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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



usage has been the subject of many serious and ex- 

 cellent investigations, but largely with the use of the 

 isolated heart. The hypothesis that the oxygen used 

 during ventricular systole is largely determined by 

 ventricular diastolic volume but not by ventricular 

 tension has been in vogue for many years. In 1927, 

 Starling & Visscher (354) showed that myocardial 

 oxygen consumption correlated with the changing 

 diastolic ventricular volume to the point of diminished 

 stroke volume from excessive ventricular distention, 

 but that the oxygen consumption of the heart had no 

 relation to its systolic volume. In isolated strips of 

 mammalian myocardium, the oxygen consumption at 

 rest increases significantly as the muscle length is 

 extended, but the tension developed as a result of 

 lengthening has a negligible effect on the oxygen 

 usage (396). Recently, others have shown an excellent 

 correlation of oxygen usage and diastolic volume in 

 the excised beating or fibrillating heart with perfused 

 coronary arteries (269). These observations also 

 apply to the heart beating within the chest. For 

 example, following partial constriction (by means 

 of a snare) of the pulmonary artery or the aorta 

 central to the two coronary ostia, the oxygen usage of 

 both ventricles, and the blood flow in right and left 

 coronary arteries, are increased, even the flow in 

 systole being considerably augmented in the right 

 coronary artery (153). However, in the excised heart 

 or the heart in situ which has been stopped by cervical 

 vagal stimulation or potassium injection, this relation 

 does not hold. Here large changes in the volume of 

 blood within the ventricular cavities can occur with- 

 out alteration of the oxygen consumption (249, 268). 

 This observation ties in with the fact that in the 

 open-chest dog the working heart's oxygen usage and 

 its coronary flow are not determined by the filling 

 pressure (atrial pressure) or the end-diastolic pressure 

 or volume, for at any given filling pressure the oxygen 

 and coronary flow can vary widely (46, 47). In ex- 

 periments in which the isolated beating heart with 

 perfused coronary arteries has been made to contract 

 isovolumetrically or isobarically, the myocardial 

 oxygen consumption is best correlated with peak 

 systolic pressure or systolic volume (269). 



that in the right atrium into which the coronary 

 blood drains approximates o to 8 mm Hg. It would be 

 expected that elevation of systemic venous or right 

 atrial pressure would decrease both right and left 

 coronary inflow. However, the influence of these 

 pressures on coronary flow is difficult to study for the 

 changes induced in them lead to other cardiodynamic 

 alterations. An approach to the problem has been 

 made by studying the effect of constriction or ligation 

 of the coronary venous drainage system on coronary 

 inflow. With the heart beating in situ, mild elevation 

 of pressure in the coronary veins draining the left 

 coronary artery by coronary sinus constriction may 

 decrease only slightly coronary inflow and increase 

 coronary A-V oxygen difference. Acute coronary 

 sinus closure causes congestion of the left ventricle 

 (but not of the right ventricle, atria, or a portion of 

 the interventricular septum), a greatly elevated venous 

 pressure in the coronary sinus and great cardiac vein 

 often approximating or exceeding aortic systolic 

 pressure (153), but the flow reduction in the left 

 coronary artery or its major branches is only moder- 

 ate, averaging 8 per cent in 10 dogs. However, the 

 venous outflow measured simultaneously in several 

 major anterior cardiac veins increases greatly. Simi- 

 lar responses occur when the major venous drainage 

 channels of the right heart, the anterior cardiac 

 veins, are occluded in acute experiments; right coro- 

 nary inflow decreases from o to 63 per cent, averaging 

 21 per cent in eight different experiments (1 53)- 



In acute experiments, pulmonary artery constric- 

 tion in the presence of previous ligation of the anterior 

 cardiac veins still causes a significant augmentation 

 of right coronary inflow. Finally, occlusion of both 

 the coronary sinus and all grossly visible anterior 

 cardiac veins reduces inflow further, but the hearts 

 generally survive and coronary inflow increases with 

 increased load. Even with chronic ligation of the 

 anterior cardiac veins and the coronary sinus, the 

 peripheral coronary venous pressure returns toward 

 normal within 30 days (1 53). 



From these observations, it does not seem likely 

 that a considerable elevation of right atrial pressure 

 will influence significantly coronary inflow in the 

 normal heart. 



Blood Pressure 



coronary' venous pressure. The venous pressure in 

 the great cardiac vein of the anesthetized dog, with 

 or without open chest, approximates (10-15)7(0-5) 

 mm Hg (153); the values for the coronary sinus and 

 anterior cardiac veins are considerably lower, while 



coronary arterial pressure. The mechanisms con- 

 cerned in alterations of coronary flow following acute 

 elevation or depression of central coronary pressure 

 have been only partially elucidated. Before con- 

 sidering the effect of coronary perfusion pressure on 

 coronary flow, attention is called to the experimental 



