Patent Application
20240000326
High arterial blood pressure (BP) afflicts many people (e.g., about one in three adults worldwide). While the incidence tends to increase with age, certain people can develop hypertension early in adulthood (e.g., about one in five US adults under 40 years old are hypertensive). The condition can be asymptomatic, but the risk for stroke and heart disease can increase monotonically with BP for a given age. Certain medications can lower BP and cardiovascular risk. However, only about three in seven people with hypertension are aware of their condition, and one of these seven has their BP under control. According to certain epidemiological data, hypertension has emerged as a leading cause of disability-adjusted life years lost.
Certain auscultatory and oscillometric BP measurement devices can be used for managing hypertension. However, these devices can lead to reduced hypertension awareness and control rates due at least in part to their reliance on an inflatable cuff. Cuff-based devices are not readily available, especially in low resource settings. As such, it can be inconvenient for people to regularly check their BP. Regular measurements during daily life are desirable, for example to circumvent white coat and masked effects in the clinic, in which patients can present with higher or lower BP than usual and to average out the large variations in BP that can occur over time, for example due to stress, physical activity, and other factors. If BP can be measured with a more convenient device, more people can be aware of their condition or be motivated to take their medications.
Therefore, there is an opportunity for methods and systems for measuring blood pressures with more convenient instrumentation.
The purpose and advantages of the disclosed subject matter will be set forth in and are apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the devices particularly pointed out in the written description and claims hereof, as well as from the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter provides devices and methods for determining the blood pressure of a subject. A device for determining blood pressure of a subject can include a force sensor configured to measure finger pressure, a camera configured to measure a finger photo-plethysmography (PPG) waveform, a screen configured to display a visual indicator to guide a subject to place a side of a finger on the camera and the screen to target a digital artery and display the finger pressure in real time such that the subject can uniformly press the finger on the camera and screen to vary external pressure of the artery, and a processor. The processor can be configured to construct an oscillogram that can be a variable-amplitude blood volume oscillations versus external finger pressure function, compute a blood pressure of the subject from the oscillogram, and display the blood pressure on the screen. In non-limiting embodiments, the processor can be configured to determine the visual indicator based on different finger placements on the camera and screen before performing the blood pressure measurement.
The disclosed subject matter provides a device for determining blood pressure of a subject that can include a skin contact area sensor configured to measure a finger area, a camera configured to measure a finger photo-plethysmography (PPG) waveform, a screen configured to display a visual indicator to guide a subject to place a fingertip on the camera and the screen to target a transverse palmar arch artery and configured to display the finger pressure in real time to guide the subject to uniformly press the fingertip on the camera and screen to vary the external pressure of the artery, and a processor. The processor can be configured to convert the finger area to finger pressure based on a pre-defined nomogram, construct an oscillogram that can be a variable-amplitude blood volume oscillations versus external finger pressure function, compute systolic and diastolic blood pressure of the subject from the oscillogram, and display the systolic and diastolic blood pressures on the screen.
As embodied herein, the nomogram can be configured to determine finger force from the finger area based on selected parameters of a parametric function and divide the determined finger force by the finger area to determine finger pressure. For purpose of illustration and not limitation, the selected parameters can be determined based on fingertip dimensions of the subject, a single cuff blood pressure reading, or a hand raising maneuver. As embodied herein, the subject can hold the device above heart level during the finger pressing, which can provide a more accurate nomogram, and the processor can be configured to use a vertical height between the device and the heart of the subject to adjust the blood pressure measurement to the heart level.
The disclosed subject matter provides a device for determining blood pressure of a subject that includes a camera configured to measure a finger photo-plethysmography (PPG) waveform, an accelerometer configured to measure a vertical height of the device relative to a heart of a subject, an output device configured to guide the subject to raise a hand to vary the transmural pressure of an artery while maintaining a finger pressure on the camera, and a processor. The processor can be configured to compute pulse pressure of the subject from the finger PPG waveform and the vertical height and display the pulse pressure on the screen.
