title: Intra-atrial conduction block along the mitral valve annulus during accessory pathway ablation: evidence for a left atrial "isthmus".
abstract: BACKGROUND
  We observed a change in the atrial activation sequence during radiofrequency (RF) energy application in patients undergoing left accessory pathway (AP) ablation. This occurred without damage to the AP and in the absence of a second AP or alternative arrhythmia mechanism. We hypothesized that block in a left atrial "isthmus" of tissue between the mitral annulus and a left inferior pulmonary vein was responsible for these findings.
  
  
  RESULTS
  Electrophysiologic studies of 159 patients who underwent RF ablation of a left free-wall AP from 1995 to 1999 were reviewed. All studies with intra-atrial conduction block resulting from RF energy delivery were identified. Fluoroscopic catheter positions were reviewed. Intra-atrial conduction block was observed following RF delivery in 11 cases (6.9%). This was evidenced by a sudden change in retrograde left atrial activation sequence despite persistent and unaffected pathway conduction. In six patients, reversal of eccentric atrial excitation during orthodromic reciprocating tachycardia falsely suggested the presence of a second (septal) AP. A multipolar coronary sinus catheter in two patients directly demonstrated conduction block along the mitral annulus during tachycardia.
  
  
  CONCLUSIONS
  An isthmus of conductive tissue is present in the low lateral left atrium of some individuals. Awareness of this structure may avoid misinterpretation of the electrogram during left AP ablation and may be useful in future therapies of atypical atrial flutter and fibrillation.
authors.affiliation: Department of Pediatric Cardiology, University of Utah, University Hospital, Salt Lake City, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Luria
authors.forename: J
authors.initials: J
authors.lastname: Nemec
authors.forename: S P
authors.initials: SP
authors.lastname: Etheridge
authors.forename: S J
authors.initials: SJ
authors.lastname: Compton
authors.forename: R C
authors.initials: RC
authors.lastname: Klein
authors.forename: S S
authors.initials: SS
authors.lastname: Chugh
authors.forename: T M
authors.initials: TM
authors.lastname: Munger
authors.forename: W K
authors.initials: WK
authors.lastname: Shen
authors.forename: D L
authors.initials: DL
authors.lastname: Packer
authors.forename: A
authors.initials: A
authors.lastname: Jahangir
authors.forename: R F
authors.initials: RF
authors.lastname: Rea
authors.forename: S C
authors.initials: SC
authors.lastname: Hammill
authors.forename: P A
authors.initials: PA
authors.lastname: Friedman
mesh_terms: D001145:Arrhythmias, Cardiac / Q000601:surgery*; D016279:Atrial Function, Left; D017115:Catheter Ablation / Q000009:adverse effects*; D004594:Electrophysiology; D006327:Heart Block / Q000209:etiology* / Q000503:physiopathology*; D006329:Heart Conduction System / Q000503:physiopathology; D006801:Humans; D008943:Mitral Valve / Q000503:physiopathology*; D012189:Retrospective Studies