Newtons third law of motion says for every action there is an equal and opposite reaction .
In vascular hemodynamics whenever there is a an obstruction , there tend to be a dilatation of the same blood vessel somewhere distally.
It may not be linked to newtons law but it is observed in many .It is more common in large vessels than small ones.
Here is a patient with a tight LAD lesion with a significantly dilated segment located immediately beyond the obstruction.This can be considered as a post stenotic dilatation. Coronary ectasia is also a possibility but since it is related to site of obstruction the former is likely .
What determines the post stenotic dilatation ?
The exact mechanism is not clear. There is a definite ,sudden pressure drop distal to the obstruction. This pressure drop recovers beyond a certain distance . At this point , the rate of increment in velocity of peaks .This somehow has an effect on the distending pressure and the adjacent vessel wall gets radially stressed and begins to dilate. (Opposite of what is expected in Bernoulli effect ?)
Is there a anatomical defect in post stenotic dilatation ?
Not every one goes for post stenotic dilatation.There is a possibility it occurs only in genetically susceptible individuals .
Significance in interventional cardiology
The post stenotic segment has a potential to misbehave in the period following PCI . If the distal instent stenosis occur (even if it is minor ! ) it can induce a cycle of post stenotic eversion of normal segment and risk of edge effect or stent thrombosis is more.
Read also Glagovian phenomenon – A form of intra stenotic coronary artery dilatation