Cuffless method for determining a user's blood pressure

Abstract

A device (1) for estimating a user's blood pressure by combining an acoustic modality, employing acoustic emitters and detectors, and an optical modality, employing light sources (15) and photodetectors (18), forming source-detector pairs. The device includes an acoustic selection unit (21), configured to determine the relevant acoustic emitter / acoustic detector pair, and an optical selection unit (22), configured to determine the relevant light source-photodetector pairs. Figure 1.

Description

Title of the invention: Cuff-free method for determining a user's blood pressure Technical field

[0001] The technical field is the characterization of blood pressure without compression. PREVIOUS ART

[0002] Most devices for characterizing blood pressure use a pressure sensor coupled to a compression cuff placed on a limb, generally an arm. The characterization of blood pressure is carried out by measuring the pressure exerted by the cuff at one or more characteristic instants. The pressure sensor or the acoustic sensor are sensitive to heartbeats and their amplitude.

[0003] The devices used by medical personnel (auscultatory method) consist of a cuff whose pressure is controlled and read by the doctor, generally associated with a stethoscope. When the cuff deflates, the appearance and disappearance of sounds, called Korotkoff sounds, are detected. The pressure applied by the cuff when the sounds appear and disappear corresponds respectively to the systolic pressure and the diastolic pressure.

[0004] In consumer blood pressure monitors, a pressure sensor determines the air pressure in the cuff. The cuff is compressed so as to first obtain arterial occlusion. When the cuff is deflated, pressure oscillations appear. The oscillations increase until they temporarily reach a maximum amplitude. At this moment, the pressure in the cuff is considered equal to the mean arterial pressure. From the maximum amplitude detected, the systolic and diastolic arterial pressures are estimated on the basis of empirical laws.

[0005] However, the use of a device comprising a cuff requires regular compression phases if continuous pressure measurement is to be performed. This constitutes a source of discomfort, linked both to the perception of compression and to the noise of the pump activating compression of the cuff. In addition, repeated occlusion at too high a frequency can present a risk.

[0006] Recently, developments have been made to enable so-called “cuffless” blood pressure measurements. The publication [1] Nabeel M. “Bi-modal arterial compliance probe for calibration-free cuffless blood pressure estimation”, IEEE transactions on biomedical engineering, Vol. 5, No. 11, November 2018, describes a device combining an acoustic modality and an optical modality to estimate a user's blood pressure, without resorting to compression of a user's limb using a cuff. The acoustic modality allows measurement of the evolution of the artery diameter between two extreme values, corresponding respectively to systole and diastole. The optical modality allows, according to the principles of PPG (Infrared Photoplethysmography), to estimate a pulse wave velocity (PWV) between two measurement points, distant from each other, along the artery. The pulse wave velocity is usually designated by the acronym PWV (Pulse Wave Velocity). The measurements resulting from the two modalities are combined in order to estimate blood pressure.

[0007] The principles set out in publication [1] can be implemented to design a device worn by a user, and allowing continuous monitoring of blood pressure, reducing the discomfort felt by the user. However, the measurements carried out in both modalities can be affected by uncertainties linked to the positioning of the active components (light sources, acoustic or optical sensors), relative to the artery. Thus, when the device is applied to the body of a user, the sensors must be correctly positioned precisely relative to the artery so that the variation in diameter and the pulse wave velocity are correctly estimated.

[0008] Also, the device described above assumes precise positioning on the user's skin. Another difficulty is related to the user's movements, which can cause a variation in the position of the sensors relative to the artery. The invention described below makes it possible to overcome these difficulties. Statement of the invention

[0009] A first object of the invention is a device for estimating a user's blood pressure, the device being intended to be worn by the user, the device comprising:

[0010] - a support, intended to be applied against the user's skin; - several light sources, arranged on the support, configured to emit light towards the user's skin when activated; - several photodetectors, arranged on the support, distant from each light source, configured to detect light emanating from the user's skin following activation of at least one light source, each photodetector forming, with said light source, a source-photodetector pair;

[0011] the device being characterized in that it comprises:

[0012] - several acoustic transducers, comprising at least: • an acoustic transmitter, configured to emit an acoustic wave through the skin; • and an acoustic detector, configured to detect an acoustic wave reflected in the user's body, propagating through the skin; - an acoustic selection unit, programmed to: • take into account an acoustic selection criterion; • select an acoustic transmitter and an acoustic detector from among the acoustic transducers, the selection being made as a function of an acoustic signal detected by each acoustic detector following emission of an acoustic wave by at least one acoustic transmitter; - an optical selection unit, configured to: • take into account an optical selection criterion; • select a first light source - photodetector pair, comprising a first light source and a first photodetector, chosen from among the light sources and the photodetectors, and a second light source - photodetector pair, comprising a second light source and a second photodetector, chosen from among the light sources and the photodetectors, the selection being made as a function of the signals detected by the first photodetector and the second photodetector following activation of the first light source and the second light source;

[0013] - a central unit, programmed to estimate blood pressure, from: • the signal detected by an acoustic detector;

[0014] • signals detected by the first photodetector and the second photodetector.

