Statistical fluctuations in heart rate variability indices.
Short-term variability in the fetal heart rate (FHR) is believed to be associated with fetal well-being, and a number of quantitative indices of variability have been proposed. Before using such an index, its sampling properties must be well understood so that apparent changes in "variability" can be confidently attributed to biological origins rather than statistical sampling fluctuations. We have compared the sampling properties of two representative variability indices on computer-synthesized "random" FHR patterns. Both indices demonstrated statistical fluctuations which decreased with increasing sample size; the coefficient of variation of a single index value is approximately Formula: see text with N being the number of cardiac cycles observed in practice, if changes in variability are to be detected with reasonable confidence, heart rate observations of several minutes will generally be required.