WO2023202766A1

WO2023202766A1 - Methods and apparatuses for determining a blood pulse travel distance and a blood pulse wave velocity

Info

Publication number: WO2023202766A1
Application number: PCT/EP2022/060415
Authority: WO
Inventors: Olli Pekka SUHONEN; Heikki Vilho NIEMINEN
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2022-04-20
Filing date: 2022-04-20
Publication date: 2023-10-26

Abstract

A method of determining a blood pulse travel distance with a wearable device (106, 502), including a position detection means (108) and being configured to be worn on a user's arm, comprises detecting a first position by the position detection means when the user places the wearable device on the user's chest on top of the user's heart and detecting a second position at a distance from the first position by the position detection means when the user fully extends the arm with the wearable device to the side. Then a blood pulse travel distance between the heart and a location of the wearable device on the user's arm is determined as the distance between the first position and the second position.

Description

METHODS AND APPARATUSES FOR DETERMINING A BLOOD PULSE TRAVEL DISTANCE AND A BLOOD PULSE WAVE VELOCITY

TECHNICAL FIELD

The disclosure relates generally to methods and apparatuses for determining a blood pulse travel distance and a blood pulse wave velocity, and more particularly, the disclosure relates to a method and an apparatus for determining the blood pulse travel distance with a wearable device, and a method and an apparatus for determining the blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. Moreover, the disclosure also relates to a method and an apparatus for determining the blood pulse wave velocity, PWV, with the wearable device, and a method and an apparatus for determining the blood pulse wave velocity, PWV, with two wearable devices.

BACKGROUND

Pulse transit time, PTT, is a potential approach for cuff-less blood pressure, BP, monitoring. The PTT is a time which is taken when the blood pulse is traveling from a heart to a distal location (e.g. a wrist or a finger). The pulse transmit time can be measured using an electrocardiogram, ECG, and a distal wearable device including an optical heart rate photoplethysmography, PPG, sensor at one device. The ECG is an electrical signal measured from the heart muscle and presents the time instance when the blood pulse starts to propagate from the heart to other parts of the body. The PTT is calculated from a time difference between a R peak observed from the ECG signal and a P-base point observed from the PPG sensor signal. Pulse wave velocity, PWV, can be calculated from the PTT if an accurate distance between the PPG sensor and the heart is known. An alternative way to calculate the PWV is to use two devices having optical PPG sensor at different locations on an arterial line of the heart and measure the time difference between P-base points observed from both the PPG sensor signals. The PWV correlates with vascular health and stiffness of the veins of the heart.

To calculate the PWV with two or more separate devices measurement, timing needs to be synchronized very accurately. To calculate the PWV from the ECG and the PPG signals, the distance between the heart and the PPG sensor and the time that the blood pulse travels from the heart to the PPG sensor needs to be known/measured accurately. Therefore, there arises a need to address the aforementioned technical problem/drawbacks in determining the blood pulse travel distance and the blood pulse wave velocity.

SUMMARY

It is an object of the disclosure to provide a method and an apparatus for determining a blood pulse travel distance with a wearable device; and a method and an apparatus for determining the blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. The disclosure further provides a method and an apparatus for determining a blood pulse wave velocity, PWV, with the wearable device; and a method and an apparatus for determining the blood pulse wave velocity, PWV, with two wearable devices while avoiding one or more disadvantages of prior art approaches.

This object is achieved by the features of the independent claims. Further, implementation forms are apparent from the dependent claims, the description, and the figures.

The disclosure provides a method and an apparatus for determining a blood pulse travel distance with a wearable device; and a method and an apparatus for determining the blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. The disclosure provides a method and an apparatus for determining a blood pulse wave velocity, PWV, with the wearable device; and a method and an apparatus for determining the blood pulse wave velocity, PWV, with two wearable devices.

According to a first aspect, there is provided a method of determining a blood pulse travel distance with a wearable device including a position detection means and being configured to be worn on a user’s arm. The method includes detecting a first position by the position detection means when the user places the wearable device on the user’s chest on top of the user’s heart. The method includes detecting a second position at a distance from the first position by the position detection means when the user fully extends the arm with the wearable device to the side. The method includes determining a blood pulse travel distance between the heart and a location of the wearable device on the user’s arm as the distance between the first position and the second position. The method enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. This method can be repeated multiple times to further improve the accuracy of determination of the blood pulse travel distance between the heart and the location of the wearable device. The method improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

Optionally, the position detection means includes an accelerometer and a gyroscope. The distance between the first position and the second position is determined using inertial navigation equations. The wearable device further includes an electrocardiogram, ECG, sensor and a photoplethysmogram, PPG, sensor. Optionally, the wearable device is configured to be worn on one of a wrist and a finger of the user’s arm.

Optionally, the method further includes detecting the first position by a position detection means of a second wearable device when the user places the second wearable device on the user’s chest on top of the user’s heart. The second wearable device includes a PPG sensor and is configured to be worn on the other one of the wrist and the finger of the user’s arm. The method further includes detecting a third position at a distance from the first position by the position detection means of the second wearable device when the user extends the arm with the second wearable device to the side at a maximum distance. The method further includes determining a second blood pulse travel distance between the heart and a location of the second wearable device on the other one of the wrist and the finger of the user’s arm as the distance between the first position and the third position. The method further includes determining a third blood pulse travel distance between the location of the wearable device and the location of the second wearable device on the user’s arm as a difference between the blood pulse travel distance and the second blood pulse travel distance.

According to a second aspect, there is provided a method of determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. Each of the two wearable devices include a position detection means and a PPG sensor. The method includes detecting a first reference position by the position detection means of one of the two wearable devices when the user places the wearable device in a reference point. The method includes detecting a second reference position by the position detection means of the same wearable device when the user places the other one of the two wearable devices in the reference point. The method includes determining a blood pulse travel distance between the locations of the wearable devices on the user’s arm as a distance between the first reference position and the second reference position.

The method enables accurate measurement of the blood pulse travel distance between the locations of the two wearable devices (e.g. a watch and a ring) using the PPG sensor. The method synchronizes timing of internal clock signals of two discrete wearable devices (e.g. the ring and the watch). With the synchronized clock signals, the blood pulse travel distance from the two wearable devices (i.e. a first wearable device and a second wearable device) can be determined more accurately. The method improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

According to a third aspect, there is provided a method of determining a blood pulse wave velocity, PWV, with a wearable device including a position detection means, an ECG sensor and a PPG sensor. The wearable device is configured to be worn on a user’s arm. The method includes registering a first time when a blood pulse starts to propagate from the user’s heart using the ECG sensor. The method includes registering a second time when the blood pulse arrives to a location of the wearable device on the user’s hand using the PPG sensor. The method includes determining a pulse arrival time, PAT, as a difference between the second time and the first time. The method includes determining a blood pulse travel distance between the heart and the location of the wearable device on the user’s arm in accordance with the above method of determining the blood pulse travel distance with a wearable device. The method includes determining the blood pulse wave velocity, PWV, based on the PAT and the blood pulse travel distance.

