title: The effect of different sampling intervals on the measurement of intrapartum fetal heart rate variability.
abstract: OBJECTIVE
  To test the hypothesis that increasing the sampling interval affects the intrapartum fetal heart rate (FHR) variability measurement.
  
  
  METHODS
  Fetal electrocardiograms were obtained from women in labor. Using the peak of the fetal R wave, the R-R interval and FHR were calculated on a beat-to-beat basis. Retrospectively, the original data were repartitioned using different intervals (2-900 seconds) to generate a window of measurement (epoch). The mean value for each epoch and the last FHR in that epoch (epochal value) were compared with published animal and human data. Errors were quantified by comparing the epochal and mean values for each epoch. Fetal heart rate variability between epochs and within each epoch was compared.
  
  
  RESULTS
  Fetal heart rate and R-R interval were measured in 146 cases. The FHR had a normal distribution (mean 140.1 beats per minute, +/- standard deviation [SD] 15.6, skew -0.07), but its inverse, the R-R interval, was not normally distributed (mean 432 milliseconds, +/- SD 52.4, skew 1.78). Using a single value for an epoch duration of 2 seconds resulted in an error that was similar to the within-epoch variability (+/- SD of 2.2 beats per minute difference between mean and epochal value compared to +/- SD of 2 beats per minute within epoch) but which increased with epoch duration.
  
  
  CONCLUSIONS
  An epoch duration of 2 seconds and a single sampled value within this period may be appropriate for measurement of both medium and long-term variability in any computerized intrapartum FHR interpretation system. Fetal heart rate (not R-R interval, because of its normal distribution) should be used to design such a computerized system.
authors.affiliation: Department of Obstetrics and Gynaecology, The Chinese University of Hong Kong.
authors.forename: M A
authors.initials: MA
authors.lastname: Wilcox
authors.forename: W
authors.initials: W
authors.lastname: Wang
authors.forename: D S
authors.initials: DS
authors.lastname: Sahota
authors.forename: R R
authors.initials: RR
authors.lastname: Dawkins
authors.forename: T K
authors.initials: TK
authors.lastname: Chung
authors.forename: A M
authors.initials: AM
authors.lastname: Chang
mesh_terms: D000328:Adult; D015982:Bias; D004562:Electrocardiography / Q000379:methods; D005260:Female; D006340:Heart Rate, Fetal; D006801:Humans; D007743:Labor, Obstetric; D011247:Pregnancy; D013997:Time Factors