The inadequacy of the normal collateral coronary circulation and the dynamic factors concerned in its development during slow coronary occlusion

Abstract

The evidence for and against the existence of a liberal collateral blood supply to the normal ventricular myocardium is reviewed. Evidence based on histological or injection studies is not conclusive as to the magnitude of such circulations. The volume flow through collateral vessels depends not only upon the size of communications but upon the magnitude of pressure differences. The presence of one without the other cannot result in flow. The experimental evidence is reviewed, and the conclusion is reached that in the normal heart collateral flow is extremely small and insufficient to support contractions in an area rendered ischemic by ligation of a main branch. The existence of a negligible collateral supply in normal hearts does not preclude the enlargement of minute potential communications nor the development of new ones when a main branch is slowly occluded. In such cases, physiological mechanisms must exist to account for the development of such circulations. The suggestion is made that, as a main branch is slowly occluded, the development of altered pressure gradients first distends normally useless channels and so furnishes a supply of blood under pressure for the newly growing vessels. The details of the probable pressure gradients are analyzed on the basis of known forces and magnitudes of the pressure variations throughout the cardiac cycle. The essential conclusions reached are: 1. 1. Dynamic pressure gradients are such that normally a transfer of blood from the coronary arteries to the ventricular cavities could occur during diastole and during the early part of isometric contractions (i.e., for perhaps 0.02 sec.) but no transfer could occur between intercoronary and coronary-extracoronary branches at any time. 2. 2. When the size of a coronary branch is reduced, the pressure gradients are favorable for developing flow through intercoronary or extracoronary anastomoses throughout the cycle; and a flow from the ventricular cavity to the artery could occur during systolic ejection. During diastole the gradient of pressure favors a flow toward the ventricular cavity although the pressure differential is decreased in proportion to the narrowing. In other words a to-and-fro movement could occur during each heart cycle. 3. 3. When a main coronary branch is completely occluded, the same pressure relations exist, except that no pressure gradient exists between coronary branches and the ventricular cavity during diastole. 4. 4. It is improbable from dynamic considerations that reversed flow from the right heart through Thebesian veins plays any rôle in collateral supply to the left ventricle.