Patent ID: 12220213

DETAILED DESCRIPTION OF THE INVENTION

FIG.1illustrates a propagation path of a blood pressure pulse from ejection from the left ventricle of a subject's heart to a wrist on which a wrist-worn blood-pressure measurement device10is worn, in accordance with many embodiments. The wrist-worn device10is configured to detect when the blood corresponding to the blood pressure pulse is ejected from the left ventricle of a subjects heart and when the blood pressure pulse arrives at the wrist-worn device10. The wrist-worn device10is configured to calculate a pulse transit time (PTT) for the blood pressure pulse for the transit of the blood pressure pulse from the left ventricle to the wrist-worn device10. The determined PTT is then used to determine one or more blood-pressure values for the subject.

In general, a PTT is the time it takes for a pulse pressure wave to propagate through a length of a subject's arterial tree. PTT has a nonlinear relationship with blood pressure. Factors that can impact how fast a blood pressure pulse will travel at a given blood-pressure in a particular artery, include, for example, arterial stiffness, arterial wall thickness, and arterial inner diameter. Equation (1) provides a functional relationship between PTT and mean arterial blood pressure (MAP).

MAP=1α⁢ln⁡[ρ⁢⁢D⁡(Δ⁢⁢d)2hE0⁡(PTT)2](1)
where: MAP is mean arterial blood pressure;PTT is Pulse Transit Time;h is arterial wall thickness;D is artery diameter;ρ is density of blood;E0is the Young's modulus of the artery at zero pressure;α is a subject dependent physiological constant; andΔd is the arterial distance between the subjects left ventricle and the wrist.

The pressure pulse travels through different arteries during its transit from the left ventricle to the wrist. As a result, variation in corresponding variables in equation (1), for example, arterial wall thickness (h), artery diameter (D), and Young's modulus of the artery at zero pressure (E0), will change the relationship between blood pressure and how fast the blood pressure pulse travels through the respective artery. Each blood pressure pulse, however, will travel through the same arteries during transit from the left ventricle to the wrist. Accordingly, a relationship between the overall PTT from the left ventricle to the wrist and MAP can be given by replacing arterial wall thickness (h), artery diameter (D), and Young's modulus of the artery at zero pressure (E0) with respective effective values suitable for the combination of all the arteries through which the pressure pulse travels from the left ventricle to the wrist. Therefore, equation (1) can be simplified to the relationship given below in equation (2).

MAP=1α⁢ln⁡[K(PTT)2](2)
where:

K=ρ⁢⁢D⁡(Δ⁢⁢d)2hE0
is suitable for the subject and the arterial tree segment over which PTT is being measured.

The values of (K) and (α) can be determined using any suitable approach. For example, an oscillometric blood pressure measurement cuff can be used to measure one or more blood pressure values for the subject at or at about the same time as when corresponding one or more PTTs are determined for the subject via the wrist-worn device10. Suitable calibration data can then be formulated using the oscillometric blood pressure measurement cuff measured blood pressure values and the corresponding one or more PTTs for the subject using known approaches. For example, a least squares method can be used to determine suitable values or relationships for determining the values of (K) and (α).

A similar approach can be used to predict MAP, systolic blood pressure (SBP), and diastolic blood pressure (DBP) values based on a measured PTT value. For example, equations (3), (4), and (5) are example regression equations that can be used to predict MAP, SBP, and DBP, respectively, from a measured PTT.
MAP=KMAP×[log(PTT)−log(PTT0)]+MAPBASELINE(3)
where: MAP is predicted mean arterial blood pressure;MAPBASELINEis a baseline measured MAP;KMAPis a subject dependent constant for MAP;PTT is the measured pulse transit time; andPTT0is the measured pulse transit time for MAPBASELINE.
SBP=KSBP×[log(PTT)−log(PTT0)]+SBPBASELINE(4)
where: SBP is predicted systolic blood pressure;SBPBASELINEis a baseline measured systolic blood pressure;KSBPis a subject dependent constant for systolic blood pressure;PTT is the measured pulse transit time; andPTT0is the measured pulse transit time for SBPBASELINE.
DBP=KDBP×[log(PTT)−log(PTT0)]+DBPBASELINE(5)
where: DBP is predicted diastolic blood pressure;DBPBASELINEis a baseline measured diastolic blood pressure;KDBPis a subject dependent constant for diastolic blood pressure;PTT is the measured pulse transit time; andPTT0is the measured pulse transit time for DBPBASELINE.

FIG.2shows an EKG trace segment12, an ICG trace segment14, and a PPG signal16relative to a pulse transit time (PTT)18for a blood pressure pulse between the left ventricle of the subject to the wrist-worn device10. In many embodiments, the wrist-worn device10includes electrodes used to generate an EKG trace and an ICG trace for the subject and a PPG sensor to generate a PPG signal for the subject. The EKG trace segment12has a segment (QRS) known as the QRS complex, which reflects the rapid depolarization of the right and left ventricles. The prominent peak (R) of the EKG trace corresponds to beginning of contraction of the left ventricle. A pulse arrival time (PAT)20is the time between the peak (R) of the EKG trace and arrival of the blood pressure pulse at the wrist-worn device10. As the left ventricle contacts, pressure builds within the left ventricle to a point where the pressure exceeds pressure in the aorta thereby causing the aortic valve to open. A pre-ejection period (PEP)22is the time period between the peak (R) of the EKG trace and the opening of the aortic valve. The PEP22correlates poorly with blood pressure. The ICG trace14provides a better indication as to when the aortic valve opens. The ejection of blood from the left-ventricle into the aorta results in a significant temporary decrease in the thoracic impedance of the subject, which corresponds to a temporary increase in the ICG trace, which is the negative of the change of impedance with time. Accordingly, in many embodiments, the ICG trace14is processes to identify a start24of the temporary increase in the ICG trace as corresponding to the opening of the aortic valve and the start of the propagation of the blood pressure pulse. In many embodiments, the arrival of the blood pressure pulse is detected via the PPG signal16, which includes an inflection point26that occurs upon arrival of the blood pressure pulse to the wrist-worn device10.

FIG.3schematically illustrates a four-electrode configuration30used to measure impedance of a subject, in accordance with many embodiments. The four-electrode configuration30includes a drive current generator32electrically coupled with a first drive current electrode34and a second drive current electrode36. In many embodiments, the drive current generator32imparts an alternating current to a subject38via the electrodes34,36. The four-electrode configuration30also includes a voltage sensor40electrically coupled with a first sense electrode42and a second sense electrode44. The use of the sense electrodes42,44, which are separated from the drive current electrodes34,36, serves to reduce the impact of impedance and contract resistance by sensing voltage with electrodes that are transferring much lower levels of current relative to the current drive electrodes34,36. In many embodiments, the alternating drive current has a frequency between 20 kHz and 100 kHz. Drive currents below 20 kHz may create muscle excitation. And while drive currents at 100 kHz produces skin-electrode impedance approximately 100 times lower than at low frequencies, applied drive currents at greater than 100 kHz may result in stray capacitance. A drive current of about 85 kHz is preferred.

FIG.4shows a side view of a wrist-worn blood-pressure measurement device50, in accordance with many embodiments. The wrist-worn device50includes a main unit52, a wrist-worn elongate band54, a first drive current electrode56, a first sense electrode58, a second drive current electrode60, a second sense electrode62, and a PPG sensor64. The first drive current electrode56, the first sense electrode58, and the PPG sensor64are: 1) supported on the wrist-worn elongate band54,2) positioned and oriented to interface with a subject's wrist upon which the wrist-worn device50is worn, and 3) operatively connected with the main unit52. The second drive current electrode60and the second sense electrode62are: 1) supported on the wrist-worn elongate band, 2) positioned and oriented to be interfaceable with the subject so that the drive current travels through the thoracic cavity of the subject (e.g., with separate fingers on the arm opposite to the arm on which the wrist-worn device50is worn), and 3) operatively connected with the main unit52. The main unit52includes circuitry and/or software for imparting drive current through the subject via the first and second drive current electrodes56,60and for processing signals from the PPG sensor64and the first and second sense electrodes58,62so as to measure a PTT and calculate one or more blood pressure values for the subject based on the PTT.

FIG.5shows a side view of another wrist-worn blood-pressure measurement device70, in accordance with many embodiments. The wrist-worn device70includes the same components as for the wrist-worn device50, but has the first drive current electrode56and the first sense electrode58located to enhance contact pressure with a wrist72of the subject. In the illustrated embodiment, the first drive current electrode56is disposed on a directly opposite inside surface of the wrist-worn band54relative to the second drive current electrode60such that contact pressure between, for example, a finger of the subject and the second drive current electrode60transfers compression through the wrist-worn band54to the first drive current electrode56, thereby increasing contact pressure between the first drive current electrode56and the wrist72. In a similar fashion, the first sense electrode58is disposed on a directly opposite inside surface of the wrist-worn band54relative to the second sense electrode62such that contact pressure between, for example, a finger of the subject and the second sense electrode62transfers compression through the wrist-worn band54to the first sense electrode58, thereby increasing contact pressure between the first sense electrode58and the wrist72. Any suitable variation can be used. For example, the locations of the first drive current electrode56and the first sense electrode58can be exchanged. As another example, the electrodes56,58,60,62can be located at any other suitable locations on the wrist-worn band54. As another example, any suitable number of the electrodes56,58,60,62can be disposed on the main unit52.

In the illustrated embodiment, the PPG sensor64is located on the wrist-worn band54so as to be disposed to sense the arrival of the blood-pressure pulse within a radial artery74of the subject. Cross sections of the ulna bone76and the radius bone78of the subject are shown for reference.

FIG.6schematically illustrates electrode locations and related body impedances in an approach for measuring chest cavity impedances, in accordance with many embodiments. In the illustrated approach, the first drive current electrode56and the first sense electrode58are held in contact with the left wrist of the subject. The second drive current electrode60is contacted by the right index finger of the subject. The second sense electrode62is contacted by the right thumb of the subject. The first and second drive current electrodes56,60impart a cross-body alternating drive current80between the drive current electrodes56,60. The cross-body drive current80propagates through the left wrist, through the left arm, through the thoracic cavity, through the right arm, and through the right index finger. The combined impedance of the left wrist local to the first drive current electrode56and the contact impedance of the first drive current electrode56and the left wrist is schematically represented as an impedance (Z1). The combined impedance of the right index finger in contact with the second drive current electrode60and the contact impedance of the second drive current electrode60and the right index finger is schematically represented as an impedance (Z3). The net cross-body impedance between the impedances (Z1and Z3) is schematically represented as an impedance (Z5). The combined impedance of the left wrist local to the first sense electrode58and the contact impedance of the first sense electrode58and the left wrist is schematically represented as an impedance (Z2). The combined impedance of the right thumb in contact with the second sense electrode62and the contact impedance of the second sense electrode62and the right thumb is schematically represented as an impedance (Z4). In many embodiments, because the first and second sense electrodes58,62are configured to measure a voltage difference without transferring any significant amount of current, the resulting voltage drops across the impedances (Z2and Z4) are small so that the voltage difference sensed by the first and second sense electrodes58,62matches the voltage difference across the impedance (Z5).

FIG.6Ashows a side view of another wrist-worn blood-pressure measurement device71, in accordance with many embodiments. The wrist-worn device71includes the same components as for the wrist-worn device70, but has the second drive current electrode60and the second sense electrode62located so that they can be engaged with another portion of the user via the user positioning the arm on which the wrist-worn device71is worn so as to press the electrodes60,62into contact with any suitable skin portion of the user. For example,FIG.6Aillustrates the electrodes60,62being pressed against a skin location on the user's thorax73(e.g., lower breast skin opposite to the arm on which the device71is worn). As another example, the electrodes60,62can be pressed against skin on the user's arm opposite to the arm on which the device71is worn.

FIG.7schematically represents an embodiment of a wrist-worn device for measuring blood pressure. In the illustrated embodiment, the wrist-worn device includes one or more processors82, memory84, a display86, one or more input/output devices88, a data bus90, an ICG/EKG unit92, the PPG sensor64, and a PPG sensor control unit94. In many embodiments, the memory84includes read only memory (ROM)96, and random access memory (RAM)98. The one or more processors82can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA).

The ICG/EKG unit92includes an ICG/EKG signal processing unit100, an ICG/EKG digital to analog unit102, an ICG/EKG analog front end unit104, and an ICG/EKG analog to digital unit106. The signal processing unit100generates a digital alternating drive signal (e.g., a digital drive signal corresponding to an 85 kHz sinusoidal drive current) and supplies the digital alternating drive signal to the digital to analog unit102. The digital to analog unit102generates a sinusoidal drive current matching the digital alternating drive signal and supplies the sinusoidal drive current to the analog front end unit104. The analog front end unit104supplies the sinusoidal drive current to the first and second drive current electrodes56,60for propagation through the subject (e.g., as the cross-body alternating drive current80illustrated inFIG.6). Resulting voltage levels are sensed via the first and second sense electrodes58,62. Signals from the sense electrodes58,62are processed by the analog front end unit104to generate an analog voltage signal supplied to the analog to digital unit106. The analog to digital unit106converts analog voltage signal to a corresponding digital signal that is supplied to the signal processing unit100. The signal processing unit100then generates corresponding ICG/EKG digital data that can be processed by the one or more processors82to determine the opening of the aortic valve and therefore the corresponding start of the propagation of a blood pressure pulse from the left ventricle to the wrist-worn device.

The PPG sensor unit64includes a PPG illumination unit108and detector line array110. The PPG illumination unit108includes two light sources112,114which transmit light having different wavelengths onto the wrist. While any suitable wavelengths can be used, the first light source112generates a beam of light having a wavelength of 525 nm. The second light source114generates a beam of light having a wavelength of 940 nm. Any suitable number of light sources and corresponding wavelengths can be used and selected to provide desired variation in tissue penetrating characteristics of the light. The detector line array110can include any suitable number of light detectors. In many embodiments, the light detectors are disposed at a plurality of different distances from the light sources112,114so that the detected light is associated with different mean penetration depths so as to enable detection of the arrival of the blood pressure pulse at different layers and/or within a layer of the wrist deeper than a layer sensed by a single light source and single detector PPG sensor. In the illustrated embodiment, the detector line array110includes four light detectors116,118,120,122, with each of the light detectors116,118,120,122being disposed at a different distance from the light sources112,114. For example, the light detectors116,118,120,122can be disposed at 2 mm, 3 mm, 4 mm, 6 mm, or 10 mm respectively, from each of the light sources112,114. Signals generated by the light detectors116,118,120,122are supplied to the PPG control unit94, which includes an analog to digital converter to generate PPG sensor digital data that can be processed by the one or more processors82to determine the arrival of the blood pressure pulse to the wrist-worn device. The PPG control unit94controls activation of the light sources112,114, and can alternately illuminate the light sources112,114at a frequency sufficiently high to enable combined assessment of the PPG sensor digital data generated by illumination of the wrist with the different wavelengths provided by the light sources112,114.

The generated ICG/EKG digital data and the PPG sensor digital data can be transferred to, and stored in, the RAM98for any suitable subsequent use. For example, the data can be: 1) processed by the one or more processors82to determine PTTs and corresponding blood pressure values for the subject, 2) displayed on the display86, and/or 3) output via the input/output devices88for any suitable purpose such as to a health care professional and/or a monitoring service. In many embodiments, the one or more processors82processes the ICG/EKG and PPG sensor digital data to generate trending data for a time period based on the one or more relative blood pressure values. Such trending data can be generated for any suitable time period, for example, for one or more days, one or more weeks, one or more months, and/or one or more years. One or more blood pressure values and/or associated trending data can be: 1) stored in the RAM98, 2) displayed on the display86, and/or 3) output via the input/output devices88for any suitable purpose such as to a health care professional and/or a monitoring service.

FIG.8shows typical EKG and ICG data traces, in accordance with many embodiments. AC body impedance Z(t) is calculated using the applied drive current I(t) and the measured resulting voltage difference signal V(t) per equation (6).
Z(t)=V(t)/I(t)  (6)

The ICG signal is then generated by calculating the negative time differential of Z(t) as shown in equation (7).
ICGSignal=−dZ/dt(7)

The EKG signal is generated by voltages generated within the body having variations at a much lower frequency (e.g., 0.05-100 Hz) in comparison to the relatively higher frequency of the impedance drive current (e.g., 85 kHz). Accordingly, signals from the first and second sense electrodes58,62can be processed to generate both the ICG and the EKG traces. When both the EKG and the ICG traces are generated, the pre-ejection period (PEP) can be determined. While the PEP time period does not correlate well with blood pressure, it may correlate with an extent to vasomotion (vasodilation and vasoconstriction) and thereby serve as an additional factor that can be used to correlate blood pressure with measured PTT. For example, a relationship can be developed where predicted blood pressure is a correlated function of both PTT and PEP.

FIG.9shows an electrocardiogram (EKG) trace segment212, a Ballisto-Cardiogram (BCG) or Seismo-Cardiogram (SCG) trace segment214, and a PPG signal216relative to a pulse transit time (PTT)218for a blood pressure pulse between the left ventricle of the subject to the wrist-worn device210. In many embodiments, the wrist-worn device210includes an accelerometer and a PPG or pulse pressure sensor. The accelerometer measures one or more accelerations used to generate a BCG and/or a SCG, which can be processed to identify when the blood pressure pulse originates from the subject's left ventricle. A PPG sensor is used to generate a PPG signal for the subject. The EKG trace segment212is shown for reference in describing the operation of the heart. The EKG trace segment212has a segment (QRS) known as the QRS complex, which reflects the rapid depolarization of the right and left ventricles. The prominent peak (R) of the EKG trace corresponds to beginning of contraction of the left ventricle. A pulse arrival time (PAT)220is the time between the peak (R) of the EKG trace and arrival of the blood pressure pulse at the wrist-worn device210. As the left ventricle contacts, pressure builds within the left ventricle to a point where the pressure exceeds pressure in the aorta thereby causing the aortic valve to open. A pre-ejection period (PEP)222is the time period between the peak (R) of the EKG trace and the opening of the aortic valve. The PEP222correlates poorly with blood pressure. The BCG/SCG trace214can be processed to identify when the aortic valve opens. The ejection of blood from the left-ventricle into the aorta results in an associated acceleration of the chest cavity that is detected via the accelerometer included in the wrist-worn device210. In many embodiments, the arrival of the blood pressure pulse is detected via the PPG signal216, which includes an inflection point224that occurs upon arrival of the blood pressure pulse to the wrist-worn device210.

FIG.10shows a schematic side view of the wrist-worn device210held in contact with a user's chest225, in accordance with many embodiments. When the wrist-worn device210is held in contact with a user's chest, SCG data is generated. When the wrist-worn device210is not held in contact with a user's chest, BCG data is generated. The wrist-worn device210includes a main unit226, a wrist-worn elongate band228, an accelerometer230, and a PPG sensor232. The accelerometer230and the PPG sensor232are supported on the wrist-worn elongate band228and operatively connected with the main unit226. The PPG sensor232is positioned and oriented to interface with a wrist234of the user when the device210is worn on the wrist234. The main unit226includes circuitry and/or software for processing output from the accelerometer230and the PPG sensor232so as to measure a PTT and calculate one or more blood pressure values for the subject based on the PTT. In the illustrated embodiment, the PPG sensor232is located on the wrist-worn band228so as to be disposed to sense the arrival of the blood-pressure pulse within a radial artery236of the subject. Cross sections of the ulna bone238and the radius bone240of the subject are shown for reference. In described embodiments, the accelerometer230is oriented to measure accelerations in each of axes Ax and Ay (in the plane of the user's chest225) and axis Az (which is perpendicular to the user's chest225).

FIG.11shows a typical time-domain SCG trace242for acceleration measured in a direction normal to a user's chest surface, in accordance with many embodiments. The SCG trace242has localized peaks244, which correspond to the opening of the aortic valve and associated ejection of blood into the aorta from the user's left ventricle. The SCG trace242can be processed to identify the localized peaks244and the associated time points at which the localized peaks occur, thereby identifying one or more time points for one or more ejections of blood from the left ventricle into the user's aorta. The identified one or more time points can be used in conjunction with one or more time points when the respective blood pressure pulses arrive at the wrist as detected by the PPG sensor232or alternatively via a pulse pressure sensor to calculate a PTT for the propagation of the blood pressure pulse from the left ventricle to the user's wrist. The calculated PTT can then be used to generate one or more blood pressure values for the user as described herein.

FIGS.12and13show additional plots that can be generated from output of the accelerometer230.FIG.12shows a typical frequency-domain SCG246generated from the output of an accelerometer held in contact with a user's chest. The frequency-domain SCG, which can be used to identify heart rate for the user, which can be used to double check that the time points corresponding to the localized peaks244are separated by a time interval consistent with the identified heart rate.FIG.13shows a typical spectrogram SCG, which can also be used to identify heart rate for the user.

FIG.14shows example x-axis acceleration, y-axis acceleration, z-axis acceleration, and vector-sum acceleration SCG plots measured using an accelerometer held in contact with a subject's chest. Each of the z-axis acceleration (normal to the subject's chest) and the vector-sum acceleration (Atotal) exhibits clear acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. The y-axis acceleration (in plane of the subject's chest) is relatively less clear with respect to having acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. And the x-axis acceleration (also in plane with the subject's chest) is the least clear with respect to having acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle.

