FI20195418A1

FI20195418A1 - An apparatus and a method for measuring cardiovascular operation

Info

Publication number: FI20195418A1
Application number: FI20195418A
Authority: FI
Inventors: Saku Levikari; Antti Immonen; Mikko Kuisma
Original assignee: Lappeenrannan Lahden Teknillinen Yliopisto Lut
Priority date: 2019-05-21
Filing date: 2019-05-21
Publication date: 2020-11-22
Also published as: WO2020234510A1

Abstract

A device for measuring cardiovascular operation comprises a thermal sensor (101) for producing a sensor signal indicative of a heat-flux (q) emitted by blood to surroundings of a blood vessel (109) conducting the blood. The device further comprises a processing system (103) for producing, based on temporal variations of the sensor signal, data indicative of a heart rate. Power consumption of the device is lower than that of corresponding devices based on electrical, optical, or mechanical measurement of cardiovascular operation. The low power consumption is advantageous especially when the device is a batteryoperated wearable device such as a wristband heart rate monitor.

Description

A device and a method for measuring cardiovascular operation Technical field The disclosure relates generally to measuring cardiovascular operation of an individual.

More particularly, the disclosure relates to a device and to a method for measuring data descriptive of cardiovascular operation.

Background People are interested in their well-being and want to monitor their health, performance, and energy consumption with wearable devices.

Typical consumer- level fitness tracker measures a heart rate and/or acceleration in order to estimate energy consumed.

Data that describes operation of a cardiovascular system can reveal several important aspects about the health of an individual.

For example, a heart rate can be used for monitoring performance and estimating energy expenditure during exercise.

On the other hand, age-related parameters and risk factors for cardiovascular diseases can be estimated by looking at the waveform of a cardiac cycle.

Common techniques for measuring cardiovascular operation include the photoplethysmography “PPG” that is an optical technique and electrocardiography “ECG” that is an electrical technique.

The PPG is more common in wearable devices such as e.g. smart watches, whereas the ECG is more often employed in medical environment.

A cardiac cycle is widely monitored using 2 the ECG, even though this is typically performed by placing several electrodes on N the body of an individual.

Some smartwatches have an integrated ECG device, but S the signal quality is limited, and the measurements are typically short duration only.

N A third commonly used technique is to measure a heart rate mechanically, e.g. with E 25 an ordinary blood pressure monitor. 0 s The above-mentioned technigues for measuring cardiovascular operation are = however not free from challenges.

One of the challenges is power consumption that N limits an operation time of a battery-operated wearable device for measuring cardiovascular operation.

For example, a device based on the PPG comprises one or more light sources for measurement of cardiovascular operation.

The light sources are advantageously light emitting diodes “LED” which have a good efficiency, but their power consumption can still be significant with respect to a battery capacity of a wearable device. Summary The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention. In accordance with the invention, there is provided a new device for measuring cardiovascular operation. The device can be, for example but not necessarily, a battery-operated wearable device. A device according to the invention comprises: - a thermal sensor for producing a sensor signal indicative of a heat-flux emitted by blood to surroundings of a blood vessel conducting the blood, and - a processing system for producing, based on temporal variations of the sensor signal, data indicative of at least a heart rate. = 20 Power consumption of the above-described device is lower than that of N corresponding devices based on electrical, optical, or mechanical measurement of ? cardiovascular operation. The low power consumption is advantageous especially - when the device is a battery-operated wearable device such as e.g. a wristband E heart rate monitor. The thermal sensor is advantageously a passive, single-chip = 25 element in which case a device according to the invention can be made cheaper

LO O and less complex than corresponding photoplethysmography “PPG” and/or

O N electrocardiography “EEG” -based devices. A device according to the invention can used in conjunction with PPG and/or ECG for redundancy.

