Spectrophotometric device for measuring oxyhemoglobin saturation

Abstract

According to a first aspect, the present invention relates to a spectrophotometric device for measuring blood oxygen saturation in a tissue of a subject, comprising: one or more light sources for emitting light signals into the tissue of the subject; one or more detectors for detecting light signals reflected by the tissue of the subject; an electronic data processing unit configured to calculate blood oxygen saturation based on the light signals measured by the one or more detectors; and a motion sensor for detecting a motion signal indicative of motion of the light source and / or the light detector. The device is characterized in that the electronic data processing unit is configured to discard the light signal detected by the motion detector if the motion signal detected by the one or more sensors exceeds a predetermined threshold. Furthermore, the present invention relates to a spectrophotometric device for measuring blood oxygen saturation in a tissue of a subject, comprising: at least one light source for emitting a light signal into the tissue of the subject; at least one detector for detecting a light signal reflected by the tissue of the subject; an electronic data processing unit configured to calculate blood oxygen saturation in consideration of the light signal measured by the at least one detector; and a contact sensor for detecting a contact signal indicative of contact between the subject's skin and the light source and / or the detector.

Description

[0001] The present invention relates to a spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, and a method for measuring blood oxygen saturation in a subject's tissue. Background Art

[0002] Monitoring blood oxygen saturation in the tissues of a subject is of clinical importance, as low blood oxygen saturation is indicative of a potentially fatal condition. This is the case, for example, with premature infants, who often suffer from injuries to the gestational tract, such as necrotizing enterocolitis or obstruction, and are at constant risk of developing shock. Therefore, in the case of premature infants, continuous and accurate monitoring of abdominal oxygen saturation is required.

[0003] The blood oxygen saturation in the subject's tissue is defined as:

[0004] StO2=c(HbO2) / (c(HbO2)+c(Hb))

[0005] where c(HbO2) and c(Hb) are the concentrations of oxyhemoglobin and deoxyhemoglobin, respectively.

[0006] Near infrared spectroscopy (NIRS) is a non-invasive technique for measuring blood oxygen saturation in a subject's tissue. NIRS relies on the different absorption properties of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) in the near infrared spectral range to determine the relative concentrations of HbO2 and Hb. NIRS can be performed non-invasively by placing a spectral sensor on the subject's skin and measuring the attenuation of the light signal after passing through the subject's tissue.

[0007] The measured light attenuation is related to the concentration of a given light-absorbing substance (chromophore) according to the Lambert-Beer law:

[0008] A λ = -log(I λ / I λ0 )=ε λ cd

[0009] Among them A λ is the light attenuation at a specific wavelength λ, c is the concentration of a specific chromophore, ε λ is the extinction coefficient of a particular chromophore at a particular wavelength, and d is the distance from the light source to the detector. Using the known extinction coefficient, the concentration of the chromophore can be calculated from the measured light attenuation. In the case of a mixture of different chromophores, the relative concentrations of the chromophores can be determined by measuring the light attenuation at several different wavelengths, at which the extinction coefficients of the chromophores are different. For a mixture containing N different chromophores, this requires measuring the attenuation at a minimum of N different wavelengths.

[0010] In a typical NIRS device, a light signal of known wavelength and intensity is transmitted into the tissue of the subject, and the light transmitted through the tissue or diffusely reflected from the tissue is detected to calculate the light attenuation. In order to accurately determine the concentration of chromophores in the tissue from the measured light attenuation, it is necessary to take into account the optical properties of the tissue, in particular the absorption due to other chromophores present in the tissue and the scattering properties of the tissue. In practice, the scattering properties of the tissue need to be taken into account by calibration measurements. In order to take into account chromophores other than HbO2 and Hb, the absorption spectra of these chromophores must be determined in order to estimate the wavelength-dependent extinction coefficient, and the light attenuation must be measured at at least 2+M wavelengths, where M is the number of additional chromophores that should be considered. Several approaches to address these problems have been developed in the prior art.

[0011] EP 1 259 791 B1 discloses a NIRS method for measuring the total blood oxygen saturation in a subject's tissue by measuring the light attenuation at three or more wavelengths and calculating the attenuation difference between the wavelengths. This scheme is also called the "differential wavelength method". This method requires measurements at N+1 different wavelengths to determine the concentrations of N different chromophores. By determining the differential attenuation, the contribution of tissue light scattering, fixed light absorption components and measurement device characteristics is minimized relative to the attenuation due to HbO2 and Hb, which improves the accuracy of the measured blood oxygen saturation.

[0012] US2012 / 0136225 A1 discloses a method for determining blood oxygen saturation in lower gastrointestinal tissue of a subject, the method comprising taking into account the presence of wavelength-dependent absorbing materials not present in the blood. Specifically, US2012 / 0136225 A1 suggests taking into account light attenuation due to the presence of feces in the lower gastrointestinal tract of the subject, particularly meconium in the gastrointestinal tract of a newborn infant.

[0013] 2012 / 0136225A1 also teaches the use of differential wavelength methods to analyze NIRS data.

[0014] Although the differential wavelength method minimizes the contribution of the scattering properties of the tissue, it still requires calibration to account for scattering as well as non-specific background absorption. This calibration is performed by determining the oxygen saturation of a given reference tissue, assuming that it is a weighted sum of the oxygen saturation of the subject's venous and arterial blood. However, this requires knowledge of the relative contributions of venous and arterial blood in the tissue. Although empirical data exist for the relative contributions of venous and arterial blood oxygen saturation, the reliability of this data is questionable. Therefore, the available calibration methods suffer from potential error sources of the differential wavelength method.