FIG.15shows example x-axis acceleration, y-axis acceleration, z-axis acceleration, and vector-sum acceleration BCG plots measured using an accelerometer coupled to a wrist-worn device that is not held in contact with the subject's chest. These BCG plots show a different order with respect to which acceleration plots exhibit acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. Specifically, the y-axis acceleration BCG plot exhibits the most clear acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. The vector-sum acceleration (Atotal) BCG plot is the next most clear after the y-axis acceleration BCG plot. Finally, each of the x-axis acceleration and the z-axis acceleration BCG plots appear to be similarly exhibit the least clear acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. As is described herein with reference toFIG.17, combinations of the component accelerations can be accomplished so as to exhibit greater signal variability, thereby having clearer acceleration peaks with respect to respective ejections of blood from the subject's left ventricle.

FIG.16schematically represents an embodiment of the wrist-worn device210. In the illustrated embodiment, the wrist-worn device210includes one or more processors282, memory284, a display286, one or more input/output devices288, a data bus290, the accelerometer230, the PPG sensor232, and a PPG sensor control unit294. In many embodiments, the memory284includes read only memory (ROM)296, and random access memory (RAM)298. The one or more processors282can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA) or integrated circuits. The accelerometer230can be any suitable accelerometer (e.g., a three-axis low noise accelerometer).

The PPG sensor unit232includes a PPG illumination unit308and detector line array310. The PPG illumination unit308includes two light sources312,314which transmit light having different wavelengths onto the wrist. While any suitable wavelengths can be used, the first light source312generates a beam of light having a wavelength of 525 nm. The second light source314generates a beam of light having a wavelength of 940 nm. Any suitable number of light sources and corresponding wavelengths can be used and selected to provide desired variation in tissue penetrating characteristics of the light. The detector line array310can include any suitable number of light detectors. In many embodiments, the light detectors are disposed at a plurality of different distances from the light sources312,314so that the detected light is associated with different mean penetration depths so as to enable detection of the arrival of the blood pressure pulse at different layers and/or within a layer of the wrist deeper than a layer sensed by a single light source and single detector PPG sensor. In the illustrated embodiment, the detector line array310includes four light detectors316,318,320,322, with each of the light detectors316,318,320,322being disposed at a different distance from the light sources312,314. For example, the light detectors316,318,320,322can be disposed at 2 mm, 3 mm, 4 mm, and 6 mm, respectively, from each of the light sources312,314. Signals generated by the light detectors316,318,320,322are supplied to the PPG control unit294, which includes an analog to digital converter to generate PPG sensor digital data that can be processed by the one or more processors282to determine the arrival of the blood pressure pulse to the wrist-worn device. The PPG control unit294controls activation of the light sources312,314, and can alternately illuminate the light sources312,314at a frequency sufficiently high to enable combined assessment of the PPG sensor digital data generated by illumination of the wrist with the different wavelengths provided by the light sources312,314.

Measured acceleration data and the PPG sensor digital data can be transferred to, and stored in, the RAM298for any suitable subsequent use. For example, the data can be: 1) processed by the one or more processors282to determine PTTs and corresponding blood pressure values for the subject, 2) displayed on the display286, and/or 3) output via the input/output devices288for any suitable purpose such as to a health care professional and/or a monitoring service. In many embodiments, the one or more processors282processes the acceleration data and PPG sensor digital data to generate trending data for a time period based on the one or more relative blood pressure values. Such trending data can be generated for any suitable time period, for example, for one or more days, one or more weeks, one or more months, and/or one or more years. One or more blood pressure values and/or associated trending data can be: 1) stored in the RAM298, 2) displayed on the display286, and/or 3) output via the input/output devices288for any suitable purpose such as to a health care professional and/or a monitoring service.

FIG.17illustrates an approach350for processing recorded acceleration data to identify when blood is ejected from the left ventricle of a user's heart, in accordance with many embodiments. In the approach350, output from the PPG sensor232is processed with a suitable bandpass filter352(e.g., a bandpass filter that attenuates frequencies less than 0.3 Hz and frequencies greater than 10 Hz) to reduce noise. The filtered PPG sensor output is then differentiated with respect to time (act354) so as to produce a signal that more clearly exhibits when the blood pressure pulse first arrives to the wrist prior to the arrival to the wrist of a reflection of the blood pressure pulse. In a similar fashion, the output from the accelerometer230(three component acceleration vector data, which varies over time) is also processed with a suitable bandpass filter356(e.g., a bandpass filter that attenuates frequencies less than 0.3 Hz and frequencies greater than 10 Hz) to reduce noise. The filtered acceleration vector data is then selectively combined so that the combined acceleration values exhibit greater variability with respect to ejections of blood from the subject's left ventricle, thereby exhibiting clearer acceleration peaks corresponding to respective ejections of blood from the subject's left ventricle. In one variation of the approach350, a magnitude trace is calculated from the three component acceleration vector data (act358). As illustrated inFIGS.14and15for each of the vector-sum acceleration data plots (Atotal) for both SCG and BCG, such a magnitude trace can exhibit clear acceleration magnitude peaks corresponding to respective ejections of blood from the subject's left ventricle. In another variation of the approach350, a principal component analysis (PCA) can be performed (act358) to identify a linear combination of the three components of the acceleration data that exhibits maximum acceleration variability, thereby increasing the likelihood that the identified combination will exhibit clear acceleration magnitude peaks corresponding to respective ejections of blood from the subject's left ventricle while allowing for flexibility in accelerometer orientation on the wrist. The principal component analysis can be accomplished by calculating the three-dimensional eigenvector associated with the maximum eigenvalue of the covariance matrix of the measured acceleration vector samples within a time window. The components of this eigenvector are used as the coefficients in the linear combination PCA-1 of the acceleration components. The resulting linear combination time samples can then be evaluated to identify peaks corresponding to respective ejections of blood from the subject's left ventricle. The PCA procedure is repeated for subsequent time windows of interest that contain measured acceleration data. In act360, identified time points for the arrival of blood pressure pulses to the wrist are correlated with respective time points for the ejection of blood from the user's left ventricle (i.e., acceleration peaks identified in the combination of the three component acceleration vector data). For example, each time point for the arrival of a blood pressure pulse can be correlated with a respective time point for the ejection of blood from the left ventricle that falls within a preselected preceding time span (e.g., from 100 ms to 300 ms prior to the arrival of the blood pressure pulse to the wrist. Any suitable preceding time span can be used. And the preceding time span used can be customized to a particular subject to reflect individual variations in pulse wave velocity related characteristics, such as relative differences in arterial stiffness.

FIG.18illustrates subsurface layers of a subject. The illustrated layers include: 1) the stratum corneum (about 20 μm thick), 2) the living epidermis (80 to 100 μm thick), 3) the papillary dermis (150 to 200 μm thick), 4) the superficial plexus (80 to 100 μm thick with a blood volume fraction of about 1.1%), 5) the reticular dermis (1400 to 1600 μm thick with a blood volume faction of about 0.83%), and 6) the deep blood net plexus (80 to 120 μm thick with a blood volume fraction of about 4.1%). Upon arrival to the wrist, the blood pressure pulse arrives at the deep blood net plexus layer before propagating to the overlying layers. As vasomotion (vasodilation and vasoconstriction) plays an important role in regulating blood flow in arterioles and capillaries further downstream in the arterial tree, using the PPG sensor to detect the arrival of the blood pressure pulse in the deep blood net plexus layer may increase the strength of the correlation between blood pressure and PTT by reducing vasomotion induced variability of PTT in shallower layers more subject to vasomotion induced variation in pulse wave velocity of the blood pressure pulse.

FIGS.19through21illustrate detection of different mean penetration depths of light emitted by a PPG sensor having returning light detectors disposed at different distances from each of two light sources of the PPG sensor, in accordance with many embodiments.FIG.19illustrates distribution of sensing depths for a combination of a 525 nm light source and a point detector disposed 2 mm from the 525 nm light source.FIG.20illustrates distributions of sensing depths for the combination of a 525 nm light source and point detectors disposed at 2 mm, 3 mm, 4 mm, and 6 mm from the 525 nm light source, as well as corresponding graphs of mean penetration depth and ratio of photons from the deep blood net plexus layer to the total detected returned light as a function of source-detector separation.FIG.21illustrates distributions of sensing depths for the combination of a 940 nm light source and point detectors disposed at 2 mm, 3 mm, 4 mm, and 6 mm from the 940 nm light source, as well as corresponding graphs of mean penetration depth and ratio of photons from the deep blood net plexus layer to the total detected returned light as a function of source-detector separation.FIGS.22and23show contribution of the total detected returned light for each layer for each wavelength and source-detector separation.FIGS.24and25show combined graphs corresponding to the graphs ofFIGS.20and21.

Using the data illustrated inFIGS.19through25, the signals from the detectors116,118,120,122,316,318,320,322generated for each of the light wavelengths generated by the light sources112,114,312,314can be processed to detect arrival of the blood pressure pulse within a selected layer (e.g., with the deep blood net plexus layer). For example, arrival of the blood pressure pulse within the reticular dermis layer can be detected first due to the large percentage of the returning light incident on the detectors116,118,120,122,316,318,320,322that returns from the reticular dermis layer. Once the arrival time to the reticular dermis layer is determined, the signals during a suitable time interval prior to the arrival time to the reticular dermis layer can be combined and/or processed to focus attention on detecting the earlier arrival of the blood pressure pulse to the deep blood plexus layer. Typically, infrared (e.g., 940 nm wavelength) light penetrates deeper into the skin compared to visible light such as green (e.g., 525 nm wavelength) or red (e.g., 660 nm wavelength). Hence, a PPG waveform recorded from infrared light corresponds to light reflected from deeper blood vessels, while a PPG waveform recorded from green light corresponds to light reflected from capillaries near the skin surface. Since the blood pulse arrives at deeper blood vessels earlier than capillaries near the skin surface, the blood pulse appears in the infrared PPG before the green PPG at the same location (e.g., on the wrist). A cross correlation of infrared and green PPG signals can be used to determine the relative delay between the arrival of the blood pulse at deeper blood vessels and the arrival of the blood pulse at capillaries near the skin surface.

The PPG signal can first be filtered in one of several ways, for example with a low-pass filter or with a regression filter. The pulse arrival can be detected as the peak of the amplitude of the PPG signal, or the “zero crossing point”. Alternatively, the PPG signal can be differentiated with respect to time and the differentiated signal used to determine a pulse arrival time. This signal processing can be performed on single pulses, leading to PTTs for each heartbeat. Or, the processing can be performed on signals that are an average from more than one pulse. One multi-beat averaging method is to first transform the signals (ICG or ECG, and also PPG) into the frequency domain using a Fourier Transform. Then a cross-correlation between the two transformed signals will give a PTT value.

FIG.26illustrates another approach for measuring a PTT that can be used to generate one or more blood pressure values for a subject. The PTT measured in this approach is for the propagation of a blood pressure pulse from an arm-worn auxiliary device430to arrival at a wrist-worn device432. The auxiliary device430and the wrist-worn device432can use any suitable approach for detecting the arrival of the blood-pressure pulse, such as via a PPG sensor as described herein.

