A device that can trigger mechanical contractions of the heart by emitting periodic electrical discharges. If the device delivers electricity through the chest wall, it is called a transcutaneous pacemaker; if it works via electrodes inserted inside the body, it is called an internal or implantable pacemaker. Pacemakers are used most often to treat patients with symptomatic slow heart rates or long pauses between heart beats (e.g., patients with third-degree heart block, symptomatic second-degree heart block, bifascicular block with first-degree heart block, carotid sinus hypersensitivity, and tachybrady syndrome) and slow ventricular response rates. Occasionally, though, they also are used for other purposes, such as to capture and override some tachyarrhythmias.