WO2023156012A1

WO2023156012A1 - Wearable electronic apparatus comprising sensors

Info

Publication number: WO2023156012A1
Application number: PCT/EP2022/054219
Authority: WO
Inventors: Heikki Vilho NIEMINEN
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2022-02-21
Filing date: 2022-02-21
Publication date: 2023-08-24
Anticipated expiration: 2024-08-21

Abstract

A wearable electronic apparatus (1) comprising a sensor group (2) comprising a plurality of temperature sensors (3), each temperature sensor (3) being configured to detect a skin temperature of a user within a sensor area of said temperature sensor (A1, A2). A processing unit (4) is configured to calculate a rate of change of a skin temperature based on said detected skin temperatures within said sensor area (A1, A2), a temperature gradient based on detected skin temperatures within at least two sensor areas (A1, A2), and/or a rate of change of said temperature gradient based on said detected skin temperatures within at least two sensor areas (A1, A2). These calculations may be used to detect coronary artery disease or acute respiratory distress syndromes, or to normalize a measured blood pressure when combined with a blood pressure detector in order to compensate for environmental conditions or different activity levels.

Description

WEARABLE ELECTRONIC APPARATUS COMPRISING SENSORS

TECHNICAL FIELD

The disclosure relates to a wearable electronic apparatus comprising sensors.

BACKGROUND

Peripheral vasoconstriction is an important autonomic response in the human body to cold exposure. It restricts heat transfer from the body core to the environment through the skin.

Microvascular reactivity describes the range of vasoconstriction/dilation of peripheral arteries, as well as the rate of change of the vasoconstriction, and has been shown to be a biomarker for coronary artery disease. In addition, a rapid change is biomarker for respiratory desease. The microvascular reactivity changes early in patients with acute respiratory distress syndrome. Such changes also correlate with the severity of respiratory desease symptoms and mortality in critically ill populations.

There are several known methods to measure microvascular reactivity. Some of the most used methods use arm cuffs to temporarily stop or reduce blood flow in the arm, for approximately 2 to 5 minutes, whereafter the arm cuff is released. The release of the arm cuff causes vasodilation when the body tries to supply oxygen and heat to tissue in which blood flow had been stopped or reduced. Different sensors are used to measure how quickly the blood flow returns oxygen and heat to the tissue. The speed of the change in sensor parameters corresponds to the vasodilation properties of the peripheral arteries and is called microvascular reactivity. One method uses SpO2 monitoring. The microvascular reactivity is measured via tissue oxygen saturation StO2 from the forearm using a PPG sensor during, and after, the aforementioned 2 to 5 minute arm cuff-induced reactive hyperemia (reduced blood flow).

Another method uses digital thermal monitoring (DTM). The microvascular reactivity is measured from the fingertip using a temperature sensor. The temperature sensor measures temperature changes during, and after, a 5 minute arm cuff-induced reactive hyperemia.

Both methods require cuff-induced reactive hyperemia which is uncomfortable and requires the subject to halt other activities for the duration of the measurement and an arm cuff to be placed onto the arm, a protocol which usually takes at least 10 minutes to execute. Therefore, it is too invasive to be adopted by the members of a large population in their daily rutines.

Furthermore, peripheral vasoconstriction depends more on core body temperature than on skin temperature. Therefore, simply measuring skin temperature and skin temperature trend in an extremety does not usually show the correct vasoconstriction status. Vasoconstriction affects the blood pressure reading in a peripheral location, e.g., it has been shown that the average finger-to-brachial pulse amplitude ratio changes from 110 % at maximal dilatation to 170 % at full constriction.

The blood pressure of one and the same person can vary depending on, for instance, the body’s reaction to cold exposure. Even mild cold exposure in a cool room can have rather significant affect on the blood pressure, even though the measurement conditions are other wise similar, conditions such as time of day, level of physical activity, and body posture.

Normalizing the blood pressure based on the vasodilation status will reduce the effects of temporary environmental conditions on the blood pressure measurements. Usually, normalization is achieved by measuring the blood pressure from the brachial artery at an upper arm since, even though the brachial artery is muscular artery and thus reacts to vasoconstriction, the effect due to vasoconstriction is much lesser in the upper arm than in the lower arm due to the shorter distance traveled by the blood in muscular arteries.

