this they fibrillate so the blood swirls in the top chambers of the heart and swirling around it can form a clot the clot can then break loose and favorite place where to go is up into the brain so patients with atrial fibrillation are at risk of stroke and we have treatments to prevent them from having a stroke if you have a sick heart valve that can be a spot where a clot can form a sticky heart valve can form a clot and if you have a big heart attack then the main pumping chambers the ventricles aren't working very well the