title: A mechanism for the upper limit of vulnerability.
abstract: BACKGROUND
  The strongest shock that induces reentry in the heart is the upper limit of vulnerability (ULV). In order to understand defibrillation, one must know what causes the ULV.
  
  
  OBJECTIVE
  The goal of this study was to examine the mechanism of the upper limit of vulnerability.
  
  
  METHODS
  Numerical simulations of cardiac tissue were performed using the bidomain model. An S2 shock was applied during the refractory period of the S1 action potential, and results using a smooth curving fiber geometry were compared with results using a smooth plus random fiber geometry.
  
  
  RESULTS
  When using a smooth fiber geometry only, no ULV was observed. However, when a random fiber geometry was included, the ULV was present. The difference arises from the fate of the shock-induced break wave front when it reaches the edge of the tissue hyperpolarized by the shock (the virtual anode).
  
  
  CONCLUSIONS
  Our numerical simulations suggest that local heterogeneities throughout the tissue may be crucial for determining the fate of the shock-induced wave front at the edge of the virtual anode, and therefore play an important role in the mechanism underlying the ULV.
authors.affiliation: Department of Physics, Oakland University, Rochester, Michigan 48309, USA.
authors.forename: Nachaat
authors.initials: N
authors.lastname: Mazeh
authors.forename: Bradley J
authors.initials: BJ
authors.lastname: Roth
mesh_terms: D003198:Computer Simulation; D004554:Electric Countershock; D004558:Electric Stimulation; D022062:Electrophysiologic Techniques, Cardiac; D006321:Heart / Q000503:physiopathology*; D008564:Membrane Potentials; D008955:Models, Cardiovascular