A term applied to rapid contraction or TREMOR of muscles, and especially to a form of abnormal action of the heart muscle in which individual bundles of fibres take up independent action. It is believed to be due to a state of excessive excitability in the muscle associated with the stretching which occurs during dilatation of the heart. The main causes are ATHEROSCLEROSIS, hypertensive heart disease and chronic rheumatic heart disease (see HEART, DISEASES OF). Fibrillation is either atrial or ventricular, depending on whether the muscle of the atria or of the ventricles is affected. When the atrium is fibrillating there is no significant contraction of the atrial muscle but the cardiac output is maintained by ventricular contraction. In ventricular fibrillation there is no significant contractile force, so that there is no cardiac output and resuscitation by cardiac massage is urgent. The commonest cause is myocardial infarction. Administration of DIGOXIN, timolol or verapamil may restore normal rhythm, and in some patients, CARDIOVERSION – a controlled direct-current electric shock given via a modified defibrillator placed on the chest wall – is effective.