EP2549926A1

EP2549926A1 - Interference reduction in monitoring a vital parameter of a patient

Info

Publication number: EP2549926A1
Application number: EP11716043A
Authority: EP
Inventors: Jeroen Veen; Theodorus Petrus Henricus Gerardus Jansen; Steven Antonie Willem Fokkenrood
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2010-03-23
Filing date: 2011-03-16
Publication date: 2013-01-30
Also published as: CN102811663A; JP2013530728A; JP6184318B2; WO2011117780A1

Abstract

The invention relates to a method of and device for monitoring a vital parameter of a patient by emitting light onto tissue of the patient with at least one light source (1, 2) and collecting light which is transmitted through the tissue and/or which is reflected from the tissue. The emitted light is multiplexed according to a predefined multiplexing scheme having a plurality of multiplexing channels, and the collected light is detected according to the predefined multiplexing scheme, resulting in a plurality of detection channels (16, 17, 18, 19). At least one of the multiplexing channels is arranged to be a dark multiplexing channel for which no light is emitted by the at least one light source (1, 2), resulting in a dark detection channel (19), and the signal of this dark detection channel (19) is used for generating a reference signal for reducing interference in the signal of at least one of the other detection channels (16, 17, 18). In this way a versatile and reliable possibility of monitoring a vital parameter of a patient with a high signal-to-interference ratio is provided.

Description

Interference reduction in monitoring a vital parameter of a patient

FIELD OF THE INVENTION

The invention relates to the field of light attenuation measurements, and especially to a method of and a device for monitoring a vital parameter of a patient by measuring attenuation of light emitted onto tissue of the patient.

BACKGROUND OF THE INVENTION

The measurement of light absorption and/or scattering when propagating through or reflecting from a certain medium forms the basis of a number of optical spectroscopic methods widely applied in various medical domains, such as patient monitoring. One illustrative example is transmissive pulse oximetry.

Pulse oximetry is an optical method for non- invasive monitoring of arterial oxygen saturation of a patient and has become one of the most commonly used techniques in clinical practice. The protein haemoglobin (Hb) binds oxygen in the red blood cells for transport through the body, and has the property of changing from dark red to bright red in color when oxygenated. By emitting and detecting light at two or more wavelengths, pulse oximeters determine the light absorbance in a peripheral vascular bed to arrive at an indirect estimate of oxygen saturation, i.e. the concentration fraction of oxyhaemoglobin (Hb0₂). Pulse oximeters rely on the changes in arterial blood volume caused by cardiac contraction and relaxation to determine the amount of light absorbed by pulsating arterial blood alone, thereby largely factoring out the contributions of tissue and venous blood.

In many applications, including oximetry, simultaneous or quasi-simultaneous attenuation measurements of an optical path at different wavelengths, i.e. of different colors, are required. To that end, typically multiple light sources are utilized which are generally combined with a single photo detector. In order to be able to distinguish between the signals from each of the emitters at the photo detector, in general, electrical multiplexing methods are employed, such as time division multiplexing (TDM), frequency division multiplexing (FDM), or code division multiplexing (CDM).

In the medical practice, light attenuation measurements applied in e.g. patient monitoring suffer from electromagnetic interference. Typically such interference comprises ambient light at various optical wavelengths and with different modulation frequencies.

Common examples include natural daylight, which is typically not modulated, as well as artificial light from incandescent lamps, which is modulated at the double mains frequency (100 Hz or 120 Hz) and 50 Hz or 60 Hz harmonics, and from fluorescent lamps with flicker rates ranging from tens to hundreds of kilohertz depending on the specific electric ballast.

Generally, in spectrometric devices measures are taken to mitigate the effect of external interference on the measurements. For example in pulse oximeters, the light sources are modulated such that at the photo detector the emitted light can be distinguished from ambient light by filtering or demodulation. Regardless of the modulation techniques applied, conventional methods rely on knowledge of the spectral modulation of the environmental light and assume that the light source modulation frequency or band that is used can remain fixed for the lifetime of the device.

