Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
  • A61B5/1455—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters
  • A61B5/14551—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using optical sensors, e.g. spectral photometrical oximeters for measuring blood gases
  • A61B5/14552—Details of sensors specially adapted therefor

Definitions

• 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.
• 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.
• 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 (Hb0 2 ).
• 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.
• TDM time division multiplexing
• FDM frequency division multiplexing
• CDM code division multiplexing
• light attenuation measurements applied in e.g. patient monitoring suffer from electromagnetic interference.
• 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.
• spectrometric devices measures are taken to mitigate the effect of external interference on the measurements.
• the light sources are modulated such that at the photo detector the emitted light can be distinguished from ambient light by filtering or demodulation.
• 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.
• 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.
• 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.
• SIR signal-to- interference ratio
• 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 comprising the following steps:
• patient does not only refer to diseased persons but to all human beings and animals, no matter whether healthy or not.
• 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.
• this light it cannot totally be avoided to collect at least some ambient light, too. This collected ambient light can cause interferences.
• 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.
• this method is used for pulse oximetry.
• 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.
• the signal of the dark detection channel could be directly used as 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.
• LMS least-mean-squares
• x(k-N+l) is the reference signal vector, taken from a dark detection channel
• 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
• 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.
• one dark channel is assigned continuously and interference reduction is activated continuously.
• 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.
• a plurality of light sources is provided for emitting light of different wavelengths.
• 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.
• one of the light sources can be switched off periodically, thus creating a dark channel periodically.
• 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.
• the structure can be expanded to take multiple reference inputs.
• 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;
• 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.
• 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.
• 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.
• a subtractor is provided which is adapted for subtracting the reference signal from the at least one of the other detection channels.
• 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.
• 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
• Fig. 6 shows a further a comparison of an original signal and a signal from which the interference is removed.
• 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.
• IR infrared
• FIG. 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the structure can be expanded to take multiple reference inputs.
• Figure 5 shows an examplatory measurement result taken from a pulse oximeter front-end.
• 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.
• 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.
• 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.
• the interference was no longer present and the inventive device stopped working, then both signals being equal.

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• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
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• Measurement Of The Respiration, Hearing Ability, Form, And Blood Characteristics Of Living Organisms (AREA)
• Measuring Pulse, Heart Rate, Blood Pressure Or Blood Flow (AREA)

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• 2011
  • 2011-03-16 CN CN2011800152102A patent/CN102811663A/zh active Pending
  • 2011-03-16 EP EP11716043A patent/EP2549926A1/en not_active Ceased
  • 2011-03-16 JP JP2013500626A patent/JP6184318B2/ja not_active Expired - Fee Related
  • 2011-03-16 WO PCT/IB2011/051100 patent/WO2011117780A1/en active Application Filing

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