title: Mechanisms of ventricular fibrillation initiation in MADIT II patients with implantable cardioverter defibrillators.
abstract: BACKGROUND
  The availability of stored intracardiac electrograms from implantable defibrillators (ICDs) has facilitated the study of the mechanisms of ventricular tachyarrhythmia onset. This study aimed to determine the patterns of initiation of ventricular fibrillation (VF) in Multicenter Automatic Defibrillator Implantation Trial (MADIT) II patients along with associated electrocardiogram (ECG) parameters and clinical characteristics.
  
  
  METHODS
  Examination of stored electrograms enabled us to evaluate the rhythm preceding each episode of VF and to calculate (intracardiac) ECG parameters including QT, QT peak (QTp), coupling interval, and prematurity index.
  
  
  RESULTS
  Sixty episodes of VF among 29 patients (mean age 64.4 +/- 2.5 years) were identified. A single ventricular premature complex (VPC) initiated 46 (77%) episodes whereas a short-long-short (SLS) sequence accounted for 14 (23%) episodes. Of the 29 patients studied, 23 patients had VF episodes preceded by a VPC only, two patients with SLS only, and four patients with both VPC and SLS-initiated episodes. There were no significant differences between initiation patterns in regards to the measured ECG parameters; a faster heart rate with SLS initiation (mean RR prior to VF of 655 +/- 104 ms for SLS and 744 +/- 222 ms for VPC) approached significance (P = 0.06). The two patients with SLS only were not on beta-blockers compared to 83% of the VPC patients.
  
  
  CONCLUSIONS
  Ventricular fibrillation is more commonly initiated by a VPC than by a SLS sequence among the MADIT II population. Current pacing modes designed to prevent bradycardia and pause-dependent arrhythmias are unlikely to decrease the incidence of VPC-initiated episodes of VF.
authors.affiliation: Department of Medicine, Case Western Reserve University/University Hospitals Case Medical Center, Cleveland, Ohio, USA.
authors.forename: Ryan
authors.initials: R
authors.lastname: Anthony
authors.forename: James P
authors.initials: JP
authors.lastname: Daubert
authors.forename: Wojciech
authors.initials: W
authors.lastname: Zareba
authors.forename: Mark L
authors.initials: ML
authors.lastname: Andrews
authors.forename: Scott
authors.initials: S
authors.lastname: McNitt
authors.forename: Ethan
authors.initials: E
authors.lastname: Levine
authors.forename: David T
authors.initials: DT
authors.lastname: Huang
authors.forename: W Jackson
authors.initials: WJ
authors.lastname: Hall
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Moss
mesh_terms: D000368:Aged; D016009:Chi-Square Distribution; D017147:Defibrillators, Implantable; D004562:Electrocardiography; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D014693:Ventricular Fibrillation / Q000503:physiopathology* / Q000517:prevention & control*; D018879:Ventricular Premature Complexes / Q000503:physiopathology