The term "stenosis" can refer to an abnormal narrowing of an artery, usually of a discrete segment. Stenosis can also refer to a reduced cross-sectional area of a heart valve when it opens. In the case of an artery, stenosis most commonly occurs in large distributing arteries such as coronary, renal, cerebral, iliac and femoral arteries. The narrowing commonly results from a chronic disease process - atherosclerosis. Sometimes a vessel can become acutely stenotic due to focal vasospasm. But in general, stenosis results from chronic vascular disease.
Stenosis increases the vascular resistance in a vascular segment as described by Poiseuille's equation, which says that resistance is inversely related to the radius to the fourth power. Therefore, if the radius (or diameter) of a vascular segment is reduced by one-half, the resistance within that narrowed segment increases by 16-fold. If this vascular segment were being perfused in isolation, the flow would be decreased 16-fold if perfusion pressure were held constant as shown in the figure to the right (green line). However, in vivo, this degree of stenosis would have much less affect on flow because the vessel is coupled in-series with other resistance vessels in the (CLICK HERE for more information). If we consider the renal artery and kidney circulation, the resistance a normal renal artery (RRA in figure equation) contributes to only a small fraction (<1%) of the kidney vascular resistance (RK). Therefore, the renal artery needs to be narrowed by more than 60% before overall renal vascular resistance is increased enough to significantly decrease renal blood flow (red line). This is also true for other organ circulations such as the heart, limbs and brain.
The term "critical stenosis" refers to a critical narrowing of an artery (stenosis) that results in a significant reduction in maximal flow capacity in a distal vascular bed. A critical stenosis may or may not reduce resting flow depending on the organ's capacity to autoregulate its blood flow and the development of collateral blood flow, both of which serve to reduce the overall resistance in the smaller resistance vessels. Clinically, a critical stenosis typically is thought of in terms of a 60-75% reduction in the diameter of the large distributing artery. This explains why interventional measures such as balloon angioplasty, stent placement, or arterial bypass surgery are not usually conducted in patients until there is at least a 75% reduction in vessel diameter.