title: Hysteresis of the RT interval with exercise: a new marker for the long-QT syndrome?
abstract: BACKGROUND
  The diagnosis of the long-QT syndrome (LQTS) may be difficult to establish in patients with normal or borderline prolongation of the QT interval. Noninvasive markers are needed to identify patients with LQTS.
  
  
  RESULTS
  Fourteen patients with known LQTS, 9 unaffected family members, and 40 control subjects underwent modified Bruce protocol exercise testing. The RT interval (peak of R wave to peak of T wave) and rate-corrected RT interval (RTc) were measured during exercise and recovery. The RT interval at 1 minute into recovery was subtracted from the RT interval at a similar heart rate during exercise (deltaRT). The RTc shortened by 61 milliseconds (ms) in the LQTS patients compared with 23 to 26 ms in the other two groups (P=.003 by ANOVA). The RT interval shortened in a linear fashion in all patients but demonstrated persistent shortening during recovery in the LQTS patients. This was manifested as a hysteresis loop in the curve relating the RT interval to cycle length. The hysteresis loop was present in 13 of 14 LQTS patients and only 4 of 40 control subjects. DeltaRT >25 ms had a sensitivity of 73%, a specificity of 92%, a positive predictive value of 79%, and a negative predictive value of 90% for LQTS.
  
  
  CONCLUSIONS
  Hysteresis of the RT interval with exercise may be useful for the diagnosis of LQTS.
authors.affiliation: Department of Medicine, University of Western Ontario, Canada. akrahn@julian.uwo.ca
authors.forename: A D
authors.initials: AD
authors.lastname: Krahn
authors.forename: G J
authors.initials: GJ
authors.lastname: Klein
authors.forename: R
authors.initials: R
authors.lastname: Yee
mesh_terms: D000293:Adolescent; D000328:Adult; D015415:Biomarkers; D002648:Child; D004562:Electrocardiography; D015444:Exercise; D005080:Exercise Test; D005260:Female; D006801:Humans; D008133:Long QT Syndrome / Q000175:diagnosis* / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D013997:Time Factors
chemical_list: D015415:Biomarkers