The purpose of this study is to evaluate the safety and effectiveness of the leadless pacemaker system in treating patients with a slow heart rate or irregular heartbeats. The Nanostim leadless pacemaker provides bradycardia pacing as a pulse generator with built-in battery and electrodes, for permanent implantation in the right ventricle. As a leadless pacemaker, it does not need a connector, pacing lead, or pulse generator pocket, but it has the same operating principles as a conventional pacemaker.