Loss of FHR variability diagnosed by frequency analysis

Aims: To determine the loss of fetal heart rate (FHR) baseline variability by frequency analysis. Methods: The FHR tracings of 12 normal fetuses and others with various conditions, as well as flat FHR tracings of a late deceleration (LD) and anencephaly recorded with Doppler fetal monitors, were scanned and processed using fast Fourier transform (FFT) analysis. The ratio of the area under the low frequency spectrum divided by the area under the whole spectrum (La/Ta) and the peak power spectrum density (PPSD) were determined. Results: Long-term variability (LTV) measures >10 bpm revealed significantly more La/Ta and PPSD than LTV <10 bpm in normal FHR tracings. The La/Ta and PPSD were >15% and 60 bpm(2)/Hz, respectively, in representative values of normal FHR cases and in those of fetal respiration, hiccupps and non-reactive FHR, whereas those of the flat baseline of LD and anencephaly were <15% and 60 bpm(2)/Hz. Conclusion: Loss of baseline variability is diagnosed when the La/Ta ratio and PPSD are <15% and 60 bpm(2)/Hz, respectively, based on the FFT frequency analysis of the FHR baseline.