CA2250331A1

CA2250331A1 - Method and apparatus for removing artifact from physiological signals

Info

Publication number: CA2250331A1
Application number: CA002250331A
Authority: CA
Inventors: Ziad Elghazzawi
Original assignee: Siemens Medical Systems, Inc.; Ziad Elghazzawi; Siemens Medical Solutions Usa, Inc.; Draeger Medical Systems, Inc.
Current assignee: Draeger Medical Systems Inc
Priority date: 1997-10-17
Filing date: 1998-10-15
Publication date: 1999-04-17
Also published as: DE69829621D1; JP4338242B2; JPH11192217A; US5971930A; EP0913121A1; DE69829621T2; ATE292413T1; EP0913121B1

Abstract

A heart rate value determined from a first physiological signal that is acquiredindependently from a second physiological signal, is used to control a band pass filter for controllably bandpass filtering the second physiological signal. In a system having sensors for independently acquiring both electrocardiogram (ECG) and pulse oximetry (SpO2) signals, a heart rate value determined from the ECG signal is used to controllably bandpass filter the red and infrared SpO2 signals, thereby reducing the level of artifact signal in the SpO2 signals.

Description

CA 022~0331 1998-10-1~

.

Memod and Apparatus for Remo~iDg Artifact From Physiological Signals BACKG~OUND OF TE~ INVENl ION
l. Field of the ~nvention The present invention relates to a method and app~udtlls for removing artifact from physiological signals, and more specifically, to using a heart rate value that is deterrnined from a first physiological signal that is acquired independently from a second physiological signal, for controllably bandpass filtering the second physiological slgna .
In a system having sensors for acquiring both electrocardiogram (ECG) and pulse oximetry (SpO2) signals, the ECG signal is used to controllably bandpass filter the red and infrared signals acquired by the pulse oximeter. As a result of the invention, more accurate measurement of blood oxygenation and pulse rate is achieved 20 by the oximeter, and the number of false alarms due to erroneous measurements is greatly reduced.

2. Description of the Prior Art:

2 5 As well known by those of ordinary skill in the art, a pulse oximeter measures arterial blood oxygen saturation and pulse rate using a sensor co~ g two LED's and a photodiode detector, which is applied directly to a well perfused part of a patient, such as at a finger or ear. The LED's of the sensor apply radiation of two di~.ent wavelenghts, commonly red and infrared, to the patient, and the photodiode

3 0 detector responsive to red and infrared light develops red and infrared electrical signals that are affected, via tr~nsmi~sion or reflection, by the patient's blood flow in the area between the two LED's and photodiode detector. The greater the oxygenation of the blood, the less of the emitted red light is detected, due to greater absorption of the red light by the patient's blood. Consequently, the acquired red and infrared signals can be 3 5 processed to develop a measurement indicative of the blood oxygenation.
-CA 022~0331 1998-10-1~

_ Additionally, by processing of the pulsatile component of the acquired light signals, a measunnent of the pulse rate of the patient can also be developed. As well known, such processing for determining blood o,sygenalion is based on the ratio of the AC and DC components of the red light colllpaled to ACr/DCr 5 the infrared light, such as: ACir/DCir The resultant value is applied to an e~e~ nt~lly determined reference table to provide the final determination of the acquired measurement of the blood oxygenation.

As well known, the blood oxygenation and pulse rate measurements made from 10 optically acquired signals are highly prone to inaccuracies due to artifacts. Such artifacts typically result from electrical interference (lights, electro-surgical and other electrical equipment near the patient), patient movement (causing a relative movement between the LED's and detector of the sensor, or even worse, the sudden admission of room light into area of the photodiode detector), as well as the fact that the AC
15 component of the acquired signals (which results from the pulsatile characteristic of the blood), is very small, typically on the order of only 1%-5% of the DC value of the acquired signals. Consequently, such artifacts are extremely detrimental to accurate pulse oximetry measurements, and furthermore7 can easily lead to the disturbing problem of false alarms.
US Patent 4,955,379 entitled MOTION ARTEFACT REJECTION SYSTEM
FOR PULSE OXIMETERS, issued September 11, 1990, discloses the a band-pass filtering (BPF) technique for removing noise artifacts from pulse oximetry signals.
More specifically, the AC components of each of the acquired red and infrared signals 2 5 is initially filtered by a BPF that is broadly tuned to the expected heart rate frequency.
The output of the BPF is applied to a frequency determining circuit, whose output is then used to cause the BPF to track the frequency determined by the frequency determining circuit. The theory of this technique is that most of the energy (and information) in the AC signal is contained in at the fundamental frequency, and since 3 0 the fundamental frequency should be the pulse rate, the frequency deterrnining circuit will determine the pulse rate as the filnd~mçnt~l frequency and control the BPF to CA 022~0331 1998-10-1~

