CUFFLESS METHOD FOR DETERMINING A USER'S BLOOD PRESSURE

Description

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, usually an arm. Blood pressure is characterized by measuring the pressure exerted by the cuff at one or more characteristic instants. The pressure sensor or acoustic sensor is 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 to first achieve arterial occlusion. When the cuff is deflated, pressure oscillations occur. The oscillations increase until they temporarily reach a maximum amplitude. At this point, the pressure in the cuff is considered equal to the mean arterial pressure. From the maximum amplitude detected, the systolic and diastolic blood pressures are estimated based on empirical laws.

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

[0006] US2010081912 and US2018078155 describe devices for measuring blood pressure by combining optical and acoustic modalities.

[0007] Recently, developments have been carried out to perform 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 a 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.Pulse wave velocity is commonly referred to as PWV (Pulse Wave Velocity). The measurements from both modalities are combined to estimate blood pressure.

[0008] The principles outlined 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, measurements made in both modalities can be affected by uncertainties related to the positioning of the active components (light sources, acoustic or optical sensors), relative to the artery. Thus, when the device is applied to a user's body, the sensors must be correctly positioned precisely relative to the artery so that the variation in diameter and pulse wave velocity are correctly estimated.

[0009] Also, the device described above requires precise positioning on the user's skin. Another difficulty is related to the user's movements, which can cause the position of the sensors to vary relative to the artery. The invention described below overcomes these difficulties. STATEMENT OF THE INVENTION

[0010] 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: 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 they are 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; several acoustic transducers, eachcomprising 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 an emission of an acoustic wave by at least one acoustic transmitter; an optical selection unit, configured to: take into account an optical selection criterion;selecting 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; a central unit, programmed to: • calculate a time shift between two echoes in the signal detected by the selected acoustic detector; calculate a time shift between signals detected by the first photodetector and the second photodetector; estimate a blood pressure from the calculated time shifts. ;

[0011] The central unit can be programmed to: estimating an arterial diameter from the time lag between the echoes of the signal detected by the selected acoustic detector; estimating a pulse wave velocity, from the time lag between the signals detected by the first photodetector and the second photodetector; estimating the arterial pressure, based on the estimated arterial diameter and pulse wave.

[0012] Preferably, the light source emits light in a spectral band between 500 nm and 1200 nm.

[0013] The device may include: 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.

[0014] The optical selection unit is then configured to: selecting the first light source and the first photodetector from among the light sources and photodetectors of the first group; selecting the second light source and the second photodetector from among the light sources and photodetectors of the second group.

[0015] According to one possibility, the acoustic 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 with the highest signal-to-noise ratio.

[0016] The acoustic criterion can be an intensity of the signal detected by an acoustic detector. The acoustic selection unit is then configured to estimate the intensity of each signal detected by an acoustic detector; select the acoustic detector with the highest intensity.

[0017] According to one possibility, the optical criterion being a correlation criterion, the optical selection unit is configured to: estimating a temporal correlation between the signals detected at different times by 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 the estimated temporal correlation.

[0018] According to one possibility, the optical criterion is an amplitude criterion, the optical selection unit is configured to: estimating an amplitude of a temporal evolution of signals detected at several instants by 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 the amplitude.

[0019] According to one possibility, the optical criterion is a shape criterion. The optical selection unit is configured to: taking into account a predetermined temporal form; determining a temporal evolution of the detected signals, 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 form.

[0020] 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: a) arrangement of the support on the skin of a user, facing an artery; 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, as a function of a comparison between each detected acoustic signal, 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) as a function of a time shift between two echoes of the acoustic signal, and of a time shift between the optical signals formed by each photodetector at different times, estimation of the user's blood pressure.

[0021] According to one possibility, step h) comprises: h1) as a function of 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 h1) and the pulse wave velocity resulting from sub-step h2), estimation of a blood pressure of the user.

[0022] 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. Steps a) to e) may constitute a calibration phase of the device, steps f) to h) being repeated between two successive calibrations.

[0023] According to one possibility, the device includes: 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.

[0024] The process can then include: 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.

[0025] According to one embodiment, the method comprises: taking into account a range of validity of the blood pressure; when the blood pressure resulting from step h) is outside the range of validity, renewal of the calibration phase.

[0026] 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 according to the acoustic signal, associated with the acoustic transmitter / acoustic detector pair, whose signal-to-noise ratio is maximum or whose intensity is maximum.

