Choice of operating parameters in heart sound removal from bowel sounds using adaptive filtering

This study is aimed at enhancing bowel sound (or gastrointestinal acoustic phenomena-[GAP]) analysis in an anesthetized rat model. The measured signal at the rat´s abdominal wall primarily contains infrequent GAP activities against a strong background of ongoing heart sounds (HS). The bandwidth of GAP events overlaps with that of HS rendering use of fixed band-pass filters inadequate. Therefore, an adaptive filter (AF) is necessary. Because no reference signal is readily available, the reference signal is generated by exploiting the low variability in HS properties. Effects of the adaptation parameter and filter order on SNR and GAP distortion due to filtering are discussed. The GAP waveform distortions were minimum for a filter dimension of 1 to 5 points and an adaptation parameter of 5 to 20% of the value at the onset of filter instability. Typical overall SNR enhancement by HS removal with the current system was 25 to 30 dB for a wide range of adaptation parameters and filter dimensions. It is concluded that an AF system can be implemented to study GAP activity in anesthetized rats