The two shockable rhythms are ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation is indicated if a shockable rhythm is present. If the victim presents in a shockable rhythm, early defibrillation is still recommended. Clinicians classify cardiac arrest into "shockable" versus "non-shockable", as determined by the ECG rhythm. CPR may succeed in inducing a heart rhythm that may be shockable.