The method enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. The method also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. The method improves vascular health and the performance of blood pulse wave velocity-based applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the registering of the first time includes registering a time of occurrence of an R peak in an electrocardiogram obtained by the ECG sensor in a cardiac cycle. The registering of the second time includes registering a time of occurrence of a virtual base point, P-base point, in a photoplethysmogram obtained by the PPG sensor in the same cardiac cycle.

According to a fourth aspect, there is provided a method of determining a blood pulse wave velocity, PWV, with two wearable devices each including a position detection means and a PPG sensor. Each of the wearable devices is configured for transmitting and receiving synchronization signals, one of the two wearable devices is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices is configured to be worn on a finger of the same arm of the user. The method includes transmitting a synchronization signal by one of the two wearable devices. The method includes receiving the synchronization signal by the other one of the two wearable devices. The method includes synchronizing clocks of the two wearable devices based on the synchronization signal. The method includes registering a first arrival time when a blood pulse arrives to the wrist using the PPG sensor of the wearable device worn on the wrist. The method includes registering a second arrival time when the blood pulse arrives to the finger using the PPG sensor of the wearable device worn on the finger. The method includes determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time. The method includes determining a blood pulse travel distance between locations of the wearable devices on the wrist and the finger on the user’s arm in accordance with the method of determining the blood pulse travel distance with the two wearable devices. The method includes determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance.

The method enables the determination of the blood pulse travel distance between the locations of the two wearable devices more accurately. The method also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. The method improves the usage of the two separate wearable devices for the pulse wave velocity, PWV, based vascular health applications. The method improves vascular health and the performance of the blood pulse wave velocitybased applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the synchronization signals are light signals that includes a pattern of light pulses. The PPG sensors of the two wearable devices include light-emitting diodes, LEDs, configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals.

Optionally, the synchronization signals are ultrasonic signals that includes a pattern of ultrasonic pulses. Each of the two wearable devices includes a speaker for transmitting the ultrasonic synchronization signals and a microphone for receiving the ultrasonic synchronization signals.

Optionally, the method further includes, after the receiving of the ultrasonic synchronization signal: (i) transmitting a second ultrasonic synchronization signal by the wearable device that has received the ultrasonic synchronization signal, and (ii) receiving the second ultrasonic synchronization signal by the other wearable device. The synchronizing of the clocks of the two wearable devices is further based on the second ultrasonic synchronization signal.

According to a fifth aspect, there is provided an apparatus for determining a blood pulse travel distance with a wearable device configured to be worn on a user’s arm and including a position detection means and a communication means. The apparatus includes a communication means and a processor. The communication means of the apparatus is configured for communicating with the wearable device. The processor is configured for instructing the wearable device to (i) detect a first position by the position detection means when the user places the wearable device on the user’s chest on top of the user’s heart, (ii) detect a second position at a distance from the first position by the position detection means when the user extends the arm with the wearable device to the side at a maximum distance, and (iii) communicate the first and second positions to the apparatus. The processor is configured for determining a blood pulse travel distance between the heart and a location of the wearable device on the user’s arm as the distance between the first position and the second position. The apparatus enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. This apparatus can be repeated multiple times to further improve the accuracy of determination of the blood pulse travel distance between the heart and the location of the wearable device. The apparatus improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

Optionally, the position detection means includes an accelerometer and a gyroscope, and the processor is configured for determining the distance between the first position and the second position using inertial navigation equations. Optionally, the wearable device further includes an electrocardiogram, ECG, sensor and a photoplethysmography, PPG, sensor. Optionally, the wearable device is configured to be worn on one of a wrist and a finger of the user’s arm.

Optionally, the communication means of the apparatus is further configured for communicating with a second wearable device configured to be worn on the other one of the wrist and the finger of the same arm of the user. The second wearable device includes a position detection means, a communication means and a photoplethysmography, PPG, sensor. The processor is configured for instructing the second wearable device to: (i) detect the first position by the position detection means when the user places the second wearable device on the user’s chest on top of the user’s heart, (ii) detect a third position at a distance from the first position by the position detection means when the user extends the arm with the second wearable device to the side at a maximum distance, and (iii) communicate the first position and the third position to the apparatus. The processor is further configured for (i) determining a second blood pulse travel distance between the heart and a location of the second wearable device on the other one of the wrist and the finger of the user’s arm as the distance between the first position and the third position, and (ii) determining a third blood pulse travel distance between the location of the wearable device and the location of the second wearable device on the user’s arm as a difference between the blood pulse travel distance and the second blood pulse travel distance.

According to a sixth aspect, there is provided an apparatus for determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. Each of the two wearable devices includes a communication means, a position detection means and a photoplethysmography, PPG, sensor. The apparatus includes a communication means and a processor. The communication means is configured for communicating with the two wearable devices. The processor is configured for instructing one of the wearable devices to (i) detect a first reference position by the position detection means when the user places the wearable device in a reference point, (ii) detect a second reference position by the position detection means when the user places the other one of the two wearable devices in the reference point, and (iii) communicate the first reference position and the second reference position to the apparatus. The processor is configured for determining a blood pulse travel distance between the locations of the two wearable devices on the user’s arm as a distance between the first reference position and the second reference position.

The apparatus enables accurate measurement of the blood pulse travel distance between the locations of the two wearable devices (e.g. a watch and a ring) using the PPG sensor. The apparatus synchronizes timing of internal clock signals of two discrete wearable devices (e.g. the ring and the watch). With the synchronized clock signals, the blood pulse travel distance from the two wearable devices (i.e. a first wearable device and a second wearable device) can be determined more accurately. The apparatus improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

According to a seventh aspect, there is provided an apparatus for determining a blood pulse wave velocity, PWV, with a wearable device configured to be worn on a user’s arm. The wearable device includes a position detection means, an ECG sensor, a PPG sensor and a communication means. The apparatus includes a communication means and a processor. The communication means is configured for communicating with the wearable device. The processor is configured for instructing the wearable device to (i) register a first time when a blood pulse starts to propagate from the user’s heart using the ECG sensor, (ii) register a second time when the blood pulse arrives to a location of the wearable device on the user’s hand using the PPG sensor, and (iii) communicate the first time and the second time to the apparatus. The processor is configured for (i) determining a pulse arrival time, PAT, as a difference between the second time and the first time, (ii) determining a blood pulse travel distance between the heart and the location of the wearable device on the user’s arm in accordance with the method of determining the blood pulse travel distance with a wearable device, and (iii) determining the blood pulse wave velocity, PWV, based on the PAT and the blood pulse travel distance.

The apparatus enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. The apparatus also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. The apparatus improves vascular health and the performance of blood pulse wave velocity-based applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the wearable device is configured for registering the first time by registering a time of occurrence of an R peak in an electrocardiogram obtained by the ECG sensor in a cardiac cycle. The wearable device is configured for registering the second time by registering a time of occurrence of a virtual base point, P-base point, in a photoplethysmogram obtained by the PPG sensor in the same cardiac cycle.

