Determining tissue properties of a body part

Abstract

According to one aspect, a computer-implemented method for determining tissue properties of a body part is provided, the method comprising the steps of controlling a shell cuff device to apply a plurality of different pressures to the body part, the shell cuff device at least partially surrounding the body part, the shell cuff device having a rigid shell portion disposed at least partially around the body part and an inflatable cuff surrounding the rigid shell portion that inflates and / or deflates to apply a plurality of different pressures to the body part via the rigid shell portion, obtaining radial measurements of a change in radius of the body part as the plurality of different pressures are applied to the body part, obtaining pressure measurements of the different pressures applied to the body part, and determining tissue properties of the body part from the radial and pressure measurements.

Description

[Technical field]

[0001] The present disclosure relates to the use of shell cuff apparatus for determining tissue properties of a body part, and in particular to computer-implemented methods, devices and systems for determining tissue properties of a body part. [Background technology]

[0002] Blood pressure (BP) is an important indicator of health in a person / subject. It is estimated that about 30% of the adult population in the United States has high blood pressure. High blood pressure is a common health problem with no obvious overt symptoms. Blood pressure generally increases with age, and older people are at a significant risk of developing high blood pressure. Persistent hypertension is one of the important risk factors for stroke, heart failure and increased mortality. A subject's condition can be improved by lifestyle changes, healthy dietary choices and drug therapy. Continuous 24-hour blood pressure monitoring is very important, especially for high-risk patients, using a system that does not interfere with normal daily activities of life. Continuous monitoring of blood pressure is also useful for patients in a medical environment, such as a hospital, for example, in the operating room (OR) or intensive care unit (ICU).

[0003] In some cases, absolute measurements of blood pressure are obtained, and in other cases, relative measurements of blood pressure can be obtained, e.g., measurements of changes in blood pressure, particularly where blood pressure changes over short time windows, e.g., on the order of minutes, and these changes are relevant for further medical examination and possibly medical intervention.

[0004] There are many different techniques available for measuring blood pressure and / or changes in blood pressure. Some of these techniques measure blood pressure itself, whereas others measure other physiological characteristics of the subject and use these physiological characteristics as surrogates for blood pressure, e.g., by relating changes or values ​​of these physiological characteristics to changes or values ​​of blood pressure. Some of the techniques for directly measuring blood pressure require invasive access to the subject's arteries, e.g., via a catheter, or the use of bulky / inconvenient equipment, e.g., inflatable cuffs. However, some of the physiological characteristics used as surrogates for blood pressure can be measured using simple and / or unobtrusive sensors applied to the subject's body.

[0005] WO 2014 / 121945 describes a blood pressure measurement system with a kink-proof shell. The blood pressure measurement system comprises a pressure means arranged to surround a body part of a patient for applying pressure to the body part and a kink-proof shell located between the pressure means and the body part. A pressure transducer or pressure sensor pad is provided on the inner surface of the shell. This arrangement, called a "shell cuff" or "shell cuff arrangement", essentially prevents attenuation of the pulsatile signal by the components of the device and / or by the air outside the device, which translates into high signal quality and allows measuring blood pressure with a similar accuracy to invasive techniques that use a catheter to directly access the blood. This is achieved by hydraulically optimized contact of the pressure transducer or pressure sensor pad with the body part suitable for the measurement.

[0006] 1 is a simplified diagram showing a shell cuff device 2 that is placed at least partially around a body part 4, in this case the arm. The shell cuff device 2 includes a hard shell that is surrounded by an inflatable cuff, and a sensor pad 6 is placed inside the hard shell to measure a pulse wave 8 passing through a blood vessel 10 (artery) in the body part 4. The pulse wave 8 creates pressure waves in the tissue of the body part 4 that are measured by the sensor pad 6.

[0007] More specifically, a pulse wave 8 in the brachial artery 10 causes a change in the movement of the skin. This change in the movement of the skin causes a compression of the liquid in the sensor pad 6. The liquid in the sensor pad 6 can be, for example, a silicone oil such as AK10. The sensor pad 6 is connected to a pressure sensor (not shown in FIG. 1 ) that measures the pressure signal through a flexible tube filled with a liquid similar to the sensor pad 6. In the current application, the liquid in the pressure sensor is a different liquid than the liquid in the sensor pad 6 for compatibility reasons. For example, the liquid in the pressure sensor can be glycerol.

[0008] The fluid filled sensor pad 6 is enclosed within the shell cuff device 2 by a hard shell, providing rigidity and causing a sufficient signal to be measured by the pressure sensor. As with a standard blood pressure cuff, there is an actuator (e.g., an air pump) that can fill the cuff with air, compressing the hard shell and therefore the arm tissue, resulting in the closure of the brachial artery.

