Yes, stents can be put in bypass grafts. Your post was brief... are you the person with the heart problem? Grafts don't collapse, they generally are blocked by cholesterol or a blood clot. Perhaps additional information will give you more difinitive opinions. Keep us informed.
Veins can collapse, it is not as uncommon as people believe. It can happen through trauma or repeated IV needles etc. In usual cases, the body will adapt by finding an alternative route through smaller veins, but if used for grafts this cannot happen. Both my vein grafts caved in and it took a CT scan to find them. When a vein collapses, it can't be opened with stents because the complete vessel is caved in.
I just realised there was a second part to your post, what happens when stents collapse. I have not heard of a stent actually collapsing, but they can certainly block just like the blood vessel it's supporting. You can form different types of blockages in a stent, again just like an artery. It can be a blood clot, it can be plaque build up and it can be scar tissue which the artery has grown. If a stent becomes over blocked, it can usually be re-opened by ballooning it and possibly inserting another stent inside it, doubling it up. The different forms of blockages tend to occur in different time frames, for example, a clot can form at any time but the correct medication keeps this from happening in nearly all cases. Scar tissue forms very quickly, within the first few weeks of stenting, and plaque is a slower progression, taking months or years.
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