title: [Application of fractal dimension in heart rate variability analysis during head-down tilt].
abstract: OBJECTIVE
  The paper applies the fractal dimension to heart rate variability (HRV), and discusses the changes of the fractal dimension of cardiovascular system during head down tilt (HDT).
  
  
  METHODS
  ECG of 10 subjects were recorded during HDT, the experiment was repeated in each subject. Fractal dimension was used to analyze the HRV.
  
  
  RESULTS
  Compared to supine before HDT, the standard deviation of all normal to normal RR intervals (SDNN) at 15-20 min, 20-25 min, 25-30 min and restoration increased significantly; the root mean square of successive differences between adjacent normal cycles (rMSSD) at 20-25 min increased significantly; the fractal dimension at 0-5 min, 5-10 min, 10-15 min, 20-25 min, 25-30 min and restoration increased significantly.
  
  
  CONCLUSIONS
  The result shows that HRV and the nonlinearity of cardiovascular system increase during HDT. It is feasible that the fractal dimension can be used for analyzing HRV.
authors.affiliation: Institute of Space Medico-Engineering, Beijing, China. liangzhonggang@163.com
authors.forename: Zhong-gang
authors.initials: ZG
authors.lastname: Liang
authors.forename: Hong
authors.initials: H
authors.lastname: Yan
authors.forename: Bin
authors.initials: B
authors.lastname: Wu
authors.forename: Ping
authors.initials: P
authors.lastname: Wu
mesh_terms: D002320:Cardiovascular Physiological Phenomena; D004562:Electrocardiography; D017709:Fractals; D018475:Head-Down Tilt; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D017711:Nonlinear Dynamics; D016683:Supine Position / Q000502:physiology; D018474:Weightlessness Simulation