The above-mentioned sensor signal produced by the thermal sensor is similar to a PPG signal which expresses changes in the volume of blood in a limb. Thus, the sensor signal related to the heat-flux can be used in the same way as a PPG signal for determining properties of a cardiovascular system and/or changes caused by aging and/or diseases. A device according to an exemplifying and non-limiting embodiment is configured to measure a waveform of the above-mentioned heat- flux. In a device according to an exemplifying and non-limiting embodiment, the processing system is configured to compute one or more numerical values indicative of the heart rate, heart rate variability “HRV”, blood pressure changes, and/or blood flow rate based on the measured waveform. The heart rate and/or one or more other quantities descriptive of cardiovascular operation can be calculated using the same algorithms as in conjunction with PPG measurements, since the output of the thermal sensor is similar to a PPG signal. However, unlike the PPG, the accuracy of the heat-flux based measurement is unaffected by skin tone. Unlike the ECG, the — heat-flux based measurement is invariant to the electrical contact between a sensor and a skin. Furthermore, the heat-flux based measurement requires only a single spot measurement, whereas both PPG and ECG require a more complex measurement arrangement on a skin. The measurement of the heat-flux can be carried out in a non-invasive way on a skin or in close vicinity of the skin, or in an invasive way. The measurement of the heat-flux can be carried out by e.g. placing the thermal sensor on the radial artery or on another suitable blood vessel. A single thermal sensor can be used for both heart rate and direct energy transfer measurements, which are useful quantities when estimating energy expenditure of = an individual. Furthermore, the thermal sensor can be used for partially powering a N 25 wearable device. The measurement can be implemented on both humans and ? animals.

N E The thermal sensor can be for example a gradient heat-flux sensor “GHFS” or a © heat-flux sensor based on thermoelectric junctions. It is also possible that the 3 thermal sensor is an indirect heat-flux sensor that comprises temperature or infrared > 30 sensors and estimation means for estimating the heat-flux based on output signals of the temperature or infrared sensors. The estimation means can be implemented with the processing system of a device according to the invention, or there can be separate estimation means that are connected to the temperature or infrared sensors and to the processing system. Therefore, the thermal sensors of devices according to the invention can be based on different direct heat-flux measuring techniques as well as different indirect heat-flux measuring techniques. Thus, in conjunction with this document, the term “thermal sensor” is to be understood in a broad sense to cover different thermal sensors for measuring a heat-flux directly as well as different thermal sensors for measuring a heat-flux indirectly e.g. based on temperature measurements. Furthermore, the thermal sensor can be a single temperature or infrared sensor for measuring for example local temperature of a — skin above the blood vessel under consideration, because temporal variation of the local temperature is indicative of temporal variation of the heat-flux emitted by blood conducted by the blood vessel. In accordance with the invention, there is provided also a new method for measuring cardiovascular operation. A method according to the invention comprises: - producing, with a thermal sensor, a sensor signal indicative of a heat-flux emitted by blood to surroundings of a blood vessel conducting the blood, and - producing, based on temporal variations of the sensor signal, data indicative of at least a heart rate. Various exemplifying and non-limiting embodiments are described in accompanied dependent claims. > Various exemplifying and non-limiting embodiments both as to constructions and to N methods of operation, together with additional objects and advantages thereof, will S be best understood from the following description of specific exemplifying and non- > limiting embodiments when read in conjunction with the accompanying drawings.

T N 25 The verbs “to comprise” and “to include” are used in this document as open s limitations that neither exclude nor reguire the existence of unrecited features. The = features recited in dependent claims are mutually freely combinable unless N otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: 5 figure 1 shows a functional block-diagram of a device according to an exemplifying and non-limiting embodiment for measuring cardiovascular operation, figure 2 illustrates an exemplifying sensor signal produced by a thermal sensor of a device according to an exemplifying and non-limiting embodiment, figures 3a, 3b, 3c, and 3d illustrate devices according to exemplifying and non- limiting embodiments for measuring cardiovascular operation, and figure 4 illustrates a flowchart of a method according to an exemplifying and non- limiting embodiment for measuring cardiovascular operation.

Description of the exemplifying embodiments The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims.

Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

Figure 1 shows a functional block-diagram of a device according to an exemplifying o and non-limiting embodiment for measuring cardiovascular operation.

The device N 20 comprises a thermal sensor 101 for producing a sensor signal 102 that is indicative 3 of a heat-flux g emitted by blood to surroundings of a blood vessel 109 conducting N the blood.

