title: Mechanisms of spontaneous shift of surface electrocardiographic configuration during ventricular tachycardia.
abstract: OBJECTIVE
  The aim of this study was to examine, with multichannel direct cardiac mapping techniques, the mechanisms of spontaneous shift of the QRS configuration in the surface electrocardiogram during episodes of ventricular tachycardia.
  
  
  BACKGROUND
  Ventricular tachycardias demonstrating a spontaneous shift in their surface electrocardiographic (ECG) features are occasionally encountered. It is not known whether such changes in configuration are primarily due to a significant change in the tachycardia site of origin or represent alterations in patterns of endocardial and epicardial activation. Knowledge of these features would be helpful, particularly when ablative therapy is considered for the arrhythmias.
  
  
  METHODS
  During map-directed cardiac surgery, episodes of ventricular tachycardia were mapped from 224 epicardial and endocardial sites. Episodes of pleomorphic tachycardia were identified and isochronal maps of endocardial and epicardial activation were constructed from representative beats before and after the change in configuration.
  
  
  RESULTS
  From 52 consecutive patients who underwent detailed intraoperative mapping, 9 patients with pleomorphic ventricular tachycardia were identified in whom 14 episodes of spontaneous shift occurred. An analysis of the epicardial activation patterns revealed that the sites of earliest epicardial breakthrough showed significant alteration at the time of QRS shift in all occurrences. In 10 of these shift episodes, however, the sites of tachycardia origin, located on the endocardial surface, remained closely adjacent (< 2 cm apart). Although these sites of origin remained relatively constant, significant alterations in the patterns of endocardial activation were seen in most episodes. These included changes in the direction of propagation of the wave front of activation and shifts between monoregional and figure eight patterns of activation.
  
  
  CONCLUSIONS
  In most episodes of pleomorphic ventricular tachycardia, the arrhythmia site of origin remains relatively constant. However, patterns of epicardial activation do undergo significant change and appear to be the major determinant of the QRS configuration on the surface ECG.
authors.affiliation: Division of Cardiology, Toronto General Hospital, University of Toronto, Ontario, Canada.
authors.forename: S K
authors.initials: SK
authors.lastname: Kimber
authors.forename: E
authors.initials: E
authors.lastname: Downar
authors.forename: L
authors.initials: L
authors.lastname: Harris
authors.forename: G
authors.initials: G
authors.lastname: Langer
authors.forename: L L
authors.initials: LL
authors.lastname: Mickleborough
authors.forename: S
authors.initials: S
authors.lastname: Masse
authors.forename: E
authors.initials: E
authors.lastname: Sevaptsidis
authors.forename: T C
authors.initials: TC
authors.lastname: Chen
mesh_terms: D002304:Cardiac Pacing, Artificial / Q000379:methods; D004562:Electrocardiography / Q000295:instrumentation / Q000379:methods*; D004566:Electrodes; D006801:Humans; D007430:Intraoperative Care / Q000379:methods; D017180:Tachycardia, Ventricular / Q000175:diagnosis* / Q000209:etiology / Q000601:surgery