Atrial fibrillation is a fast, irregular rhythm where single muscle fibers in your heart’s upper chambers twitch or contract. According to the American Heart Association (AHA), atrial fibrillation is a major cause of stroke, especially among older people. This irregular rhythm may cause blood to pool in the heart’s upper chambers. The pooled blood can lead to clumps of blood called blood clots. A stroke can occur if a blood clot travels from the heart and blocks a smaller artery in the brain.