The purpose of this project was to develop a noninvasive method to detect fetal heart activity in utero.
ABSTRACT
The purpose of this project was to develop a noninvasive method to detect fetal heart activity in utero. Developing such a method is of high importance, since most of the current methods suffer from a high percentage of incorrect diagnoses: false alarms resulting in operational deliveries, and a small but significant percentage of undetected fetal distress.
BACKGROUND
ECG (Electrocardiogram) is a signal, produced by the electrical activity of the heart. It is measured between two electrodes attached to the skin. The ECG has a known shape, and clinical diagnoses can be made by its monitoring. The abdominal ECG measured from a pregnant woman contains the maternal signal, the fetal signal, and additive noise. The noise is mainly due to the muscles electrical activity – EMG. The fetal ECG signal has an amplitude 10-100 times smaller than the maternal. Currently a Doppler Ultrasound device is used for monitoring fetal heart rate. Our goal was to use the Doppler signal for detection of the fetal ECG.
BASIC APPROACH
The Doppler signal is filtered to produce envelopes from the bursts. Adjacent envelopes are cross-correlated to give the interval between fetal heart beats. Using these intervals, it is possible to average a fetal ECG template, starting the averaging from an arbitrary point, and then fine tuning it. By searching above a threshold, the maternal beats are identified, and averaged to a template. The additional noise is modeled as an autoregressive stochastic process, using noise only segments. The model we use to reconstruct each fetal ECG complex, is based on a model used by Lange et. al. for brain signals – EEG. The block diagram of the model is shown in Fig. 1.
The filters coefficients are found using the Least Squares solution. Then, the fetal complex shape is given by the output of its filter.
This algorithm was realized in a Graphical User Interface. The output of the interface is shown in the Fig. 2. In the reconstructed fetal ECG signal, the dotted line is the measured abdominal ECG, and the continuous line is the reconstructed one.
TOOLS
The signals were sampled with a Data Translation A/D card. The simulations and the Graphical User Interface were programmed in MATLAB for Windows, on a PC.
CONCLUSION
The algorithms were found to be efficient, and the results were satisfactory. The developed interface gave excellent results in several patients tested so far, and in several situations. The dependence of the algorithm on the Doppler signal, requires special attention in measuring it, and making sure that its quality is sufficient. The interface is a tool for achieving good results fairly easily from the measured signals.
ACKNOWLEDGMENTS
We would like to thank our supervisor Danny Lange for his guidance throughout the project, and Dr. Reuven Lewinsky for providing the signals, and for giving us insight into the problem. We would also like to thank the laboratory staff for the technical support.
This work was supported by the Ollendorff Center Research Fund.