Apparatus and method for measuring vascular flow resistance

Abstract

The apparatus for measuring the flow resistance of a blood vessel includes a photoplethysmography sensor (101) that emits electromagnetic radiation having various wavelengths into the blood vessel, a pressure device (102) that applies a controllable mechanical pressure to the blood vessel, and a control system (103) that estimates the compliance of the blood vessel based on the electromagnetic radiation reflected by the blood vessel. Shorter wavelengths of electromagnetic radiation are reflected by smaller blood vessels compared to longer wavelengths. The control system optimizes resistor values ​​of the circuit model to minimize the difference between the node voltage waveforms of the circuit model and the measured blood pressure waveform prevailing in the blood vessel that reflects the various wavelengths. The capacitor values ​​of the circuit model are based on the estimated compliance of the blood vessel, and the optimized resistor values ​​of the circuit model are indicative of the flow resistance of the blood vessel.

Description

[Technical field]

[0001] The present disclosure relates to an apparatus and method for measuring flow resistance in a blood vessel, and further to a computer program for measuring flow resistance in a blood vessel. [Background technology]

[0002] Quantitative assessment of vascular pathology and its progression often involves the measurement and estimation of various quantitative criteria, such as blood pressure, vascular cross-sectional area, vascular mean diameter, vascular stiffness, and vascular flow resistance. For example, abnormal vascular stiffness and flow resistance have been associated with an increased risk of cardiovascular events, such as myocardial infarction and stroke, which are the two leading causes of death in the developed world. Increased arterial stiffness and flow resistance can increase the workload of the heart, as the heart must do more work to maintain the required blood flow. Over time, this increased workload can lead to left ventricular hypertrophy and left ventricular remodeling, which can lead to heart failure. Increased workload can also be associated with higher heart rates, proportionally longer systolic periods, and reduced diastolic periods. This reduces the amount of time available for perfusion of cardiac tissue, which occurs primarily during diastole. Thus, an enlarged heart with greater oxygen demand can reduce the supply of oxygen and nutrients.

[0003] For the above mentioned kind of reasons, several techniques have been developed to measure and / or estimate various quantitative criteria of blood vessels. Jukka-Pekka Sirkia et al.: A Multi-Wavelength Photoplethysmography Device for Measuring Pulse Wave Transit Time in Cutaneous Microvasculature, Computing in Cardiology, September 16, 2020, describes a multi-wavelength photoplethysmography (NWPPG) device for studying the dermal microvasculature. The device exploits the fact that the penetration depth of light into the skin depends on the light wavelength. Thus, the device makes it possible to study blood vessels at different depths.

[0004] US Patent Application Publication No. 2017172430 describes a method for cuffless blood pressure measurement that includes recording a physiological signal and a multi-wavelength photoplethysmography "PPG" signal from a given body part, deriving a depth-specific PPG signal reflective of arterial blood volume with reference to the physiological signal, calculating a pulse wave transit time "PTT" from the physiological signal and the derived arterial blood PPG signal, and calculating blood pressure from the relationship between the calculated PTT and blood pressure.

[0005] US Patent Application Publication No. 2019336016 describes a device for non-invasive capillary blood pressure measurement, which includes a front end that contacts the body to pressurize and depressurize the capillaries in the tissue, a pressure control module that adjusts the contact pressure between the front end and the tissue, a pressure transducer connected to the front end to measure the contact pressure, a capillary sensing module that detects capillary pulsation under contact pressure regulation, and a computing system that executes an algorithm to determine the capillary pressure based on the capillary pulsation and the contact pressure regulation.

[0006] However, a need remains for a technique that rapidly and cost-effectively measures vascular flow resistance. Summary of the Invention

[0007] 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 not intended to identify key or critical elements of the invention, nor is it intended 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 example embodiments of the invention.

[0008] According to the present invention, a new device for measuring the flow resistance of a blood vessel is provided. - a photoplethysmography "PPG" sensor configured to emit electromagnetic radiation into a blood vessel, receive a portion of the electromagnetic radiation reflected from the blood vessel, and generate a measurement signal indicative of the received portion of the electromagnetic radiation, the electromagnetic radiation having various wavelengths, the measurement signal comprising wavelength specific component signals indicative of the wavelengths reflected and received from the blood vessel, shorter ones of the wavelengths being reflected from smaller ones of the blood vessels compared to longer ones of the wavelengths; a pressure device configured to generate a controllable mechanical pressure applied to a blood vessel; a control system configured to find from each of the wavelength-specific component signals a portion where the envelope, i.e. the curve outlining the extrema of the wavelength-specific component signals, has an exponential change with time, i.e. an exponential growth or an exponential decrease, when the mechanical pressure is changing linearly with time, i.e. decreasing or increasing, and to generate for each of the wavelength-specific component signals an estimate of the time coefficient of the exponential change, the time coefficient being indicative of the compliance of the blood vessel reflecting the various wavelengths; Equipped with.

[0009] The control system is configured to optimize a resistance value of the circuit model to minimize a difference between a node voltage waveform of the circuit model and a measured waveform of blood pressure in blood vessels reflecting various wavelengths among the wavelengths, the capacitor value of the circuit model being based on the above-mentioned compliance of the blood vessels reflecting the various wavelengths, and the optimized resistance value represents the flow resistance of the blood vessels reflecting the various wavelengths.

