BLOOD SUPPLY TO THE HEART 



I53 1 



be estimated by observing the decrease in total left 

 coronary inflow after occlusion of the septal artery or 

 by direct cannulation (265 and Gregg, unpublished 

 observations). In either method, the volume of flow 

 is from 11 to 21 per cent of total left coronary flow. 



Radioactive cations (Na 22 , K 42 , Mg 2S , Rb 86 , Fe 56 ) 

 and anions (P 32 , I 131 ) and D 2 have been applied to 

 the coronary circulation as a means of determining 

 distribution of blood and plasma flow, and metabo- 

 lism of the involved myocardial bed. Tissue uptake 

 and turnover rates of the radioactive substances have 

 revealed a heterogeneous myocardial distribution (21, 

 122, 1 97, 234, 244, 245). All left ventricular areas in- 

 cluding base, apex, septum, and free walls have a 50 to 

 100 per cent higher uptake and turnover rate than 

 the right ventricle and atria. In descending order of 

 activity are the right ventricle, left atrium, right 

 atrium, His bundle and, lastly, the sino-atrial and 

 atrioventricular nodes. In most instances, the myo- 

 cardial uptake is nearly instantaneous since a plateau 

 is reached after a single systemic circulation and, 

 thereafter, remains relatively constant with only- 

 minor differences between the 20-sec and 10-min 

 determinations. D 2 similarly reaches equilibrium 

 between plasma and tissue water after a single circula- 

 tion and can also be calculated within 10 to 20 sec 

 following injection (197). 



Radio-rubidium (Rb'* 6 ) has been found to be the 

 most versatile for myocardial flow determinations 

 because of its long half-life (T 1 2 = 19.5 days), rapid 

 myocardial uptake (in exchange for intracellular 

 potassium), and relatively fixed myocardial extraction 

 despite varying arterial concentrations (230, 245). In 

 addition to the tissue concentrations, Rb 86 and Na 22 

 and D 2 have been used for coronary blood flow 

 determinations, and in those instances where checks 

 against a standard reference method (i.e., N 2 and 

 flowmeters) were done, good correlations were ob- 

 tained (274). Flow values vary from 0.4 to 1.6 ml 

 per g per min, with an average of 0.7 to 1 .0 ml per g 

 per min for dog and man, while in rats values four 

 times this have been found, supposedly related to the 

 four-fold greater energy output of the rodent myo- 

 cardium, i.e., 1 .00 joules per g per min versus 0.27 

 joules per g per min (185). 



The Ye, 



Circuit 



In addition to the regional differences in rate of up- 

 take, there also exists a concentration gradient be- 

 tween the endocardial and epicardial surfaces, the 

 former having the higher uptake and turnover of 



radioactive cations (244). The disparity is most 

 marked in the right ventricle since the concentration 

 of Thebesian vessels is highest in this chamber, and 

 also, a favorable pressure gradient exists for blood to 

 flow from the myocardium to the cavities during 

 systole. It has therefore been argued that this is sup- 

 portive evidence for utilization of the deep vascular 

 communications of the heart. The role played by the 

 deep vascular structures, however, is probably quite 

 small for several reasons. Balance studies in which an 

 attempt was made to measure coronary inflow and 

 outflow simultaneously with rotameters in the super- 

 ficial coronary vessels of the open-chest dog have 

 shown that a) coronary sinus flow ceases when both 

 the right and left coronary arteries are occluded with 

 the heart beating in situ; b) the left coronary artery 

 accounts for all but 5 to 10 per cent of coronary sinus 

 outflow; c) 80 to 85 per cent of left coronary inflow is 

 reflected in the coronary sinus outflow while some of 

 the remainder is accounted for by the anterior cardiac 

 veins; d) 90 per cent or more of the right coronary 

 inflow drains via the anterior cardiac veins; and 

 e ) there is no evidence of significant Thebesian drain- 

 age of the right coronary system (153). These studies 

 in the open-chest dog are technically quite difficult 

 and although recovery is usually of the order of 80 to 

 85 per cent (300), comparison of total coronary inflow 

 with outflow in the superficial veins is subject to 

 considerable error. At the same time, in other experi- 

 ments following acute coronary sinus ligation it was 

 observed that although the lateral wall of the left 

 ventricle was markedly congested, portions of the 

 interventricular septum showed less evidence of con- 

 gestion. This observation of 20 years ago was not fol- 

 lowed up until recently when it was found that the 

 portion of left coronary inflow (about 15%) not re- 

 covered in the coronary sinus could be largely ac- 

 counted for by the fact that a portion of the left 

 anterior atrial artery flow drains into the left atrium, 

 and that most of the septal artery and some branches 

 of the left descendens artery which perfuse the septum 

 drain into the right ventricular cavity (265, 266). The 

 finding concerning drainage of the left atrial coronary 

 flow is in line with observations with an illuminated 

 cardioscope in humans and dogs at the time of cardiac 

 surgery, that very small streams of dark blood can be 

 seen entering the left atrium but not the left ventricle 



(53)- 



The deep drainage channels could have an im- 

 portant functional role if they served as arterial 

 channels from the left ventricular cavity to the myo- 

 cardium during coronary artery constriction or 