In figure 1, a skin of an individual under consideration is denoted with a E reference 118. The blood vessel 109 can be for example an artery such as e.g. a o radial or ulnar artery of a wrist, a digital artery of a finger, or some other suitable D 25 blood vessel.

The device comprises a processing system 103 for producing, based > on temporal variations of the sensor signal 102, data indicative of at least a heart rate.

Furthermore, the data may express e.g. heart rate variability “HRV”, blood pressure changes, blood flow rate, and/or one or more other guantities related to cardiovascular operation. The thermal sensor 101 can be any suitable sensor that is fast and sensitive enough to detect a cardiac cycle. The response time of the thermal sensor 101 is advantageously less than 0.3 seconds, or more advantageously less than 0.1 seconds.

In a device according to an exemplifying and non-limiting embodiment, the thermal sensor 101 is a gradient heat-flux sensor “GHFS” based on thermoelectric anisotropy and adapted to generate the sensor signal proportional to the heat-flux through the gradient heat-flux sensor. The gradient heat-flux sensor may comprise for example anisotropic material where a thermal electromotive force is created from — the heat-flux by the Seebeck effect. The anisotropic material can be for example single-crystal bismuth. It is also possible that the gradient heat-flux sensor comprises a multilayer structure whose layers are obligue with respect to a surface of the gradient heat-flux sensor for receiving the heat-flux. The multilayer structure generates a thermal electromotive force from the heat-flux by the Seebeck effect.

The multilayer structure comprises first layers and second layers that are interleaved with the first layers. The first layers can be made of e.g. metal, metal alloy, or semiconductor material, and the second layers can be made of e.g. semiconductor material. The response time of a gradient heat-flux sensor can be in the range from 1 ns to 10 ns. Thus, the response time is sufficiently small for achieving sufficiently fast heat-flux measurements. Furthermore, the thickness of a gradient heat-flux sensor in the direction of the heat-flux can be small, e.g. about 0.1 mm. Thus, the gradient heat-flux sensor does not represent a significant heat storage capacity o which would disturb and slow down the heat-flux measurement. Further details of S gradient heat-flux sensors can be found for example from the publication: "Local 3 25 Heat Flux Measurement in a Permanent Magnet Motor at No Load’, Hanne K.

N Jussila, Andrey V. Mityakov, Sergey Z. Sapozhnikov, Vladimir Y. Mityakov and Juha E Pyrhönen, Institute of Electrical and Electronics Engineers “IEEE” Transactions on © Industrial Electronics, Volume: 60, pp. 4852 — 4860, 2013.

D > In a device according to an exemplifying and non-limiting embodiment, the thermal N 30 sensor 101 comprises multiple thermoelectric junctions for generating a thermal electromotive force representing the sensor signal. The thermal sensor based on thermoelectric junctions may have tens, hundreds, or even thousands of thermoelectric junctions that are connected in series.

In a device according to an exemplifying and non-limiting embodiment, the thermal sensor 101 comprises temperature or infrared sensors and estimation means for estimating the heat-flux based on output signals of the temperature or infrared sensors.

In this exemplifying case, the operation of the thermal sensor can be based on e.g. a measured temperature difference over a heat conducting path and a known thermal resistance of the heat conducting path.

A device according to an exemplifying and non-limiting embodiment comprises a — heat-sink element 104 for receiving the heat-flux from the thermal sensor 101. The heat-sink element attenuates temperature variations on a surface of the heat-flux 101 sensor facing towards the heat-sink element.

Thus, the hear-sink element stabilizes the operating conditions of the thermal sensor 101. The mass and the heat capacity of the heat-sink element 104 are advantageously so big that a thermal — time constant of the heat-sink element is at least 5 seconds, i.e. the temperature of the heat-sink element is substantially constant during a heart-beat period.

A device according to an exemplifying and non-limiting embodiment comprises thermally insulating material 105 surrounding the heat-sink element 104. The thermally insulating material 105 insulates the heat sink element 104 from changes of ambient temperature, i.e. the temperature of the surrounding air.