[0010] The pressure device described above can be, for example, a device that directs mechanical pressure to a fingertip or toe, or a device that includes a cuff and a pump system that controls the gas pressure inside the cuff to direct mechanical pressure to the arm.

[0011] Photoplethysmography "PPG" sensors may be configured to emit electromagnetic radiation having a wavelength ranging from 625 nm to 1000 nm, i.e., red or infrared light, for measuring the flow resistance of arteries located in the subcutaneous tissue, to emit electromagnetic radiation having a wavelength ranging from 565 nm to 590 nm, i.e., yellow light, for measuring the flow resistance of blood vessels located in the upper portion of the subcutaneous tissue, to emit electromagnetic radiation having a wavelength ranging from 500 nm to 565 nm, i.e., green light, for measuring the flow resistance of small arteries located in the dermis, and to emit electromagnetic radiation having a wavelength ranging from 450 nm to 485 nm, i.e., blue light, for measuring the flow resistance of capillaries located in the upper portion of the dermis.

[0012] The present invention also provides a new method for measuring vascular flow resistance, comprising the steps of: - emitting electromagnetic radiation into the blood vessels such that the electromagnetic radiation has different wavelengths; receiving a portion of the electromagnetic radiation reflected by the blood vessel; - generating a measurement signal indicative of a received portion of the electromagnetic radiation such that the measurement signal comprises wavelength-specific component signals indicative of wavelengths reflected from the blood vessels and received, the shorter ones of the wavelengths being reflected from smaller ones of the blood vessels compared to the longer ones of the wavelengths; generating a mechanical pressure applied to a blood vessel; - finding from each of the wavelength-specific component signals a portion whose envelope has an exponential variation with time when the mechanical pressure varies linearly with time; - generating an estimate of the time coefficient of exponential change for each of the wavelength-specific component signals, the time coefficient being indicative of the compliance of the blood vessel reflecting the wavelength under consideration; - optimizing the resistance value of the circuit model to minimize the difference between the waveform of the node voltage of the circuit model and the measured waveform of the blood pressure in the blood vessel reflecting various wavelengths of the wavelengths, the capacitor value of the circuit model being based on the compliance of the blood vessel, the optimized resistance value being indicative of the flow resistance of the blood vessel; Includes.

[0013] According to the present invention, there is also provided a new computer program for measuring the flow resistance of a blood vessel. - controlling a photoplethysmography sensor to emit electromagnetic radiation into a blood vessel, to receive a portion of the electromagnetic radiation reflected by the blood vessel, and to generate a measurement signal indicative of the received portion of the electromagnetic radiation, the electromagnetic radiation having various wavelengths, the measurement signal comprising wavelength specific component signals indicative of the wavelengths reflected and received by the blood vessel, shorter ones of the wavelengths being reflected by smaller ones of the blood vessels compared to longer ones of the wavelengths; - controlling a pressure device to generate a mechanical pressure applied to a blood vessel; - controlling the programmable processing system to find from each of the wavelength-specific component signals a portion where the envelope has an exponential change with time when the mechanical pressure varies linearly with time, and to generate for each of the wavelength-specific component signals an estimate of the time coefficient of the exponential change, the time coefficient being indicative of the compliance of the blood vessel reflecting the wavelength under consideration; - optimizing the resistance value of the circuit model to minimize the difference between the waveform of the node voltage of the circuit model and the measured waveform of the blood pressure in the blood vessel reflecting various wavelengths of the wavelengths, the capacitor value of the circuit model being based on the compliance of the blood vessel, the optimized resistance value being indicative of the flow resistance of the blood vessel; The method comprises the steps of: providing a programmable processing system having computer-executable instructions for controlling the programmable processing system to perform the steps of the method;

[0014] According to the invention there is also provided a novel computer program product comprising a non-volatile computer readable medium, for example a compact disc "CD", encoded with a computer program according to the invention.

[0015] Exemplary and non-limiting embodiments are set forth in the accompanying dependent claims.

[0016] Various exemplary and non-limiting embodiments, both as to structure and method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplary embodiments when read in connection with the accompanying drawings.

[0017] The verbs "comprise" and "include" are used herein as open limitations which neither exclude nor require the presence of unrecited features.

[0018] The features recited in the accompanying dependent claims are mutually freely combinable, unless expressly stated otherwise.

[0019] Furthermore, it will be understood that throughout this specification the use of "a" or "an", i.e. the singular forms, does not exclude a plurality. [Brief description of the drawings]

[0020] Exemplary and non-limiting embodiments and their advantages are described in more detail below with reference to the accompanying drawings. [Figure 1a] 1 shows an apparatus for measuring flow resistance in a blood vessel according to an exemplary and non-limiting embodiment. [Figure 1b] 1 shows an exemplary graph illustrating electromagnetic radiation having various wavelengths reflected from blood vessels in a fingertip as a function of time. [Figure 1c] 1B shows an exemplary graph converted to a logarithmic vertical scale from FIG. [Figure 1d] 1 shows an exemplary circuit model used to estimate vascular flow resistance. [Diagram 2]1 shows an apparatus for measuring flow resistance in a blood vessel according to an exemplary and non-limiting embodiment. [Diagram 3] 1 shows a flowchart of a method for measuring flow resistance in a blood vessel according to an exemplary and non-limiting embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The specific examples provided in the following description should not be construed as limiting the scope and / or applicability of the appended claims. The lists and groups of examples provided in the description are not exhaustive unless expressly stated otherwise.

