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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



made by measuring oxygen uptake, first in the beating 

 heart during repetitive systoles and diastoles, and then 

 in the relaxed heart or during prolonged diastole, 

 thus obtaining the oxygen usage during systole by 

 difference. Estimation of the metabolism of the myo- 

 cardium in the absence of a heart beat, that is during 

 prolonged diastole, has been made in the vagus-ar- 

 rested heart (see section on Physical Determinants of 

 Coronary Flow). The oxygen saturation of the arterial 

 blood and coronary sinus blood is also measured 

 continuously. This permits left coronary arteriovenous 

 oxygen difference as well as coronary inflow to be 

 measured continuously, first in the beating heart and 

 then in the stopped heart until a new equilibrium is 

 established, usually within 20 to 25 sec (155, 156, 

 249). As coronary inflow rises immediately with 

 asystole, the oxygen saturation of blood in the coro- 

 nary sinus also rises, thus greatly reducing the coro- 

 nary arteriovenous oxygen difference. Calculations 

 in many experiments show that as the result of the 

 combination of an increased coronary flow and a 

 decreased coronary arteriovenous oxygen difference, 

 the oxygen usage per 100 g of left ventricle per min 

 decreases from the average control level of 8. 1 ml in 

 the working beating heart to 2.3 ml in the resting 

 heart, or to 30 per cent of the control. This oxygen 

 consumption in diastole is about one-third that in 

 systole for an equivalent time period (249). 



Attention is also directed to the values for oxygen 

 usage obtained in the same type of preparation but in 

 which the external work of the heart is reduced to 

 zero by other means. In the potassium-stopped heart, 

 the oxygen usage of 2 ml during diastole is about the 

 same as in vagal asystole. In the beating heart emp- 

 tied by suction and hemorrhage, and in the heart 

 with induced ventricular fibrillation, the oxygen 

 usages of 3.4 and 3.8 ml are much greater (249) (see 

 the paragraphs under Heart Rate for more detailed 

 consideration). 



The metabolism of the heart is predominantly 

 aerobic. With abrupt vagal stoppage, however, during 

 constant pressure perfusion of the coronary arteries, 

 an excess of oxygen (oxygen debt) over that in the 

 asystolic state is taken up by the heart from the onset 

 of asystole to the time of appearance of the final 

 resting metabolism. This volume of oxygen, which 

 is quite small (estimated as 8% compared to the 

 maximum oxygen debt for an equivalent weight of 

 skeletal muscle of man), might be greater in a heart 

 working to capacity. Whether under prolonged hy- 

 poxia the anaerobic component of myocardial metab- 



olism can be extended has not been determined 



( ] 55)- 



As in any muscle, the mechanical efficiency of the 

 left ventricle is estimated by dividing its external 

 work by the difference between its oxygen consump- 

 tion during activity and during its resting state. Pub- 

 lished data (31) which indicate efficiency approxi- 

 mating 10 to 20 per cent in the normal heart include 

 only the first two measurements. Since the resting 

 metabolism is considerable and variable, and such 

 values are generally not available, interpretation of 

 the relation of cardiac work to oxygen uptake is diffi- 

 cult. 



RESPONSE OF THE CORONARY CIRCULATION 

 TO VARIOUS STIMULI 



The information has been obtained from the un- 

 anesthetized dog and man and from the anesthetized 

 open- or closed-chest dog. 



Resting Stalt- 

 As already pointed out, the levels of coronary flow 

 and oxygen usage of the myocardium are quite 

 fluctuant, varying greatly with the different types of 

 preparation and the prevailing stimuli. For values of 

 coronary flow and myocardial metabolism representa- 

 tive of the basal or resting state, selection of data in 

 the dog and man must be restricted to those in which 

 the systemic arterial blood pressure, cardiac index, 

 cardiac work index, heart rate, and body oxygen 

 uptake roughly approximated those figures for the 

 resting state. In the abnormal or diseased state in 

 human beings, data have been restricted to those in 

 which systemic blood pressure, cardiac index, cardiac 

 work index, and heart rate approximate values re- 

 garded as acceptable for the basal state when there 

 was no known reason for it to be elevated. These 

 criteria exclude a considerable volume of published 

 work, especially in man. While most of the data 

 comparing left coronary flow to the oxygen usage 

 per 100 g left ventricle per min, for the resting state 

 in man and dog, have been obtained by the N 2 

 method, the excellent correlation of A-V oxygen dif- 

 ferences yielded by this method with those from the 

 more precise methods used on the dog supports the 

 accuracy of the former when properly used. 