FIGS.27and28show side views of the auxiliary device430and the wrist-worn device432. The auxiliary device430includes an arm-worn elongate band434and an auxiliary PPG sensor436coupled to the band434. The auxiliary device430can include one or more reference features or marks to as to enable reliable positioning and/or orientation of the auxiliary PPG sensor436relative to a selected underlying artery so as to detect arrival of the blood pressure pulse within the selected underlying artery. The wrist-worn device432can be configured similar to any of the wrist-worn devices described herein with respect to the PPG sensor464and can have a main unit438that is configured similar to any of the main units described herein with respect to all relevant functionality thereof.

FIG.29illustrates an exemplary method510for calculating a mean arterial pressure with a wrist-worn pressure sensor. At step510, after the wrist-worn device is coupled with a user's wrist, a constant pressure may be applied to the wrist with a pressure sensor coupled with a pressure actuator. Pressure measurements from the wrist may be received from the pressure sensor once it is urged against the wrist514. The user may then be instructed to sweep their arm between a first height and a second height516to vary the hydrostatic pressure experienced at the wrist. As the user sweeps their arm from the first height to the second height, a swept pressure signal may be received from the pressure sensor where the pressure pulses vary in amplitude due to the changes in hydrostatic pressure experienced at the wrist as the user moves their arm. The swept pressure signal may be analyzed to identify a maximum pressure pulse in the swept pressure signal520. A hydrostatic pressure associated with the maximum pressure pulse is obtained522after identifying the maximum pressure pulse. A mean arterial pressure may then be calculated524based on the obtained hydrostatic pressure and the constant pressure applied to the wrist. An indication may then be outputted526to provide a user an indication of the obtained mean arterial pressure. It will be appreciated however that a PPG sensor of the wrist-worn devices described above may alternatively be utilized, instead of a pressure sensor, to provide optical volume waveform signals, wherein a maximum volume waveform signal is identified to determine the mean arterial pressure according toFIG.29.

The exemplary method510utilizes the changes in hydrostatic pressure for applanation of an artery of the user. In many embodiments, the method510may be used for applanation of the radial artery or other superficial artery with sufficient bony support of a user. As the wrist changes in height relative to the heart of the user, the amount of hydrostatic pressure will vary and apply different amounts of pressure at the wrist of the user for applanation of the target artery. This exemplary method510for calculating mean arterial pressure is counterintuitive as many prior non-invasive methods of measuring and monitoring blood pressure teach away from arm movement during blood pressure monitoring. More specifically, many prior methods require or suggest that a user maintain their arm in preferred position throughout the measurement and/or monitoring of the user's blood pressure. Further, some methods of monitoring or measuring blood pressure may require wrist harnesses that lock the user's wrist in a preferred orientation while the measurements are taken. A method where the user may obtain blood pressure measurements and/or monitoring without the need for bulky wrist harnesses may provide a more convenient method in which users can easily measure their own arterial pressure on the go and outside of a clinic setting.

In many embodiments, after the user has coupled the device to their wrist, a constant pressure may be applied512by urging a pressure sensor against the wrist of the user. The constant pressure may be applied by a number of different ways. For example, wrist-worn device straps may be manually tightened (e.g., a Velcro strap, adjustable strap, or the like etc.) or mechanically tightened (e.g., through a ratcheting mechanism, or the like, etc.). The straps can be tightened using micro-linear actuator, or electroactive polymer (artificial muscles). In many embodiments a pressure actuator may be used to urge the pressure sensor against the wrist of the user. For example, solenoids, linear actuators, fluid bladders or the like may be coupled with a pressure sensor and actuated to urge the pressure sensor against the wrist and may also be actuated to reduce an amount of pressure applied.

In some embodiments, the applied constant pressure could be selected in the range 80-120 mmHg, which is close to the range of mean arterial pressures of interest. The use of applanation tonometry to determine mean arterial pressure requires that the transmural pressure equals zero, P_transmural=0. The transmural pressure acting across an arterial wall is defined as the difference between the internal pressure and external pressure, P_transmural=P_internal−P_external. Under the assumption of negligible resistance from the aorta to large peripheral arteries, the internal pressure P_internal at a peripheral artery is the sum of the central aortic blood pressure and the hydrostatic pressure at the peripheral artery relative to the aorta. Hence, the internal pressure of a peripheral artery that is below the aorta is greater than the blood pressure of the aorta; similarly, the internal pressure of a peripheral artery that is above the aorta is less than the blood pressure of the aorta. For a constant external pressure, the transmural pressure is largest when the peripheral artery is at its lowest point and smallest when the peripheral artery is at its highest point. When the artery is at its lowest point, the transmural pressure is typically greater than zero. As the artery is raised from its lowest point, the transmural pressure decreases until it reaches zero and begins to become negative. It follows that for a constant external pressure P_external, the transmural pressure will reach zero at a height that depends on the central aortic blood pressure. As the central aortic blood pressure increases, the transmural pressure equals zero at increasing peripheral artery heights. Conversely, as the central aortic blood pressure decreases, the transmural pressure equals zero at decreasing peripheral artery heights. For example, a constant pressure may be applied at the wrist such that transmural pressure at the wrist is positive when the user's arm is at a resting position (e.g., by the user's side when standing). The constant pressure may also be configured to allow the transmural pressure to turn negative after the user raises their arm a height relative to the user's heart. With such a configuration, an applanation of a target artery where the arterial wall is flattened and transmural pressure turns to zero. Here, the arterial pressure is perpendicular to the surface may occur at a height between the resting position where transmural pressure is positive state and the raised position where transmural pressure is negative. At the this height of the wrist, the hydrostatic pressure acting on the user's wrist and the constant pressure applied at the wrist may applanate the artery such that the arterial pressure is the only pressure detected by the pressure sensor (e.g., a desired applanation).

Once the pressure sensor is coupled with the wrist of the user, a pressure signal/measurement may be received from the pressure sensor514. The received pressure signal may correspond to an arterial pressure of the user. In some embodiments, the pressure sensor may be a capacitive pressure sensor, a piezoelectric film pressure sensor, a piezoresistive microelectromechanical system (MEMS) pressure sensor, bladder fluid or gas pressure sensor, or the like.FIG.29Ashows an exemplary piezoelectric film sensor that may be used with embodiments of the present invention described herein.FIG.29Bshows an exemplary piezoresistive pressure sensor that may be used with embodiments of the present invention described herein.

In some embodiments a piezoelectric film pressure sensor may be preferable as the film may be thin and may better conform to the contours of the user's wrists. When using a piezoelectric film pressure sensor, some embodiments may actuate the piezoelectric film pressure sensor with a fluid bladder. A fluid bladder pressure sensor identifying an applied pressure by the fluid bladder may be used to measure static pressure while the piezoelectric film pressure sensor measures dynamic pressure. The piezoelectric film measures the dynamic pressure oscillations from the artery, while the fluid bladder pressure sensor measures the static applied pressure from the fluid bladder.

In some embodiments a piezoresistive may be preferable as the film may also conform to the contours of the user's wrist and may further measure a static and dynamic pressure.

In many embodiments, an array of pressure sensors may be used to ensure that at least one of the pressure sensors of the array is positioned at a preferable location relative the target artery of the user. For example, in some embodiments, a 12×1 array, two 12×1 arrays, a 3×4 array, two 3×4 arrays, or the like of pressure sensors may be applied transverse to the radial artery of the wrist. In some embodiments, a single pressure actuator may be used to urge the entire array of sensors against the target artery. In other embodiments, multiple pressure actuators may be used to urge portions of the array of sensors against the target artery. For example, some embodiments of the wrist-worn device may have each pressure sensor coupled with a pressure actuator such that each individual pressure sensor may be individually urged against and away from the wrist by a desired amount and at different times. Further details of exemplary devices are discussed further below.

The user may be instructed to sweep their arm between a first height and a second height516. The first height and second heights may be, for example, a resting position where the user's arm rests against their side when standing and a raised position where the user's arm is raised above their head. In many embodiments, it may be preferable to instruct that the user slowly sweep their hand to different heights so that a plurality of pressure pulses may be measured at different heights. Further, while not essential, it may be preferable to instruct the user to maintain their arm in an extended position or straight orientation (e.g., where the elbow is locked) so that a wrist height measurement, relative to the user's shoulder, may be calculated using an angle of the arm and a shoulder-to-wrist length.

As the user moves their arm to different heights, a swept pressure signal may be received518. The swept pressure signal may include a plurality of pressure pulses that vary in amplitude due to changing hydrostatic pressure experienced at the wrist at the different heights.

As discussed above, a desired applanation of a target artery where the arterial wall is flattened and the arterial pressure is perpendicular to the surface may occur at a desired height between the first wrist height (e.g., resting position where the arm is positioned by the user's side) where the transmural pressure is positive and a second wrist height (e.g., a raised position above the resting position) where the transmural pressure is negative or vice-versa. At this desired height where the transmural pressure is zero, the hydrostatic pressure acting on the user's wrist and the constant pressure applied at the wrist may applanate the artery such that the arterial pressure stress is measured by the pressure sensor. Accordingly, in a height swept pressure signal with a plurality of pressure pulses measured at different heights, the desired applanation of the target artery is associated with the pressure pulse with the largest amplitude (i.e., “maximum pressure pulse”). Thus, after receiving the swept pressure signal518, a maximum pressure pulse in the swept pressure signal is identified520as it is associated with the desired applanation of the target artery and a corresponding hand height, location, and/or orientation may be recorded for calculating a hydrostatic pressure.

To calculate a mean arterial pressure524, the applied constant pressure and a hydrostatic pressure acting on the wrist during the measurement of the maximum pressure pulse are obtained. The mean arterial pressure (MAP) may be calculated by the following formula:
MAP=Papplied−Phydrostatic,  (8)
where: Pappliedis the constant pressure applied at the wrist and Phydrostaticis the hydrostatic pressure acting on the wrist during the measurement of the maximum pressure pulse.Phydrostaticmay be calculated by:
Phydrostatic=ρgh,(8a)
where: ρ is the density of blood, g is the gravitational constant, and h is the height difference between the heart and the wrist of the user (“heart-to-wrist height”). The average density of blood is approximately 1060 kg/m3. The gravitational constant is approximately 9.8 m/s2. The height difference, h, may be defined as:
h=Heightheart−Heightwrist,  (9)
where h is obtained in centimeters (cm) and where MAP is outputted in mmHg, equation (1) may be rewritten to:

MAP⁢⁢(mmHg)=Pressureapplied-0.78⁢(mmHgcm)*h⁡(cm),(10)

Accordingly, MAP may be calculated by obtaining the constant pressure applied at the wrist and by obtaining the heart-to-wrist height of the user that is associated with the measurement of the maximum pressure pulse.

