Abnormally fast, slow or irregular heartbeat, caused by disturbed impulses from the sinoatrial node or irregularities in their transmission. Under certain circumstances impulses may be produced in parts of the heart other than the sinoatrial node, in which case the regular rhythm is disturbed by extra beats (extrasystole). Abnormalities in the transmission system impair the passage of impulses from the auricles to the ventricles, and in serious cases they can be entirely blocked (heart block), meaning that the ventricles do not contract, or contract only at a very slow pace. If the muscle cells do not contract simultaneously, the condition is known as atrial or ventricular fibrillation, a life-endangering condition, because it usually leads to cardiac arrest. Heart rhythms are checked by an electrocardiogram (ECG). Most can be treated by drugs; ventricular fibrillation can be stopped only by a powerful electric shock to the heart.