title: Phase dependencies between longitudinal corneal apex displacement and cardiovascular signals: is the ocular pulse influenced by the electrical activity of the heart?
abstract: BACKGROUND
  The aim was to establish phase relationships between the principal harmonic, related to the heart rate, of synchronically registered longitudinal corneal apex displacement (LCAD), blood pulsation (BP) and electrical heart activity signals in a group of healthy subjects.
  
  
  METHODS
  Longitudinal corneal apex displacement was non-invasively measured using an ultrasonic distance sensor. Synchronously, electrocardiographic (ECG) and blood pulsation signals were acquired. As all considered signals are non-stationary (that is, their spectral characteristics vary in time), a reliable and repeatable phase estimation method was sought. For this, a range of phase estimators were tested in the windowed regime of simulated non-stationary signals. Two robust estimators that showed minimum mean square error performance, were selected for further analysis of real signals registered for seven subjects participating in the study.
  
  
  RESULTS
  The windowed cross-correlation and the windowed minimum sum of squared error method achieved the best results among the estimators considered and their outputs were averaged to arrive at a robust phase estimator. Across the subjects, it was found that an increase in the time delay between the principal harmonic of BP and ECG signals, θ(BP,ECG), corresponds to a slight time delay increase between the corresponding harmonics of longitudinal corneal apex displacement and blood pulsation signals, θ(LCAD,BP) and a decrease in the time delay between those of longitudinal corneal apex displacement and ECG signals, θ(LCAD,ECG). Significant correlation (paired t-test, p < 0.05) were found between θ(BP,ECG) and θ(LCAD,BP) as well as between θ(BP,ECG) and θ(LCAD,ECG). There was no significant correlation found between θ(LCAD,BP) and θ(LCAD,ECG).
  
  
  CONCLUSIONS
  The results indicate that longitudinal corneal apex displacement and correspondingly the ocular pulse phenomenon have not only a vascular origin but could also be influenced by the electrical activity of the heart.
authors: Institute of Physics, Wroclaw University of Technology, Wroclaw, Poland. monika.danielewska@pwr.wroc.pl
authors: Monika E
authors: ME
authors: Danielewska
authors: D Robert
authors: DR
authors: Iskander
authors: Malgorzata
authors: M
authors: Kowalska
authors: Henryk T
authors: HT
authors: Kasprzak
mesh_terms: D000328:Adult; D003315:Cornea / Q000502:physiology*; D019781:Corneal Topography / Q000379:methods*; D004562:Electrocardiography; D005123:Eye / Q000098:blood supply; D005133:Eye Movements / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult