The incidence of patients who present to the office with arrhythmia and hemodynamic instability is unknown. Emergency medical systems data, based on ambulance runs, are available only for patients who have had a cardiac arrest. When faced with an unstable or potentially unstable patient, however, we must be prepared to act quickly, safely, and accurately. This article addresses the general approach to such a patient; provides necessary information on office emergency preparation, including training, rapid response team protocol, and the use of automated external defibrillators; and addresses the identification and initial office management of the various rhythms that are capable of threatening a patient's life. © 2006 Elsevier Inc. All rights reserved.