title: Rate-related changes in QRS morphology in patients with fixed bundle branch block: implications for differential diagnosis of wide QRS complex tachycardia.
abstract: OBJECTIVE
  To analyse QRS morphology in response to rapid atrial pacing (RAP) and supraventricular tachycardia (SVT) in patients with pre-existing bundle branch block (BBB).
  
  
  RESULTS
  We prospectively studied 59 patients in sinus rhythm (SR), with QRS > or = 120 ms, and no pre-excitation. Trains of RAP were introduced at increasing rates until atrioventricular block. QRS during SR and last QRS complex of each RAP train were compared on the 12-leads. Previously described criteria for minor and major configuration differences were used to identify QRS changes. During RAP minor QRS changes were seen in 22 (37%) and major changes in 23 (39%) subjects. One patient showed major axis shifts and no one showed a change to the contralateral BBB pattern. QRS changes were significantly and independently related to RAP rate and type of BBB (more frequent if right-BBB). Of 14 subjects (24%) with SVT, 13 displayed the same QRS changes during RAP.
  
  
  CONCLUSIONS
  In patients with organic BBB, important changes in QRS morphology, except for a change in the contralateral BBB, can appear during RAP and SVT. Thus, in these patients, a change in QRS morphology during tachycardia does not necessarily imply that it is ventricular tachycardia.
authors.affiliation: Cardiology Department, Hospital General Universitario, Gregorio Marañón, Doctor Esquerdo 46, Madrid 28007, Spain.
authors.forename: Tomás
authors.initials: T
authors.lastname: Datino
authors.forename: Jesús
authors.initials: J
authors.lastname: Almendral
authors.forename: Esteban
authors.initials: E
authors.lastname: González-Torrecilla
authors.forename: Felipe
authors.initials: F
authors.lastname: Atienza
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: García-Fernández
authors.forename: Angel
authors.initials: A
authors.lastname: Arenal
authors.forename: Leonardo
authors.initials: L
authors.lastname: Atea
authors.forename: Francisco
authors.initials: F
authors.lastname: Fernández-Avilés
mesh_terms: D000368:Aged; D002037:Bundle-Branch Block / Q000175:diagnosis* / Q000503:physiopathology; D003937:Diagnosis, Differential; D004562:Electrocardiography / Q000379:methods; D005260:Female; D006334:Heart Function Tests; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013617:Tachycardia, Supraventricular / Q000175:diagnosis* / Q000503:physiopathology; D017180:Tachycardia, Ventricular / Q000175:diagnosis* / Q000503:physiopathology