Hence, there is a need for an improved wearable apparatus as well as an improved system and method for measuring blood pressure.

SUMMARY

It is an object to provide an improved wearable electronic apparatus. The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided a wearable electronic apparatus comprising a sensor group comprising a plurality of temperature sensors, each temperature sensor being configured to detect a skin temperature of a user within a sensor area of the temperature sensor; and a processing unit configured to calculate a rate of change of a skin temperature based on the detected skin temperatures within the sensor area, a temperature gradient based on detected skin temperatures within at least two sensor areas, and/or a rate of change of the temperature gradient based on the detected skin temperatures within at least two sensor areas.

Such a solution facilitates continues and/or repeated monitoring of a user’s health which may be executed without affecting the user’s daily routines and/or without being physically noticeable.

In a possible implementation form of the first aspect, the temperature gradient is calculated based on at least one detection of the skin temperature in each sensor area, allowing the temperature gradient to be calculated only once or repeatedly such that the user is monitored continously. In a further possible implementation form of the first aspect, the rate of change of the temperature gradient is calculated based on several detections detections of the skin temperatures in each sensor area, facilitating a simple way of calculating the change in temperature distribution across the user’s skin.

In a further possible implementation form of the first aspect, the temperature gradient is used to detect a vasoconstriction status of the user and the rate of change of the temperature gradient is used to detect a microvascular reactivity of the user, in the sensor areas, when the wearable electronic apparatus is worn by the user, allowing the apparatus to monitor several health conditions using the same data.

In a further possible implementation form of the first aspect, the vasoconstriction status is used to normalize a measured blood pressure of the user, the microvascular reactivity is used to detect coronary artery disease of the user, and/or a rate of change of the microvascular reactivity is used to detect an acute respiratory distress syndrome of the user, allowing the apparatus to monitor several health conditions using only skin temperature data.

In a further possible implementation form of the first aspect, the temperature sensors are arranged in the form of an array or a matrix, allowing differeny configurations for different wearables.

In a further possible implementation form of the first aspect, wherein at least one temperature sensor is configured to be in direct contact with a skin of the user and wherein the temperature sensor is configured such that the sensor area of the temperature sensor extends adjacent an artery of the user when the apparatus is worn, or the sensor area of the temperature sensor partially overlaps the artery of the user when the apparatus is worn. This allows the sensors to be arranged such that it suits the form factor of the apparatus.

In a further possible implementation form of the first aspect, the wearable electronic apparatus further comprises a user activity detection arrangement configured to detect an activity level of the user, the user activity detection arrangement comprising an accelerometer, a temperature sensor, and/or a heart-rate monitor, preventing daily activities from causing measurement errors by allowing the apparatus to execute its measurements at predefined and suitable times or at certain activity levels.

In a further possible implementation form of the first aspect, the detected activity level is used to initiate repeated measurements of the rate of change of the temperature gradient during a predefined time of day and/or to initiate the repeated calculations in response to user activity reaching a predefined user activity level. This is a simple way of ensuring measurements are made when the user is at rest.

In a further possible implementation form of the first aspect, the wearable electronic apparatus further comprises a carrier substrate configured to accommodate the sensor group and the processing unit, the carrier substrate being configured to enclose a limb of the user when worn by the user. This allows the sensor group to be built into any common wearable item.

In a further possible implementation form of the first aspect, the carrier substrate is configured to enclose a wrist or a finger of the user, allowing the sensor group to be part of common accessories such as smart watches or rings.

In a further possible implementation form of the first aspect, the temperature sensors are at least partially arranged along a direction allowing the sensor group to extend perpendicular to a main extent of the limb of the user when the electronic apparatus is worn. This facilitates simple calculations of the skin temperature gradient and its rate of change, as the skin temperature peaks on top of the artery extending through the limb and then decreases noticeablably as the distance to the artery increases.

In a further possible implementation form of the first aspect, the temperature sensors are conduction-based or convection-based, the conduction-based sensor being one of a thermistor, a resistance temperature detector, a thermocouple, or an integrated circuit based sensing circuit, the convection-based sensor being an infrared sensor. This allows maximum flexibility, user-friendliness, and reliability due to adaptation to type of apparatus.

