title: Pacing after shocks stronger than the upper limit of vulnerability: impact on fibrillation induction.
abstract: BACKGROUND
  After upper-limit-of-vulnerability (ULV) shocks of the same strength and coupling interval (CI) during the T wave, (1) the epicardial activation pattern (EAP) for the first postshock cycle is indistinguishable between shocks that do (VF) and do not (NoVF) induce ventricular fibrillation (VF) and (2) >/=3 cycles in rapid succession always occur during VF but not during NoVF episodes. To study the role of these rapid cycles, rapid pacing was performed after a shock stronger than the ULV that by itself did not induce rapid cycles and VF.
  
  
  RESULTS
  A 504-electrode sock was sutured to the heart in 6 pigs to map EAPs. The S2 shock strength and S1-S2 CI at the ULV were determined by T-wave scanning with an up/down protocol. Ten shocks 50 to 100 V above the ULV (aULV) were delivered at the same S1-S2 CI to confirm that VF was not induced. Then, the postshock interval after aULV shocks was scanned with an S3 pacing stimulus from the LV apex until the shortest S2-S3 CI that captured was reached. This was repeated for S4, S5, etc, until VF was induced. To induce VF, 3 pacing stimuli (S3-S5) with progressively shorter CIs were required; S3 or S3, S4 never induced VF. After cycle S5, which induced VF, 2 EAP types occurred: focal (74%) and reentrant (26%).
  
  
  CONCLUSIONS
  At least 3 cycles with short CIs are necessary for VF induction after aULV shocks. Cycles S3-S4 may create the substrate for cycle S5 to initiate VF.
authors.affiliation: Departments of Medicine, University of Alabama at Birmingham, Birmingham, AL, USA. toon@crml.uab.edu
authors.forename: N
authors.initials: N
authors.lastname: Chattipakorn
authors.forename: P C
authors.initials: PC
authors.lastname: Fotuhi
authors.forename: C M
authors.initials: CM
authors.lastname: Sreenan
authors.forename: J B
authors.initials: JB
authors.lastname: White
authors.forename: R E
authors.initials: RE
authors.lastname: Ideker
mesh_terms: D000818:Animals; D002304:Cardiac Pacing, Artificial / Q000379:methods*; D004198:Disease Susceptibility; D004594:Electrophysiology; D010496:Pericardium / Q000503:physiopathology; D013552:Swine; D014693:Ventricular Fibrillation / Q000209:etiology*