In emergency cardiac care, any of the following cardiac rhythm disturbances: ventricular fibrillation, pulseless ventricular tachycardia or some poorly tolerated supraventricular tachycardias (e.g., some instances of rapid atrial fibrillation, atrial flutter, or AV nodal re-entrant tachycardia). By contrast, asystole, pulseless electrical activity, heart blocks, and the bradycardias are not shockable. Defibrillation or cardioversion of these latter rhythms may result in injury to the patient.