[0009] The blood pressure measurements made by the shell cuff device 2 need to be calibrated against blood pressure measurements taken using invasive techniques, which are taken simultaneously. Since tissue stiffness affects the transmission of pressure signals from the artery 10 to the outside of the body part 4, said calibration can be done for a wide range of patients with different levels of tissue stiffness. Because the shell cuff device 2 includes a rigid shell, different sizes of shell cuffs are provided to accommodate patients (subjects) with different arm diameters. Summary of the Invention [Problem to be solved by the invention]

[0010] Currently, the effects of individual variations in tissue properties cannot be taken into account in measurements taken with shell cuff devices. Instead, variations in tissue properties are only taken into account according to the expected average tissue properties for a particular size shell cuff. However, for a single size shell cuff, there are significant differences in tissue properties between subjects. For example, subjects with large obese arms will have different tissue properties compared to subjects with similar size muscular arms.

[0011] It is therefore desirable to be able to determine the tissue properties of a subject's body part so as to be able to take into account any estimates or measurements of physiological characteristics (e.g., blood pressure) derived from measurements obtained using, for example, a shell cuff device. [Means for solving the problem]

[0012] The technology described herein uses a shell cuff device to accommodate many different pressures. A sensor is used to obtain a measurement of the radius or change in radius of the body part as it is applied to the body part, and tissue properties of the body part can be determined from a function or relationship between the change in radius of the body part and the pressure applied by the shell cuff device.

[0013] According to a first aspect, there is provided a computer-implemented method for determining tissue properties of a body part, the method comprising the steps of controlling a shell cuff device to apply a plurality of different pressures to the body part, the shell cuff device at least partially surrounding the body part, the shell cuff device having a rigid shell portion disposed at least partially around the body part and an inflatable cuff surrounding the rigid shell portion that inflates and / or deflates to apply a plurality of different pressures to the body part via the rigid shell portion, obtaining radial measurements of a radius of the body part or a change in radius of the body part as the plurality of different pressures are applied to the body part, obtaining pressure measurements of the different pressures applied to the body part, and determining tissue properties of the body part from the radial measurements and the pressure measurements.

[0014] According to a second aspect, there is provided a computer implemented method for determining a measure of a physiological characteristic of a subject, the method comprising determining tissue properties of a body part of a subject according to the first aspect or any embodiment thereof, using said determined tissue properties to calibrate pressure measurements by a sensor pad positioned inside a rigid shell portion, and determining a measure of a physiological characteristic using said calibrated pressure measurements.

[0015] According to a third aspect, there is provided a device configured for determining tissue properties of a body part, the device being configured or having a processing unit configured to control a shell cuff device (which may have a processing unit that outputs control signals to a shell cuff device) to apply a plurality of different pressures to the body part, to obtain (e.g., receive) radius measurements of a radius or change in radius of the body part as the plurality of different pressures are applied to the body part, to obtain (e.g., receive) pressure measurements of the different pressures applied to the body part, and to determine tissue properties of the body part from the radius measurements and the pressure measurements, wherein the shell cuff device at least partially surrounds the body part, the shell cuff device having a rigid shell portion disposed around at least a portion of the body part and an inflatable cuff surrounding the rigid shell portion that inflates and / or deflates to apply a plurality of different pressures to the body part via the rigid shell portion.

[0016] According to a fourth aspect, there is provided a device configured to determine a measure of a physiological characteristic of a subject, the device being configured according to the fourth aspect or any embodiment of this fourth aspect, and also configured to use the determined tissue characteristic to calibrate a pressure measurement by a sensor pad positioned inside the rigid shell portion, and to determine the measure of said physiological characteristic using the calibrated pressure measurement.

[0017] According to a fifth aspect, there is provided a system for determining tissue properties of a body part, comprising a device according to the third or fourth aspect, a shell cuff apparatus configured to at least partially surround the body part, and one or more sensors for providing (e.g. obtaining) radius and pressure measurements, the shell cuff apparatus comprising a rigid shell portion configured to be disposed around at least a portion of the body part, an inflatable cuff surrounding the rigid shell portion configured to inflate and / or deflate to apply a plurality of different pressures to the body part via the rigid shell portion, and one or more sensors for providing (e.g. obtaining) the radius and pressure measurements.

[0018] According to a sixth aspect there is provided a computer program product having a computer readable medium having computer readable code embodied therein which is configured, upon execution of instructions by a computer, or a processing unit of a device of the third or fourth aspect or any embodiment of these aspects, to cause the computer or device to perform a method according to the first aspect, the second aspect or any embodiment of these aspects.

[0019] Various embodiments of devices according to the fourth and fifth aspects are also envisaged, where the device is additionally or further configured to operate according to various embodiments of the methods described herein.

[0020] These and other aspects will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief description of the drawings]

[0021] Exemplary embodiments will now be described, by way of example only, with reference to the following drawings, in which: [Figure 1] FIG. 1 is a simplified diagram of a shell cuff device around a body part of a subject. [Diagram 2] FIG. 2 is a block diagram of a system for determining tissue properties of a body part having a device and a shell cuff apparatus. [Diagram 3] FIG. 3 illustrates an exemplary hard shell portion equipped with a radial sensor in the form of a strain gauge. [Figure 4] FIG. 4 shows a cross section of a hard shell portion around a simplified model of a body portion. [Diagram 5] FIG. 5 shows the sensor pad being pressed into a body part. [Figure 6] FIG. 6 is a flow chart illustrating a method for determining tissue properties of a body part. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] As mentioned above, the techniques described herein use sensors to obtain measurements of the radius of the body part or the change in radius of the body part as many different pressures are applied to the body part using a shell cuff device. Tissue properties of the body part can be determined from a function or relationship between the change in radius of the body part and the pressure applied by the shell cuff device. Tissue properties can include elastic modulus, nonlinear E-modulus, Poisson's ratio, v arm , fat free mass (FFM) and adipose fat mass (AFM).

