Abstract:
The present invention provides non-invasive devices, methods, and systems for determining a pressure of blood within a cardiovascular system of a user, the cardiovascular system including a heart and the user having a wrist covered by skin. More particularly, the present invention discloses a variety of wrist-worn devices having a variety of sensors configured to non-invasively engage the skin on the wrist of the user for sensing a variety of user signals from the cardiovascular system of the user. Generally, approaches disclosed herein may passively track blood pressure values without any interaction required on the part of the user or may allow for on demand or point measurements of blood pressure values by having a user actively interact with the sensors of the wrist-worn device. Approaches disclosed herein further allow for absolute blood pressure values to be determined directly without the requirement for any periodic calibrations or for relative blood pressure values to be tracked so as to provide relative blood pressure indices.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Appln. No. 62/047,452 filed Sep. 8, 2014; the full disclosure which is incorporated herein by reference in its entirety for all purposes. 
         [0002]    The present application is related to U.S. Provisional Appln. Nos. 62/047,431 entitled “Systems, Devices, and Methods for Measuring Blood Pressure of a User;” 62/047,472 entitled “Wrist Worn Accelerometer For Pulse Transit Time (PTT) Measurements of Blood Pressure;” and 62/047,486 entitled “Electrical Coupling of Pulse Transit Time (PTT) Measurement System to Heart for Blood Pressure Measurement;” all of which were filed on Sep. 8, 2014, and are incorporated herein by reference in their entirety for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Elevated blood pressure (a.k.a. hypertension) is a major risk factor for cardiovascular disease. As a result, blood pressure measurement is a routine task in many medical examinations. Timely detection of hypertension can help inhibit related cardiovascular damage via accomplishment of effective efforts in treating and/or controlling the subject&#39;s hypertension. 
         [0004]    A person&#39;s blood pressure is a continuously changing vital parameter. As a result, sporadic office blood pressure measurements may be insufficient to detect some forms of hypertension. For example, hypertension can occur in a pattern that evades detection via isolated office blood pressure measurement. Common hypertension patterns include white coat hypertension (elevated only during a limited period of time), borderline hypertension (fluctuating above and below definitional levels over time), nocturnal hypertension (elevated only during sleeping hours or not showing the normal drop in pressure during sleep), isolated systolic hypertension (elevated systolic pressure with non-elevated diastolic pressure), and isolated diastolic hypertension (elevated diastolic pressure with non-elevated systolic pressure). To detect such hypertension patterns, it may be necessary to perform additional blood pressure measurements over time to obtain a more complete view of a person&#39;s blood pressure characteristics. Although continuous measurement of blood pressure can be achieved by invasive means, for example, via an intra-arterial pressure sensing catheter, noninvasive blood pressure measurement approaches are more typically used. 
         [0005]    Current noninvasive blood pressure measurement approaches include ambulatory and home blood pressure measurement strategies. These strategies provide such a more complete view of a person&#39;s blood pressure characteristics and are often employed in recommended situations. Ambulatory blood pressure measurement is performed while the person performs daily life activities. Currently, ambulatory blood pressure measurements are typically performed every 20 to 30 minutes using brachial oscillometric blood pressure measurement cuffs. Ambulatory blood pressure measurement may be recommended where there is large variability in office blood pressure measurements, where a high office blood pressure measurement is made in a person with otherwise low cardiovascular risk, when office and home blood pressure measurements vary, where resistance to drug treatment of blood pressure is noted or suspected, where hypotensive episodes are suspected, or where pre-clampsia is suspected in pregnant women. Home blood pressure measurements include isolated self-measurements performed by a person at home. Home blood pressure measurements may be recommended where information is desired regarding the effectiveness of blood pressure lowering medication over one or more dose-to-dose intervals and/or where doubt exists as to the reliability of ambulatory blood pressure measurement. 
         [0006]    Current ambulatory and home blood pressure measurement approaches, however, fail to provide continuous measurement of blood pressure. Thus, convenient and effective approaches for noninvasive continuous measurement of blood pressure remain of interest. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention provides non-invasive devices, methods, and systems for determining a pressure of blood within a cardiovascular system of a user, the cardiovascular system including a heart and the user having a wrist covered by skin. More particularly, the present invention discloses a variety of wrist-worn devices having a variety of sensors configured to non-invasively engage the skin on the wrist of the user for sensing a variety of user signals from the cardiovascular system of the user. Generally, approaches disclosed herein may passively track blood pressure values without any interaction required on the part of the user, which is of particular benefit during overnight monitoring when the user is asleep or for other periods of extended monitoring. Passive tracking is particularly ideal as blood pressure values may be obtained consistently, frequently, and/or continuously over a period of time for potentially longer and more accurate and complete data sets as this approach is not dependent on user compliance and eliminates any artifacts (e.g., artificially elevated blood pressure value) associated with the act of taking the actual blood pressure measurement (e.g., white coat syndrome). Alternatively, approaches may allow for on demand or point measurements of blood pressure values by having a user actively interact with the sensors of the wrist-worn device to initiate the blood pressure measurements. For example, the user may engage sensors of the wrist-worn device with another part of their body (e.g., arm, fingers, sternum, ear) or the user may need to engage the arm on which the wrist device is worn (e.g., volume or pressure oscillometry). 
         [0008]    Approaches disclosed herein further allow for absolute blood pressure values to be determined directly without the requirement for any periodic calibrations (e.g., applanation tonometry as described in greater detail below) or for relative blood pressure values to be tracked so as to provide relative blood pressure indices. The relative blood pressure values may be calibrated with a reference measurement to determine blood pressure values on an absolute scale. However, relative blood pressure values, even if not calibrated to provide absolute blood pressure values, can be of clinical benefit to the user or the health care professional. For example, providing a blood pressure index can show variations or patterns over time (e.g., trending data) which may be of particular diagnostic or therapeutic value for the user or health care professional. Still further, the present invention provides wrist-worn devices that are portable and compact in design and can be easily and comfortably worn for extended of periods of time. In particular, the wrist-worn devices of the present invention provide accurate and robust blood pressure monitoring and tracking outside the conventional hospital setting, which in turn reduces health care costs and empowers users and their caregivers and/or health care professionals to make more informed decisions. 
         [0009]    Methods utilizing hydrostatic pressure changes to determine a mean or absolute blood pressure, and more specifically employing modified volume or pressure oscillometry techniques, are disclosed. In particular, such methodologies advantageously utilize the pressure changes associated with the natural vertical movement of the user&#39;s arm (e.g., actively raising and lowering their fully extended arm) not as a source of error, but instead to non-invasively measure a mean blood pressure. Methods of the present invention for determining a pressure of blood within a cardiovascular system of a user may comprise receiving a plurality of user signals from the cardiovascular system of the user with a sensor. The sensor non-invasively engages the skin of the user over the wrist of the user, each of the user signals being received by the sensor while the sensor has an associated height relative to the heart of the user. The user moves the wrist between the signals so that the heights of the sensor differ within a range of heights relative to the heart of the user. The different heights are maintained for a sufficient length of time for the device to measure blood pressure at each height. For example, the user may slowly raise their arm from a starting position below the heart to and end position above their head or vice versa, wherein the range of heights relative to the heart of the user may comprises a range from about 1 cm to about 40 cm resulting in a hydrostatic pressure differential in range from just below 1 mmHg to about 31 mmHg. A signal variation amplitude of the plurality of signals associated with the range of heights is identified and a standard pressure of the blood of the user based on the signal variation amplitude and the plurality of signals is determined, the standard pressure having an associated standard blood pressure measurement height relative to the heart. 
         [0010]    The plurality of user signals may comprise volume or pressure waveform signals from at least one photoplethysmogram (PPG) or pressure sensor (e.g., pressure sweep for applanation tonometry approaches disclosed in greater below) respectively non-invasively engaging the skin of the user over the wrist. In this example, the signal variation amplitude may be identified from a maximum volume or pressure waveform signal based on an oscillation or amplitude of the plurality of volume or pressure waveform signals of the user. In particular, the volume or pressure waveform signal associated with the highest oscillation or amplitude comprises the maximum volume or pressure waveform signal. 
