Stenosis

Stenosis is defined as the narrowing of the lumen due to a thrombus or clot in a vessel. The narrowing can occur in the aorta, pulmonary, pyloric, or spinal. Stenosis can be divided into three regions: proximal stenosis, at/mid stenosis, and distal stenosis.

In proximal stenosis, laminar flow is seen associated with high pressure and low speed thus exhibiting a dark color when using color doppler. As the blood approaches at the stenosis site, flow speed increases and pressure drops (i.e. Bernoulli Effect), and the flow changes from laminar to disturbed flow, which can be described as a "forward curved flow". When color doppler is applied, the color will appear bright d/t the high speed at the stenosis site thus exhibiting a high or positive doppler shift. As the flow speed increases at the region of stenosis, turbulent flow will eventually occur. Turbulent flow can be seen distal to the stenosis site, and is describe as choatic or blood circular flow (i.e. eddie's). In distal stenosis, pressure is mediated as well as flow speed and when color doppler is applied, the image will display mixed colors. The onset of turbulence flow can be predicted by Reynold's number as it induces a critical rate of 2000. The sounds produced by turbulence can be auscultated and described as bruits, which is an abnormal blowing or swishing sound caused by atherosclerosis. If palpated by hand, the sound is described as "thrill".

A person who is diagnosed with stenosis will show no symptoms or is asymptomatic due to the "continuity rule", which states that blood is neither created nor destroyed as it flows through the vessel. The flow is close to normal due to the volumetric flow rate that is held constant at 100 cm/s throughout the vessel. The volumetric flow rate (or Poiseuille's Law) is determined by adding up the velocities from the three regions (proximal, at, and distal to the stenosis) to give the average flow speed. The average flow speed multiplied by the cross sectional area of the vessel is equal to the volumetric flow rate. In other words, during stenosis, the volumetric flow rate is converted to the average flow speed, which is related to Doppler Shift. Therefore, doppler shift is directly related to flow speed, not volumetric flow rate. The volumetric flow rate is only used for long straight vessels without stenosis and the diameter in Poiseuille's law depends on the diameter to the fourth power (entire vessel). By contrast, the continuity rule applies to vessels with stenosis and the flow speed depends on the diameter to the second power (short portion of a vessel).

If, however, the clot enlarges and causes the vessel to become occluded (thus no doppler shift and no flow), then the pt will exhibit symptoms and the condition should be treated to prevent future AAA, CVA, or TIA.



read info on stroke/CVA

read info on carotid artery stenosis