Detection of fetal breathing movements with joint time-frequency analysis of cardiotocogram records

Most of biological signals do not conform to the requirement of stationarity. This fact is the main reason of not appropriate results of analysis of these signals in frequency domain. On the other hand researchers are interested not only if some periodical components exist but when they occur and how long they last. A lot of different joint time-frequency algorithms are available to obtain such information [1] but most of them produce complex structure of cross-term interference. The amplitudes of particular rhythms existing in biological signals have not only wide range but they change in time. These phenomena cause that frequently useful time-dependent pattern is completely destroyed and very difficult for further interpretation. The appropriate method of signal preprocessing may help the researcher to enhance some features and to find valuable information. Two simple algorithms of fetal heart rate filtering are described. Both of them allow to refine fetal breathing movements component from complex structure of the joint time-frequency spectrogram.