However, if the ambient light modulation spectrum is only partly known or not known a priori, such as is the case when the spectrometric device operates in the vicinity of light communication systems, then interference may be present in the modulation spectrum of the detected light at the device operation frequency. Similarly, new operation schemes of high- intensity discharge (HID) lamps might result in an interference signal with a wide frequency range. If an interferer contaminates the operation frequency band, the signal-to- interference ratio (SIR) may decrease to a large extent, thereby degrading the measurement quality.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of monitoring a vital parameter of a patient by measuring attenuation of light emitted onto tissue of the patient and an according device which allow for a high signal-to-interference ratio in a versatile and reliable way.

This object is achieved by the subject matter of the independent claims.

Preferred embodiments are described in the sub claims.

This means that according to the invention, a method of monitoring a vital parameter of a patient by emitting light onto tissue of the patient with at least one light source and collecting light which is transmitted through the tissue and/or which is reflected from the tissue is provided, the method comprising the following steps:

multiplexing the emitted light according to a predefined multiplexing scheme having a plurality of multiplexing channels; detecting the collected light according to the predefined multiplexing scheme, resulting in a plurality of detection channels;

arranging at least one of the multiplexing channels to be a dark multiplexing channel for which no light is emitted by the at least one light source, resulting in a dark detection channel; and

using the signal of the dark detection channel for generating a reference signal for reducing interference in the signal of at least one of the other detection channels.

It should be emphasized that the term "patient" does not only refer to diseased persons but to all human beings and animals, no matter whether healthy or not.

According to the invention, light, which is transmitted through the tissue or/and which is reflected from the tissue, is collected which is necessary for the attenuation measurement in order to monitor the vital parameter of the patient. However, when collecting this light, it cannot totally be avoided to collect at least some ambient light, too. This collected ambient light can cause interferences.

Hence, it is an idea of the invention to adapt a signal provided by a dark detection channel, i.e. a detection channel which is at least temporarily not used for spectrometric purposes, to reduce interferences caused by ambient light or other sources influencing the detection signals.

According to an embodiment of the invention, this method is used for pulse oximetry. However, the invention does not only apply to pulse oximetry, but can also be used for other spectroscopic methods for monitoring a vital parameter of a patient where a dark channel can be assigned, and where interference components present in the dark channel output relate in some way to the interference components in the other outputs. The dark channel output can therefore be used as a reference for reduction of interference components in another channel.

In general, the signal of the dark detection channel could be directly used as the reference signal. However, according to an embodiment of the invention, for generating the reference signal, the signal of the dark detection channel is adaptively filtered, and the reference signal is preferably subtracted from the signal of the at least one of the other detection channels.

For the adaptive filtering, in general, a wide variety of different methods can be used. However, according to an embodiment of the invention, a least-mean-squares (LMS) algorithm for generating the reference signal is used. The LMS algorithm and its derivatives are well known to those skilled in the art. According to an embodiment of the invention, the LMS algorithm updates an N-tap weight vector w = [wo wi ... according to w(k+l) = w(k) + 2px(k)d(k) where μ represents an adaptation constant, x(k) = [x(k) X4(k-1) ... x(k-N+l) is the reference signal vector, taken from a dark detection channel, and d(k) = x(k) - w (k)x(k) is the difference of the channel/detector signal and the filtered reference. The output of the subtraction is d(k), giving the result of the light extinction measurement for the light source, but from which the interference is removed. The adaptation constant determines the convergence speed of the algorithm as well as the final misadjustment and can be

dynamically set to determine the properties of the algorithm. By this operation, the interference in the output signal can be reduced significantly.

The signals of the detection channels can be processed without any pre- processing. However, according to an embodiment of the invention, the signals of the respective detection channels are low-pass filtered. In this way, reduction out-of-band signals can be achieved.

Various strategies can be followed to operate the reference reduction structure. According to one strategy, one dark channel is assigned continuously and interference reduction is activated continuously. According to an embodiment of the invention the interference level is estimated on the basis of the dark detection channel signal and/or the reference signal, and the signal of the dark detection channel is used for generating the reference signal for reducing interference of the at least one of the other detection channels only when the interference level exceeds a predefined threshold.

