Home > Conditions & Treatments > Arrhythmia > Ventricular Fibrillation. Ventricular fibrillation is a condition in which the heart's electrical activity becomes disordered. When this happens, the heart's ventricles, the lower chambers that pump blood, contract in a rapid, unsynchronized way. The ventricles flutter rather than beat, causing the heart to pump little or no blood. Ventricular fibrillation is life-threatening and requires prompt treatment. Collapse and sudden cardiac death will follow in minutes unless medical help is provided immediately.