[0027] The optical selection criterion may include a time correlation criterion, up to a time shift, so that the selection of each source-detector pair includes: an estimation of a temporal correlation between the signals detected at different times by the photodetectors of each light source-photodetector pair; a determination of the light source-photodetector pairs for which the signal resulting from the photodetector has the highest temporal correlation.

[0028] The optical selection criterion may be one of the selection criteria described in connection with the first subject of the invention.

[0029] The process can be as follows: 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.

[0030] The invention will be better understood by reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0031] There figure 1 is an example of a device according to the invention. The Figures 2A to 2E illustrate an implementation of the acoustic modality of the device. The Figures 3A and 3B illustrate an implementation of the optical modality of the device. The figure 4is another example of a device according to the invention. The Figure 5 schematizes the main steps of implementing a method for determining a user's blood pressure using a device as described in connection with the figure 1 or the figure 4 . PRESENTATION OF SPECIAL EMBODIMENTS

[0032] There figure 1 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 against the skin, such as an elastic armband or bracelet.

[0033] The support includes components allowing the implementation of acoustic or optical modalities as described in [1].

[0034] 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 not necessarily, each acoustic transducer can function as both an acoustic transmitter or an acoustic detector.

[0035] 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 the Figures 2A to 2E .

[0036] 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 the . Figures 3A and 3B .

[0037] 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).

[0038] 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 P 1 , P 2 separated by a separation distance Δ, 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: a time shift Δt between the signals respectively detected by each photodetector 18; the separation distance d(P 1 , P 2 ) between the two measurement points P 1 and P 2 .

[0039] The support 10 is placed 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: a relevant acoustic transmitter and detector, i.e. correctly positioned relative to the artery A, so as to obtain a correct estimate of the temporal variation in 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 the blood volume at the two measurement points P 1 and P 2 . Comparison of the signals from each photodetector allows an estimate of the speed of the pulse wave.

[0040] The device comprises an acoustic selection unit 21, connected to each acoustic detector. 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. On the figure 1, an acoustic transducer is shown diagrammatically, functioning 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 the artery A. The signal resulting from the selected transducer is then transmitted to the acoustic processing unit 31 so as to determine the temporal evolution D(t) of the diameter of the artery.

[0041] 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 measuring points respectively defined by each light source pair. photodetector. On the figure 1 , we have schematized two light source-photodetector pairs selected by two dotted contours. We consider that each measurement point P 1 and P 2 is located opposite the middle of a straight line segment connecting the light source to the photodetector of the selected light source-photodetector pair. On the figure 1 , we have represented: a double arrow materializing a first distance, called backscattering, d 1 between the source and the photodetector of the first selected light source-photodetector pair; a double arrow materializing a second backscattering distance d 2 between the source and the photodetector of the second selected light source-photodetector pair.

[0042] Preferably, each selected light source-photodetector pair extends on either side of the artery, or along the artery.

[0043] The measuring points P 1 and P 2 respectively formed by the two selected light source - photodetector pairs are spaced apart from each other. The distance d(P 1 , P 2 ), along the artery, between the two measuring points, is preferably greater than 1 cm or 5 cm.

[0044] 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.

[0045] Preferably, the light sources 15 and the photodetectors 18 are distributed on the support 10 forming a first group G 1 and a second group G 2 . The optical selection unit 22 is programmed to select: a first light source-photodetector pair among the light sources 15 and photodetectors 18 of the first group G 1 ; a second light source-photodetector pair among the light sources 15 and photodetectors 18 of the second group G 2 .

[0046] This division into two groups makes it possible to guarantee a minimum separation distance d(P 1 , P 2 ) min between the first and second measuring points P 1 , P 2 respectively defined by each selected light source-photodetector pair. The distance d(P 1 ,P 2 ) between the measuring points P 1 and P 2 is assumed to be known by knowledge of the geometry of the device. It is represented on the figure 1 by a double dotted arrow.