According to a preferred embodiment of the invention a plurality of light sources is provided for emitting light of different wavelengths. Further, according to a preferred embodiment of the invention, a plurality, preferably all, of the multiplexing channels are consecutively arranged to be the dark channel for which no light is emitted by the at least one light source, respectively, resulting in an alternately changing dark detection channel. In a system comprising a plurality of light sources emitting light of different wavelengths and a corresponding plurality of detection channels, one of the light sources can be switched off periodically, thus creating a dark channel periodically. Thus, any of the channels can become the dark channel by switching off the respective light source. If the dark channel is rotated among the channels, still spectrometric information for all wavelengths can be obtained while reducing interference. In addition the structure can be expanded to take multiple reference inputs.

Above mentioned object is further addressed by a device for monitoring a vital parameter of a patient, comprising:

at least one light source for emitting light onto tissue of the patient;

at least one light detector for collecting light which is transmitted through the tissue and/or which is reflected from the tissue;

a multiplexer adapted for multiplexing the emitted light according to a predefined multiplexing scheme having a plurality of multiplexing channels, wherein at least one of the multiplexing channels is a dark multiplexing channel for which no light is emitted by the at least one light source;

a plurality of detection channels, being connected to the least one light detector and being adapted for detecting the collected light according to the predefined multiplexing scheme, wherein at least one of the detection channels relating to the at least one dark multiplexing channels is a dark detection channel; and

a reference signal generator adapted for using the signal of the dark detection channel as a reference signal for reducing interference in the signal of at least one of the other detection channels.

According to a preferred embodiment of the invention, the device is adapted for emitting light with at least two different wavelengths, e.g. by comprising two different light sources. Further, it is especially preferred that the device comprise a pulse oximeter.

The reference signal generator can be designed in different ways. According to an embodiment of the invention, the reference signal generator comprises an adaptive filter which is adapted for adaptively filtering the signal of the dark channel for generating the reference signal. Further, according to an embodiment of the invention, a subtractor is provided which is adapted for subtracting the reference signal from the at least one of the other detection channels.

Furthermore, according to a preferred embodiment of the invention, the device comprises a low-pass filter which is adapted for filtering the signals of the respective detection channels. In this way out-of-band signals can be reduced and, thus, to a certain degree excluded from further processing. It is preferred that the device comprises a plurality of light sources for emitting light of different wavelengths, respectively. Moreover, it is preferred that a common light detector for all detection channels is provided. According to a preferred embodiment of the invention, the adaptive filter is adapted to provide the reference signal based on a least-mean-square algorithm. This has shown to be advantageous to many applications of the invention, since the least-mean square algorithm is especially useful to significantly reduce interferences from the signal coming from a light detector without reducing the information content relevant to the monitoring of a vital parameter of a patient.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 shows a typical setup for transmission pulse oximetry;

Fig. 2 depicts a generalized block diagram of a transmission pulse oximetry method according to an embodiment of the invention;

Fig. 3 shows a demodulator with a periodic square wave reference signal;

Fig. 4 shows a block diagram of a four-channel device for monitoring a vital parameter of a patient;

Fig. 5 shows a comparison of an original signal and a signal from which the interference is removed; and

Fig. 6 shows a further a comparison of an original signal and a signal from which the interference is removed.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a typical setup for transmission pulse oximetry: A red light source 1 and an infrared (IR) light source 2 are used for irradiating red light of 660 nm and IR light of 940 nm onto tissue of a patient, respectively, i.e. onto a finger 3. The part of the light which is transmitted through the finger 3 is then collected with a common light detector 4.

Figure 2 depicts a general block diagram of a transmission pulse oximeter. The system comprises a processing unit 5 that adjusts the parameters of a light modulator 6 which acts a multiplexer and a pulse controller and modulates the light sources 1, 2. The

configuration of the light modulator 6 depends on the specific multiplexing scheme applied, e.g. in case of TDM the light sources 1, 2 are activated alternatingly, whereas for FDM the light sources 1, 2 radiate light simultaneously but with different modulation frequencies. The reason for applying such a multiplexing scheme is that in this way the single light detector 4 can be used to estimate the attenuation of the light from both light sources 1 , 2.

