title: Validation of a noninvasive measure of local myocardial repolarization in a conscious human model: adaptation of repolarization to changes in rate.
abstract: BACKGROUND
  A commercial pacemaker sensor measure of the unipolar endocardial stimulus to T wave interval may accurately reflect changes in the monophasic action potential duration at 90% repolarization (APD90). This sensor system was used to study the kinetics of adaptation of repolarization duration to changes in heart rate in humans.
  
  
  RESULTS
  Patients were studied using an external pacemaker capable of displaying all stimulus to T wave intervals for each paced beat. Right ventricular stimulation was delivered via the pacemaker and compared simultaneously to APD90. Steady-state pacing was simulated by 60 seconds of pacing at cycle lengths (CLs) 350 to 700 msec. Adaptation to a new ventricular rate was analyzed with a sudden 200-msec decrease in CL. The relation between repolarization measure and steady-state CL (n = 16) was linear with a slope of 0.16 and 0.19 for APD90 and stimulus to T wave interval, respectively (P = NS). The adaptation of both repolarization measures to a sudden change in rate were best modeled by a biexponential function. Stimulus to T wave interval exhibited a parallel course to APD90, and an analysis of normalized differences between APD90 and stimulus to T wave interval followed an approximately normal distribution, with 93.5% of the paired differences within 2 SD of the mean.
  
  
  CONCLUSIONS
  A pacemaker sensor measure of stimulus to T wave interval accurately parallels APD90 during both steady-state and sudden changes in rate. Repolarization in human endocardium follows a linear relation to steady-state CL and adapts to a new rate with a biexponential function. This model represents a novel method for studying human cardiac repolarization.
authors.affiliation: Division of Cardiology, St. Michael's Hospital, University of Toronto, Canada.
authors.forename: S D
authors.initials: SD
authors.lastname: Lee
authors.forename: P
authors.initials: P
authors.lastname: Dorian
authors.forename: M
authors.initials: M
authors.lastname: Geist
authors.forename: E
authors.initials: E
authors.lastname: Davies
authors.forename: A
authors.initials: A
authors.lastname: Barr
authors.forename: C
authors.initials: C
authors.lastname: Dunne
authors.forename: M
authors.initials: M
authors.lastname: Paquette
authors.forename: D
authors.initials: D
authors.lastname: Newman
mesh_terms: D000200:Action Potentials; D000293:Adolescent; D000328:Adult; D000368:Aged; D004562:Electrocardiography; D005260:Female; D006321:Heart / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010138:Pacemaker, Artificial