As embodied herein, the processor can be further configured to guide the subject to apply hard finger pressure on the camera, guide the subject to change a level of finger pressure based on the measured AC and/or DC value of the PPG waveform and the PPG measurement during the hard finger pressure, and identify a finger pressure corresponding to when a blood volume oscillation is near maximal. As embodied herein, the processor can be further configured to compare the PPG waveform during hand raising with the PPG waveform during finger pressing to assess a level of the accuracy of the device.
As embodied herein, the processor can be configured to construct a shifted oscillogram to relate variable-amplitude blood volume oscillations to a hydrostatic pressure change measured using the vertical height. The pulse pressure can be computed from the shifted oscillogram. As embodied herein, the accelerometer can be configured to measure the vertical height of the device relative to the heart. For purpose of illustration and not limitation, the processor can be configured to convert the pulse pressure to brachial artery pulse pressure using a transfer function.
The disclosed subject matter provides a device for determining blood pressure of a subject that includes a force sensor configured to measure finger pressure of the subject, a PPG sensor configured to measure a finger PPG waveform of the subject, a barometric pressure sensor configured to measure a vertical height of the device relative to a heart of the subject, and a processor. The processor can be configured to measure readings of the barometric pressure sensor during finger pressing and without the finger pressing while holding the device at heart level, adjust the blood pressure measured during the finger pressing to a heart level using the readings of the barometric pressure sensor, and display the adjusted blood pressure of the subject on a screen. As embodied herein, the blood pressure can be adjusted based on blood density, gravity, and/or the readings of the barometric sensor.
The disclosed subject matter provides a device for determining blood pressure of a subject that includes a force sensor configured to measure finger pressure and finger pressure oscillation, a visual indicator to guide a subject to place a fingertip on the force sensor, a screen configured to display the finger pressure in real time to guide the subject to press the finger on the sensor to vary the external pressure of the underlying artery, and a processor. The processor can be configured to measure AC and DC components of the finger pressure, identify an AC finger pressure pulse of maximal oscillation and a DC finger pressure at the maximal oscillation, determine a blood pressure of the subject based on the AC finger pressure pulse of maximal oscillation, and the DC finger pressure at the maximal oscillation, and display the blood pressure of the subject on the screen.
As embodied herein, the processor can be configured to determine the blood pressure based on fingertip dimensions of the subject and/or a single cuff blood pressure reading of the subject. As embodied herein, the processor can be further configured to compute diastolic blood pressure from the variable-amplitude finger pressure pulse oscillations and to compute systolic blood pressure from a blood pressure waveform
As embodied herein, the blood pressure waveform is converted to the brachial artery blood pressure waveform using a transfer function and regression equation. For purpose of illustration and not limitation, the device can further include a barometric pressure sensor to detect blood pressure at the heart level.
The disclosed subject matter provides a device for determining blood pressure of a subject that includes an array of force sensors configured to measure finger pressure and finger pressure pulse over each sensing element of the array, a visual indicator to guide the person to place a fingertip of the subject on the sensor array, a screen configured to display the finger pressure in real time to guide the subject to press the fingertip on the sensor to vary the external pressure of the underlying artery, and a processor. The processor can be configured to measure AC and DC components of the finger pressure at each sensing element of the array, determine a blood pressure of the subject from the AC and DC components, and display the blood pressure of the subject on the screen.
As embodied herein, the blood pressure can be determined based on maximal pressure pulse oscillation over the sensing elements and the DC components of the finger pressures. For purpose of illustration and not limitation, the processor can be further configured to generate a finger blood pressure waveform based on the AC and DC components and convert the blood pressure waveform to a brachial artery blood pressure waveform using a transfer function and regression model. As embodied herein, the device can further include a barometric pressure sensor to detect the blood pressure at the heart level.
The disclosed subject matter provides a device for determining blood pressure of a subject that includes a force sensor configured to measure finger pressure and a finger pressure pulse, a finger photo-plethysmography (PPG) sensor configured to measure a PPG waveform, a visual indicator to guide a subject in placing a fingertip on the sensors, a screen configured to display the finger pressure in real time to guide the subject to press the fingertip on the sensors to vary the external pressure of the underlying artery, and a processor. The processor can be configured to measure AC and DC components of the finger pressure and the PPG waveform, compute an arterial compliance curve using the AC finger pressure component and PPG waveform, compute a blood pressure of the subject using the arterial compliance curve, and display the blood pressure of the subject on the screen.
