WEARABLE, AMBULATORY. CONTINUOUS. NONINVASIVE BLOOD PRESSURE MEASURING METHOD

AND SYSTEM

Field of the Invention

The present invention relates to a method and system for blood pressure measuring. Specifically, this invention relates to a method and system for a wearable, ambulatory, continuous, non-invasive blood pressure monitoring.

Background of the Invention

Blood pressure is the force within the arterial system of an individual that ensures the flow of blood and delivery of oxygen and nutrients to the tissue. Prolonged reduction or loss of pressure severely limits the amount of tissue perfusion and could therefore result in damage or even death of the tissue. Although some tissues can tolerate hypo-perfusion for long periods of time, the brain, heart and kidneys are very sensitive to a reduction in blood flow. Therefore, patients, and specially brain, heart and kidneys patients are very sensitive to blood flow variations.

Because of changes in the patient's blood pressure, constant monitoring is important. Further, in order to provide a patient with the ability to maintain his daily lifestyle the need has arisen for a wearable, ambulatory, continuous, non-invasive blood pressure monitoring.

The traditional methods of measuring blood pressure use inflatable cuffs either auscultator or oscillometry based. Thus, the measurement of blood pressure is with a stethoscope, occlusive cuff and pressure monometer (or equivalent). These are highly obtrusive techniques and can only provide sparsely sampled measurements, which are extremely sensitive to any

exerted, normal or sleep -related motion. Further, these techniques are slow, subjective in nature, require the intervention of a skilled clinician and do not provide timely readings frequently required in critical situations.

For these reasons, two methods of measuring blood pressure have been developed: noninvasive, intermitted methods that use an automated cuff device such as an oscillometric cuff; and invasive, continuous (beat-to-beat) measurements that use a catheter.

The invasive methods have inherent disadvantages including risk of emboli, infection, bleeding and vessel wall damage. Nevertheless, they are considered to be accurate and are used as gold standard in establishing Blood Pressure values.

The common noninvasive measurement techniques can be classified into the auscultative technique, oscillometric technique the method of tonometry and PWV-based methods.

The auscultative technique is usually used with a mercury sphygmomanometer. The basic components in the mercury sphygmomanometer include an inflatable cuff and a mercury barometer. In some of the newer products, the mercury barometer is replaced by an electronic pressure sensor. Nonetheless, the working principle remains the same: arterial blood pressure is to be measured on the upper arm of a person; air in the cuff is first released, and then the cuff is wrapped around an upper limb; a stethoscope is need for the detection of the Korotkoff sounds; the head of the stethoscope is placed over the brachial artery; air is pumped into the cuff until the level of mercury in the barometer reaches a predetermined threshold; the Korotkoff sounds are to be detected while

the cuff is being slowly deflated; the cuff pressures at which the first and the fifth Korotkoff sounds are detected represent the systolic BP (SBP) and diastolic BP (DBP) of the subject respectively.

Although this method can be used to identify the systolic and the diastolic blood pressure, it is not suitable for measuring the blood pressure of those people with weaker 5th Korotkoff sounds.

The oscillometric technique can overcome the shortcoming of the auscultative technique in the way that it can measure blood pressure of those who have weak Korotkoff sounds. However, the technique also requires an inflatable cuff to be wrapped around the upper limb of the subject. While the inflated cuff is slowly deflated, the pressure in the cuff oscillates as a result of arterial pulsation. From the pattern of oscillation, the mean blood pressure, systolic blood pressure and diastolic blood pressure are determined. Mean blood pressure is the cuff pressure at which the maximum amplitude of oscillation is detected. Systolic blood pressure and diastolic blood pressure are estimated from the mean blood pressure and the oscillation pattern.

Nevertheless, the oscillometric cuff method typically requires 15 to 45 seconds to obtain a measurement and should allow sufficient time for venous recovery. Thus, at best there is typically 0.5 to 1 minute between updated pressure measurements. This is an inordinately long amount of time to wait for an updated pressure reading when fast acting medications are administrated. Also, too frequent cuff inflations over extended periods may result in ecchymosis and/or nerve damage in the area underlying the cuff.

