Atrial fibrillation (AF) is an abnormal heart rhythm that is common among patients who are admitted to an intensive care unit (ICU) of a hospital. It is usually a transient occurrence that resolves as the patient recovers from their underlying condition. However, patients who develop AF can present with a very rapid heart rate that in some cases can put stress on the heart which can lead to life threatening heart attacks, low blood pressure or breathing problems. Not all patients with AF will have unstable heart function but those who have rapid heart rates can worsen quickly. The goals of treatment for AF with a rapid heart rate but no unstable heart function are two fold. Patients can be treated by controlling the heart rate and/or by attempting to convert the AF to a normal heart rhythm. The heart rate can be controlled by medication and the AF can be converted by either electrical cardioversion (an electric shock that jump-starts the heart) or medication. Currently it is unknown if the goal of treatment should be to simply control the heart rate and wait for the patient to spontaneously convert to a normal heart rhythm or convert the AF with medication for patients who only have the rapid heart rate.