[0015] The central unit can be programmed to:

[0016] • estimate an arterial diameter from the signal detected by the selected acoustic detector; • estimate a pulse wave velocity, from the signals detected by the first photodetector and the second photodetector; • estimate blood pressure, based on estimated arterial diameter and pulse rate.

[0017] Preferably, the light source emits light in a spectral band between 500 nm and 1200 nm.

[0018] The device may comprise:

[0019] - a first group of light sources and photodetectors; - a second group of light sources and photodetectors, distant from the first group of light sources and photodetectors.

[0020] The optical selection unit is then configured to:

[0021] • select the first light source and the first photodetector from among the light sources and photodetectors of the first group; • select the second light source and the second photodetector from the light sources and photodetectors of the second group.

[0022] According to one possibility, the acoustic criterion being a signal-to-noise ratio, the acoustic selection unit is configured to:

[0023] - estimate a signal-to-noise ratio of each signal detected by an acoustic detector; - select the acoustic detector with the highest signal-to-noise ratio.

[0024] The acoustic criterion may be an intensity of the signal detected by an acoustic detector, the acoustic selection unit is then configured to

[0025] - estimate an intensity of each signal detected by an acoustic detector; - select the acoustic detector with the highest intensity.

[0026] According to one possibility, the optical criterion being a correlation criterion, the optical selection unit is configured to:

[0027] - estimate a temporal correlation between the signals detected at different times by photodetectors of each light source-photodetector pair; - select the first light source and the first photodetector as well as the second light source and the second photodetector according to the estimated time correlation.

[0028] According to one possibility, the optical criterion is an amplitude criterion, the optical selection unit is configured to:

[0029] - estimate an amplitude of a temporal evolution of signals detected at several instants by photodetectors of each light source-photodetector pair; - select the first light source and the first photodetector as well as the second light source and the second photodetector according to the amplitude.

[0030] According to one possibility, the optical criterion is a shape criterion. The optical selection unit is configured to:

[0031] - take into account a predetermined temporal form; - determine a temporal evolution of the detected signals, in different instants by the photodetectors of each light source-photodetector pair; - selecting the first light source and the first photodetector as well as the second light source and the second photodetector according to a correlation between the temporal evolution of the detected signals and the predetermined temporal shape.

[0032] Another object of the invention is a method for estimating blood pressure using a device according to the first object of the invention, the method comprising:

[0033] a. arrangement of the support on the skin of a user, facing an artery; b. emission of at least one incident acoustic wave by a transmitter acoustics and acquisition of acoustic signals by an acoustic detector, each detected acoustic signal comprising echoes representative of reflections of acoustic background incident by the artery, step b) being carried out for different acoustic transmitters and / or different acoustic detectors, such that each detected acoustic signal is associated with an acoustic transmitter and an acoustic detector; c. using the acoustic selection unit: - taking into account an acoustic selection criterion; - selection of an acoustic transmitter and an acoustic detector, in function of a comparison between each acoustic signal detected, during step b), and the acoustic selection criterion; d. for each light source, emission of incident light towards the user's skin and detection of backscattered radiation by at least one photodetector, each photodetector generating an optical signal representative of the intensity of the backscattered radiation; e. using the optical selection unit: - taking into account an optical selection criterion; - selection of two light source - photodetector pairs, each pair comprising a light source and a photodetector, based on a comparison between each optical signal from each photodetector and the optical selection criterion; f. emission of an incident acoustic wave by the acoustic transducer selected in c), and formation of an acoustic signal representative of echoes following reflections of the incident acoustic wave by the artery; g. activation of the light sources of each light source-photodetector pair selected during e) and formation, by each photodetector of each pair, of an optical signal representative of the intensity of the radiation backscattered by the artery; h. based on the acoustic signal and the optical signals formed by each photodetector at different times, estimation of the user's blood pressure.

[0034] According to one possibility, step h) comprises:

[0035] - hl) depending on the acoustic signal formed, estimation of the diameter of the artery; - h2) as a function of the optical signals formed, at different times, by each selected photodetector, estimation of a pulse wave velocity; - h3) from the diameter of the artery resulting from sub-step hl) and the pulse wave velocity resulting from substep h2), estimation of a user's blood pressure.

[0036] Sub-step h2) may comprise an estimation of a time shift between the optical signals respectively formed by the first photodetector and the second photodetector.

[0037] Steps a) to e) may constitute a calibration phase of the device, steps f) to h) being repeated between two successive calibrations.

[0038] According to one possibility, the device comprises:

[0039] - a first group of light sources and photodetectors; - a second group of light sources and photodetectors, distant from the first group of light sources and photodetectors.

[0040] The method can then comprise:

[0041] - a selection of a first light source / photodetector pair in the first group; - a selection of a second light source / photodetector pair in the second group.

[0042] According to one embodiment, the method comprises:

[0043] - taking into account a range of validity of the blood pressure; - when the blood pressure resulting from step h) is outside the validity range, repeat the calibration phase.

[0044] According to one possibility, the acoustic selection criterion is a maximum signal-to-noise ratio, or a maximum intensity of a detected acoustic signal, the selection of the transmitter and the acoustic detector being carried out as a function of the acoustic signal, associated with the acoustic transmitter / acoustic transmitter pair, whose signal-to-noise ratio is maximum or whose intensity is maximum.