[0022] FIG. 1a shows a schematic diagram of an exemplary and non-limiting embodiment of a device for measuring blood vessel flow resistance. The device comprises a photoplethysmography "PPG" sensor 101 that emits electromagnetic radiation onto a person's fingertip 108 and receives a portion of the electromagnetic radiation reflected by blood vessels at the fingertip 108. The PPG sensor 101 comprises a radiation emitter 106 and a photodetector 107. The radiation emitter 106 may comprise, for example, one or more light emitting diodes "LEDs" and the photodetector 107 may comprise, for example, one or more photodiodes or phototransistors. FIG. 1a also shows an enlarged schematic cross-sectional view 110 of the fingertip. The cross-section is parallel to the yz-plane of a coordinate system 199.

[0023] In the exemplary device shown in FIG. 1a, the PPG sensor 101 is configured to emit electromagnetic radiation such that the electromagnetic radiation includes radiation components having five different wavelengths. In the cross-sectional view 110, the radiation components are depicted by polyline arrows 127, 128, 129, 130, and 131. The first radiation component 127 may be, for example, infrared radiation having a wavelength in the range of 700 nm to 1000 nm, the second radiation component 128 may be, for example, red radiation having a wavelength in the range of 625 nm to 700 nm, the third radiation component 129 may be, for example, yellow radiation having a wavelength in the range of 565 nm to 590 nm, the fourth radiation component 130 may be, for example, green radiation having a wavelength in the range of 500 nm to 565 nm, and the fifth radiation component 131 may be, for example, blue radiation having a wavelength in the range of 450 nm to 485 nm. For another example, first radiation component 127 may be infrared radiation having a wavelength in the range of 800 nm to 900 nm, second radiation component 128 may be red radiation having a wavelength in the range of 650 nm to 675 nm, third radiation component 129 may be yellow radiation having a wavelength in the range of 575 nm to 580 nm, fourth radiation component 130 may be green radiation having a wavelength in the range of 530 nm to 545 nm, and fifth radiation component 131 may be blue radiation having a wavelength in the range of 460 nm to 475 nm.

[0024] As shown in cross-sectional view 130, red and infrared radiation components 127 and 128 reach arteries 111 located in subcutaneous tissue 114, yellow radiation component 129 reaches blood vessels located in a portion of subcutaneous tissue 114 adjacent to dermis 115, green radiation component 130 reaches arterioles 112 located in dermis 115, and blue radiation component 131 reaches capillaries 112 located in a portion of dermis 115 adjacent to epidermis 116. Thus, shorter wavelengths are reflected by smaller blood vessels, i.e., blood vessels closer to the skin surface, than longer wavelengths. Photodetector 107 of PPG sensor 101 is configured to generate a measurement signal, which comprises wavelength-specific component signals indicative of wavelengths reflected and received by blood vessels. FIG. 1b shows a graph illustrating exemplary wavelength-specific component signals 117, 118, 119, 120, and 121 of an exemplary measurement signal 122. Wavelength-specific component signal 117 corresponds to infrared radiation, wavelength-specific component signal 118 corresponds to red radiation, wavelength-specific component signal 119 corresponds to yellow radiation, wavelength-specific component signal 120 corresponds to green radiation, and wavelength-specific component signal 121 corresponds to blue radiation. Photodetector 107 may, for example, comprise a number of photodiodes or phototransistors responsive to different wavelengths, or photodetector 107 may comprise filters to perform wavelength separation.

[0025] The apparatus comprises a pressure device 102 configured to generate a controllable mechanical pressure P applied to the blood vessel. The apparatus comprises a control system 103 configured to control the pressure device 102 to vary, i.e. decrease or increase, the mechanical pressure linearly with respect to time t as electromagnetic radiation is emitted into the blood vessel and a reflected portion of the electromagnetic radiation is received from the blood vessel. Figure 1b shows a line 132 illustrating the time dependence of the mechanical pressure in an exemplary situation where the mechanical pressure is decreasing linearly and the above-mentioned wavelength-specific component signals 117-121 are measured.

[0026] 1a, the pressure device 102 comprises a pressure sensor 109 that measures a mechanical pressure P directed by a fingertip 108 to the pressure sensor 108, and pressure means for controllably pressing the fingertip 108 against the PPG sensor 101 and the pressure sensor 109. In this exemplary device, the pressure means comprises a pressure element 105 and a force generator 104 that directs a force to the pressure element 105. The force generator 104 may comprise, for example, an electric stepper motor and a threaded rod or any other element suitable for generating forces.

[0027] The control system 103 is configured to find, from each of the wavelength-specific component signals 117-121, a portion where the envelope has an exponential change with respect to time, i.e. an exponential growth or an exponential decrease, when the mechanical pressure is decreasing linearly with respect to time. The control system 103 is configured to generate, for each of the wavelength-specific component signals 117-121, an estimate for the coefficient of time of said exponential change. The coefficient of time is indicative of the compliance of the blood vessel reflecting the wavelength corresponding to the wavelength-specific component signal under consideration, and thereby also of the stiffness of said vessel.

