title: Averaging improves the quality of impedance stroke volume measurements during the head up tilt test.
abstract: OBJECTIVE
  To assess the improvement in quality following averaging data from two or more tilts in the stroke volume (SV) response curve during a head-up tilt test.
  
  
  METHODS
  Ten adult male subjects were tilted head-up to 55 degrees four times. They were kept head-up for 3-minutes with intervening rests of 3-minutes. Impedance measurements were recorded by the RheoCardioMonitor every 10-seconds. The percentage changes in impedance variables with tilting and coefficients of variability (CV) were calculated. The SV data were divided into four single tilt response curves, which were used to assess the variability of the data when these tilts were analyzed separately, in pairs or in triplets. Confidence intervals reflecting this variability were estimated.
  
  
  RESULTS
  Head-up tilting resulted in a mean (SD) decrease in SV of 31 (11)%. Significant variability existed between SV readings (CV = 8 (2)%). Averaging improved the resolution of the wavelets and in most subjects (7 out of 10) allowed dynamic and static phases of the response to be identified. Upper confidence intervals (mean (range)) were reduced from 15 (10-23)% for single wavelets to 8 (3-10)% for pairs to 5 (3-8)% for triplets.
  
  
  CONCLUSIONS
  Impedance measurements can be very variable, making the assessment of SV changes during a head-up tilt test difficult. By averaging the data from several tilts one can improved the quality of the SV wavelet sufficiently to identify important postural changes.
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine and Therapeutics, Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, PRC.
authors.forename: Ye
authors.initials: Y
authors.lastname: Zhang
authors.forename: Lester A H
authors.initials: LA
authors.lastname: Critchley
authors.forename: Julian A J H
authors.initials: JA
authors.lastname: Critchley
mesh_terms: D000328:Adult; D002307:Cardiography, Impedance; D004562:Electrocardiography; D006801:Humans; D008297:Male; D011187:Posture / Q000502:physiology; D013318:Stroke Volume; D018667:Tilt-Table Test; D013997:Time Factors