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The importance of the collateral circulation for myocardial survival
Insights
Myocardial survival during acute coronary occlusion hinges on collateral blood flow. Factors like arterial pressure and extravascular resistance influence this flow, with smaller ischemic areas receiving more, delaying but not preventing necrosis.
Area of Science:
- Cardiovascular Physiology
- Myocardial Ischemia Research
- Coronary Circulation Dynamics
Background:
- Myocardial survival post-coronary occlusion is critically dependent on collateral blood flow and oxygen supply.
- Ischemic myocardium is believed to induce maximal vasodilation in collateral blood vessels.
Purpose of the Study:
- To elucidate the determinants of collateral blood flow in acute coronary occlusion.
- To understand the factors influencing myocardial survival time and infarct development.
Main Methods:
- Analysis of factors affecting collateral blood flow, including hydraulic resistance, driving pressure, extravascular resistance, and ischemic bed size.
- Examination of transmural pressure distribution and its impact on subendocardial versus subepicardial flow.
Main Results:
- Collateral blood flow is governed by hydraulic resistance, arterial pressure, extravascular resistance, and ischemic bed size.
- Smaller ischemic beds receive proportionally more collateral flow, delaying necrosis.
- Subendocardial collateral flow is lower than subepicardial flow due to tissue pressure, leading to earlier damage and no-reflow in the subendocardium.
Conclusions:
- Collateral blood flow dynamics are complex, influenced by both vessel resistance and tissue pressure.
- Differential flow distribution between subendocardium and subepicardium dictates the pattern and progression of myocardial infarction.
- Understanding these factors is crucial for developing strategies to improve outcomes in acute coronary occlusion.
Abstract:
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.