[0028] As mentioned above, in the exemplary case shown in FIG. 1b, the mechanical pressure decreases linearly, and therefore each of the wavelength-specific component signals 117-121 has a portion whose envelope has exponential growth. FIG. 1b shows an expanded view of a portion of the wavelength-specific component signal 119. In FIG. 1b, the envelopes of the wavelength-specific component signals 117-121 are depicted in bold. The exponential growth in the portion of the envelope of the wavelength-specific component signal 119 is shown in FIG. αYt is depicted by a dashed line. In this case, the time coefficient for exponential growth is α Y Therefore, the time coefficient α Y indicates the compliance, and thereby also the stiffness, of the blood vessels through which the yellow light is reflected.

[0029] There are many ways to find the portion of the envelope that has an exponential change when the mechanical pressure is decreasing linearly and generate an estimate for the coefficient of time for the exponential change. For example, curve fitting based on the least mean squares "LMS" method may be used. Thus, the apparatus according to the embodiment of the present invention is not limited to any particular method of finding the portion of the envelope that has an exponential change and generate an estimate for the coefficient of time for the exponential change. For example, in the apparatus according to the exemplary 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 a graph illustrating the converted wavelength-specific component signals 117', 118', 119', 120', and 121', which correspond to the wavelength-specific component signals 117, 118, 119, 120, and 121 shown in FIG. 1b, respectively. The logarithmic transformation converts the exponential changes into linear changes depicted by lines 117'', 118'', 119'', 120'', and 121'' shown in FIG. 1c. The control system 103 is configured to estimate the slope of the linear change for the envelope of each transformed wavelength-specific component signal. The slope is the coefficient of time for the exponential change. In the exemplary case shown in FIGS. 1b and 1c, the slope, i.e., the coefficient of time, for infrared light is α IR and the time coefficient for red light is α R and the time coefficient for yellow light is α Y and the time coefficient for green light is α G where the time coefficient for blue light is α B Therefore, for example, α R indicates the compliance of the artery and therefore its stiffness, and α G indicates the compliance of the arteriole and therefore its stiffness, and α B indicates the compliance of the capillary and thereby also its stiffness.

[0030] The control system 103 is configured to optimize the resistance value of the circuit model to minimize the difference between the waveform of the node voltage of the circuit model and the measured waveform of the blood pressure prevailing in the blood vessel reflecting various of the wavelengths. The capacitor value of the circuit model is based on the above-mentioned compliance of the blood vessel, and the optimized resistance value indicates the flow resistance of the blood vessel. FIG. 1d shows an exemplary circuit model used to estimate the flow resistance of the blood vessel. In this exemplary case, the circuit model comprises three circuit segments 150, 151, and 152. The circuit segment 150 may model, for example, an artery located in the subcutaneous tissue, the circuit segment 151 may model, for example, a small artery located in the dermis, and the circuit segment 152 may model, for example, a capillary located in the upper part of the dermis. The node voltage U1 may model the blood pressure prevailing in the blood vessel reflecting infrared and red light, the node voltage U2 may model the blood pressure prevailing in the blood vessel reflecting green light, and the node voltage U3 may model the blood pressure prevailing in the blood vessel reflecting blue light. The node voltage U0 may have a waveform whose maximum value is the systolic measured blood pressure and whose minimum value is the diastolic measured blood pressure. The shape of the node voltage U0 waveform may be modeled, for example, as U0=V1_AC+V2_AC+V1_DC, where V1_AC=V1sin(2π×HR / 60×t), V2_AC=V2sin(2π×2HR / 60×t), where t is time in seconds, HR is the heart rate in beats per minute, and V1, V2, and V1_DC are selected such that the maximum value of U0 is the systolic measured blood pressure and the minimum value of U0 is the diastolic measured blood pressure. The voltage levels of the circuit model are preferably selected such that volts directly correspond to millimeters of mercury "mmHg". In an exemplary case, HR=60 / min, V1=10 volts, V2=10 volts, V1_DC=80 volts to 120 volts, e.g., 100 volts. In this exemplary case, U0 peak-to-peak is 25 to 40 volts, e.g., about 35 V. The voltage V2_DC models the blood pressure after the capillary tube, which may range from 0 to 10 volts, or from 3 to 8 volts.As mentioned above, the capacitor values ​​of the circuit model are based on the above-mentioned compliance of the blood vessel. In this exemplary case, the capacitor values ​​of the circuit model are, for example, C1,1 = C1,2 = α. IR / 2 or α R / 2, C2,1=C2,2=α G / 2, C3=α B It could be.

[0031] It should be noted that the above-mentioned method of modeling the input voltage U0 of the circuit model is merely an example. It is also possible to measure or estimate the waveform of the blood pressure in the first largest blood vessel modeled by the circuit model, e.g., an artery, and then use the measured or estimated blood pressure waveform as the input voltage U0 of the circuit model. Furthermore, the characteristic values, e.g., amplitude and phase, of the various sinusoidal AC and DC components of the input voltage U0 can be parameters that are optimized together with the resistance values ​​of the circuit model. Furthermore, the instantaneous values ​​that form the waveform of the input voltage U0 can be parameters that are optimized together with the resistance values ​​of the circuit model. Thus, the present invention is not limited to any particular method of obtaining the input voltage U0 of the circuit model. The measurements are preferably calibrated, e.g., so that the measured pressure values ​​correspond directly to the voltage values ​​of the circuit model.

