AN APPARATUS AND A METHOD FOR MEASURING COMPLIANCE OF BLOOD VESSELS

Abstract

An apparatus for measuring compliance of blood vessels includes a photoplethysmography sensor for emitting electromagnetic radiation to the blood vessels, for receiving electromagnetic radiation reflected off the blood vessels, and for producing a measurement signal indicative of the received electromagnetic radiation. The apparatus further includes a pressure instrument for producing mechanical pressure applied on the blood vessels, and a control system for controlling the pressure instrument to change the mechanical pressure linearly with respect to time during emission of electromagnetic radiation to the blood vessels and reception of reflected electromagnetic radiation from the blood vessels. The control system finds, from the measurement signal, a portion whose envelope has exponential change with respect to time and produces an estimate for an exponent coefficient of time during the exponential change. The coefficient is indicative of the compliance, as well as stiffness, of the blood vessels.

Claims

1. Apparatus for measuring compliance of blood vessels, the apparatus comprising: a photoplethysmography sensor configured to emit electromagnetic radiation to the blood vessels, to receive a part of the electromagnetic radiation reflected off the blood vessels, and to produce a measurement signal indicative of the received part of the electromagnetic radiation, a pressure instrument configured to produce mechanical pressure applied on the blood vessels, and a control system configured to control the pressure instrument to change the mechanical pressure linearly with respect to time during emission of the electromagnetic radiation to the blood vessels and reception of the part of the electromagnetic radiation reflected off the blood vessels, wherein the control system is configured to find, from the measurement signal, a portion whose envelope has exponential change with respect to time and to produce an estimate for a coefficient of time related to the exponential change, the coefficient of time being indicative of the compliance of the blood vessels.

2. The apparatus according to claim 1, wherein the photoplethysmography sensor is configured to emit, to the blood vessels, the electromagnetic radiation so that the electromagnetic radiation has wavelengths selected from at least one of the following ranges: from 625 nm to 1000 nm, from 565 nm to 590 nm, from 500 nm to 565 nm, and from 450 nm to 485 nm.

3. The apparatus according to claim 1, wherein the photoplethysmography sensor is configured to emit, to the blood vessels, the electromagnetic radiation so that the electromagnetic radiation has different wavelengths and to produce the measurement signal to comprise wavelength-specific component signals being indicative of received wavelengths reflected off the blood vessels, and the control system is configured to produce the coefficient of time for each of the wavelength-specific component signals corresponding to the different wavelengths where a shorter one of the wavelengths relates to smaller ones of the blood vessels than a longer one of the wavelengths.

4. The apparatus according to claim 1, wherein the control system is configured to control the photoplethysmography sensor to variate wavelength of the electromagnetic radiation, and to produce the coefficient of time for each value of the wavelength where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

5. The apparatus according to claim 3, wherein the control system is configured to compute a ratio of at least one pair of the coefficients of time corresponding to different wavelengths, each ratio expressing a stiffness mismatch between ones of the blood vessels having different sizes where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

6. The apparatus according to claim 1, wherein the control system is configured to convert the measurement signal to a logarithmic scale, to find from the converted measurement signal a portion whose envelope has linear change with respect to time, and to produce an estimate for a slope of the envelope of the converted measurement signal related to the linear change, the slope being the coefficient of time related to the exponential change.

7. The apparatus according to claim 1, wherein the pressure instrument comprises a force generator and a pressing element configured to direct the mechanical pressure to a fingertip or a toe in accordance with a control signal generated by the control system.

8. The apparatus according to claim 1, wherein the pressure instrument comprises a cuff and a pump system configured to control gas pressure inside the cuff to direct the mechanical pressure to an arm and to change the mechanical pressure when the photoplethysmography sensor emits and receives the electromagnetic radiation to and from the arm, the photoplethysmography sensor being located on an inner surface of the cuff.

9. A method for measuring compliance of blood vessels, the method comprising: emitting electromagnetic radiation to the blood vessels, receiving a part of the electromagnetic radiation reflected off the blood vessels, producing a measurement signal indicative of the received part of the electromagnetic radiation, producing mechanical pressure applied on the blood vessels and changing linearly with respect to time (t) during the emitting the electromagnetic radiation to the blood vessels and the receiving the part of the electromagnetic radiation reflected off the blood vessels, and finding, from the measurement signal, a portion whose envelope has exponential change with respect to time and producing an estimate for a coefficient of time related to the exponential change, the coefficient of time being indicative of the compliance of the blood vessels.

10. The method according to claim 9, wherein the electromagnetic radiation has wavelengths selected from at least one of the following ranges: from 625 nm to 1000 nm, from 565 nm to 590 nm, from 500 nm to 565 nm, and from 450 nm to 485 nm.

