The Effect of Different types
of Digital Filters on the Results of High-Resolution Electrocardiography
The type of the digital filter used in high resolution electrocardiography may strongly affect signal shape, and may cause excessive filter ringing, thus corrupting the signal and deteriorating measurement precision. This is even more important in the case when the signals of interest are of low-amplitude, relatively short duration and higher frequency. Such is the case with the late potentials, which are expected after a large and steep QRS deflection. Simson's "bidirectional" infinite-impulse-response (IIR) filter is of good spectral resolution, but of poor phase. It does not cause spread out of the QRS, but pushes artefactual ringing into it from both sides, and in addition introduces a discontinuity in the middle of the QRS complex. This is a serious drawback.
The finite-impulse-response (FIR) filters, on the other hand, offer a linear phase thus preserving signal morphology, but good frequency resolution comes at the cost of poor time resolution. This may cause unreal widening of the QRS complex (especially after a large QRS ending abruptly), and may affect the precision of analysis.
In an effort to optimize the filters used in HiRes signal-averaged electrocardiography, we studied different IIR and FIR filters that we synthesized using our own development software (written in Forth language). We developed the following filters:
We then compared how the
QRS shape of the vector magnitude of the filtered XYZ was affected by the
type of filter, and how the diagnostical measures QRSduration, V40 and
LAS are influenced. Although all three parameters are modified by the
filter type, it is possible to translate the criteria by redefining the
tresholds.
Discussion and conclusion:
The FIR filter with Fourier series coefficients
with number of taps between 30 and 50 might be optimal for the detection
of late potentials. For n=51 no misclassification
occurs when the critical QRS treshold is increased by 25 ms, the LAS border
is prolonged by 2 ms, and 7 uV are added to the V40 treshold.
Related Publications:
[1] Gramatikov, B. Digital filters for the detection of late potentials. Medical and Biological Engineering and Computing, Vol.31, No.4, July 1993, pp.416-420.
[2] Gramatikov, B. Comparison of some linear phase FIR-filters for real-time ECG processing. Sixth National Conference on biomedical Physics and Engineering with international Participation. Sofia, 22-24 October 1992 (in Engl.) In Proceedings - pp.27-31.
[3] Gramatikov, B. Methods of generating and testing of digital filters for biological signals (Bulg.) Fourth national scientific session Automation in biotechnological processes and biomedical research, Sofia, 17-19 Sept. 1991; in Proceedings, pp. 210-217.