[0032] In the device according to the exemplary and non-limiting embodiment, the control system 103 is configured to vary the resistance values ​​R1,1, R1,2, R2,1, R2,2, R3,1, and R3,2 as long as the sum of the squared time integrals of the difference between the waveforms of the node voltages U1, U2, and U3 and the measured waveform of the blood pressure is decreasing. The optimized resistance values ​​indicate the flow resistance of the blood vessels reflecting the various waveforms. In the above exemplary case, R1,1+R1,2 is the flow resistance of the artery, R2,1+R2,2 is the flow resistance of the arteriole, and R3,1+R3,2 is the flow resistance of the capillary. The resistance values ​​may be varied, for example, stochastically, in which case the optimization process is a simulated evolutionary process. It is noted that any suitable multivariate optimization technique may be used, and the invention is not limited to any particular multivariate optimization technique. It is further noted that the circuit model shown in FIG. 1d is merely a non-limiting example, and that various circuit models may be used in connection with various embodiments of the invention. For example, a blood vessel reflecting a given wavelength, such as an artery, may be modeled using only one resistor-capacitor "RC" loop, or using three or more cascaded RC loops.

[0033] The waveforms of the blood pressures prevailing in the blood vessels reflecting various wavelengths can be measured, for example, in the following manner: The maximum value of the blood pressures prevailing in the blood vessels reflecting a given wavelength is the reduced pressure, e.g., the value of pressure 132 in FIG. 1b, at which the corresponding wavelength-specific component signal 117, 118, 119, 120, or 121 has 50% of its maximum peak-to-peak value. For example, in the exemplary case shown in FIG. 1b, the maximum value of the blood pressures prevailing in the blood vessels reflecting infrared and red light is P1, the maximum value of the blood pressures prevailing in the blood vessels reflecting yellow light is P2, and the maximum value of the blood pressures prevailing in the blood vessels reflecting green light is P3. The pressure values ​​P1, P2, and P3 can be, for example, 127 mmHg (about 16.9 kPa), 100 mmHg (about 13.3 kPa), and 81 mmHg (about 10.8 kPa), respectively. The average value of the blood pressure prevailing in a vessel reflecting a given wavelength is the value of the reduced pressure 132 at which the corresponding wavelength-specific component signal 117, 118, 119, 120, or 121 has its maximum peak-to-peak value. The shape of the waveform of the blood pressure prevailing in a vessel reflecting a given wavelength is the shape of the corresponding wavelength-specific component signal 117, 118, 119, 120, or 121 when the mechanical pressure is kept constant, preferably zero. The waveform of each wavelength-specific component signal can then be scaled and shifted so that its maximum value is the aforementioned maximum value and its average value is the aforementioned average value. More information on how to measure the waveform of the blood pressure prevailing in a vessel reflecting various wavelengths can be found, for example, in the literature Panula, T. et al.: "Device for measuring blood pressure from the fingertip to assess cardiovascular health", Biosensors and Bioelectronics, Vol. 167, November 1, 2020, 112483. It should be noted that the present invention is not limited to any particular method of measuring the blood pressure waveform prevailing in a blood vessel that reflects various wavelengths.

[0034] In the exemplary device shown in FIG. 1a, the PPG sensor 101 is configured to emit and receive different wavelengths simultaneously. However, a control system of the device according to an exemplary and non-limiting embodiment may control the PPG sensor to vary the wavelength of electromagnetic radiation and to obtain α-phase measurements for the different wavelengths and the blood pressure waveforms prevailing in the blood vessels that reflect the different wavelengths. IR , α R , α Y , α G , and α B It is also possible to configure the time coefficients, such as:

[0035] 2 shows a schematic diagram of an exemplary, non-limiting embodiment of a device for measuring blood vessel flow resistance. The device comprises a photoplethysmography (PPG) sensor 201 that emits electromagnetic radiation having various wavelengths into the blood vessels of the arm and receives a portion of the electromagnetic radiation reflected by the blood vessels. The PPG sensor 201 is configured to generate a measurement signal indicative of the received portion of the electromagnetic radiation such that the measurement signal comprises wavelength-specific component signals indicative of the wavelengths reflected and received by the blood vessels, with shorter ones of the wavelengths being reflected by smaller ones of the blood vessels compared to longer ones of the wavelengths.

[0036] The device comprises a pressure device 202 configured to generate a controllable mechanical pressure applied to a blood vessel. The device comprises a control system 203 configured to control the pressure device 202 to decrease the mechanical pressure linearly with time when electromagnetic radiation is emitted to the blood vessel and reflected electromagnetic radiation is received from the blood vessel. The control system 203 is configured to find a portion of each wavelength-specific component signal where the envelope has an exponential change with time when the mechanical pressure is changing linearly with time, and generate an estimate for each wavelength-specific component signal for a coefficient of time for the exponential change. The coefficient of time is indicative of the compliance of the blood vessel reflecting the wavelength under consideration, and thereby the coefficient of time is indicative of the stiffness of the blood vessel. The control system 203 is configured to optimize a resistor value of the circuit model to minimize the difference between the waveform of the node voltage of the circuit model and the measured waveform of the blood pressure in the blood vessel reflecting the various ones of the wavelengths. The capacitor value of the circuit model is based on the above-mentioned compliance of the blood vessel, and the optimized resistor value is indicative of the flow resistance of the blood vessel.

