Atrial fibrillation is the most common clinical arrhythmia. An electrocardiogram reveals abnormal fluctuations in the atria, which are fast and disordered with some fibrillating waves, known as atrial fibrillation. Diseases causing atrial fibrillation are often associated with structural heart disease. Common conditions include rheumatic heart disease, heart failure, pulmonary heart disease, viral myocarditis, coronary artery disease, and congenital heart disease. Atrial fibrillation can also occur in conditions that cause sympathetic or parasympathetic nerve activity changes, with hyperthyroidism being a common related disease.