JP6184318B2 - Reduce interference in monitoring patient life parameters - Google Patents

Description

  The present invention relates to the field of light attenuation measurement, and more particularly to a method and device for monitoring a patient's life parameters by measuring the attenuation of light emitted onto the patient's tissue.

  Measuring light absorption and / or scattering when passing through or reflecting from a particular medium forms the basis for multiple optical spectroscopy methods that are widely applied in various medical fields such as patient monitoring. To do. One illustrative example is transmission pulse oximetry.

Pulse oximetry is an optical method for non-invasive monitoring of patient arterial oxygen saturation and is one of the most commonly used techniques in clinical practice. Protein hemoglobin (Hb) binds to oxygen in red blood cells for transport through the body and has the property of changing color from dark red to light red when oxidized. By releasing and detecting light at two or more wavelengths, pulse oximeters, indirect estimation of oxygen saturation, i.e. oxyhemoglobin (Hb0 ₂₎ to reach a concentration ratio of the light absorbance in the peripheral vascular beds To decide. A pulse oximeter relies on changes in arterial blood volume caused by cardiac contraction and relaxation to determine the amount of light absorbed by pulsating only arterial blood. This largely excludes tissue and venous blood contributions.

  In many applications, including oxygen measurements, different wavelength optical paths are required, ie simultaneous or quasi-simultaneous attenuation measurements of different colors. Therefore, usually a plurality of light sources that are generally combined with a single photodetector are utilized. Generally, an electrical multiplexing method is employed so that the signals from each of the emitters at the photodetector can be distinguished. For example, time division multiplexing (TDM), frequency division multiplexing (FDM), or code division multiplexing (CDM).

  Optical attenuation measurements applied in medical settings, for example in patient monitoring, are subject to electromagnetic interference. Typically, such interference has ambient light at various optical wavelengths and with different modulation frequencies. Common examples are based not only on natural light, which is not normally modulated, but also artificial light from incandescent lamps that are modulated at twice the main frequency (100 Hz or 120 Hz) and overtones of 50 Hz or 60 Hz, and specific electrical ballasts. Includes artificial light from fluorescent lamps with flicker rates in the range of tens to hundreds of kilohertz.

  In general, in spectrometer devices, measurements are made to mitigate the effects of external interference on the measurement. For example, in a photodetector, the light source is modulated in a pulse oximeter so that the emitted light can be distinguished from ambient light by filtering or demodulation. Regardless of the modulation technique applied, conventional methods rely on information regarding spectral modulation of ambient light and assume that the light source modulation frequency or band used can remain fixed with respect to the lifetime of the device.

  However, if the ambient light modulation spectrum is only partially known or not a priori known, such as when the spectrometer device operates near an optical communication system, it was detected at the device operating frequency. There may be interference in the modulation spectrum of light. Similarly, new operating schemes for high intensity discharge (HID) lamps may cause interference signals with a wide frequency range. If interferers contaminate the operating frequency band, the signal to interference ratio (SIR) may be significantly reduced. Thereby, measurement quality deteriorates.

  It is an object of the present invention to provide a method for monitoring a patient's life parameters by measuring the attenuation of light emitted onto the patient's tissue and a high signal-to-interference ratio in a versatile and reliable manner. It is to provide a corresponding device that enables.

  This is achieved by the subject matter of the separate independent claims. Preferred embodiments are set forth in the dependent claims.

This is in accordance with the invention by emitting light to the patient's tissue using at least one light source and collecting light transmitted through and / or reflected from the tissue, It means that a method for monitoring a patient's life parameters is provided,
Multiplexing the emitted light based on a predetermined multiplexing scheme having a plurality of multiplexing channels;
Detecting the collected light based on the predetermined multiplexing scheme to produce a plurality of detection channels;
Configuring at least one of the multiplexing channels to be a dark multiplexing channel without light emitted by the at least one light source, resulting in a dark detection channel;
Generating a reference signal for reducing interference in at least one signal of another detection channel using the signal of the dark detection channel.

  It should be understood that the term “patient” refers to all humans and animals, regardless of whether they are healthy, as well as those who are ill.

  According to the present invention, light transmitted through and / or reflected from tissue is collected. This is necessary for attenuation measurements in order to monitor patient life parameters. However, when collecting this light, it is not possible to completely avoid collecting at least some ambient light. This collected ambient light can cause interference.

  Thus, the idea of the present invention is provided by a dark detection channel, i.e. a detection channel that is at least temporarily not used for spectroscopic purposes, in order to reduce interference caused by ambient light or other sources affecting the detection signal. Is to adapt the signal.