11. The method according to claim 9, wherein the electromagnetic radiation has different wavelengths and the measurement signal comprises wavelength-specific component signals being indicative of received wavelengths reflected off the blood vessels, and the coefficient of time is produced for each of the wavelength-specific component signals corresponding to the different wavelengths where a shorter one of the wavelengths relates to smaller ones of the blood vessels than a longer one of the wavelengths.

12. The method according to claim 9, wherein wavelength of the electromagnetic radiation is varied, and the coefficient of time is produced for each value of the wavelength where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

13. The method according to claim 11, wherein the method comprises computing a ratio of at least one pair of the coefficients of time corresponding to different wavelengths, each ratio expressing a stiffness mismatch between ones of the blood vessels having different sizes where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

14. A non-transitory computer readable medium encoded with a computer program for measuring compliance of blood vessels, the computer program comprising computer executable instructions for controlling a programmable processing system to: control a photoplethysmography sensor to emit electromagnetic radiation to the blood vessels, to receive a part of the electromagnetic radiation reflected off the blood vessels, and to produce a measurement signal indicative of the received part of the electromagnetic radiation, control a pressure instrument to produce mechanical pressure applied on the blood vessels and to change the mechanical pressure linearly with respect to time during emission of the electromagnetic radiation to the blood vessels and reception of the part of the electromagnetic radiation reflected off the blood vessels, and find, from the measurement signal, a portion whose envelope has exponential change with respect to time and to produce an estimate for a coefficient (a) of time related to the exponential change, the coefficient of time being indicative of the compliance of the blood vessels.

15. (canceled)

16. The apparatus according to claim 2, wherein the photoplethysmography sensor is configured to emit, to the blood vessels, the electromagnetic radiation so that the electromagnetic radiation has different wavelengths and to produce the measurement signal to comprise wavelength-specific component signals being indicative of received wavelengths reflected off the blood vessels, and the control system is configured to produce the coefficient of time for each of the wavelength-specific component signals corresponding to the different wavelengths where a shorter one of the wavelengths relates to smaller ones of the blood vessels than a longer one of the wavelengths.

17. The apparatus according to claim 2, wherein the control system is configured to control the photoplethysmography sensor to variate wavelength of the electromagnetic radiation, and to produce the coefficient of time for each value of the wavelength where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

18. The apparatus according to claim 4, wherein the control system is configured to compute a ratio of at least one pair of the coefficients of time corresponding to different wavelengths, each ratio expressing a stiffness mismatch between ones of the blood vessels having different sizes where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

19. The method according to claim 10, wherein the electromagnetic radiation has different wavelengths and the measurement signal comprises wavelength-specific component signals being indicative of received wavelengths reflected off the blood vessels, and the coefficient of time is produced for each of the wavelength-specific component signals corresponding to the different wavelengths where a shorter one of the wavelengths relates to smaller ones of the blood vessels than a longer one of the wavelengths.

20. The method according to claim 12, wherein the method comprises computing a ratio of at least one pair of the coefficients of time corresponding to different wavelengths, each ratio expressing a stiffness mismatch between ones of the blood vessels having different sizes where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

Description

BRIEF DESCRIPTION OF FIGURES

[0036] Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below with reference to the accompanying drawings, in which:

[0037] FIG. 1a illustrates an apparatus according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels,

[0038] FIG. 1b shows exemplifying graphs illustrating electromagnetic radiations having different wavelengths and reflected off blood vessels of a fingertip as a function of time,

[0039] FIG. 1c shows the exemplifying graphs of FIG. 1b converted to a logarithmic vertical scale,

[0040] FIG. 2 illustrates an apparatus according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels,

[0041] FIG. 3 shows a flowchart of a method according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels.

DESCRIPTION OF EXEMPLIFYING AND NON-LIMITING EMBODIMENTS

[0042] The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated.

[0043] FIG. 1a shows a schematic illustration of an apparatus according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels. The apparatus comprises a photoplethysmography PPG sensor 101 for emitting, to a fingertip 108 of an individual, electromagnetic radiation and for receiving a part of the electromagnetic radiation reflected off the blood vessels of the fingertip 108. The PPG sensor 101 comprises a radiation emitter 106 and a photodetector 107. The radiation emitter 106 may comprises e.g. one or more light emitting diodes LED and the photodetector 107 may comprise e.g. one or more photodiodes or phototransistors. FIG. 1a shows also a magnified, schematic section view 110 of the fingertip. The section plane is parallel with the yz-plane of a coordinate system 199.