Thus, the thermally insulating material 105 stabilizes the temperature of the heat sink element 104. The thermally insulating material 105 can be for example Styrofoam or some other 2 suitable material having a high thermal resistivity. 8 Figure 2 illustrates an exemplifying sensor signal produced by the thermal sensor N 25 101. In a device according to an exemplifying and non-limiting embodiment, the = processing system 103 is configured to bandpass filter the sensor signal prior to © producing the data indicative of the heart rate and/or other quantities related to D cardiovascular operation.

In figure 2, the sensor signal that is outputted by the S thermal sensor 101 is denoted with a reference 202 and the bandpass-filtered sensor signal is denoted with a reference 220. The sensor signal, or the bandpass- filtered sensor signal, is similar to a photoplethysmogram “PPG” signal which expresses changes in the volume of blood in a limb. Thus, the waveform of the sensor signal can be used for determining properties of a cardiovascular system and/or changes with aging and/or diseases. The above-mentioned waveform can be observed by e.g. placing the thermal sensor on the radial artery. Based on the measurement, the heart rate and/or other quantities related to cardiovascular operation can be calculated.

In a device according to an exemplifying and non-limiting embodiment, the processing system 103 is configured to detect whether the sensor signal exhibits a dicrotic notch between each systolic peak and a local minimum corresponding to an end-diastolic pressure and preceding a next systolic peak. In a device according to an exemplifying and non-limiting embodiment, the processing system 103 is configured produce a numerical value indicative of a time interval At from the systolic peak to a diastolic peak subsequent to the dicrotic notch. The time interval At is illustrated in figure 2 and it represents a pulse propagation time “PPT” in a cardiovascular system under consideration.

In a device according to an exemplifying and non-limiting embodiment, the processing system 103 is configured to find local minima and local maxima from the sensor signal and to produce, based on the found local minima and local maxima, the data indicative of the heart rate. In figure 2, the heart-beat period is denoted with T.

In a device according to an exemplifying and non-limiting embodiment, the > processing system 103 is configured to produce a Fourier transformation of the N sensor signal and to produce, based on the Fourier transformation, the data S indicative of the heart rate.

N I 25 A device according to an exemplifying and non-limiting embodiment comprises a N memory device 106 for recording a time series indicative of the waveform of the s sensor signal. Furthermore, the device may comprise a battery element 115 and a 2 user interface 108. It is also possible that the device comprises a data interface for © communicatively connecting to an external device and/or a power supply terminal receiving electric power from an external power source.

In the above-described examples, the sensor signal under consideration is advantageously a bandpass-filtered sensor signal.

The implementation of the processing system 103 can be based on one or more processor circuits, each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as for example an application specific integrated circuit “ASIC”, or a configurable hardware processor such as for example a field programmable gate array “FPGA”. The memory device 106 may comprise one or more memory circuits such as e.g. a random-access memory “RAM” circuit.

Figures 3a, 3b, 3c, and 3d illustrate devices according to exemplifying and non- limiting embodiments for measuring cardiovascular operation. Each of the devices illustrated in figures 3a-3d comprises a holder element for holding the thermal sensor of the device in an operating position with respect to an appropriate blood vessel. In the exemplifying device illustrated in figure 3a, the holder element 307a is a wristband for holding two thermal sensors 301a and 301b so that the thermal sensor 301a is on the radial artery 309 of a wrist and the other thermal sensor 301b is on the ulnar artery 310 of the wrist. The wristband is presented as a partial section view so that the thermal sensors 301a and 301b and heat-sink elements 304a and 304b are visible. The use of the two thermal sensors 301a and 301b improves the accuracy and reliability of results which are computed based on sensor signals produced by the two thermal sensors 301a and 301b. The surfaces of the device which are against a skin area above the radial artery 309 and a skin area above the = ulnar artery 310 are advantageously rounded, e.g. such as spherical calotte N surfaces, to improve thermal contacts between the device and the skin. The device ? 25 further comprises electronics embedded in the wristband. A device of the kind 2 illustrated in figure 3a can be as well a device provided with an ankle strap.

a © In the exemplifying device illustrated in figure 3b, the holder element 307b is a ring D that is presented as a partial section view so that a thermal sensor 301c is visible. > The device further comprises electronics embedded in the ring. When the ring is around a finger in an appropriate position, the thermal sensor 301c measures a heat-flux emitted by blood conducted by a digital artery of the finger.

