Atrial fibrillation (AF) is a disorganized rhythm in the upper chambers of the heart (atria). The rate at which the atria fibrillate is very rapid (the equivalent of 600-800 beats per minute). Intermittently, one of these impulses travels through the AV node (the electrical connection between the upper and lower chambers of the heart) and into the ventricles. This leads to the typical irregularly irregular pulse in patients with atrial fibrillation. The rate at which these impulses travel through the AV node and into the ventricles is highly variable and depends on many different factors including the amount of scarring around the AV node, medications, and the patient's activity. The rate can sometimes be rapid which can lead to symptoms of palpitations, shortness of breath, dizziness, and lightheadedness. Some patients might have no symptoms at all when in atrial fibrillation and it is sometimes discovered at a routine physical exam. AF is most common in patients with heart disease (myocardial infarction, hypertension, and valvular heart disease) although it can be seen in individuals with apparently normal hearts. In all patients with atrial fibrillation, a thorough workup should be performed to attempt to identify any heart disease that might be present. This typically includes an electrocardiogram, an echocardiogram, and possibly a stress test. Since hyperthyroidism (an overactive thyroid gland) can cause atrial fibrillation in some people, thyroid function tests (a blood test) should also be sent. If hyperthyroidism is suspected as the cause of the AF then treatment of the thyroid disorder can typically eliminate future episodes of AF. If hypertension is suspected then treatment with antihypertensive medications could be warranted. Atrial fibrillation is associated with an increased risk of stroke in many patients. Since the atrial muscle is fibrillating in an uncoordinated manner, the blood in the atria can form into clots. If these clots break off from the atria they can travel to the brain and cause a stroke. Most patients with atrial fibrillation will need to be on a blood thinner to prevent blood clots from forming. Treatment of AF can consist of control of the heart rate while in AF or restoration and maintenance of a normal heart rhythm with medications. In some patients with symptomatic atrial fibrillation, a radiofrequency ablation can be performed to help maintain normal rhythm.