Abnormalities or damage to the heart's structure are the most common cause of atrial fibrillation. There are many other possible causes, including high blood pressure, heart attack, coronary artery disease, abnormal heart valves, heart defects you're born with, an overactive thyroid gland or other metabolic imbalance, and exposure to stimulants, such as medications, caffeine, tobacco or alcohol. Besides, sick sinus syndrome, lung diseases, previous heart surgery, viral infections, sleep apnea, stress due to surgery, pneumonia or other illnesses, may also cause atrial fibrillation.