If you ever wonder why I do what I do, there's probably no better example than to see 20 year old Belgian soccer player Anthony Van Loo saved by his implantable cardiac defibrillator (ICD):
In this video, Van Loo is seen walking from the field and then collapsing at 7 seconds, his legs are seen twitching at 15 seconds as his automatic defibrillator fires to restore his heart rhythm to normal, and then by 21 seconds after the event he regains conciousness and sits up. According to some reports, Van Loo was not allowed to return to soccer unless he had an ICD implanted due to his known cardiac disorder.
Despite all of the press out there, this is NOT a heart attack, but rather the result of a life-threatenting heart rhythm disorder like ventricular fibrillation or ventricular tachycardia. In young people, the cause of these life-threatening arrhythmias include right ventricular dysplasia, catecholamine-induced ventricular tachycardia, idiopathic cardiomyopathy, and long QT syndrome (to name just a few). At maximum output, a defibrillator can deliver about 830 volts in a tenth of a second to restore the heart rhythm back to normal.
h/t: JC - A faithful reader.
Addendum: Video - Van Loo describes what he felt.