[0045] The optical selection criterion may include a time correlation criterion, with a time shift, such that the selection of each source-detector pair includes:

[0046] - an estimation of a temporal correlation between the signals detected at different times by the photodetectors of each source pair of light-photodetector; - a determination of the light source-photodetector pairs for which the signal resulting from the photodetector has the highest temporal correlation.

[0047] The optical selection criterion may be one of the selection criteria described in connection with the first subject of the invention.

[0048] The method may be such that:

[0049] - the first light source - photodetector pair defines a first measuring point; - the second light source - photodetector pair defines a second measuring point; - the first measuring point and the second measuring point are distant from each other.

[0050] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0051] [Fig.l] [Fig.l] is an example of a device according to the invention.

[0052] [Fig.2A]

[0053] [Fig.2B]

[0054] [Fig.2C]

[0055] [Fig.2D]

[0056] [Fig.2E] Figures 2A to 2E illustrate an implementation of the acoustic modality of the device.

[0057] [Fig.3A]

[0058] [Fig.3B] Figures 3A and 3B illustrate an implementation of the optical modality of the device.

[0059] [Fig.4] [Fig.4] is another example of a device according to the invention.

[0060] [Fig.5] [Fig.5] shows the main stages of implementing a process determining a user's blood pressure using a device as described in connection with [Fig.l] or [Fig.4]. PRESENTATION OF SPECIAL EMBODIMENTS

[0061] [Fig.l] shows an example of a device 1 allowing an estimation of the blood pressure of a user. The device 1 comprises a support 10, intended to be placed against the skin of a user, facing an artery A. This may for example be the carotid artery, as described in [1] or the radial artery for a wrist device. The support 10 is configured to be placed permanently in contact with the skin. It is preferably a flexible support, so as to fit the contour of the skin. The support 10 may be adhesive or be connected to a means of holding it against the skin, such as an elastic armband or bracelet.

[0062] The support comprises components allowing implementation of acoustic or optical modalities such as described in [1].

[0063] The acoustic modality is implemented using acoustic transducers 11, distributed on the support. Each acoustic transducer is configured to emit and / or detect an ultrasonic acoustic wave, so as to determine a temporal evolution of the diameter D(t) of the artery A under the effect of cardiac activity. The acoustic transducers can be piezoelectric transducers or electromechanical sensors of the MEMS type. The acoustic transducers are connected to an acoustic processing unit 31. Preferably, but without this being necessary, each acoustic transducer can function both as an acoustic transmitter or an acoustic detector.

[0064] The device comprises an acoustic processing unit 31, configured to receive signals detected by at least one acoustic transducer 11, operating as an acoustic detector, so as to estimate, at different times, the diameter of the artery D(t). The acoustic modality is described in more detail in connection with FIGS. 2A to 2E.

[0065] The optical modality is implemented using light sources 15 and photodetectors 18, distributed on the support 10, and distant from each other. Each light source is configured to emit light towards the user's skin when it has been activated. Each photodetector is distant from a light source. Each photodetector is configured to detect light emanating from the user's skin following activation of at least one light source. Each photodetector can thus detect light emitted by a light source, and having propagated through the user's body, before emerging from the user's skin, facing the photodetector. When the photons detected by a photodetector have propagated between the skin and the artery, the detected light undergoes periodic variations under the effect of the periodic variation of the blood volume in the tissues probed (artery, but also veins, capillaries, etc.)), induced by cardiac activity. The optical modality is described in more detail in connection with Figures 3A and 3B.

[0066] Generally speaking, the optical modality assumes the consideration of light source-photodetector pairs, each pair associating a light source and a photodetector, distant from the light source. The distance between a light source and a photodetector of the same pair can be of the order of a few millimeters to a few cm. A photodetector (respectively a light source) can form different pairs with different light sources (respectively different photodetectors).

[0067] The photodetectors are connected to an optical processing unit 32. The optical processing unit 32 is configured to receive signals detected by at least one photodetector 18, so as to estimate, at different times, the time taken by the disturbance of the blood volume in the tissues induced by the cardiac activity to propagate between two measurement points Pb P2 separated by a separation distance A, along an arterial segment. Each measurement point is located between a light source and a photodetector. The pulse wave velocity VOP is estimated from a ratio between:

[0068] - a time shift At between the signals respectively detected by each photodetector 18; the separation distance d(Pb P2) between the two measuring points Pi and P2.

[0069] The support 10 is arranged on the user's skin, facing the analyzed artery A. However, the position of the artery relative to the support 10 is not known precisely, in particular when the user is likely to move. An important aspect of the invention is to be able to select:

[0070] a relevant acoustic transmitter and detector, i.e. correctly positioned relative to artery A, so as to obtain a correct estimate of the temporal variation of the diameter of the artery; two relevant light source-photodetector pairs, each pair comprising a light source 15 and a photodetector 18, each pair being positioned so as to determine a variation in blood volume at the two measurement points Pi and P2. Comparison of the signals from each photodetector allows an estimation of the pulse wave velocity.