[0023] The radius of the body part or the change in radius of this body part can be measured using a sensor referred to herein as a "radius sensor." Many different types of sensors can be used for this purpose. For example, the radius sensor can be or include either a strain gauge incorporated into the rigid shell portion of the shell cuff device, or a separate measuring device based on a metal strip that is used to measure both the initial radius of the body part (i.e., before pressure is applied) and the change in radius of the body part as pressure is applied to the body part.

[0024] The pressure applied to the body part can be measured directly using a sensor pad associated with the shell cuff device, or indirectly through the measurement of the pressure in the inflatable cuff combined with the known coefficient of friction between the overlapping parts of the rigid shell part, the total length of the rigid shell part and the change in the radius of the body part measured with the cuff pressure. The total length of the rigid shell part is a fixed parameter of the rigid shell part and is the length of the rigid shell part surrounding said body part (approximately given by 2πR, where R is the radius of the body part) plus the amount of overlap of both sides of the rigid shell part. Too large a discrepancy between both methods indicates an error in the pressure measured by the sensor pad, which automatically introduces errors in the cardiac output or other physiological characteristics derived from the measured pressure. This can happen especially in small adults with stiff arms. With the proposed technology the exact pressure on the body part can be determined so that potential errors can be avoided or compensated for.

[0025] Thus, embodiments of these techniques allow for the determination of one or more tissue properties that can be used for individual calibration of the sensor pad pressure measurements to determine actual cardiac output or other physiological properties. Additionally, embodiments provide for detection of potential errors in the measurement of cardiac output or other physiological properties, particularly for small adults with stiff arms, and for correcting these errors.

[0026] 2 illustrates an exemplary system 20 for determining tissue properties of a body part. System 20 includes a shell cuff apparatus 22 and a device 24. In some embodiments, system 20 is also used to determine or measure one or more physiological properties of a subject based on the determined tissue properties.

[0027] The shell cuff device 22 includes an inflatable cuff 26 and a rigid shell portion 28. The rigid shell portion 28 should be disposed around at least a portion of a body part, such as an arm or leg, and the inflatable cuff 26 surrounds the rigid shell portion 28 (i.e., the inflatable cuff 26 is disposed around the rigid shell portion 28 such that when the inflatable cuff 26 is inflated, the inflatable cuff presses the rigid shell portion 28 against the body part). The rigid shell portion 28 may be formed from any suitable material, for example, a plastic material having a modulus of elasticity in the range of 0.4-5 GPa. Such a plastic material may be high density polyethylene (HDPE). The length and / or thickness of the rigid shell portion 28 may vary depending on the diameter of the body part (e.g., arm or leg) in which the shell cuff device 22 is to be used. In particular, the larger the diameter of the body part, the greater the length and thickness of the rigid shell portion 28. Typical thicknesses may be in the range 0.5 to 5 mm, or such as in the range 1 to 2 mm, and typical overall lengths may be in the range 0.25 to 0.7 metres, and such as in the range 0.3 to 0.5 metres.

[0028] The shell cuff apparatus 22 also has a pump 30 connected (e.g., via a connecting tube 32) to the inflatable cuff 26. The pump 30 is controllable (e.g., in response to a control signal) to selectively inflate the inflatable cuff 26. The pump 30 may also selectively deflate the inflatable cuff 26, and / or a valve (not shown) may be provided that allows the inflatable cuff 26 to be selectively deflated.

[0029] To enable determination of tissue properties of the body portion underlying the inflatable cuff 26 and rigid shell portion 28, a radius sensor 34 is provided for measuring the radius of the body portion, or the change in radius of the body portion, when a number of different pressures are applied to the body portion by the inflatable cuff 26 and rigid shell portion 28. The radius sensor 34 may take any of several different forms, as outlined further below. The radius sensor 34 outputs a measurement signal having a measurement of the radius or change in radius over time.

[0030] In some embodiments, for example when the system 20 is used to determine a physiological characteristic of a subject taking into account the determined tissue characteristics, the shell cuff device 22 may further comprise a sensor pad 36 positioned on the inner surface of the rigid shell portion 28, i.e., on the side of the rigid shell portion 28 that faces the body portion when the rigid shell portion 28 is placed around the body portion. When the inflatable cuff 26 is inflated, the inflatable cuff 26 and the rigid shell portion 28 press the sensor pad 36 against the body portion, allowing the sensor pad 36 to measure the pressure in the body portion, for example by a pulse wave passing through the body portion. The sensor pad 36 outputs a measurement signal having a measurement of said pulse wave or pressure wave over time. The sensor pad 36 may be a liquid-filled pad. The sensor pad 36 may be connected via a flexible tube filled with a liquid similar to that of the sensor pad 36 to a pressure sensor that is on the outside of the rigid shell portion 28 and measures the pressure. The liquid in the sensor pad 36 may be, for example, a silicone oil such as AK10, and the liquid in the pressure sensor may be a different liquid than the liquid in the sensor pad 36, for example, glycerol.