In the exemplifying device illustrated in figure 3c, the holder element 307c is a body of a subdermal implant that comprises a thermal sensor 301d and electronics 313. In figure 3c, a hand is presented as transparent so that an artery 311 and the device, i.e. the subdermal implant, are visible. The electronics 313 may comprise means for a wireless connection with an external device, e.g. an inductive, capacitive, or radio connection. In the exemplifying device illustrated in figure 3d, the holder element 307d is a sticker patch that comprises a thermal sensor 301e and electronics 314. The thermal sensor 301e and the electronics 314 are inside the sticker patch, and the thermal sensor 301e and the electronics 314 are presented with dashed lines. The sticker patch comprises adhesive substance for attaching the sticker patch on a skin 318 of an individual. In figure 3d, a blood vessel 312 that is below the skin is presented with dashed lines. Figure 4 illustrates a flowchart of a method according to an exemplifying and non- limiting embodiment for measuring cardiovascular operation. The method comprises the following actions: - action 401: producing, with a thermal sensor, a sensor signal indicative of a heat-flux emitted by blood to surroundings of a blood vessel conducting the blood, and - action 402: producing, based on temporal variations of the sensor signal, o data indicative of at least a heart rate. N In a method according to an exemplifying and non-limiting embodiment, the 5 response time of the thermal sensor is less than 0.3 seconds.

N z In a method according to an exemplifying and non-limiting embodiment, the thermal N 25 — sensor is a gradient heat-flux sensor based on thermoelectric anisotropy and 3 adapted to generate the sensor signal proportional to the heat-flux through the D gradient heat-flux sensor. In a method according to an exemplifying and non-limiting N embodiment, the gradient heat-flux sensor comprises anisotropic material, e.g. single-crystal bismuth, for generating a thermal electromotive force representing the sensor signal. In a method according to an exemplifying and non-limiting embodiment, the gradient heat-flux sensor comprises a multilayer structure for generating a thermal electromotive force representing the sensor signal. Layers of the multilayer structure are oblique with respect to a surface of the gradient heat- flux sensor for receiving the heat-flux.

In a method according to an exemplifying and non-limiting embodiment, the thermal sensor comprises thermoelectric junctions for generating a thermal electromotive force representing the sensor signal.

In a method according to an exemplifying and non-limiting embodiment, the heat- flux is conducted from the thermal sensor to a heat-sink element. In a method according to an exemplifying and non-limiting embodiment, the mass and the heat capacity of the heat-sink element are so big that a thermal time constant of the heat- sink element is at least 5 seconds. In a method according to an exemplifying and non-limiting embodiment, the heat-sink element is surrounded with thermally insulating material.

A method according to an exemplifying and non-limiting embodiment comprises detecting whether the sensor signal exhibits a dicrotic notch between each systolic peak and a local minimum corresponding to an end-diastolic pressure and preceding a next systolic peak.

A method according to an exemplifying and non-limiting embodiment comprises producing a numerical value indicative of a time interval from the systolic peak to a o diastolic peak subsequent to the dicrotic notch.

N A method according to an exemplifying and non-limiting embodiment comprises S bandpass filtering the sensor signal prior to producing the data indicative of the heart > rate.

T N 25 A method according to an exemplifying and non-limiting embodiment comprises s finding local minima and local maxima from the sensor signal and producing, based 2 on the found local minima and local maxima, the data indicative of the heart rate.

A method according to an exemplifying and non-limiting embodiment comprises producing a Fourier transformation of the sensor signal and producing, based on the Fourier transformation, the data indicative of the heart rate. A method according to an exemplifying and non-limiting embodiment comprises recording a time series indicative of a waveform of the sensor signal. In a method according to an exemplifying and non-limiting embodiment, the thermal sensor is held with a holder element in an operating position with respect to the blood vessel conducting the blood. The specific examples provided in the description given above should not be construed as limiting the applicability and/or interpretation of the appended claims.

It is to be noted that lists and groups of examples given in this document are non- exhaustive lists and groups unless otherwise explicitly stated.

