High-Bandpass Filters in Electrocardiography: Source of Error in the Interpretation of the ST Segment
Figure 2
(a) The application of high-pass filters with cut-off frequencies of 0.5 Hz produces minimal attenuation of the amplitude of the first harmonic in the frequency spectrum of our signal but yields phase shifts that affect up to the fifth or sixth harmonic (corresponding to a frequency of ~5 Hz). (b) Attenuation of a wave is a decrease of the peak-to-peak amplitude. Phase shift is a displacement of the wave. (c) High-pass filters of 0.05 Hz do not cause attenuation or phase shifts in the frequency spectrum of the signal. (d) Here is shown the correspondence between the digital signals obtained by scanning a trace before and after application of a 0.5 Hz high-pass filter, using the MATLAB 5.0 program, with those obtained in real traces by manual recording with high-pass filters of 0.05 and 0.5 Hz. “Manual” mode means real-time print.