[0031] In some embodiments, measurements from the sensor pad 36 can be used to determine the radius of the body part or a change in the radius of the body part, in which case a separate radius sensor 34 is not required.

[0032] The measurement signals from the radius sensor 34 and sensor pad 36 (whichever combination of radius sensor 34 and sensor pad 36 is present in the shell cuff apparatus 22) are provided to the device 24 for analysis.

[0033] Device 24 operates in accordance with the techniques described herein to determine tissue properties of a body portion of a subject. Device 24 is configured to receive measurement signals from radius sensor 34 and / or measurement signals from sensor pad 36. In some embodiments, device 24 is configured to control operation of pump 30 (e.g., by sending control signals to shell cuff apparatus 22 / pump 30) to thereby initiate inflation of inflatable cuff 26 at the appropriate time.

[0034] Device 24 may be in the form of, or be part of, a computing device such as, for example, a server, a desktop computer, a laptop, a tablet computer, a smartphone, a smartwatch, or other types of devices typically found in a clinical environment, such as a patient monitoring device (e.g., a monitoring device located at a patient's bedside in a clinical environment) used to monitor (and optionally display) various physiological characteristics of a subject / patient, including blood pressure, cardiac output (CO), stroke volume (SV), stroke volume variation (SVV), or pulse contour stroke volume (PCSV).

[0035] The device 24 includes a processing unit 38 configured to control the operation of the device 24 and to execute or perform the methods described herein. The processing unit 38 may be implemented in numerous ways using software and / or hardware to perform the various functions described herein. The processing unit 38 includes one or more microprocessors or digital signal processors (DSPs) that are programmed using software or computer program code to perform the necessary functions and / or to control the components of the processing unit 38 to perform the necessary functions. The processing unit 38 may be implemented as a combination of dedicated hardware (e.g., amplifiers, preamplifiers, analog-to-digital converters (ADCs) and / or digital-to-analog converters (DACs)) to perform some functions and processors (e.g., one or more programmed microprocessors, controllers, DSPs and associated circuitry) to perform other functions. Examples of components that may be used in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, DSPs, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), hardware for implementing neural networks, and / or so-called artificial intelligence (AI) hardware accelerators (i.e., processors or other hardware designed specifically for AI applications for use in conjunction with a main processor).

[0036] The processing unit 38 is connected to a memory unit 40 that can store data, information and / or signals used by the processing unit 38 when controlling the operation of the device 24 and / or when executing or performing the methods described herein. For example, the memory unit 40 can store any of radius measurements, pressure measurements, and measurement signals from one or both of the radius sensor 34 and the pressure sensor 36 for later use by the processing unit 38. In some implementations, the memory unit 40 stores computer readable code that is executed by the processing unit 38 to cause the processing unit 38 to perform one or more functions, including the methods described herein. In certain embodiments, the program code can be in the form of an application for a smartwatch, smartphone, tablet, laptop, or computer. The memory unit 40 may comprise any type of non-transitory machine-readable medium, such as a cache or system memory, including volatile and non-volatile computer memory, such as RAM, static RAM (SRAM), dynamic RAM (DRAM), ROM, programmable ROM (PROM), erasable PROM (EPROM), and electrically erasable PROM (EEPROM), and the memory unit 40 may be implemented in the form of a memory chip, an optical disk (e.g., a compact disk (CD), a digital versatile disk (DVD) or a Blu-ray disk), a hard disk, a tape storage solution or a solid-state device, including a memory stick, a solid-state drive (SSD), a memory card, etc.

[0037] In some embodiments, device 24 has a user interface 42 that includes one or more components that allow a user of device 24 to input information, data, and / or instructions to device 24 and / or that allow device 24 to output information or data to a user of device 24. Information that may be input via user interface 42 may include any of gender, age, height, weight, body mass index (BMI), and the like. Information that may be output by user interface 42 may include any of information regarding tissue characteristics, values ​​of tissue characteristics, information regarding physiological characteristics of the subject, values ​​of physiological characteristics, and the like. User interface 42 may include any suitable input components, including, but not limited to, a keyboard, a keypad, one or more buttons, switches or dials, a mouse, a trackpad, a touch screen, a stylus, a camera, a microphone, and the like, and / or user interface 42 may include any suitable output components, including, but not limited to, a display screen, a light or light elements, one or more speakers, a vibration element, and the like.

[0038] It will be understood that practical embodiments of device 24 and / or shell cuff apparatus 22 may include components in addition to those shown in Figure 2. For example, device 24 may include a power source, such as a battery, or components to enable device 24 to be connected to a mains power source. Device 24 may also include interface circuitry to enable data connections and / or exchanges with other devices, including shell cuff apparatus 22 (e.g., if shell cuff apparatus 22 is not directly connected to device 24), servers, databases, and user devices.

[0039] In some embodiments, the shell cuff device 22 may have one or more additional sensors to measure other properties. For example, in embodiments in which the radius sensor 34 is a strain gauge, a temperature sensor may also be provided that can be used to measure the temperature of the environment in which the shell cuff device 22 is being used. The measured temperature may be used to correct the strain gauge measurements for changes in temperature. Alternatively, multiple strain gauges may be provided that are coupled in a half or full Wheatstone bridge configuration to automatically correct for changes in temperature.

[0040] In another approach, in implementations where the rigid shell portion 28 is tapered to better match the shape of a body part (e.g., an arm), multiple radius sensors 34 can be provided at different portions of the rigid shell portion 28 to measure the radius or change in radius of those portions.

[0041] FIG. 3 illustrates an exemplary rigid shell portion 28 with a radius sensor 34 in the form of a strain gauge. The rigid shell portion 28 comprises a sheet of material that is curved or "rolled" into a cylindrical shape so that the rigid shell portion 28 can be placed around a body part. As shown in FIG. 3, the leading edges of the material overlap at region 50. When the inflatable cuff 26 is inflated, the rigid shell portion 28 is compressed against the body part, the amount of overlap at region 50 increases, and the diameter of the cylinder formed by the rigid shell portion 28 decreases. The radius sensor 34 in the form of a strain gauge can be attached to an outer or inner surface of the rigid shell portion 28 to measure the strain caused by the deformation of the rigid shell portion 28. Alternatively, the strain gauge 34 can be integral with the rigid shell portion 28. Preferably, the rigid shell portion 28 is made of a material that has a low coefficient of friction when the material overlaps. This allows the diameter or size of the rigid shell portion 28 to change more smoothly as the pressure applied to the outside of the rigid shell portion 28 changes.

[0042] FIG. 3 also illustrates an alternative approach for use with an existing shell cuff device that does not include a radius sensor 34 to enable the techniques described herein to be used. In this approach, a mounting portion 52 is provided that may be attached or disposed on an exterior or interior surface of the rigid shell portion 28, such as at location 54, and that has a radius sensor 34 in the form of a strain gauge. Attachment of the mounting portion 52 may be accomplished using any suitable attachment means, such as, for example, adhesives, clamps, screws, bolts, etc. Deformation of the rigid shell portion 28 as the inflatable cuff 26 inflates causes a corresponding deformation of the mounting portion 52, and the radius sensor 34 measures the strain associated with this deformation. In a particular implementation, the mounting portion 52 is formed from steel having a thickness of 0.01 mm to 5 mm. The force with which the mounting portion 52 is attached to the rigid shell portion 28 (clamping force) should be small compared to the pressure exerted by the inflatable cuff 26 as it inflates, to avoid affecting the pressure within the inflatable cuff 26.

[0043] The processing unit 38 or the radius sensor 34 may have suitable electronic components or software that allow it to make and / or interpret measurements of radius or changes in radius by the radius sensor 34. For example, in an embodiment in which the radius sensor 34 is a strain gauge, the radius sensor 34 or the processing unit 38 may have an electrical circuit in the form of a Wheatstone bridge that is used to obtain measurements from the strain gauge. A low pass frequency filter may be provided in the processing unit 38 or the radius sensor 34 to filter low frequency noise, such as, for example, pressure pulses from the pump 30 and / or pulses of a heartbeat. A typical cut-off frequency is preferably lower than 20 Hertz (Hz), more preferably lower than 0.5 Hz.

[0044] As mentioned above, the radius sensor 34 can be any of several different forms, with a strain gauge arranged as shown in Figure 3 being one possibility. Other types of radius sensor 34 can be one or more sensors for measuring one or more dimensions or changes in dimensions of the rigid shell portion 28. The dimensions measured to determine the radius or change in radius include the diameter of the rigid shell portion 28, the circumference of the rigid shell portion 28, the tangential displacement of the rigid shell portion 28, or the amount of overlap of the rigid shell portion 28 in region 50. Types of sensors used to measure one or more dimensions include optical sensors (e.g., cameras, imaging units, or laser-based systems, which may or may not require the placement or presence of a visual marker on the rigid shell portion 28), proximity sensors disposed at different locations on the rigid shell portion 28 to measure the amount of overlap in the area 50, pressure (touch) sensors, electrical property sensors (e.g., measuring resistance, conductance, impedance, etc. between the two parts of the rigid shell portion 28 in the overlap area 50), accelerometers that measure the movement of the rigid shell portion 28 from which deformation of the rigid shell portion 28 is derived, and volume sensors that measure the volume of fluid flowing to and / or from the body portion underlying the rigid shell portion 28 as the inflatable cuff 26 inflates and deflates (wherein a change in the volume of the measured body portion indicates a change in the volume of the rigid shell portion 28 surrounding the body portion).

[0045] In embodiments where a separate radius sensor 34 is not provided and measurements from the sensor pad 36 are used to determine the radius or change in radius of the body part, measurements of the total air (or other fluid) volume in the inflatable cuff 26 and measurements of the pressure in the inflatable cuff 26 can be combined to provide a relationship between the change in radius of the body part and the pressure in the body part. The amount of air (or other fluid) in the inflatable cuff 26 can be measured using a flow meter and measurements of air (fluid) flow are integrated over time to provide the volume of air (fluid) in the inflatable cuff 26.

