An automated external defibrillator (AED) is a portable device that analyzes the heart's rhythm and can deliver an electric shock to restore a normal rhythm for cardiac arrest. An AED is used for arrhythmias like ventricular tachycardia and ventricular fibrillation that can lead to cardiac arrest. To operate an AED, call 911, check for a pulse, locate the AED, turn it on and follow its instructions to deliver a shock if advised or continue CPR until help arrives. AEDs are designed to be simple to use by bystanders to provide emergency treatment until emergency medical services arrive.