I have read your query
I will try to help you understand it.
is blocked when some material is blocked in.
This is flushed out of it.
When this happens, there is some discomfort in the vein.
Besides, the material is flushed into the system.
And there is a small increase in pressure when the flushing
Together a heaviness is felt.
Ideally it is supposed to be changed everytime such a thing happens (blocking of cannula). But to save the discomfort to patients, this tecnique is used (flushing).
A heploc may be used to avoid this/minimize it.
Best of luck