Method and apparatus for removing baseline wander from an ECG signal

• US 20070078354A1
• Filed: 11/10/2005
• Published: 04/05/2007
• Est. Priority Date: 10/04/2005
• Status: Abandoned Application

• Current Assignee: Welch Allyn Incorporated
• Original Assignee: Welch Allyn Incorporated
• Inventors: Holland, Alexander
• Application Number:
  US11/270,902
• Publication Number:
  US 20070078354A1
• Time in Patent Office: Days
• US Class Current: 600/509
• CPC Class Codes: A61B 5/316 Modalities, i.e. specific diagnostic methods
• CPC Class Codes: A61B 5/318 Heart-related electrical modalities, e.g. electrocardiography [ECG]
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F

Abstract

According to one aspect of the invention, an improved ECG monitor includes a plurality of electrodes to be affixed to a patient'"'"'s body to pick up ECG signals in an ECG signal band. The electrodes are electrically coupled to a plurality of input amplifiers. At least one analog to digital converter (“ADC”) is electrically coupled to the input amplifiers to digitize the ECG signals. A digital baseline wander filter has an internal finite impulse response (“FIR”) low pass filter characterized by a substantially trapezoidal impulse response. The baseline wander filter substantially removes a baseline wander signal component having a range of frequency components below the ECG signal band. The ECG waveform output signal is a baseline filtered ECG waveform representing the one or more of the ECG signals. The ECG waveform output signal from the improved ECG monitor is delayed less than 2 seconds from the ECG signals.

Claims

1. An improved ECG monitor comprising:

a plurality of electrodes to be affixed to a patient'"'"'s body to pick up ECG signals in an ECG signal band, the electrodes electrically coupled to a plurality of input amplifiers;

at least one analog to digital converter (“

ADC”

), the ADC electrically coupled to the input amplifiers to digitize the ECG signals;

a digital baseline wander filter electrically coupled to the at least one ADC to receive the digitized ECG signals, the baseline wander filter having an internal finite impulse response (“

FIR”

) low pass filter characterized by a substantially trapezoidal impulse response, the baseline wander filter to substantially remove a baseline wander signal component having a range of frequency components below the ECG signal band; and

an ECG waveform output signal, the ECG waveform output signal being a baseline filtered ECG waveform representing the one or more of the ECG signals, wherein the ECG waveform output signal from the improved ECG monitor is delayed less than 2 seconds from the ECG signals.

2. The ECG monitor of claim 1 wherein the baseline wander filter is of a low pass to high pass digital filter architecture.

3. The ECG monitor of claim 2 wherein the low pass to high pass digital filter architecture comprises a first signal path and a second signal path, the first signal path comprising a gain and delay element (all pass filter) and the second signal path comprising a cascade of two or more FIR low pass filters.

4. The ECG monitor of claim 3 wherein the substantially trapezoidal impulse results from two cascade boxcar filters in the second signal path or a trapezoidal impulse response with rounded corners resulting from more than two cascade boxcar filters in the second signal path.

5. The ECG monitor of claim 4 wherein the two or more FIR low pass filters are implemented as two FIR low pass filters using an infinite impulse response (“

IIR”

) computationally efficient filter topology.

6. The ECG monitor of claim 5 wherein the baseline wander filter transfer function is represented by the equation:

$H<br><br>\left(z\right)=\left(\frac{1}{N_1}<br><br>\frac{1-z^{-N_1}}{1-z^{-1}}\right)<br><br>\left(\frac{1}{N_2}<br><br>\frac{1-z^{-N_2}}{1-z^{-1}}\right)=\frac{\left(1+z^{-1}+\mathrm{\Lambda <br><br>}+z^{-N_1+1}\right)<br><br>\left(1+z^{-1}+\mathrm{\Lambda <br><br>}+z^{-N_2+1}\right)}{N_1<br><br>N_2}$

7. The ECG monitor of claim 6 wherein

$N_2\ge$

wA D ⁢

 

⁢

and ⁢

 

⁢

N 2 - 2 ⁢

w ≥

N 1 ≥

f s ⁢

wA N 2 ⁢

S .

8. The ECG monitor of claim 6 wherein the sample rate f_s is 500 Hz and N₂>

1500 and 1400>

N₁>

500/3≈

167.

9. The ECG monitor of claim 1 further comprising a power line AC noise filter to remove power line noise from the ECG waveform signal output.

10. The ECG monitor of claim 1 further comprising a high frequency noise filter to remove high frequency noise from the ECG waveform signal output.

