The importance of the collateral circulation for myocardial survival.

In acute coronary occlusion the survival time of ischemic myocardium depends critically upon collateral blood flow and on oxygen uptake at the moment of, and during, occlusion. There are good reasons to believe that ischemic myocardium provides the stimulus for near-maximal vasodilation of collateral blood vessels. Under these conditions the determinants of collateral blood flow are: a) the anatomically fixed hydraulic resistance of the collaterals proper, b) the arterial driving pressure, c) extravascular resistance (radial stress, pressure transmission across the LV wall, tissue pressure) and d) size of the ischemic bed. Under ideal conditions (maximal dilation of collaterals) overall collateral resistance is 3.5 resistance units, i.e. theoretically a perfusion pressure of 350 mmHg is needed to drive 100 ml of blood per minute through 100 g of tissue. Small ischemic beds receive a relatively larger amount of collateral flow and vice versa. This delays necrosis (but does not prevent it) following occlusion of small coronary arteries. The reason for this is the more favorable ratio of epicardial circumference (of the ischemic area) to ischemic volume because canine collaterals are exclusively located on the epicardial surface.-Tissue pressure in acute occlusion is distributed in such a way that subendocardial collateral flow is lower than subepicardial flow. This leads to an earlier onset of irreversible damage in the subendocardium, earlier damage to subendocardial microvessels, i.e. earlier subendocardial no-reflow phenomenon. Flow "offered" to but not "taken" by the subendocardium is at the disposal of the subepicardium which thereby increases its chances of survival. As a rule subendocardial flow decreases as a function of time after occlusion and subepicardial flow increases. In certain cases even subepicardial flow is too low shortly after occlusion. In this case it decreases further with time and a truly transmural infarct develops.