As embodied herein, the processor can be further configured to compute the blood pressure by forming an oscillogram based on external finger pressure and PPG waveform, performing a cross-correlation between the arterial compliance curve and the derivative of the oscillogram with respect to pressure, and determining a minimum value and a maximum value of the cross-correlation as systolic and diastolic blood pressures.
Reference will now be made in detail to the various exemplary embodiments of the disclosed subject matter, which are illustrated in the accompanying drawings.
The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosed subject matter, and in the specific context where each term is used. Certain terms are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner in describing the compositions and methods of the disclosed subject matter.
As used herein, the use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” Still further, the terms “having,” “including,” “containing,” and “comprising” are interchangeable, and one of skill in the art is cognizant that these terms are open-ended terms.
The term “about” or “approximately” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, “about” can mean within 3 or more than 3 standard deviations, per the practice in the art. Alternatively, “about” can mean a range of up to 20%, preferably up to 10%, more preferably up to 5%, and more preferably still up to 1% of a given value. Alternatively, particularly with respect to biological systems or processes, the term can mean within an order of magnitude, preferably within 5-fold, and more preferably within 2-fold, of a value.
A “user” or “subject” herein is a vertebrate, such as a human or non-human animal, for example, a mammal. Mammals include, but are not limited to, humans, primates, farm animals, sport animals, rodents, and pets.
The disclosed subject matter provides techniques for determining the blood pressure of a subject. The disclosed subject matter provides systems and methods for determining blood pressure of a subject using a non-invasive cuff-less device. A non-invasive cuff-less device as embodied herein can be configured as or utilize a portable device, which can be configured as a stand-alone medical device using specialized hardware and/or software as described herein. Additionally or alternatively, a non-invasive cuff-less device can utilize a general-purpose mobile or wearable device, such as a smartphone, portable computer, or other suitable general-purpose device. An example non-invasive cuff-less device 100 can include a camera 304, a sensor 101, a screen 102, and a processor.
As embodied herein, and as shown for example in
For example, and as embodied herein, the device 100 can provide a one-time or periodic initialization to determine optimal finger placement for the user. In this initialization, a user is guided to incrementally place more of the finger on the screen. The device 100 can identify the finger positioning that provides a suitable area of screen contact without approaching force saturation. For example and without limitation, device 100 can provide a visual indicator that can guide the user to place the finger that can yield a largest area of screen contact without force saturation.
As embodied herein, the disclosed device 200 can include a force sensor 101. The force sensor can measure the finger pressure on the force sensor. For example and without limitation, with reference to
Referring still to
As embodied herein, the device 100 can include a processor. The processor can be configured to form an oscillogram based on the measured PPG waveform and finger pressure. For example, the oscillogram can be a variable-amplitude blood volume oscillations versus an external finger pressure function 105. The variable-amplitude blood volume oscillations can be obtained from the PPG waveform as the user presses their finger on the sensors to vary the external pressure of the underlying artery. The processor can determine the blood pressure from the oscillogram and display the determined blood pressure on the screen. For example, the processor can estimate systolic and diastolic blood pressure from the oscillogram using the standard fixed ratio algorithm, a patient-specific algorithm, or another suitable oscillometric BP estimation algorithm. Additionally or alternatively, mean blood pressure can be estimated using similar techniques.
The processor can alternatively estimate the pulse pressure (e.g., PP=systolic blood pressure minus diastolic blood pressure) based on the PPG waveform that can be obtained through a PPG sensor (e.g., camera or finger PPG sensor) without using a force sensor. The finger PPG waveform can include alternating current (AC) and direct current (DC) components during increasing external finger pressure. For example, the processor can use the DC and/or AC components of the PPG waveform to determine how much finger pressure the user needs to apply on the PPG sensor (e.g., camera or finger PPG sensor). As shown for example in
As embodied herein, the processor can convert the finger BP measurement into brachial artery BP. For example, to obtain brachial blood pressure, the processor can extract the PPG waveform beat with maximal oscillation. The PPG waveform beat can be then calibrated so that its minimum and maximum can correspond to the computed finger diastolic and systolic BP. A transfer function to account for a wave reflection and a regression model to account for the resistive pressure drop, can be applied to convert the finger waveform beat to a brachial BP waveform beat. The minimum and maximum of the brachial BP waveform beat can be defined as systolic and diastolic blood pressure. Finger PP can be converted to brachial artery PP in a similar way but without using the regression model.
