BLOOD SUPPLY TO THE HEART 



'553 



consumption are reduced. Atropine raises each of these 

 parameters fas well as the heart rate) to normal. 

 These experiments are difficult to interpret. Howevei , 

 since the stroke coronary flow and stroke coronary 

 oxygen usage are unaffected by hypothyroidism, the 

 reduced flow and oxygen consumption are probably 

 related in part to the altered myocardial metabolism. 



pitressin. There is agreement that Pitressin increases 

 resistance to flow in the coronary circulation (146, 

 153, 218, 384). In the revived human heart perfused 

 by the Langendorff method, and in the perfused dog 

 heart in ventricular fibrillation, Pitressin decreases 

 coronary flow. In the open- (146) or closed-chest dog, 

 coronary inflow decreases, the reduction occurring 

 throughout the cardiac cycle in the presence of an 

 increased central coronary pressure and, sometimes, a 

 mild reduction in heart rate. Selective angiography 

 demonstrates visible vasoconstriction of the superficial 

 coronary arterial tree (see fig. 13) (393). Although it 

 seems reasonably sure that this hormone decreases 

 coronary flow by a direct constrictive action on the 

 coronary bed, simultaneous studies have not been 

 made of the associated work and metabolism, and the 

 possibility of a reduced metabolic influence has not 

 been ruled out. If Pitressin has a direct action on the 

 coronary vessels, presumably it acts at the arteriolar 

 level. This is so since in the isolated perfused rabbit 

 heart (1 76) Pitressin does not change the intracellular 

 and extracellular Na and K values. If resistance 

 increased at the venules or distal end of the capillaries, 

 one might expect an increase in the extravascular 

 space. 



Exercise and Excitement 



Most of the information thus far considered is 

 based upon observations obtained from the resting 

 human and the anesthetized, open-chest dog. It is not 

 known to what extent it applies to normal situa- 

 tions, since the information has been obtained either 

 under conditions far removed from normal, as the 

 result of insults from anesthesia, surgery, and trauma 

 in the last situation, and hence, it does not contain 

 information from normal humans and animals as to 

 the regulation of the coronary circulation exposed to 

 the stresses of everyday life such as exercise, excite- 

 ment, and positional changes. For example, in exercise 

 and excitement, the heart rate is greatly increased. 

 It is disturbing that in only two of all the conditions 

 of stress in which heart rate increases, in the open- 

 chest dog, do the stroke coronary flow and stroke 

 coronary oxygen increase. These are in thyrotoxicosis 



and with cardiac sympathetic nerve stimulation. In 

 the others, stroke coronary flow and stroke coronary 

 oxygen decrease. This would mean that coronary flow 

 and oxygen usage are completely limited by the 

 heart rate. For example, if the heart rate is tripled, 

 coronary flow can only be increased three times. It is 

 difficult to conceive that the heart works in this way, 

 but rather that additional mechanisms can also 

 increase the coronary flow per heart beat. 



Accordingly, considerable effort has been ex- 

 pended to make appropriate measurements in the 

 normal state. It is not to be expected that new pa- 

 rameters of control will necessarily exist in these 

 stresses imposed by everyday life, but it is possible that 

 their weighting will be quite different. Early observa- 

 tions indicated that in man (240) and in the dog 

 (105) left coronary flow and myocardial oxygen 

 consumption increased, while the coronary arterio- 

 venous oxygen showed little change. More recently, an 

 appropriate flowmeter has been applied to the 

 coronary system of an essentially normal animal. 

 Initially, it was believed that no flowmeter would 

 operate properly if applied directly to the ventricle 

 on the surface of the heart because of its violent 

 motion. Therefore, in large dogs a systemic artery, 

 either the carotid or internal mammary, was anasto- 

 mosed by a nonsuture technique to the left circumflex 

 coronary artery branch so that a flowmeter could 

 later be mounted on it in a quiescent region off the 

 surface of the heart. Angiograms and postmortem ex- 

 amination of the hearts indicated the patency of the 

 anastomoses and the normalcy of the other coronary 

 vessels and the myocardium. Of y^ dogs, 6 died of 

 technical errors on the table or shortly thereafter, 

 3 died of thrombosis at the site of the anastomosis, 2 to 

 1 3 days postoperative. The remaining 24 dogs were 

 sacrificed 1 2 to 24 months later. Prior to sacrifice an 

 electromagnetic flowmeter, modeled after that of 

 Kolin (217), was placed on the anastomosed internal 

 mammary artery and the coronary blood flow meas- 

 ured daily for periods up to 2 months. Zero blood 

 flow was obtained when desired by temporarily 

 occluding the flow by means of a special rubber pneu- 

 matic cuff placed around the internal mammary 

 artery just distal to the flow transducer at the time of 

 its implantation (171). 



These preliminary experiments in 1958 were 

 encouraging. Electromagnetic flowmeters of the 

 sine-wave type, but greatly modified and improved 

 from the standpoint of miniaturization, sensitivity, 

 stability, and ruggedness, were constructed (212a). 

 The flow probes used on the left coronary artery were 

 necessarily somewhat smaller than an aspirin tablet, 



