Re-entry is the third, and possibly the most common cause of premature beats and tachyarrhythmias. Under normal circumstances, an impulse arising in the atria excites the atria and then the ventricles via the AV node and His –Purkinje conducting system. It then dies out because the tissues, once excited, cannot be re-excited until they have recovered their excitability. In addition, the mitral and tricuspid valve rings,shown in the figure in blue and which together form the fibrous skeleton of the heart, separate the atria from the ventricles and prevent the impulse from traveling between these chambers via any route other than the AV nodal pathway or AV nodal bypass tracts.