[0037] 2, the pressure device 202 includes a cuff 241 and a pump system 242 configured to control the gas pressure inside the cuff, thereby controlling the mechanical pressure directed at the blood vessel when the PPG sensor 201 is emitting electromagnetic radiation to and receiving electromagnetic radiation from the arm. The PPG sensor is disposed on the inner surface of the cuff.

[0038] Each of the control systems 103 and 203 shown in Figures la and 2 may be implemented, for example, with one or more processor circuits, each of which may be a programmable processor circuit provided with appropriate software, a dedicated hardware processor, such as an application specific integrated circuit "ASIC," or a configurable hardware processor, such as a field programmable gate array "FPGA," etc. Each of the control systems 103 and 203 may further comprise a memory, for example implemented with one or more memory circuits, each of which may be, for example, a random access memory "RAM" device.

[0039] According to an exemplary, non-limiting embodiment, an apparatus for measuring flow resistance in a blood vessel includes a control system, the control system comprising: a)R cr is the autocorrelation of the arterial blood pressure waveform, and τ min is the value of the zero shift in the autocorrelation R cr (0), the minimum time shift in the autocorrelation that gives a peak in the autocorrelation, C is a calibration constant that can be determined, for example, experimentally, according to the formula CO=stroke volume "SV" x heart rate "HR" x α C ×(√(R cr (0))+√(-R cr (τ min ))) estimate the cardiac output "CO" according to b) estimating the mean arterial pressure "MAP" using any suitable known method; c) Calculate systemic vascular resistance (SVR) = MAP / CO. d) Calculate the blood flow through the tube, F = arterial blood pressure / SVR. e) It is configured to calculate the flow resistance R at a given point in the vasculature, e.g., an artery, an arteriole, or a capillary, = blood pressure at the given point / F.

[0040] 3 shows a flow chart of a method for measuring the flow resistance of a blood vessel according to an exemplary and non-limiting embodiment, the method comprising the following operations: - action 301: emitting electromagnetic radiation into a blood vessel such that the electromagnetic radiation has different wavelengths; Action 302: receiving a portion of electromagnetic radiation reflected by a blood vessel; - operation 303: generating a measurement signal indicative of a received portion of the electromagnetic radiation such that the measurement signal comprises wavelength-specific component signals indicative of wavelengths reflected from blood vessels and received, where shorter ones of the wavelengths are reflected from smaller ones of the blood vessels compared to longer ones of the wavelengths; Action 304: Generating a mechanical pressure applied to a blood vessel; - action 305: finding from each of the wavelength-specific component signals a portion whose envelope has an exponential change with respect to time when the mechanical pressure changes linearly with respect to time; - operation 306: generating an estimate of a time coefficient of exponential change for each of the wavelength-specific component signals, the time coefficient being indicative of compliance of a blood vessel reflecting the wavelength under consideration; - operation 307: optimizing a resistance value of the circuit model to minimize a difference between a waveform of node voltages of the circuit model and a measured waveform of a blood pressure prevailing in the blood vessel reflecting various wavelengths of the wavelengths, the capacitance value of the circuit model being based on the compliance of the blood vessel, the optimized resistance value being indicative of the flow resistance of the blood vessel; Includes.

[0041] In methods according to exemplary and non-limiting embodiments, the electromagnetic radiation has a wavelength selected from the following ranges: 625 nm to 1000 nm, 565 nm to 590 nm, 500 nm to 565 nm, and 450 nm to 485 nm.

[0042] A method according to an exemplary and non-limiting embodiment includes transforming a measurement signal to a logarithmic scale, finding from the transformed measurement signal a portion whose envelope has a linear variation with respect to time, and generating an estimate for the slope of the envelope of the transformed measurement signal with respect to the linear variation, the slope being the coefficient of time with respect to the exponential variation.

[0043] In a method according to an exemplary and non-limiting embodiment, mechanical pressure is directed to a person's fingertips or toes.

[0044] In a method according to an exemplary and non-limiting embodiment, mechanical pressure is directed to the person's arm using a cuff and a pump system configured to control gas pressure inside the cuff, in this exemplary case, a photoplethysmography sensor is disposed on the inner surface of the cuff.

[0045] In a method according to an exemplary and non-limiting embodiment, the resistance value of the circuit model is optimized by changing the resistance value as long as the sum of the squared time integral of the difference between the node voltage waveform and the measured blood pressure waveform is reduced.