[0071] The device comprises an acoustic selection unit 21, connected to each detector acoustic. The acoustic selection unit 21 is programmed to take into account an acoustic selection criterion and select an acoustic transmitter and an acoustic detector from among the acoustic transducers. The selected acoustic transmitter and detector are those for which, following the emission of an acoustic wave by the acoustic transmitter, the acoustic detector detects an acoustic signal considered to best satisfy the acoustic selection criterion. The acoustic selection criterion is, for example, a maximum signal-to-noise ratio. In this case, the acoustic detector selected by the acoustic selection unit is, for example, the one which generates, during a measurement period, the acoustic signal having the highest signal-to-noise ratio. In [Fig.l], an acoustic transducer is shown diagrammatically, operating both as an acoustic transmitter and as an acoustic detector, selected by a dotted outline.The selected acoustic transducer is centered (or considered as such) relative to artery A. The resulting signal from the selected transducer is then transmitted to the processing unit. acoustic 31 in order to determine the temporal evolution D(t) of the diameter of the artery.

[0072] The device comprises an optical selection unit 22, connected to each photodetector 18. The optical selection unit 22 is programmed to take into account an optical selection criterion and select two optical signals resulting from the respective photodetectors of two light source-photodetector pairs. The selected pairs are those for which the photodetectors generate an optical signal considered to best satisfy the optical selection criterion. The optical selection criterion is for example a correlation between the signals respectively generated by the photodetector of each pair. In this case, the light source-photodetector pairs selected by the optical selection unit 22 are those generating temporally correlated optical signals, apart from the time shift. By temporally correlated, it is meant that the changes, as a function of time, of the signals respectively at the two measurement points, are correlated.The time shift depends on the distance between the measurement points respectively defined by each light source - photodetector pair. In [Fig.l], two light source-photodetector pairs are shown schematically, selected by two dotted contours. Each measurement point Pi and P2 is considered to be located opposite the middle of a straight line segment connecting the light source to the photodetector of the selected light source-photodetector pair. In [Fig.l], we have represented: .

[0073] - a double arrow materializing a first distance, called backscattering, di between the source and the photodetector of the first selected light source-photodetector pair; - a double arrow materializing a second backscattering distance d2 between the source and the photodetector of the second selected light source-photodetector pair.

[0074] Preferably, each selected light source-photodetector pair extends on either side of the artery, or along the artery.

[0075] The measuring points Pi and P2 respectively formed by the two selected light source - photodetector pairs are spaced apart from each other. The distance d(Pb P2 ), along the artery, between the two measuring points, is preferably greater than 1 cm or 5 cm. The distance

[0076] The signals resulting from the photodetectors of each selected light source-photodetector pair are addressed to the optical processing unit 32, so as to estimate the pulse wave velocity VOP.

[0077] Preferably, the light sources 15 and the photodetectors 18 are distributed on the support 10 forming a first group Gi and a second group G2. The optical selection unit 22 is programmed to select:

[0078] - a first light source-photodetector pair among the light sources 15 and photodetectors 18 of the first group Gp - a second light source-photodetector pair among the light sources 15 and photodetectors 18 of the second group G2.

[0079] This division into two groups makes it possible to guarantee a minimum separation distance d(Pb P2)min between the first and second measuring points Ph P2 respectively defined by each selected light source-photodetector pair. The distance d(Pi,P2) between the measuring points Pi and P2 is assumed to be known by knowledge of the geometry of the device. It is represented in [Fig.l] by a double dotted arrow.

[0080] Figures 2A, 2B and 2C illustrate the operation of the acoustic modality. Each transducer emits an incident acoustic wave, symbolized by the arrow Fp. The latter propagates through the user's skin, towards the artery A. A part of the incident acoustic wave is successively reflected by a proximal portion Ap and a distal portion Ad, diametrically opposed, of the wall of the artery A. In [Fig.2A], the arrows F2 and F3 correspond respectively to the parts of the incident wave respectively reflected by the proximal portions Ap and distal Ad. The transducer (or another transducer) detects the waves successively reflected by the proximal portions Ap and distal Ad, the latter forming respectively a proximal echo Ep and a distal echo Ed. [Fig.2B] schematizes the echoes. In [Fig.2B], the ordinate axis corresponds to the amplitude of the detected wave and the abscissa axis corresponds to time.The time shift dt between the two echoes Ep and Ed makes it possible to estimate the diameter of the artery, knowing the propagation speed of the acoustic wave. The measurements shown in [Fig.2B] schematize an acoustic signal sent by each transducer 11 to the selection unit 21.

[0081] The measurements shown in [Fig.2B] are repeated at different measurement times, using a sampling frequency that may be, for example, 1 kHz. [Fig.2C] represents a temporal evolution of the diameter D(t) of the artery (y-axis - units pm) as a function of time t (unit: second). The maximum diameter corresponds to systole, while the minimum diameter corresponds to diastole. In this example, the variation in diameter AD between systole and diastole is of the order of 0.6 mm.