FIG.30illustrates an exemplary method528of calculating the hydrostatic pressure at the wrist522. At step530, a signal indicative of an angle of the pressure sensor may be received while the pressure sensor obtains the swept pressure signal. A shoulder-to-wrist length of user may be obtained532. A height of the sensor relative to the user's shoulder may be calculated534using the signal indicative of the angle of the pressure sensor and the obtained shoulder-to-wrist length. A height of the user's shoulder may then be obtained536for use in calculating a wrist height38based on the shoulder height and the sensor height relative to the shoulder. A user's heart height may then be obtained540. A height difference between the pressure sensor/wrist and the heart may then be calculated542based on the obtained user heart height540and the calculated wrist height538. Using the calculated height difference, a hydrostatic pressure acting on the wrist at the height of the sensor may be calculated544and used to calculate the MAP524(e.g., using equation 10).

In some embodiments, an accelerometer may be coupled with the wrist-worn device and may output an angle of the pressure sensor530while receiving the swept pressure signal. The received angle information530may be used with an obtained shoulder-to-wrist height532to identify a height of the pressure sensor and wrist of the user relative to the shoulder of the user. For example, a shoulder-to-wrist height (Heightshoulder-to-wrist) may be calculated with the following:
Heightshoulder-to-wrist=lshoulder-to-wrist*sin θwrist;  (11)
where: lshoulder-to-wristis the length of the shoulder to the wrist of the user, and theta is the angle of the wrist/pressure sensor relative to horizontal identified by the accelerometer.

Optionally, if the accelerometer returned an angle, φ, of the pressure sensor530relative to vertical (e.g., where an arm raised straight up returns an angle of 0° and an arm position straight down returns an angle of 180°), shoulder-to-wrist height may be calculated with the following:
Heightshoulder-to-wrist=lshoulder-to-wrist*cos φwrist.  (12)

The length of the shoulder to the wrist of the user may be obtained532directly from a user input546for use in equation (11) or (12). For example, a user interface may be provided that requests the user to input a shoulder-to-wrist length. In response to a user input indicative of the shoulder-to-wrist length, the device may store the received user input for use in equation (11) and/or (12).

In some embodiments of the invention, the user may input anthropometric data548and the length of the shoulder to the wrist of the user may be estimated based on the user inputted anthropometric data. For example, in some embodiments, a user may input a gender and a height. In further embodiments, other anthropometric data may be obtained such as a user's age, weight, ethnicity, etc. Based on received anthropometric data, shoulder-to-wrist length may be estimated. For example, in some embodiments, a shoulder-to-wrist length of a male user may be estimated as approximately 30%-36% of the user's inputted height, and in some embodiments preferably about 33%-34% of the user's inputted height and in further embodiments, even more preferably about 33.4%-33.5% of the user's inputted height. For some embodiments, a shoulder-to-wrist length of a female user may be estimated as approximately 31%-37% of the user's inputted height, and in some embodiments, even more preferably about 33%-35% of the user's inputted height, and in further embodiments, even more preferably about 33.3%-34.5% of the user's inputted height.

Thereafter, a user's wrist height (Heightwrist) may be calculated538by obtaining a user shoulder height536with the following:
Heightwrist=Heightshoulder+Heightshoulder-to-wrist.  (13)

Optionally, equation (13) may be substituted into equation (9) to provide:
h=Heightheart-wrist=Heightheart−(Heightshoulder+Heightshoulder-to-wrist).   (14)

In a similar manner to receiving a shoulder to wrist length, a shoulder height may be requested and received through a user input546or may be estimated using received anthropometric data548. For example, in some embodiments, a shoulder height of a male user may be estimated as approximately between 80%-84% of the user's height, and in further embodiments, preferably between about 81.5%-82.5% of the user's height, and even more preferably about 81.9%-82% of the users height. For a female user, a shoulder height may be estimated as approximately between 81.5%-83.5% of the user's inputted height, and in further embodiments, preferably between 82%-83% of the user's inputted height, and even more preferably about 82.4%-82.6% of the user's inputted height.

To calculate for Heightheart-wristusing equation (13) or equation (14), a user heart height540may be obtained directly through user input542(user inputted and stored for subsequent use) or may be estimated based on anthropometric data inputted by the user548(e.g., gender, height, or the like). In some embodiments, a height of the user's heart may be estimated as approximately 70-75% of the user inputted height, in further embodiments, preferably about 72%-73% of the user inputted height and even more preferably about 72.5% of the user inputted height.

Once Heightheart-wristis obtained, a hydrostatic pressure acting on the wrist may be calculated544using equation (8a) and a MAP may be calculated524using equation (10).

After calculating an MAP for a user, the method510may then proceed to output an indication to the user that is indicative of the calculated MAP526. The output may comprise the calculated MAP. Alternatively, the output may be a general indicator that indicates where the calculated MAP falls on a spectrum (e.g., good MAP, intermediate MAP, bad MAP). The output may be audio (e.g., a voice or other audio indicator) or visual. For example, the output may be outputted to a display of the device or LEDs may be illuminated to provide the indication. In some embodiments, the output may be communicated to a separate wearable device coupled with the wrist-worn blood pressure monitoring device. For example, in some embodiments, the wrist-worn blood pressure monitoring device may be coupled with a separate wrist-worn electronics device. The separate device may include a separate power source, processor, communications port, memory, and inputs/outputs, etc. In further embodiments, the output may be transmitted (e.g., wirelessly) to a mobile device of a user. For example, an indication of the calculated MAP may be transmitted to a smartphone, or other portable electronic device (e.g., tablets, PDAs, laptops, or the like) for recordation, analysis, and documentation.

In some embodiments, the wrist-worn blood pressure monitor may output or otherwise transmit received sensor signals (e.g., wrist angle, pressure signal, swept pressure signal or the like) to a separate device for further processing and recordation. This may be advantageous in reducing the processing power needed in the wrist-worn device, thereby allowing the device to have a smaller footprint and may allow the device to be operated for longer periods of time due to a lower power consumption. Further, by transmitting the data to a secondary device (e.g., watch, phone, tablet, or the like) on-board storage and battery requirements may be reduced, thereby further allowing the device to have a smaller footprint.

While generally discussed as instructing the user to actively, intentionally, and/or knowingly carry out the arm sweep for generating the swept pressure pulse, other embodiments may be passive where the pressure signals may be received throughout a period of time as the user carries out daily activities. Other sensor data (e.g., accelerometer data) may indicate the movement of the sensor to different heights and may indicate the receipt of a swept pressure signal. The passively received swept pressure signal (e.g., where the user does not carry out the arm sweep in response to instructions), may then be analyzed for calculating a MAP of the user per the methods described above.

Optionally, in some embodiments, an accelerometer and gyroscope on the wrist could be used to trace the trajectory of the wrist during daily movements and, hence, determine the height between the wrist and the shoulder, the heart-to-wrist height can then be determined by a single measurement of the shoulder-to-heart height.

FIGS.31A-31Cillustrate a user550sweeping his arm for producing the swept pressure signal for the exemplary method510.FIG.31Aillustrates the user550with a wrist-worn device552at a first height554relative to his heart556where the wrist/wrist-worn device552is below the user's heart556.FIG.31Billustrates the user550with the wrist-worn device552at an height558where the wrist/wrist-worn device552is approximately equal to a height of his heart556.FIG.31Cillustrates the user550with the wrist-worn device552at a second height560relative to his heart556where the wrist/wrist-worn device552is above the user's heart556.

InFIG.31A, Heightheart-wristhas a positive value as the heart height is greater than the wrist height. Accordingly, per equation (8a), the user550experiences a positive hydrostatic pressure at the wrist when the wrist is below the heart556of the user. For example, using equation (8a), the user experiences +40 mmHg of hydrostatic pressure at the wrist when the wrist is about 51.28 cm below the heart556. Thus if the desired applanation of the target artery (or a measurement of the maximum pressure pulse) occurs when the wrist is below the heart height556, the calculated MAP is less than the applied pressure.

InFIG.31B, Heightheart-wristis approximately zero. Accordingly, per equation (8a), at this height, no hydrostatic pressure acts on the wrist relative to the heart556. If the desired applanation of the target artery (or a measurement of the maximum pressure pulse) occurs when the wrist height is equal to the heart height, the calculated MAP is equal to the applied pressure.

InFIG.31C, Heightheart-wristhas a negative value as the heart height is less than the wrist height, (see equation (9)). Accordingly, per equation (8a), the user550experiences a negative hydrostatic pressure at the wrist relative to the heart when the wrist is above the heart556of the user550. For example, using equation (8a), the user experiences −40 mmHg of hydrostatic pressure at the wrist when the wrist is about 51.28 cm above the heart556. If the desired applanation of the target artery (or a measurement of the maximum pressure pulse) occurs when the wrist is above the heart height556, the calculated MAP is greater than the applied pressure.

In many embodiments, the transmural pressure at a low end of the arm sweep may be positive where the wrist and device are positioned below the heart (e.g.,FIG.31A) and may be negative at a high end of the arm sweep where the wrist and device are positioned above the heart (e.g.,FIG.31C). In such instances, the desired applanation of the target artery and measurement of the maximum pressure pulse will occur at an intermediate height between the low end of the arm sweep and the high end of the arm sweep where the transmural pressure is zero.

FIG.32shows an exemplary device562for monitoring and/or measuring blood pressure of a user. The device562may include a wrist strap564and an actuator system566supported by the wrist strap564. The actuator system566may include a tip567for coupling with a pressure sensor (not shown) and may be configured to position the pressure sensor at a desired location relative to a coupled wrist.

The wrist strap564may be provided for coupling with a wrist of the user. While illustrated as configured to partially wrap around a user's wrists, other embodiments may fully wrap around a user's wrist. As discussed above, wrist strap564may be tightened around the wrist of a user to apply the constant pressure during an MAP measurement. The wrist strap564may include clasps, ratcheting mechanisms, or other engagement/tightening features for coupling and/or tightening the device562with a wrist of the user.

In some embodiments, the wrist strap564may be configured to couple with/modify a separate wearable device with a strap. For example, the wrist strap564may couple to the inner surface/contact surface of a strap of a separate wearable device. In some embodiments, the separate device may also be a wrist worn device, such as a watch or the like.

Actuator system566may be supported relative to a wrist of the user via wrist strap564. The actuator system566may provide a number of degrees of freedom to a pressure sensor coupled a tip567of the actuator system566relative to the wrist so that a pressure sensor may be preferentially placed at a desired location on the wrist and with a desired amount of pressure. For example, as illustrated actuator system566includes a first rail568for positioning a coupled pressure sensor perpendicular or transverse to a coupled wrist of a user. Actuator system566may further include a second rail570for positioning the tip567along the length of a target artery. Further, actuator system566may include a linear actuator572for urging a pressure sensor coupled thereto against a wrist of a user (e.g., for applying the constant pressure for measuring MAP). In some embodiments, the 2 rail system can be replaced by an automatic step controlled linear stage positioning system. And the linear actuator572can be replaced with a voice coil actuator (VCA) or a piezoelectric stack actuator.

The exemplary device562may be configured to carry out the exemplary method510. In some embodiments, the exemplary device562may be used to monitor blood pressure using applanation tonometry where the actuator572is configured to perform a pressure sweep in the Z direction (i.e. into the wrist) for identifying an MAP and then actuated to apply a preferred pressure so that the pressure sensor provides continuous blood pressure monitoring.

