title: Instantaneous phase difference analysis between thoracic and abdominal movement signals based on complementary ensemble empirical mode decomposition.
abstract: BACKGROUND
  Thoracoabdominal asynchrony is often adopted to discriminate respiratory diseases in clinics. Conventionally, Lissajous figure analysis is the most frequently used estimation of the phase difference in thoracoabdominal asynchrony. However, the temporal resolution of the produced results is low and the estimation error increases when the signals are not sinusoidal. Other previous studies have reported time-domain procedures with the use of band-pass filters for phase-angle estimation. Nevertheless, the band-pass filters need calibration for phase delay elimination.
  
  
  METHODS
  To improve the estimation, we propose a novel method (named as instantaneous phase difference) that is based on complementary ensemble empirical mode decomposition for estimating the instantaneous phase relation between measured thoracic wall movement and abdominal wall movement. To validate the proposed method, experiments on simulated time series and human-subject respiratory data with two breathing types (i.e., thoracic breathing and abdominal breathing) were conducted. Latest version of Lissajous figure analysis and automatic phase estimation procedure were compared.
  
  
  RESULTS
  The simulation results show that the standard deviations of the proposed method were lower than those of two other conventional methods. The proposed method performed more accurately than the two conventional methods. For the human-subject respiratory data, the results of the proposed method are in line with those in the literature, and the correlation analysis result reveals that they were positively correlated with the results generated by the two conventional methods. Furthermore, the standard deviation of the proposed method was also the smallest.
  
  
  CONCLUSIONS
  To summarize, this study proposes a novel method for estimating instantaneous phase differences. According to the findings from both the simulation and human-subject data, our approach was demonstrated to be effective. The method offers the following advantages: (1) improves the temporal resolution, (2) does not introduce a phase delay, (3) works with non-sinusoidal signals, (4) provides quantitative phase estimation without estimating the embedded frequency of breathing signals, and (5) works without calibrated measurements. The results demonstrate a higher temporal resolution of the phase difference estimation for the evaluation of thoracoabdominal asynchrony.
authors.affiliation: Institute of Computer Science and Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan.
authors.forename: Ya-Chen
authors.initials: YC
authors.lastname: Chen
authors.affiliation: Department of Computer Science, National Chiao Tung University, Hsinchu, 30010, Taiwan. labview@cs.nctu.edu.tw.|Institute of Biomedical Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan. labview@cs.nctu.edu.tw.|Biomedical Electronics Translational Research Center and Biomimetic Systems Research Center, National Chiao Tung University, Hsinchu, 30010, Taiwan. labview@cs.nctu.edu.tw.
authors.forename: Tzu-Chien
authors.identifier: http://orcid.org/0000-0003-2493-3780
authors.initials: TC
authors.lastname: Hsiao
mesh_terms: D034861:Abdominal Wall / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement; D012119:Respiration; D012815:Signal Processing, Computer-Assisted; D035441:Thoracic Wall / Q000502:physiology*; D013997:Time Factors; D055815:Young Adult
keywords: Complementary ensemble empirical mode decomposition; Instantaneous phase difference; Lissajous figure; Thoracoabdominal motion