According to an eighth aspect, there is provided an apparatus for determining a blood pulse wave velocity, PWV, with two wearable devices each including a position detection means, a PPG sensor and a communication means. Each of the wearable devices is configured for transmitting and receiving synchronization signals, one of the two wearable devices is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices is configured to be worn on a finger of the same arm of the user. The apparatus includes a communication means and a processor. The communication means of the apparatus is configured for communicating with the two wearable devices. The processor is configured for instructing the two wearable devices to (i) transmit a synchronization signal by one of the two wearable devices, (ii) receive the synchronization signal by the other one of the two wearable devices, (iii) synchronize clocks of the two wearable devices based on the synchronization signal, (iv) register a first arrival time when a blood pulse arrives to the wrist using the PPG sensor of the wearable device worn on the wrist, (v) register a second arrival time when the blood pulse arrives to the finger using the PPG sensor of the wearable device worn on the finger, and (vi) communicate the first arrival time and the second arrival time to the apparatus. The processor is configured for (i) determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time, (ii) determining a blood pulse travel distance between locations of the two wearable devices on the wrist and the finger on the user’s arm in accordance with the method of determining the blood pulse travel distance with the two wearable devices, and (iii) determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance.

The apparatus enables the determination of the blood pulse travel distance between the locations of the two wearable devices more accurately. The apparatus also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the locations of the wearable devices on the user’s arm more accurately. The apparatus improves the usage of the two separate wearable devices for the pulse wave velocity, PWV, based vascular health applications. The apparatus improves vascular health and the performance of the blood pulse wave velocity-based applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the synchronization signals are light signals that includes a pattern of light pulses. The PPG sensors of the two wearable devices include LEDs configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals. Optionally, the synchronization signals are ultrasonic signals that includes a pattern of ultrasonic pulses. Each of the two wearable devices includes a speaker configured for transmitting the ultrasonic synchronization signals and a microphone configured for receiving the ultrasonic synchronization signals.

Optionally, the processor is configured for instructing the two wearable devices, after the receiving of the ultrasonic synchronization signal, to (i) transmit a second ultrasonic synchronization signal by the wearable device that has received the ultrasonic synchronization signal, and (ii) receive the second ultrasonic synchronization signal by the other wearable device. The synchronizing of the clocks of the two wearable devices is further based on the second ultrasonic synchronization signal.

A technical problem in the prior art is resolved, where the technical problem is that determining a blood pulse travel distance and a blood pulse wave velocity more accurately. Therefore, in contradistinction to the prior art, according to a method of determining a blood pulse travel distance with a wearable device, the method enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. This method can be repeated multiple times to further improve the accuracy of determination of the blood pulse travel distance between the heart and the location of the wearable device. The method improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement. The method also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately.

These and other aspects of the disclosure will be apparent from and the implementation(s) described below.

BRIEF DESCRIPTION OF DRAWINGS

Implementations of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of an apparatus for determining a blood pulse travel distance with a wearable device configured to be worn on a user’s arm in accordance with an implementation of the disclosure;

FIG. 2 is a block diagram of an apparatus for determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure;

FIG. 3 is a block diagram of an apparatus for determining a blood pulse wave velocity, PWV, with a wearable device configured to be worn on a user’s arm in accordance with an implementation of the disclosure;

FIG. 4 is a block diagram of an apparatus for determining a blood pulse wave velocity, PWV, with two wearable devices in accordance with an implementation of the disclosure;

FIG. 5 illustrates an exemplary view of an apparatus for determining a blood pulse travel distance with a wearable device configured to be worn on a user’s arm in accordance with an implementation of the disclosure;

FIGS. 6A-6C illustrate an exemplary view of an apparatus for determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure;

FIGS. 7A-7D illustrate an exemplary view of an apparatus for determining a blood pulse wave velocity, PWV, with two wearable devices and a corresponding graphical representation that indicates a time of flight of a synchronization signal in accordance with an implementation of the disclosure;

FIG. 8 is a flow diagram that illustrates a method of determining a blood pulse travel distance with a wearable device in accordance with an implementation of the disclosure;

FIG. 9 is a flow diagram that illustrates a method of determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure;

FIGS. 10A-10B are flow diagrams that illustrate a method of determining a blood pulse wave velocity, PWV, with a wearable device in accordance with an implementation of the disclosure;

FIGS. 11A-1 IB are flow diagrams that illustrate a method of determining a blood pulse wave velocity, PWV, with two wearable devices in accordance with an implementation of the disclosure; and

FIG. 12 is an illustration of a computer system (e.g. an apparatus) in which the various architectures and functionalities of the various previous implementations may be implemented.

DETAILED DESCRIPTION OF THE DRAWINGS

Implementations of the disclosure provide methods and apparatuses for determining a blood pulse travel distance and a blood pulse wave velocity, and more particularly, the disclosure relates to a method and an apparatus for determining the blood pulse travel distance with a wearable device; and a method and an apparatus for determining the blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user. Moreover, the disclosure also relates to a method and an apparatus for determining the blood pulse wave velocity, PWV, with the wearable device; and a method and an apparatus for determining the blood pulse wave velocity, PWV, with two wearable devices.

To make solutions of the disclosure more comprehensible for a person skilled in the art, the following implementations of the disclosure are described with reference to the accompanying drawings.

Terms such as "a first", "a second", "a third", and "a fourth" (if any) in the summary, claims, and foregoing accompanying drawings of the disclosure are used to distinguish between similar objects and are not necessarily used to describe a specific sequence or order. It should be understood that the terms so used are interchangeable under appropriate circumstances, so that the implementations of the disclosure described herein are, for example, capable of being implemented in sequences other than the sequences illustrated or described herein. Furthermore, the terms "include" and "have" and any variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, a method, a system, a product, or a device that includes a series of steps or units, is not necessarily limited to expressly listed steps or units but may include other steps or units that are not expressly listed or that are inherent to such process, method, product, or device.

FIG. 1 is a block diagram of an apparatus 100 for determining a blood pulse travel distance with a wearable device 106 configured to be worn on a user’ s arm in accordance with an implementation of the disclosure. The wearable device 106 includes a position detection means 108 and a communication means 110. The apparatus 100 includes a communication means 102 and a processor 104. The communication means 102 of the apparatus 100 is configured for communicating with the wearable device 106. The processor 104 is configured for instructing the wearable device 106 to (i) detect a first position by the position detection means 108 when the user places the wearable device 106 on the user’s chest on top of the user’s heart, (ii) detect a second position at a distance from the first position by the position detection means 108 when the user extends the arm with the wearable device 106 to the side at a maximum distance, and (iii) communicate the first and second positions to the apparatus 100. The processor 104 is configured for determining a blood pulse travel distance between the heart and a location of the wearable device 106 on the user’s arm as the distance between the first position and the second position.