FIG.33illustrates another exemplary device574for monitoring and/or measuring blood pressure of a user. The device574may include a housing576with a curved configuration with an inner surface578configured to match the curvature of the underside of the wrist of a user. Housing576may include slots or engagement features580for coupling with a wrist strap (not shown). The housing576may include recessed surfaces/slots582for receiving a sensor array and corresponding recessed surfaces/slots584for receiving sensor leads of a received sensor array. Further, in some embodiments, housing576may include a recessed surface/slot586for receiving a pressure actuator for urging a received sensor array against a wrist of a user.

Slots580may be configured to receive a wrist strap for coupling the device574to a wrist of the user. The slot may, for example, receive a hook-and-loop fastener strap (e.g., Velcro® tape, or the like) for securing the device574to the wrist.

The recessed surface582may be configured for receiving a pressure sensor array. In some embodiments the pressure sensor array may comprise capacitive pressure sensors, piezoresistive MEMS pressure sensors, piezoelectric film pressure sensors, or the like. In some embodiments a 12×1 pressure sensor array may be received. The recessed surface582may align a received sensor array parallel with the wrist strap so that the sensor array traverses the target artery (e.g., radial artery). This may ensure that at least one of the pressure sensors of the pressure sensor array is positioned over the target artery. In the illustrated embodiment, two recessed surfaces582are provided for two 12×1 sensor arrays. While illustrated with two recessed surfaces582for receiving 12×1 sensor arrays, it should be understood that other embodiments may include single recessed surface582or may include three or more recessed surfaces582for receiving sensor arrays. Further, while the recessed surfaces582are described as configured to receive 12×1 sensor arrays, it should be understood that embodiments are not limited to receiving 12×1 sensor arrays. Embodiments may have recessed surfaces to receive other sensor arrays configurations (e.g., 2×1 sensor arrays, 3×3 sensor arrays, 4×4 sensor arrays, 4×3 sensor arrays, 4×6 sensors arrays or the like). Examples of array geometries include, but are not limited to, rectangular, hexagonal, and arrays with staggered rows or columns.

Recessed surface586may be further recessed than recessed surface582so that the received pressure actuator may urge the received pressure sensors against the wrist of the user. In some embodiments, the recessed surface586may be configured to receive a fluid bladder pressure actuator. The fluid bladder actuator may be configured to be filled with various amounts of fluid to urge a received pressure sensor against a wrist with vary amounts of pressure. Some embodiments may include a fluid bladder pressure sensor for providing a signal indicative of the fluid pressure within the bladder. The recessed surface586and the received fluid bladder may extend transverse to the recessed surfaces582so that a single fluid bladder may be actuated to urge a plurality of received pressure sensor arrays against the wrist of the user with a single actuation. The bladder actuator in recessed surface586may also be configured as an array of bladders to actuate the pressure sensor or sensor array.

The device574may be configured to carry out the exemplary method510. In some embodiments, the exemplary device574may be used to monitor blood pressure using applanation tonometry where a received pressure actuator in recess586is configured to perform a pressure sweep in the Z direction for identifying an MAP and then actuated to apply a preferred pressure so that the pressure sensor(s) provide continuous blood pressure monitoring.

FIG.34illustrates another exemplary device588for monitoring and/or measuring blood pressure of a user. Exemplary device588may include an enclosure590having slots592for receiving a wrist strap for coupling the device588to a wrist of a user. Enclosure590may include a slot594for receiving a pressure bladder or other type of actuator. Enclosure590may further house a driver596and disposed between the received pressure actuator and pressure sensor. The device588may further include a pressure sensor (not shown) coupled to a surface of the driver596that is opposite a surface that couples with the received pressure actuator. The pressure sensor or pressure sensor array can be attached to the moving part596, then be urged against artery.

Similar to the embodiment574illustrated inFIG.33, device588may receive straps through slots592for coupling the device588with a wrist of the user. Further, the received straps may be used to tighten or to urge the device588and a pressure sensor of the device588against the wrist of the user. The enclosure590may position a driver596between a pressure actuator (e.g., a fluid bladder) and a pressure sensor. The driver596may be configured to evenly distribute forces from the pressure actuator across the pressure sensor. This may be preferred when device588couples with a plurality of pressure sensors and where the pressure actuator comprises a pressure bladder. In some embodiments, a pressure bladder surface may project and retract unevenly or otherwise have a bulge that applies different amounts of pressure depending on a contact location along the bladder surface. Thus, with a pressure sensor array, some pressure sensors may be applied to a wrist with a different pressure compared to other pressure sensors in the array. A rigid driver596disposed between a fluid bladder and one or more pressure sensors of device588may alleviate these issues by evenly distributing pressure from the fluid bladder across the pressure sensor array.

In the illustrated embodiment, the driver596may have a cross section that generally resembles a “T,” however other configurations are possible. The enclosure590may include a T opening598in a sidewall600of the enclosure590. The opening598may be dimensioned to receive driver596during assembly of enclosure590. Once the driver596is inserted within the enclosure590, an insert602may be positioned between the driver596and the opening598to secure the driver596within the enclosure590.

Device588may couple with capacitive, piezoelectric film, piezoresistive pressure sensors or the like for measuring pressure. Further while discussed as using a fluid bladder as a pressure actuator, other actuators may be used (e.g., linear actuators, solenoids or the like). In some embodiments, utilizing one or more fluid bladders, fluid bladder pressure sensors may be used to provide a signal indicative of a fluid pressure with the one or more bladders.

Similar to the embodiments described above, the device588may be used to carry out method510. Further in some embodiments, the exemplary device588may be used to monitor blood pressure using applanation tonometry where a received pressure actuator (e.g., fluid bladder) in slot584is configured to perform a pressure sweep in the Z direction by urging driver596and coupled pressure sensors against the wrist for identifying an MAP and then actuated to apply a preferred pressure so that the pressure sensor(s) provide continuous blood pressure monitoring.

FIG.35illustrates yet another exemplary device604for measuring or monitoring blood pressure of a user. The exemplary device604includes an elastic housing band606configured to couple with a wrist of a user. The elastic housing band606may include engagement features608for coupling to a wrist strap. The elastic housing band606may further define a housing for receiving a fluid bladder610. An inflation port612may extend from the fluid bladder housing610to an outer surface of the elastic housing band606.

Elastic housing band606may generally have a curved configuration with an inner surface614configured to match the curvature of a user's wrist. The outer surface of the elastic housing band606may include ribs618and grooves620that run transverse to a length of the elastic housing band606. The ribs618and grooves620may be configured to provide additional flexibility in elastic housing band606, thereby allowing elastic housing band606to better conform to the curvature of a user's wrists.

Fluid bladder housing610may be configured to receive a fluid bladder. In many embodiments the device604may include an accordion bladder for urging one or more pressure sensors against the wrist of the user. An accordion bladder may avoid applying varying pressure along a contact face of the bladder and may thereby provide even distribution of pressure along a pressure sensor or pressure sensor array.

FIG.36illustrates an exemplary accordion bladder622. Accordion bladder622may have side walls624that generally define a volume for receiving fluid for expanding accordion bladder622a desired amount. The defined volume may be in fluid communication with inflation port612. The side walls624may be generally defined by a plurality of pleats or bellows that expand and contract with the filling and removal of fluid from the bladder622. Accordion bladder622may further include a generally flat distal face626for coupling with a pressure sensor or pressure sensor array. Due to the accordion configuration of the bladder622, fluid filling of the bladder622projects the distal face626of the bladder622linearly and evenly, thus increasing surface contact between the bladder622and a pressure sensor or array of sensors and reducing a bladder intramural stress. In this case the fluid pressure inside the bladder will be evenly exerted on surface626and been acting directly on the sensor or sensor array, and in turn to the artery. Pressure may then be applied to the pressure sensor/pressure sensor array and the wrist evenly. Accordingly, in some embodiments, a need for a driver disposed between the pressure actuator and the pressure sensor/pressure sensor array may be avoided by using such a bladder622. The accordion type bladder can be made of thermoplastics (e.g. nylon, polyethylene, Teflon, etc.).

Device604may couple with capacitive, piezoelectric film, piezoresistive MEMS pressure sensors or the like for measuring pressure. Further while discussed as using a fluid bladder as a pressure actuator, other actuators may be used (e.g., linear actuators, solenoids or the like). In some embodiments, utilizing one or more fluid bladders, fluid bladder pressure sensors may be used to provide a signal indicative of a fluid pressure with the one or more bladders and the signal may be used for calibrating one or more pressure sensors of the device.

Similar to the embodiments described above, the device604may be used to carry out method510. Further in some embodiments, the exemplary device604may be used to monitor blood pressure using applanation tonometry where a received pressure actuator (e.g., accordion fluid bladder) in fluid bladder housing610is configured to perform a pressure sweep in the Z direction by urging a coupled pressure sensor/pressure sensor array against the wrist for identifying an MAP and then actuated to apply a preferred pressure so that the pressure sensor(s) provide continuous blood pressure monitoring.

FIG.37shows an exemplary pressure sensor array628that may be used with the devices and methods described above. Pressure sensor array628may be 46 mm×46 mm in dimension and may comprises a plurality of capacitive pressure sensors630arranged in a 16×16 array. The pressure sensor array628may include a cable632to couple the pressure sensor array to a processing device (controller).

Each element may be approximately 2 mm×2 mm in size, thus providing an active area size of 32 mm×32 mm. The thickness of the active area may be approximately 1 mm. A scan rate may be up to 39 Hz.

FIG.38illustrates another exemplary pressure sensor array634. The array634comprises a first array636and a second array638. The first array636may comprise a 4×3 capacitive pressure sensor array and the second array638may similarly comprise a 4×3 capacitive pressure sensor array. Each pressure sensor may be 2×2 mm. Accordingly the array634may have an active area size of 16 mm×6 mm. The wiring640associated with the first array636may be routed to a first side of the pressure sensor array634and the wiring642associated with the second array638may be routed to a second side of the pressure sensor array634. Wiring640,642may each comprise twelve wires that correspond to each of the pressure sensors in the respective arrays.

The first array636and the second array638may be symmetric so that the application of this sensor array634against the user's wrist may also symmetric. This type of array634may reduce the cantilever beam loading situation (when sensor array with only one side wiring structure is been pressed against artery, the array will undergo a bending mode between sensor array and wiring pack) and provide a more symmetric load on the sensor array634.

The wiring640,642for the sensor array634may be backed by a fabric material644(e.g., a cloth material). A fabric backing material644may facilitate installation within a monitoring device and may also reduce undesired bending or stretching loads being applied to the sensor array634.

