BLOOD SUPPLY TO THE HEART 



1543 



fact that in the presence of a declining coronary 

 perfusion pressure, coronary flow ceases even when 

 coronary perfusion pressure is still sizeable, an ob- 

 servation also made in the renal and mesenteric beds 

 (150) (see values under Physical Determinants of 

 Coronary Flow). Coronary inflow (right or left 

 coronary artery) immediately increases throughout 

 the cardiac cycle with a rising perfusion pressure and 

 decreases with a falling perfusion pressure in all 

 preparations studied. 



In both right and left hearts, however, there is no 

 set relationship between coronary flow and change in 

 central coronary perfusion pressure, the effect on 

 flow of a given pressure change varying from zero to 

 a maximum. The degree of change and its duration 

 will depend upon the extent of passive and active 

 changes in resistance within the myocardium asso- 

 ciated with the alteration of perfusion pressure. In 

 the heart doing no external work (empty, beating, 

 or fibrillating), the change in coronary flow is sizeable 

 with moderate change in coronary perfusion pressure, 

 but various relationships are observed. The resistance 

 may be semilogarithmic (80, 283, 341), i.e., it de- 

 creases with increasing flow, or at the highest flow 

 rates resistance may be constant or may increase. 

 Associated changes in resistance in the coronary bed 

 can be demonstrated when the vessels are perfused at 

 various pressures with the cardiac work kept con- 

 stant or not varying greatly. In the open-chest dog, 

 there is a rapid and marked change in coronary 

 flow within a few seconds following change of the 

 perfusion pressure. The induced change in coronary 

 flow may remain for some time (1 to 2 min) or it 

 may return toward, to, above, or below the control 

 flow level, thus showing large resistance changes in 

 the coronary vascular bed (89, 158, 249, 315). A 

 similar autoregulation of blood flow in the presence of 

 a mechanically induced change in perfusion pressure 

 has been observed in other regions such as the kidney 

 (363) and skeletal muscle (353). 



It is not surprising that in these various situations, 

 most of which are highly abnormal, a variable rela- 

 tion of pressure to flow exists. It is believed that these 

 changes represent automatic shifts in the size of the 

 coronary vascular bed and in vascular resistance 

 (passive and active blood vessel changes) which 

 serve to meet the metabolic needs of the myocardium. 

 Whether they are related to the oxygen supply and 

 demand, to the relative amount of metabolites washed 

 away, or to some other control, is not known. 



One of the largest changes in coronary flow from 

 altered coronary perfusion pressure occurs during 



aortic constriction with the heart beating and working 

 in situ. In general, in such instances in which a change 

 in coronary flow resulting from alteration of coronary 

 perfusion pressure is associated with a change in 

 ventricular stress (ventricular size or systolic pres- 

 sure, or both), the coronary A-V oxygen is the same 

 or increased slightly while the oxygen consumption 

 changes considerably in the same direction as the 

 flow. Since the heart rate (generally fast) does not 

 alter greatly, both stroke coronary flow and stroke 

 coronary oxygen usage show large increases. 



The oxygen uptake of a heart in the open-chest dog 

 performing external work can apparently be altered 

 by changing abruptly or gradually the level of a 

 constant coronary perfusion pressure by 5 to 35 mm 

 Hg for periods up to several minutes. The apparent 

 oxygen uptake of the left ventricle increases signifi- 

 cantly when coronary perfusion pressure increases. 

 When coronary perfusion pressure decreases, oxygen 

 uptake decreases. This change in oxygen uptake by 

 the myocardium associated with the opposite change 

 in coronary A-V oxygen occurs in the presence of a 

 constant arterial blood pressure, heart rate, stroke 

 volume, and cardiac work. Similarly, in isolated 

 hearts not performing external work, the change in 

 coronary flow from alteration of coronary perfusion 

 pressure is counterbalanced by a shift in the coronary 

 A-V oxygen in the opposite direction, but the oxygen 

 uptake is changed significantly especially at the 

 higher levels of coronary perfusion pressure. As yet, 

 experimental testing has not been able to ascribe 

 this apparent change in oxygen uptake to an arti- 

 factual happening (158, and unpublished observa- 

 tions on the isolated heart). These findings, which 

 have been confirmed (7), would seem to make suspect 

 various observations, especially in the isolated heart, 

 in which perfusion pressure has been varied. 



Many observers have reported that a given increase 

 in work of the heart, produced by raising aortic 

 pressure (aortic clamp) while holding cardiac output 

 constant, results in a much higher coronary flow and 

 oxygen usage per minute and per beat than when a 

 similar increase in cardiac work is effected by ele- 

 vating cardiac output through increased venous 

 return at a constant aortic blood pressure (208). 

 This discrepancy between the relative oxygen costs of 

 pressure and flow work is observed in the isolated 

 heart as well as in the dog with a complete circulation. 

 In experiments with the isolated, supported heart 

 with a constant heart rate, an increase in left ven- 

 tricular work, by augmenting cardiac output while 

 at the same time lowering aortic pressure by aortic 



