Ventricular fibrillation :A condition in which the heart's electrical activity and consequent contraction becomes disordered and ineffective. One mechanism believed to be responsible for the rapid unsynchronized disorderly contraction of the ventricles is multiple reentrant waves (multiple spiral waves of electrical activity, as shown in the figure). Ventricular fibrillation is very serious. Since the heart pumps little or no blood, collapse and sudden cardiac death will follow within minutes unless medical help is provided immediately.