The principle of the tonometry is to gently press a vessel against the underlying bone in order to counteract the circumferential stress at the vessel wall. There should be an external pressure at which the internal and external pressures of the vessel are thereby equalized and the external pressure is modulated such that the equalization is maintained. The applied pressures are recorded by an array of sensors positioned on the surface of the artery, to thereby determine the blood pressure. Nevertheless, the accuracy of the measurement is readily affected by positions and angles of the sensors.

Applanation Tonomety based Blood Pressure measurement is a non invasive and simple to measure method. Several companies had launched commercial products for the doctor's office (where the subject is supposed to be still), such at Hypertension Diagnostics, MetdTech, and others capable to continuously measure blood pressure. However, tonometry necessitates the application of controlled and steady counter pressure on the measuring sensor, and in addition it exercises high sensitivity to motion artifacts in the detected signals. Tonometry based devices failed to replace older oscillometric or auscultation based devices fundamentally because they are as bulky and in addition, they failed to prove reliable. Unlike older methods, tonometry based devices can provide continuous BP measurement. However, because of their sensitivity to motion, they might be useful at rest only, and as such are inadequate for a mobile device.

PTT (or Pulse Transition Time) claims to be more robust to motion and less sensitive to absolute pressure (amplitude) values. All PTT variants fundamentally measure Pulse Transition Time (PTT) between two (or more) body points close to arteries. Some variants of PTT have been explored over the years and have been perfected using faster and more accurate technology and better algorithms. Examples of such studies are

Yio-Wlia Shau, Noninvasive Assessment of the Viscoelasticity of Peripheral Arteries, Ultrasound in Med. & Biol., Vol. 25, No.9, pp. 1377-1388, 1999; Michael F. O'Rourke, Aortic Diameter, Aortic Stiffness, and Wave Reflection Increase With Age and Isolated Systolic Hypertension, 2005; C. J. Mills, Pressure-Flow Relationship and Vascular Impedance in Man, Cardiovascular Research, 1970, 4, 405-417; B.S. Brook, A Model for Time- Dependent Flowing (Giraffe Jugular) Veins: Uniform Tube Properties, Journal of Biomechanics 35, 2002; S.J. Sherwin, One-Dimensional Modeling of a Vascular Network in Space-Time Variables, Kluwer Academic Publishers, 2002; S.J. Sherwin, Computational Modeling of ID Blood Flow with Variable Mechanical Properties and its Application to the Simulation of Wave Propagation in the Human Arterial System, International Journal For Numerical Methods In Fluids, 43: 673-700, 2003.

Since the patient is in motion, in addition to modifications to the conventional measuring process, additional fundamental arguments in the PTT measuring process need further consideration: the effects of gravity on the cardiovascular system due to continuous dynamics of the extremities and barometric pressure impact on the measuring process in reference to the calibration point, as discussed in the article The Role of Body Position and Gravity in the Symptoms and Treatment of Various Medical Diseases, R'my C. Martin-Du Pana, Raymond Benoitb, Lucia Girardierc, SWISS MED WKLY, 134:543-551, 2004.

U.S. Pat. No. 5,564 427 (Aso et al.) proposed the use of a linear equation to calculate blood pressure (BP) using the pulse transit time (PTT). This method was further developed by U.S. Pat. No. 5,649,543 (Hosaka et al.). U.S. Pat. No. 5,709,212 (Sugo et al.) introduced a multi-parameter approach to determine the parameters at different blood pressure levels for systolic and diastolic pressures respectively in order to calibrate the linear

measurement system. Japanese patent No. 10-151118 (Shirasaki et al.) introduced another method to calibrate the parameters based on the multiple blood pressure reference inputs in. Nevertheless, some of the variants necessitate synchronization to the heart beat while others use controlled counter pressure (called vascular unloading).

Australian Atcor Medical has launched the SphygmoCor family which is all based on PWV (pressure wave velocity, a variant of PTT, that calculates the pressure wave velocity between two locations at a known distance by measuring the Pulse Transition Time, or PTT between these two sites) measurements between body's extremities. However, Atcor Medical products have been proved to be effective for stationary measurements exclusively.