[0047] THE Figures 2A, 2B and 2Cillustrate the operation of the acoustic modality. Each transducer emits an incident acoustic wave, symbolized by the arrow F 1 . 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 A p and a distal portion A d , diametrically opposed, of the wall of the artery A. On the Figure 2A , the arrows F 2 and F 3 correspond respectively to the parts of the incident wave respectively reflected by the proximal portions A p and distal A d . The transducer (or another transducer) detects the waves successively reflected by the proximal portions A p and distal A d , the latter forming respectively a proximal echo E p and a distal echo E d . The Figure 2B schematizes the echoes. On the Figure 2B, the ordinate axis corresponding to the amplitude of the detected wave and the abscissa axis corresponding to time. The time shift dt between the two echoes E p and E d makes it possible to estimate the diameter of the artery, knowing the propagation speed of the acoustic wave. The measurements represented on the Figure 2B schematize an acoustic signal addressed by each transducer 11 to the selection unit 21.

[0048] The measures shown on the Figure 2B are repeated at different measurement times, according to a sampling frequency which can be, for example, 1 kHz. The Figure 2C represents a temporal evolution of the diameter D(t) of the artery (y-axis - units µm) 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 ΔD between systole and diastole is of the order of 0.6 mm.

[0049] 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). The 2D figureshows a simulation configuration. An acoustic probe comprising a line of 25 transducers 11, each operating in emitter / detector mode, arranged in contact with a cylindrical skin S, with a radius of approximately 13 mm, was simulated. 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 (provided by Extende), the intensity of the echoes produced by each transducer in response to an incident acoustic pulse was evaluated. The densities and acoustic propagation velocities in the modeled media S, M and A are representative of those of blood, muscles and skin respectively. Figure 2Erepresents 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 with the maximum signal among the other transducers. This results in a precise estimation of the temporal evolution D(t) of the diameter D of the artery.

[0050] Alternatively, several transducers, out of phase with each other, are used simultaneously. The selection unit allows a reference transducer to be selected, and then a time phase shift between the different transducers relative to the reference transducer to be estimated.

[0051] THE Figures 3A and 3Billustrate 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.

[0052] 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 may be a laser diode, a VECSEL, or the end of a light guide, for example an optical fiber.

[0053] 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 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. On the Figure 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.

[0054] 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 probed tissues, 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.

[0055] 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.

[0056] As shown in the figure 1, the device makes it possible to select two light source-photodetector pairs, respectively defining two measurement points P 1 and P 2 , along the artery, spaced by a separation distance d(P 1 , P 2 ). 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 Δt. Knowing the separation distance d(P 1 , P 2 ), 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: VOP = d P 1 P 2 Δt

[0057] There Figure 3B represents: a first optical signal SO 1 resulting from the photodetector of the first light source-photodetector pair; a second optical signal SO 2 resulting from the photodetector of the second light source-photodetector pair.

[0058] On the Figure 3B, each signal corresponds to the opposite of the detected signal: thus, each signal SO 1 , SO 2 is representative of the periodic absorption of the tissues at the measurement points P 1 and P 2 . The time shift can be estimated from the time difference Δt between remarkable points of each curve, for example maxima or minima.

[0059] Preferably the separation distance d(P 1 , P 2 ) is greater than 5 mm. It is preferably less than 10 cm or 5 cm.

[0060] 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.

[0061] 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 user's movements. Thus, it is difficult to determine a priori the most relevant source-photodetector pairs to accurately estimate the pulse wave velocity. It is understood, from the Figure 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 blood volume disturbance induced by cardiac activity. Il It is also preferable that the source and the photodetector of the same pair are arranged either along the artery or on either side of the artery, so that the measuring point, located between the source and the photodetector, is located close to the artery.

[0062] The optical selection unit 22 takes into account different detected signals as shown in the Figure 3B . These are optical signals representative of the intensity detected by the photodetectors of different light source-photodetector pairs. The detected signals include a so-called pulsatile component, which can be considered periodic, due to the repetition of 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.

[0063] The optical selection criterion can be a temporal correlation. Indeed, the most relevant detected signals have a comparable temporal evolution, which corresponds to the cardiac activity. Selecting detected signals with a high temporal correlation facilitates the determination of the time shift Δt illustrated in the Figure 3B . By temporal correlation we mean a correlation within a time shift which corresponds to the distance between the measurement points along the artery.

[0064] Other optical selection criteria can be applied in addition to or instead 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 above a certain threshold, or located within a previously defined acceptance range.

[0065] The optical selection criterion can 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 which present the best correlation with a predetermined temporal shape.

[0066] 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.

[0067] The optical selection criteria previously described can be combined.

[0068] 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.

[0069] 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 P = f D VOP where f is a previously defined function.

