title: Heart rate-independent prolongation of QTc interval in women with syndrome X.
abstract: BACKGROUND
  Cardiac syndrome X is usually diagnosed in the presence of typical exertional chest pain, a positive response to exercise testing, and normal coronary angiograms. The underlying pathogenic mechanisms are speculative, but myocardial ischemia and increased sympathetic activity have been implicated.
  
  
  OBJECTIVE
  The present study examined whether QTc interval is prolonged in women with syndrome X when confounding factors such as heart rate, gender, and environmental conditions are accounted for.
  
  
  METHODS
  Maximum QTc interval and its relationship to clinical and exercise variables were evaluated in 32 women with syndrome X (exertional chest pain, positive exercise testing, and completely normal coronary arteries) and 34 normal controls.
  
  
  RESULTS
  Patients with syndrome X had significantly longer QTc interval (440 +/- 24 ms) than normal subjects (410 +/- 26 ms, p < 0.004). However, this QTc prolongation appears to be heart rate-independent as patients with syndrome X showed longer QT and QTc intervals than controls despite an identical heart rate. No relation was found between QTc prolongation and clinical or exercise test variables in patients with syndrome X.
  
  
  CONCLUSIONS
  Although increased sympathetic drive is present in syndrome X, and this variable is likely to modulate QTc duration, the mechanism and clinical implications of QTc interval prolongation in syndrome X remain speculative.
authors.affiliation: Department of Cardiological Sciences, St. George's Hospital Medical School, London, United Kingdom.
authors.forename: C
authors.initials: C
authors.lastname: Mammana
authors.forename: O A
authors.initials: OA
authors.lastname: Salomone
authors.forename: J
authors.initials: J
authors.lastname: Kautzner
authors.forename: R A
authors.initials: RA
authors.lastname: Schwartzman
authors.forename: J C
authors.initials: JC
authors.lastname: Kaski
mesh_terms: D016022:Case-Control Studies; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D017566:Microvascular Angina / Q000175:diagnosis* / Q000503:physiopathology; D008875:Middle Aged