Therefore, the need has arisen for a wearable, ambulatory, continuous, non-invasive blood pressure monitoring in order to provide a constant blood pressure monitoring while providing a patient with the ability to maintain his daily lifestyle.

Therefore, the need has arisen for a wearable, ambulatory, continuous, non-invasive blood pressure monitoring in order to provide a constant blood pressure monitoring while providing a patient with the ability to maintain his daily lifestyle.

It is another object of the present invention to provide a method and a system for ambulatory blood pressure monitoring.

This and other advantages and objects of the present invention will become apparent from the description and claims to follow.

Summarv of the Invention

The present invention relates to a method for the wearable, ambulatory, continuous, non-invasive blood pressure and pulse rate monitoring under patient's normal day to day activity, comprising the steps of: sensing the patient's blood pressure wave signals along with the patient's motion- induced signals, using a sensor surface array located along the patient's Artery; sensing the patient's motion-induced signals, using a sensor surface array located perpendicularly to the patient's Artery; receiving each sensor's signals through its associated channel; amplifying both the motion- induced and the blood pressure wave sensed signals; filtering both the motion-induced and the blood pressure wave amplified signals; transforming both the motion-induced and the blood pressure wave filtered signals from analog to digital representation; Band Pass filtering both the motion-induced and the blood pressure wave digitized signals; transforming both the motion-induced and the blood pressure wave filtered signals into a map of vectors; transforming the motion-induced vectored signals by 90 degrees; subtracting the transformed motion-induced vectored signals from the blood pressure wave vectored signals; calculating the time lag between the different sensor blood pressure wave signals; calculating the patient's pressure wave velocity using said time lag and the distance between the sensors located along the patient's Artery; calculating the patient's blood pressure and pulse rate using the PWV method.

Preferably, the sensor array located along the patient's Artery comprises an array pressure surface sensors arranged as an XY surface.

In an embodiment, the sensor array located along the patient's Radial Artery comprises and array of optical surface sensors arranged as an XY surface.

Preferably, the sensor array located perpendicularly to the patient's Artery comprises pressure surface sensors.

In an embodiment, the sensor array located perpendicularly to the patient's Artery comprises optical surface sensors.

Preferably, the compensating for the effects of gravity on the cardiovascular system and for the barometric pressure impact in reference to the calibration point, using acceleration and hydrostatic pressure gauges which are attached to the hybrid sensor's center.

The invention further relates to a system for a wearable, ambulatory, continuous, non-invasive blood pressure and pulse rate monitoring under patient's normal day to day activity, comprising: a surface sensor array located along the patient's Artery, to sense the patient's blood pressure wave signals along with the patient's motion-induced signals; a sensor surface array located perpendicularly to the patient's Artery, to sense the patient's motion-induced signals; associated channels for receiving each sensor's signals; means for amplifying both the motion-induced and the blood pressure wave sensed signals; means for filtering both the motion- induced and the blood pressure wave amplified signals; means for transforming both the motion-induced and the blood pressure wave filtered signals from analog to digital representation; a Band Pass filter for filtering both the motion-induced and the blood pressure wave digitized signals; means for transforming both the motion-induced and the blood pressure wave filtered signals into a map of vectors; means for transforming the motion-induced vectored signals by 90 degrees; means for subtracting the transformed motion-induced vectored signals from the blood pressure wave vectored signals; means for calculating the time lag between the different sensors' motion- induced signals; means for calculating the patient's

pressure wave velocity using said time lag and the distance between the sensors placed along the patient's Artery; means for calculating the patient's blood pressure and pulse rate using the PWV method.

Preferably, the sensor surface array located along the patient's Artery comprises pressure sensors.

In an embodiment, the sensor surface array located along the patient's Artery comprises optical sensors.

Preferably, the sensor surface array located perpendicularly to the patient's Artery comprises pressure sensors.

In an embodiment, the sensor surface array located perpendicularly to the patient's Artery comprises optical sensors.