[0046] An alternative method for determining the radius of a body part without using a radius sensor 34 is the case of overlapping hard shell parts 28 .

[0047] Relationship R(Δp i )teeth,

number

[0048] As mentioned above, the response of the pressure sensor (sensor pad 36) within the rigid shell portion 28 is strongly influenced by the tissue properties of the subject, and so it is useful to determine one or more tissue properties of the subject. In principle, it is possible to determine the nonlinear elastic moduli (E-modules) of a body part from the change in radius of the body part due to pressure applied to the outside of the body part. The details of these relationships are discussed further below, but it will be understood that a relatively large decrease in radius of a body part with increasing pressure is indicative of a subject with different tissue properties compared to a subject who experiences a smaller decrease in radius for the same change in pressure.

[0049] The diagram of Figure 4 shows a cross-section of the hard shell portion 28 around a simplified model of a body portion 60. The body portion 60 is assumed to have a circular cross-section and is bounded by a radius R b2, the rigid shell portion 28 includes bones 62 having a radius of curvature 64, arteries 64 and other soft tissues 66. In this model, the soft tissues 66 are considered to be homogenous (i.e., uniform density and other material properties), although it will be appreciated that more complex models including different tissue sections representing fat, muscle, etc. may be used. The radius sensors 34 are provided on the rigid shell portion 28 and may be, for example, strain gauges. The sensor pads 36 are positioned on the inner surface of the rigid shell portion 28 and pressed into the body part 60. The rigid shell portion 28 is wrapped around the body part 60 such that there is an overlap region 50 where the leading edges of the rigid shell portions 28 overlap one another. The amount of overlap (i.e., the length L0 that the shells overlap) is L0=L t -2πR i (Δp i ) (2) where L t is the total length of the hard shell portion 28 in the circumferential direction of the body part 60, and R i (Δp i ) is the pressure applied to the inside of the body part 60 (Δp i The radius of the body portion as a function of the total length L of the hard shell portion 28. t is a fixed parameter of the rigid shell portion 28 measured from one side of the rigid shell portion 28 to the other side of the rigid shell portion 28. With reference to FIG. t is the total length of the rigid shell portion 28 around the body part 60, including the length of the rigid shell portion 28 in the overlap region 50. Pressure applied to the interior of the body part 60 is measured by the sensor pad 36. Although not shown in FIG 4, the inflatable cuff 26 is disposed around the rigid shell portion 28 such that when the inflatable cuff 26 is inflated, pressure is applied to the rigid shell portion 28 (as indicated by the arrow pointing towards the rigid shell portion 28).

[0050] The thickness t of the hard shell portion 28 is t=R0-R i (3) where R is the distance from the center of the body portion 60 to the outer surface of the hard shell portion 28. i0The initial radius of the body part, indicated by can be measured or determined using radius sensor 34 when no pressure or very little pressure is applied to body part 60 by inflatable cuff 26.

[0051] Radius sensor 34 is used to measure the radius or change in radius of body part 60 as the pressure (Δp) in inflatable cuff 26 changes. Sensor pad 36 is used to measure the pressure (Δp i ) is measured according to the following formula (4).

number

[0052] It will be appreciated that the relationship shown in equation (4) is based on the simplified body part model of FIG. 4 and that different relationships can be derived for more complex models, e.g., where the body part is considered to be composed of different types of tissue, such as muscle and fat.

[0053] In the above-described method, the pressure (Δp i) in the inflatable cuff 26. However, in an alternative approach, the pressure on the body part 60 can be determined by measuring the pressure inside the inflatable cuff 26 (the "cuff pressure"), represented as Δp0. Due to the properties of the rigid shell portion 28, the cuff pressure Δp0 is a function of the pressure inside the rigid shell portion 28 compressing the body part 60, Δp i Different (larger) than.

[0054] A simple analytical relationship has been found between the pressure measured by the sensor pad 36 and the cuff pressure. The ratio between these pressures is called the effectiveness ε. Effectiveness ε depends on the coefficient of friction, the overall length of the hard shell portion (L t ) and the radius of the actual body part (which is a function of the applied pressure).

number

[0055] The derivation of the validity ε allows the measurement of pressure by the sensor pad 36 to be verified. In particular for smaller sized body parts where the change in radius upon application of pressure is relatively small, there is a risk that the pressure measurement by the sensor pad 36 is not the actual pressure acting on the body part. This is illustrated in FIG. 5. For small, hard body parts, the sensor pad 36 may not be pressed completely into the body part, and the pressure in the sensor pad 36 may be greater than the pressure acting on the rest of the body part. The use of the validity ε therefore allows the measurement of pressure by the sensor pad 36 to be verified based on the measured cuff pressure.

[0056] 6 illustrates a method for determining tissue properties of a body part according to the techniques described herein. The method can be performed by device 24, and in particular by processing unit 38. The method is performed as a result of device 24 or processing unit 38 executing suitable computer readable code, e.g., stored in memory unit 40.

