title: Bradycardia pacing-induced short-long-short sequences at the onset of ventricular tachyarrhythmias: a possible mechanism of proarrhythmia?
abstract: OBJECTIVE
  The purpose of this study was to characterize interactions between normal pacing system operation and the initiating sequence of ventricular tachycardia (VT)/ventricular fibrillation (VF).
  
  
  BACKGROUND
  Abrupt changes in ventricular cycle lengths (short-long-short, S-L-S) might initiate VT/VF. The S-L-S sequences might be passively permitted or actively facilitated by bradycardia pacing.
  
  
  METHODS
  Initiating sequences of 1,356 VT/VF episodes in the PainFree Rx II (n = 634) and EnTrust Trial (n = 421) were analyzed with stored electrograms and by pacing mode (DDD/R, VVI/R, and Managed Ventricular Pacing [MVP]). Interactions between pacing and VT/VF initiation were classified as: non-pacing associated, pacing associated, pacing permitted, and pacing facilitated.
  
  
  RESULTS
  Non-pacing associated (no pacing, no S-L-S) and pacing associated (ventricular pacing without S-L-S) onset accounted for 44.0% and 29.8% of all VT/VF, respectively. Pacing permitted (S-L-S sequences without ventricular pacing) episodes accounted for 6.4% (DDD/R), 20.0% (MVP), and 25.6% (VVI/R) of 1,356 VT/VF episodes. Pacing facilitated onset (S-L-S sequences actively facilitated by ventricular pacing including the terminal beat after a pause) accounted for 8.2% (MVP), 9.4% (VVI/R), and 14.8% (DDD/R) of 1,356 VT/VF episodes. Pacing facilitated S-L-S VT/VF occurred in 2.6% (MVP), 3.3% (VVI/R), and 5.2% (DDD/R) of patients with episodes and was the sole initiating sequence in approximately 1% of patients. Pause durations during pacing facilitated S-L-S differed between modes (DDD/R 793 +/- 172 ms vs. MVP 865 +/- 278 ms vs. VVI/R 1180 +/- 414 ms, p = 0.002). The majority of these episodes were monomorphic VT.
  
  
  CONCLUSIONS
  Ventricular tachycardia/VF in some implantable cardioverter-defibrillator patients might be initiated by S-L-S sequences that are actively facilitated by bradycardia pacing operation and might constitute an important mechanism of ventricular proarrhythmia.
authors: Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA. mosweeney@partners.org
authors: Michael O
authors: MO
authors: Sweeney
authors: Linda L
authors: LL
authors: Ruetz
authors: Paul
authors: P
authors: Belk
authors: Thomas J
authors: TJ
authors: Mullen
authors: James W
authors: JW
authors: Johnson
authors: Todd
authors: T
authors: Sheldon
mesh_terms: D000368:Aged; D001919:Bradycardia / Q000503:physiopathology / Q000628:therapy*; D017147:Defibrillators, Implantable / Q000009:adverse effects*; D004868:Equipment Failure; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D017180:Tachycardia, Ventricular / Q000209:etiology* / Q000503:physiopathology; D016896:Treatment Outcome; D014693:Ventricular Fibrillation / Q000209:etiology* / Q000503:physiopathology