Detecting and Processing the Foetal Electro-cardiogram Obtained Using Abdominal Electrodes

Abstract

The signal detected from electrodes placed on the abdomen of an expectant mother consists of the foetal electro-cardiogram, estimated between 5 and 60 microvolts, a larger maternal complex, potentials originating from breathing and abdominal muscles and electronic noise. In this thesis the development of an amplifier to improve the quality of the foetal signal to facilitate its interpretation is described. The basic amplifier, of the balanced input type, is designed with complementary dual field effect transistors and is temperature compensated. The input impedance is about 1010ohms and the bandwidth extends from d.c. to 1O5Hz. For electrocardiographic purposes these were limited to 107 ohms for each input and 1Hz to 130Hz respectively. The use of negative common mode feedback gives a rejection ratio estimated to be 120 db. The electronic noise is less than 1 microvolt peak to peak. The output is single ended and the overall gain is 104. Spurious signals due to interaction between metallic electrode materials and body electrolytes are reduced by the use of graphite electrodes in the form of blocks and cloth drastically cleaned initially to remove other metals with which it had come into contact during manufacture. F.M. tape recording and averaging using a PDP8/I computer was undertaken to produce noise-free foetal electro-cardiograms for diagnostic purposes. The various shapes of the averaged electrocardiograms obtained are discussed. The effect of bandwidth on the averaged waveform and on the diagnosis of foetal presentation is demonstrated. The system has also been used for detecting foetal life, multiple pregnancies, presentation and for measuring heart rate. Recent methods for improving the computer processing techniques are also discussed. It is concluded that foetal electro-cardiography using abdominal electrodes can provide a single and safe method for obtaining invaluable information for the obstetrician. The basic amplifier with its high input impedance, wide bandwidth and low noise level and additional processing techniques so developed may have applications in general electrophysiology.