[0015] Another way to perform NIRS measurements is to measure the light attenuation at several wavelengths and at several different distances between the light source and the light detector. a,λ It can be calculated according to the following formula:

[0016]

[0017] where μ s,λ is an empirically determined value that takes into account the attenuation of the optical signal due to light scattering in the subject's tissue at a specific wavelength λ, λ is the attenuation at a specific wavelength λ, d is the average distance between the source and the detector, and is the slope of the attenuation with respect to the distance from the source to the detector. The concentration of the chromophore can be determined from the absorption μ using the Beer-Lambert law. a,λ This scheme is also called the "multi-distance method". It has been applied to measure the oxygen saturation of muscle tissue (Tachtsidis, Ilias et al., "A Hybrid Multi-Distance Phase and Broadband Spatially Resolved Spectrometer and Algorithm for Resolving Absolute Concentrations of Chromophores in the Near-Infrared Light Spectrum"

[0018] Advances in Experimental Medicine and Biology 662 (2010): 169–175.

[0019] Conventional spectrophotometric devices for measuring blood oxygen saturation can be attached to a subject's finger, or laid flat on the subject's skin. In order to measure the blood oxygen content of a newborn or premature infant, it is important to attach the device as loosely as possible to prevent any discomfort or harm to the infant. Therefore, clamp devices attached to a finger are not very suitable for monitoring newborns or premature infants. In contrast, spectrophotometric devices that can be placed more or less loosely on the abdomen of a subject are more suitable for monitoring newborns or premature infants. However, the devices must be attached tightly enough to prevent the device from falling and to prevent interference from ambient light. Because they cannot be attached too tightly, conventional spectrophotometric devices for measuring blood oxygen saturation sometimes experience measurement artifacts caused by, for example, movement of the subject or loss of contact with the subject's skin.

[0020] Technical issues

[0021] An object of the present invention is to provide an improved spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, in particular for measuring blood oxygen saturation in a newborn or premature infant. The spectrophotometric device should have improved measurement accuracy and / or should provide information on other important parameters besides blood oxygen saturation. Summary of the invention

[0022] According to a first aspect, the present invention relates to a spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, comprising: one or more light sources for emitting light signals into the subject's tissue; one or more detectors for detecting light signals reflected by the subject's tissue; an electronic data processing unit configured to calculate blood oxygen saturation based on the light signals measured by the one or more detectors; and a motion sensor for detecting motion signals indicating motion of the light sources and / or light detectors. The device is characterized in that the electronic data processing unit is configured to discard the light signal measured by the motion detector if the motion signal detected by the one or more sensors exceeds a predetermined threshold.

[0023] According to a first aspect, the present invention also relates to a method for non-invasively measuring blood oxygen saturation in a subject's tissue, comprising the following steps: using at least one light source to emit a light signal into the subject's tissue, using at least one detector to detect the light signal reflected from the subject's tissue, using an electronic data processing unit to calculate the blood oxygen saturation based on the light signal measured by the one or more detectors, using a motion sensor to detect a motion signal indicating the motion of the light source and / or the light detector. The method is characterized in that if the motion signal detected by the one or more sensors exceeds a predetermined threshold, the electronic data processing unit discards the light signal measured by the motion detector.

[0024] By detecting and taking into account motion signals indicative of motion of the light source and / or light detector, the spectrophotometric device can eliminate one source of measurement artifacts and thereby improve the accuracy of the measured blood oxygen saturation. This is particularly important when monitoring newborns or premature infants, who often experience sudden movements that lead to measurement artifacts in conventional spectrophotometric devices.

[0025] In the context of the present invention, discarding an optical signal means that the normal processing of the optical signal is interrupted. This may mean, for example, that the oxygen saturation is not calculated, that the calculation of the oxygen saturation is interrupted, that the oxygen saturation is calculated but not reported to the user, or that the oxygen saturation is calculated and reported but marked as an uncertain value. If the oxygen saturation is calculated under these conditions, it can be saved in the electronic record for later use, but it is preferably marked as an uncertain value.

[0026] The spectrophotometric device is configured to transmit the light signal into the subject tissue and detect the light signal after the light signal passes through the subject tissue. Therefore, the spectrophotometric device can measure the attenuation of the light signal after it passes through the subject tissue. The light signal emitted into the subject tissue is partially absorbed, partially transmitted and partially reflected by the subject tissue. The spectrophotometric device according to the present invention measures the light signal reflected from the subject tissue. Therefore, at least one light source and at least one detector are preferably located on the same side relative to the subject tissue.

[0027] Therefore, the spectrophotometric device is different from a conventional clip-on device in which the light source and the detector are located on opposite sides of the subject's tissue in order to measure the light signal transmitted through the subject's tissue. Such conventional clip-on devices can only be attached to the subject's limbs, such as the subject's fingers, and therefore cannot be used to measure the blood oxygen saturation of other body parts, such as the abdomen. In contrast, the spectrophotometric device according to the present disclosure can be placed on any body part, such as the subject's abdomen, to specifically measure the blood oxygen saturation of that body part.

[0028] Preferably, the spectrophotometric device is configured to measure the blood oxygen saturation of the subject's head, trunk, abdomen, arms or legs, more preferably, the blood oxygen saturation of the subject's head, trunk or abdomen. Most preferably, the spectrophotometric device is configured to measure the blood oxygen saturation of the subject's abdomen. This requires that the spectrophotometric device is configured so that the light source and the light detector can be placed flat on the subject's skin. Preferably, the spectrophotometric device also includes a fixing device, with which the light source and the light detector can be fixed on the subject's skin. This can be, for example, a belt or an adhesive patch. In a preferred embodiment, the spectrophotometric device includes an adhesive patch, preferably a Velcro patch or a Velcro tape, which can be used to attach the spectrophotometric device to the subject's clothing, for example, attaching the spectrophotometric device under the subject's diaper.

