Ventricular fibrillation is a situation in which the ventricles of the heart beat in an abnormal rhythm because of disorganized electrical activity. This is a dangerous condition because it can quickly result in a cardiac arrest and death, if left untreated.
Under normal circumstances, the electrical activity of the heart is initiated by the sinus node. After the start of the electrical signal in these cells, it travels to the AV-node which passes the signal through the bundle of His, and towards the bundle branches. However, in ventricular fibrillation the electrical activity does not start in the sinus node; instead, it starts in random sites of the heart. This eventually causes the abnormal rhythm. Most of the time, this happens as a result of cardiac diseases like cardiomyopathy or myocarditis. It can also be seen in cases of electrolyte imbalance, drug overdoses, or major trauma. When ventricular fibrillation occurs, it must be treated with CPR and defibrillation immediately.