According to a second aspect, there is provided a blood pressure measurement system configured to be worn by a user, the blood pressure measurement system comprising the wearable electronic apparatus according to the above and a blood pressure detector. The blood pressure detector is configured to measure a first blood pressure of the user, the wearable electronic apparatus is configured to detect a vasoconstriction status of the user, and the blood pressure measurement system is configured to calculate a second normalized blood pressure using the first measured blood pressure and the detected vasoconstriction status.

By normalizing the blood pressure measurement using vasoconstriction status, any environmental impact on the measurement, such as that of being in a cold room, can be compensated for, leading to a more accurate blood pressure measurement.

In a possible implementation form of the second aspect, the blood pressure detector is part of the wearable electronic apparatus, facilitating a blood pressure measurement system which is more or less constatly located on the user and which does not require much action from the user.

In a further possible implementation form of the second aspect the first blood pressure is measured by registering a pulse transit time between a first photoplethysmogram sensor and a second photoplethysmogram sensor arranged within the wearable electronic apparatus, facilitating a solution with as few components as possible.

In a further possible implementation form of the second aspect the first blood pressure is measured by registering a pulse arrival time between an electrocardiogram sensor and a photoplethysmogram sensor arranged within the wearable electronic apparatus, allowing use of the system together with conventional ECG equipment. According to a third aspect, there is provided a method for measuring blood pressure comprising the steps of measuring a first blood pressure of a user by a blood pressure measuring apparatus, detecting a vasoconstriction status of the user by a wearable electronic apparatus, calculating a second normalized blood pressure using the first measured blood pressure and the detected vasoconstriction status.

In a possible implementation form of the third aspect the wearable electronic apparatus is configured to be worn on a first limb of the user, and wherein the wearable electronic apparatus comprises a first sensor and a second sensor, the first sensor being arranged such that it is in contact with the first limb, the first blood pressure being measured when the user places a second limb onto the second sensor. This allows for a very simple and user friendly way of measuring blood pressure, since the measurement circuit is closed by a simple move of an opposite limb such as an arm.

In a further possible implementation form of the third aspect, the first sensor and the second sensor is one of a photoplethysmogram sensor and an electrocardiogram sensor, allowing the measurements to be made either using as little equipment as possible or using conventional ECG equipment.

These and other aspects will be apparent from the embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which: Fig. 1 shows an illustration of a wearable electronic apparatus in accordance with an example of the embodiments of the disclosure, where the apparatus is worn by a user;

Fig. 2a shows a schematic bottom view of a wearable electronic apparatus in accordance with an example of the embodiments of the disclosure;

Fig. 2b shows a schematic top view of a wearable electronic apparatus in accordance with an example of the embodiments of the disclosure;

Fig. 3 illustrates differences in skin temperature distribution, and different temperature gradients, as detected by a wearable electronic apparatus in accordance with an example of the embodiments of the disclosure, along a direction perpendicular to a limb of a user when wearing said electronic apparatus;

Fig. 4 illustrates differences in skin temperature distribution, and the rate of change of the distribution, as detected by a wearable electronic apparatus in accordance with an example of the embodiments of the disclosure, along a direction perpendicular to a limb of a user when wearing said electronic apparatus;

DETAILED DESCRIPTION

The present invention relates to a wearable electronic apparatus 1 comprising a sensor group 2 comprising a plurality of temperature sensors 3, each temperature sensor 3 being configured to detect a skin temperature of a user within a sensor area of the temperature sensor Al, A2 and a processing unit 4 configured to calculate a rate of change of a skin temperature based on the detected skin temperatures within the sensor area Al, A2, a temperature gradient based on detected skin temperatures within at least two sensor areas Al, A2, and/or a rate of change of the temperature gradient based on the detected skin temperatures within at least two sensor areas Al, A2. Fig. 1 illustrates a wearable electronic apparatus 1 in the form of a bracelet or watch, the wearable electronic apparatus 1 may nevertheless have the form of any suitable wearable element such as a ring.