As embodied herein, the device 100 can include a skin contact area sensor, and the processor can be configured to measure systolic and diastolic blood pressures using the skin contact area sensor. The processor can convert the measured finger area to finger pressure using a pre-determined nomogram. The nomogram can include a parametric function (e.g., an exponential) to predict the finger force/pressure from the finger area. The processor can determine the parameters of this function by using an empirical equation with fingertip dimensions as input and the parameter(s) as the output and/or utilizing two-finger pressure measurements at two different heights relative to the heart for a known blood pressure change or a single cuff blood pressure measurement. The empirical equation can be derived from a training dataset from a cohort of subjects. Finger pressure can be obtained by computing force from the area measurement via the parametric function and dividing this value by the measured area. For purpose of illustration and not limitation, the user can hold the device 100 above the heart level to improve the accuracy of the device 100. For example, the user can lie down and hold the device 100 upward with arms straight while performing the finger pressing actuation. As embodied herein, the computed blood pressure can be adjusted for the hydrostatic blood pressure change using the arm length.
As embodied herein, the device 100 can include a barometric pressure sensor for detecting BP at heart level. For example, the barometric pressure sensor can detect height differences of less than about 5 cm, which can correspond to a minor error of blood pressure (e.g., less than about 3.5 mmHg error). A user can hold the device 100 with the barometric pressure sensor at heart level in a pledge of allegiance pose for about 5-20 sec. The barometric pressure measurement can be averaged. The user can perform the finger pressing technique as described herein to measure blood pressure while holding the smartphone in a static, arbitrary way including any vertical level relative to the heart. For purpose of illustration and not limitation, the barometric pressure can be averaged over the finger actuation. The difference in the two measurements can provide the vertical height for correcting the blood pressure measurement for the hydrostatic BP difference. As embodied herein, the blood pressure measurement can be corrected to heart level based on the blood density, gravity, and barometric sensor readings. For example, values of rho-g-h, where rho is the known blood density, g is gravity, and h is the second barometric sensor reading minus the first barometric sensor reading, can be added to the blood pressure measurement to correct it to heart level. For purpose of illustration and not limitation, the disclosed device 100 can include a temperature sensor that can be used to assess the quality of PPG and barometric measurements.
As embodied herein, the barometric pressure sensor can be used alternatively or in addition to the accelerometer to determine the vertical height of the disclosed device 100 relative to the heart during the hand raising technique 601, 602.
As embodied herein, the disclosed device 100 can determine the blood pressure of a user/subject with a pressure sensor alone. For example, the user can press on a force sensor of the device 100 on an artery. For purpose of illustration and not limitation, the disclosed device 100 can include the force sensor of a known area. For example, a user can perform the finger pressing as described herein, and congruent with the conventional applanation tonometry principle, the AC pressure waveform can increase with reducing wall tension and then decrease with arterial occlusion. The AC waveform beat at maximal amplitude can correspond to a zero-mean blood pressure waveform beat (ΔP(t)) scaled by the unknown constant that could be related to the area of the artery divided by the area of the sensor (k). Mean blood pressure (P_m) can be given by the DC pressure at which the AC amplitude is maximal according to applanation tonometry. The processor can generate a waveform indicating kΔP(t)+P_m based on the measurements. The parameter k can be determined to have a fully defined finger blood pressure waveform beat. For purpose of illustration and not limitation, the parameter k can be determined via an empirical equation relating arterial area to fingertip dimensions or by calibration with a single cuff blood pressure measurement. As embodied herein, the parameter k can be determined based on the measured diastolic blood pressure (P_d) from the AC pressure waveform. The minimum of ΔP(t) can be scaled to equal P_d-P_m, and P_m can be added to the waveform to generate the finger BP waveform. The peak of the waveform can be systolic blood pressure (P_s).