  According to an embodiment of the invention, this method is used for pulse oximetry. However, the present invention is not only applied to pulse oximetry, but the patient is assigned a dark channel and the interfering elements present in the dark channel output are related in some way to the interfering elements at other outputs. It can also be used for other spectroscopic methods to monitor the life parameters of The dark channel output can therefore be used as a reference for the reduction of interference elements in another channel.

  In general, the dark detection channel signal can be used directly as a reference signal. However, according to an embodiment of the present invention, the signal of the dark detection channel is adaptively filtered to generate the reference signal, and the reference signal is preferably derived from at least one signal of another detection channel. Subtracted.

  For adaptive filtering, a wide variety of different methods can generally be used. However, according to one embodiment of the present invention, a least mean square (LMS) algorithm is used to generate the reference signal. The LMS algorithm and its derivatives are known to those skilled in the art.

を

に基づき更新する。ここで、μは、適合定数を表し、

は、暗い検出チャネルからとられる参照信号ベクトルであり、

は、チャネル／検出器信号及びフィルタリングされた参照の差である。減算の出力はｄ（ｋ）である。これは、光源に関する光減衰測定の結果を与える。しかし、この測定から干渉は除去される。適合定数は、アルゴリズムの収束速度だけでなく最終的な不良調整率（misadjustment）を決定し、アルゴリズムの特性を決定するため、動的にセットされることができる。この動作により、出力信号における干渉は、明らかに減らされることができる。 According to an embodiment of the present invention, the LMS algorithm is an N-tap weight vector.

The

Update based on Where μ represents the adaptation constant,

Is the reference signal vector taken from the dark detection channel,

Is the difference between the channel / detector signal and the filtered reference. The output of the subtraction is d (k). This gives the result of a light attenuation measurement for the light source. However, interference is removed from this measurement. The adaptation constant can be set dynamically to determine the final misadjustment as well as the convergence speed of the algorithm and to determine the characteristics of the algorithm. With this operation, interference in the output signal can be clearly reduced.

  The detection channel signal can be processed without any pre-processing. However, according to one embodiment of the invention, the signals of the individual detection channels are low pass filtered. In this way, deletion of out-of-band signals can be realized.

  Various strategies can be followed to operate the reference reduction structure. According to one strategy, one dark channel is continuously allocated and interference reduction is continuously activated. According to an embodiment of the present invention, based on the dark detection channel signal and / or the reference signal, the interference level is estimated, and only when the interference level exceeds a predetermined threshold, the signal of the dark detection channel is A reference signal is generated to reduce interference in at least one of the other detection channels.

  According to a preferred embodiment of the present invention, a plurality of light sources are provided that emit light of different wavelengths. Further in accordance with a preferred embodiment of the present invention the plurality of multiplexing channels, preferably all of the multiplexing channels, are continuous such that they are the dark multiplexing channels without light emitted by the at least one light source. Resulting in a dark detection channel that is constructed and alternated. In a system having multiple light sources that emit light of different wavelengths and corresponding multiple detection channels, one of the light sources can be periodically switched off, thus creating a dark channel periodically. The Thus, by switching off individual light sources, any of the channels can become a dark channel. If a dark channel is rotated between channels, spectral information for all wavelengths can still be obtained while reducing interference. Furthermore, this structure can be expanded to take multiple reference inputs.

The above objective is further solved by a device for monitoring a patient's life parameters, which device comprises:
At least one light source that emits light to the patient's tissue;
At least one photodetector that collects light transmitted through and / or reflected from the tissue;
A multiplexer configured to multiplex the emitted light based on a predetermined multiplexing scheme having a plurality of multiplexing channels, wherein at least one of the multiplexing channels is provided by the at least one light source; A multiplexer, which is a dark multiplexing channel with no light emitted;
A plurality of detection channels connected to the at least one photodetector and configured to detect the collected light based on the predetermined multiplexing scheme, the at least one dark multiplexing channel A plurality of detection channels, wherein at least one of the detection channels is a dark detection channel;
A reference signal generator configured to use the dark detection channel signal as a reference signal for reducing interference in at least one signal of another detection channel.

  According to a preferred embodiment of the invention, the device is configured to emit light having at least two different wavelengths, for example by having two different light sources. Furthermore, it is particularly preferred that the device has 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 configured to adaptively filter the dark channel signal to generate the reference signal. Further in accordance with an embodiment of the present invention, a subtractor is provided that is configured to subtract a reference signal from at least one of the other detection channels.