The apparatus 100 enables the determination of the blood pulse travel distance between the heart and the location of the wearable device 106 on the user’s arm more accurately. This apparatus 100 can be repeated multiple times to further improve the accuracy of determination of the blood pulse travel distance between the heart and the location of the wearable device 106. The apparatus 100 improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

Optionally, the position detection means 108 includes an accelerometer and a gyroscope, and the processor 104 is configured for determining the distance between the first position and the second position using inertial navigation equations. Optionally, the wearable device 106 further includes an electrocardiogram, ECG, sensor and a photoplethysmography, PPG, sensor. Optionally, the wearable device 106 is configured to be worn on one of a wrist and a finger of the user’s arm.

Optionally, the communication means 102 of the apparatus 100 is further configured for communicating with a second wearable device configured to be worn on the other one of the wrist and the finger of the same arm of the user. The second wearable device includes a position detection means, a communication means and a photoplethysmography, PPG, sensor. The processor 104 is configured for instructing the second wearable device to: (i) detect the first position by the position detection means when the user places the second wearable device on the user’s chest on top of the user’s heart, (ii) detect a third position at a distance from the first position by the position detection means when the user extends the arm with the second wearable device to the side at a maximum distance, and (iii) communicate the first position and the third position to the apparatus 100. The processor 104 is further configured for (i) determining a second blood pulse travel distance between the heart and a location of the second wearable device on the other one of the wrist and the finger of the user’s arm as the distance between the first position and the third position, and (ii) determining a third blood pulse travel distance between the location of the wearable device 106 and the location of the second wearable device on the user’s arm as a difference between the blood pulse travel distance and the second blood pulse travel distance. FIG. 2 is a block diagram of an apparatus 200 for determining a blood pulse travel distance with two wearable devices (e.g. a first wearable device 206A and a second wearable device 206B) configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure. Each of the two wearable devices (e.g. 206A & 206B) includes a position detection means (e.g. 208A & 208B), a communication means (e.g. 210A & 210B), and a photoplethysmography, PPG, sensor (e.g. 212A & 212B). The apparatus 200 includes a communication means 202 and a processor 204. The communication means 202 of the apparatus 200 is configured for communicating with the two wearable devices (e.g. 206A & 206B). The processor 204 is configured for instructing one of the wearable devices (e.g. 206A) to (i) detect a first reference position by the position detection means (e.g. 208A) when the user places the wearable device (e.g. 206A) in a reference point, (ii) detect a second reference position by the position detection means (e.g. 208B) when the user places the other one of the two wearable devices (e.g. 206B) in the reference point, and (iii) communicate the first reference position and the second reference position to the apparatus 200. The processor 204 is configured for determining a blood pulse travel distance between the locations of the two wearable devices (e.g. 206A & 206B) on the user’s arm as a distance between the first reference position and the second reference position.

The apparatus 200 enables accurate measurement of the blood pulse travel distance between the locations of the two wearable devices (e.g. a watch 206A and a ring 206B) using the PPG sensor (e.g. 212A & 212B). The apparatus 200 synchronizes timing of internal clock signals of two discrete wearable devices (e.g. the watch 206A and the ring 206B). With the synchronized clock signals, the blood pulse travel distance from the two wearable devices (i.e. the first wearable device 206A and the second wearable device 206B) can be determined more accurately. The apparatus 200 improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

FIG. 3 is a block diagram of an apparatus 300 for determining a blood pulse wave velocity, PWV, with a wearable device 306 configured to be worn on a user’s arm in accordance with an implementation of the disclosure. The wearable device 306 includes a position detection means 308, a communication means 310, an ECG sensor 312 and a PPG sensor 314. The apparatus 300 includes a communication means 302 and a processor 304. The communication means 302 of the apparatus 300 is configured for communicating with the wearable device 306. The processor 304 is configured for instructing the wearable device 306 to (i) register a first time when a blood pulse starts to propagate from the user’s heart using the ECG sensor 312, (ii) register a second time when the blood pulse arrives to a location of the wearable device 306 on the user’s hand using the PPG sensor 314, and (iii) communicate the first time and the second time to the apparatus 300. The processor 304 is configured for (i) determining a pulse arrival time, PAT, as a difference between the second time and the first time, (ii) determining a blood pulse travel distance between the heart and the location of the wearable device 306 on the user’s arm as described above, and (iii) determining the blood pulse wave velocity, PWV, based on the PAT and the blood pulse travel distance.

The apparatus 300 enables the determination of the blood pulse travel distance between the heart and the location of the wearable device 306 on the user’s arm more accurately. The apparatus 300 also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the location of the wearable device 306 on the user’s arm more accurately. The apparatus 300 improves vascular health and the performance of blood pulse wave velocity-based applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the wearable device 306 is configured for registering the first time by registering a time of occurrence of an R peak in an electrocardiogram obtained by the ECG sensor 312 in a cardiac cycle. The wearable device 306 is configured for registering the second time by registering a time of occurrence of a virtual base point, P-base point, in a photoplethysmogram obtained by the PPG sensor 314 in the same cardiac cycle.

FIG. 4 is a block diagram of an apparatus 400 for determining a blood pulse wave velocity, PWV, with two wearable devices (e.g. a first wearable device 406A and a second wearable device 406B) in accordance with an implementation of the disclosure. Each wearable devices (e.g. 406A & 406B) include a position detection means (e.g. 408A & 408B), and a communication means (e.g. 410A & 410B), and a PPG sensor (e.g. 412A & 412B). Each of the wearable devices (e.g. 406A & 406B) is configured for transmitting and receiving synchronization signals, one of the two wearable devices (e.g. 406A) is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices (e.g. 406B) is configured to be worn on a finger of the same arm of the user. The apparatus 400 includes a communication means 402 and a processor 404. The communication means 402 of the apparatus 400 is configured for communicating with the two wearable devices (e.g. 406A & 406B). The processor 404 is configured for instructing the two wearable devices (e.g. 406A & 406B) to (i) transmit a synchronization signal by one of the two wearable devices (e.g. 406A), (ii) receive the synchronization signal by the other one of the two wearable devices (e.g. 406B), (iii) synchronize clocks of the two wearable devices (e.g. 406A & 406B) based on the synchronization signal, (iv) register a first arrival time when a blood pulse arrives to the wrist using the PPG sensor (e.g. 412A) of the wearable device (e.g. 406A) worn on the wrist, (v) register a second arrival time when the blood pulse arrives to the finger using the PPG sensor (e.g. 412B) of the wearable device (e.g. 406B) worn on the finger, and (vi) communicate the first arrival time and the second arrival time to the apparatus 400. The processor 404 is configured for (i) determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time, (ii) determining a blood pulse travel distance between locations of the two wearable devices (e.g. 406A & 406B) on the wrist and the finger on the user’s arm in accordance with the method of determining the blood pulse travel distance with the two wearable devices (e.g. 406A & 406B), and (iii) determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance.

The apparatus 400 enables the determination of the blood pulse travel distance between the locations of the two wearable devices (e.g. 406A & 406B) more accurately. The apparatus 400 also enables the accurate determination of pulse wave velocity, PWV, by measuring the blood pulse travel distance between the heart and the locations of the wearable devices (e.g. 406A & 406B) on the user’s arm more accurately. The apparatus 400 improves the usage of the two separate wearable devices (e.g. 406A & 406B) for the pulse wave velocity, PWV, based vascular health applications. The apparatus 400 improves vascular health and the performance of the blood pulse wave velocity-based applications by improving the accuracy of the blood pulse wave velocity measurement.