FIG.39illustrates an exemplary pressure actuator-pressure sensor assembly646that may be used with the devices and methods disclosed herein. Assembly646may include an actuator array648coupled with a sensor array650. Each actuator652of the actuator array648may be coupled to a pressure sensor654in the pressure sensor array650. Each of the actuators652in the pressure actuator array648may be individually controlled to urge each of the pressure sensors654of the pressure sensor array650against a wrist/target artery of the user by different amounts. For example, different sensors may be urged different distances or amounts depending on the curvature, contours, or location on the wrist where the sensor is to be urged against. Thus some embodiments, may be configured to tailor to different user wrist curves and contours and may thereby provide more accurate pressure measurements. Accordingly, subsets of the pressure sensor array may be urged against different portions of the wrist. Based on pressure sensor readings, a preferred sensor, sensor location, or sensor signal may be identified and used for blood pressure measurements and/or monitoring.

In some instances when a constant actuation pressure (e.g., 80 mmHg) is applied, the sensor array element with the largest static pressure value may be different from the element with the largest dynamic pressure value. In such instances, the actuator can be moved or a different actuator can be used at a different position until the same element exhibits the largest static pressure as well as the largest dynamic pressure when a constant actuation pressure is applied.

While the array of actuators648is illustrated as a 5×9 array and the array of sensors650similarly illustrated as a 5×9 array, other array sizes are possible (e.g., smaller or larger). Further, the actuators652are illustrated as linear actuators, however other actuators may be used, including but not limited to, fluid bladders, rails actuators, solenoids, or the like. The pressure sensors654may be capacitive, piezoresistive, piezoelectric film sensor or the like. The pressure sensor array can be mounted entirely with some backing material to the linear actuator array, or individual elements may be mounted on individual actuators to form the entire array.

FIG.40illustrates an exemplary method660of operating the exemplary assembly646ofFIG.39. At step662, a first subset of the actuators are activated to urge a first subset of the sensors against the wrist. Pressure signals from the first subset of pressure sensors may then be received664. One or more swept pressure signals may be received by varying an applied pressure with the first subset of actuators666. Thereafter, a second subset of the actuators may be activated to urge a second subset of the sensors against the wrist668. One or more pressure signals from the second subset of sensors may then be received670. One or more swept pressure signals may be generated by varying the applied pressure with the second subset of actuators672. A maximum pressure pulse may then be identified in each of the swept pressure signals674. A maximum pressure pulse with the largest amplitude out of the identified maximum pressure pulses may then be identified676. In some embodiments, the method may include identifying the pressure sensor that recorded the maximum pressure pulse with the largest amplitude678and identifying a location of the identified sensor relative to the wrist of the user680. In some embodiments, the identified sensor and the identified location may be a preferred sensor and location that most closely identifies a blood pressure of the user and may be used for MAP measurements and blood pressure monitoring via applanation tonometry.

The first/second subset of actuators and the first/second subset of pressure sensors may be a single actuator and a single pressure sensor or may be more than one actuator and more than one sensor. In some embodiments, the first subset of actuators and sensors may be a first half of an array of actuator-sensor assemblies, while the second subset of actuators and sensors may be a second half of the array of actuator-sensor assemblies. In some embodiments, the first subset may be a quarter of an array of actuator-sensor assemblies, and the second subset may be another quarter of the array of actuator-sensor assemblies. Where the first subset and the second subset of actuator-sensor assemblies are less than the total number of actuator-sensor assemblies of the device, the method660may be repeated for additional subsets of actuator-sensor assemblies that remain.

While discussed as generating the swept pressure signal by varying the pressure applied by a coupled actuator, a swept pressure signal may, in some embodiments be generated by a change in height of the wrist relative to the heart of the user similar to embodiments described above. However, in many embodiments, a passive method (i.e., that does not require user arm movement) may be preferable as such methods may be performed with little to no inconvenience to the user.

Further, in some embodiments, prior to receiving the one or more pressure signals from the second subset of sensors670, the first subset of sensors may be retracted away from the wrist.

Additionally, while method660is described with steps for processing the data by identifying a maximum pressure pulse with the largest amplitude out of a plurality of identified maximum pressure pulses within each pressure signal, other methods of signal analysis may be provided.

FIG.41illustrates the coupling of a device682having a plurality of sensor-actuator assemblies684to a wrist686of a user according to embodiments of the present invention. The device682may be configured to measure the blood pressure of a user through applanation of the radial artery688.

The device682includes a strap690extends around the wrist686and supports each of the plurality sensor-actuator assemblies684against the wrist686. The sensor-actuator assemblies684may comprise an actuator692coupled with a pressure sensor694. The plurality of sensor-actuator assemblies684may couple with the wrist686at a device skin interface696.

The actuators692may be configured to selectively and/or sequentially urge regions of the skin interface696adjacent the respective actuators692and disposed between the actuators692and the wrist against the wrist686of the user. The coupled pressure sensor694may measure pressure experienced between the actuators692and the wrist686and provide a respective pressure signal to a processer (not shown). Accordingly, the skin interface696may comprise a plurality of regions along the wrist686. While illustrated as a cross-section, it should be understood that skin interface696may comprise an array of regions that correspond to an array of actuators692.

As illustrated, the skin interface696of the device682is generally disposed over the radial artery688. While the radial artery688has a small footprint, a sensor or sensor array that covers a large region of the wrist circumference may ensure that the sensor or at least one sensor of a sensor array is positioned and/or oriented over the radial artery688in a desired manner. In some embodiments, given that not all sensors694of the device682are in a preferred position (e.g., where the face of the sensor is perpendicular to a pressure pulse from the target artery), it may be preferable to identify a preferred sensor694and a preferred region for applanation of the radial artery688. This may be carried out by analyzing and comparing the signals from the plurality of sensors694. For example, the sensors694disposed further from the radial artery688may provide weaker pressure signals that are not as meaningful for determining a blood pressure of a user.

In the illustrated embodiment with a plurality of sensors694, the actuators692may be selectively and/or sequentially activated to urge different regions of the skin interface696against the wrist686in order to identify a preferred region for applanation of the radial artery688. The preferred region for applanation of the radial artery688may be identified based on pressure signals received from the one or more sensors694of the device682. For example, the skin interface region disposed between sensor-actuator assembly698may be urged against the wrist686and a signal may be received from the corresponding sensor694of sensor-actuator assembly698. Additionally, the skin interface region disposed between the sensor-actuator assembly700may be urged against the wrist686and a signal may be received from the corresponding sensor694of the sensor-actuator assembly700. The signals from the sensor of assembly698and the sensor of assembly700may then be compared to determine which signal is stronger and/or preferred. Given that the sensor-actuator assembly700is positioned closer to radial artery688and that the surface face of the sensor of assembly700is more perpendicular to pressure pulses from the radial artery688, the signal from the sensor of assembly700may be stronger and preferred in comparison to the signal of the sensor of assembly698as it is further from the radial artery688and oriented at an angle relative to pressure pulses from the artery688and may suffer from increased signal loss.

The regions of the skin interface696may be selectively urged such that subsets of the regions of the skin interface696are urged against the wrist686at a time. The subsets of regions may be urged by multiple actuators692where a subset of the actuators692are activated (e.g., half the actuators, a quarter of the actuators, a single actuator etc.). Accordingly, in some embodiments the subsets of regions may each be urged selectively and sequentially by a single actuator692for identifying a preferred region and sensor694.

FIG.42illustrates the selective actuation of a single region of a skin interface710against a wrist of a user according to embodiments of the present invention. Device701may include pressure sensors702that may be coupled with one of a plurality of actuators704. The actuators704may be supported adjacent the wrist by a strap706. The sensors702may couple with the skin708of the user via skin interface710. As illustrated inFIG.42, in some embodiments, a single region of the skin interface710disposed between an actuator704and the wrist may be urged against the wrist for applanation of the artery712using a single actuator704. While applanating the artery712with the single actuator704, the remaining actuators704may not be actively urging respective regions of the skin interface710against the wrist. This manner of actuation of regions of the skin interface710against the wrist may be performed selectively and sequentially in order to identify a preferred region for applanation of the artery712and a preferred sensor signal from one of the sensors702.

FIG.43illustrates device701selectively actuating more than one region of a skin interface710against a wrist of the user according to embodiments of the present invention. As illustrated inFIG.43, a subset of regions (e.g., the right half the regions) of the skin interface710positioned between actuators704and the wrist are urged against a wrist of a user by activating two of the actuators704while the other two actuators704may not be actively urging respective regions of the skin interface710against the wrist. In some embodiments, pressure signals may only be processed from the advanced pressure sensors702. In some embodiments, pressure signals may only be received from the advanced pressure sensors702. In some embodiments, the received pressure signals may be processed to identify a blood pressure of the user or compared to identify a preferred pressure sensor702between the two advanced pressure sensors702and a preferred region for applanation. In such a method, processing time may be reduced as only a subset of pressure signals may be received from the subset urged regions.

WhileFIG.41-FIG.43illustrate devices with a plurality of individual sensors702, other embodiments may utilize a sensor system comprising a pressure film sensor. For example,FIG.44illustrates a device800that includes a pressure film sensor802that may be coupled with a plurality of actuators804. The actuators804may be supported adjacent the wrist by a strap806. The sensor802may couple with the skin808of the user via skin interface810. As illustrated inFIG.44, in some embodiments, a single region of pressure film sensor802and a single region of the skin interface810may be urged against the wrist for applanation of the artery812using a single actuator804. While applanating the artery812with the single actuator804, the remaining actuators804may not be actively urging respective regions of the pressure film sensor802and the skin interface810against the wrist. This selective actuation of regions of the pressure film sensor802against the wrist may be performed selectively and sequentially in order to identify a preferred region of the pressure film sensor802and skin interface810for applanation of the artery812.

FIG.45illustrates device800selectively actuating a subset of regions of a skin interface810and pressure film sensor802against a wrist of the user according to embodiments of the present invention. As illustrated inFIG.45, a subset of regions (e.g., the right half the regions) of the skin interface810are urged against a wrist of a user by activating two of the actuators804on the right while the other two actuators804on the left may not be actively urging the respective regions of the pressure film sensor802against the wrist. Regions of the pressure film sensor802may be selectively and/or sequentially urged against the wrist to identify a preferred region of the skin interface810for applanation of the target artery812and a preferred region of the pressure film sensor802for receiving pressure signals.

FIG.46A-46Cshow sensor data obtained from an array of pressure sensors applied to a user according to embodiments of the present invention. The data was received from a 1×12 array of pressure sensors applied to a subject's wrist at the radial artery. The pressure actuator was a linear actuator that traveled approximately 6 mm perpendicularly to the wrist surface with a speed of 25 steps/s (each step was approximately 38 μm). The wrist was approximately 15 cm below the heart. The reference blood pressure taken from an oscillometric brachial monitor was systolic blood pressure (123 mmHg) and diastolic blood pressure (78 mmHg). The reference mean arterial pressure was estimated by mean arterial pressure=⅓*(systolic blood pressure)+⅔*(diastolic blood pressure). The total (i.e., AC and baseline) pressure waveform from the sensor element with the strongest pulsatile (i.e., AC) component is illustrated in the pressure vs. time chart shown inFIG.46A. The AC pressure waveform versus time for the same sensor element is illustrated inFIG.46B.FIG.46Cshows the relative AC amplitude vs. baseline from the same sensor element. Element 20 had the largest pressure amplitude measurements while the remaining received relatively weaker pressure signals Accordingly, element 20 may be a preferred sensor and may be considered to be placed at a preferred region and/or orientation adjacent the target artery. Thus, in some embodiments, a blood pressure measurement may be calculated based on this pressure signal alone.