  Further in accordance with a preferred embodiment of the present invention the device has a low pass filter configured to filter the signals of the individual detection channels. In this way, out-of-band signals can be reduced and thus excluded to some extent from additional processing. Preferably, the device has a plurality of light sources each emitting light of a different wavelength. More preferably, a common photodetector is provided for all detection channels.

  According to a preferred embodiment of the present invention, the adaptive filter is configured to provide a reference signal based on a least mean square algorithm. This has been shown to be advantageous for many applications of the present invention. This is because the least mean square algorithm is particularly beneficial in obviously reducing interference from the signal resulting from the photodetector without reducing the information content associated with monitoring the patient's life parameters.

FIG. 2 shows an exemplary setup for transmitted pulse oximetry. FIG. 2 shows a generalized block diagram of a transmitted pulse oximetry method according to an embodiment of the present invention. It is a figure which shows the demodulator with a periodic square wave reference signal. FIG. 6 shows a block diagram of a four channel device that monitors patient life parameters. It is a figure which shows the comparison of the signal from which an original signal and interference are removed. FIG. 5 shows a further comparison of the original signal and the signal from which interference is removed.

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

  FIG. 1 shows a typical setup for transmitted pulse oximetry. A red light source 1 and an infrared (IR) light source 2 are used to irradiate 660 nm red light and 940 nm infrared light onto a patient's tissue, i.e., finger 3, respectively. A part of the light transmitted through the finger 3 is collected by a general photodetector 4.

  FIG. 2 shows a general block diagram of a transmission pulse oximeter. This system has a processing unit 5 which functions as a multiplexer and a pulse controller and adjusts the parameters of the light modulator 6 which modulates the light sources 1, 2. The configuration of the light modulator 6 depends on the specific multiplexing scheme applied. For example, in the case of TDM, the light sources 1 and 2 are activated alternately. On the other hand, in the case of FDM, the light sources 1 and 2 emit light at the same time but with different modulation frequencies. The reason for applying such a multiplexing scheme is thus that a single photodetector 4 can be used to estimate the attenuation of light from both light sources 1, 2.

  The photodetector 4 detects the light propagated through the finger 3 and converts it into an electrical signal. This signal is preprocessed by the signal conditioning block 8. This processing block has an analog amplifier and a bandpass filter, and this signal is suitable for conversion into the digital domain by an analog-to-digital converter (ADC) 9. A correlator 10, each having a demodulator 11 and a demultiplexer 12, is used to simultaneously demodulate and demultiplex the detected light, and the result is provided to the processing unit 5. This unit determines the parameter of interest by evaluating the transmitted and demodulated signal.

FIG. 3 shows a demodulator 11 with a periodic square wave reference signal. Here, the information regarding the optical attenuation becomes present in the baseband by multiplying the received signal by using the local reference of the same fundamental frequency (f _m = 1 / T _m ). Subsequently, only the base-band signal is stored by passing the signal through the low-pass filter 13. Thereby, out-of-band interference is ignored.

  FIG. 3 shows a general operating scheme. The exact implementation of this function can be optimized for the particular modulation scheme applied. Note that the square wave in FIG. 3 is merely illustrative. This is because any periodic signal can be applied to modulate the light source and demodulate the received signal as long as the fundamental frequency and / or harmonics match.

  FIG. 4 shows a block diagram of a four channel device for monitoring patient life parameters according to an embodiment of the present invention. FIG. 4 shows that four detection channels 16, 17, 18, 19 are available, but only three detection channels 16, 17, 18 are in use for spectroscopic measurements, and the fourth detection channel 19 is An example used as a reference for interference reduction is shown. During the time slot assigned to the fourth detection channel 19, no light source is active.

  Due to the demodulation of the different detection channels 16, 17, 18, 19, the fundamental frequency is shifted 90 ° in the shift device 20. The output of the dark detection channel 19 is fed to the adaptive filter 14 to provide a reference signal. As a result, when subtracted from one signal of the other detection channels 16, 17, 18, the interference on these signals is reduced.

  In this embodiment, the adaptive filter 14 is based on a least mean square algorithm. However, the method according to the invention is not limited to this adaptation.

  In the subtracter 15, the reference signal is subtracted from the signal originating from the detection channels 16, 17, 18. Use of the low pass filter 13 is an optional feature. Note that the subtractor 15 can also be fed directly after the demodulator 11.

  Various strategies can be applied to activate the subtractor 15. According to one strategy, one dark detection channel is continuously allocated and the subtractor 15 is activated continuously or becomes active when a certain interference level is detected at the dark channel output. . Alternatively, one of the light sources 1, 2 is switched periodically. This creates a periodically dark channel and this output is used as an input to the adaptive filter 14 only when significant interference is present in the dark channel. Of course, any channel can become a dark channel by switching off individual light sources. If the dark channel is switched between channels, the spectral information for all wavelengths can still be obtained while the interference is reduced. Furthermore, this structure can be expanded to take multiple reference inputs.

