title: Effects of shock strengths on ventricular defibrillation failure.
abstract: BACKGROUND
  The mechanism of defibrillation is controversial. Reentry appearing immediately after the shock has been shown to be responsible for defibrillation failure in some studies while other studies have demonstrated that a rapid train of focal activations with the first focus appearing >50 ms after the shock is responsible for failed defibrillation. We tested the hypothesis that both patterns can occur, but at different shock strengths.
  
  
  RESULTS
  Biphasic 6/4 ms shocks of 100-900 V in 100-V increments were given after 10 s of ventricular fibrillation from electrodes in right ventricular apex and right atrium in five isolated pig hearts. Transmembrane activity was optically mapped from the anterior and posterior epicardium using two CCD cameras. The defibrillation threshold (DFT) was 786+/-199 V. The interval from the shock to the earliest post-shock activation was zero for shocks <400 V but increased with increasing shock voltage to 62+/-6 ms at 800 V. The number of post-shock phase singularities, which is related to reentry incidence, decreased continuously from pre-shock values for 100-V shocks to zero as the shock strength increased to 600 V. Focal activations were observed after shocks >600 V with no epicardial reentry present.
  
  
  CONCLUSIONS
  Reentry is responsible for defibrillation failure for low-strength shocks. As the shock strength approaches the DFT, a focal epicardial activation pattern becomes responsible for failed defibrillation. Thus, the mechanism of defibrillation failure depends on shock strength, with focal activation as the mechanism for the clinically important near-DFT strength shocks.
authors.affiliation: Department of Medicine, University of Alabama at Birmingham, Birmingham, AL 35294-0019, USA. toon@crml.uab.edu
authors.forename: Nipon
authors.initials: N
authors.lastname: Chattipakorn
authors.forename: Isabelle
authors.initials: I
authors.lastname: Banville
authors.forename: Richard A
authors.initials: RA
authors.lastname: Gray
authors.forename: Raymond E
authors.initials: RE
authors.lastname: Ideker
mesh_terms: D000818:Animals; D004554:Electric Countershock / Q000379:methods*; D004594:Electrophysiology; D005260:Female; D006321:Heart / Q000503:physiopathology*; D008297:Male; D008856:Microscopy, Fluorescence; D010477:Perfusion; D013552:Swine; D017211:Treatment Failure; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy*