FR3118409A1 - Device and method for measuring blood pressure and a state of stress - Google Patents

Abstract

Device for detecting blood pressure and intended to be worn on the skin of a user, the device comprising: a first module (10), configured to estimate a pulse wave speed and/or blood pressure of the user, comprising: at least one detector (112, 122), configured to measure a quantity depending on the blood pressure or pulse wave velocity;a first processing unit (18), programmed to estimate blood pressure from the signal emitted by the detector; the device being characterized in that it also comprises: a second module (20), intended to measure a physiological characteristic of the user, and to estimate a stress level of the user according to the measured physiological characteristic; a central unit (50) being programmed to generate an alert signal, depending on blood pressure and stress level.

Description

Device and method for measuring blood pressure and a state of stress

The technical field of the invention is pulse wave velocity (PWV) and/or blood pressure measurement correlated to an estimate of a user's state of stress.

PRIOR ART

High blood pressure disease is responsible for nine million deaths worldwide. Patient monitoring usually consists of spot measurements. Automated blood pressure monitors using the principle of oscillometric measurement are widely used by the medical profession and are also available to the general public. In this type of device, a cuff positioned on the arm, forearm or wrist is inflated until it blocks blood circulation, then deflated. During deflation or inflation, oscillations in the cuff pressure are measured, these are maximum when the cuff pressure cancels the mean arterial pressure. From the mean arterial pressure, the systolic pressure and the diastolic pressure are estimated according to rules of thumb. The main constraints related to this type of device are the pressure exerted by the device which can be painful when the measurement is repeated regularly and the noise of the pump which can be annoying in the long run. These drawbacks make the use of an oscillometric cuff difficult to reconcile with continuous measurement.

Many alternatives exist, called "cuffless" (without inflatable cuff), based on pulse wave transit time measurements, for example by optical methods, of the PPG (PhotoPlethysmoGraphy) type, or by acoustic methods, or electric. This opens the way to blood pressure measurements performed continuously, by less constraining devices worn on a user. This type of device can be integrated into a patch, a headband or a watch.

However, the measurement of a blood pressure can be influenced by the state of a person, in particular a state of stress. Indeed, when a person is in a state of stress, blood pressure increases. A known effect is emotional high blood pressure, of the "white coat effect" type, which can cause a sudden increase in blood pressure under the effect of an emotion, for example when measuring blood pressure at a doctor's office. . It is a transient hypertension, which disappears when the effects of the emotion have disappeared.

The inventors propose a device and a method, allowing consideration of the stress factor in the event of measurements indicating high arterial pressure.

A first object of the invention is a device for detecting arterial pressure, the device being intended to be worn on the skin of a user, the device comprising:

• a first module, configured to estimate a pulse wave velocity and/or blood pressure of the user, the first module comprising:
  • at least one detector, configured to measure a magnitude representative of the pulse wave velocity and/or the blood pressure of the user;
  • a first processing unit, programmed to estimate a pulse wave velocity or an arterial pressure from the quantity measured by the detector;

the device being characterized in that it also comprises:

• a second module, configured to measure a physiological characteristic of the user, the value of the physiological characteristic being able to vary according to a stress level of the user, the second module comprising a second processing unit, programmed to estimating a stress level of the user as a function of the measured physiological characteristic;
• a central unit, connected to the first module and to the second module, the central unit being programmed to generate an alert signal, for the attention of the user, when:
  • the blood pressure exceeds a predetermined blood pressure threshold, or the pulse wave velocity exceeds a predetermined pulse wave velocity;
  • and when the stress level exceeds a predetermined stress threshold.

Advantageously, the device may include a control module, the control module being configured to be activated, by the central unit following the generation of the alert signal, the control module being such that:

• the control module is intended to follow a relaxation activity, carried out by the user, following the emission of the alert signal;
• the control module comprises a sensor, intended to measure a physiological characteristic of the user, the value of the physiological characteristic being liable to vary according to the level of stress of the user;
• the control module is configured to send the user an accompanying signal, representative of a change in the examiner's state of stress during the relaxation activity.

According to one possibility, the control module is configured to measure heart rate variability. The control module can include:

• a light source, configured to emit light towards the body of the user;
• a photodetector, configured to detect light backscattered or transmitted by a part of the user's body, following illumination by the light source;
• a processing unit, configured to process a signal generated by the photodetector, representative of the light detected by the latter, so as to extract a quantitative value relating to a variability of the heart rate.

The control module can be configured to activate the second module, the accompanying signal then being determined according to the physiological characteristic measured by the second module.

According to one embodiment,

• the first module comprises at least one transmitter, configured to emit an optical, or electrical, or acoustic signal so that the signal propagates through a part of the user's body;
• the detector of the first module being configured to detect the signal emitted by the transmitter, and having propagated through a part of the body of the user.

According to one embodiment, the device comprises an actimetry module, the actimetry module comprising a motion sensor configured to detect a movement of the user, the actimetry module being configured to detect a physical activity exerted by the user, the central unit being connected to the actimetry module and configured to invalidate the blood pressure measurement when a physical activity considered significant has been detected.

According to one embodiment,

• the first module comprises a first sensor and a second sensor, the first sensor and the second sensor being remote from each other, each sensor being configured to generate a signal representative of a variation in blood volume in a part of the body of the user in front of which he is disposed;
• the first processing unit is configured to estimate a pulse wave velocity, between the first sensor and the second sensor, from the signals respectively generated by each sensor.

According to one embodiment,

• the first sensor comprises a first light source and a first photodetector, the first photodetector being arranged to detect light backscattered or transmitted by the part of the user's body facing which it is positioned, following illumination by the first source from light ;
• a second sensor comprises a second light source and a first photodetector, the second photodetector being arranged to detect light backscattered or transmitted by the part of the user's body facing which it is positioned, following illumination by the second source from light.

According to one embodiment, the second module comprises:

• two electrodes, spaced from each other, intended to be applied against the skin of the user, as well as an impedance measurement circuit arranged to estimate an impedance of the skin between the electrodes;
• a second processing unit, configured to estimate a state of stress as a function of the impedance measured by the impedance measurement circuit.

A second object of the invention is a method for measuring a blood pressure of a user, the user wearing a device according to the first object of the invention, the method comprising

• a) estimation of the user's blood pressure at a measurement instant, using the first module;
• b) determination of a state of the user, the state being chosen from a state of rest and a state of stress, using the second module;
• c) when the user is in a state of stress, generating an alert signal.

According to one possibility,

• the alert signal includes an invitation addressed to the user to perform a relaxation activity;
• the device comprises the control module described in connection with the first object of the invention;
• the method comprises, following the generation of the alert signal, an activation of the control module, so as to send the user an accompanying signal, representative of a change in the state of the user.

The method may comprise, following the generation of the alert signal:

• d) estimation, by the control module, of a variability of the heart rate of the user during a determined period;
• e) when the heart rate variability is within an acceptance range, repeating steps a) to c).

The control module can be configured to repeat step b) and to emit the accompanying signal according to the state of stress determined during each repeated step b), until the state of the user is considered a state of rest.

The invention will be better understood on reading the description of the embodiments presented, in the remainder of the description, in connection with the figures listed below.

FIGURES

There shows an example of a device comprising a first module, placed at the arm of a user, and a second module, placed at the wrist of the user.

There schematizes the main components of the first module.

FIGS. 1C and 1D illustrate the operation of the first module respectively according to a backscatter configuration and according to a transmission configuration.

There shows the time shift of two signals resulting respectively from two sensors, distant from each other, of the first module.

There is an example of a first one-piece module.

There shows an example of measurement of the time shift of two signals resulting respectively from two sensors of the first module.

There schematizes a method for determining a characteristic instant of each signal resulting from the sensors of the first module.

There schematizes the main components of the second module.

There shows a change in a conductance of a user's skin as a function of time from measurements taken by the second module.

There schematizes the main steps of a method of using a device according to the invention.

There schematizes four modules of a device according to the invention, connected to a central unit.

There represents a control module.

There represents a signal resulting from the control module when the user is in a state of stress (top curve) or when he is in a state considered to be at rest (bottom curve).

DESCRIPTION OF PARTICULAR EMBODIMENTS

The device 1 according to the invention combines a first module 10, allowing a measurement of the arterial pressure as well as a second module 20, allowing a measurement of a physiological characteristic varying in the case of the occurrence of a state of stress.

There represents an example of a first module 10, dedicated to the measurement of arterial pressure. It is preferably a cuffless module (without inflation). On the , the first module 10 comprises a first sensor 11 and a second sensor 12 remote from each other. In this example, the first module 10 is placed against a person's skin, at arm level. Each sensor can be attached to the arm by a band 15. Other arrangements are possible, as described below. When the device is placed on the arm, it is preferably placed facing the brachial artery. The headband can be replaced by another type of tie, making it possible to hold the sensor on the skin, in contact with the tissues of the user. It can be a biocompatible adhesive, a bracelet.

There schematizes the first sensor 11 and the second sensor 12. Each sensor operates according to the same principle. The first sensor 11 comprises a first light source 111 as well as a first photodetector 112. The first light source is for example a light emitting diode (LED: Light Emitting Diode). The light source 111 emits an incident light beam 13 propagating towards the user's tissues, along a propagation axis Z, for example perpendicular to the surface 2s of the user's body. The light beam is preferably emitted in a narrow spectral band, preferably <50 nm, in a spectral range between 400 nm and 1100 nm.

As shown on the , the photons of the incident light beam 13 are emitted along a propagation axis Z. They penetrate the tissues 2 and part of them is backscattered, for example along a direction parallel to the propagation axis Z, in a direction opposed to the latter. The backscattered photons constitute a backscattered radiation 14. The backscattered radiation 14 can be detected by the photodetector 112, placed facing a surface 2s of the user's body. The photodetector 112 can be configured in such a way as to detect backscattered radiation emanating from the sample at a distance d, called the backscattering distance, generally non-zero and less than a few millimeters, or a few tens of millimeters. The photodetector 112 thus makes it possible to measure the intensity of the backscattered radiation. The photodetector 112 is for example a photodiode, or a set of photodiodes. On the the dotted curved arrow represents an optical path of photons emitted by the light source 111, then diffused in the body of the user.

According to an alternative, represented on the , a part of the user's body extends between the light source 111 and the photodetector 112. According to such a configuration, called in transmission, the photodetector 112 measures an intensity of the light having passed through a part of the user's body. user. Such a configuration assumes that the part of the user's body, between the light source and the photodetector, is thin enough for a sufficient quantity of photons to emanate from the medium and be detected by the photodetector.

Whatever the embodiment, on the first sensor or the second sensor, the photodetector is arranged to measure an intensity of light formed by photons having propagated through a part of the user's body: this is either backscattered photons, or photons having passed through biological tissues. In the remainder of the description, the backscattering configuration represented in FIGS. 1A to 1C is used.

The second sensor 12 comprises a second light source 12 ₁ as well as a second photodetector 12 ₂ . The second light source and the second photodetector are preferably identical to those described in connection with the first sensor 11. The first sensor 11 and the second sensor 12 are preferably identical to each other.

The first module 10 includes a processing unit 18 configured to control the first sensor 11 and the second sensor 12. The processing unit 18 is configured to collect the signals acquired by the first photodetector 11 ₂ and the second photodetector 12 ₂ . The processing unit 18 is linked to a memory, in which are stored instructions for implementing the method described below. The processing unit may include a microprocessor.

The first sensor 11 and the second sensor 12 are spaced apart by a distance D. When the first module 10 is placed at arm level, as shown in the , the distance D can be between 1 cm and 10 cm. The center of each sensor corresponds to a point equidistant from the light source and the photodetector of said sensor. The distance between two sensors is center to center.

The principle of operation of each sensor is based on the fact that at each heartbeat, the blood flow in front of each sensor leads to a modulation of the light propagating through the biological tissues forming the body 2 of the user. This results in a modulation of the light detected by the photodetector associated with the light source (photodetector 11 ₂ or photodetector 12 ₂ ). Thus, the intensity detected by each photodetector forms a periodic signal, the fundamental frequency of which corresponds to the heart rate. The spacing D between the two sensors 11, 12 allows a time shift Δt to be determined between the periodic signals resulting respectively from the first sensor and from the second sensor. Estimating the time lag, called the pulse wave transit time, allows an estimation of a velocity, known as the "pulse wave velocity" (PWV).

There represents a first signal S1 resulting from the first sensor 11 (solid line) and a second signal S2 resulting from the second sensor 12 (dotted line). The offset Δt makes it possible to determine the pulse wave velocity, the latter allowing an estimation of the arterial pressure. On the , a signal corresponding to the opposite of the detected signal, representative of tissue absorption, has been shown.

Depending on the distance between each sensor 11, 12, the frequency of acquisition of the signals resulting from each sensor can be between 100 Hz and 100 kHz, which allows an estimation of the pulse wave velocity with sufficient precision. .

According to a variant, the first module 10 is placed at the wrist of a person. In this case, the first sensor 11 and the second sensor 12 are spaced apart by a distance D of between 5 mm and 5 cm. Each sensor is preferably placed facing the radial artery.

According to another possibility, the first module 10 is arranged at the neck of a person, facing the carotid artery. In this case, the first sensor 11 and the second sensor 12 are spaced apart by a distance D of between 5 mm and 5 cm. Each sensor can be maintained by a patch.

According to another possibility, there are two sensors 11 and 12 respectively arranged at two different parts of the body: for example carotid and wrist or carotid and femoral artery.

Each sensor 11, 12 of the first module 10 can be configured to perform other optical measurements relating to other physiological characteristics. It may for example be a measurement of the heart rate, or of the arterial oxygen saturation (oximetry), or of a variability of the heart rate. Each sensor can include several light sources, of different wavelengths, to perform this type of measurement. Each sensor can comprise for example five light-emitting diodes, respectively emitting in spectral bands centered, typically, on the wavelengths 525 nm, 660 nm, 740 nm, 805 nm, 850 nm. The heart rate can for example be taken using the diode emitting at the 525 nm wavelength. The 660 nm, 805 nm and 850 nm wavelengths can be used for oximetry measurements. The 740 nm wavelength can be used to measure pulse wave velocity VOP. The 525 nm wavelength can be used to measure heart rate.

According to one possibility, each sensor may comprise several photodetectors, for example several photodiodes, respectively arranged at different backscatter distances d from a light source. The backscatter distances can be for example 6 mm, 12 mm, 18 mm. It also allows different depths of tissue to be addressed, with the depth examined increasing with backscatter distance.

There shows a first one-piece module, the first sensor 11 and the second sensor 12 being arranged on the same support. The support can for example be a support fixed to the body of the user by a headband. It can also be an adhesive support, of the patch type.

There represents two curves S1, S2, respectively shifted by a time shift Δt. The time lag can be estimated by detecting, on each curve, a characteristic instant. This is for example a time at which the measured signal reaches a minimum level. The curves of the correspond to an attenuation signal (axis of ordinates – arbitrary unit), as a function of time (axis of abscissas – unit: second). Each minimum corresponds to a minimum attenuation, that is to say an instant at which the volume of blood is minimum. At this instant, the backscattered signal detected by the sensor is maximum. Each of these instants can be determined as described in the publication Chiù Y, “Determination of pulse wave velocities with computerized algorithms”, Am. Heart J. 121(5), 1460-1470 (1991).

There shows an example of an S attenuation curve as described in connection with the . The identification of the characteristic moment can be carried out by determining an intersection between:

• a horizontal tangent T ₁ to the attenuation curve.
• a straight line T ₂ considered as representative of the increasing part of the attenuation curve, the increasing part being designated “systolic rise”. The systolic rise corresponds to a rapid dilation of the artery, causing a rapid increase in blood volume, which causes an increase in attenuation.

From the measured PWV, it is possible to estimate blood pressure, as described in the following publications:

• Mukkamala R. et al “Toward Ubiquitous Blood Pressure Monitoring via Pulse Transit Time: Theory and Practice”, IEEE Transactions on Biomedical Engineering, Vol. 62, No. 8, August 2015.
• Block R. et al “Conventional pulse transit times as markers of blood pressure changes in humans”, Scientific Reports (2020), 20-16373.

The first module 10 can allow measurement of the pulse wave velocity by a method different from the optical method, as described below. The first module 10 can also be configured to determine blood pressure (BP), without prior estimation of the pulse wave velocity. The blood pressure PA can for example be determined by optical means, as described in the publication Lubin M. et al “Blood pressure measurement by coupling an external pressure and photo-plethysmographic signals”, EMBC, July 2020.

In addition to the first module 10, the device comprises a second module 20, intended to evaluate a state of stress of the user wearing the device. The second module 20 is configured to measure a physiological signal, at different measurement instants, from which a value of a physiological characteristic of the user is estimated. The value of the physiological characteristic considered can vary according to a level of stress of the user. The physiological characteristic can be:

• a characteristic of the cardiac activity, measured for example by an electrocardiogram (ECG) or a simple determination of a heart rate or of an inter-beat interval and its variations obtained for example with one of the optical sensors of the first module of determination of VOP or PA;
• a characteristic of muscle activity, measured by an electromyogram (EMG);
• a characteristic of the user's electrodermal activity, representative of electrical conduction or electrical impedance properties of the skin (eg electrodermal conductance or impedance);
• a characteristic of skin temperature;
• a characteristic of a respiratory signal.

In the example considered, the characteristic is an electrical impedance of the skin. During the onset of a state of stress, the eccrine sweat glands produce an increased quantity of sweat, which decreases the impedance of the skin. As shown on the , The second module comprises two electrodes 21, 22. An impedance measurement circuit 23 makes it possible to apply a potential difference between the electrodes, and to measure the electric current flowing in the circuit 23. Alternatively, an electric current in the circuit 23 and the potential difference between the electrodes is measured. The applied potential difference is sinusoidal, with zero mean value. The induced current has an effective intensity of less than 10 µA. The frequency of the potential difference is for example 1 kHz, the amplitude being 1 V.

A processing unit 28 allows a calculation of a complex impedance of the skin. It can also determine the occurrence of a state of stress. The link between the value of the measured characteristic and the occurrence of a state of stress can be established by a membership function, as described in the publication: Charbonnier, Sylvie et al "A Multi-feature Fuzzy Index to Assess Stress Level from Bio-signals. Conference proceedings: Annual International Conference of the IEEE Engineering in Medicine and Biology Society". IEEE Engineering in Medicine and Biology Society. Conference. 2018. 1086-1089, or in patent application US20200015729A1. As described in the aforementioned references, the processing unit 28 can generate a membership function, quantifying the state of stress, ranging between 0 (state of rest) and 1. The membership function quantifies a probability that the user is in a state of stress. Beyond a certain threshold, for example 0.6, the user is considered to be in a state of stress.

The electrodes 21, 22, the impedance measurement circuit 23 and the processing unit 28 can be carried by the same support 25, for example an elastic bracelet.

There represents a real part of the impedance during the occurrence of a stress. This figure represents an increase in conductance (ordinate axis – nanoSiemens nS unit), as a function of time (abscissa axis – second unit), under the effect of stress.

Thus, the device 1 makes it possible to simultaneously perform a measurement of the arterial pressure or at least of the pulse wave velocity as well as an estimation of a state of stress.

There shows the main steps in the implementation of device 1. These steps are now described.

Step 100 : blood pressure measurement.

During this step, the blood pressure and/or the VOP of the user is measured, for example using a first module 10 as described in connection with FIGS. 1A to 1F and 2A and 2B.

This step is implemented at a measurement instant t. The measurement times can be spread throughout the day, on a regular basis. Several measurements can be implemented at close times, so as to be averaged: the arterial pressure then corresponds to an average or a combination of the arterial pressures measured.

Step 110 : estimation of a state of stress

Simultaneously or consecutively to step 100, an estimation of a stress state of the user is carried out, using a second module 20, for example as described in connection with FIGS. 3A and 3B.

Step 120 : estimation of a movement

This step is optional. It is a question of evaluating a physical activity carried out by the user during steps 100 and 110. The device 1 comprises an actimetry circuit 30, allowing detection of a movement. The actimetry circuit may include one or more motion sensors, of the accelerometer, gyrometer or magnetometer type. The actimetry circuit can detect that the user is carrying out a significant physical activity at the time of measurement, for example a sports activity, likely to impact the measurement of blood pressure or the estimation of the state of stress.

Step 130 : centralization

The measurements resulting from steps 100 (blood pressure), 110 (state of stress) and 120 (movement) are centralized in a central processing unit 50, connected to the various modules 10 (blood pressure), 20 (stress), 30 (actimetry ) of device 1.

When an arterial pressure (or a PWV) greater than a predetermined pressure threshold is detected, or a significant variation greater than a predetermined threshold, the central unit 50 analyzes the level of stress resulting from the module 20. The module 20 can carry out an estimation of the state of stress simultaneously with the measurement of the blood pressure, or being triggered when a blood pressure is higher than the pressure threshold. When the module 20 detects a probable state of stress, an alert signal is sent, by the central unit 50, to the user. The alert message corresponds to a concomitance between a high PA (or PWV) and a state of stress of the user. Step 140 is activated, so as to invite the user to relax. In the absence of detection of a state of stress, step 150 is activated.

When the actimetry circuit detects probable physical activity, step 160 is activated.

The central processing unit can perform an interface function with the user: addressing the alert signal and addressing the accompanying signal described later.

Step 1 4 0

This step is activated when a pressure above the threshold has been measured, and when a state of stress has been detected. It can for example be:

• an increase in blood pressure greater than 20 mmHg on average over the last 30 minutes;
• a value greater than 0.6 of the membership function resulting from modulus 20, and this for more than one hour, the membership function varying between 0 (state of rest) and 1 (state of stress).
• the absence of physical activity possibly detected by the actimetry module.

This step constitutes an important aspect of the invention: it is a question of accompanying the user in a process of stress management, on the basis of a control module 40. The control module 40 is intended to provide measures quantifying an evolution of the stress state of the user when the latter follows a relaxation activity. The relaxation activity is for example a cardiac coherence activity. The objective of the control module 40 is to interact with the user, so that the latter becomes aware of the evolution of his state of stress. The control module interacts with the user by establishing an accompanying signal, perceptible by the user: audible signal and/or visible signal. The accompanying signal is representative of the evolution of the stress state of the user during the relaxation activity.

The control module can use the stress state determination module 20. In this case, the module 20 is activated during the relaxation of the user, so that the latter can be informed of the evolution of his state. of stress.

The control module 40 can be separated from the module 20. An example of control module 40 has been represented on the . The control module 40 is in this example an optical module, similar to the sensors 11 and 12 described in connection with FIGS. 1A to 1C. The control module can use all or part of the components of sensors 11 and 12

In this example, the control module 40 comprises a light source 41, for example of the LED type, and a photodetector 42, for example of the photodiode type. It is carried by a support 44, so as to be applied against the body of the user. It comprises a processing unit 48 allowing processing of the light information detected by the photodetector 42, so as to estimate the heart rate of the user. The heart rate is estimated on the basis of variations in the light transmitted or backscattered by the tissues using the photodetector 42. The signal detected by the photodetector 42 comprises a continuous component, to which is added a pulsatile component, the latter varying with heart rate. Heart rate information is contained in the pulsatile component. It can be extracted by high pass filtering of the signal detected by the photodetector.

From the pulsatile component, amplitude thresholding makes it possible to identify characteristic instants, typically extremums (maxima or minima). The time interval separating the characteristic instants allows an estimation of a heart rate. An example of application is for example given in US9778111B2

There represents pulsatile components of detected optical signals, representative of heartbeats, corresponding respectively to a stress situation (top) and to a normal situation (bottom), ie at rest. The curve corresponding to the rest situation presents relatively regular undulations. The heart rate is modulated by respiratory sinus arrhythmia (RSA), causing a slight increase in heart rate during inspiration (reduction of the inter-beat interval) and a slight decrease in heart rate during expiration (lengthening of the inter-beat interval).

The occurrence of a stressful situation can lead to a decrease in heart rate variability, as shown in the (top curve), and disappearance of the respiratory sinus arrhythmia.