[0082] The position of the transducer 11 relative to the artery has an influence on the quality of the measurement of D(t). Simulations have shown that when the transducer is not centered relative to the artery, the intensity of the reflected acoustic wave decreases. Given the measurement noise, this increases the uncertainty in determining the time interval dt, and therefore the time evolution D(t). [Fig.2D] shows a simulation configuration. An acoustic probe comprising a line was simulated of 25 transducers 11, each operating in emitter / detector mode, arranged in contact with a skin S, of cylindrical shape, and radius approximately 13 mm. The artery A was modeled in the center of the cylinder modeling the skin S. Between the skin and the artery extends a medium M corresponding to a muscle. Thus, the artery A is located at a depth of between 10 mm and 15 mm under the skin S, which corresponds to a usual artery depth. Using the CIVA software (provider Extende), the intensity of the echoes produced by each transducer in response to an incident acoustic pulse was evaluated. The densities and acoustic propagation speeds in the modeled media S, M and A are representative of those of the blood, muscles and skin respectively. [Fig.2E] represents the evolution of the signal amplitude (y-axis - arbitrary units) as a function of the transducer (x-axis).In this case, transmission and reception are carried out by the same transducer. Transducers 1, 13 and 25 correspond respectively to the extreme left, centered and extreme right positions of the probe. It is observed that a centered position of the acoustic transducer, relative to the artery, makes it possible to obtain a maximum echo intensity. Thus, the position of the transducer has a significant influence on the quality of the measurement carried out. The acoustic selection unit 21 allows selection of the acoustic transducer generating the echoes whose signal is maximum among the other transducers. This results in a precise estimation of the temporal evolution D(t) of the diameter D of the artery.

[0083] According to one possibility, several transducers, phase-shifted from each other, are used simultaneously. The selection unit makes it possible to select a reference transducer, then to estimate a time phase shift between the different transducers relative to the reference transducer.

[0084] Figures 3A and 3B illustrate the operation of the optical modality. A light source 15 and a photodetector 18 are shown forming a light source-photodetector pair. The light source 15 emits an incident light beam 16 propagating, through the skin, towards the user's artery A. The emission direction is generally perpendicular to the skin. The light beam is preferably emitted according to a narrow spectral emission band, preferably <50 nm, in a spectral range between 500 nm and 1200 nm, and preferably between 500 nm and 1000 nm. This spectral range corresponds to significant absorption of light by hemoglobin. The spectral bands can be centered on the following wavelengths: 525 nm, 660 nm, 740 nm, 805 nm, 850 nm.

[0085] Each light source may be an LED (light emitting diode), or one end of an optical fiber, the other end of which is arranged facing a light source. Each photodetector may be a photodiode, or a light end of an optical fiber, the other end of which is connected to a light sensor. Alternatively, each light source can be a laser diode, a VECSEL, or the end of a light guide, such as an optical fiber.

[0086] The photons of the incident light beam 16 penetrate into the user's body. They propagate in the tissues between the skin and the artery A, the latter being located at a depth, under the skin, of the order of 10 mm. A portion of the incident photons is backscattered in a direction parallel to the emission direction. The re-backscattered photons constitute backscattered radiation 17. The backscattered radiation 17 can be detected by the photodetector 18, placed opposite the user's skin. The distance d between the light source and the photodetector, called the backscattering distance, is generally non-zero and is generally between 5 mm and a few cm. The photodetector 18 thus makes it possible to measure the intensity of the backscattered radiation according to the backscattering distance d. In [Fig.3A], the dotted curved arrows represent optical paths of photons emitted by the light source 15 and detected by the photodetector 18. The photodetector is thus arranged to measure an intensity of a light beam formed by backscattered photons.

[0087] The greater the backscattering distance, the more the photons constituting the backscattered radiation 17 penetrate the user's tissues to a greater depth. The intensity of the backscattered radiation depends on the variation in blood volume in the artery. The greater the quantity of blood, the greater the quantity of photons absorbed by the hemoglobin, and the more the intensity of the backscattered radiation decreases. With each heartbeat, the influx of blood causes, in the tissues probed, a modulation of the absorption of the light propagating in the tissues. This results in a modulation of the intensity of the backscattered radiation detected by the photodetector 18 associated with the light source.

[0088] The signal detected by the photodetector 18 comprises a continuous component, to which is added a pulsatile component, the latter varying according to the cardiac activity. The intensity detected by the photodetector thus comprises a periodic component, the fundamental frequency of which corresponds to the heart rate.

[0089] As shown in Figure 1, the device makes it possible to select two light source-photodetector pairs, respectively defining two measurement points Pi and P2, along the artery, spaced apart by a separation distance d(P15 P2). Due to the separation distance, the periodic signals resulting respectively from the first and second light source-photodetector pairs are shifted by a time shift At. Knowing the separation distance d(Ps, P2), the estimation of the time shift, called the pulse wave transit time, allows an estimation of a speed, called the “pulse wave velocity” (PWV), according to the expression:

[0090] vop= d(P| P,)

[0091] [Fig.3B] represents:

[0092] - a first optical signal SOi resulting from the photodetector of the first light source-photodetector pair; - a second optical signal SO2 resulting from the photodetector of the second light source-photodetector pair.

