Ventricular fibrillation is an immediately life-threatening arrhythmia in which the heart's electrical activity and associated contractions become disordered and ineffective. It is characterized by rapid, irregular activation of the ventricles and thereby prevents an effective mechanical contraction. Blood pressure instantaneously drops to zero, leading to death within minutes due to lack of cardiac output unless successful electrical defibrillation is performed; spontaneous conversion to sinus rhythm is rare.

During ventricular fibrillation, the ECG has no distinctive QRS complexes but instead consists of an undulating baseline of variable amplitude. Although the sinus node continues to function properly, P waves cannot be discerned in the VF waveform. Ventricular tachycardia in many cases can degenerate to ventricular fibrillation.

One mechanism believed to be responsible for the rapid, unsynchronized contraction of the ventricles is the continuous reactivation of one or more electrical circuits in the ventricles, which can appear as three-dimensional spiral or scroll waves of electrical activity. These waves prevent the ventricles from contracting in a coordinated manner, thereby compromising the heart's ability to pump blood.