Optionally, the synchronization signals are light signals that includes a pattern of light pulses. The PPG sensors (e.g. 412A & 412B) of the two wearable devices (e.g. 406A & 406B) include LEDs configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals. Optionally, the synchronization signals are ultrasonic signals that includes a pattern of ultrasonic pulses. Each of the two wearable devices (e.g. 406A & 406B) includes a speaker configured for transmitting the ultrasonic synchronization signals and a microphone configured for receiving the ultrasonic synchronization signals.

Optionally, the processor 404 is configured for instructing the two wearable devices (e.g. 406A & 406B), after the receiving of the ultrasonic synchronization signal, to (i) transmit a second ultrasonic synchronization signal by the wearable device (e.g. 406B) that has received the ultrasonic synchronization signal, and (ii) receive the second ultrasonic synchronization signal by the other wearable device (e.g. 406A). The synchronizing of the clocks of the two wearable devices (e.g. 406A & 406B) is further based on the second ultrasonic synchronization signal.

FIG. 5 illustrates an exemplary view of an apparatus for determining a blood pulse travel distance with a wearable device 502 configured to be worn on a user’ s arm in accordance with an implementation of the disclosure. The wearable device 502 includes a position detection means and a communication means. The apparatus includes a communication means and a processor. The communication means of the apparatus is configured for communicating with the wearable device 502. The processor is configured for instructing the wearable device 502 to (i) detect a first position by the position detection means when the user places the wearable device 502 on the user’s chest on top of the user’s heart (e.g. place A), (ii) detect a second position at a distance from the first position by the position detection means when the user extends the arm with the wearable device 502 to the side at a maximum distance (e.g. place B), and (iii) communicate the first and second positions to the apparatus. The processor is configured for determining a blood pulse travel distance between the heart (e.g. place A) and a location of the wearable device on the user’s arm (e.g. place B) as the distance between the first position and the second position.

Optionally, the position detection means includes an accelerometer and a gyroscope, and the processor is configured for determining the distance between the first position and the second position using inertial navigation equations. Optionally, the wearable device 502 further includes an electrocardiogram, ECG, sensor and a photoplethysmography, PPG sensor. Optionally, the wearable device 502 is configured to be worn a wrist of the user’s arm. For example, if the wearable device 502 is a watch and worn on the user’s arm, the user places the watch on the chest on the top of the heart (i.e. place A) and then stretches out the hand to extremity (i.e. place B). The wearable device 502 detects the first position when the user places the wearable device 502 on place A, detects the second position at a distance from the first position when the user extends the arm with the wearable device 502 to the place B, and communicates the first and second positions to the apparatus. The processor determines the blood pulse travel distance between the place A and the place B as the distance between the first position and the second position. The accuracy of the apparatus is improved if the measurement is repeated for multiple times. Optionally, if the user is instructed to move hand with the wearable device 502 to chest and back, then the apparatus determines both the start and end point of the movement and determines the distance that the wearable device 502 moved quite accurately using a fixed-interval smoothing technique.

FIGS. 6A-6C illustrate an exemplary view of an apparatus for determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure. Each of the two wearable devices includes a communication means, a position detection means and a PPG sensor. The apparatus includes a communication means and a processor. The communication means is configured for communicating with the two wearable devices. Optionally, the position detection means includes an accelerometer and a gyroscope, and the processor is configured for determining the distance between the first position and the second position using inertial navigation equations. Optionally, the wearable device is configured to be worn on one of a wrist and a finger of the user’s arm.

In FIG. 6A, the two wearable devices (e.g. a watch 602A & a ring 602B) are configured to be worn in different locations on one arm of the user. For example, the watch 602A and the ring 602B are worn in different locations on one arm of the user. Optionally, the watch 602A is worn on the wrist (i.e. place B) of the user and the ring 602B on the finger of the same arm (i.e. place C) of the user. Optionally, a reference point 604 is on the chest on the top of the heart (i.e. place A). Optionally, the user places one wearable device (e.g. the ring 602B) after another wearable device (e.g. the watch 602A) on top of the reference point 604. Optionally, one of the wearable devices (e.g. the watch 602A) detects the first reference position using a position detection means when the user places the wearable device (e.g. the watch 602A) in the reference point 604 and then extend his hand. Optionally, the other one of the wearable devices (e.g. the ring 602B) detects the second reference position using its position detection means when the user places the other one of the two wearable devices (e.g. the ring 602B) in the reference point 604 and extend his hand. The two wearable devices (e.g. the watch 602A & the ring 602B) communicate the first reference position and the second reference position to the apparatus. The delta between these distances (i.e. the first reference position and the second reference position) is the distance between the ring 602B and the watch 602A. The processor of the apparatus is configured for determining a blood pulse travel distance between the locations (i.e. the place B and the place C) of the two wearable devices (e.g. 602A & 602B) on the user’s arm as a distance between the first reference position and the second reference position.

In FIGS. 6B and 6C, the two wearable devices (e.g. a watch 606A & a ring 606B) are configured to be worn in different locations on one arm of the user. For example, the watch 606A and the ring 606B are worn in different locations on one arm of the user. Optionally, the watch 606A is worn on the wrist of the user and the ring 606B on the finger of the same arm of the user. Optionally, a reference point 608 is on a table. In FIG. 6B, one of the wearable devices (e.g. the ring 606B) is on top of the reference point 608. Optionally, when the user moves the hand forward, other one of the wearable devices (e.g. the watch 606A) is on top of the reference point 608. The movement of the hand of the user is depicted in FIG. 6C. Optionally, the apparatus comprises an accelerometer that logs the blood pulse travel distance between the ring 606B and the watch 606A.

FIGS. 7A-7D illustrate an exemplary view of an apparatus for determining a blood pulse wave velocity, PWV, with two wearable devices (e.g. a watch 702A & a ring 702B) and a corresponding graphical representation that indicates a time of flight of a synchronization signal in accordance with an implementation of the disclosure. The apparatus includes a communication means and a processor. The communication means of the apparatus is configured for communicating with the two wearable devices (e.g. the watch 702A & the ring 702B). The watch 702A and the ring 702B each include a position detection means, a PPG sensor (e.g. 704) and a communication means. Each of the wearable devices (e.g. the watch 702A & the ring 702B) is configured for transmitting and receiving synchronization signals. For example, the watch 702A is configured to be worn on a wrist of an arm of a user and the ring 702B is configured to be worn on a finger of the same arm of the user. The watch 702A transmits a synchronization signal to the ring 702B. The ring 702B receives the synchronization signal from the watch 702A. The processor of the apparatus synchronizes clock signals of the the watch 702A and the ring 702B based on the synchronization signal. The processor registers a first arrival time when a blood pulse arrives to the wrist using the PPG sensor of the watch 702A worn on the wrist, and registers a second arrival time when the blood pulse arrives to the finger using the PPG sensor 704 of the ring 702B worn on the finger. The processor communicates the first arrival time and the second arrival time to the apparatus. The processor is configured for (i) determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time, (ii) determining a blood pulse travel distance between locations of the watch 702A on the wrist and the ring 702B on the finger on the user’s arm, and (iii) determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance. Optionally, the two wearable devices (e.g. a watch 702A & a ring 702B) are unconnected wearable devices.