[0093] In [Fig.3B], each signal corresponds to the opposite of the detected signal: thus, each signal SOi, SO2 is representative of the periodic absorption of the tissues at the measurement points Pi and P2. The time shift can be estimated from the time difference At between remarkable points of each curve, for example maxima or minima.

[0094] Preferably the separation distance d(P1? P2) is greater than 5 mm. It is preferably less than 10 cm or 5 cm.

[0095] The acquisition frequency of the signals resulting from each photodetector can be between 100 Hz and 100 kHz, which allows an estimation of the pulse wave velocity with sufficient temporal resolution.

[0096] However, the depth of the artery under the skin is not known precisely. Similarly, the position of the artery, parallel to the support 10, is not known, or may vary depending on the movements of the user. Thus, it is difficult to determine a priori the most relevant source-photodetector pairs for accurately estimating the pulse wave velocity. It is understood from [Fig. 3A] that the backscattering distance must be sufficient to allow propagation of the detected photons through the artery or a volume of tissue impacted by the disturbance of the blood volume induced by cardiac activity. It is also preferable for the source and the photodetector of the same pair to be arranged either along the artery or on either side of the artery, so that the measurement point, located between the source and the photodetector, is located close to the artery.

[0097] The optical selection unit 22 takes into account different detected signals as shown in [Fig.3B]. These are optical signals representative of the intensity detected by the photodetectors of different light source-photodetector pairs. The detected signals comprise a component, called pulsatile, which can be considered as periodic, due to the repetition of the cardiac cycles. The selection of the relevant source-photodetector pairs is carried out by taking into account an optical selection criterion and by determining two light source-photodetector pairs for which the optical selection criterion is satisfied.

[0098] The optical selection criterion can be a temporal correlation. Indeed, the most relevant detected signals have a comparable temporal evolution, which corresponds to cardiac activity. Selecting detected signals with high temporal correlation facilitates the determination of the time shift At shown in [Fig.3B]. By temporal correlation is meant a correlation within a time shift that corresponds to the distance between the measurement points along the artery.

[0099] Other optical selection criteria can be applied in addition to or in replacement of the correlation criterion. During each period, the pulsatile component of each detected signal describes an oscillation of a certain amplitude Amp . The acceptance criterion can be an amplitude greater than a certain threshold, or located within a previously defined acceptance range.

[0100] The optical selection criterion may be a correlation of the temporal evolution of each detected signal, over a period, with a predetermined shape. Two light source-photodetector pairs are then selected having the best correlation with a predetermined temporal shape.

[0101] Another optical selection criterion may be a minimum distance between the source and the photodetector forming a light source-photodetector pair, so that the addressed depth is sufficient.

[0102] The optical selection criteria previously described can be combined.

[0103] The optical processing unit 32 then estimates the pulse wave velocity from optical signals generated by the photodetectors of the selected light source-photodetector pairs.

[0104] The device comprises a calculation unit 35, configured to estimate a blood pressure value from the temporal evolution of diameter D(t) resulting from the acoustic processing unit 31 and the VOP resulting from the optical processing unit 32. The blood pressure P is such that

[0105] P = f(D, VOP) (2)

[0106] where f is a previously defined function.

[0107] For example, the function f can be such that:

[0108] P ( t ) .p0^ 2p (VOP)hn (3)

[0109] where:

[0110] - P ( t) is the blood pressure at an instant; - Dq is a reference diameter, for example the diameter at the end of diastole - P(> is a reference pressure corresponding to D = Dq.

[0111] The blood pressure P ( t ) obtained by equation (3) describes a periodic function, each period corresponding to a heartbeat. During each period, the maximum pressure corresponds to the systolic blood pressure, the minimum pressure corresponds to the diastolic blood pressure. The blood pressure average is the average of the pressure over a period.

[0112] The function / previously mentioned can be determined by calibration, in the presence of a reference measurement of the user's blood pressure. A link can then be established between the measured values ​​(D, VOP) at different times with a reference blood pressure measured by the reference method. The comparison between the measured values ​​and the measured reference pressure makes it possible to establish the calibration function.

[0113] [Fig. 4] represents another example of a device suitable for implementing the invention. [Fig. 4] shows the main dimensions. The device allows the analysis of an artery A extending between the two extreme positions Asup and Ainf shown in [Fig. 4]. The two groups Gi and G2 are represented by a bracket, in which the two light source-photo-detector pairs are to be identified according to the position of the artery relative to the support 10.

[0114] According to a variant, the support 10 is divided into two elementary supports distant from each other. The first elementary support comprises a first group Gi of photodetectors and light source. A second elementary support comprises a second group of photodetectors and light sources, the transducers can be fixed on one of the elementary supports or on both elementary supports. Thus, the support 10 may not be monolithic and comprise different elementary supports.

[0115] The acoustic or optical selection units, as well as the central unit, can form a single unit, implemented by a microprocessor. Alternatively, each of these units implements a microprocessor. According to one possibility, all or part of the selection units or the central unit is remote, at a distance from the support 10. The device comprises a transmission unit so as to transmit the signals from the transducers and the photodetectors by wired or wireless connection.