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Claims

What is claimed is:

1. A device for measuring cardiovascular operation, characterized in that the device comprises: - a thermal sensor (101, 301a-301e) for producing a sensor signal (102) indicative of a heat-flux (q) emitted by blood to surroundings of a blood vessel conducting the blood, and - a processing system (103) for producing, based on temporal variations of the sensor signal, data indicative of at least a heart rate.

2. A device according to claim 1, wherein a response time of the thermal sensor is less than 0.3 seconds.

3. A device according to claim 1 or 2, wherein the thermal sensor (101) is a gradient heat-flux sensor based on thermoelectric anisotropy and adapted to generate the sensor signal proportional to the heat-flux through the gradient heat- flux sensor.

4 Adevice according to claim 3, wherein the gradient heat-flux sensor comprises anisotropic material for generating a thermal electromotive force representing the sensor signal.

5. Adevice according to claim 3, wherein the gradient heat-flux sensor comprises a multilayer structure for generating a thermal electromotive force representing the = 20 sensor signal, layers of the multilayer structure being oblique with respect to a N surface of the gradient heat-flux sensor for receiving the heat-flux.

O N 6. A device according to claim 5, wherein the multilayer structure comprises first E layers and second layers, the second layers being made of semiconductor material co and being interleaved with the first layers and the first layers being made of metal, < . . O 25 metal alloy, or semiconductor material.

O N 7. Adevice according to claim 1 or 2, wherein the thermal sensor (101) comprises thermoelectric junctions for generating a thermal electromotive force representing the sensor signal.

8. A device according to any one of claims 1-7, wherein the thermal sensor (101) is suitable for receiving the heat-flux from a tissue surrounding the blood vessel when being in mechanical contact with the tissue.

9. Adevice according to claim 1 or 2, wherein the thermal sensor (101) comprises an infrared detector for obtaining information about temperature or about a temperature differential.

10. A device according to any one of claims 1-9, wherein the device comprises a heat-sink element (104, 304a, 304b) for receiving the heat-flux from the thermal sensor (101) and for attenuating temperature variations on a surface of the thermal sensor facing towards the heat-sink element.

11. A device according to claim 10, wherein a mass and a heat capacity of the heat-sink element are so big that a thermal time constant of the heat-sink element is at least 5 seconds.

12. A device according to claim 10 or 11, wherein the device comprises thermally insulating material (105) surrounding the heat-sink element and thermally insulating the heat sink element from changes of ambient temperature.

13. A device according to any one of claims 1-12, wherein the processing system (103) is configured to detect whether the sensor signal exhibits a dicrotic notch between each systolic peak and a local minimum corresponding to an end-diastolic pressure and preceding a next systolic peak. 2 S 14. A device according to claim 13, wherein the processing system (103) is 3 configured produce a numerical value indicative of a time interval (At) from the N systolic peak to a diastolic peak subsequent to the dicrotic notch.

I a 15. Adevice according to any one of claims 1-14, wherein the processing system 2 25 (103) is configured to bandpass filter the sensor signal prior to producing the data 2 indicative of the heart rate.

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16. A device according to any one of claims 1-15, wherein the processing system (103) is configured to find local minima and local maxima from the sensor signal and produce, based on the found local minima and local maxima, the data indicative of the heart rate.

17. A device according to any of one of claims 1-16, wherein the processing system (103) is configured to produce a Fourier transformation of the sensor signal and produce, based on the Fourier transformation, the data indicative of the heart rate.

18. A device according to any of one of claims 1-17, wherein the device comprises a memory device (106) for recording a time series indicative of a waveform of the sensor signal.

19. Adevice according to any of one of claims 1-18, wherein the device comprises a holder element (307a-307d) for holding the thermal sensor in an operating position with respect to the blood vessel conducting the blood.

20. A method for measuring cardiovascular operation, characterized in that the method comprises: - producing (401), with a thermal sensor, a sensor signal indicative of a heat- flux emitted by blood to surroundings of a blood vessel conducting the blood, and - producing (402), based on temporal variations of the sensor signal, data indicative of at least a heart rate. oO

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