[0029] The light source may be a broadband light source that emits light within a certain wavelength range. Alternatively, the light source may be a collection of light sources that each emit light with a narrow spectral bandwidth, such as a collection of light emitting diodes. In a preferred embodiment, the light source comprises a collection of light emitting diodes, each light emitting diode emitting light at a different wavelength.

[0030] A spectrophotometric device may include one or more such light sources, each of which is either a broadband light source or a collection of light sources that emit light within a narrow spectral bandwidth.

[0031] The photodetector can be, for example, a CCD sensor or a photodiode or any other device that can convert light into an electric current or voltage. The detector can include a collection of individual detectors, each of which is configured to detect light of different wavelengths. For example, the detector can include a photodiode array, each photodiode being equipped with a filter that transmits light of a specific wavelength to a corresponding photodiode. Alternatively, the detector can include a photodiode array or a CCD sensor and a diffraction grating that separates the optical signal into different optical components of different wavelengths and images each optical component onto different photodiodes of the photodiode array or onto different regions of the CCD sensor. Alternatively, the spectrophotometric device can use time or frequency multiplexing to perform wavelength separation. Therefore, the detector can measure the intensity of the optical signal as a function of wavelength. This enables the spectrophotometric device to measure the attenuation of the optical signal as a function of wavelength.

[0032] The spectrophotometric device may comprise one or more such photodetectors, wherein each photodetector may comprise, for example, a photodiode array, a single CCD chip or a single photodiode. Preferably, the spectrophotometric device comprises two to six, most preferably four, such photodetectors, which are located at different distances from the light source.

[0033] Preferably, the spectrophotometric device is configured to measure the attenuation of the reflected light signal at two or more different wavelengths. Measuring the attenuation at two or more different wavelengths allows calculation of the relative concentrations of oxyhemoglobin and deoxyhemoglobin in the subject's tissue, and thereby blood oxygen saturation.

[0034] In order to measure the attenuation at a given number of different wavelengths, it is sufficient to configure the light source and the light detector to measure the attenuation at different wavelength ranges, the different wavelength ranges comprising at least the specified wavelength. The spectral bandwidth of each wavelength range may vary, as long as the wavelength ranges can be clearly distinguished. Preferably, the attenuation at different wavelength ranges with a bandwidth of ±25nm or less, more preferably ±15nm or less, even more preferably ±10nm or less, most preferably ±5nm or less is measured.

[0035] To improve the accuracy of the measurement, the attenuation is preferably measured at three or more different wavelengths, more preferably at four or more different wavelengths, more preferably at five or more wavelengths, and most preferably at seven or more wavelengths. Measuring at three or more different wavelengths enables the relative concentrations of oxygenated hemoglobin and deoxygenated hemoglobin to be calculated using a differential wavelength method.

[0036] In order to distinguish between deoxyhemoglobin and oxyhemoglobin, it is preferred to measure at least at two different wavelengths where the extinction coefficients of these two substances are significantly different, that is, at least one wavelength where the extinction coefficient of deoxyhemoglobin is higher than the extinction coefficient of oxyhemoglobin, and at least a second wavelength where the extinction coefficient of oxyhemoglobin is higher than the extinction coefficient of deoxyhemoglobin.

[0037] Preferably, the spectrophotometric device is configured to measure the attenuation of the reflected optical signal at different wavelengths in the range of 650 nm to 3 μm, more preferably in the range of 650 nm to 1 μm, most preferably in the range of 690 nm to 910 nm.

[0038] In the case of measuring the abdomen of a subject, a combination of several wavelengths that provide increased measurement accuracy has been found. These wavelengths can be selected to better distinguish between Hb, HbO2 and other absorbents (such as feces) present in the abdomen of the subject. These optimized wavelength combinations are described below.

[0039] In one embodiment, the spectrophotometric device is configured to measure the attenuation of the optical signal at different wavelengths selected from 695nm, 712nm, 733nm, 743nm, 762nm, 783nm, 790nm, 805nm, 880nm, 894nm and 909nm. Preferably, the wavelength is selected from 695nm, 733nm, 762nm, 790nm, 805nm, 894nm and 909nm. Preferably, these wavelengths are measured with a spectral bandwidth of ±10nm, more preferably ±5nm. Preferably, the spectrophotometric device is configured to measure the attenuation of the optical signal at at least three different wavelengths selected from these wavelengths.

[0040] In one embodiment, the spectrophotometric device is configured to measure the attenuation of optical signals at least at 712 nm, 733 nm, 762 nm, 783 nm and 909 nm. Preferably, these wavelengths are measured with a spectral bandwidth of ±10 nm, more preferably ±5 nm.

[0041] In another embodiment, the spectrophotometric device is configured to measure the attenuation of optical signals at least at 712nm, 733nm, 762nm, 783nm, 894nm and 909nm. Preferably, these wavelengths are measured with a spectral bandwidth of ±10nm, more preferably ±5nm.

[0042] Most preferably, the spectrophotometric device is configured to measure the attenuation of optical signals at 695nm, 733nm, 762nm, 790nm, 805nm, 894nm and 909nm.Preferably, these wavelengths are measured with a spectral bandwidth of ±10nm, more preferably ±5nm.

[0043] In a preferred embodiment, the spectrophotometric device is configured to measure the attenuation of the light signal at two or more light source-to-detector distances. This enables the use of a multi-distance method to calculate the relative concentrations of oxy- and deoxy-haemoglobin.