exclude all other frequencies, along with artifacts. Unfortunately, it is quite possible that the fundamental frequency determined by the frequency determining circuit may in fact be an artifact signal, such as one that is generated by electrical equipment, causing the oximeter to process the artifact signal and report erroneous infol..,a~ion.
5 Consequently, this technique is undesirable.

US Patent 4,928,692 entitled METHOD AND APPARATUS FOR
DETECTING OPTICAL PULSES, issued May, 29, 1990, discloses a technique wherein the R-wave portion of a patient's ECG waveform is correlated in time with the 10 optical signals acquired by a pulse oximeter. The correlation is used to develop an enabling signal for processing of the acquired optical signals by the oximeter. The theory is that since the pulse component of the optical signals contain the i~ollllaLion, - ~ and the occurrence of the optical pulses can be predicted to follow an ECG R-wave by a certain amount, selective timing of oximeter enablement will prevent artifact from 15 being admitted into the oximeter and erroneously processed. Unfortunately, since artifacts can occur at any time, and in general are not in any way correlated so as to have any relation to occurrence of an ECG R-wave, this technique is undesirable.

20 SUMMARY OF TH~; INVENTION
Tn ~ccord~nc~w}ththe-principlcs ofth~present-invention-~heart ratc value-that is determined from a first physiological signal that is acquired independently from a second physiological signal, is used for controllably bandpass filtering the second physiological signal. In a system having sensors for independently acquiring both 25 electrocardiogram (ECG) and pulse oximetry (SpO2) signals, a heart rate valuedetermined from the ECG signal is used to controllably bandpass filter the red and infrared SpO2 signals.

As a result of the invention, more accurate measurement of blood oxygenation 30 and pulse rate is achieved by the oximeter, and the number of false alarms due to erroneous measurements is greatly reduced.

CA 022~0331 1998-10-1~

In a pr~e,~ed embodiment, the heart rate value derived from the ECG signal is used to determine the main frequency of the heartbeat. APter dete~ .ng the main frequency of the heartbeat, a bandpass filter (BPF) having a center frequency centered at the main heartbeat frequency, has one or more of its filter characteristics controlled 5 by value of the determined heartbeat frequency, for filtering the red and infrared SpO2 signals. This method dramatically improves the determination of the AC values of the red and infrared signals. In accordance with a further aspect of the invention the DC
part of the red and infrared SpO2 signals are determined as the mean of these signals over a fixed time interval, or by low pass filtering. Once the AC and DC components 10 of the red and infrared signals are determined, the ratio (ACred/DCred)/(ACir/DCir) is used to determine the blood saturation, as well known. By improving the determination of the AC and DC components, the ratio accuracy is improved, and - - therefore the blood saturation measurement accuracy is improved, specifically for the case where there are small amplitude SpO2 signals and high levels of artifact.

BRIEF DESCRIPTION OF Tl~E DR~WING
The sole FIGURE illustrates in block diagram forrn the operation of a pulse oximeter in accordanc~ with the principles of the invention.

DETAILED DESCRIPTION OF TIIE PREFERRED EMBODIlV~ENT
Referring to the FIGURE, a physiological monitor 2 is shown having a first sensor arrangement 4 for acquiring electrocardiogram ~ECG) signals 5, and a second 2 5 sensor arrangement 6 for acquiring red and infrared pulse oximetry (SpO2) electrical signals 7a and 7b. Sensor arrangement 4 may comprise one or more ECG electrodes attached to the chest of a patient, as well known to those in the art, and sensor arrangement 6 may comprise two LED's and a photodiode detector, attached to a well perfused part of the patient, such as a finger or ear, as also well known to those in the 30 art. As previously described, the acquired red and infrared SpO2 signals can be processed to develop measurements indicative of the blood oxygenation and pulse rate of the patient. As well known, such processing for determining blood o~ygenalion is CA 02250331 1998-10-1~

based on the ratio of the AC and DC components of the red signal compared to the ACr/DCr inr.aled signal, Such as. ~Ci,/DCi, The resl-lt~nt value is applied to an experimentally-determined reference table to provide the final deterrnination of the acquired measurement of the blood oxygenation.