The light detector 4 detects the light that has propagated through the finger 3 and converts it into an electrical signal. This signal is then pre-processed by a signal- conditioning block 8, which comprises analog amplifiers and band-pass filters, which make the signal suitable for conversion to the digital domain by an analog-to-digital converter (ADC) 9. Correlators 10, each comprising a demodulator 1 1 and a demultiplexer 12, are used to simultaneously demodulate and demultiplex the detected light, and the results are presented to the processing unit 5, which determines the parameters of interest by evaluating the transmitted and demodulated signals.

Figure 3 shows a demodulator 1 1 with a periodic square wave reference signal. Here, the information on the light attenuation becomes present in the base-band by multiplying the received signal with a local reference of the same fundamental frequency (f_m = 1/T_m). Subsequently, only the base-band signal is preserved by passing the signal through a low-pass filter 13, thereby disregarding out-of-band interference.

Figure 3 depicts a generic operational scheme; the exact implementation of the functionality can be optimized for the specific modulation scheme being applied. It should be noted that the square wave in Fig. 3 is only illustrative, as any periodic signal can be applied to both modulate the light sources and demodulate the received signal as long as the fundamental frequencies and/or harmonics coincide.

Figure 4 shows a block diagram of a four-channel device for monitoring a vital parameter of a patient according to an embodiment of the invention. Fig. 4 shows an example where four detection channels 16, 17, 18, 19 are available, but only three detection channels 16, 17, 18 are in use for spectrometric measurement and the fourth detection channel 19 is used as the reference for interference reduction. During the time-slot assigned to the fourth detection channel 19, no light source is active.

For demodulation of the different detection channels 16, 17, 18, 19 the fundamental frequency is shifted by 90° in a shifting device 20. The output of the dark detection channel 19 feeds to adaptive filters 14 to provide a reference signal such that when subtracted from the signal of one of the other detection channels 16, 17, 18, interference for these signals is reduced.

In this embodiment the adaptive filter 14 is based on a least-mean-square algorithm, but the method according to the invention is by no means limited to this adaption. In a subtracter 15, the reference signal is subtracted from the signal coming from the detection channels 16, 17, 18. The use of a low pass filter 13 is an optional feature. It should be noted that the subtractor 15 can also be fed with the signals directly after the demodulator 11.

Various strategies can be followed to operate the subtractor 15. According to one strategy one dark detection channel is assigned continuously, and the subtractor 15 is activated continuously, or becomes active when a certain interference level is detected in the dark channel output. Alternatively, one of the light sources 1 , 2 is switched off periodically, thus creating a dark channel periodically, and the output is used as an input for the adaptive filter 14 only if significant interference is present in the dark channel. Of course, any of the channels can become the dark channel by switching off a respective light source. If the dark channel is switched among the channels, still spectrometric information for all wavelengths can be obtained while interference is reduced. In addition the structure can be expanded to take multiple reference inputs.

Figure 5 shows an examplatory measurement result taken from a pulse oximeter front-end. Here a large 100 Hz interference pulse pattern was deliberately applied to the instrument. Some of the harmonics of the interference coincided with harmonics of the 275 Hz pulse wave used as the demodulation reference, and as a result the channel outputs contain interference originating from 100 Hz harmonics. In Fig. 5, one of the original channel outputs is depicted as a thin line. One dark channel was assigned and its output was taken as the reference for an LMS algorithm with a 64 tap weight vector, and the result of reduction is shown as a thick line. After its initial convergence behavior the algorithm is able to suppress the interference component in the channel by 20 dB, such that it is close to the background noise level. Is should be noted that the signals are DC free because of a high-pass filter applied to the channel output in this case.