[0044] In order to achieve this feature, the spectrophotometric device preferably includes at least two photodetectors, which are located at different fixed distances from the light source. In a preferred embodiment, the spectrophotometric device includes a light source located at a first position and two or more photodetectors located at different distances from the light source. However, it is also possible that the spectrophotometric device includes at least two light sources, which are located at different fixed distances from the photodetectors. In addition, the spectrophotometric device may include a single light source and a single photodetector, wherein the light source and / or the photodetector are movable so as to change the distance from the light source to the detector during measurement. The advantage of this embodiment is that the attenuation of the optical signal as a function of the distance from the light source to the detector can be sampled over a wide range and a large number of data points.

[0045] In a preferred embodiment, the spectrophotometric device is configured to measure attenuation at more than two source-to-detector distances in order to improve the accuracy of the multi-distance method. In a preferred embodiment, the spectrophotometric device is configured to measure attenuation at three source-to-detector distances.

[0046] The distance between the light source and the light detector can be optimized based on, for example, the sensitivity of the detector and the optical properties of the subject's tissue. In the case of a spectrophotometric device for measuring blood oxygen saturation in the abdomen of a newborn, the shortest distance from the light source to the detector is preferably at least 0.8 cm, more preferably at least 0.9 cm, and most preferably at least 1.0 cm. Preferably, the shortest distance between the light source and the detector is in the range of 0.8 to 2 cm, more preferably at least 0.9 to 1.5 cm, and most preferably 0.95 to 1.2 cm. The longest distance from the light source to the detector is preferably in the range of 2 to 10 cm, preferably 2.2 to 6 cm, and most preferably 2.5 to 5 cm.

[0047] Preferably, the at least one light source and the at least one light detector are configured such that they can be in direct contact with the subject's skin to avoid any interference from ambient light.

[0048] The one or more light sources and the one or more light detectors are preferably located in a single housing. The housing is preferably configured to be attached to or positioned on the skin of a subject. More specifically, the housing is preferably configured to be attached to or positioned on the abdomen of a subject.

[0049] The housing may be made of any suitable material, preferably a plastic material. The housing has at least one surface facing the subject's skin, and one or more light sources and one or more light detectors are located on the surface. Preferably, the housing has a single surface facing the subject's skin, and both the light source and the light detector are located on the surface.

[0050] The surface facing the subject's skin may be planar, or its shape may correspond to the surface of a body part on which the spectrophotometric device is placed. For example, the surface of the housing may be concavely curved to correspond to the convex curvature of the subject's abdomen.

[0051] In a preferred embodiment, the housing is provided with an adhesive patch for attaching the housing to the patient's clothing. The adhesive patch can be preferably arranged on the surface of the housing away from the subject's skin, preferably arranged on a surface opposite to the above-mentioned surface on which the light source and the light detector are arranged. For example, the adhesive patch can be used for attaching the housing to a diaper worn by the subject. Therefore, the spectrophotometric device can be fixed below the subject's clothing (e.g., diaper). The adhesive patch is preferably a Velcro patch or a Velcro tape.

[0052] Preferably, the housing is made of a rigid material so that the distance between the light source and the detector remains constant. This is particularly preferred if the spectrophotometric device uses a multi-distance method to calculate blood oxygen saturation. Alternatively, the housing may be made of a flexible material. This has the advantage that the shape of the housing can be adapted to the shape of individual subjects and individual body parts. This allows for a better fit between the spectrophotometric device and the subject's skin and may help to eliminate measurement artifacts due to ambient light hitting the light detector.

[0053] According to a first aspect, the spectrophotometric device further comprises a motion sensor. The motion sensor generates a motion signal indicating the motion of the light source and / or the light detector. Therefore, the motion sensor is preferably located near the light source and / or the light detector. More preferably, the motion sensor is arranged in the above-mentioned housing together with one or more light sources and one or more light detectors. In this case, the motion signal preferably indicates the movement of the entire housing. However, it is also possible that the motion signal specifically indicates the motion of one of the light sources and / or one of the light detectors.

[0054] The motion sensor may include, for example, a linear accelerometer configured to measure linear displacement or a gyroscope configured to measure rotational displacement. A combined linear accelerometer and gyroscope may also be used to measure both linear and rotational displacements. Preferably, the motion sensor is a piezoelectric device or a micro-electromechanical system (MEMS).

[0055] Preferably, a linear accelerometer is used. Suitable linear accelerometers can measure displacement along three axes, two axes or a single axis. Most preferably, a linear 3-axis accelerometer is used.

[0056] The motion sensor preferably has a sampling rate of 1 kHz to 5.3 kHz.The motion sensor preferably measures acceleration in the range of ±16g, more preferably ±8g, most preferably ±4g, more preferably ±2g.

[0057] The electronic data processing unit configured to calculate the blood oxygen saturation is configured to receive data from the motion sensor and one or more light sources and one or more light detectors. The electronic data processing unit can be physically connected to the motion sensor, light source and light detector, for example, by wires. However, the electronic data processing unit can also be located at a remote location and wirelessly connected to the motion sensor, light source and light detector. The electronic data processing unit can also be a central processing unit that is part of many spectrophotometric devices. In this case, the electronic data processing unit receives signals from multiple motion sensors, light sources and light detectors to calculate the blood oxygen saturation of multiple subjects.

[0058] The electronic data processing unit may be implemented, for example, on a personal computer comprising a central processing unit, a random access memory, a data storage device and a data connection for receiving measurement data from the motion sensor and the light detector.

