Pacemakers are small—weighing about an ounce—sources of electrical stimulation implanted in the heart to maintain a healthy rhythm. Pacemakers are used to treat bradycardia; when the heart rate is slow, as well as atrial fibrillation, heart failure and syncope. They work by delivering an electrical shock to the heart to retrigger a healthy rhythm. Pacemakers rest just under the collarbone and are implanted during an outpatient procedure. Your heart health is critical: Make an appointment with a cardiologist at the MedStar Heart and Vascular Institute today.