As embodied herein, the disclosed device 200 can measure finger blood pressure using tonometric finger pressing. According to the applanation tonometry principle, a force sensor can flatten or applanate the artery so that the wall tension can be perpendicular to the force sensor and can be encompassed by the flattened artery so that pressure can be derived as the ratio of the measured force to the known sensor area. For example, the disclosed device 200 can include a multi-sensor force array. The multi-sensor array can be attached to the back of the device 200 (e.g., a mobile device or smartphone). A user can perform the finger pressing actuation, and the maximum force oscillation beat over finger pressures, and all sensors can be detected. This beat can be divided by the sensing element area to yield a finger BP waveform beat. For purpose of illustration and not limitation, the sensor array can include sensing elements that can be smaller than the entire sensor. The smaller sensing elements can thus be suitable for use in the sensor array without as high resolution specifications.
As embodied herein, the disclosed device 100 can utilize both a sensitive force sensor and the PPG sensor for the improved accuracy of blood pressure computation. The PPG sensor can be the camera or the finger PPG sensor. The processor can compute systolic and diastolic blood pressures from the AC components of both PPG and pressure measurements. For example, the processor can use an oscillometry model to compute the blood pressure based on the AC components of the PPG and pressure measurements. The device 100 can include a PPG-force sensor unit that can detect the AC pressure waveform. The user can perform the finger pressing method with the sensor unit as described herein. The AC pressure waveform beat of maximal amplitude, which can be concatenated to correspond with the multiple PPG waveform beats, can be selected. The derivative of the PPG waveform can be selected with respect to the concatenated pressure waveform and plotted against the DC pressure measurement to yield data points indicating a shifted arterial compliance curve. For example, a parametric function can be fitted to the data points, and the function can be shifted so that its peak is at zero transmural pressure to arrive at the scaled arterial compliance curve. For purpose of illustration and not limitation, the blood pressure can be computed by forming the oscillogram, taking its derivative with respect to external finger pressure, and performing a cross-correlation between the arterial compliance curve and derivative of the oscillogram. The peak location of the cross-correlation function can denote diastolic BP, and the valley location can denote systolic blood pressure. Alternatively, an oscillometry model with the compliance curve can be fitted to the measured oscillogram or its derivative in an optimal sense to estimate blood pressure.
The following examples are provided for purpose of illustration and confirmation of the disclosed subject matter only and without limitation thereof.
High arterial blood pressure (BP) afflicts about one in three adults worldwide. While the incidence increases with age, many people develop hypertension early in adulthood (e.g., more than one in five US adults under 40 years old are hypertensive). The condition is usually asymptomatic, but the risk for stroke and heart disease increases monotonically with BP for a given age. Lifestyle changes and many inexpensive, once-daily medications can lower BP and cardiovascular risk. Yet, only about three in seven people with hypertension are aware of their condition, and just one of these seven has their BP under control. Epidemiological data on hypertension in low resource settings are more alarming. As a result, hypertension has emerged as the leading cause of disability-adjusted life years lost.
Auscultatory and oscillometric BP measurement devices have been instrumental in managing hypertension. At the same time, these devices may bear responsibility for the abysmal hypertension awareness and control rates due to their reliance on an inflatable cuff. Cuff-based devices are not readily available, especially in low resource settings. Hence, most people do not regularly check their BP. Regular measurements during daily life are needed to circumvent white coat and masked effects in the clinic in which patients present with higher or lower BP than usual and to average out the large variations in BP that occur over time due to stress, physical activity, and other factors. If BP could be measured more conveniently, then many people would become aware of their condition or motivated to take their medications.
Hence, cuff-less BP monitoring devices are being widely pursued. However, the devices under investigation generally suffer from the debilitating limitation of requiring calibrations with cuff devices in order to output a measurement in units of mmHg.
The oscillometric principle has been extended for cuff-less and calibration-free BP monitoring via readily available smartphones.
A device was developed comprised of a custom PPG-force sensor unit affixed to the back of a smartphone to implement this “oscillometric finger pressing method”. The device could yield BP measurements with a level of accuracy comparable to an FDA-cleared finger cuff volume clamping device over the normotensive range.
