decrease and stroke 

 volume and atrial and ventricular diastolic pressures 

 may undergo only limited changes. 



Anemia 



The coronary system participates actively in the 

 circulatory adjustments to anemia. For hemoglobin 

 values of 10 g or more, the systemic circulation is 

 essentially unaltered and the compensation of the 

 coronary system to the decreased oxygen-carrying 

 capacity is similar to that with hypoxia, i.e., an 

 increased coronary flow without change in oxygen 

 uptake. When the hemoglobin values reach 6 to 8 g, 

 the response of the systemic circulation is manifested 

 by tachycardia, increased cardiac output and cardiac 

 work, and a fall in peripheral resistance. The coronary 

 flow may now triple; coronary venous blood may 

 contain less than 2 vol per cent oxygen, the coronary 

 arteriovenous oxygen difference may be 4 ml or less, 

 and oxygen uptake may be considerably increased. 

 The increase in coronary flow is related in part to the 

 decreased blood viscosity, and in larger part to the 

 active dilatation associated with myocardial hypoxia, 

 which in turn arises from the low hematocrit and 

 from the increased metabolism. Ultimately, myo- 

 cardial failure will occur in severe anemia when the 

 coronary vessels have approached maximal dilatation 

 and cannot further compensate for the decreased 

 oxygen-carrying capacity of the blood either by 

 increased flow or by increased oxygen extraction. In 

 the presence of coronary stenosis associated with 

 anemia, the effect of coronary arteriolar dilatation in 

 increasing flow is minimized by the high fixed resis- 

 tance of the stenotic artery, and myocardial depression 

 and failure occur at lesser degrees of anemia (31, 57). 



Very little information is available regarding the 

 coronary circulation in the presence of polycythemia 

 vera. The expanded red cell mass has been associated 

 with a considerable reduction in coronary blood flow 

 and an increased oxygen extraction without change 

 in oxygen usage. Allocation of these changes to an 

 enhanced oxygen-carrying capacity or to a viscosity 

 effect has not been made (305). 



Xarvits Influences 



The control of the coronary circulation by para- 

 sympathetic and sympathetic nerves has been the 

 subject of intensive investigation and dispute. Many 

 experiments, however, have been interpreted with 

 difficulty since cardiac output and cardiac work were 

 not determined, and heart rate and arterial blood 

 pressure which affect coronary flow and oxygen 

 usage varied widely (go, 187, 340, 405). There is 

 some evidence to indicate that the over-all nature and 

 extent of neural cardiogenic control is some degree of 

 coronary vasoconstriction since a) an outstanding 

 characteristic of the isolated heart or heart-lung 

 preparation is a very high coronary blood flow and 

 low myocardial oxygen extraction, and b) in chronic 

 dogs the procedure of pericoronary denervation 

 results in a relative increase in coronary flow and 

 decrease in oxygen extraction (43). For the most part, 

 the nervous system influences on the coronary cir- 

 culation have been studied by observing the coronary 

 flow, oxygen usage, and contractility responses fol- 

 lowing electrical stimulation or severance of the 

 nerves. Although such procedures are not paralleled 

 by normal occurrences in the animal, the observed 

 responses are presumed to indicate the functions 

 which the nerves are capable of exercising in the 

 intact animal. Further difficulty in interpretation 

 arises from the fact that the specific effects upon the 

 heart muscle and on the coronary vascular system 

 are largely experimentally inseparable and only the 

 net effect can be observed. Differences in methods 

 and preparations are additional variables which may 

 account for the discordant results of different in- 

 vestigators. 



vagus. Recent studies of the effect of the vagus nerves 

 on the heart have gradually clarified our view of their 

 effect on the coronary circulation. Early evidence 

 indicated that the vagus nerves contain both dilator 

 and constrictor fibers (401). That the vagus exerts a 

 vasoconstrictor effect is based on observations that 

 abolition of the parasympathetic pathways in the 

 heart-lung preparation (by mechanical and chemical 

 means) results in augmentation of heart rate or 

 coronary flow, while stimulation of the peripheral 

 ends of the cut vagi decreases coronary flow (10). 

 The evidence that it exerts a vasodilator effect arises 

 from the observation that in the fibrillating heart with 

 coronary arteries perfused with blood under a con- 

 stant pressure, vagal section usually decreases coro- 

 nary inflow but vagal stimulation usually increases 