The wearable electronic apparatus may comprise a carrier substrate 6 configured to accommodate the sensor group 2 and a processing unit 4, described in more detail below. The carrier substrate 6 is configured to enclose a limb, of the user when worn by the user, e.g. a wrist or a finger of the user. As shown in Fig. 2a, the sensor group 2 may be arranged at a bottom of the carrier substrate 6, e.g. the bottom of a watch body or in a wist strap such that the temperature sensors 3 can be in contact with the underside of the user’s wrist. The carrier substrate 6 may comprise any suitable material such as fabric, plastic, or metal.

As mentioned above, the wearable electronic apparatus comprises a sensor group 2. The sensor group 2 comprises a plurality of interspaced temperature sensors 3. The temperature sensors 3 are interspaced, i.e. each temperature sensor 3 is arranged at a suitable distance from adjacent temperature sensors 3.

Each temperature sensor 3 is configured to detect the skin temperature of the user within a sensor area Al, A2, i.e. each temperature sensor 3 is configured to detect skin temperatures within its own sensor area.

The temperature sensors 3 may be arranged in the form of an array, as shown in Figs. 1 and 2a, or in the form of a matrix (not shown). The temperature sensors 3 may be at least partially arranged along a direction DI allowing the sensor group 2 to extend perpendicular to a main extent of the limb of the user when the electronic apparatus is worn. The main extent of the limb of the user may extend in a direction D2, as shown in Fig. 1.

The temperature sensors 3 may be conduction-based or convection-based. The conductionbased sensor may be one of a thermistor, a resistance temperature detector, a thermocouple, or an integrated circuit based sensing circuit. The convection-based sensor may be an infrared sensor. At least one temperature sensor 3 is configured to be in direct contact with the skin of the user. Furthermore, the at least one temperature sensor 3 is configured such that the sensor area Al, A2 of the temperature sensor 3 extends adjacent an artery of the user when the apparatus is worn, or the sensor area Al , A2 of the temperature sensor 3 partially overlaps the artery of the user when the apparatus is worn, as shown in Fig. 1.

The processing unit 4 is configured to calculate a rate of change of a skin temperature based on the detected skin temperatures within the sensor area Al, A2, a temperature gradient based on detected skin temperatures within at least two sensor areas Al, A2, and/or a rate of change of the temperature gradient based on the detected skin temperatures within at least two sensor areas Al , A2. The processing unit 4 may be any suitable processor or other technical solution capable of making such calculations.

The above-mentioned rate of change of a skin temperature is calculated by detecting an instantaneous skin temperature at at least two separate points in time. A change in skin temperature alone may be an indication of vasodilation or vasoconstriction. However, the rate of change, or speed at which the skin temperature changes, i.e. the difference in temperature over a predefined time interval, is a better indication of whether vasodilation or vasoconstriction takes place. The rate of change of the skin temperature can be calculated for just one sensor area Al, or for several sensor areas Al, A2. For example, the apparatus 1 may be configured to detect a first temperature T1 and, after a period of time t, detect a second temperature T2 within a sensor area Al, A2. As an example, a first temperature T1 is illustrated by a specific point on the lower curve in Fig. 3 (i.e. in one sensor area), while a second, higher temperature T2 is illustrated by a correspondning point on the upper curve. The rate of change, or speed, of the temperature change may thereafter be calculated as (T2-Tl)/t.

The above-mentioned temperature gradient is an even better indicator for vasodilation or vasoconstriction, since it does not depend on hysteresis effects caused by parameters such as how much of the user’s body is covered by clothing or whether the user has just eaten. The temperature gradient is calculated using the detected skin temperatures within at least two sensor areas Al, A2. When, e.g., the temperature sensors 3 of a sensor group are positioned perpendicular to a main artery, the skin temperature gradient describes the status of vasodilation or vasoconstriction. The temperature gradient is calculated by simultaneously detecting the instantaneous skin temperature in at least two sensor areas. The difference in temperature, i.e. the temperature distribution, between the sensor areas Al, A2, i.e. the temperature gradient, is illustrated in Fig. 3 by two curves. During vasodilation, the skin temperature distribution is quite flat, see the upper curve in Fig. 3, and the temperature gradient is small. During vasoconstriction, the skin temperature has a different shape as shown in the bottom curve in Fig. 3. The skin temperature peaks on top of the artery A and decreases noticeablably as the distance to the artery A increases. Hence, the temperature gradient is larger.

