title: QT Adaptation and Intrinsic QT Variability in Congenital Long QT Syndrome.
abstract: BACKGROUND
  Increased variability of QT interval (QTV) has been linked to arrhythmias in animal experiments and multiple clinical situations. Congenital long QT syndrome (LQTS), a pure repolarization disease, may provide important information on the relationship between delayed repolarization and QTV.
  
  
  RESULTS
  Twenty-four-hour Holter monitor tracings from 78 genotyped congenital LQTS patients (52 females; 51 LQT1, 23 LQT2, 2 LQT5, 2 JLN, 27 symptomatic; age, 35.2±12.3 years) were evaluated with computer-assisted annotation of RR and QT intervals. Several models of RR-QT relationship were tested in all patients. A model assuming exponential decrease of past RR interval contributions to QT duration with 60-second time constant provided the best data fit. This model was used to calculate QTc and residual "intrinsic" QTV, which cannot be explained by heart rate change. The intrinsic QTV was higher in patients with long QTc (r=0.68; P<10(-4)), and in LQT2 than in LQT1/5 patients (5.65±1.28 vs 4.46±0.82; P<0.0002). Both QTc and intrinsic QTV were similar in symptomatic and asymptomatic patients (467±52 vs 459±53 ms and 5.10±1.19 vs 4.74±1.09, respectively).
  
  
  CONCLUSIONS
  In LQTS patients, QT interval adaptation to heart rate changes occurs with time constant ≈60 seconds, similar to results reported in control subjects. Intrinsic QTV correlates with the degree of repolarization delay and might reflect action potential instability observed in animal models of LQTS.
authors.affiliation: Scripps Clinic, San Diego, CA (S.S.) University of Tennessee, College of Medicine, Chattanooga, TN.
authors.forename: Srikanth
authors.initials: S
authors.lastname: Seethala
authors.affiliation: University of Pittsburgh, PA (P.S., J.).
authors.forename: Prabhpreet
authors.initials: P
authors.lastname: Singh
authors.affiliation: University of Iowa, Iowa City, IA (V.S.).
authors.forename: Vladimir
authors.initials: V
authors.lastname: Shusterman
authors.affiliation: Department of Cardiology, Center for Cardiological Innovation and Institute for Surgical Research, Oslo University Hospital, Rikshospitalet, Oslo, Norway (M.R., K.H.H.).
authors.forename: Margareth
authors.initials: M
authors.lastname: Ribe
authors.affiliation: Department of Cardiology, Center for Cardiological Innovation and Institute for Surgical Research, Oslo University Hospital, Rikshospitalet, Oslo, Norway (M.R., K.H.H.) University of Oslo, Norway (K.H.H.).
authors.forename: Kristina H
authors.initials: KH
authors.lastname: Haugaa
authors.affiliation: University of Pittsburgh, PA (P.S., J.).
authors.forename: Jan
authors.initials: J
authors.lastname: Němec
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000293:Adolescent; D000328:Adult; D004562:Electrocardiography; D015716:Electrocardiography, Ambulatory; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008133:Long QT Syndrome / Q000151:congenital* / Q000235:genetics / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D055815:Young Adult
keywords: QT adaptation; QT variability; congenital long QT syndrome