title: Respiratory-phase domain analysis of heart rate variability can accurately estimate cardiac vagal activity during a mental arithmetic task.
abstract: OBJECTIVE
  The objectives of this paper were to present a method to extract the amplitude of RSA in the respiratory-phase domain, to compare that with subjective or objective indices of the MWL (mental workload), and to compare that with a conventional frequency analysis in terms of its accuracy during a mental arithmetic task.
  
  
  METHODS
  HRV (heart rate variability), ILV (instantaneous lung volume), and motion of the throat were measured under a mental arithmetic experiment and subjective and objective indices were also obtained. The amplitude of RSA was extracted in the respiratory-phase domain, and its correlation with the load level was compared with the results of the frequency domain analysis, which is the standard analysis of the HRV.
  
  
  RESULTS
  The subjective and objective indices decreased as the load level increased, showing that the experimental protocol was appropriate. Then, the amplitude of RSA in the respiratory-phase domain also decreased with the increase in the load level. The results of the correlation analysis showed that the respiratory-phase domain analysis has higher negative correlations, -0.84 and -0.82, with the load level as determined by simple correlation and rank correlation, respectively, than does frequency analysis, for which the correlations were found to be -0.54 and -0.63, respectively. In addition, it was demonstrated that the proposed method could be applied to the short-term extraction of RSA amplitude.
  
  
  CONCLUSIONS
  We proposed a simple and effective method to extract the amplitude of the respiratory sinus arrhythmia (RSA) in the respiratory-phase domain and the results show that this method can estimate cardiac vagal activity more accurately than frequency analysis.
authors.affiliation: Graduate School of Information Science and Technology, University of Tokyo, Tokyo, Japan. kota@nano.pe.u-tokyo.ac.jp
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Kotani
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Takamasu
authors.forename: Makoto
authors.initials: M
authors.lastname: Tachibana
mesh_terms: D006321:Heart / Q000294:innervation*; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D007564:Japan; D008433:Mathematics; D008606:Mental Processes; D015656:Respiratory Mechanics / Q000502:physiology*; D014630:Vagus Nerve / Q000502:physiology*