High-pass filtering of the electrogastrogram.

The recording and processing of an electrogastrogram require adequate band-pass filtering, to suppress unwanted artefacts but preserve the original signal waveform. High-pass filtering of various types, of different time constants tau, filter order, analogue and digital implementation, have been used to obtain higher baseline stability and faster signal recovery after strong artefacts. Special attention should be given to possible signal amplitude and phase distortions due to high-pass filtering, which can strongly influence accurate amplitude measurements or studies of signal propagation from multichannel recordings. Synthesised and original signals are used to demonstrate the effect of high-pass filtering. The use of a first-order filter with tau = 5 s is recommended for EGG studies if not especially directed to investigation of bradygastria. In the opposite case, tau = 15 s should be used, and with backward filtering a full restoration of the original signal can be obtained. The same is valid for recording the electrical activity of the colon. Lower time constants (tau = 5 s or less) can be applied to acquire signals from the small intestines. A radical solution is the use of a DC amplifier with controllable subtraction of the DC component.