The control module 40 is programmed to quantify a score representative of the variability of the heart rate, for example the RMSSD (Root Mean Square of Successive Differences of the interbeat interval – quadratic mean of successive differences of the interbeat interval). In order for the user to regain an RMSSD score considered normal, the device can generate an accompanying signal, inviting the user to adopt regular breathing, typically between 5 and 7 breaths per minute. The accompanying signal, visual and/or auditory, can be modulated by the RMSSD score. During this relaxation phase, the control module quantifies a relaxation score representative of the variation in the heart rate. The relaxation score, quantified, is an indication transmitted to the user during the relaxation phase. When the relaxation score enters an acceptable value range, the user can again trigger step 100 for a new blood pressure measurement.

The device makes it possible to operate in a closed loop. This allows the user to take action during the relaxation phase by monitoring the relaxation score. It can then start a blood pressure measurement when the physiological conditions are more favorable.

Step 150 : Validation of the measurement

During step 150, the blood pressure measurement is validated, because the user is considered to be in a state of rest, i.e. not in a stressful situation.

Step 160 : Invalidation of the measurement

During this step, the pressure measurement is invalidated by the detection of significant physical activity by the actimetry circuit 30.

Although the application has been described using a module 10 for determining blood pressure or VOP based on an optical method, of the PPG type, the invention can be applied to other measurement methods sensitive to a periodic variation. blood volume in the tissues under the effect of cardiac activity. These include methods for establishing a response of biological tissues to an excitation signal. When applying a PPG-type method, an optical response of the tissues to an optical excitation is measured. The optical response, corresponding to the backscattered beam (or to the transmitted beam in the case of a transmission configuration) is modulated by the quantity of blood present in the illuminated tissues.

Alternatively, the determination of the VOP can be carried out by a passive measurement, for example by using a mechanical sensor of the tonometer type: with this type of sensor, a deformation of the tissue is measured under the effect of the pulse wave , without excitation signal.

Other measurement methods can be based on an electrical excitation of the tissues, and on a measurement of a response, also electrical, of the tissues, according to the principle of electrical impedance measurements. Under the effect of a periodic variation in blood volume, the electrical impedance varies periodically. Thus, a measurement of electrical impedance allows monitoring of cardiac activity. The module 10 can comprise two sensors 11, 12, remote from each other. Each sensor makes it possible to obtain a periodic variation of the electrical impedance. The offset between the periodic variations respectively determined by each sensor makes it possible to estimate the pulse wave velocity. This type of modality is described in the publication Bassem I. et al “Multi-source multi-frequency bio-impedance measurement method for localized pulse wave monitoring”, 2020.

The measurement of a pulse wave velocity can also be obtained by an acoustic method, by determining a periodic variation in the acoustic impedance of tissues induced by cardiac activity. The principle is then based on the emission of an acoustic wave propagating towards the tissues, and on the detection of an acoustic wave reflected by the tissues. The module 10 can comprise two sensors 11, 12, remote from each other. Each sensor makes it possible to obtain a periodic variation of the acoustic impedance. The offset between the periodic variations respectively determined by each sensor makes it possible to estimate the pulse wave velocity. This type of modality is described in the publication Hermeling E et al. “The dicrotic notch as alternative time-reference point to measure local pulse wave velocity in the carotid artery by means of ultrasonography”, Journal of hypertension, 2009, 2028-2035.

Claims (14)

Device for detecting arterial pressure, the device being intended to be worn on the skin of a user, the device comprising:

• a first module (10), configured to estimate a pulse wave velocity and/or blood pressure of the user, the first module comprising:
  • at least one detector (11 ₂ , 12 ₂ ), configured to measure a quantity representative of the pulse wave velocity and/or of the blood pressure of the user;
  • a first processing unit (18), programmed to estimate a pulse wave velocity or an arterial pressure from the quantity measured by the detector;

the device being characterized in that it also comprises:

• a second module (20), configured to measure a physiological characteristic of the user, the value of the physiological characteristic being able to vary according to a stress level of the user, the second module comprising a second processing unit (28), programmed to estimate a stress level of the user as a function of the measured physiological characteristic;
• a central unit (50), connected to the first module and to the second module, the central unit being programmed to generate an alert signal, for the attention of the user, when:
  • the blood pressure exceeds a predetermined blood pressure threshold, or the pulse wave velocity exceeds a predetermined pulse wave velocity;
  • and when the stress level exceeds a predetermined stress threshold.

