HIGH-PASS FILTERING OF THE ELECTROGASTROGRAM

The recording and processing of an electrogastrogram require adequate bandpass filtering, to suppress unwanted artefacts but preserve the or iginal signal waveform. High-pass filtering of various types, of diffe rent time constants tau, filter order, analogue and digital implementa tion, have been used to obtain higher baseline stability and faster si gnal recovery after strong artefacts. Special attention should be give n to possible signal amplitude and phase distortions due to high-pass filtering, which can strongly influence accurate amplitude measurement s or studies of signal propagation from multichannel recordings, Synth esised 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 rec ommended for EGG studies if not especially directed to investigation o f bradygastria. In the opposite case, tau = 15 s should be used, and w ith backward filtering a full restoration of the original signal can b e 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 componen t.