Optionally, the synchronization signals are light signals that includes a pattern of light pulses. The PPG sensors of the two wearable devices (e.g. the watch 702A & the ring 702B) include LEDs configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals. In FIG. 7B, the graphical representation shows a time of flight of the synchronization signal (e.g. LED synchronization signal) transmitting from the watch 702A to the ring 702B to determine the blood pulse wave velocity, PWV. Optionally, the synchronization signals are ultrasonic signals that includes a pattern of ultrasonic pulses. In FIG. 7C, the exemplary view shows an ultrasonic signal transmitting from the watch 702A to the ring 702B to determine a blood pulse wave velocity, PWV. Each of the two wearable devices includes a speaker configured for transmitting the ultrasonic synchronization signals and a microphone configured for receiving the ultrasonic synchronization signals.

Optionally, the processor is configured for instructing the two wearable devices, after the receiving of the ultrasonic synchronization signal, to (i) transmit a second ultrasonic synchronization signal by the wearable device (e.g. the watch 702A) that has received the ultrasonic synchronization signal, and (ii) receive the second ultrasonic synchronization signal by the other wearable device (e.g. the ring 702B). The synchronizing of the clocks of the watch 702A and the ring 702B is further based on the second ultrasonic synchronization signal. In FIG. 7D, the graphical representation shows a time of flight of the ultrasonic synchronization signals of the watch 702A and the ring 702B to determine the blood pulse wave velocity, PWV. Optionally, the watch 702A transmits the ultrasonic synchronization signals to the ring 702B and the ring 702B responses immediately.

FIG. 8 is a flow diagram that illustrates a method of determining a blood pulse travel distance with a wearable device in accordance with an implementation of the disclosure. The wearable device includes a position detection means and is being configured to be worn on a user’s arm. At a step 802, a first position is detected by the position detection means when the user places the wearable device on the user’s chest on top of the user’s heart. At a step 804, a second position is detected at a distance from the first position by the position detection means when the user fully extends the arm with the wearable device to the side. At a step 806, a blood pulse travel distance is determined between the heart and a location of the wearable device on the user’s arm as the distance between the first position and the second position.

The method enables the determination of the blood pulse travel distance between the heart and the location of the wearable device on the user’s arm more accurately. This method can be repeated multiple times to further improve the accuracy of determination of the blood pulse travel distance between the heart and the location of the wearable device. The method improves vascular health and the performance of blood pulse travel distance-based applications by improving the accuracy of the blood pulse travel distance measurement.

FIG. 9 is a flow diagram that illustrates a method of determining a blood pulse travel distance with two wearable devices configured to be worn in different locations on one arm of a user in accordance with an implementation of the disclosure. Each of the two wearable devices include a position detection means and a PPG sensor. At a step 902, a first reference position is detected by the position detection means of one of the two wearable devices when the user places the wearable device in a reference point. At a step 904, a second reference position is detected by the position detection means of the same wearable device when the user places the other one of the two wearable devices in the reference point. At a step 906, a blood pulse travel distance is determined between the locations of the wearable devices on the user’s arm as a distance between the first reference position and the second reference position.

FIGS. 10A-10B are flow diagrams that illustrate a method of determining a blood pulse wave velocity, PWV, with a wearable device in accordance with an implementation of the disclosure. The wearable device includes a position detection means, an ECG sensor and a PPG sensor. The wearable device is configured to be worn on a user’s arm. At a step 1002, a first time is registered when a blood pulse starts to propagate from the user’ s heart using the ECG sensor. At a step 1004, a second time is registered when the blood pulse arrives to a location of the wearable device on the user’s hand using the PPG sensor. At a step 1006, a pulse arrival time, PAT, is determined as a difference between the second time and the first time. At a step 1008, a blood pulse travel distance is determined between the heart and the location of the wearable device on the user’s arm in accordance with the above method of determining the blood pulse travel distance with a wearable device. At a step 1010, the blood pulse wave velocity, PWV, is determined based on the PAT and the blood pulse travel distance.

FIGS. 11A-1 IB are flow diagrams that illustrate a method of determining a blood pulse wave velocity, PWV, with two wearable devices in accordance with an implementation of the disclosure. Two wearable devices each include a position detection means and a PPG sensor. Each of the wearable devices is configured for transmitting and receiving synchronization signals. One of the two wearable devices is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices is configured to be worn on a finger of the same arm of the user. At a step 1102, a synchronization signal is transmitted by one of the two wearable devices. At a step 1104, the synchronization signal is received by the other one of the two wearable devices. At a step 1106, clocks of the two wearable devices are synchronized based on the synchronization signal. At a step 1108, a first arrival time is registered when a blood pulse arrives to the wrist using the PPG sensor of the wearable device worn on the wrist. At a step 1110, a second arrival time is registered when the blood pulse arrives to the finger using the PPG sensor of the wearable device worn on the finger. At a step 1112, a pulse transit time, PTT, is determined during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time. At a step 1114, a blood pulse travel distance is determined between locations of the wearable devices on the wrist and the finger on the user’s arm in accordance with the method of determining the blood pulse travel distance with the two wearable devices. At a step 1116, a blood pulse wave velocity, PWV, is determined based on the determined PTT and the blood pulse travel distance.

FIG. 12 is an illustration of a computer system (e.g. an apparatus) in which the various architectures and functionalities of the various previous implementations may be implemented. As shown, the computer system 1200 includes at least one processor 1204 that is connected to a bus 1202, wherein the computer system 1200 may be implemented using any suitable protocol, such as PCI (Peripheral Component Interconnect), PCI- Express, AGP (Accelerated Graphics Port), Hyper Transport, or any other bus or point- to-point communication protocol (s). The computer system 1200 also includes a memory 1206.

Control logic (software) and data are stored in the memory 1206 which may take a form of random-access memory (RAM). In the disclosure, a single semiconductor platform may refer to a sole unitary semiconductor-based integrated circuit or chip. It should be noted that the term single semiconductor platform may also refer to multi-chip modules with increased connectivity which simulate on-chip modules with increased connectivity which simulate on-chip operation, and make substantial improvements over utilizing a conventional central processing unit (CPU) and bus implementation. Of course, the various modules may also be situated separately or in various combinations of semiconductor platforms per the desires of the user.

The computer system 1200 may also include a secondary storage 1210. The secondary storage 1210 includes, for example, a hard disk drive and a removable storage drive, representing a floppy disk drive, a magnetic tape drive, a compact disk drive, digital versatile disk (DVD) drive, recording device, universal serial bus (USB) flash memory. The removable storage drive at least one of reads from and writes to a removable storage unit in a well-known manner.

