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Study objective The protocol for the use of the automated external defibrillator calls for a period of "hands-off" time, during which no cardiopulmonary resuscitation (CPR) can be performed. We assessed the actual interruption time of CPR during the use of the automated external defibrillator in patients in out-of-hospital cardiac arrest. Methods This study included 184 patients experiencing out-of-hospital cardiac arrest in which an automated external defibrillator was applied by first responders. ECG and voice recordings from the automated external defibrillator were downloaded and analyzed. Start and end times of CPR were recorded, as were intervals measured from the recordings concerning the programmed interruption time and the interruption time related to performance. Results The automated external defibrillators were connected for a median time of 4 minutes 47 seconds (range 31 to 1,404 seconds). CPR was performed during 45%±15% (mean±SD) of the connected time or until return of spontaneous circulation. During the automated external defibrillator connection time in the 96 patients with a shockable rhythm, CPR was performed 36%±20% of the time. Programmed interruption of CPR took 40%±15% of the automated external defibrillator connection time, and no CPR was performed related to performance during 23%±15% of the time. A palpable pulse was never present immediately after a shock, and return of spontaneous circulation was observed in 3 of 184 patients before arrival of the ambulance. Ultimately, return of spontaneous circulation occurred in 87 of 184 patients. Conclusion First responders using automated external defibrillator voice prompts provide CPR less than half the time that the automated external defibrillator is connected to the patient. Technical improvements in automated external defibrillator rhythm analysis, more efficient resuscitation algorithms, and first-responder education could increase CPR delivery and, perhaps, improve outcome.