  FIG. 5 shows an exemplary measurement taken from a front-end pulse oximeter. Here, a large 100 Hz interference pulse pattern was deliberately applied to the instrument. Some of the overtones of the interference coincide with the overtones of the 275 Hz pulse wave used as a demodulation reference, and as a result, the channel output includes interference resulting from the overtones of 100 Hz. In FIG. 5, one of the original channel outputs is shown as a thin line. One dark channel is assigned and its output is taken as a reference to the LMS algorithm using a 64-tap weight vector and the reduction results are shown as thick lines. After its initial convergence behavior, the algorithm can suppress the interference elements in the 20 dB channel. As a result, the noise level is close to the background noise level. Note that the signal is DC free. This is because in this case a high pass filter is applied to the channel output.

  FIG. 6 shows the measurement results under the same conditions as in FIG. However, contrary to the case shown in FIG. 5, a pulse oximeter probe is attached to a human finger. The pulsating waveform produced by the blood volume pulse is clearly present at the output (thick line) processed by the method of the present invention. On the other hand, at the original output (thin line), this signal is flooded by interference. Around the time constant 80s there is no longer any interference. The device of the present invention stops and both signals are 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.

  From studying the drawings, disclosure and appended claims, other variations to the disclosed embodiments can be understood and implemented by those skilled in the art practicing the claimed invention. 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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims (12)

In a method for monitoring a patient's life parameters by emitting light onto a patient's tissue using at least one light source and collecting light transmitted through and / or reflected from the tissue ,
Multiplexing the emitted light based on a predetermined multiplexing scheme having a plurality of multiplexing channels;
Detecting the collected light based on the predetermined multiplexing scheme to produce a plurality of detection channels;
Configuring one of the multiplexing channels to produce a dark detection channel to be a dark multiplexing channel without any light emitted by the at least one light source;
Applying an adaptive filter to the signal of the dark detection channel and generating a reference signal using a least mean square algorithm;
Reducing interference in at least one signal of other detection channels only when the interference level in the reference signal exceeds a predetermined threshold ;
The method wherein the dark detection channel is switched between the plurality of detection channels.   The method of claim 1, comprising low-pass filtering the signal of the individual detection channel to reduce out-of-band signals. Comprising providing a plurality of light sources that emit light of different wavelengths, the method according to claim 1 or 2. 4. A method according to any of claims 1 to 3 , comprising providing a common photodetector for all detection channels. 2. The step of alternately configuring the plurality of multiplexing channels to produce an alternating dark detection channel so that the dark multiplexing channel is free of light emitted by the at least one light source. The method in any one of thru | or 4 . Based on the signals of at least two detection channels are used as a reference signal for the signal of the dark detection channels reduce interference comprises the step of estimating the oxygen saturation of arterial of the patient by the pulse oximetry, claims 1 to 5 The method in any one of. A device for monitoring a patient's life parameters,
At least one light source that emits light into the patient's tissue;
At least one photodetector that collects light transmitted through and / or reflected from the tissue;
A multiplexer configured to multiplex the emitted light based on a predetermined multiplexing scheme having a plurality of multiplexing channels, wherein at least one of the multiplexing channels is provided by the at least one light source; A multiplexer, which is a dark multiplexing channel with no emitted light,
A plurality of detection channels connected to the at least one photodetector and configured to detect the collected light based on the predetermined multiplexing scheme, the at least one dark multiplexing channel A plurality of detection channels, wherein one of the detection channels is a dark detection channel;
Applying an adaptive filter to the signal of the dark detection channel, generating a reference signal using a least mean square algorithm, and at least of other detection channels only when the interference level in the reference signal exceeds a predetermined threshold A reference signal generator configured to reduce interference in one signal;
The device, wherein the dark detection channel is switched between the plurality of detection channels. The reference signal generator is
The device of claim 7 , comprising a subtractor configured to subtract the reference signal from at least one of the signals of the other detection channels. 9. A device according to claim 7 or 8 , wherein a low-pass filter configured to filter the signal of the individual detection channel is provided to reduce out-of-band signals. 10. A device according to any of claims 7 to 9 , wherein a plurality of light sources are provided, each emitting light of a different wavelength. 11. A device according to any of claims 7 to 10, wherein a common photodetector is provided for all detection channels. The device has a pulse oximeter, according to any one of claims 7 to 11 devices.

Images