Differences in foetal heart rate variability from phonocardiography and abdominal electrocardiography

The agreement of the phonocardiographic method to provide foetal heart rate variability (FHRV) indices equivalent to those derived from abdominal electrocardiography was tested. 15 pregnant women were recruited in order to obtain antepartum foetal phonocardiograms and abdominal electrocardiograms three minutes long. From the respective sound (SS) and electric (RR) time series, typical temporal and spectral indices of FHRV were computed and compared. Means of the SS and RR intervals were not significant (P> 0.05) and these showed a correlation r=0.98. However, the temporal indices, P(TOT), HF and LF/HF presented differences (P< 0.05), since SS values were higher. Spectral coherence decreased below 0.5 for frequencies above 0.28+ 0.07 Hz, where bands resembling maternal and foetal breathing movements were noted. Particularly above 0.28Hz, temporal and spectral FHRV indices derived from phonocardiography and electrocardiography show differences. Quality of the signal, processing techniques, and maternal and foetal respiratory factors could account to explain these.