Validity of mathematical methods of quantitating fetal heart rate variability

Twenty-two mathematical formulas purporting to quantitate fetal heart rate variability, 13 for short-term and nine for long-term variability, were examined by using artificially generated numbers representing either short-term variability or long-term variability. Numbers representing short-term variability consisted of alternating beats, and those representing long-term variability were sine waves of differing amplitude or frequency spanning the generally accepted clinical range of fetal heart rate variability. The validity of the indices was determined by applying certain selected criteria. The results showed that 11 of the 13 short-term variability indices validly measured short-term variability, whereas only one of the nine long-term variability indices did so. The two major defects in the long-term variability indices were that there was an increase in the long-term variability index as the short-term variability increased, and the long-term variability indices did not increase linearly with increasing frequency of long-term variability complexes. This study suggests that short-term variability is relatively easy to quantify, but there is little relationship between the long-term variability indices and what is clinically regarded as long-term variability.