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Perfusion can be defined as tissue blood flow at the capillary level. There are two components of flow—volume and velocity. Blood flow is a volume of blood moving at a certain velocity. Approximately 8% of the left ventricular mass at rest is blood (the blood volume fraction of the myocardium1), about 90% of which is resident in capillaries.2 The red blood cell velocity in the myocardium is approximately 1 mm/s at rest.3 Myocardial blood flow at rest is approximately 1 ml/min/g.
Because there are no arteriovenous connections in the human heart, any blood entering the coronary artery will reach the capillaries under normal circumstances. Consequently, flow measured at any level—coronary arteries, arterioles, or capillaries—will reflect tissue perfusion. If flow is reduced in a coronary artery in the absence of infarction (such as in the setting of a critical stenosis) myocardial perfusion may still be normal at rest because of collateral flow. In this instance flow measured in the stenotic coronary artery will not correlate with either arteriolar or capillary flow (perfusion). Because the coronary artery from which the collaterals are emanating is now supplying flow to its own perfusion bed in addition to the bed with stenosis, its flow will increase. The flow measured in the small arterioles (9–11 μm diameter where radiolabelled microspheres lodge) and capillaries will underestimate total flow in this coronary artery since some of it is shunted to the other bed before reaching these levels.
The physiology of coronary blood flow is even more complex immediately after reperfusion in patients with acute myocardial infarction. In this situation, myocardial blood volume decreases in proportion to the extent of necrosis,4 while blood velocity …
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