[0059] The electronic data processing unit executes an algorithm for calculating the blood oxygen saturation. In this context, the blood oxygen saturation StO2 is defined as the ratio of the concentration of oxygenated hemoglobin HbO2 to the total concentration of oxygenated hemoglobin HbO2 and deoxygenated hemoglobin Hb according to the following formula:

[0060] StO2=c(HbO2) / (c(HbO2)+c(Hb))

[0061] Where c(HbO2) is the concentration of oxygenated hemoglobin and c(Hb) is the concentration of deoxygenated hemoglobin.

[0062] The algorithm comprises determining the optical attenuation A of an optical signal emitted by one or more light sources and detected by one or more light detectors according to the wavelength λ. λ According to the light attenuation A λ, the algorithm then calculates the concentration of oxyhemoglobin c (HbO2) and the concentration of deoxyhemoglobin c (Hb) based on the Lambert-Beer law. Specifically, the algorithm uses empirically determined extinction coefficients of oxyhemoglobin and deoxyhemoglobin and empirically determined calibration data to account for light scattering in the subject's tissue. The extinction coefficients of oxyhemoglobin and deoxyhemoglobin can be determined individually from the absorption spectra of oxyhemoglobin and deoxyhemoglobin. Preferably, the algorithm only determines the relative concentrations of oxyhemoglobin and deoxyhemoglobin. Since blood oxygen saturation is defined as the ratio of the concentrations of oxyhemoglobin and deoxyhemoglobin, it is not necessary to determine the absolute concentration.

[0063] In one embodiment, the algorithm includes calculating the concentration of oxygenated hemoglobin c (HbO2) and the concentration of deoxygenated hemoglobin c (Hb) using a differential wavelength method. This includes measuring the attenuation of light at at least three different wavelengths and calculating the attenuation difference between the wavelengths. The differential wavelength method is described, for example, in EP 1259791B1.

[0064] In another embodiment, the algorithm includes calculating the concentration of oxygenated hemoglobin c (HbO2) and the concentration of deoxygenated hemoglobin c (Hb) using a multi-distance method. This includes measuring the light attenuation at two or more different light source-to-detector distances, calculating the slope of the light signal attenuation relative to the light source-to-detector distance according to the wavelength, and calculating the blood oxygen saturation in the subject's tissue based on the slope of the light signal attenuation. For example, the multi-distance method is described in Tachtsidis, Ilias et al., "A Hybrid Multi-Distance Phase and Broadband Spatially Resolved Spectrometer and Algorithm for Resolving Absolute Concentrations of Chromophores in the Near-Infrared Light Spectrum", Advances in Experimental Medicine and Biology 662 (2010): 169–175.

[0065] By measuring the attenuation as a function of the source-to-detector distance, the relative absorption μ can be calculated using the multi-distance method. a，λThis eliminates the need to perform calibration by determining the oxygen saturation of a given reference tissue, which is assumed to be a weighted sum of the oxygen saturation of the subject's venous and arterial blood. This approach only needs to take into account light scattering by the tissue. This eliminates a source of systematic error because it no longer requires any assumptions about the relative contributions of venous and arterial blood.

[0066] The electronic data processing unit may also be configured to use the pulsatile portion of the measured light attenuation to determine the pulse and / or respiratory rate of the subject. In this case, the electronic data processing unit is required to continuously sample the light signal detected by the light detector within a given period of time.

[0067] In a preferred embodiment, the algorithm takes into account absorption from other chromophores in addition to oxyhemoglobin and deoxyhemoglobin. The algorithm preferably takes into account empirically determined data that account for wavelength-dependent light absorption by light absorbers other than hemoglobin. Preferably, the algorithm takes into account light absorption due to other light absorbing substances in the abdomen of the subject. Most preferably, this includes absorption by feces, infant transition feces, meconium, and / or biliverdin.

[0068] In a specific embodiment, the algorithm takes into account the wavelength-dependent extinction coefficients of these other light absorbers. The extinction coefficients can be determined by measuring the absorption spectrum of a separate sample of, for example, feces, transition feces, meconium and / or biliverdin. In a preferred example, the extinction coefficient is determined by measuring the absorption spectrum of a separate sample of meconium.

[0069] Meconium is the earliest feces of a mammalian infant. Meconium is composed of materials ingested during the time the infant spends in the womb: intestinal epithelial cells, fetal hair, mucus, amniotic fluid, bile and water. It has been found that the average absorption spectrum of meconium samples taken from many different subjects can be used as a source of extinction data for the above calculations. In one embodiment, the data illustrating the attenuation of the optical signal due to the light absorber therefore includes the wavelength-dependent extinction coefficient of the meconium sample taken from the newborn.

[0070] Transition stool is produced by newborn infants in the first few days of life. The composition of transition stool differs from meconium in that it contains a large amount of biliverdin. Therefore, the data illustrating the attenuation of the optical signal due to the light absorber preferably include wavelength-dependent extinction coefficients of transition stool samples taken from newborn infants preferably within the first two weeks after birth, more preferably within the first week after birth, and most preferably within the first five days after birth.

[0071] In another preferred embodiment, the data describing the attenuation of the optical signal due to the light absorber therefore comprise the wavelength-dependent extinction coefficient of biliverdin.

[0072] The electronic data processing unit is preferably configured to continuously sample the measurement data of the one or more light detectors and calculate the blood oxygen saturation. The sampling rate is preferably in the range of 0.1kHz to 10kHz, more preferably in the range of 0.5kHz to 3kHz, and most preferably in the range of 0.6kHz to 2kHz. The electronic data processing unit may also perform some form of downsampling, for example by calculating an average light signal from a set of continuous, discrete measurements provided by the one or more light detectors. Preferably, the electronic data processing unit calculates a moving average of the measurement data of the one or more light detectors and calculates the light attenuation and the blood oxygen saturation based on the moving average.

