References
External links
Shire Pharmaceuticals Agrylin/Xagrid website for international visitors
Shire Pharmaceuticals Xagrid website for UK visitors
AOP Orphan Pharmceuticals website
Re-entry ventricular arrhythmia is a type of paroxysmal tachycardia occurring in the  ventricle where the cause of the arrhythmia is due to the electric signal not completing the normal circuit, but rather an alternative circuit looping back upon itself. There develops a self-perpetuating rapid and abnormal activation. ("Circus Movement" is another term for this.) Conditions necessary for re-entry include a combination of unidirectional block and slowed conduction. Circus movement may also occur on a smaller scale within the AV node (dual AV nodal physiology), a large bypass tract is not necessary.Re-entry is divided into two major types: [Anatomically Defined] re-entry and [Functionally Defined] re-entry. The circus movement can occur around an anatomical or functional core. Either type may occur alone, or together.Anatomically defined re-entry has a fixed anatomic pathway. Anomalous conduction via accessory pathways (APs) creates the re-entry circuit (which are also called bypass tracts), that exists between the atria and ventricles. Wolff–Parkinson–White syndrome (WPW) is an example of anatomically defined re-entry. WPW syndrome is an atrioventricular re-entrant tachycardia (AVRT), secondary to an accessory pathway that connects the epicardial surfaces of the atrium and ventricle along the AV groove. The majority of time symptomatic WPW fits the definition of AVRT (Supraventricular tachycardia) however AVNRT (dual AV nodal physiology) exist in ~10% of patients with WPW syndrome creating the possibility of spontaneous atrial fibrillation degenerating into ventricular fibrillation (VF).