11. The ECG monitor of claim 1 further comprising a pulse detection and analysis function block to generate one or more ECG waveform synchronization signals.

12. The ECG monitor of claim 11 further comprising an electrical connection to send the ECG waveform signals to another device.

13. The ECG monitor of claim 12 wherein the electrical connection to send the ECG waveform signals is selected from the group of electrical connections consisting of a cable, a wired network, a wireless network, an optical link, an infrared link, an acoustic link, and an RF wireless link.

14. The ECG monitor of claim 13 wherein the device is a defibrillator.

15. A method to design an ECG baseline wander filter having near optimum minimal delay while meeting industry requirements for ECG monitors comprising the steps of:

providing a set of relevant parameters from an ECG monitor performance specification;

converting the relevant parameters to impulse response constraints on a set of discrete signal equations for a finite impulse response filter;

providing a transfer function for a filter architecture; and

reducing the impulse response constraints to a final set of equations for the filter architecture, to determine the parameters defining a finite impulse response of the ECG baseline wander filter.

16. The method of claim 15 wherein providing relevant parameters from an ECG monitor performance specification comprises providing relevant parameters from the American National Standards Institute/Association for the Advancement of Medical Instrumentation (“

ANSI/AAMI”

) EC11 specification.

17. The method of claim 16 wherein providing a set of relevant parameters comprises providing A, the amplitude of an exciting test pulse;

w, the number of samples specifying the width of this pulse;

D, the maximal allowed displacement error from the actual ECG waveform; and

S, the maximal slope allowed at the end of the waveform.

18. The method of claim 17 wherein converting the relevant parameters to impulse response constraints comprises converting the relevant parameters to impulse response constraints as follows:

$\underset{m=0}{\overset{w-1}{\sum <br><br>}}<br><br>h<br><br>\left(n-m\right)<<br><br>D<br><br>\text{/}<br><br>A<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}\left|\underset{m=0}{\overset{w-1}{\sum <br><br>}}<br><br>\left(h<br><br>\left(n-m\right)-h<br><br>\left(n-1-m\right)\right)\right|<<br><br>\frac{S}{f_s<br><br>A}$

19. The method of claim 18 wherein reducing the impulse response constraints to a final set of equations for the filter architecture, to determine the parameters comprises reducing the impulse response constraints to a final set of equations to determine the parameters for a concatenated filter topology.

20. The method of claim 19 wherein reducing the impulse response constraints to a final set of equations comprises reducing the impulse response constraints (for a baseline wander filter having two boxcar filters in a low pass filter path) to a final set of equations:

$N_2\ge <br><br>\frac{\mathrm{wA}}{D}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>N_2-2<br><br>w\ge <br><br>N_1\ge <br><br>\frac{f_s<br><br>\mathrm{wA}}{N_2<br><br>S}.$

21. An improved digital baseline wander (restoration) filter comprising:

a low pass filter to high pass filter digital architecture having a first signal path and a second signal path, the first signal path comprising a gain and delay element (all pass filter) and the second signal path comprising a cascade of two or more FIR low pass filters wherein the improvement is to the impulse response of the low pass filter in the form of a finite impulse response (“

FIR”

) that is substantially trapezoidal in shape; and

a digital input signal coupled to the first and second signal paths, the digital input signal having a signal band of interest of frequencies above a frequency f_c and a baseline wander including frequencies below f_c the baseline wander filter to substantially remove a baseline wander signal component having a frequency components below f_c and to pass the signal band of frequencies above f_c to generate a baseline wander filtered output signal having only a signal band of frequencies substantially above f_c.

22. The digital baseline wander filter of claim 21 wherein f_c is in the range of 0.1 Hz to 0.9 Hz.

23. The digital baseline wander filter of claim 21 wherein the two FIR low pass filters are implemented using an infinite impulse response (“

IIR”

) computationally efficient filter topology.

24. The digital baseline wander filter of claim 21 wherein the baseline wander filter transfer function is represented by the equation:

$H<br><br>\left(z\right)=\left(\frac{1}{N_1}<br><br>\frac{1-z^{-N_1}}{1-z^{-1}}\right)<br><br>\left(\frac{1}{N_2}<br><br>\frac{1-z^{-N_2}}{1-z^{-1}}\right)=\frac{\left(1+z^{-1}+\mathrm{\Lambda <br><br>}+z^{-N_1+1}\right)<br><br>\left(1+z^{-1}+\mathrm{\Lambda <br><br>}+z^{-N_2+1}\right)}{N_1<br><br>N_2}$