Pulse rate can be determined by applying the AC component to a zero-crossing detector and accuml-l~ting the count during intervals of one minute or less. A
processor 8 is illustrated as being responsive to the AC and DC components of the acquired red and infrared SpO2 signals. Processor 8, using the above noted equation, 10 then develops measurements indicative of the blood oxygenation and pulse rate of the patient, and provides them at outputs 10 and 12, respectively, for application to a - ~ display or other interface, not specifically shown, as indications to a user of monitor 2.

As previously noted, the AC components of the acquired SpO2 signals are 15 easily cont~min~ted with artifact signals which disturb their processin;" and can easily result in determination of inaccurate measured values and even the generation of false alarms by monitor 2.

As also previously noted, since most of the desired information in the acquired 2 0 SpO2 signals is located at the fi~nd~mental frequency of the component representative of the pulsing blood in the patient, bandpass filtering of the acquired SpO2 signals to select only the fundamental frequency of the pulsatile component for further processing should greatly reduce the level of artifacts that cont~min~te the acquired SpO2 signals.
However, in the forenoted US Patent 4,955,379 the fundamental frequency of the 25 pulsatile component is deterrnined by processing of the acquired SpO2 signals. As previously noted, this technique is undesirable.

Thus, in accordance with a first aspect of the present invention, a heart rate value is obtained independently from the acquired SpO2 signals, namely, from the3 0 ECG signal S acquired by the separate sensor arrangement 4, and that value is used to controllably bandpass filter each of the acquired SpO2 signals.

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More specifically, a heart rate calculator 14 of any of several designs well known to those skilled in the art, calculates the frequency of the heartbeat in units of Hertz. The calculation is done as follows:

HR

5 where HMF is the main frequency of the heartbeat in Hertz, and ~ is the heart rate in beats per minute.

Next, the filter characteristics of bandpass filters (BPF's) 16a and 16b are - controlled by the calculated HMF value, for controllably filtering the red and infrared 10 SpO2 electrical signals 7a and 7b, respectively, before they are applied to processor 8.
The same bandpass filter characteristics are used for both of the red and infrared signals. In response to the HMF value, at least one of the passband filter shape, center frequency or bandwidth of each of BPF filters 1 6a and 1 6b is controlled in an adaptive manner in response to changes in the frequency of the heartbeat signal as determined by heart rate calculator 14. In a preferred embodiment, filters 16a and 16b may each comprise digital filters, having their coefficients controlled by the output of a look-up t~b~e (no~specificall~shown). Thc filler c~~ t~ Iha~ ~ ~uLpul by ~I,e look-u~
table are determined by application of the HMF value thereto as an address, and illustratively cause each filter to have a bandwidth of 10.5Hz centered at the ~ nging 2 0 value of the deterrnined frequency of the heartbeat signal. The thus f~ltered red and infrared signals are then applied to processor 8. The narrowness of filters 16a and 16b substantially reduce the amplitude of artifact signals in the red and infrared signals, yet, because the passband is centered at the fun-l~m~nt~l frequency of the heartbeat signal, the information content of the red and infrared signals is only incignificantly affected.
In accordance with a further aspect of the present invention, in parallel with the forenoted bandpass filtering, the DC components of the red and infrared SpO2 electrical signals are determined using low pass filters (LPF) 18a and 18b having a cut-off frequency of less than 0.5 Hz. Alternatively, the DC components can be CA 022~0331 1998-10-1~

determined by calc~ ting the mean value of these signals over a fLYed interval of time, such as over a 10 seconds.

Finally, processor B determines the pulse rate from the AC components of the S red and infrared SpO2 electrical signals, and determines a measurement of the blood ox~ygenaLion by the ratio of AC and DC components of the red and infrared signals applied thereto.

