Defibrillation is a common treatment for life-threatening cardiac arrhythmias including ventricular fibrillation and ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the heart with a device called a defibrillator. Defibrillators can be external or implanted (implantable cardioverter-defibrillator), depending on the type of device needed. Some external units, known as automated external defibrillators (AEDs), automate the diagnosis of treatable rhythms, meaning that lay responders or bystanders are able to use them successfully with little or no training at all.