An irregular heartbeat that occurs when the electrical signals in the atria (the two upper chambers of the heart) fire rapidly at the same time. This causes the heart to beat faster than normal. Although atrial fibrillation can occur without any signs or symptoms, it may sometimes cause fatigue, chest pain, heart palpitations, trouble breathing, low blood pressure, dizziness or fainting, weakness, and other signs and symptoms. Atrial fibrillation may be caused by certain conditions, such as high blood pressure, heart attack, coronary artery disease, abnormal heart valves, congenital heart disease, lung disease, viral infections, or an overactive thyroid gland. Previous heart surgery, stress, or exposure to caffeine, tobacco, alcohol, or certain medicines may also lead to atrial fibrillation. It may go away on its own or it may last for many years. If not treated, atrial fibrillation can lead to serious problems, such as blood clots, stroke, and heart failure. It is more common in older adults. Also called afib.