title: Characterization of the excitable gap in human type I atrial flutter.
abstract: OBJECTIVE
  We sought to characterize the excitable gap of the reentrant circuit in atrial flutter.
  
  
  BACKGROUND
  The electrophysiologic substrate of typical atrial flutter has not been well characterized. Specifically, it is not known whether the properties of the tricuspid valve isthmus differ from those of the remainder of the circuit.
  
  
  METHODS
  Resetting was performed from two sites within the circuit: proximal (site A) and distal (site B) to the isthmus in 14 patients with type I atrial flutter. Resetting response patterns and the location where interval-dependent conduction slowing occurred were assessed.
  
  
  RESULTS
  Some duration of a flat resetting response (mean +/- SD 40.1 +/- 20.9 ms, 16 +/- 8% of the cycle length) was observed in 13 of 14 patients; 1 patient had a purely increasing response. During the increasing portion of the resetting curve, interval-dependent conduction delay most commonly occurred in the isthmus. In most cases, the resetting response was similar at both sites. In three patients, the resetting response differed significantly between the two sites; this finding suggests that paced beats may transiently change conduction within the circuit or the circuit path, or both.
  
  
  CONCLUSIONS
  Some duration of a flat resetting response was observed in most cases of type I atrial flutter, signifying a fully excitable gap in all portions of the circuit. The isthmus represents the portion of the circuit most vulnerable to interval-dependent conduction delay at short coupling intervals.
authors.affiliation: Philadelphia Heart Institute and the Sidney Kimmel Cardiovascular Research Center, Pennsylvania 19029, USA. callans@auhs.edu
authors.forename: D J
authors.initials: DJ
authors.lastname: Callans
authors.forename: D
authors.initials: D
authors.lastname: Schwartzman
authors.forename: C D
authors.initials: CD
authors.lastname: Gottlieb
authors.forename: S M
authors.initials: SM
authors.lastname: Dillon
authors.forename: F E
authors.initials: FE
authors.lastname: Marchlinski
mesh_terms: D000200:Action Potentials / Q000502:physiology; D001282:Atrial Flutter / Q000175:diagnosis / Q000503:physiopathology*; D016275:Atrial Function / Q000502:physiology; D006328:Cardiac Catheterization; D002304:Cardiac Pacing, Artificial; D004562:Electrocardiography; D004594:Electrophysiology; D005260:Female; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D012032:Refractory Period, Electrophysiological / Q000502:physiology