Computer programs, or computer control logic algorithms, may be stored in at least one of the memory 1206 and the secondary storage 1210. Such computer programs, when executed, enable the computer system 1200 to perform various functions as described in the foregoing. The memory 1206, the secondary storage 1210, and any other storage are possible examples of computer-readable media. In an implementation, the architectures and functionalities depicted in the various previous figures may be implemented in the context of the processor 1204, a graphics processor coupled to a communication interface 1212, an integrated circuit (not shown) that is capable of at least a portion of the capabilities of both the processor 1204 and a graphics processor, a chipset (namely, a group of integrated circuits designed to work and sold as a unit for performing related functions, and so forth).

Furthermore, the architectures and functionalities depicted in the various previous- described figures may be implemented in a context of a general computer system, a circuit board system, a game console system dedicated for entertainment purposes, an application-specific system. For example, the computer system 1200 may take the form of a desktop computer, a laptop computer, a server, a workstation, a game console, an embedded system.

Furthermore, the computer system 1200 may take the form of various other devices including, but not limited to a personal digital assistant (PDA) device, a mobile phone device, a smart phone, a television, and so forth. Additionally, although not shown, the computer system 1200 may be coupled to a network (for example, a telecommunications network, a local area network (LAN), a wireless network, a wide area network (WAN) such as the Internet, a peer-to-peer network, a cable network, or the like) for communication purposes through an I/O interface 1208.

It should be understood that the arrangement of components illustrated in the figures described are exemplary and that other arrangement may be possible. It should also be understood that the various system components (and means) defined by the claims, described below, and illustrated in the various block diagrams represent components in some systems configured according to the subject matter disclosed herein. For example, one or more of these system components (and means) may be realized, in whole or in part, by at least some of the components illustrated in the arrangements illustrated in the described figures.

In addition, while at least one of these components are implemented at least partially as an electronic hardware component, and therefore constitutes a machine, the other components may be implemented in software that when included in an execution environment constitutes a machine, hardware, or a combination of software and hardware. Although the disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

1. A method of determining a blood pulse travel distance with a wearable device (106, 502) comprising a position detection means (108) and being configured to be worn on a user’s arm, the method comprising: detecting a first position by the position detection means when the user places the wearable device on the user’s chest on top of the user’s heart, detecting a second position at a distance from the first position by the position detection means when the user fully extends the arm with the wearable device to the side, and determining a blood pulse travel distance between the heart and a location of the wearable device on the user’s arm as the distance between the first position and the second position.

2. The method of claim 1, wherein the position detection means (108) comprises an accelerometer and a gyroscope, wherein the distance between the first position and the second position is determined using inertial navigation equations.

3. The method of claim 1 or 2, wherein the wearable device (106, 502) further comprises an electrocardiogram, ECG, sensor and a photoplethysmogram, PPG, sensor.

4. The method of any of claims 1 to 3, wherein the wearable device (106, 502) is configured to be worn on one of a wrist and a finger of the user’s arm.

5. The method of claim 4, further comprising: detecting the first position by a position detection means (108) of a second wearable device when the user places the second wearable device on the user’s chest on top of the user’s heart, wherein the second wearable device comprises a PPG sensor and is configured to be worn on the other one of the wrist and the finger of the user’s arm, detecting a third position at a distance from the first position by the position detection means of the second wearable device when the user extends the arm with the second wearable device to the side at a maximum distance, determining a second blood pulse travel distance between the heart and a location of the second wearable device on the other one of the wrist and the finger of the user’s arm as the distance between the first position and the third position, and determining a third blood pulse travel distance between the location of the wearable device (106, 502) and the location of the second wearable device on the user’s arm as a difference between the blood pulse travel distance and the second blood pulse travel distance.

6. A method of determining a blood pulse travel distance with two wearable devices (206A, 206B, 602A, 602B, 606A, 606B) configured to be worn in different locations on one arm of a user, wherein each of the two wearable devices comprises a position detection means (208A, 208B) and a PPG sensor (212A, 212B), the method comprising: detecting a first reference position by the position detection means of one of the two wearable devices when the user places said wearable device in a reference point (604, 608), detecting a second reference position by the position detection means of the same wearable device when the user places the other one of the two wearable devices in the reference point, and determining a blood pulse travel distance between the locations of the wearable devices on the user’s arm as a distance between the first reference position and the second reference position.

7. A method of determining a blood pulse wave velocity, PWV, with a wearable device (306) comprising a position detection means (308), an ECG sensor (312) and a PPG sensor (314), wherein the wearable device is configured to be worn on a user’s arm, the method comprising: registering a first time when a blood pulse starts to propagate from the user’s heart using the ECG sensor, registering a second time when the blood pulse arrives to a location of the wearable device on the user’s hand using the PPG sensor, determining a pulse arrival time, PAT, as a difference between the second time and the first time, determining a blood pulse travel distance between the heart and the location of the wearable device on the user’s arm in accordance with the method of any of claims 1 to 4, and determining the blood pulse wave velocity, PWV, based on the PAT and the blood pulse travel distance.

8. The method of claim 7, wherein the registering of the first time comprises registering a time of occurrence of an R peak in an electrocardiogram obtained by the ECG sensor (312) in a cardiac cycle, and the registering of the second time comprises registering a time of occurrence of a virtual base point, P-base point, in a photoplethysmogram obtained by the PPG sensor (314) in the same cardiac cycle.

9. A method of determining a blood pulse wave velocity, PWV, with two wearable devices (406A, 406B, 702A, 702B) each comprising a position detection means (408A, 408B) and a PPG sensor (412 A, 412B), wherein each of the wearable devices is configured for transmitting and receiving synchronization signals, one of the two wearable devices is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices is configured to be worn on a finger of the same arm of the user, the method comprising: transmitting a synchronization signal by one of the two wearable devices, receiving the synchronization signal by the other one of the two wearable devices, synchronizing clocks of the two wearable devices based on the synchronization signal, registering a first arrival time when a blood pulse arrives to the wrist using the PPG sensor of the wearable device worn on the wrist, registering a second arrival time when the blood pulse arrives to the finger using the PPG sensor of the wearable device worn on the finger, determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time, determining a blood pulse travel distance between locations of the wearable devices on the wrist and the finger on the user’s arm in accordance with the method of claim 5 or 6, and determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance.

10. The method of claim 9, wherein the synchronization signals are light signals comprising a pattern of light pulses, wherein the PPG sensors (412A, 412B) of the two wearable devices (406A, 406B, 702A, 702B) comprise light-emitting diodes, LEDs, configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals.

11. The method of claim 10, wherein the synchronization signals are ultrasonic signals comprising a pattern of ultrasonic pulses, wherein each of the two wearable devices (406A, 406B, 702A, 702B) comprises a speaker for transmitting the ultrasonic synchronization signals and a microphone for receiving the ultrasonic synchronization signals.