[0073] Preferably, the spectrophotometric device includes an electronic storage device connected to an electronic data processing unit, which stores data identifying the spectrophotometric device and calibration data specific to the spectrophotometric device. During operation, the electronic data processing unit can use this information to calculate the blood oxygen saturation. Preferably, the electronic storage device is an integral part of the electronic data processing unit. However, this is not required. The electronic storage device does not necessarily have to be located in the same location or near the electronic data processing unit. The electronic storage device can also be arranged in the above-mentioned housing containing the light source and the detector, or it can be located in a connector that connects the housing to the electronic data processing unit. The electronic storage device can also be located at a remote location and can be connected to the electronic data processing unit, for example, by a wireless location.

[0074] According to a first aspect of the invention, the electronic data processing unit is configured to discard the light signal measured by at least one motion sensor if the motion signal detected by the detector exceeds a predetermined threshold. This improves the accuracy of the measurement and reduces false alarms, because measurement artifacts caused by movement of the spectrophotometric device and / or the subject can be identified and discarded.

[0075] The electronic data processing unit is preferably configured to continuously sample the motion signal provided by the motion sensor. This preferably occurs at the same sampling rate as the sampling of the measurements of the one or more light detectors. The electronic data processing unit may perform downsampling of the measurement data received from the motion sensor as described above for the light signal received by the light detector, for example by calculating a moving average of the motion signal.

[0076] Preferably, the electronic data processing unit is configured to continuously sample the measurement data of one or more light detectors to calculate the blood oxygen saturation, and continuously sample the motion signal provided by the motion sensor. In this embodiment, if the motion signal exceeds a predetermined threshold, the calculation of the blood oxygen saturation is interrupted.

[0077] In one embodiment, the electronic data processing unit discards the light signal as soon as the motion signal exceeds a predetermined first threshold value, and resumes processing of the light signal as soon as the motion signal falls below a predetermined second threshold value. The first threshold value and the second threshold value do not have to be the same. Instead, the first threshold value may be greater than or less than the second threshold value. In one embodiment, the first threshold value is greater than the second threshold value. In this case, the light signal is discarded only if the motion signal is relatively high. Thus, the processing of the light signal is not unnecessarily interrupted by small fluctuations in the motion signal. At the same time, the processing of the light signal is resumed only if the motion signal is below a relatively low second threshold value. Thus, processing is resumed only when interference of motion with the light measurement can be safely excluded.

[0078] In one embodiment, the electronic data processing unit is configured to issue a warning to the user if the motion signal exceeds a predetermined threshold. This may be an optical warning, such as a light indicator or message displayed on an electronic display, and / or an acoustic warning.

[0079] In a preferred embodiment, the electronic data processing unit performs an iterative algorithm, wherein each iteration comprises the following steps:

[0080] 1. Receiving a light signal measured by at least one detector and a motion signal detected by a motion sensor;

[0081] 2. Check the quality of light and motion signals;

[0082] 3. If the quality of the light signal and the motion signal is sufficient, accept the blood oxygen saturation calculation based on the light signal, optionally report the calculated blood oxygen saturation to the user, and reset the value of the signal quality timer to zero;

[0083] 4. If the quality of the light signal and / or motion signal is not sufficient, discard the light signal or blood oxygen saturation calculation and increment the value of the signal quality timer; and

[0084] 5. If the value of the signal quality timer exceeds a predetermined threshold, a warning is issued to the user.

[0085] In this embodiment, the electronic data processing unit stores and modifies the value of the signal quality timer during each iteration of the algorithm described above. This allows the electronic data processing unit to warn the user if the signal quality timer exceeds a predetermined threshold. In other words, if the signal quality is not sufficient for several consecutive iterations of the algorithm, the user is warned. Typical thresholds for the signal quality timer are in the range of 1 to 10 minutes.

[0086] To check the quality of the light signal and the motion signal, if the motion signal exceeds a predetermined threshold, the algorithm reports insufficient quality. Otherwise, it reports sufficient quality.

[0087] Preferably, the algorithm also checks the quality of the optical signal.

[0088] Preferably, the algorithm performs the following steps to check the quality of the light signal and the motion signal:

[0089] 1. If the motion signal exceeds a predetermined threshold, report insufficient quality;

[0090] 2. Otherwise, if the signal-to-noise ratio of the optical signal is below a predetermined threshold, reporting insufficient quality;

[0091] 3. Otherwise, report sufficient quality.

[0092] Here, the noise level used to calculate the signal-to-noise ratio is determined based on a measurement of the ambient light when all light sources of the spectrophotometric device are turned off.

[0093] More preferably, the algorithm performs the following steps to check the quality of the light signal and the motion signal:

[0094] 1. If the motion signal exceeds a predetermined threshold, report insufficient quality;

[0095] 2. Otherwise, if the signal-to-noise ratio of the optical signal is below a predetermined threshold, reporting insufficient quality;

[0096] 3. Otherwise, if the optical signal degradation as a function of the source-to-detector distance does not follow an exponential law, an insufficient signal quality is reported;

[0097] 4. Otherwise, report sufficient quality.

[0098] Here, the reduction of the optical signal as a function of the light source to the detector distance is preferably evaluated by curve fitting. Preferably, the logarithm of the signal amplitude as a function of the light source to the detector distance is determined, a linear regression model is used for curve fitting, and the coefficient of determination is determined. The coefficient of determination (R-squared) is determined according to known statistical methods. If the coefficient of determination is above a predetermined threshold, preferably above a threshold of 0.97, the reduction of the optical signal as a function of the light source to the detector distance is considered to follow an exponential law.