[0046] According to an exemplary and non-limiting embodiment, a method for measuring flow resistance in a blood vessel comprises: a)R cr is the autocorrelation of the arterial blood pressure waveform, and τ min is the value of the zero shift in the autocorrelation R cr (0), the minimum time shift in the autocorrelation that gives a peak in the autocorrelation, C is a calibration constant that can be determined, for example, experimentally, according to the formula CO=stroke volume "SV" x heart rate "HR" x α C ×(√(R cr (0))+√(-R cr (τ min estimating the cardiac output "CO" according to b) estimating the mean arterial pressure "MAP" using any suitable known method; c) Calculate systemic vascular resistance (SVR) = MAP / CO; d) Calculating the blood flow through the vessel F=arterial blood pressure / SVR; e) calculating the flow resistance R at a given point in the vascular structure, e.g., an artery, an arteriole, or a capillary, = blood pressure at the given point / F; Includes.

[0047] A computer program according to the exemplary and non-limiting embodiments comprises computer-executable instructions for controlling a programmable processing system to perform operations related to the methods according to any of the exemplary and non-limiting embodiments described above.

[0048] According to an exemplary and non-limiting embodiment, a computer program comprises a software module for measuring the flow resistance of a blood vessel. The software module comprises: - controlling a photoplethysmography sensor to emit electromagnetic radiation into a blood vessel, to receive a portion of the electromagnetic radiation reflected by the blood vessel, and to generate a measurement signal indicative of the received portion of the electromagnetic radiation, the electromagnetic radiation having various wavelengths, the measurement signal comprising wavelength specific component signals indicative of the wavelengths reflected and received by the blood vessel, shorter ones of the wavelengths being reflected by smaller ones of the blood vessels compared to longer ones of the wavelengths; - controlling a pressure device to generate a mechanical pressure applied to a blood vessel; - controlling the programmable processing system to find from each of the wavelength-specific component signals a portion where the envelope has an exponential change with time when the mechanical pressure varies linearly with time, and to generate for each of the wavelength-specific component signals an estimate of the time coefficient of the exponential change, the time coefficient being indicative of the compliance of the blood vessel reflecting the wavelength under consideration; - optimizing the resistance value of the circuit model to minimize the difference between the waveform of the node voltage of the circuit model and the measured waveform of the blood pressure in the blood vessel reflecting various wavelengths of the wavelengths, the capacitor value of the circuit model being based on the compliance of the blood vessel, the optimized resistance value being indicative of the flow resistance of the blood vessel; The method comprises the steps of: providing a programmable processing system having computer-executable instructions for controlling the programmable processing system to perform the steps of the method;

[0049] A software module may be, for example, a subroutine or function implemented with suitable programming tools in a programmable processing system.

[0050] A computer program product according to an exemplary and non-limiting embodiment comprises a computer readable medium, for example a compact disc "CD", encoded with a computer program according to an exemplary embodiment.

[0051] A signal in accordance with the exemplary and non-limiting embodiments is encoded to carry information defining a computer program in accordance with the exemplary embodiments.

[0052] The computer program according to the exemplary and non-limiting embodiment may for example constitute part of the software of a mobile device, for example a smartphone or a wearable device.

[0053] According to an exemplary and non-limiting embodiment, the computer program comprises: a)R cr is the autocorrelation of the arterial blood pressure waveform, and τ min is the value of the zero shift in the autocorrelation R cr (0), the minimum time shift in the autocorrelation that gives a peak in the autocorrelation, C is a calibration constant that can be determined, for example, experimentally, according to the formula CO=stroke volume "SV" x heart rate "HR" x α C ×(√(R cr (0))+√(-R cr (τ min))) estimate the cardiac output "CO" according to b) Calculate systemic vascular resistance "SVR" = MAP / CO, where MAP is the mean arterial pressure determined using any suitable known method; c) Calculate the blood flow through the tube, F = arterial blood pressure / SVR. d) providing computer executable instructions for controlling the programmable processing system to calculate flow resistance R=blood pressure at a given point / F in a vasculature, e.g., an artery, arteriole, or capillary.

[0054] The specific examples provided in the description given above should not be construed as limiting the scope and / or applicability of the appended claims. The lists and groups of examples provided in the description given above are not exhaustive unless expressly stated otherwise.

Claims

1. A device for measuring vascular flow resistance, comprising: a photoplethysmography sensor (101, 201) configured to emit electromagnetic radiation into the blood vessel, receive a portion of the electromagnetic radiation reflected by the blood vessel, and generate a measurement signal (122) indicative of the received portion of the electromagnetic radiation, the electromagnetic radiation having various wavelengths, the measurement signal comprising wavelength-specific component signals (117-121) indicative of the wavelengths reflected and received by the blood vessel, shorter ones of the wavelengths being reflected by smaller ones of the blood vessels compared to longer ones of the wavelengths; a pressure device (102, 202) configured to generate a controllable mechanical pressure applied to the blood vessel; From each of the wavelength-specific component signals, it is possible to determine that the envelope changes exponentially with respect to time (t) (~e αＹｔ ) and find the time coefficient (α IR , α R , α Y , α G , α B and a control system (103, 203) configured to generate an estimate for each of the wavelength-specific component signals of a time constant, the coefficient of time being indicative of compliance of the blood vessel reflective of the wavelength under consideration, The control system is configured to optimize a resistance value of the circuit model to minimize a difference between a node voltage waveform of the circuit model and a measured waveform of blood pressure in the blood vessel reflecting various ones of the wavelengths, wherein a capacitor value of the circuit model is based on the compliance of the blood vessel, and the optimized resistance value represents the flow resistance of the blood vessel.

