Coronary atherosclerosis causes gradual reduction in vascular cross-section area. Among the factors governing supply and demand, coronary perfusion pressure is the only parameter that appears to be affected by progressing coronary artery disease in the early stages. Of course, in later stages, essentially all aspects of coronary blood flow and metabolism are affected by coronary artery disease. Coronary obstruction causes a drop in perfusion pressure. The pressure drop across a plaque is proportional to the radius (fourth power), the length of stenosis, and the magnitude of flow. As flow increases across a stenotic lesion, a larger pressure drop develops. This reduction in perfusion pressure is compensated by distal bed vasodilation until the critical lower threshold of coronary perfusion pressure is reached (~60 mm Hg). At this point, autoregulation disappears and flow becomes dependent on perfusion pressure in a linear fashion. Therefore, the significance of coronary artery disease becomes clinically apparent when the perfusion pressure of coronary vasculature falls below this threshold. ventolin inhalers
The area of myocardium that is most vulnerable to the effects of coronary artery disease and reduced perfusion pressure is subendocardium (inner one third or one fourth of the myocardium).