Techniques for measurement of blood pressures have included using a mobile device with PPG and force sensors and visual indicia to indicate where to place the fingertip on the sensor unit, steadily varying fingertip pressure under guidance of the smartphone, forming an oscillogram (variable-amplitude blood volume oscillations versus applied pressure function), and computing BP from the oscillogram. These techniques provided a convenient site for BP measurement via finger pressing—the transverse palmar arch artery in the fingertip (see
The disclosed subject matter includes related improvements to further advance measurement of blood pressures. These improvements generally circumvent prior techniques.
Improved Apps for Smartphones with Existing PPG and Force Sensors
There are many smartphones with 3D Touch capabilities including iPhone 6s-X models and select Huawei and Xiaomi models. In these phones, the front camera for PPG sensing is at some distance (e.g., 2-8 mm) from the force sensor under the screen, and the base of the nail on the fingertip should be above the front camera for high-fidelity measurement of the blood volume oscillations. Hence, only a small portion of the finger will be on the force sensor under the screen, which will degrade the measurement of force while rendering small changes in finger force to translate to large changes in pressure (i.e., more difficult finger actuation). Conversely, placing a greater portion of the fingertip on the screen will degrade the PPG measurement.
The disclosed subject matter provides techniques configured to target a digital artery running along a side of the finger (see
Apps for Conventional Smartphones without Force Sensors
Given that virtually every adult has real risk for developing hypertension and that smartphones are available to billions of people including those in low resource settings, it is desirable to make standalone smartphones into BP monitors. However, most smartphones do not include 3D Touch or similarly sensitive force sensors. As such, the disclosed subject matter provides techniques to measure absolute BP in units of mmHg using only standard smartphones.
One example uses arm rather than finger actuation. In oscillometry, the cuff compresses the artery to vary its external pressure. During this process, the device also measures the cuff pressure, which indicates both the blood volume oscillations in the artery (AC cuff pressure) and the external pressure (DC cuff pressure). BP is estimated from the resulting oscillogram, which is again the function relating the variable-amplitude blood volume oscillations to the applied pressure. Note that the abscissa of the oscillogram may be viewed more generally as a change in transmural pressure of the artery (i.e., internal BP minus external cuff pressure in this case). Certain methods thus involve varying the internal rather than external pressure of an artery to change the transmural pressure. As a user of a finger worn ring device lowers their hand with arm straight, the internal BP in the finger increases due to the weight of the arm blood column (“hydrostatic effect”) by an amount equal to pgh, where p is the known density of blood, g is gravity, and h is the vertical distance between the hand position and heart. In this way, arterial transmural pressure is varied without a cuff. The device includes a PPG sensor, force sensor, and accelerometer. The accelerometer allows measurement of the hydrostatic BP change (i.e., plg sin θ, where l is the measured arm length, θ is the angle between the arm and horizontal plane, and g sin θ is the accelerometer output). The BP changes for typical arm lengths is about ±50 mmHg with respect to heart level. For a mean BP of 80 mmHg, the transmural pressure variation is about 30 to 130 mmHg. However, the oscillogram in both the positive and negative transmural pressure regimes is needed to compute BP accurately. The ring must thus be worn tight enough to generate negative transmural pressures. The force sensor of known area measures the ring contact pressure on the finger, which is subtracted from the hydrostatic BP change. BP may then be estimated from the PPG oscillations as a function of the transmural pressure change. The main problem is that the ring should be applied with a pressure equal to around mean BP, but BP is what is sought for measurement.
Another problem with bringing the hand raising actuation to a smartphone is eliminating the need for the force sensor. However, note that all smartphones have PPG sensors in the form of a camera or a dedicated sensor (e.g., Samsung Galaxy S series) and three-axis accelerometer/gyroscope combination.
To solve these and other problems, the disclosed subject matter can limit the measurement to pulse pressure (PP=systolic BP−diastolic BP). PP would be useful to detect isolated systolic hypertension, which is a common form of hypertension that occurs with aging.
