'554 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



since the maximum space available for implantation 

 on the main left coronary approximates 2 to 2.5 mm. 

 A large electromagnetic flowmeter implanted on the 

 ascending aorta (or pulmonary artery) gave simul- 

 taneous cardiac output per heartbeat. For phasic 

 arterial pressure a plastic tube filled with heparin 

 was implanted in the aorta just beyond the aortic 

 flow transducer. 



Strips of record in figure 1 1 illustrate the flow 

 through the left circumflex artery anastomosed to the 

 internal mammary artery in a large greyhound at 

 standing rest, and running on a treadmill at 1 2 mph 

 on a 5 per cent grade for 3 min (mild to moderate 

 exercise for such a dog). As the heart rate almost 

 triples, the mean coronary flow also triples. Despite 

 this, stroke coronary flow does not increase but 

 decreases mildly. The fact that the stroke coronary 

 flow did not increase with exercise cannot be ascribed 

 to increased resistance through the much longer 

 anastomosed circuit because tests in acute experi- 

 ments showed that coronary flow was the same in 

 the long and short circuits up to levels of about 650 

 ml per min (344). Similar flow changes during 

 exercise occur in the main left coronary artery and 

 in the descendens and circumflex branches without 

 anastomosis (212). This suggests that the coronary 

 flow is limited by the heart rate. It is expected (as 

 yet without proof) that with quite heavy exercise, 

 the coronary flow per heart beat will rise. 



The coronary flow response to varying degrees of 

 excitement is quite different from that to exercise. 

 In the experiment illustrated (fig. 12), a dog at rest 

 underwent spontaneous excitement. As the heart 

 rate increases from 98 to 250 beats per min, but 

 without change in mean blood pressure, the left 

 circumflex coronary flow increases from 94 to 344 

 ml per min, and despite the shortened diastole 

 during which most of the coronary flow occurs, the 

 coronary flow is more than tripled. In contrast to the 

 response in exercise, however, stroke coronary flow- 

 increases from 1.0 to 1.3, this occurring during systole 

 and diastole. Later, as a moderate increase in blood 

 pressure occurs, the stroke coronary flow is approxi- 

 mately tripled. Similar flow responses to excitement 

 have been observed in the main left coronary artery 

 with or without a large blood pressure change. These 

 experiments indicate that the heart is able to obtain 

 an increased coronary flow during excitement not 

 only because of the increased number of heartbeats, 

 but also because of an increased flow per heartbeat. 



\'alvular Disease 



It is difficult, if not impossible, to duplicate human 

 valvular disorders experimentally because of lack of 

 methodologies to assess accurately the degree of 

 insufficiency and stenosis in both man and beast, to 

 measure the coronary blood flow, and to make the 



REST 



(STANDING) 



180 



120 



60 







•"-I 



EXERCISE 



RECOVERY 



5 SEC 



fo 



l\ 



10 SEC 



20 SEC 



5 SEC 



: 



M 



HYPEREMIA 



m 

 (5 SEC OCCLUSION) 



HEART 

 RATE 



MEAN 

 FLOW 



STROKE 

 COR FLOW 



80 

 45.5 

 0.57 



172 



54 7 



0.32 



214 

 70.5 

 0.33 



220 



110.7 



48 



146 



108 



0.74 



89 



45 



0.51 



fig. 1 1. Reproduction of sections of records taken from an exercising dog showing phasic coronary 

 flow obtained by an electromagnetic flowmeter mounted on an internal mammary artery anasto- 

 mosed to the left circumflex coronary artery. Anastomosis performed 1 7 months earlier and flow 

 probe implanted 7 weeks before. Large greyhound at standing rest, running 3 min on treadmill at 

 10 mph, recovery, 5-sec occlusion of anastomosis to observe reactive hyperemia. [Khouri ft al. 



(212).] 



79 

 108 

 1.37 



