BLOOD SUPPLY TO THE HEART 



■541 



the sinus node naturally occurring remains to be 

 determined. 



In the open-chest dog, various arrhythmias, either- 

 occurring naturally or induced by electrical means, 

 by mechanical stroking of the heart, or by aconite 

 application, significantly reduce systemic blood pres- 

 sure and coronary flow when the irregularity is 

 marked or the rate rapid (above 190 per min) (72, 

 387). These arrhythmias include incomplete heart 

 block, premature auricular and ventricular systoles, 

 auricular fibrillation and flutter, auricular and ven- 

 tricular tachycardia. 



Heart Doing Xo Externa! Work 



Knowledge of the metabolic state of the heart 

 doing no external work is important because: a) 

 ventricular fibrillation and asystole are frequent 

 experimental and clinical occurrences; b) in the 

 empty beating heart or the asystolic arrested heart, 

 the magnitude of oxygen utilization could seriously 

 affect the potential for normal external efficiency of 

 the myocardium; c) with the advent of open-heart 

 surgery one must be certain, in the hearts made 

 dynamically quiescent by means of cardiac bypass, 

 ventricular fibrillation, or ventricular arrest, that the 

 metabolic requirements are met by the available 

 oxygen and myocardial damage does not occur. 



The relative length of time the A-V node and myo- 

 cardium can withstand complete ischemia and still 

 function normally on return of their blood supply 

 has been studied in dogs whose hearts were maintained 

 on an extracorporeal circulation. Myocardial anoxia 

 (by clamping the coronary inflow) leads to somewhat 

 earlier damage to the myocardial muscle than to the 

 conducting system, for after 80 to 90 min of anoxia, 

 blood pressure cannot be maintained on removal of 

 the heart from the extracorporeal circulation, while 

 conduction is normal after as much as 100 min of 

 anoxia. The former is due to development of an 

 unusual firmness of the left ventricular muscle which 

 is not reversible upon reperfusion of the heart (65). 

 At the same time, ventricular distensibility, as re- 

 vealed by ventricular pressure-volume curves, is 

 sharply reduced (161). If ventricular fibrillation is 

 induced without maintenance of coronary flow, myo- 

 cardial substances such as adenosine triphosphate 

 and glycogen (which are maintained with coronary 

 perfusion) fall progressively within 15 to 30 min and 

 are partially resynthesized upon reinstitution of 

 perfusion (288). The oxygen usage has been deter- 

 mined for the left ventricle, the external work of 



which has been reduced to zero by four different 

 procedures — vagal stimulation, intracoronary po- 

 tassium injection, ventricular fibrillation, and hemor- 

 rhage combined with suction to give an empty but 

 beating heart. Results have been rather variable for 

 the same procedure in the hands of different investi- 

 gators and generally one investigator has used only 

 two of the procedures. For example, the values for 

 oxygen usage for 100 g myocardium during fibrilla- 

 tion vary from 3.7 to 14.6 ml, in the empty beating 

 heart from 1.5 to 3.5 ml, during vagal asystole from 

 0.8 ml to 3.7 ml, intracoronary potassium injection 

 from 1.4 to 2.5 ml (17, 22, 26, 29, 179, 192, 249, 

 268, 288, 369). However, comparing the four pro- 

 cedures in the same series of experiments using the 

 open-chest dog, the oxygen uptake per 100 g left 

 ventricle in the working heart is 8 to 10 ml per min, 

 the resting metabolism (absence of heart beat, cardiac 

 output, and arterial blood pressure) during cardiac 

 arrest by vagal stimulation or potassium injection, 

 approximates 2.5 ml per 100 gleft ventricle per min, or 

 about 25 to 30 per cent of that at the prior working 

 level (249). The metabolism of the nonworking (but 

 slowly beating) heart obtained by rapid exsanguina- 

 tion is about 3.4 ml, and of the fibrillating heart 3.8 

 ml. Where measured, oxygen values were the resultant 

 of a simultaneous coronary flow increase and coro- 

 nary A-V oxygen decrease. Although the relative 

 values may hold, too much stress should not be placed 

 on the absolute values. While the various determi- 

 nants of each are probably still largely unknown, 

 knowledge is sufficient to indicate that each should 

 be widely variable. For example, values for the empty 

 beating heart are grossly affected by the prevailing 

 heart rate; values for the fibrillating heart depend 

 upon the type and frequency of fibrillation and upon 

 the ventricular diastolic size (268); values for the 

 vagus-arrested heart or following removal of an 

 artificial pacemaker are not affected by ventricular 

 systolic or diastolic size but vary greatly with the 

 previous level of metabolism in the working heart. 

 With a large elevation of myocardial metabolism by 

 intracoronary artery injection of epinephrine or 

 norepinephrine, the values are especially high and 

 may equal 50 per cent of those in the control state 

 (249)- 



I entmular I olume or Fiber Length 



Correlation of the left ventricular diastolic or 

 systolic fiber length (volume of a ventricle), or ven- 

 tricular tension, with the coronary flow and oxygen 