To explain the overall idea,
Alternatively, the hydrostatic BP change can be estimated without using an accelerometer/gyroscope or any other sensor. While maintaining the constant finger pressure, the user lowers the phone to the floor and raises it upwards in intuitive and fixed increments (e.g., approximately 45 degrees for a 3-5 s at a time guided by the smartphone via audio cues). The hydrostatic BP change can then be estimated based on the known increments. The advantage here is that phone orientation, which can affect accelerometer/gyroscope usage, becomes unimportant such that the hand raising may be easier to perform. In addition, the initial step of determining the constant finger pressure may not be necessary. The user may simply press firmly on the PPG sensor and perform the hand raising. If an inverted U-shaped oscillogram is not observed, then the phone can ask the user to try again or perform the initial step. As another alternative, a smartwatch (e.g., an Apple Watch) with a PPG sensor to measure the PPG waveform from the back of the wrist could be used instead. The initial step can be performed by tightening the watch. The same watch tightness can be used for subsequent BP measurement.
Another example includes techniques to measure systolic and diastolic BP via a standard smartphone by exploiting the existing capacitive sensor array under the screen for accurately measuring finger contact area in addition to the front camera. As shown in
For either example, it is the finger BP that is measured. But brachial BP can be clinically important. Finger BP is lower by about 10 mmHg than brachial BP due to a resistive pressure drop. Finger PP is higher than brachial PP due to wave reflection, especially in more compliant arteries. Hence, finger diastolic and mean BP are lower than brachial diastolic and mean BP, while finger systolic BP is variable relative to brachial systolic BP. To obtain brachial BP, the PPG waveform beat with maximal oscillation is extracted. This beat may best but imperfectly correspond to a finger BP waveform beat. The PPG waveform beat is then calibrated so that its minimum and maximum correspond to the computed finger diastolic and systolic BP. A transfer function (to account for wave reflection) and a regression equation (to account for the resistive pressure drop) or other similar transformations are then applied to convert the finger waveform beat to a brachial BP waveform beat. The minimum and maximum of the brachial BP waveform beat are taken as systolic and diastolic BP. If only finger PP is available, then the PPG waveform beat of maximal amplitude is calibrated so that its amplitude equals finger PP, and a transfer function is then applied to obtain a zero-mean brachial BP waveform. The peak-to-peak amplitude of this waveform gives brachial PP.
Convenient Sensor for BP Measurement at Heart Level
Prior techniques included steps towards ensuring BP measurement at heart level via processing of images of the user. Such image processing is difficult and may not be accurate enough. The disclosed subject matter provides techniques to use sensitive barometric pressure sensors having increased sensitivity for hydrostatic BP correction. These sensors are able to detect height differences of <5 cm, which corresponds to only about <3.5 mmHg error. Exemplary steps are as follows. A device like the one in
Custom Device with Pressure Sensor Alone
PPG sensors, especially those employing visible light, do not work well in low signal conditions, such as in cold environments (e.g., air-conditioned rooms) and dark skin. Using only a pressure sensor that can measure the pulse and pressure over the BP range (e.g., 0-250 mmHg) could overcome the limitations of PPG while simplifying the sensor design or even providing greater accuracy in BP measurement. The disclosed subject matter can include implementing the finger pressing method with a pressure sensor alone based on the applanation tonometry principle. The general principle involves pressing a force sensor on an artery. In this example, the sensor must (i) flatten or “applanate” the artery so that the wall tension is perpendicular to the sensor and (ii) be encompassed by the flattened artery so that pressure may be derived as the ratio of the measured force to the known sensor area.
A device similar to
The parameter k may be determined via an empirical equation relating arterial area to fingertip dimensions (e.g., as measured with a smartphone as shown in
Another way to determine the parameters is to detect diastolic BP (Pd) from the AC pressure waveform, similar to oscillometric algorithms like the fixed-ratio algorithm. Then, the minimum of ΔP(t) is scaled to equal Pd-Pm. Adding Pm to the waveform gives the finger BP waveform, and the peak of this waveform denotes systolic BP (Ps). Systolic BP is typically most difficult to measure via oscillometric algorithms.
In all cases, the finger BP waveform may be transformed to brachial BP and corrected to heart level as described in earlier sections.
