Reviewed by: Jonathan Melamed, M.D NYU Medical Center Ventricular Fibrillation (also known as “V-Fib”) is a very rapid, uncoordinated series of contractions throughout the lower chambers of the heart caused by chaotic electrical impulses. In this case, the rhythm is very fast and irregular causing the heart to effectively stop beating. Ventricular fibrillation is the worst kind of abnormal heart rhythm, and is a form of cardiac arrest. ">