Thus, what has been shown and described is a novel method and apparalus for 10 reducing artifact signals in a physiological signal in a manner which fulfills all the advantageous and objects sought therefore. Many changes, modifications, variations ., .. , , . . . = . . . . . . . . .
and other uses and applications of the subject invention will however be apparent to -~ those of ordinary skill in the art after consideration of this specification and its accompanying drawings, which disclose a p-efelled embodiment thereof. For example, 15 although in the illustrated embodiment artifact is reduced in an SpO2 physiological signal, the invention can find applicability for reducing artifact in any physiological signal having a pulsatile component. In this regard, although the invention is illustrated in an SpO2 monitor modified to also acquire an ECG signal, the monitor may in fact comprise a mulLip~,neter monitor. Furthermore, it should be understood that the 2 0 heart rate value can be determined by means other than by use of an ECG signal, and i~r PY~ple7 ~n ultr~soulld~accelerometer, nu~lear--n-~neti~reson~lce, elcctric~limpedance, or other signal may be used instead. Additionally, although in the illustrated embodiment only the center frequency of PBF's 1 6a and 1 6b was controlled, other characteristics of the filters could be adaptively modified, such as bandwidth 2 5 and/or band shape. In view of the above, the scope of the invention is inte~ded to be limited only by the following claims.

Claims

What I claim is:

1. A method for reducing the level of artifact signal in a physiological signal,comprising the following steps:
acquiring a first physiological signal;
acquiring a second physiological signal using an acquisition technique that is different and independant from the technique used to acquire the first physiological signal;
processing the second physiological signal so as to determine a heart rate valuetherefrom; and using said determined heart rate value for controllably bandpass filtering the first physiological signal and developing an AC component thereof having a reduced level of artifact signal therein.

2. The method of claim 1, wherein:
said first-noted acquiring step acquires red and infrared pulse oximetry signals;
and said second-noted acquiring step acquires electrocardiogram signals.

3. The method of claim 2, including a processing step comprising processing of the red and infrared pulse oximetry signals after being band pass-filtered, to develop a measurement of blood oxygenation.

4. The method of claim 3, wherein said processing step processed the red and infrared pulse oximetry signals after being band pass filtered, to develop a measurement of pulse rate.

5. The method of claim 3, wherein the processing step includes processing AC
and DC components of the red and infrared pulse oximetry signals in accordance with the following equation:

for developing said measurement of blood oxygenation.

6. The method of claim 3, including a filtering step comprising low pass filtering said red and infrared pulse oximetry signals in parallel with their band pass filtering, to develop DC components thereof.

7. The method of claim 6, wherein said processing step includes processing the AC and DC components of the red and infrared pulse oximetry signals in accordance with the following equation:

for developing said measurement of blood oxygenation.

8. The method of claim 7, wherein said processing step processes the red and infrared pulse oximetry signals after being band pass filtered, to develop a measurement of pulse rate.

9. The method of claim 7, wherein said processing step develops said DC
components by determining the mean value of the red and infrared signals over a fixed interval of time.

10. Apparatus for reducing, the level of artifact signal in an acquired physiological signal, comprising:
a first sensor arrangement for acquiring a first physiological signal;
a second sensor arrangement for acquiring a second physiological signal using an acquisition technique that is different and independant from the acquisition technique used to acquire the first physiological signal;
a processor responsive to the second physiological signal for determining a heart rate value therefrom; and a bandpass filter responsive to said determined heart rate value for controllably bandpass filtering the first physiological signal and developing an AC componentthereof having a reduced level of artifact signal therein.

11. The apparatus of claim 10, wherein:
said first sensor arrangement acquires red and infrared pulse oximetry signals;
and said second sensor arrangement acquires electrocardiogram signals.

12. The apparatus of claim 11, wherein said processor is responsive to the red and infrared pulse oximetry signals after they are band pass filtered, for developing a measurement of blood oxygenation.

13. The apparatus of claim 12, wherein said processor is responsive to the red and infrared pulse oximetry signals after they are band pass filtered, for developing a measurement of purse rate.

14. The apparatus of claim 12, wherein said processor processes AC and DC
components of the red and infrared pulse oximetry signals in accordance with thefollowing equation:

for developing said measurement of blood oxygenation.

15. The apparatus of claim 12, further including a low pass filter for filtering said red and infrared pulse oximetry signals in parallel with their band pass filtering, to develop DC components thereof.

16. The apparatus of claim 15, wherein said processor processes AC and DC
components of the red and infrared pulse oximetry signals in accordance with thefollowing equation for developing said measurement of blood oxygenation.

17. The apparatus of claim 16, wherein said processor is responsive to the red and infrared pulse oximetry signals after they are band pass filtered, for developing a measurement of pulse rate.

18. The apparatus of claim 16, wherein said processor develops said DC
components by determining the mean value of the red and infrared signals over a fixed interval of time.