[0044] In the exemplifying apparatus illustrated in FIG. 1a, the PPG sensor 101 is configured to emit the electromagnetic radiation so that the electromagnetic radiation contains radiation components with five different wavelengths. In the section view 110, the radiation components are depicted with polyline arrows 127, 128, 129, 130, and 131. The first radiation component 127 can be for example infrared radiation having a wavelength on the range from 700 nm to 1000 nm, the second radiation component 128 can be for example red radiation having a wavelength on the range from 625 nm to 700 nm, the third radiation component 129 can be for example yellow radiation having a wavelength on the range from 565 nm to 590 nm, the fourth radiation component 130 can be for example green radiation having a wavelength on the range from 500 nm to 565 nm, and the fifth radiation component 131 can be for example blue radiation having a wavelength on the range from 450 nm to 485 nm. For another example, the first radiation component 127 can be infrared radiation having a wavelength on the range from 800 nm to 900 nm, the second radiation component 128 can be red radiation having a wavelength on the range from 650 nm to 675 nm, the third radiation component 129 can be yellow radiation having a wavelength on the range from 575 nm to 580 nm, the fourth radiation component 130 can be green radiation having a wavelength on the range from 530 nm to 545 nm, and the fifth radiation component 131 can be blue radiation having a wavelength on the range from 460 nm to 475 nm.

[0045] As illustrated in the section view 130, the red and infrared radiation components 127 and 128 reach arteries 111 located in the hypodermis 114, the yellow radiation component 129 reach blood vessels located in a portion of the hypodermis 114 adjacent to the dermis 115, the green radiation component 130 reach arterioles 112 located in the dermis 115, and the blue radiation component 131 reach capillaries 112 located in a portion of the dermis 115 adjacent to the epidermis 116. Therefore, shorter wavelengths relate to smaller blood vessels, i.e. blood vessels nearer to a skin surface, than longer wavelengths. In the exemplifying apparatus illustrated in FIG. 1a, the photodetector 107 of the PPG sensor 101 is configured produce a measurement signal that comprises wavelength-specific component signals indicative of received wavelengths reflected off the blood vessels. FIG. 1b shows graphs illustrating exemplifying wavelength-specific component signals 117, 118, 119, 120, and 121 of an exemplifying measurement signal 122. The wavelength-specific component signal 117 corresponds to infrared radiation, the wavelength-specific component signal 118 corresponds to red radiation, the wavelength-specific component signal 119 corresponds to yellow radiation, the wavelength-specific component signal 120 corresponds to green radiation, and the wavelength-specific component signal 121 corresponds to blue radiation. The photodetector 107 may comprise for example many photodiodes or phototransistors that are sensitive to different wavelengths, or the photodetector 107 may comprise filters to implement wavelength separation.

[0046] The apparatus comprises a pressure instrument 102 configured to produce mechanical pressure P applied on the blood vessels. The apparatus comprises a control system 103 configured to control the pressure instrument 102 to change, i.e. to decrease or increase, the mechanical pressure linearly with respect to time t during emission of the electromagnetic radiation to the blood vessels and reception of the reflected electromagnetic radiation from the blood vessels. FIG. 1b shows a line 132 illustrating the time dependence of the mechanical pressure in the exemplifying case in which the mechanical pressure has been linearly decreased and the above-mentioned wavelength-specific component signals 117-121 have been measured. The pressure values P1, P2, and P3 can be e.g. 127 mmHg, 100 mmHg, and 81 mmHg, respectively.

[0047] In the exemplifying apparatus illustrated in FIG. 1a, the pressure instrument 102 comprises a pressure sensor 109 for measuring the mechanical pressure P directed by the fingertip 108 to the pressure sensor 108 and pressing means for controllably pressing the fingertip 108 against the PPG sensor 101 and the pressure sensor 109. In this exemplifying apparatus, the pressing means comprise a pressing element 105 and a force generator 104 for directing force to the pressing element 105. The force generator 104 may comprise for example an electric stepper motor and a threaded rod or some other suitable elements for generating force.

[0048] The control system 103 is configured to find, from each wavelength-specific component of the measurement signal, a portion whose envelope has exponential change, i.e. exponential growth or exponential decrease, with respect to time and to produce an estimate for a coefficient of time related to the exponential change. The coefficient of time is indicative of the compliance of the blood vessels and thereby also the stiffness of the blood vessels.