Device according to claim 1, comprising a control module (40), the control module being configured to be activated, by the central unit (50) following the generation of the alert signal, the control module being such that :

• the control module is intended to follow a relaxation activity, carried out by the user, following the transmission of the alert signal;
• the control module comprises a sensor, intended to measure a physiological characteristic of the user, the value of the physiological characteristic being capable of varying according to the level of stress of the user;
• the control module is configured to send the user an accompanying signal, representative of a change in the state of stress of the examiner during the relaxation activity.

A device according to claim 2, wherein the monitoring module is configured to measure heart rate variability. Device according to Claim 3, in which the control module comprises

• a light source (41), configured to emit light toward the user's body;
• a photodetector (42), configured to detect light backscattered or transmitted from a part of the user's body, following illumination by the light source (41);
• a processing unit (48), configured to process a signal generated by the photodetector (42), representative of the light detected by the latter, so as to extract a quantitative value relating to a variability of the heart rate.

Device according to claim 2, in which the control module is configured to activate the second module (20), the accompanying signal being determined according to the physiological characteristic measured by the second module (20). Device according to any one of the preceding claims, in which:

• the first module comprises at least one transmitter (11 ₁ , 12 ₁ ), configured to emit an optical, or electrical, or acoustic signal so that the signal propagates through a part of the user's body;
• the detector of the first module being configured to detect the signal emitted by the transmitter, and having propagated through a part of the body of the user.

Device according to any one of the preceding claims, comprising an actimetry module (30), the actimetry module comprising a movement sensor configured to detect a movement of the user, the actimetry module being configured to detect a physical activity exercised by the user, the central unit (50) being connected to the actimetry module and configured to invalidate the blood pressure measurement when a physical activity considered significant has been detected. Device according to any one of the preceding claims, in which:

• the first module (10) comprises a first sensor (11) and a second sensor (12), the first sensor and the second sensor being remote from each other, each sensor being configured to generate a signal representative of a variation in blood volume in a part of the user's body facing which he is positioned;
• the first processing unit (18) is configured to estimate a pulse wave velocity, between the first sensor and the second sensor, from the signals respectively generated by each sensor.

Device according to claim 8, in which:

• the first sensor (11) comprises a first light source (11 ₁ ) and a first photodetector (11 ₂ ), the first photodetector being arranged to detect light backscattered or transmitted by the part of the user's body facing which it is is disposed, following illumination by the first light source;
• a second sensor (12) comprises a second light source (12 ₁ ) and a first photodetector (12 ₂ ), the second photodetector being arranged to detect light backscattered or transmitted by the part of the user's body facing which he is is arranged, following illumination by the second light source.

Device according to any one of the preceding claims, in which the second module comprises:

• two electrodes (21, 22), spaced from each other, intended to be applied against the skin of the user, as well as an impedance measurement circuit (23) arranged to estimate an impedance of the skin between the electrodes;
• a second processing unit (28), configured to estimate a state of stress based on the impedance measured by the impedance measurement circuit (23).

A method of measuring a blood pressure of a user, the user wearing a device according to any preceding claim, the method comprising

• a) estimating the blood pressure of the user at a measurement instant, using the first module (10);
• b) determining a state of the user, the state being chosen from a state of rest and a state of stress, using the second module (20);
• c) when the user is in a state of stress, generation of an alert signal by the central unit (50).

A method according to claim 11, wherein

• the alert signal comprises an invitation addressed to the user, to carry out a relaxation activity;
• the device is a device according to any one of claims 2 to 5;
• the method comprises, following the generation of the alert signal, an activation of the control module, so as to send the user an accompanying signal, representative of a change in the state of the user.

Method according to claim 12, comprising, following the generation of the alert signal,

• d) estimation, by the control module, of a variability of the heart rate of the user during a determined period;
• e) when the heart rate variability is within an acceptance range, repeating steps a) to c).

Method according to claim 12, in which the control module is configured to reiterate step b) and to emit the accompanying signal as a function of the state of stress determined during each repeated step b), until that the state of the user is considered as a state of rest.