Underlying causes of atrial fibrillation and flutter include dysfunction of the sinus node (the "natural pacemaker" of the heart) and a number of heart and lung disorders, including coronary artery disease, rheumatic heart disease, mitral valve disorders, pericarditis and others.
Hyperthyroidism, hypertension and other diseases can cause arrhythmias, as can recent heavy alcohol use (binge drinking). Some cases of atrial fibrillation or flutter occur in the setting of a heart attack or soon after surgery on the heart.