12. The method of claim 11, further comprising, after the receiving of the ultrasonic synchronization signal: transmitting a second ultrasonic synchronization signal by the wearable device that has received the ultrasonic synchronization signal, and receiving the second ultrasonic synchronization signal by the other wearable device, wherein the synchronizing of the clocks of the two wearable devices is further based on the second ultrasonic synchronization signal.

13. An apparatus (100) for determining a blood pulse travel distance with awearable device (106, 502) configured to be worn on a user’s arm and comprising a position detection means (108) and a communication means (110), the apparatus comprising: a communication means (102) configured for communicating with the wearable device, and a processor (104) configured for instructing the wearable device to detect a first position by the position detection means when the user places the wearable device on the user’s chest on top of the user’s heart, detect a second position at a distance from the first position by the position detection means when the user extends the arm with the wearable device to the side at a maximum distance, and communicate the first and second positions to the apparatus, wherein the processor is configured for determining a blood pulse travel distance between the heart and a location of the wearable device on the user’s arm as the distance between the first position and the second position.

14. The apparatus (100) of claim 13, wherein the position detection means (108) comprises an accelerometer and a gyroscope, and the processor (104) is configured for determining the distance between the first position and the second position using inertial navigation equations.

15. The apparatus (100) of claim 13 or 14, wherein the wearable device (106, 502) further comprises an ECG sensor and a PPG sensor.

16. The apparatus (100) of any of claims 13 to 15, wherein the wearable device (106, 502) is configured to be worn on one of a wrist and a finger of the user’s arm.

17. The apparatus (100) of claim 16, wherein the communication means (102) is further configured for communicating with a second wearable device configured to be worn on the other one of the wrist and the finger of the same arm of the user, wherein the second wearable device comprises a position detection means, a communication means and a PPG sensor, the processor (104) is configured for instructing the second wearable device to: detect the first position by the position detection means when the user places the second wearable device on the user’s chest on top of the user’s heart, detect a third position at a distance from the first position by the position detection means when the user extends the arm with the second wearable device to the side at a maximum distance, and communicate the first position and the third position to the apparatus, the processor is further configured for determining a second blood pulse travel distance between the heart and a location of the second wearable device on the other one of the wrist and the finger of the user’s arm as the distance between the first position and the third position, and determining a third blood pulse travel distance between the location of the wearable device (106, 502) and the location of the second wearable device on the user’s arm as a difference between the blood pulse travel distance and the second blood pulse travel distance.

18. An apparatus (200) for determining a blood pulse travel distance with two wearable devices (206A, 206B, 602A, 602B, 606A, 606B) configured to be worn in different locations on one arm of a user, wherein each of the two wearable devices comprises a communication means (210A, 210B), a position detection means (208A, 208B) and a PPG sensor (212A, 212B), the apparatus comprising: a communication means (202) configured for communicating with the two wearable devices, and a processor (204) configured for instructing one of the wearable devices to detect a first reference position by the position detection means when the user places said wearable device in a reference point (604, 608), detect a second reference position by the position detection means when the user places the other one of the two wearable devices in the reference point, and communicate the first reference position and the second reference position to the apparatus, wherein the processor is configured for determining a blood pulse travel distance between the locations of the two wearable devices on the user’s arm as a distance between the first reference position and the second reference position.

19. An apparatus (300) for determining a blood pulse wave velocity, PWV, with a wearable device (306) configured to be worn on a user’s arm, wherein the wearable device comprises a position detection means (308), an ECG sensor (312), a PPG sensor (314) and a communication means (310), the apparatus comprising: a communication means (302) configured for communicating with the wearable device, and a processor (304) configured for instructing the wearable device to register a first time when a blood pulse starts to propagate from the user’s heart using the ECG sensor, register a second time when the blood pulse arrives to a location of the wearable device on the user’s hand using the PPG sensor, and communicate the first time and the second time to the apparatus, wherein the processor is configured for determining a pulse arrival time, PAT, as a difference between the second time and the first time, determining a blood pulse travel distance between the heart and the location of the wearable device on the user’s arm in accordance with the method of any of claims 1 to 5, and determining the blood pulse wave velocity, PWV, based on the PAT and the blood pulse travel distance.

20. The apparatus (300) of claim 19, wherein the wearable device (306) is configured for registering the first time by registering a time of occurrence of an R peak in an electrocardiogram obtained by the ECG sensor (312) in a cardiac cycle, and the wearable device is configured for registering the second time by registering a time of occurrence of a virtual base point, P-base point, in a photoplethysmogram obtained by the PPG sensor (314) in the same cardiac cycle.

21. An apparatus (400) for determining a blood pulse wave velocity, PWV, with two wearable devices (406A, 406B, 702A, 702B) each comprising a position detection means (408A, 408B), a PPG sensor (412A, 412B) and a communication means (410A, 410B), wherein each of the wearable devices is configured for transmitting and receiving synchronization signals, one of the two wearable devices is configured to be worn on a wrist of an arm of a user and the other one of the two wearable devices is configured to be worn on a finger of the same arm of the user, the apparatus comprising: a communication means (402) configured for communicating with the two wearable devices, and a processor (404) configured for instructing the two wearable devices to transmit a synchronization signal by one of the two wearable devices, receive the synchronization signal by the other one of the two wearable devices, synchronize clocks of the two wearable devices based on the synchronization signal, register a first arrival time when a blood pulse arrives to the wrist using the PPG sensor of the wearable device worn on the wrist, register a second arrival time when the blood pulse arrives to the finger using the PPG sensor of the wearable device worn on the finger, and communicate the first arrival time and the second arrival time to the apparatus, wherein the processor is configured for determining a pulse transit time, PTT, during which the blood pulse travels from the wrist to the finger as a difference between the second arrival time and the first arrival time, determining a blood pulse travel distance between locations of the two wearable devices on the wrist and the finger on the user’s arm in accordance with the method of any of claims 6 to 8, and determining a blood pulse wave velocity, PWV, based on the determined PTT and the blood pulse travel distance.

22. The apparatus (400) of claim 21, wherein the synchronization signals are light signals comprising a pattern of light pulses, wherein the PPG sensors (412A, 412B) of the two wearable devices (406 A, 406B, 702A, 702B) comprise LEDs configured for transmitting the light synchronization signals and photodetectors configured for receiving the light synchronization signals.

23. The apparatus (400) of claim 21, wherein the synchronization signals are ultrasonic signals comprising a pattern of ultrasonic pulses, wherein each of the two wearable devices (406A, 406B, 702A, 702B) comprises a speaker configured for transmitting the ultrasonic synchronization signals and a microphone configured for receiving the ultrasonic synchronization signals.

24. The apparatus (400) of claim 23, wherein the processor (404) is configured for instructing the two wearable devices (406 A, 406B, 702 A, 702B), after the receiving of the ultrasonic synchronization signal, to transmit a second ultrasonic synchronization signal by the wearable device that has received the ultrasonic synchronization signal, and receive the second ultrasonic synchronization signal by the other wearable device, wherein the synchronizing of the clocks of the two wearable devices is further based on the second ultrasonic synchronization signal.