[0099] If the electronic data processing unit receives a motion signal from a linear accelerometer and / or a gyroscope, the electronic data processing unit may calculate a processed motion signal indicating motion in a particular direction. The processed motion signal is then compared with a predetermined threshold to determine whether to interrupt the calculation of blood oxygen saturation. For example, the electronic data processing unit may calculate a processed motion signal indicating motion tangentially or perpendicularly to the subject's skin. For example, this may be achieved by using a gyroscope to determine the three-dimensional orientation of the motion sensor relative to the subject's skin and by using a linear accelerometer to determine linear motion in a certain direction relative to the subject's skin. The electronic data processing unit may also calculate a processed motion signal based on a weighted average of motions in different directions. For example, motion in a direction perpendicular to the subject's skin may be given a greater weight than motion tangentially to the subject's skin. Therefore, the algorithm for calculating blood oxygen saturation is more sensitive to motion in a direction perpendicular to the subject's skin than to motion in a direction tangential to the subject's skin. Motion perpendicular to the skin may indicate that the motion sensor is separated from the skin and that the measurement of the optical signal is impaired.

[0100] In another embodiment, the electronic data processing unit is configured to determine the respiratory rate of the subject based on a motion signal provided by the motion sensor. Preferably, the electronic data processing unit is configured to determine the respiratory rate based on periodic variations of the motion signal. More preferably, the measurement is based on a motion signal provided by a linear motion sensor. More preferably, the measurement is based on a measurement of a linear motion perpendicular to the subject's skin. Periodic variations in the motion signal indicate periodic motion of the motion sensor, which in turn is caused by movement of the subject's body (e.g., the subject's abdomen), and therefore also indicate the respiratory rate.

[0101] The spectrophotometric device may also include an electronic display for displaying the currently calculated blood oxygen saturation level to a user.

[0102] The spectrophotometric device may also include a temperature sensor for measuring the skin temperature and / or the body core temperature of the subject. For example, this may be a resistance temperature detector, a thermocouple, or a pyrometer. To determine the body core temperature, a heat flux sensor may be used.

[0103] According to a second aspect, the present invention relates to a spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, comprising: at least one light source for emitting a light signal into the subject's tissue; at least one detector for detecting the light signal reflected by the subject's tissue; an electronic data processing unit configured to calculate the blood oxygen saturation taking into account the light signal measured by the at least one detector; and a contact sensor for detecting a contact signal indicating contact between the subject's skin and the light source and / or the detector.

[0104] According to a second aspect, the present invention also relates to a method for non-invasively measuring blood oxygen saturation in a subject's tissue, comprising the following steps: using at least one light source to emit a light signal into the subject's tissue, using at least one detector to detect the light signal reflected from the subject's tissue, using an electronic data processing unit to calculate the blood oxygen saturation based on the light signal measured by the one or more detectors, using a contact sensor to detect a contact signal indicating contact between the subject's skin and at least one light source and / or at least one detector. The method is characterized in that if the contact signal indicates that at least one light source and / or at least one detector has lost contact with the subject's skin, the electronic data processing unit discards the light signal measured by at least one detector and / or issues a warning to the user.

[0105] The spectrophotometric device according to the second aspect corresponds to the spectrophotometric device according to the first aspect disclosed above, with the modification that the spectrophotometric device according to the second aspect does not necessarily include the motion sensor disclosed above. Therefore, the spectrophotometric device according to the second aspect may have all the optional features described for the spectrophotometric device according to the first aspect described above. The spectrophotometric device according to the second aspect may also include the above-mentioned motion sensor. Similarly, the spectrophotometric device according to the first aspect may optionally include the contact sensor disclosed for the second aspect.

[0106] According to the second aspect, the spectrophotometric device includes a contact sensor for detecting a contact signal indicating contact between the subject's skin and the light source and / or the detector. Thus, it is possible to continuously monitor the correct placement of the light source and / or the detector. For example, due to the influence of ambient light, loss of contact between the subject's skin and the light source or detector may lead to an erroneous determination of light attenuation. This source of error can be eliminated by continuously monitoring the contact signal measured by the contact sensor. Therefore, the spectrophotometric device according to the second aspect improves the accuracy of the measured blood oxygen saturation. This is particularly important when monitoring newborns or premature infants, who often show sudden movements that lead to loss of skin contact in conventional spectrophotometric devices.

[0107] In this document, the term "skin contact" in relation to light sources and detectors refers to an optimal distance of said light sources and detectors from the skin. This may mean that the surfaces of said light sources and / or detectors are in direct physical contact with said skin, or that they are placed at a certain narrow distance, which allows for optimal transmission of light signals to and from the subject's skin.

[0108] The contact sensor may for example be a capacitive sensor, a conductivity sensor or a pressure sensor. Preferably, the contact sensor provides a distance signal indicating the distance between the contact sensor and the subject's skin. A distance signal below a predetermined threshold indicates that the contact sensor has lost contact with the user's skin. Preferably, the contact sensor is arranged such that loss of skin contact of the contact sensor indicates that at least one light source and / or at least one detector has also lost contact with the subject's skin.

[0109] In a preferred embodiment, the contact sensor is arranged in a housing together with the light source and the detector.The housing may be constructed as explained above with respect to the first embodiment.Preferably, the contact sensor is arranged on a surface of the housing facing the subject's skin.

