An EKG showing ventricular fibrillation, courtesy Andersat, via Wikipedia Commons
Ventricular Fibrillation (VF) or v-fib is the uncoordinated contraction of the heart muscles. It is one of 2 rhythms that an Automated External Defibrillator can shock, the other being pulseless ventricular tachycardia (VT).
When the heart is fibrillating, it is not able to act as an effective pump; instead it makes a motion that has been described as "an uncoordinated jelly". Early defibrillation can significantly increase the chances of patient survival.
A standard sinus rhythm is displayed on an electrocardiogram (ECG) as a "normal" heartbeat. VT will show as a sped up sinus rhythm. VF displays as an uncoordinated zigzag, more like a seismograph.
Defibrillation stops the heart by overloading its electrical system, with the hope that the heart's natural pacemaker, the sinoatrial node, will take control again and proper functioning resume.
An infarction on the myocardial muscle is a common cause for VF; dead muscle does not contract thus affecting the contraction of the remaining muscle. This has an effect on the timing of the signals passing through the Purkinje fibres.