Ventricular tachydysrhythmias are the most common cause of sudden cardiac death in adults. Defibrillation is the most effective treatment for VT and VF. The effectiveness of defibrillation diminishes rapidly over time, and unless treated promptly, VF becomes less coarse and eventually converts to the less treatable rhythm of fine VF or asystole. Adding field ACLS capability appears to affect survival from out-of-hospital cardiac arrest favorably, although the degree of benefit is relatively minimal compared to the powerful effect of early defibrillation.