[0116] [Fig.5] shows schematically the main steps of a method implementing a device as shown in figures 1 or 4.

[0117] Step 100: arrangement of the support on the skin of a user, facing an artery.

[0118] Step 110: emission of an acoustic wave by an acoustic transmitter and detection of an acoustic wave reflected by an acoustic detector, step 110 being repeated for different acoustic transmitters and / or different acoustic detectors. Each detected acoustic signal is likely to include echoes representative of the reflection, by the artery, of the acoustic wave emitted by the acoustic transmitter.

[0119] Step 120: processing of the acoustic signals by the acoustic selection unit, so as to identify the relevant acoustic transmitter / detector pair. The latter corresponds to the detector whose detected acoustic signal, following the emission of a acoustic wave by the acoustic transmitter, most satisfies the acoustic selection criterion previously mentioned.

[0120] Step 130: acquisition of optical signals by the different photodetectors of different light source-photodetector pairs. During this step, a light source can be sequentially activated and optical signals from different photodetectors considered to be sufficiently close to the source to detect usable backscattered radiation are acquired. The light sources are activated successively. Optical signals are then obtained for each light source-photodetector pair.

[0121] Step 140: Processing of the optical signals resulting from each photodetector, during step 130 by the optical selection unit, so as to identify the two relevant light source-photodetector pairs. These correspond to the pairs whose signals, resulting from the photodetector, most closely satisfy the optical selection criterion.

[0122] Steps 110 to 140 correspond to a calibration phase, based on the calibration measurements acquired during steps 110 and 130. The acquisition of the calibration measurements can be carried out during a predefined calibration period, which can last a few minutes for example.

[0123] Steps 150 to 170 correspond to the implementation of the device, following the calibration phase.

[0124] Step 150: determination of the temporal evolution of the diameter D(t) of the artery, by implementing the acoustic modality. During this step, the acoustic processing unit 31 receives the acoustic signal resulting from the acoustic detector selected during step 120.

[0125] Step 160: determination of the evolution of the pulse wave velocity VOP of the artery, by implementing the optical modality. During this step, the optical processing unit 32 receives the optical signals resulting from the respective photodetectors of the two light source-photodetector pairs selected during step 140.

[0126] Steps 150 and 160 may be performed simultaneously or in any order.

[0127] Step 170: estimation of the arterial pressure as a function of the diameter of the artery D(t) and the VOP resulting from steps 150 and 160, for example using expression (3) or another calibration function

[0128] Steps 150 to 170 may be repeated at successive measurement times, so as to perform a “continuous” blood pressure measurement, i.e. at sufficiently close times. The calibration phase (steps 110 to 140) may be repeated periodically, so as to identify the most relevant acoustic transducers and light source / photodetector pairs. The calibration phase may also be carried out in the event of the occurrence of a blood pressure measurement considered abnormal.

[0129] The invention takes advantage of the fact that the device allows the estimation of blood pressure without resorting to compression of a limb of the user. It is robust to uncertainties in the positioning of the support of the device on the skin of the user, as well as to possible movements of the user wearing the support.

Claims

Claims

1. Device for estimating a user's blood pressure, the device being intended to be worn by the user, the device comprising: - a support (10), intended to be applied against the user's skin; - several light sources (15), arranged on the support, configured to emit light (16) towards the user's skin when activated; - several photodetectors (18), arranged on the support, distant from each light source, configured to detect a light (17) emanating from the user's skin following activation of at least one light source, each photodetector forming, with said light source, a source-photodetector pair; the device being characterized in that it comprises: - several acoustic transducers (11), comprising at least: • an acoustic transmitter, configured to emit an acoustic wave through the skin; • and an acoustic detector, configured to detect an acoustic wave reflected in the user's body, propagating through the skin; - an acoustic selection unit (21), programmed to: • take into account an acoustic selection criterion; • select an acoustic transmitter and an acoustic detector from among the acoustic transducers, the selection being made as a function of an acoustic signal detected by each acoustic detector following emission of an acoustic wave by at least one acoustic transmitter; - an optical selection unit (22), configured to: • take into account an optical selection criterion; • select a first light source-photodetector pair, comprising a first light source light and a first photodetector, chosen from among the light sources and the photodetectors, and a second light source - photodetector pair, comprising a second light source and a second photodetector, chosen from among the light sources and the photodetectors, the selection being made according to the signals detected by the first photodetector and the second photodetector following activation of the first light source and the second light source; - a central unit (35), programmed to estimate a blood pressure, from: • the signal detected by the selected acoustic detector; • the signals detected by the first photodetector and the second photodetector.

2. Device according to claim 1, in which the central unit is programmed to: • estimate an arterial diameter (D(t)) from the signal detected by the selected acoustic detector; • estimate a pulse wave velocity (PWV), from the signals detected by the first photodetector and the second photodetector; • estimate the arterial pressure (P(t)), as a function of the estimated arterial diameter and pulse wave velocity.

3. A device according to any one of claims 1 or 2, wherein each light source emits light in a spectral band between 500 nm and 1200 nm.