[0070] For example, the function f can be such that: P t − P 0 = 2 ρ VOP 2 ln D t D 0 Or : P ( t ) is the blood pressure at a given time t D 0 is a reference diameter, for example the diameter at the end of diastole P 0 is a reference pressure corresponding to D = D 0 .

[0071] 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 mean blood pressure is the average of the pressure over a period.

[0072] The function f 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.

[0073] There figure 4 represents another example of a device suitable for implementing the invention. It has been indicated, on the figure 4 , the main dimensions. The device allows the analysis of an artery A extending between the two extreme positions represented A sup and A inf represented on the figure 4 . The two groups G 1 and G 2 are represented by a brace, in which the two light source-photodetector pairs are to be identified according to the position of the artery relative to the support 10.

[0074] 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 G 1 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.

[0075] 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.

[0076] There Figure 5 schematizes the main stages of a process implementing a device as represented in the figures 1 Or 4 .

[0077] Step 100: Place the support on a user's skin, facing an artery.

[0078] 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.

[0079] 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 an acoustic wave by the acoustic transmitter, best satisfies the previously mentioned acoustic selection criterion.

[0080] Step 130: Acquisition of optical signals by the different photodetectors of different light source-photodetector pairs. In this step, a light source can be sequentially activated and optical signals are acquired from different photodetectors considered to be sufficiently close to the source to detect usable backscattered radiation. The light sources are activated successively. Optical signals are then obtained for each light source-photodetector pair.

[0081] 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.

[0082] 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.

[0083] Steps 150 to 170 correspond to the implementation of the device, following the calibration phase.

[0084] 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.

[0085] 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.

[0086] Steps 150 and 160 can be performed simultaneously or in any order.

[0087] Step 170: Estimation of blood pressure as a function of artery diameter D(t) and PWV resulting from steps 150 and 160, for example using expression (3) or another calibration function

[0088] 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 performed in the event of the occurrence of a blood pressure measurement considered to be abnormal.

[0089] The invention takes advantage of the fact that the device allows the estimation of blood pressure without resorting to compression of a user's limb. It is robust to uncertainties in the positioning of the device support on the user's skin, as well as to possible movements of the user wearing the support.

Claims

1. A device for estimation of a blood pressure of a user, the device being intended to be worn by the user, the device including: - a support (10) intended to be applied against the skin of the user; - a plurality of light sources (15) disposed on the support and configured to emit light (16) toward the skin of the user when they are activated; - a plurality of photodetectors (18) disposed on the support at a distance from each light source and configured to detect light (17) emanating from the skin of the user following activation of at least one light source, each photodetector forming with said light source a source - photodetector pair; - a plurality of acoustic transducers (11), each including at least: • an acoustic emitter configured to emit an acoustic wave through the skin; and • an acoustic detector configured to detect an acoustic wave reflected in the body of the user and propagating through the skin; - an acoustic selection unit (21) programmed: • to take into account an acoustic selection criterion; • in accordance with the acoustic selection criterion, to select an acoustic emitter and an acoustic detector from among the acoustic transducers, the selection being effected as a function of an acoustic signal detected by each acoustic detector following emission of an acoustic wave by at least one acoustic emitter; - an optical selection unit (22) configured: • to take into account an optical selection criterion; • in accordance with the optical selection criterion, to select a first light source - photodetector pair including a first light source and a first photodetector chosen from among the light sources and the photodetectors and a second light source - photodetector pair including a second light source and a second photodetector chosen from among the light sources and the photodetectors, the selection being effected as a function of the signals detected by the first photodetector and the second photodetector following activation of the first light source and of the second light source; - a central unit (35) programmed to : • calculate a temporal offset between two echoes in the signal detected by the selected acoustic detector; • calculate a temporal offset between the signals detected by the first photodetector and the second photodetector • estimate a blood pressure from the calculated temporal offsets.

2. The device according to claim 1, wherein the central unit is programmed: • to estimate an arterial diameter (D(t)) from the temporal offset between the echoes in the signal detected by the selected acoustic detector; • to estimate a pulse wave velocity (VOP) from the temporal offset between the signals detected by the first photodetector and the second photodetector; • to estimate the blood pressure (P(t)) as a function of the estimated arterial diameter and the estimated pulse wave velocity.

