Main Page | See live article | Alphabetical index Fibrillation is the rapid, irregular, and unsynchronized contraction of the muscle fibers of the heart. There are two major classes of fibrillation, atrial fibrillation and ventricular fibrillation. The former can be a chronic condition; the latter is fatal if not reversed by defibrillation. This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.