Hemodynamic principles in the control of coronary blood flow.

The effects of atherosclerotic epicardial stenoses on coronary vascular resistance can be understood in terms of basic principles of fluid mechanics. Resistance is directly related to the pressure drop across the stenosis and inversely related to flow. Even with a fixed anatomic stenosis, however, resistance is not fixed; it increases as flow across the stenosis increases. This exacerbates the pressure drop across the stenosis that develops as a result of flow; at high flows, large pressure drops can occur. This characteristic of flow through stenotic lesions can contribute to a "steal" phenomenon between either epicardial or intramural coronary arteries. Studies have also shown the clinical importance and influence of dynamic alterations in coronary resistance, occurring either at the large or small vessel level. In addition, compressive forces exerted by the myocardium or by elevated intraventricular pressures can increase coronary vascular resistance, and thus interfere with myocardial perfusion. All of these factors must be considered in order to obtain a comprehensive understanding of the mechanisms leading to myocardial ischemia and, therefore, to the clinical syndrome of angina pectoris.