Preferably, the method further comprises acceleration and hydrostatic pressure gauges which are attached to the hybrid sensor's center, compensating for the effects of gravity on the cardiovascular system and for the barometric pressure impact on the sensor in reference to the calibration point using.

Brief Description of the Drawings

In the drawings:

- Fig. 1 is a schematic top view of the logical measuring process of the hybrid sensor for PWV signal sensing with the orthogonal motion compensation method (on the X axis) according to a preferred embodiment of the present invention;

Fig. 2A is a schematic top view of the mechanical measuring process of a hybrid sensor for PWV signal sensing with the orthogonal

motion compensation method, according to a preferred embodiment of the present invention;

- Fig. 2B is a schematic lateral view of the mechanical measuring process of a hybrid sensor for PWV signal sensing with the orthogonal motion compensation method, according to a preferred embodiment of the present invention;

- Fig. 2 C is a schematic axial view of the mechanical measuring process of a hybrid sensor for PWV signal sensing with the orthogonal motion compensation method, according to a preferred embodiment of the present invention;

- Fig. 3 is a flow diagram showing the key processing steps of the hybrid sensor's detected signals which includes the orthogonal motion compensation method, according to a preferred embodiment of the present invention;

Fig. 4A is a schematic view of the present invention's four points sensing device;

- Fig. 4B is a schematic view showing the signals' readings of the four points sensing device, according to a preferred embodiment of the present invention; and

- Fig. 4C is a schematic view showing the signals' processing of the four points sensing device, according to a preferred embodiment of the present invention.

Detailed Description of Preferred Embodiments

In order to provide a real time, continuous, beat-to-beat blood pressure measurements and pulse rate, under patient's normal day to day activity and, if desired, around-the-clock, a designated, miniature, wearable sensor that facilitates the PWV (Pressure Wave Velocity) measurement in motion in accordance with a customized motion noise reduction process is described hereinafter with reference to Figs. 1-7.

The customized motion reduction process of the invention is termed "orthogonal motion compensation method". A short description is provided hereinafter.

The Sensor described here uses a plurality of optical (or pressure) sensing elements alongside the artery, designated schematically as Yi and Y2 array (in Figs 2A) to measure the arterial pulsation. This array generates a two dimensional sensing surface. The arterial pulsation measured signal(s) by Yi and Y2 array consist a superposition of arterial pulsation information as well as motion induced signals within the same frequency band and phase.

In addition, the Sensor described here uses a plurality of optical (or pressure) sensing elements perpendicular to the artery measured, designated schematically as Xi and X2 array (in Drawing 2A). This array generates a two dimensional sensing surface. The array is aimed to measure motion induced signals in close proximity to the artery area measured by the Yi and Y2 array. The signal(s) measured by Xi and X2 array is composed mainly of motion induced signal within the same frequency band as signals from Yi and Y2 array.

The motion induced signals measured by the Xi and X2 array isn't white (random) but limited by the degrees of mechanical motion allowed by the body area being measured while moving thus creating what is called "pink noise". The signals recorded from this surface are first filtered and then transformed into a map of local x',y' vectors. The majority of those vectors will then describe motion signals on a surface space. Because of the hypothesis that the motion noise is pink, the Xi and X2 array will generate a two dimensional motion vectors map to be called the motion fingerprint.

The Xi and X2 array is located perpendicular to the Yi and Y ₂ arterial pulsation array and covering the same area However, because of its location, the Xi and X2 array carries a far lesser S/N of the pulsation signal and measures the same motion noises but perpendicular to the Yi and Y2 array.

When transforming the motion induced signals of the Xi and X2 array by 90 degrees the same motion fingerprint is obtained as from the Yi and Y2 array. Subtracting the transformed motion signal vectors obtained from the Xi ⁹⁰ and X2 ⁹⁰ array from the Yi and Y2 array map allows for a dramatic reduction of motion induced artifacts, thus improving S/N and accuracy.

The validity of this method stipulates that Xi and X2 and Yi and Y2 arrays cover the same area and that the area is as small as possible.