Accurate BP Computation Algorithms via PPG and Sensitive Pressure Sensors
A sensitive pressure sensor for measuring the AC pressure waveform and DC external pressure may alternatively be used in conjunction with a PPG sensor to improve the accuracy of BP computation. As previously mentioned, oscillometric algorithms are a bottleneck in achieving clinical accuracy.
Similar to an example method described in the preceding section, one way is to compute diastolic and mean BP from the oscillogram using the fixed-ratio or another algorithm, find the AC pressure beat of maximal amplitude, and scale it so that its minimum and mean equal diastolic and mean BP. Systolic BP is then given as the peak of the calibrated finger BP waveform.
Another way is to invoke a mathematical model of oscillometry. A useful oscillogram model represents the oscillation amplitude (ΔO) as the difference in the arterial blood volume-transmural pressure relationship (f(·)) evaluated at SP and DP (Ps and Pd) as follows:
ΔO=cf(Ps−Pe)−cf(Pd−Pe), (1)
where Pe is the external pressure of the artery and c is a scale factor to convert blood volume to the PPG units. Differentiating this equation with respect to P e yields the following model of the derivative of the oscillogram:
where g(·) is the derivative of f(·) and represents the arterial compliance curve. One exemplary parametric model for the compliance curve is as follows:
where u(·) is the unit-step function, α and β reflect the arterial compliance curve widths over negative and positive transmural pressures, and γ denotes the height of the curve. This model may be fitted to a measured oscillogram alone to determine systolic and diastolic BP and the arterial compliance parameters. Hence, in contrast to conventional oscillometric algorithms, this example algorithm is specific to the person in the sense that both BP and compliance are measured. However, estimating five parameters from the limited information in the oscillogram can be challenging.
Mobile Devices for Venous Blood Pressure Measurement
All of the aforementioned examples pertain to arterial blood pressure (BP) measurement. However, venous blood pressure (VP) is also important. VP may be used to predict the onset of symptoms due to congestion in patients with heart failure (bilateral failure, right heart failure, or left heart failure leading to high pulmonary afterload) and thereby avert costly hospitalizations. VP may also be used to manage pulmonary arterial hypertension patients. However, VP typically requires invasive procedures for its measurement. The disclosed subject matter provides techniques to translate the above concepts for non-invasive measurement of VP.
One method is based on
To achieve these and other advantages, the disclosed subject matter provides techniques to create a finger worn ring sensor including an infrared PPG transducer for digital artery measurement, a force sensor with known contact area, and an accelerometer/gyroscope. It may be put on the finger with a Velcro strap or latching like a belt. The arm length of the person is measured to determine the pgh difference when the hand is at heart level versus fully lowered. The ring is tightened by the user to be roughly equal to this value under audio or visual guidance from the device. A marker may be used, such as belt hole number, to indicate the level of tightness for future use. Then, the user lowers their arm and slowly raises it to heart level as described earlier. The relative vertical height is measured with the accelerometer as also described previously. The plot of the DC and AC components of the PPG waveform versus the hydrostatic BP change minus the finger contact pressure of the sensor, which will range from 0 mmHg to pgh, is then used to detect VP.
Another solution for standard smartphones is to invoke the front camera and capacitive sensor array to measure the full PPG waveform, finger contact area, and finger force as described in previous sections. In this case, because VP is low, the useful ROI of the function relating area to force (see
For both solutions, the AC component of the PPG may also be useful in detecting VP. For example, the maximal oscillation during the steep drop could be reflective of mean VP.
Another example method is based on the finger cuff volume clamping principle, also shown in
In addition to the specific embodiments claimed below, the disclosed subject matter is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features presented in the dependent claims and disclosed above can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter should be recognized as also specifically directed to other embodiments having any other possible combinations. Thus, the foregoing description of specific embodiments of the disclosed subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed.
It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the disclosed subject matter without departing from the spirit or scope of the disclosed subject matter. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents.
This application is a continuation of International Patent Application No. PCT/US2022/011657, filed Jan. 7, 2022, which claims priority to U.S. Provisional patent application Ser. No. 63/135,430, filed Jan. 8, 2021, all of which are hereby incorporated by reference herein in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 63135430 | Jan 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2022/011657 | Jan 2022 | US |
| Child | 18219154 | US |
