Human fetal phonocardiography and the detection of fetal activity
Antepartum detection of the fetus at risk of death or damage in-utero remains a major challenge in modern obstetrics. Ultrasonic monitoring of individual fetal biophysical activities (such as fetal heart rate, fetal breathing or fetal body movements) has become widely applied as a method for evaluating fetal well-being. The combined assessment of these fetal activities and the relations between them, however, may well be more useful both in predicting imminent fetal death and in preventing it. The results of ultrasound studies, however, are hampered (e.g. from a safety viewpoint) by the natural periodicity of fetal activities. Fetal phonocardiographic techniques, on the other hand, can be easily used over long time periods and there can be no doubt of their safety. Furthermore, the fetal phonocardiogram may contain more information that fetal heart rate alone. This thesis describes the design and development of a new high-fidelity fetal phono-sensor, based on a piezo-electric PVDF transducer, which offers a completely non-invasive and reliable method of assessing the fetus over the long term. This sensor has been optimised to record faithfully the acoustic output of the fetus and to maximise the signal energy transfer across the maternal abdominal wall, where hitherto this has not been achieved. This is done by matching the compliance of the sensor to that of the maternal abdominal wall. Using a purpose built measuring device, abdominal wall compliance was measured clinically as 3.5 mm/N (averaged over 76 patients). Theoretical and experimental techniques were used to adjust the sensor's compliance to match that of the maternal abdominal wall [to within 4:1], as well as to minimise noise and maximise signal capture. The sensor's force and displacement senstivities were measured as 2183 V/N and 2480 mm/N (much greater than for any present or past phono-sensors). Using a new experimental rig developed to simulate the transmission of the fetal phono-signals through the maternal abdominal wall, the dynamic performance and frequency response of the sensor were also optimised. Clinical studies on 18 patients from 28-41 weeks gestation, showed that the fetal phonocardiogram contains not only fetal heart rate information, but also information about fetal breathing movements (FBM) and fetal body movements (FM), as well as detailed beat-to-beat heart sound interval information. By comparison with real-time ultrasound, various phono-signal patterns were shown to be caused by the fetal activities: regular and cyclic for FBM; and intermittent and noise-like for FM. By using new computerised signal processing techniques in the time domain (such as template-matching and zero-crossing functions), these recorded fetal activities were detected automatically (in over 80% of the time) and their timing periodicities analysed. Frequency domain analysis (using techniques such as the Hilbert transform and cepstral analysis) of the timing periodicities of the fetal heart sounds, showed that diastolic and beat-to-beat time intervals (as wll as their variabilities) are significantly increased during FBM. Fetal heart rate is also decreased [from 147 to 141 bpm] during FBM episodes. Fetal body movements, on the other hand, are associated with significant decreases in systolic, diastolic and beat-to-beat time intervals, whilst their variabilities are increased. Fetal heart rate, in this case, is found to increase [from 144 to 157 bpm]. Although these techniques do not run in real-time at present, they would be capable of doing so if transferred onto fast computers (or transputers). As a result of this work, one is now perhaps in a better position to envisage an automatic real-time fetal phono-based monitoring system for the routine clinical assessment of fetal well-being.