Fig. 1 is a schematic top view of a hybrid sensor for PWV signal sensing, with orthogonal motion compensation, according to a preferred embodiment of the present invention. The measurement is done between very close points (distance d) apart on a single artery 101, no artery ramifications being present at a location at least 10 times the distance d upstream, and the mean diameter of the measured artery being fairly constant for said upstream distance. For this purpose, the wrist or ankles are regions of choice, thus, the single artery 101 is preferably the Radial Artery wherein the distance d approximately equals 5 times the artery diameter D. Furthermore, the physiology of said regions is known to carry longitudinal muscle and tendons, and as such the stress induced by motion on those regions has predictable angular and longitudinal degrees of freedom. Thus, the sensed noise isn't isotropic nor random in nature but directional. The directional noise is filtered out by means of the orthogonal motion compensation method and software that uses methods of signal processing

to determine systolic and diastolic PWV signals. For example see "Determination of Pulse Wave Velocities with computerized algorithms", Y Christopher Chiu, MD, Patricia W. Arand Phd., Sanjeev G. Shroff Phd., Ted Feldman MD, and John D. Carroll MD., Chicago III. The section of Cardiology, Department of Medicine, University of Chicago hospitals, American Heart Journal, May 1991.

In fig. 1 Y indicates the arterial pulse detection axis (pressure/optical) sensing, and X the motion detection axis (pressure/optical) sensing.

A Sensor array (Yi and Y2) 202 is placed along the distance d on the artery 101 to sense the blood pressure wave creating the sensed vectored signal 102. Another set of sensor array (Xi and X2) 201 are placed along the distance d, orthogonally to the Radial Artery 101, sense the motion- induced noise signals 103. The signal 103 is 90 degrees transformed, creating a new signal 104. The signal 104 is subtracted from signal 102. The outcome generates a new signal set 105, which is used to calculate the PWV which in turn results consequently in calculating patient's blood pressure. A reference to how to determine PWV was provided hereinabove.

In one embodiment, the hybrid sensor further comprises acceleration and hydrostatic pressure gauges, which are attached to the hybrid sensor's center. Effects of gravity on the cardiovascular system due to continuous dynamics (motion) of the extremities and barometric pressure impact in reference to the calibration point, cause hemodynamic changes. The acceleration and hydrostatic pressure gauges compensate for the abovementioned hemodynamic changes which affect the measured blood pressure.

The hydrostatic pressure gauges output compensates the blood pressure calculations to assume for body elevation changes from a calibration position, while the accelerometer output compensates the blood pressure calculations to assume hand instantaneous location relative to the heart and initial calibration position.

Figs. 2A-2C are schematic views of the mechanical arrangement of a hybrid sensor 203 for PWV signal sensing with the orthogonal motion compensation method , according to a preferred embodiment of the present invention. Pressure/optical sensors (201 and 202), are placed orthogonally to each other. Signal sensor array 202 is placed along the Radial Artery 101 in order to sense the blood pressure wave along with the patient's motion- induced noises. Motion sensor array 201 is placed orthogonally to the Radial Artery 101, in order to sense the motion-induced noises. The motion- induced noises are deducted from the blood pressure wave (containing the patient's motion-induced noises) in order to calculate the PWV.

In the figure 202 is a pulsation sensing array, 201 is a motion sensing array, 205 represents the tissue/fat and 206 the wrist.

Fig. 3 is a flow diagram showing the key processing steps of the hybrid sensor's detected signals, according to a preferred embodiment of the present invention. The motion-induced signals, detected by the motion sensors 201 shown in Figs. 2A-2C, are amplified and filtered, in block 301. In block 302 the amplified and filtered signals are transferred from analog to digital representation. The digitized signals are filtered once more in block 303, using a Band Pass filtering. The motion-induced noise measured by the pressure sensors is not white noise (random), but it is vectored and limited by the degrees of mechanical motion allowed by the wrist while moving.

