Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States.  Although several theories exist as to the cause of induction and maintenance of ventricular arrhythmias, the role of Purkinje fibers during these processes remains unclear.  Purkinje fiber potentials are considered viable targets for ablation during ventricular tachycardia and ventricular fibrillation (VF), indicating that they play a critical role in these arrhythmias.  Using 3-dimensional modeling, Berenfeld and Jalife reported that focal activity may arise at the Purkinje muscle junctions, leading to the initiation of VF.  After initiation, the Purkinje fibers appear to become irrelevant once intramyocardial reentry is established during the early stages of VF.  However, global ischemia progresses rapidly as VF progresses and because Purkinje fibers are less resistant to ischemia than the surrounding ventricular myocardium, Purkinje fibers may begin to take on an important role in maintaining VF.  Using optical mapping, I am investigating the role of Purkinje fibers during VF.