hello i have gone through your complaints and noted your history. Mitral valve problems cause the left upper chamber heart to enlarge and produce some rhythm disturbances. most common is atrial fibrillation which means irregular fast impulses from upper chamber of the heart. this irregular heart rate may cause dizziness due to irregular supply of blood to the brain. this also can lead to formation of clot in the upper chamber of heart which may dislodge into the brain and cause a stroke. but since you are on coumadin thats less likely. another cause would be low blood pressure - hypotension. i dont know your ejection fraction from your 2 d echo but i suppose it would be less- in view of your history of 4 CABGs, and MVR. this may lead to low blood pressure and giddiness.. I would also like to confirm if the valve is functioning well because its 10 years since then. A stenosed or narrow valve can also lead to dizziness spells. i want you to do an EKG, 2D echo and holter monitoring if rhythm disturbances on EKG. Further course of action can be decided after that. if above tests are inconclusive CT BRAIN should be done. wishing you good health thank you