The temperature gradient may be calculated based on one detection of the skin temperature in each sensor area Al, A2, or on several detections of the skin temperature in each sensor area Al, A2, allowing the temperature gradient to be calculated repeatedly and the user to be monitored continously. The gradient of a curve is calculated as Ay/ Ax. In the example shown in Fig. 3, Ax corresponds to a distance between sensors, or rather sensor areas, since the sensors are arranged along the x-axis (DI). Ay corresponds to a change in temperature between these sensor areas since each instantaneous temperature, measured at substantially the same time in each sensor area, are shown along the y-axis in the form of points along on eof the curves. Hence, a first temperature T1 may be detected in a first sensor area Al and a second temperature T2 may be detected in a second sensor area A2. The distance between the first sensor area Al and the second sensor area A2 is Ax. The temperature gradient of one of the two curves shown in Fig. 3 may thereafter be calculated as (T2- Tl)/Ax.

The above-mentioned rate of change of the temperature gradient is also calculated using the detected skin temperatures within at least two sensor areas Al, A2. The rate of change of the temperature gradient is illustrated in Fig. 4, and may be calculated based on several subsequent detections of the skin temperatures in the two or more sensor areas Al, A2. In other words, the rate of change of the temperature gradient is the speed at which the temperature gradient changes, i.e. the change in curve slope as illustrated in Fig. 4. The speed at which the temperature gradient, or distribution, changes describes how quickly the change from vasodition to vasoconstriction (and the reverse) takes place, which in turn provides a reliable indication of the user’s micro vascular reactivity. As mentioned above, the temperature gradient is calculated as Ay/ Ax, i.e. a change in detected temperature divided by a distance between sensor areas, shown as one of the curves in Fig. 4, referred to below as a first curve. When making an identical calculation after some time t, be it immediately after the first calculation of several hours or days thereafter, this is illustrated as one of the other curves in Fig. 4, referred to below as a second curve. When there is a difference in shape between the first curve and the second curve, in any given sensor area, the two curves have two different temperature gradients, i.e. the first curve has a first temperature gradient G1 and the second curve has a second temperature gradient G2.

The rate of change, or speed, of the change in temperature gradient is calculated as (G2- Gl)/t, t being the time lapsed between measurements.

In summary, the temperature gradient may be used to detect the vasoconstriction status of the user and the rate of change of the temperature gradient may be used to detect the microvascular reactivity of the user, in the sensor areas Al, A2, when the wearable electronic apparatus 1 is worn by the user. The vasoconstriction status may be used to normalize a measured blood pressure of the user, which is described in more detail below. Furthermore, the microvascular reactivity may be used to detect coronary artery disease of the user and the rate of change of the microvascular reactivity may be used to detect an acute respiratory distress syndrome of the user. For example, slow changes over weeks and months may indicate coronary artery disease while quick changes over hours or days may be early signs of acute respiratory distress.

The wearable electronic apparatus 1 may further comprise a user activity detection arrangement 5 configured to detect an activity level of the user. The user activity detection arrangement 5 may comprise an accelerometer, a temperature sensor, and/or a heart-rate monitor. Furthermore, the user activity detection arrangement 5 may be arranged separately on the user’s body, i.e. outside of the wearable electronic apparatus 1, or it may be part of the wearable electronic apparatus 1. There are many daily activities which can cause measurement errors in the temperature gradient. It is therefore preferable to find a time or a user activity level that is repeated on a daily basis, such as sleep. This allows the microvascular reactivity measurements between different days to be the most comparable.

The detected activity level is used to initiate repeated calculations of the rate of change of the temperature gradient during a predefined time of day, e.g. during a period which the user has predefined as a sleep period. Furthermore, the calculations may be initiated in response to user activity reaching a predefined user activity level, e.g. a level which indicates rest or being seated.