2. 2. The device of claim 1, wherein the photoplethysmography sensor (101) is configured to emit the electromagnetic radiation into the blood vessel such that the electromagnetic radiation has wavelengths selected from the following ranges: 625 nm to 1000 nm, 565 nm to 590 nm, 500 nm to 565 nm, and 450 nm to 485 nm.

3. The control system (103) converts the wavelength-specific component signals to a logarithmic scale, finds from each converted wavelength-specific component signal a portion where the envelope has a linear change with respect to time, and calculates a slope (α IR , α R , α Y , α G , α B 3. The apparatus of claim 1, further configured to generate an estimate for a wavelength-specific component signal (S1) of the wavelength-specific component signal under consideration, the slope being the coefficient of time for the exponential change of the wavelength-specific component signal under consideration.

4. The apparatus according to any one of claims 1 to 3, wherein the pressure device (102) comprises a force generator (104) and a pressure element (105) configured to direct the mechanical pressure to a fingertip or toe according to a control signal generated by the control system (103).

5. The apparatus of any one of claims 1 to 3, wherein the pressure device (202) comprises a cuff (241) and a pump system (242) configured to control gas pressure inside the cuff to direct the mechanical pressure to the arm.

6. 6. The apparatus of claim 1, wherein the control system (103) is configured to vary the resistance value as long as the sum of the time integral of the square of the difference between the waveform of the node voltage and the measured waveform of blood pressure is decreasing.

7. 1. A method for measuring vascular flow resistance, comprising: Emitting electromagnetic radiation into the blood vessel (301), such that the electromagnetic radiation has different wavelengths; receiving (302) a portion of the electromagnetic radiation reflected from the blood vessel; generating a measurement signal indicative of the received portion of the electromagnetic radiation, the measurement signal comprising wavelength specific component signals indicative of wavelengths reflected from the received blood vessels, shorter ones of the wavelengths being reflected from smaller ones of the blood vessels compared to longer ones of the wavelengths; generating a mechanical pressure applied to the blood vessel (304); From each of the wavelength-specific component signals, it was found that the envelope changes exponentially with respect to time (~e αＹｔ ) (305); and The time coefficient for the exponential change (α IR , α R , α Y , α G , α B ) for each of the wavelength specific component signals, where the coefficient of time is indicative of compliance of the blood vessel reflective of the wavelength under consideration (306). The method includes optimizing a resistance value of a circuit model to minimize a difference between a node voltage waveform of the circuit model and a measured waveform of blood pressure in the blood vessel reflecting various ones of the wavelengths, wherein a capacitor value of the circuit model is based on the compliance of the blood vessel, and the optimized resistance value represents the flow resistance of the blood vessel (307).

8. 8. The method of claim 7, wherein the electromagnetic radiation has a wavelength selected from the following ranges: 625 nm to 1000 nm, 565 nm to 590 nm, 500 nm to 565 nm, and 450 nm to 485 nm.

9. The method includes converting the wavelength-specific component signals to a logarithmic scale, finding from each converted wavelength-specific component signal a portion where the envelope has a linear variation with respect to time, and calculating a slope (α IR , α R , α Y , α G , α B 9. The method of claim 7, comprising: generating an estimate for the slope of the wavelength-specific component signal under consideration;

10. The method according to any one of claims 7 to 9, wherein the mechanical pressure is directed to the fingertips or toes.

11. 11. The method of claim 7, wherein the method comprises varying the resistance as long as the sum of the time integral of the square of the difference between the waveform of the node voltages and the measured waveform of the blood pressure is decreasing.

12. A computer program for measuring vascular flow resistance, comprising: controlling a photoplethysmography sensor to emit electromagnetic radiation into the blood vessels, to receive a portion of the electromagnetic radiation reflected by the blood vessels, and to generate a measurement signal indicative of the received portion of the electromagnetic radiation, the electromagnetic radiation having various wavelengths, the measurement signal comprising wavelength specific component signals indicative of wavelengths reflected and received by the blood vessels, shorter ones of the wavelengths being reflected by smaller ones of the blood vessels compared to longer ones of the wavelengths; controlling a pressure device to generate a mechanical pressure applied to the blood vessel; A programmable processing system is controlled to extract from each of the wavelength-specific component signals an envelope that exhibits an exponential change with time (t) (~e αＹｔ ) and find the time coefficient (α IR , α R , α Y , α G , α B 16. A computer program comprising computer executable instructions for controlling a programmable processing system to generate an estimate of a compliance of the blood vessel reflective of the wavelength under consideration for each of the wavelength specific component signals, the coefficient of time being indicative of the compliance of the blood vessel reflective of the wavelength under consideration, the computer program comprising: the computer program comprising computer executable instructions for controlling the programmable processing system to optimize a resistance value of a circuit model to minimize a difference between a node voltage waveform of the circuit model and a measured waveform of blood pressure in the blood vessel reflecting various ones of the wavelengths, wherein a capacitor value of the circuit model is based on the compliance of the blood vessel, and the optimized resistance value is indicative of the flow resistance of the blood vessel.

13. 13. A computer program product comprising a non-transitory computer readable medium encoded with the computer program of claim 12.

Images