BLOOD SUPPLY TO THE HEART 



1567 



could, however, be interpreted differently. There are 

 many reasons to favor the view that in this situation, 

 circulatory failure not due to severe irregularity of 

 the heart beat is due successively to: a) defection of 

 useful contractions in the ischemic area, b) a loss of 

 contractile energy through expansion of the affected 

 area and d) failure of the still viable fractions to 

 compensate adequately. 



Since protracted hypotension can, at times, lead to 

 myocardial damage and failure, and since experi- 

 mentally the coronary collateral flow varies passively 

 with the systemic blood pressure, attempts have been 

 made to improve such hearts experimentally and 

 clinically by drugs and a venoarterial perfusion. 



The state of the heavily stressed normal myo- 

 cardium could be improved with drugs either by 

 increasing its oxygen supply or by using the available 

 oxygen more economically. The major mechanism 

 for increasing the oxygen supply is by increased 

 coronary flow since, normally, the oxygen is largely 

 extracted from blood passing through the myo- 

 cardium. The drugs would have to promote coronary 

 flow in the heavily loaded normal myocardium in 

 which oxygen usage, coronary flow, and coronary 

 A-V oxygen difference are already at a high level. 

 Whether any drug has the desired type of dilatation 

 (active myocardial vessel relaxation, decreased extra- 

 vascular compression, minimal increase in metab- 

 olism and cardiac work, minimal effect on other 

 vascular beds), and whether it also increases ventricu- 

 lar efficiency remains to be determined. In the normal 

 dog, drugs such as papaverine, nitroglycerin, epineph- 

 rine, aminophylline, Coramine, and khellin augment 

 the myocardial coronary flow and oxygen supply, but 

 generally at a considerable expense to the heart 

 through decreased coronary sinus oxygen (with 

 nitroglycerin coronary sinus oxygen is increased), 

 and increased cardiac work and metabolism. In 

 normal man, sublingual nitroglycerin leads to an 

 increased myocardial oxygen usage (increased 

 coronary flow and constant coronary A-V oxygen 

 difference), with little or no change in cardiac output 

 and cardiac work, and with a decreased efficiency 

 (42). In patients at rest, with coronary artery disease, 

 coronary flow is normal. Following nitroglycerin, 

 coronary flow and oxygen usage are unchanged but 

 systemic blood pressure, cardiac work, and cardiac 

 output are reduced; hence, coronary resistance is not 

 changed while efficiency is decreased (138). It would 

 thus seem that the dilator capacity of the coronary- 

 tree with coronary artery disease is exhausted. The 



mechanism whereby nitroglycerin relieves pain is not 

 that of general coronary dilatation and is unknown. 



The incidence of cardiogenic shock complicating 

 acute myocardial infarction has been reported as 1 2 

 per cent, and mortality associated with this com- 

 plication may be in excess of 80 per cent (3, 121). 

 Vasopressor drugs have been widely employed in 

 this situation (see table 1 for details). The improve- 

 ment that occurs in the human heart with drugs such 

 as neosynephrine and norepinephrine, in the presence 

 of coronary insufficiency and infarction, arises 

 because of a good dynamic response in the normal but 

 overstretched myocardium. This presumably aug- 

 ments the coronary collateral flow by increasing the 

 coronary perfusion pressure and by making the heart 

 smaller (see section on coronary collaterals). 



Although the use of vasopressor agents may 

 reduce mortality in myocardial infarction with 

 shock, at least half fail to respond. In such patients, 

 extracorporeal circulatory support is being tried 

 whereby blood is pumped from a convenient vein to 

 an artery (14, 357). The major objective is to produce 

 a sustained increase in aortic pressure and, hence, an 

 increase in coronary, cerebral, and other important 

 regional circulations, and yet, without an increase in 

 left ventricular work that might cause further cardiac 

 deterioration. Conclusive evidence of the benefit of 

 this procedure has not yet been obtained. In dogs 

 subjected to coronary embolization, use of a closed- 

 chest extracorporeal circulation with blood transfer 

 from the veins to the abdominal aorta has been 

 effective in restoring central aortic pressure only if 

 the aorta is occluded beyond the pump (219). 



Coronary Artery Collateral Circulation 



PREPARATIONS AND METHODOLOGIES FOR COLLATERAL 



flow in animal and man. Most studies have been 

 prophylactic in nature, i.e., a potential stimulus has 

 been applied to the normal coronary circulation 

 without interruption of coronary flow to determine 

 whether, following subsequent coronary artery ob- 

 struction, the coronary collateral flow will be in- 

 creased. In only a few instances has the effect on 

 collateral flow of different variables been studied some 

 time after creation of coronary insufficiency. It is 

 unfortunate that a standardized preparation of 

 coronary insufficiency has not been generally em- 

 ployed since this is the situation existing in man with 

 coronary artery disease. 



The experimental tools for study of the collateral 

 circulation leave much to be desired. In the experi- 



