BLOOD SUPPLY TO THE HEART 



'533 



and oxygen content of coronary sinus blood and of the 

 remaining coronary venous blood including that 

 from the right coronary artery. Obviously, these ob- 

 servations are germane only to the problem of whether 

 an increase in right ventricular metabolism asso- 

 ciated with increased right ventricular pressure is 

 reflected in the coronary sinus blood (195, 264). This 

 might not be expected because of the very small 

 drainage of right coronary flow into the coronary 

 sinus. These experiments are certainly not germane 

 to the problem of whether the two coronary venous 

 drainage fractions from the left coronary artery have 

 the same chemical composition, for such measure- 

 ments were not made. 



This question for the left myocardium has been 

 answered by simultaneously and continuously meas- 

 uring, under different circumstances, left coronary 

 artery flow, and the flow and oxygen content of the 

 two coronary venous fractions derived from the left 

 coronary artery. In these experiments, the systemic 

 venous return bypassed the right heart, and the right 

 coronary artery was generally clamped. The oxygen 

 uptake calculated on the basis of left coronary artery 

 flow times the difference between the arterial and 

 coronary sinus oxygen content agrees quite well with 

 the oxygen uptake based on the sum of the respective 

 volume flows and the oxygen content of the two left 

 coronary venous drainage fractions. This is effected 

 by a combination of a generally lower oxygen content 

 in the coronary sinus and a considerably greater 

 coronary sinus flow (300). Hence, in the open-chest 

 dog a combination of left coronary artery flow and 

 coronary sinus arteriovenous oxygen difference gives 

 a reasonably precise value for uptake of oxygen by 

 the left ventricle. 



From data such as these it has been reasonably- 

 assumed that measurement of coronary sinus flow 

 could be substituted for left coronary inflow and, 

 together with the coronary sinus, A-V oxygen differ- 

 ence could also serve as an index of metabolic events 

 in the left myocardium of man and beast. The authors, 

 however, in no way recommend this procedure. 

 Although widely used in man, it has never been 

 demonstrated that the flow, composition, and sources 

 of coronary sinus blood fulfill the requirements as laid 

 down and found to exist in the dog. [Actually in 

 early experiments with the isolated dog heart sig- 

 nificant right coronary artery drainage into the 

 coronary sinus was demonstrated (92).] In addition, 

 accurate measurement of coronary sinus flow is 

 extremely hazardous whether done indirectly by 

 means of the nitrous oxide method or directly by 



cannulation. In the first case there is the ever present 

 danger of contamination with right atrial blood. In 

 the second instance, without knowledge of the in- 

 vestigator, coronary sinus flow may be reduced 1 >\ 

 shrinkage and partial closure of the sinus. This 

 diminishes only slightly the left coronary inflow, 

 which now drains preferentially by the anterior 

 cardiac veins. 



PHYSICAL DETERMINANTS OF CORONARY FLOW 



Coronary flow is related to the pressure difference 

 (effective pressure) between the central coronary 

 artery (identical to aortic pressure) and the right 

 atrium divided by the sum of the viscous resistances 

 to flow in the epicardial portion of the artery and in 

 the peripheral coronary bed. Viscous resistance to 

 flow, aside from change in hematocrit, is mainly 

 governed by the mean caliber of the coronary vascular 

 bed. Since the arterial resistance is negligible, the 

 mean coronary diameter and, hence, flow are con- 

 trolled by the effective intravessel pressure and by two 

 peripheral mechanisms, i.e., active changes in the 

 state of the small mass of intramural smooth muscle 

 built into the coronary vessels, and the mechanical or 

 passive effect on flow exerted during ventricular 

 systole by the large muscle mass around the coronary 

 vessels. 



Insight into the complexity of the integrating action 

 of central and peripheral flow determinants has been 

 obtained from the recording of the peripheral coro- 

 nary pressure and the phasic or moment-to-moment 

 changes in coronary inflow and outflow in the epi- 

 cardial arteries and veins (151, 153, 158, 212, 301). 

 These curves were obtained from the open-chest dog 

 and from the resting unanesthetized dog some days 

 postoperatively, after implanting an electromagnetic 

 flowmeter on the left coronary artery (fig. 6). At the 

 onset of isometric contraction of the left ventricle in 

 the unanesthetized dog, there is an abrupt decrease in 

 left coronary inflow and, although at times backflow 

 may appear, a considerable forward flow generally 

 persists throughout systole. With the rise in aortic 

 pressure, forward flow increases initially and rapidly, 

 only to decrease to a new intermediate level in late 

 systole. With the onset of isometric relaxation, 

 coronary flow increases significantly, peaking at 

 early diastole and then declining progressively. These 

 demarcations of flow are much less obvious in the 

 right coronary inflow pattern, which roughly re- 

 sembles the prevailing aortic pressure curve. The flow 



