[ 534 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



AORTIC PRESS 



LEFT COR FLOW 



CARDIAC OUTPUT 



105 mm Hq MEAN 



225 ml/min 



90 ml/IOOg/min 



2 8 ml/HEART 



BEAT 



3000 ml/mm 



37 ml/HEART 



BEAT 



HEART RATE 80 



fig. 6. Reproduction of a retrace of an original record taken 

 in the conscious dog, 14 days postoperative, showing phasic 

 aortic blood pressures recorded by a strain gauge connected to 

 a chronically implanted aortic catheter, and phasic left coronary 

 artery flow and stroke cardiac output by electromagnetic flow- 

 meters chronically implanted, respectively, on the main left 

 coronary artery and ascending aorta. (Unpublished observa- 

 tions.) 



patterns just indicated for the conscious dog are 

 similar to those in the open-chest dog except that in 

 the left coronary artery of the latter, systolic flow is 

 minimal and backflow is usually present during iso- 

 metric contraction (153). 



The flow patterns of the left coronary artery are a 

 complex of events happening in the total distribution 

 of flow in the left myocardium and a small portion of 

 the right ventricle. Regional variations of flow pattern 

 might be expected based on anatomical and func- 

 tional differences in the areas supplied. Flow patterns 

 of the main left coronary and its circumflex and 

 descendens branches are essentially similar. Phasic 

 flow, however, in the left anterior atrial artery shows 

 a forward flow in both systole and diastole with the 

 flow pattern resembling an aortic pressure pulse (349). 

 About 40 per cent of this arterial flow (5 '"< of left 

 circumflex flow) drains into the left atrium (266). 

 Patterns of flow through the canine septal artery are 

 not available. It would, however, be predicted that 

 the pattern would differ from that in the circumflex 

 and descendens by having a much smaller systolic 

 flow since this artery has essentially no epicardial 

 component. Most of the flow in this very small artery 

 drains into the right ventricle (265). 



The finding of a significant and variable coronary 

 flow during systole in the left coronary artery of the 

 unanesthetized dog deserves further comment. In the 



past, the view based on work in the open-chest dog 

 has been that flow in the left coronary artery is very 

 small during systole, that it does not vary significantly 

 with different dynamic conditions, and that it can 

 be accounted for largely on the basis of radial en- 

 largement of the epicardial vessels and their filling 

 during ventricular contraction (153). This meant 

 that events in systole could be and were largely ig- 

 nored and that the only important considerations for 

 regulation of left coronary flow were happenings 

 during diastole. Recent work using chronically im- 

 planted electromagnetic flowmeters indicates that 

 although the coronary flow in systole in the unanesthe- 

 tized dog at rest can, at times, be rather small, in 

 many dogs it may approximate 30 per cent of that 

 during diastole. In the presence of mild exercise, it 

 does not appreciably increase, but following release of 

 coronary artery occlusion, and during excitement and 

 chronic stimulation of the cardiac sympathetic nerves, 

 the volume of systolic flow increases 300 to 400 per 

 cent, as does the diastolic flow, the ratio between the 

 two remaining about the same (139, 212, 301 ). Finally, 

 in irreversible hemorrhagic shock, late in the period 

 of spontaneous cardiovascular decay after blood 

 reinfusion, the systolic flow may approach that during 

 diastole for an equivalent time interval, and eventually 

 the flow pattern may resemble somewhat the prevail- 

 ing aortic pressure pulse with most of the coronary 

 flow occurring in systole rather than in diastole ( 1 59)- 

 The proper explanation of these findings awaits 

 future experimentation (fig. 7). 



The preceding account indicates that the coronary 

 bed has a fluctuating resistance to flow. Flow curve 

 inspection shows the obvious importance of left 

 ventricular contraction in controlling coronary flow, 

 because during systole left coronary flow is reduced 

 while coronary sinus flow is increased. The increase 

 in coronary sinus flow suggests that ventricular con- 

 traction acts to aid coronary flow by massaging blood 

 through its wall; the reduction in coronary inflow sug- 

 gests that it acts to throttle coronary flow. The 

 answer depends upon the relative changes of inflow 

 and outflow volume during systole. Unfortunately, 

 this is impossible to determine because of the incom- 

 plete and variable drainage of the left coronary artery 

 through the coronary sinus. However, actual meas- 

 urements in the left coronary artery of the open-chest 

 dog show that the peripheral coronary maximal 

 systolic and minimal diastolic pressure values approxi- 

 mate 80 20 mm Hg, and inflow is cut off at these 

 pressure levels when the left coronary artery is per- 

 fused through its distal end under constant pressure. 