The sensor array is spread orthogonally to the Radial Artery, thus creating an X, Y, surface. In block 304 the signals recorded from this surface are transformed into a map of vectors. The vectored noise part of the signals generates specific vectored fingerprint in the overall pressure pulsation map. The perpendicular motion sensing array 201 covers the same area and measures the same vectored motion noise, as the arterial pulsation sensing array 202 with 90 degrees shift. Therefore, the vectored motion noise is transformed by 90 degrees in block 305 in order to be subtracted in block 309 from the arterial pulsation sensing signals 202, thus allowing a dramatic reduction of motion-induced artifacts and thus improving signal- to-noise ratios and accuracy.

The blood pressure wave signals, detected by signal sensors 202 shown in Figs. 2A-2C, are amplified and filtered, in block 306 similarly to block 301. In block 307 the amplified and filtered signals are transferred from analog to digital representation and the digitized signals are filtered once more with a Band Pass filtering. In block 308 the signals recorded by the signal sensors are transformed into a map of vectors. The signals recorded by the motion sensors 201 (shown in figs. 2.a-2.c) are subtracted from the signals recorded by the Signal sensors 202 (shown in Figs. 2A-2C) in block 309.

The processed signal 310 is transferred for the PWV computation wherein the pressure wave velocity is revealed and accordingly the patient's blood pressure and pulse rate.

Fig 4A is a logical schematic view of the four points sensing method according to a preferred embodiment of the present invention. The wave velocity of the heart pulse signal is found using a hybrid sensor containing two (or more) sensor arrays. An array comprising two (or more) sensors 401

is placed perpendicularly to another array containing two signal sensors 402 and 403. Signal sensors 402 and 403 are placed along the patient's Artery 400, measuring both the blood pulse wave and the patient's motion induced noises. The motion sensors 401 are placed perpendicularly to the patient's Radial Artery, measuring the patient's motion-induced noises. Since the signal measured by the signal sensors 402 and 403 includes both the patient's motion-induced noise and the patient's heart pulse signal, the signal-to-noise ratio of the measured signal is low. Therefore, the patient's motion-induced noise must be deducted from the signal measured by the signal sensors 402 and 403. Since the motion induced noise measured by the array of motion sensors 401 is the same as the motion- induced noise contained in the signal measured by the signal sensors 402 and 403, the patient's motion-induced noise is deducted from the signal measured by the signal sensors 402 and 403 by deducting the motion-induced noise measured by motion sensors 401 array from the signal measured by the Signal sensors 402 and 403. Hence, a dramatic reduction of motion- induced artifacts is achieved, and thus, the signal-to-noise ratio and the measured patient's heart pulse signal accuracy are dramatically improved.

Fig 4B schematically shows the signals' readings of the four (or more) points sensing method, according to a preferred embodiment of the present invention. Plot 404 describes the signal detected by signal sensor 403, located on the patient's Artery. Plot 405 describes the signal detected by signal sensor 402, located on the patient's Radial Artery with distance dx from Signal sensor 403. Plot 406 describes the average of the signals detected by the array of Motion sensors 401, located perpendicularly to the patient's Artery.

Fig. 4C schematically shows the signals' processing of the four (or more) points sensing method, according to a preferred embodiment of the present

- 17 - invention. Plot 407 describes the correlation between the signal 404 detected by signal sensors 403 located along the patient's Artery and signal 405 detected by signal sensors 402 located along the patient's Artery with distance dx from signal sensor 403. The average of the signals detected by the array of motion sensors 401, which is described by plot 406 (shown in fig. 4.b), is deducted from both plot 404 and from plot 405 creating plots 409 and plot 410 respectively. Hence, plots 409 and plot 410 respectively represent the patient's heart pulse signal free of the patient's motion- induced noise. Plot 408 describes the new correlation between plot 409 and plot 410. The system identifies corresponding points on both plot 409 and plot 410. The system measures the time lag (dt) between the abovementioned corresponding points. The system calculates the patient's heart pressure wave velocity using the abovementioned time lag (dt) and the distance between the signal sensors 402 and 403.

Although embodiments of the present invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without departing from its spirit or exceeding the scope of the claims.