High-Bandpass Filters in Electrocardiography: Source of Error in the Interpretation of the ST Segment

Figure 1

(a) The ECG corresponding to Figure 7, which was used to obtain a sample of the signal, recording the coordinates of 514 points in one PP cycle. The cardiac frequency is 55 bpm, which implies an RR cycle or PP cycle of 1.08 s. The first harmonic represented in (b) and (c) in red colour has a frequency of 0.9 Hz (1/1.08), and on the basis of this frequency the others are multiples (1.8, 2.7 Hz) of the first. (b) Spectrum of frequencies, by amplitude, corresponding to the 169 harmonics into which the signal was decomposed. The zone corresponding to the first 40 harmonics, which are those that contribute to the basic shape of the ECG signal, is shown—the harmonics beyond the first 25–30 having a very low amplitude. (c, d) Graphic and tabular presentation of the first 5 harmonics. Each one is composed of 514 coordinates corresponding to the PP cycle sample previously defined. Compare (b) and (c) (matching colors). The frequency spectrum is the amplitude representation (peak-to-peak distance) of each one of the harmonics mentioned. The sum of the 5 waves in each one of the 514 points is presented in tabular form in the last column of (d) and in graphic form superimposed in red colour on the digitized ECG signal in (e).