Observations in perinatal heart rate monitoring. I. A quantitative method of describing baseline variability of the fetal heart rate.

Simultaneous magnetic tape recordings of fetal scalp ECG, maternal ECG and uterine contractions were obtained from 23 patients in active labor. Computer-derived distributions of beat-to-beat changes were used to develop a simple numerical and graphical description of the long and short-term variability patterns. The visually observed variability may be described by the R-R interval change (beat-to-beat change), which is greater than 90% of the total number of beat-to-beat changes. Thus, the 90th percentile of R-R interval changes quantitatively describes the baseline variability. The variability may be classified as fixed, minimal, moderate or marked, based on this simple calculation. Graphs of cumulative percentage of R-R interval changes provide additional visual information. The effects of meperidine HCl on the observed variability were easily detected and visualized by these methods.