FIG.47illustrates a method of calibrating relative blood pressure signals according to embodiments of the present invention. As described above, relative blood pressure values may be calibrated with a reference measurement to determine blood pressure values on an absolute scale. At step910, a first sensor of a wrist-worn device non-invasively engaging the skin on the wrist of the user, senses a first user signal indicative of ventricular ejection of blood from the heart of the user. The first sensed ventricular ejection signal has an associated ventricular ejection time. At step912, a second sensor of the wrist-worn device non-invasively engaging the skin on the wrist of the user, senses a second user signal indicative of arrival of a pressure pulse in the wrist. The second sensed pressure pulse signal is associated with the first sensed ventricular ejection signal and has an associated pulse arrival time. A relative blood pressure value may be then determined in response to a first PTT identified from a difference between the ventricular ejection time and the pulse arrival time per step914.

At step916, an absolute reference blood pressure measurement obtained in coordination with the relative blood pressure may be received from an accurate reference measurement device. The absolute reference blood pressure measurement may be obtained from a variety of sources including volume oscillometry (as described herein), an oscillometric cuff, or an input by the user. In step918, the absolute blood pressure of the relative blood pressure value may then be determined in response to a difference between the relative blood pressure and the absolute reference blood pressure. The determined absolute blood pressure may be compared to a standard performance threshold (e.g., reference measurement) per step920. For example, if the difference between the threshold value is greater than ±5 mmHg mean error or ±8 mmHg sigma error, a blood pressure index of the relative blood pressure values may be transmitted instead of the absolute blood pressure values per step926. In addition, a plurality of relative blood pressure values determined prior to or subsequent the first PTT may further be calibrated based on the difference between the relative blood pressure associated with the first PTT and the absolute reference blood pressure for backward or retroactive calibration of existing data or forward calibration of new data per step922.

The blood pressure signals may be filtered based on contextual information associated with the user per step924. As described above, contextual filtering may be based on a variety of information that may provide context for any measured blood pressure changes or artifacts. Accordingly, the filtered blood pressure signals may be masked, discarded, or automatically annotated. The plurality of calibrated and/or non-filtered blood pressure values may then be transmitted to a second electronic device (e.g., watch, mobile device, tablet, or computer) or database for further processing (e.g., absolute blood pressure tracking), storage (e.g., electronic medical record), retrieval by other devices or programs (e.g., health software application), and/or display to the user or their health care professional per step926. It will be appreciated that in some situations, PTT measurements from step914may be directly filtered per step924and/or transmitted per step926directly to the second electronic device or database in a non-calibrated (e.g., non-manipulated) format. The second electronic device or database may be better suited in some instances to store individual calibration equations and process the PTT measurements to determine absolute blood pressure values. As discussed above, the second electronic device or database may not only process the PTT measurements (e.g., calibration of relative blood pressure signals), but also allow for storage of the data in a variety of formats (e.g., non-calibrated PTT measurements, trending data, absolute blood pressure values), retrieval of the data by other devices or programs, and/or display of the data.

FIG.48illustrates a schematic example of an overall system including a first wrist-worn band928, a second wrist-worn electronic device (e.g., watch930), and a third non-wrist device (e.g., a mobile device932) according to embodiments of the present invention. The first wrist-worn band928may comprise any one of the blood pressure monitoring sensor arrangements disclosed herein and is configured to non-invasively engage the skin on the wrist of the user. The elongate band928is releasably coupleable to a second wrist-worn watch930as described in greater detail below. At least one PTT or pressure sensor934may be coupled to the elongate band928, the sensor non-invasively engaging the skin over the wrist of the user for measuring user signals from the cardiovascular system of the user. In addition, a height sensor944may be coupled to the elongate band928so as to account for any hydrostatic pressure effects associated with the measured cardiovascular user signals. One or more processors936may be coupled to the elongate band928and the at least one PTT or pressure sensor934for determining relative or absolute blood pressure signals based on the user signals. The one or more processors936can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA). The elongate band928may further include memory938, such as read only memory (ROM) and/or random access memory (RAM). A power source940may also be coupled to the elongate band928and the processor936or the at least one PTT or pressure sensor934for providing power to the wrist-worn band928. A telemetry/wireless interface942(e.g., Bluetooth or WiFi) may also be coupled to the elongate band928and the processor936.

The second wrist-worn watch930may comprise one or more heart rate monitor sensor(s)946, a second processor948, a second power source950, a second memory952, a second telemetry interface954, and/or a user display956that are enclosed within a distinct and separate housing from the first wrist-worn blood pressure monitoring band928. The first wrist-worn band928may easily communicate (e.g., transmit blood pressure values, receive updated instructions, such as new calibration equations, etc.) with the second wrist-worn watch930via WiFi or Bluetooth. Still further, the telemetry interface942of the elongate band928may be configured to communicate not only with the second wrist-worn watch930, but also with the third non-wrist device (mobile device932). For example, the telemetry interface942of the elongate band928may be configured to transmit the relative or absolute blood pressure signals to a health application software958on the mobile device932. The mobile device932may in turn display the relative blood pressure signals960and/or the absolute blood pressure signals962in a graphical format dictated by the health application software958for a time period of a day, week, month, or year. The blood pressure graphs960,962may then be viewable by the user or a health care professional for use in diagnostic or therapeutic decision making. Still further, the mobile device932may be configured to receive the blood pressure signals from the wrist-worn band928and/or wrist-worn watch930and in turn re-transmit this data to a cloud database964for further processing, storage, or retrieval by other devices or programs. For example, the blood pressure measurements may be transmitted specifically to an electronic health or medical record database966.

Referring now toFIGS.49A-49C, providing bands928that are releasably coupleable to the watch930provides for user customization of the watch930based on the desired sensor monitoring (e.g., absolute, relative, passive, active, etc.). For example, a first applanation tonometry band968as illustrated inFIG.49Amay comprise a plurality of pressure sensors970and actuators972for measuring absolute blood pressure values. The pressure sensors970may comprise pressure transducers as illustrated or still further a piezoelectric film or piezoresistive film for sensing. The pressure sensors970are configured to non-invasively engage an anterior surface of the wrist of the user and be positioned over a radial artery so as to passively or actively measure the absolute blood pressure signals. The actuators972urge each of the pressure sensors970against the wrist of the user by applying a constant or variable pressure thereto.

FIGS.49B and49Cillustrate bands974,982for measuring relative blood pressure values. As described above, the least one PTT sensor may comprise first and second sensors. The first sensor is configured to measure a first user signal indicative of ventricular ejection of blood from the heart of the user, the first sensed ventricular ejection signal having an associated ventricular ejection time. The second sensor is configured to measure a second user signal indicative of arrival of a pressure pulse in the wrist, the second sensed pressure pulse signal associated with the first sensed ventricular ejection and having an associated pulse arrival time, wherein the relative blood pressure signal is determined from a difference between the ventricular ejection time and the pulse arrival time. The first sensor may comprises at least one (or combination thereof) ICG, ECG, BCG, PCG, and/or SCG sensor coupled to the elongate band. The second sensor may comprise at least one PPG sensor or physical pressure pulse sensor coupled to the elongate band.

With reference toFIG.49B, a second band974may comprise an ICG/PPG sensor arrangement for measuring relative blood pressure values. In particular, the at least one ICG sensor may comprise at least a first pair of dry electrodes976non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes978contacted by at least two separate fingers (or a thumb, palm, or wrist) of a hand opposite a hand on which the device is worn to provide cross-body dynamic impedance measurements. The PPG sensor980may comprise at least one infra-red, red, or green optical source and a detector positioned over a radial artery of the wrist (or the finger or arm) of the user. With reference toFIG.49C, a third band982may comprise a BCG/PPG sensor arrangement for passive monitoring of relative blood pressure values. The BCG sensor984may comprise an accelerometer non-invasively engaging an anterior surface of the wrist so as to passively measure a relative blood pressure. At least one height sensor996may be coupled to the elongate band982so as to account for hydrostatic pressure effects. The user may selectively choose between the first968, second974, or third bands982for the desired sensor monitoring and may further interchange the bands at any time period as desired via a releasable coupling feature994. The at least one releasable connection or coupling feature994of the elongate bands968,974, or982may help secure the selected band986to the heart rate monitor watch device930.

As shown inFIG.50, a fourth selected band986is releasably coupleable to the watch device930and includes two types of sensor monitoring arrangements. An ECG sensor arrangement is provided for cross-body electrical potential measurements and a SCG sensor arrangement for comparison of the ECG measurement to another active measurement that has little or no error due to hydrostatic pressure changes as the SCG measurement is made at the chest which is relatively aligned with a height of the heart. The ECG sensor comprises a first pair of dry electrodes988non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes990contacted by at least two separate fingers (or a thumb, palm, or wrist) of a hand opposite a hand on which the device is worn. The SCG sensor992comprises an accelerometer and the accelerometer992, wrist-worn band986and/or hand of the wrist-worn device non-invasively engage the sternum.

It will be appreciated that personal information data may be utilized in a number of ways to provide benefits to a user of a device. For example, personal information such as health or biometric data may be utilized for convenient authentication and/or access to the device without the need of a user having to enter a password. Still further, collection of user health or biometric data (e.g., blood pressure measurements) may be used to provide feedback about the user's health and/or fitness levels. It will further be appreciated that entities responsible for collecting, analyzing, storing, transferring, disclosing, and/or otherwise utilizing personal information data are in compliance with established privacy and security policies and/or practices that meet or exceed industry and/or government standards, such as data encryption. For example, personal information data should be collected only after receiving user informed consent and for legitimate and reasonable uses of the entity and not shared or sold outside those legitimate and reasonable uses. Still further, such entities would take the necessary measures for safeguarding and securing access to collected personal information data and for ensuring that those with access to personal information data adhere to established privacy and security policies and/or practices. In addition, such entities may be audited by a third party to certify adherence to established privacy and security policies and/or practices. It is also contemplated that a user may selectively prevent or block the use of or access to personal information data. Hardware and/or software elements or features may be configured to block use or access. For instance, a user may select to remove, disable, or restrict access to certain health related applications that collect personal information, such as health or fitness data. Alternatively, a user may optionally bypass biometric authentication methods by providing other secure information such as passwords, personal identification numbers, touch gestures, or other authentication methods known to those skilled in the art.

In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention can be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.

While the exemplary embodiments have been described in some detail, by way of example and for clarity of understanding, those of skill in the art will recognize that a variety of modifications, adaptations, and changes may be employed.