[0049] As mentioned above, in the exemplifying case illustrated in FIG. 1b, the mechanical pressure is linearly decreased and thus each of the wavelength-specific component signals 117-121 has a portion whose envelope has exponential growth. FIG. 1b shows a magnification of a part of the wavelength-specific component signal 119. In FIG. 1b, the envelopes of the wavelength-specific component signals 117-121 are depicted with thick lines. The exponential growth ?e.sup.?Yt on a part of the envelope of the wavelength-specific component signal 119 is depicted with a dashed line. In this case, the coefficient of time related to the exponential growth is ?.sub.Y. Thus, the coefficient of time ?.sub.Y is indicative of the compliance of the blood vessels and thereby also the stiffness of the blood vessels from which yellow light is reflected off.

[0050] There are many ways to find the portion whose envelope has the exponential change and to produce the estimate for the coefficient of time related to the exponential change. For example, curve fitting based on e.g. the least-mean-square LMS method can be used. Thus, apparatuses according to embodiments of the invention are not limited to any specific ways to find the portion whose envelope has the exponential change and to produce the estimate for the coefficient of time related to the exponential change.

[0051] In an apparatus according to an exemplifying and non-limiting embodiment, the control system 103 is configured to convert the wavelength-specific components 117-121 of the measurement signal to a logarithmic scale. FIG. 1c shows graphs illustrating the converted wavelength-specific component signals 117, 118, 119, 120, and 121 which corresponds to the wavelength-specific component signals 117, 118, 119, 120, and 121 shown in FIG. 1b, respectively. Due to the logarithmic conversion, the exponential growth is converted into linear growth that is depicted with lines 117, 118, 119, 120, and 121 shown in FIG. 1c. Concerning the linear growth, the control system 103 is configured to produce an estimate for a slope of the envelope of each converted wavelength-specific component signal. The slope is the coefficient of time related to the exponential growth. In the exemplifying case shown in FIGS. 1b and 1c, the slope i.e. the coefficient of time related to the infrared light is UR, the coefficient of time related to the red light is ?.sub.R, the coefficient of time related to the yellow light is ay, the coefficient of time related to the green light is ac, and the coefficient of time related to the blue light is ?.sub.B. Thus, for example, ?.sub.R is the indicative of compliance of arteries and thereby also the stiffness of the arteries, ac is indicative of compliance of arterioles and thereby also the stiffness of the arterioles, and ?.sub.B is indicative of compliance of capillaries and thereby also the stiffness of the capillaries.

[0052] In an apparatus according to an exemplifying and non-limiting embodiment, the control system 103 is configured to compute a ratio of at least one pair of the coefficients of time corresponding to different wavelengths. Each ratio expresses a stiffness mismatch between blood vessels having different sizes where a shorter wavelength relates to smaller blood vessels and a longer wavelength relates to larger blood vessels. For example, the ratio ?.sub.R/?.sub.G is indicative of the stiffness mismatch between arteries and arterioles.

[0053] In the exemplifying apparatus illustrated in FIG. 1a, the PPG sensor 101 is configured to emit and receive different wavelengths simultaneously. It is however also possible that the control system of an apparatus according to an exemplifying and non-limiting embodiment is configured to control the PPG sensor to variate the wavelength of the electromagnetic radiation, and to produce the coefficients of time for different wavelengths successively.

[0054] FIG. 2 shows a schematic illustration of an apparatus according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels. The apparatus comprises a photoplethysmography PPG sensor 201 for emitting electromagnetic radiation and for receiving a part of the electromagnetic radiation reflected off blood vessels of an arm. The PPG sensor 201 is configured to produce a measurement signal indicative of the received part of the electromagnetic radiation. The apparatus comprises a pressure instrument 202 configured to produce mechanical pressure applied on the blood vessels. The apparatus comprises a control system 203 configured to control the pressure instrument 202 to change the mechanical pressure linearly with respect to time during emission of the electromagnetic radiation to the blood vessels and reception of the reflected electromagnetic radiation from the blood vessels. The control system 203 is configured to find, from the above-mentioned measurement signal, a portion whose envelope has exponential change, ?e.sup.?t or ?e.sup.??t, with respect to time and to produce an estimate for a coefficient ? of time related to the exponential change. The coefficient ? of time is indicative of the compliance of the blood vessels and thereby indicative of the stiffness of the blood vessels.

[0055] In the exemplifying apparatus illustrated in FIG. 2, the pressure instrument 202 comprises a cuff and a pump system 242 configured to control gas pressure inside the cuff to direct the mechanical pressure to the arm and to change the mechanical pressure when the PPG sensor 201 emits and receives the electromagnetic radiation to and from the arm. The PPG sensor is located on an inner surface of the cuff.

[0056] Each of the control systems 103 and 203 shown in FIGS. 1a and 2 can be implemented for example with 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. Each of the control systems 103 and 203 may further comprise memory implemented for example with one or more memory circuits each of which can be e.g. a random-access memory RAM device.

