n. a very small fibre or a constituent thread of a fibre (for example, a *myofibril of a muscle fibre). —fibrillar, fibrillary adj.
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 ?bres 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 in dilatation of the heart. The main causes are ATHEROSCLEROSIS, chronic rheumatic heart disease and hypertensive heart disease (see HEART, DISEASES OF). Fibrillation is distinguished as atrial or ventricular, depending on whether the muscle of the atria or of the ventricles is affected. In atrial ?brillation, the heartbeats and the pulse become extremely irregular, both as regards time and force; when the atrium is ?brillating there is no signi?cant contraction of the atrial muscle but the cardiac output is maintained by ventricular contraction. In ventricular ?brillation there is no signi?cant contractile force, so that there is no cardiac output. 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 modi?ed de?brillator placed on the chest wall – is e?ective.... fibrillation
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