An external defibrillator for delivering a defibrillation shock to the heart of a patient under cardiac arrest is provided. The defibrillator comprises an energy source for providing electrical energy, at least one capacitor for building up and storing an electrical charge for delivery to the patient and at least one set of electrodes to be applied externally to the patient's skin. The external defibrillator is adapted to provide at least one pulse train comprising at least one individual pulse of a duration of from 0.4 to 3 ms. Waveforms and methods of operating an automated external defibrillator are further provided.