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

4. The device according to any one of the preceding claims, including - a first group (G1) of light sources and of photodetectors; - a second group (G2) of light sources and of photodetectors at a distance from the first group of light sources and of photodetectors; - the optical selection unit is then configured • to select the first light source and the first photodetector from among the light sources and the photodetectors of the first group; • to select the second light source and the second photodetector from among the light sources and the photodetectors of the second group.

5. The device according to any one of the preceding claims, wherein 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; - to select the acoustic detector for which the signal-to-noise ratio is the highest.

6. The device according to any one of the preceding claims, wherein 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; - to select the first light source and the first photodetector as well as the second light source and the second photodetector as a function of the estimated temporal correlation.

7. The device according to any one of the preceding claims, 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 various times by photodetectors of each light source - photodetector pair; - to select the first light source and the first photodetector as well as the second light source and the second photodetector as a function of the amplitude.

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

9. A method of estimation of a blood pressure using a device according to any one of the preceding claims, the method including: a) disposing the support (10) on the skin of a user, facing an artery (A); b) emitting at least one incident acoustic wave by means of an acoustic emitter and acquiring acoustic signals by means of an acoustic detector, each acoustic signal detected including echoes representative of reflections of the incident acoustic wave by the artery, the step b) being carried out for different acoustic emitters and / or different acoustic detectors so that each acoustic signal detected is associated with an acoustic emitter and an acoustic detector; c) using the acoustic selection unit: - taking into account an acoustic selection criterion; - selecting an acoustic emitter and an acoustic detector as a function of a confrontation between each acoustic signal detected during the step b) and the acoustic selection criterion; d) for each light source, emitting incident light toward the skin of a user and detecting back-scattered radiation by means of at least one photodetector, each photodetector generating an optical signal representative of the intensity of the back-scattered radiation; e) using the optical selection unit: - taking into account an optical selection criterion; - selecting two light source - photodetector pairs, each pair including a light source and a photodetector, as a function of a confrontation between each optical signal from each photodetector and the optical selection criterion; f) emitting an incident acoustic wave from the acoustic transducer selected in c) and forming an acoustic signal representative of echoes following reflection of the incident acoustic wave by the artery; g) activating light sources of each light source - photodetector pair selected in e) and each photodetector of each pair forming an optical signal representative of the intensity of the radiation back-scattered by the artery; h) estimating the blood pressure of the user as a function of the a temporal offset between two echoes in the acoustic signal and from a temporal offset between the optical signals formed by each photodetector.

10. The method according to claim 9, in which the step h) includes: - h1) estimating the diameter of the artery as a function of the formed acoustic signal; - h2) estimating a pulse wave velocity as a function of the optical signals formed at different times by each selective photodetector; - h3) estimating a blood pressure of the user from the resulting diameter of the artery from the sub-step h1) and from the resulting pulse wave velocity from the substep h2).

11. The method according to claim 10, wherein the substep h2) includes estimating a temporal offset (Δt) between the optical signals respectively formed by the first photodetector and the second photodetector.

12. The method according to any one of claims 9 to 11, wherein the steps a) to e) constitute a phase of calibration of the device, the steps f) to h) being reiterated between two successive calibrations.

13. The method according to any one of claims 9 to 12, wherein the device is a device according to claim 4, the method including: - selecting a first light source - photodetector pair in the first group (G1); - selecting a second light source - photodetector pair in the second group (G2).

14. The method according to any one of claims 9 to 12, including: - taking into account a range of validity of the blood pressure; - if the resulting blood pressure from the step h) is situated outside the range of validity, repeating the calibration phase.

15. The method according to any one of claims 9 to 14, wherein the acoustic selection criterion is a maximum signal-to-noise ratio, the selection of the acoustic emitter and of the acoustic detector being effected as a function of the acoustic signal associated with the acoustic emitter - acoustic detector pair the signal-to-noise ratio of which is the maximum.

16. The method according to any one of claims 9 to 15, wherein the optical selection criterion includes a temporal correlation criterion ignoring a temporal offset so that the selection of each source - detector pair includes: - estimating a temporal correlation between the signals detected at different times by the photodetectors of each light source - photodetector pair; - determining the light source - photodetector pairs for which the resulting signal from the photodetector has the highest temporal correlation.

17. The method according to any one of claims 9 to 16, wherein: - the first light source - photodetector pair defines a first measurement point (P1); - the second light source - photodetector pair defines a second measurement point (P2); - the first measurement point and the second measurement point are at a distance from one another.