Figure 6 shows a measurement result under the same conditions as in Fig. 5, but contrary to the case shown in Fig. 5, now the pulse oximeter probe was attached to a human finger. The pulsating wave form caused by the blood volume pulse is clearly present in the output processed according to the inventive method (thick line), whereas in the original output (thin line) the signal is swamped by interference. Around time instant 80 s, the interference was no longer present and the inventive device stopped working, then both signals being equal. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method of monitoring a vital parameter of a patient by emitting light onto tissue of the patient with at least one light source (1, 2) and collecting light which is transmitted through the tissue and/or which is reflected from the tissue, the method comprising the steps of:

multiplexing the emitted light according to a predefined multiplexing scheme having a plurality of multiplexing channels;

detecting the collected light according to the predefined multiplexing scheme, resulting in a plurality of detection channels (16, 17, 18, 19);

arranging at least one of the multiplexing channels to be a dark multiplexing channel for which no light is emitted by the at least one light source, resulting in a dark detection channel (19); and

using the signal of the dark detection channel (19) for generating a reference signal for reducing interference in the signal of at least one of the other detection channels (16, 17, 18).

2. The method according claim 1, comprising the steps of

adaptively filtering the signal of the dark detection channel (19) for generating the reference signal; and

subtracting the reference signal from the signal of the at least one of the other detection channels (16, 17, 18).

3. The method according to claim 2, comprising the step of using a least-mean- squares algorithm for generating the reference signal.

4. The method according to any one of the claims 1 or 2, comprising the step of low-pass filtering the signals of the respective detection channels (16, 17, 18, 19) in order to reduce out-of-band signals.

5. The method according to any of claims 1 to 4, comprising the steps of estimating the interference level on the basis of the dark detection channel signal and/or the reference signal; and

using the signal of the dark detection channel (19) for generating the reference signal for reducing interference in the at least one of the other detection channels (16, 17, 18) only when the interference level exceeds a predefined threshold.

6. The method according to any of claims 1 to 5, comprising the step of providing a plurality of light sources (1, 2) for emitting light of different wavelengths, respectively.

7. The method according to any of claims 1 to 6, comprising the step of providing a common light detector (4) for all detection channels (16, 17, 18, 19).

8. The method according to any of claims 1 to 7, comprising the step of alternately arranging a plurality, preferably all, of the multiplexing channels to be the dark multiplexing channel for which no light is emitted by the at least one light source (1, 2), respectively, resulting in an alternately changing dark detection channel (16, 17, 18, 19).

9. The method according to any of claims 1 to 8, comprising the step of estimating arterial oxygen saturation of the patient by pulse oximetry based on the signals of at least two detection channels (16, 17, 18) for which the signal of the dark detection channel (19) has been used as a reference signal for reducing interference.

10. A device for monitoring a vital parameter of a patient, comprising

at least one light source (1, 2) for emitting light onto tissue of the patient; at least one light detector (4) for collecting light which is transmitted through the tissue and/or which is reflected from the tissue;

a multiplexer adapted for multiplexing the emitted light according to a predefined multiplexing scheme having a plurality of multiplexing channels, wherein at least one of the multiplexing channels is a dark multiplexing channel for which no light is emitted by the at least one light source (1, 2);

a plurality of detection channels (16, 17, 18, 19), being connected to the least one light detector (4) and being adapted for detecting the collected light according to the predefined multiplexing scheme, wherein at least one of the detection channels (16, 17, 18, 19) relating to the at least one dark multiplexing channels is a dark detection channel (19); and

11. The device according claim 10, wherein the reference signal generator comprises

an adaptive filter (14) which is adapted for adaptively filtering the signal of the dark detection channel (19) for generating the reference signal, and

a subtractor (15) which is adapted for subtracting the reference signal from the at least one of the signals of the other detection channels (16, 17, 18).

12. The device according to any of claims 10 or 11, wherein a low-pass filter (13) is provided which is adapted for filtering the signals of the respective detection channels (16, 17, 18, 19) in order to reduce out-of-band signals.

13. The device according to any of claims 10 to 12, wherein a plurality of light sources (1, 2) is provided for emitting light of different wavelengths, respectively.

14. The device according to any of claims 10 to 13, wherein a common light detector (4) for all detection channels is provided.

15. The device according to any of claims 10 to 14, wherein the device comprises a pulse oximeter.