blood clot left ventrical

10 years ago I had a heart attack and i'm monitored regularly. I had just had an angiogram and they said I needed a pacemaker/ defibulator.  the day they were preping me for surgery they did an echo and said I had a blood clot in my left ventrical.  my question is how long dose it take a clot to form  and way did they not find it before? they say i have to go on coumadin for 3 month before I can have the pacemaker/defib. does this seem right?

[Cleveland Clinic]

It depends on the source: some people will not place a defibrillator while a clot is present, while others do not feel that this is a contraindication. Clots form because the muscle underneath is dead and this causes stasis of blood and a subsequent clot. It may have not been there before, and you need coumadin to prevent it from embolizing and causing you sudden cardiac death, stroke or a heart attack.