title: Sedentary long-duration head-down bed rest and ECG repolarization heterogeneity.
abstract: BACKGROUND
  We studied the effects of 90 d of 6 degrees head-down bed rest (HDBR) on cardiac autonomic and advanced electrocardiographic (ECC) function, especially on repolarization heterogeneity as assessed by beat-to-beat QT interval variability (QTV), T-wave complexity, and 3-dimensional ECG. Based on prior observations of lengthening of the QTc interval during long-duration spaceflight, we hypothesized that abnormalities in ECG repolarization would also occur during long-duration HDBR.
  
  
  METHODS
  During controlled breathing, 5-min supine high-fidelity 12-lead ECGs were obtained from 20 healthy subjects (14 men and 6 women) together with measurements of plasma volume and electrolytes at 5 points in time: within 10 d before; 28-30, 60, and 90 d into; and 3-5 d after HDBR.
  
  
  RESULTS
  By repeated measures ANOVA, 90 d of sedentary HDBR significantly increased the QTV index (from -1.87 +/- 0.33 to - 1.53 +/- 0.39 units), the index of unexplained QTV (from 0.61 +/- 0.48 to 1.21 +/- 0.40 units), the T-wave complexity intradipolar ratio (from 0.344 +/- 0.260 to 2.04 +/- 4.01%), and the spatial QRS-T angle (from 49.1 +/- 23.8 to 58.7 +/- 31.0 degrees), and significantly decreased the spatial ventricular gradient (from 91.3 +/- 26.5 to 59.1 +/- 23.0 mV x ms(-1)). These changes resolved in part by 3-5 d after resumption of ambulation, but unlike concomitant changes in the QTc interval itself and in heart rate variability, they did not significantly relate to changes in electrolytes or plasma volume.
  
  
  CONCLUSIONS
  Sedentary, long-duration HDBR reversibly increases ECG repolarization heterogeneity and by inference ventricular arrhythmic risk.
authors.affiliation: University of Texas Medical Branch, Galveston, TX, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Sakowski
authors.forename: Vito
authors.initials: V
authors.lastname: Starc
authors.forename: Scott M
authors.initials: SM
authors.lastname: Smith
authors.forename: Todd T
authors.initials: TT
authors.lastname: Schlegel
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D000704:Analysis of Variance; D001145:Arrhythmias, Cardiac / Q000503:physiopathology; D001341:Autonomic Nervous System / Q000502:physiology*; D001510:Bed Rest / Q000009:adverse effects*; D004562:Electrocardiography; D005260:Female; D018475:Head-Down Tilt / Q000009:adverse effects*; D006339:Heart Rate; D006801:Humans; D008297:Male; D010953:Plasma Volume; D013026:Space Flight