4. Device according to any one of the preceding claims, comprising a first group (GJ of light sources and photode- detectors; a second group (G2) of light sources and photodetectors, distant from the first group of light sources and photodetectors; the optical selection unit is configured to • select the first light source and the first photodetector from among the light sources and photodetectors of the first group; • select the second light source and the second photodetector from the light sources and photodetectors of the second group.

5. Device according to any one of the preceding claims, in which the acoustic selection criterion being a signal-to-noise ratio, the acoustic selection unit is configured to: - estimate a signal-to-noise ratio of each signal detected by an acoustic detector; - select the acoustic detector for which the signal-to-noise ratio is the highest.

6. Device according to any one of the preceding claims, in which the optical selection criterion being a correlation criterion, the optical selection unit is configured to: - estimate a temporal correlation between the signals detected at different times by photodetectors of each light source-photodetector pair; - select the first light source and the first photodetector as well as the second light source and the second photodetector according to the estimated temporal correlation.

7. Device according to any one of claims 1 to 6, wherein the optical selection criterion being an amplitude criterion, the optical selection unit is configured to: estimate an amplitude of a temporal evolution of signals detected at different times by photodetectors of each light source-photodetector pair; - select the first light source and the first photodetector as well as the second light source and the second photodetector according to the amplitude.

8. Device according to any one of the preceding claims, in which the optical selection criterion being a shape criterion, the optical selection unit is configured to: - take into account a predetermined temporal form; - determine a temporal evolution of the signals detected, at different times, by the photodetectors of each light source-photodetector pair; - selecting the first light source and the first photodetector as well as the second light source and the second photodetector according to a correlation between the temporal evolution of the detected signals and the predetermined temporal shape.

9. A method of estimating blood pressure using a device according to any one of the preceding claims, the method comprising: a. arrangement of the support (10) on the skin of a user, facing an artery (A); b. emission of at least one incident acoustic wave by an acoustic transmitter and acquisition of acoustic signals by an acoustic detector, each detected acoustic signal comprising echoes representative of reflections of the incident acoustic wave by the artery, step b) being carried out for different acoustic transmitters and / or different acoustic detectors, such that each detected acoustic signal is associated with an acoustic transmitter and an acoustic detector; c. using the acoustic selection unit: - taking into account an acoustic selection criterion; - selection of an acoustic transmitter and an acoustic detector, based on a comparison between each acoustic signal detected, during step b), and the acoustic selection criterion; d. for each light source, emission of incident light towards the user's skin and detection of backscattered radiation by at least one photodetector, each photodetector generating an optical signal representative of the intensity of the backscattered radiation; e. using the optical selection unit: - taking into account an optical selection criterion; - selection of two light source - photodetector pairs, each pair comprising a light source and a photodetector, based on a comparison between each optical signal from each photodetector and the optical selection criterion; f. emission of an incident acoustic wave by the acoustic transducer selected during c), and formation of an acoustic signal representative of echoes following reflections of the incident acoustic wave by the artery; g. activation of the light sources of each light source-photodetector pair selected during e) and formation, by each photodetector of each pair, of an optical signal representative of the intensity of the radiation backscattered by the artery; h. based on the acoustic signal and the optical signals formed by each photodetector at different times, estimation of the user's blood pressure.

10. The method of claim 9, wherein step h) comprises: - hl) depending on the formed acoustic signal, estimation of the diameter of the artery; - h2) depending on the optical signals formed, in different instants, by each selected photodetector, estimation of a pulse wave velocity; - h3) from the diameter of the artery resulting from the sub-step hl) and the pulse wave velocity resulting from sub-step h2), estimation of a user's blood pressure.

11. Method according to claim 10, in which sub-step h2) comprises an estimation of a time shift (At) between the optical signals respectively formed by the first photodetector and the second photodetector.

12. Method according to any one of claims 9 to 11, in which steps a) to e) constitute a calibration phase of the device, steps f) to h) being repeated between two successive calibrations.

13. Method according to any one of claims 9 to 12, in which the device is a device according to claim 4, the method comprising: - a selection of a first light source / photodetector pair in the first group (GJ; - a selection of a second light source / photodetector pair in the second group (G2).

14. Method according to any one of claims 9 to 12, comprising - taking into account a validity range of the blood pressure; - when the blood pressure resulting from step h) is located outside the validity range, renewal of the calibration phase.

15. Method according to any one of claims 9 to 14, in which the acoustic selection criterion is a maximum signal-to-noise ratio, the selection of the transmitter and the acoustic detector being carried out as a function of the acoustic signal, associated with the acoustic transmitter / acoustic transmitter pair, whose signal-to-noise ratio is maximum.

16. Method according to any one of claims 9 to 15, in which the optical selection criterion comprises a temporal correlation criterion, to within a time shift, such that the selection of each source-detector pair comprises: - an estimation of a temporal correlation between the signals detected by the photodetectors of each light source-photodetector pair at different times; - a determination of the light source-photo- detector for which the resulting signal from the photodetector has the highest time correlation.

17. A method according to any one of claims 9 to 16, wherein: - the first light source - photodetector pair defines a first measurement point (Pi); - the second light source - photodetector pair defines a second measuring point (P2); - the first measuring point and the second measuring point are distant from each other.

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