An external defibrillator is a machine that is used to deliver an electrical shock to the heart to restore a normal heart rhythm. It is used to treat life threatening heart rhythms such as ventricular fibrillation or ventricular tachycardia. It can also be used for electric conversion of atrial fibrillation to normal sinus rhythm.

Figure 1: A normal ECG pattern compared to and life threatening arrhythmia ECG (ventricular fibrillation shown here).

Figure 2: An external defibrillator used to shock the heart and restore a normal heart rhythm.