[0110] Preferably, the electronic data processing unit is configured to discard the light signal measured by the at least one detector and / or to issue a warning to the user if the contact signal indicates that the at least one light source and / or the at least one detector has lost contact with the subject's skin.

[0111] In one embodiment, the electronic data processing unit is configured to continuously sample the measurement data of one or more light detectors to calculate the blood oxygen saturation, and continuously sample the contact signal provided by the contact sensor. If the contact signal indicates that at least one light source and / or at least one detector has lost contact with the subject's skin, the light signal is discarded. More preferably, the contact sensor is configured to determine which light source and / or light detector has lost contact with the subject's skin, and only discard the light signal emitted or measured by the specific light source and / or light detector.

[0112] Furthermore, the electronic data processing unit may also be configured to issue a warning to the user if the contact signal indicates that the at least one light source and / or the at least one detector has lost contact with the subject's skin. This may be an optical warning, such as a light indicator or message displayed on an electronic display, and / or an acoustic warning.

Claims

1. A spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, comprising: one or more light sources for emitting light signals into tissue of the subject; one or more detectors for detecting light signals reflected by tissue of the subject; An electronic data processing unit configured to calculate blood oxygen saturation based on the light signal measured by the one or more detectors; and a motion sensor for detecting a motion signal indicating movement of the light source and / or the light detector, characterized in that the electronic data processing unit is configured to discard the light signal measured by the motion detector if the motion signal detected by the one or more sensors exceeds a predetermined threshold.

2. The spectrophotometric device according to claim 1, characterized in that The spectrophotometric device is configured to measure blood oxygen saturation on the abdomen of a subject.

3. The spectrophotometric device according to any one of claims 1 or 2, characterized in that: The spectrophotometric device is configured to measure the attenuation of the optical signal at two or more wavelengths and / or two or more source-to-detector distances.

4. The spectrophotometric device according to any one of claims 1 to 3, characterized in that: The motion sensor includes a linear accelerometer configured to measure linear displacement and / or a gyroscope configured to measure rotational displacement.

5. The spectrophotometric device according to any one of claims 1 to 4, characterized in that: The electronic data processing unit is configured to continuously sample the measurement data of the one or more light detectors to calculate the blood oxygen saturation, and to continuously sample the motion signal provided by the motion sensor, wherein the electronic data processing unit is configured to discard the light signal if the motion signal detected by the motion sensor exceeds the predetermined threshold.

6. The spectrophotometric device according to any one of claims 1 to 5, characterized in that The electronic data processing unit is configured to discard the light signal once the motion signal exceeds a predetermined first threshold and to resume processing of the light signal once the motion signal drops below a predetermined second threshold.

7. The spectrophotometric device according to claim 6, characterized in that The first threshold is greater than the second threshold.

8. The spectrophotometric device according to any one of claims 1 to 7, characterized in that Also included is an electronic storage device connected to the electronic data processing unit, the electronic storage device storing data identifying the spectrophotometric device and calibration data specific to the spectrophotometric device.

9. The spectrophotometric device according to any one of claims 1 to 8, characterized in that The electronic data processing unit is configured to determine a breathing rate of the subject based on the motion signal provided by the motion sensor.

10. The spectrophotometric device according to any one of claims 1 to 9, characterized in that The electronic data processing unit is configured to issue a warning to the user if the motion signal exceeds the predetermined threshold.

11. The spectrophotometric device according to any one of claims 1 to 10, characterized in that The one or more light sources and the one or more light detectors are located in a single housing, and the housing includes an adhesive patch for attaching the housing to clothing of the subject.

12. The spectrophotometric device according to any one of claims 1 to 11, characterized in that The electronic data processing unit is configured to determine the quality of the light signal and to discard the light signal only if the quality of the light signal is insufficient and the motion signal exceeds the predetermined threshold.

13. A method for non-invasively measuring blood oxygen saturation in a subject's tissue, comprising the steps of: Use at least one light source to emit a light signal into a subject's tissue, use at least one detector to detect the light signal reflected from the subject's tissue, use an electronic data processing unit to calculate blood oxygen saturation based on the light signal measured by one or more detectors, and use a motion sensor to detect a motion signal indicating movement of the light source and / or the light detector, characterized in that if the motion signal detected by the one or more sensors exceeds a predetermined threshold, the electronic data processing unit discards the light signal measured by the motion detector.

14. A spectrophotometric device for measuring blood oxygen saturation in a subject's tissue, comprising: at least one light source for emitting a light signal into tissue of a subject; at least one detector for detecting light signals reflected from tissue of the subject; an electronic data processing unit configured to calculate the blood oxygen saturation taking into account the light signal measured by the at least one detector; and a contact sensor for detecting a contact signal indicative of contact between the subject's skin and the light source and / or the detector.

15. The spectrophotometric device according to claim 14, characterized in that The electronic data processing unit is configured to discard the light signal measured by the at least one detector and / or to issue a warning to a user if the contact signal indicates that the at least one light source and / or the at least one detector have lost contact with the subject's skin.

16. A method for non-invasively measuring blood oxygen saturation in a subject's tissue, comprising the steps of: Using at least one light source to emit a light signal into a subject's tissue, using at least one detector to detect the light signal reflected from the subject's tissue, using an electronic data processing unit to calculate blood oxygen saturation based on the light signal measured by one or more detectors, using a contact sensor to detect a contact signal indicating contact between the subject's skin and the at least one light source and / or the at least one detector, characterized in that if the contact signal indicates that the at least one light source and / or the at least one detector has lost contact with the subject's skin, the electronic data processing unit discards the light signal measured by the at least one detector and / or issues a warning to a user.