[0057] FIG. 3 shows a flowchart of a method according to an exemplifying and non-limiting embodiment for measuring compliance of blood vessels. The method comprises the following actions: [0058] action 301: emitting electromagnetic radiation to the blood vessels, [0059] action 302: receiving a part of the electromagnetic radiation reflected off the blood vessels, [0060] action 303: producing a measurement signal indicative of the received part of the electromagnetic radiation, [0061] action 304: producing mechanical pressure applied on the blood vessels and changing linearly with respect to time t during the emitting the electromagnetic radiation to the blood vessels and the receiving the part of the electromagnetic radiation reflected off the blood vessels, [0062] action 305: finding, from the measurement signal, a portion whose envelope has exponential change, ?e.sup.?t or ?e.sup.??t, with respect to time, and [0063] action 306: producing an estimate for a coefficient ? of time related to the exponential change, the coefficient of time being indicative of the compliance of the blood vessels.

[0064] In a method according to an exemplifying and non-limiting embodiment, the electromagnetic radiation has wavelengths selected from at least two of the following ranges: from 625 nm to 1000 nm, from 565 nm to 590 nm, from 500 nm to 565 nm, and from 450 nm to 485 nm.

[0065] In a method according to an exemplifying and non-limiting embodiment, the electromagnetic radiation has different wavelengths and the measurement signal comprises wavelength-specific component signals being indicative of received wavelengths reflected off the blood vessels, and the coefficient of time is produced for each of the wavelength-specific component signals corresponding to the different wavelengths where a shorter one of the wavelengths relates to smaller ones of the blood vessels than a longer one of the wavelengths.

[0066] In a method according to an exemplifying and non-limiting embodiment, the wavelength of the electromagnetic radiation is varied, and the coefficient of time is produced for each value of the wavelength where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

[0067] A method according to an exemplifying and non-limiting embodiment comprises computing a ratio of at least one pair of the coefficients of time corresponding to different wavelengths, each ratio expressing a stiffness mismatch between ones of the blood vessels having different sizes where a shorter wavelength relates to smaller ones of the blood vessels than a longer wavelength.

[0068] A method according to an exemplifying and non-limiting embodiment comprises converting the measurement signal to a logarithmic scale, finding from the converted measurement signal a portion whose envelope has linear change with respect to time, and producing an estimate for a slope of the envelope of the converted measurement signal related to the linear change. The slope of the linear change is the coefficient of time related to the exponential change.

[0069] In a method according to an exemplifying and non-limiting embodiment, the mechanical pressure is directed to a fingertip or a toe of an individual.

[0070] In a method according to an exemplifying and non-limiting embodiment, the mechanical pressure is directed to an arm of an individual with a cuff and a pump system configured to control gas pressure inside the cuff. In this exemplifying case, the photoplethysmography sensor is located on an inner surface of the cuff.

[0071] A computer program according to an exemplifying and non-limiting embodiment comprises computer executable instructions for controlling a programmable processing system to carry out actions related to a method according to any of the above-described exemplifying and non-limiting embodiments.

[0072] A computer program according to an exemplifying and non-limiting embodiment comprises software modules for measuring compliance of blood vessels. The software modules comprise computer executable instructions for controlling a programmable processing system to: [0073] control a photoplethysmography PPG sensor to emit electromagnetic radiation to the blood vessels, to receive a part of the electromagnetic radiation reflected off the blood vessels, and to produce a measurement signal indicative of the received part of the electromagnetic radiation, [0074] control a pressure instrument to produce mechanical pressure applied on the blood vessels and to change the mechanical pressure linearly with respect to time t during emission of the electromagnetic radiation to the blood vessels and reception of the part of the electromagnetic radiation reflected off the blood vessels, [0075] find, from the measurement signal, a portion whose envelope has exponential change, ?e.sup.?t or ?e.sup.??t, with respect to time, and [0076] produce an estimate for a coefficient ? of time related to the exponential change, the coefficient of time being indicative of the compliance of the blood vessels.

[0077] The software modules can be for example subroutines or functions implemented with programming tools suitable for the programmable processing equipment.

[0078] A computer program product according to an exemplifying and non-limiting embodiment comprises a computer readable medium, e.g. a compact disc CD, encoded with a computer program according to an exemplifying embodiment.

[0079] A signal according to an exemplifying and non-limiting embodiment is encoded to carry information defining a computer program according to an exemplifying embodiment.

[0080] A computer program according to an exemplifying and non-limiting embodiment may constitute e.g. a part of a software of a mobile device, e.g. a smart phone or a wearable device.

[0081] The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.