Synchronized cardioversion is a procedure that delivers a controlled electrical shock to the heart to restore normal rhythm in atrial fibrillation. The shock is timed (“synchronized”) with the heart’s electrical cycle to avoid causing more dangerous arrhythmias. It is typically used for patients who are unstable with low blood pressure, chest pain, or severe symptoms, or when medications have failed to convert the rhythm. Sedation or anesthesia is usually given to make the procedure more comfortable. Cardioversion is highly effective, though atrial fibrillation can sometimes return, so ongoing treatment is often needed.