[0057] During the measurement stage of the method, represented by steps 101, 103 and 105, a shell cuff device 22 is placed partially or completely around a body part for which one or more tissue properties are to be determined. The shell cuff device 22 has a rigid shell portion 28 that is placed partially or completely around the body part with an inflatable cuff 26 surrounding the rigid shell portion 28. The inflatable cuff 26 is inflated and / or deflated via a pump 30 to apply a number of different pressures to the body part over time via the rigid shell portion 28.

[0058] Thus, in a first step, the shell cuff apparatus 22 is controlled to apply a plurality of different pressures to the body part. The shell cuff apparatus 22 may be controlled by the device 24 or the processing unit 38, in particular the pump 30 may be controlled to inflate and / or deflate the inflatable cuff 26. In step 101, the inflatable cuff 26 may be continuously inflated or deflated, or the inflatable cuff 26 may be inflated or deflated in steps (i.e., inflating / deflating the inflatable cuff 26 to a first pressure, maintaining that pressure for a short period of time, inflating / deflating the inflatable cuff 26 to another pressure, etc.). The processing unit 38 may control the application of a plurality of different pressures by the shell cuff apparatus 22 by outputting appropriate control signals to the shell cuff apparatus 22.

[0059] In step 103, measurements of the radius of the body part or measurements of the change in radius of the body part are taken. These measurements are referred to as "radius measurements." The radius measurements are taken when a number of different pressures are applied to the body part (i.e., when the inflatable cuff 26 is inflated and / or deflated), i.e., a number of radius measurements are taken, one for each different applied pressure.

[0060] In some embodiments, the radius measurements are obtained using one or more radius sensors 34. The radius sensors 34 can be sensors configured to measure deformation of the rigid shell portion 28. As the rigid shell portion 28 is disposed around the body part, deformation of the rigid shell portion 28 results in a change in the radius of the body part underlying the rigid shell portion 28. The radius sensors 34 are configured or arranged such that deformation of the rigid shell portion 28 changes one or more electrical properties of the radius sensors 34, such as, for example, resistance, capacitance, inductance. The radius measurements can be derived from the changes in these electrical properties. In some embodiments, the radius sensors 34 are strain gauges.

[0061] In alternative embodiments, the radius measurements may be obtained from measurements of the pressure in the inflatable cuff 26 and measurements of the flow of fluid into and out of the inflatable cuff 26. In these embodiments, a cuff pressure sensor may be provided to measure the pressure in the inflatable cuff 26, and a flow meter may be provided to measure the flow of fluid into and / or out of the inflatable cuff 26. Thus, in these alternative embodiments, the radius measurements are obtained by processing the measurements of the pressure in the inflatable cuff 26 and the measurements of the flow of fluid into and out of the inflatable cuff 26.

[0062] In step 105, measurements of different pressures applied to the body part are taken. These measurements are called "pressure measurements." These pressure measurements are taken simultaneously with the radius measurements, so that for each radius measurement there is a corresponding measurement of the pressure applied to the body part at that time. This allows the change in radius of the body part to be observed in response to changes in applied pressure.

[0063] In some embodiments, pressure measurements are taken using a sensor pad 36 positioned inside the rigid shell portion 28. In an alternative embodiment, cuff pressure sensors may be used to take measurements of pressure within the inflatable cuff 26 as the inflatable cuff 26 is inflated and / or deflated, and pressure measurements may be determined from the respective cuff pressure sensor measurements and corresponding radius measurements (i.e., radius measurements taken generally simultaneously with the cuff pressure sensor measurements). In particular, pressure measurements may be determined from the cuff pressure sensor measurements, the corresponding radius measurements, the coefficient of friction of the rigid shell portion 28, and the overall length of the rigid shell portion 28.

[0064] Once these measurements have been taken in steps 103 and 105, tissue properties of the body part can be determined from the radius and pressure measurements in step 107. In some embodiments, step 107 includes using a function that relates changes in radius of the body part to one or more tissue properties and pressure applied to the body part. The function can be based on a model of the structure of the body part. For example, the model can assume that the tissue properties are uniform throughout the body part, or that certain parts of the body part have fat and other parts have muscle, each with different tissue properties.

[0065] In some embodiments, step 107 includes determining the tissue properties E, k, and v arm , evaluating equation (4).

[0066] In some embodiments, once the tissue characteristics are determined, they are used to calibrate pressure measurements obtained using the shell cuff device 22 to determine a physiological characteristic of the subject. The physiological characteristic can be either blood pressure, cardiac output, stroke volume (SV), stroke volume variation, or pulse contour stroke volume (PCSV). Those skilled in the art will know techniques for calibrating pressure measurements according to the determined tissue characteristics. For example, WO 2019 / 211210 describes the calibration of SV and PCSV based on the tissue characteristics FFM (fat free mass) and AFM (fat mass), and the techniques described herein can be used to determine these tissue characteristics and thereby improve said calibration.

