Abstract
We observed that an ST-T change of the ECG occurred just before ventricular premature contraction (VPC) in ambulatory ECG reproduced using a fast linear phase finite impulse response (FIR) filtering procession. We analyzed the characteristics of the waveform distortion induced by the FIR filtering and compared it with the contour effect, which is reported to induce a similar waveform distortion. Ambulatory ECG equipment used in the experiments were System 1 with the FIR filter and System 2 witout one. FIR filtering by System 1 caused pseudo ST-T abnormality immediately before VPC, while the System 1 without FIR filtering did not show a significant waveform change. The System 2 produced a substantial waveform distortion by the contour effect. In computer simulation, waveform distortions of square pulses depended on cut off frequency. To identify this phenomenon more clearly, we used a square pulse, which contains low frequency components as VPC does, as the test pulse. Large waveform distortions of the baselines before square pulses were observed both with the System 1 and the FIR filter and with the System 2. Computed simulation of the waveform distortion induced by the FIR filtering was also performed. The computed signal simulated the FIR filter-induced waveform change well. We conclude that the waveform distortion before VPC was produced by the FIR filtering procession, which we have been using to remove a baseline fluctuation of ambulatory ECG. This ECG wave distortion was similar to one induced by the contour effect. We should pay attention to this waveform distortion by the FIR filter in reading ambulatory ECG, in addition to the contour effect-induced distortion.
