Heart Rhythm O2 is an online-only, open access publication of the Heart Rhythm Society and is one of three open access companion titles to the respected Heart Rhythm. With a focus on cardiac arrhythmias and electrophysiology, Heart Rhythm O2 publishes original clinical and experimental research, review articles, design papers, brief reports, commentaries, and perspectives in contrast. As an open access journal, Heart Rhythm O2 serves as a major venue for sharing information on the latest research and technologies for electrophysiologists, physicians, and allied health professionals who care for patients with rhythm disorders. Case reports, illustrations, and educational vignettes can be submitted to the journal's companion title, Heart Rhythm Case Reports.