[0067] Variations to the disclosed embodiments can be understood and implemented by those skilled in the art when implementing the principles and techniques described herein, by studying the drawings, the disclosure and the appended claims. In the claims, the term "comprising" does not exclude other elements or steps, nor does it exclude a plurality of them, even if a plurality is not stated. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be used to advantage. A computer program can be stored or distributed on a suitable medium, such as an optical storage medium or a solid-state medium, for example supplied together with or as part of other hardware, but can also be distributed in other forms, for example via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be interpreted as limiting the scope thereof.

Claims

1. A computer-implemented method for determining tissue characteristics of a body part, the method comprising: Controlling a shell cuff device to apply a plurality of different pressures to the body part, the shell cuff device at least partially surrounding the body part, the shell cuff device having a rigid shell portion at least partially disposed around the body part and an inflatable cuff surrounding the rigid shell portion that inflates and / or deflates to apply the plurality of different pressures to the body part via the rigid shell portion, the controlling step; Obtaining a radius measurement of the body part or a change in the radius of the body part using measurements from a radius sensor or a sensor pad positioned inside the rigid shell portion when the plurality of different pressures are applied to the body part; Obtaining pressure measurements of the different pressures applied to the body part, the pressure measurements being obtained using the sensor pad or, when the inflatable cuff inflates and / or deflates, obtaining a measurement of the pressure inside the inflatable cuff using a cuff pressure sensor and determining the pressure measurements from the measurements of each cuff pressure sensor and the corresponding radius measurements, the obtaining step; and Determining the tissue characteristics of the body part from the radius measurements and the pressure measurements A computer-implemented method.

2. Determining the pressure measurements comprises: The method of claim 1, comprising determining the pressure measurements as a function of cuff pressure sensor measurements, corresponding radius measurements, the coefficient of friction of the rigid shell portion, and the total length of the rigid shell portion.

3. The method of claim 1, wherein the radius sensor is a sensor configured to measure deformation of the rigid shell portion.

4. The method of claim 1 or 3, wherein the radius sensor is configured or arranged such that deformation of the rigid shell portion changes one or more electrical characteristics of the radius sensor.

5. The method of claim 1, wherein the radius sensor is a strain gauge.

6. The method of claim 1 or 2, wherein the radius measurements are obtained from measurements of the pressure inside the inflatable cuff and measurements of the flow rate of fluid into and out of the inflatable cuff.

7. The method according to claim 1, wherein the radius measurement value is obtained from the pressure measurement value inside the inflatable cuff, the pressure measurement value obtained using the sensor pad, the coefficient of friction of the rigid shell portion, and the total length of the rigid shell portion.

8. The step of determining the tissue characteristics of the body part from the radius measurement value and the pressure measurement value includes using a function that associates a change in the radius of the body part with one or more tissue characteristics and the pressure applied to the body part. The method according to claim 1.

9. The step of determining the tissue characteristics uses the acquired radius measurement value ΔR ｉ and the acquired pressure measurement value Δp ｉ to use E ａｒｍ (Δp ｉ ) = E ０ + k · Δp ｉ to use the following formula 【Number 4】 having a step of obtaining the value of E ０ where E is the elastic modulus of the tissue in the body part, k is a parameter representing the non-linear elastic modulus of the tissue, v ａｒｍ is the Poisson's ratio of the tissue, R ｉ０ is the initial radius of the body part when the pressure by the inflatable cuff is not applied to the body part, and R ｂ is the radius of the bone in the body part, the method according to claim 1.

10. A computer-implemented method for determining a measurement value of a physiological characteristic of a subject, the method comprising: Determining the tissue characteristics of the body part of the subject according to the method according to claim 1; Calibrating the pressure measurement value by the sensor pad using the determined tissue characteristics; and Determining the measurement value of the physiological characteristic using the calibrated pressure measurement value A computer-implemented method having.

11. A system for determining the tissue characteristics of a body part, the system comprising: A shell cuff device configured to at least partially surround a body part, the shell cuff device including a rigid shell portion configured to be disposed around at least a portion of the body part, and the rigid shell portion. An inflatable cuff surrounding the rigid shell portion, configured to expand and / or contract to apply the plurality of different pressures to the body part through the rigid shell portion; a shell cuff device; One or more sensors for supplying the radius measurement value and the pressure measurement value; and A device configured to determine the tissue characteristics of the body part, the device having a processing unit configured to execute the method according to claim 1. A system having.

12. A system for determining a measurement value of a physiological characteristic of a subject, the system comprising: A shell cuff device configured to at least partially surround a body part, the shell cuff device including a rigid shell portion configured to be disposed around at least a portion of the body part, and the rigid shell portion. An inflatable cuff surrounding the rigid shell portion, configured to expand and / or contract to apply the plurality of different pressures to the body part through the rigid shell portion; a shell cuff device; one or more sensors for supplying the radius measurement value and the pressure measurement value, and a device configured to determine a measurement value of a physiological characteristic of the subject, the device having a processing unit configured to execute the method according to claim 10 A system having. **Claim 13** A computer program product, having instructions for causing the system according to claim 11 to execute the steps of the method according to claim 1. **Claim 14** A computer program product, having instructions for causing the system according to claim 12 to execute the steps of the method according to claim 10.

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