The present invention furthermore relates to a blood pressure measurement system configured to be worn by a user. The blood pressure measurement system comprises the above-described wearable electronic apparatus 1 and a blood pressure detector 7.

The blood pressure detector 7 is configured to measure a first blood pressure of the user.

The blood pressure detector 7 may be a conventional, separate device for detecting blood pressure. The blood pressure detector 7 may also be part of the wearable electronic apparatus 1 , wholly or partially.

For example, the first blood pressure may be measured by registering a pulse transit time between a first photoplethysmogram sensor 7a and a second photoplethysmogram sensor 7b arranged within the wearable electronic apparatus 1.

Furthermore, the first blood pressure may be measured by registering a pulse arrival time between an electrocardiogram sensor 7c and a photoplethysmogram sensor 7b arranged within the wearable electronic apparatus 1.

The first photoplethysmogram sensor 7a and the second photoplethysmogram sensor, as well as the electrocardiogram sensor 7c and the photoplethysmogram sensor 7b, are arranged at a distance from each other on the user’s body along the artery system. The wearable electronic apparatus 1 may be configured to detect the microvascular reactivity of the user, as described above. The blood pressure measurement system is, in turn, configured to calculate a second normalized blood pressure using the first measured blood pressure and the detected microvascular reactivity. The physiology of the user and the placement of sensors affects the calculations wherefore some calibration of the system may be necessary.

The present invention furthermore relates to a method for measuring blood pressure, the method comprising the steps of measuring a first blood pressure of a user by a blood pressure detector 7, detecting the vasoconstriction status of the user by a wearable electronic apparatus 1, and calculating a second normalized blood pressure using the first measured blood pressure and the detected vasoconstriction status.

The wearable electronic apparatus 1 may be configured to be worn on a first limb of the user, such as an arm. The wearable electronic apparatus 1 may comprise a first sensor 7b and a second sensor 7a, 7c, the first sensor 7b being arranged such that it is in contact with the first limb, e.g. the underside of the wrist, and the first blood pressure may be measured when the user places a second limb, such as a finger of the other arm, onto the second sensor 7a, 7c. As illustrated in Fig. 2b, the second sensor 7a, 7c may be arranged on a top side or part of the wearable electronic apparatus 1 which is separate from a bottom part, or underside, of the wearable electronic apparatus 1 as illustrated in Fig. 2a. However, the first sensor 7b and the second sensor 7a, 7c may also be arranged at opposite surfaces of one and the same part of the wearable electronic apparatus 1, such as adjacent an inside surface and an outside surface of a watch strap.

The first sensor 7b and the second sensor 7a, 7c may be one of a photoplethysmogram sensor and an electrocardiogram sensor.

The various aspects and implementations have been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

The reference signs used in the claims shall not be construed as limiting the scope. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure. As used in the description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate.

Claims

1. A wearable electronic apparatus (1) comprising:

-a sensor group (2) comprising a plurality of temperature sensors (3), each temperature sensor (3) being configured to detect a skin temperature of a user within a sensor area (Al , A2 ) of said temperature sensor (3); and

-a processing unit (4) configured to calculate

—a rate of change of a skin temperature based on said detected skin temperatures within said sensor area (Al, A2), and/or

—a temperature gradient based on detected skin temperatures within at least two sensor areas (Al, A2), and/or

—a rate of change of said temperature gradient based on detected skin temperatures within at least two sensor areas (Al, A2).

2. The wearable electronic apparatus (1) according to claim 1, wherein said temperature gradient is calculated based on at least one detection of said skin temperature in each sensor area (Al, A2).

3. The wearable electronic apparatus (1) according to claim 1 or 2, wherein said rate of change of said temperature gradient is calculated based on several detections of said skin temperatures in each sensor area (Al, A2).

4. The wearable electronic apparatus (1) according to any one of the previous claims, wherein said temperature gradient is used to detect a vasoconstriction status of said user and said rate of change of said temperature gradient is used to detect a microvascular reactivity of said user, in said sensor areas (Al, A2), when said wearable electronic apparatus (1) is worn by said user.

5. The wearable electronic apparatus (1) according to claim 4, wherein said vasoconstriction status is used to normalize a measured blood pressure of said user, said microvascular reactivity is used to detect coronary artery disease of said user, and/or a rate of change of said microvascular reactivity is used to detect an acute respiratory distress syndrome of said user.

