Method and device for filtering out baseline fluctuations from a

Details Method and device for filtering out baseline fluctuations from a Method and device for filtering out baseline fluctuations from a

1993-12-22

1995-11-28

Kamm, William E.

Surgery

Truss

Pad

A61B 504

Patent

active

054698561

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates generally to a method and a device for filtering out baseline fluctuations from an electrocardiogram by means of a high pass or band pass filter.
2. Description of the Related Art
Electrocardiograms derived from patients are normally overlaid by disturbance signals such as, for example, the 50 Hz (60 Hz in the United States) alternating voltage induced by the alternating current grid, electrical muscle potentials and artifacts resulting from taking the electrocardiogram from the patient. To the extent that these disturbances are of low-frequency type in comparison with the heartbeat rate, they are expressed in the form of baseline fluctuations in the respectively recorded electrocardiogram.
It is known for example from "Journal of Clinical Engineering", Volume 7, No. 3, July-September 1982, pages 235-240, to filter out these baseline fluctuations from the recorded electrocardiogram by means of a high pass filter. In this case, the lower cut off frequency of the high pass filter is set to a value which is below the frequencies which are characteristic of the heart signals at the lowest occurring heart rate. At high heartbeat rates, there is therefore a disturbance frequency range which is not covered by the high pass filtering.


SUMMARY OF THE INVENTION

It is an object of the present invention to improve the effectiveness of the baseline filtering of an electrocardiogram signal.
The above object is inventively achieved in a method having the steps of determining the current heartbeat rate from the electrocardiogram signal taken from the patient and altering the lower cut off frequency depending on the current heartbeat rate. The lower cut off frequency is increasingly altered in the case of an increasing heartbeat rate and decreasingly altered in the case of a decreasing heartbeat rate.
The above object is inventively achieved in an apparatus for filtering out baseline fluctuations from an electrocardiogram signal having a high pass or band pass filter with a variable low cut off frequency. The input of the filter, which is acted upon by the electrocardiogram signal connects with the input of a device for determining the heartbeat rate from the electrocardiogram signal. This device connects on the output side with a control output of the filter to control the lower cut off frequency.
An advantage of the method of the present invention or of the apparatus the present invention is the matching of the lower cut off frequency of the filter to the determined heartbeat rate whereby the maximum possible component of the baseline fluctuations is continuously filtered out from the electrocardiogram signal.
The determination of the heartbeat rate advantageously occurs by subjecting the electrocardiogram signal to a frequency analysis, or an alternative thereto. Such frequency analysis can be performed by using a frequency analyzer. In doing so, component frequencies in the electrocardiogram signal are determined and the lowest "useful" component can be fixed and used to determine the lower cut-off frequency of the filter. The components below the cut-off frequency are in the unwanted baseline fluctuations and are thus filtered out. In an alternative embodiment, a QRS detector, a downstream stage and a reciprocal value former are used. A largest period in the "useful" electrocardiogram signal is determined by measuring the temporal spacing of the detected QRS complexes with the downstream stage. The reciprocal value from the reciprocal value former forms the lower cut-off frequency by taking the reciprocal value of the period. The occurrence of QRS complexes in the electrocardiogram signal is detected and the reciprocal value is thus formed from the temporal spacing of successive, detected QRS complexes.


BRIEF DESCRIPTION OF THE DRAWING

The exemplary embodiments of the method and apparatus for filtering out baseline fluctuations from an electrocardiogram signal of the present invention shall be set forth in greater detail below with refe

REFERENCES:
patent: 4261369 (1981-04-01), Allor
patent: 4281664 (1981-08-01), Duggan
patent: 5016642 (1991-05-01), Dukes et al.
patent: 5074303 (1991-12-01), Hauck
patent: 5259387 (1993-11-01), dePinto

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