6. The wearable electronic apparatus (1) according to any one of the previous claims, wherein said temperature sensors (3) are arranged in the form of an array or a matrix.

7. The wearable electronic apparatus (1) according to any one of the previous claims, wherein at least one temperature sensor (3) is configured to be in direct contact with a skin of said user and wherein said temperature sensor (3) is configured such that said sensor area (Al, A2) of said temperature sensor (3) extends adjacent an artery of said user when said apparatus is worn, or said sensor area (Al, A2) of said temperature sensor (3) partially overlaps said artery of said user when said apparatus is worn.

8. The wearable electronic apparatus (1) according to any one of the previous claims, further comprising a user activity detection arrangement (5) configured to detect an activity level of said user, said user activity detection arrangement (5) comprising an accelerometer, a temperature sensor, and/or a heart-rate monitor.

9. The wearable electronic apparatus (1) according to claim 8, wherein said detected activity level is used to initiate repeated calculations of said rate of change of said temperature gradient during a predefined time of day and/or to initiate said repeated calculations in response to user activity reaching a predefined user activity level.

10. The wearable electronic apparatus (1) according to any one of the previous claims, further comprising a carrier substrate (6) configured to accommodate said sensor group (2) and said processing unit (4), said carrier substrate (6) being configured to enclose a limb of said user when worn by said user.

11. The wearable electronic apparatus (1) according to claim 10, wherein said carrier substrate (6) is configured to enclose a wrist or a finger of said user.

12. The wearable electronic apparatus (1) according to claim 10 or 11, wherein said temperature sensors (3) are at least partially arranged along a direction (DI) allowing said sensor group (2) to extend perpendicular to a main extent of said limb of said user when said electronic apparatus is worn.

13. The wearable electronic apparatus (1) according to any one of the previous claims, wherein said temperature sensors (3) are conduction-based or convection -based, said conduction-based sensor being one of a thermistor, a resistance temperature detector, a thermocouple, or an integrated circuit based sensing circuit, said convection-based sensor being an infrared sensor.

14. A blood pressure measurement system configured to be worn by a user, said blood pressure measurement system comprising the wearable electronic apparatus (1) according to anyone of claims 1 to 13 and a blood pressure detector (7), said blood pressure detector (7) being configured to measure a first blood pressure of said user, said wearable electronic apparatus (1) being configured to detect a vasoconstriction status of said user, said blood pressure measurement system being configured to calculate a second normalized blood pressure using said first measured blood pressure and said detected vasoconstriction status.

15. The blood pressure measurement system according to claim 14, wherein said blood pressure detector (7) is part of said wearable electronic apparatus (1).

16. The blood pressure measurement system according to claim 14 or 15, wherein said first blood pressure is measured by registering a pulse transit time between a first photoplethysmogram sensor (7a) and a second photoplethysmogram sensor (7b) arranged within said wearable electronic apparatus (1).

17. The blood pressure measurement system according to claim 14 or 15, wherein said first blood pressure is measured by registering a pulse arrival time between an electrocardiogram sensor (7c) and a photoplethysmogram sensor (7b) arranged within said wearable electronic apparatus (1).

18. A method for measuring blood pressure comprising the flowing steps:

- measuring a first blood pressure of a user by a blood pressure detector 7,

- detecting a vasoconstriction status of said user by a wearable electronic apparatus (1),

- calculating a second normalized blood pressure using said first measured blood pressure and said detected vasoconstriction status.

19. The method for measuring blood pressure according to claim 15, wherein said wearable electronic apparatus (1) is configured to be worn on a first limb of said user, and wherein said wearable electronic apparatus (1) comprises a first sensor (7b) and a second sensor (7a, 7c), said first sensor (7b) being arranged such that it is in contact with said first limb, said first blood pressure being measured when said user places a second limb onto said second sensor (7a, 7c).

20. The method for measuring blood pressure according to claim 19, wherein said first sensor (7b) and said second sensor (7a, 7c) is one of a photoplethysmogram sensor and an electrocardiogram sensor.