         [0011]    A signal indicative of the height of the sensor relative to the heart associated with the maximum volume or pressure waveform signal may be received and/or calculated from at least a height sensor, accelerometer, and/or a barometric pressure sensor coupled to the wrist-worn device. Still further, user input (e.g., length of arm, height from heart to shoulder, etc.), or other anthropometric data may also be utilized in combination with the height sensor, accelerometer, and/or a barometric pressure sensor signals to determine a height measurement associated with the highest oscillation or amplitude. Ideally, the height measurement provides accuracy of ±6 cm for ensuring pressure errors of less than 3-5 mmHg. The standard or mean arterial pressure may be determined based on the maximum volume or pressure waveform signal and the signal indicative of the height of the sensor relative to the heart associated with the maximum volume or pressure waveform signal (e.g., hydrostatic pressure component). 
         [0012]    The mean arterial pressure may be generally correlated to the hydrostatic pressure component determined above plus a relatively constant, low pressure applied externally to a radial artery beneath the skin of the wrist of the user as the user raises or lowers their arm though the range of heights relative to the heart. This relatively constant pressure may be applied over the radial artery by an actuator coupled to the wrist-worn device or by user actuation, such as snugly tightening the band of the device around their wrist. This constant pressure range should be within the range of known or expected mean arterial pressure, so that as the local pressure changes with changes in the arm height, the applied pressure becomes equal to the temporary local pressure at some height of the arm relative to the heart. A pressure sensor or an array thereof may be coupled to the wrist-worn device and non-invasively engaging the skin of the wrist to measure the pressure applied to the wrist as the at least one PPG or pressure sensor is swept through the range of heights relative to the heart of the user for determining the mean arterial pressure. The mean arterial pressure point measurement may further be utilized as a reference blood pressure measurement for calibrating relative blood pressure signals, as described in greater detail below. Still further, the determined mean arterial pressure may be transmitted to a second wrist-worn device (e.g., watch), mobile device, tablet, computer, or database for further processing (e.g., calibration of relative blood pressure signals 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. 
         [0013]    As described above, relative blood pressure values may be calibrated with a reference measurement to determine blood pressure values on an absolute scale. Methods of the present invention for obtaining a blood pressure measurement of a user comprise sensing, with a first sensor of a wrist-worn device non-invasively engaging the skin on the wrist of the user, a first user signal indicative of ventricular ejection of blood (or when a pressure pulse begins propagation) from the heart of the user, the first sensed ventricular ejection signal having an associated ventricular ejection time. The method may further comprise sensing, with a second sensor of the wrist-worn device non-invasively engaging the skin on the wrist of the user, 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 signal and having an associated pulse arrival time. A relative blood pressure value may be determined in response to a first pulse transit time (PTT) identified from a difference between the ventricular ejection time and the pulse arrival time. An absolute reference blood pressure measurement obtained in coordination with the relative blood pressure may be received from an accurate reference measurement device and the absolute blood pressure of the relative blood pressure value determined in response to a difference between the relative blood pressure and the absolute reference blood pressure. 
         [0014]    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 (e.g., backward or retroactive calibration of existing data or forward calibration of new data). For example, a second PTT may be determined using the first and second sensors of the wrist-worn device, and the absolute blood pressure of the second PTT determined in response to the difference between the relative blood pressure and the absolute reference blood pressure. In another example, an absolute blood pressure of a second PTT determined from the first and second sensors of the wrist-worn device and prior to the first PTT is determined in response to the difference between the relative blood pressure and the absolute reference blood pressure. It will be appreciated that the plurality of relative blood pressure values may further be adjusted based on a variety of other factors, such as anthropometric information, vasomotor effects, hydrostatic effects, ambient temperature, user actively level, skin perfusion, skin temperature, or body posture. 
         [0015]    Ideally, the plurality of relative blood pressure values are measured when the user is relatively stationary for a short period of time, for example 30 seconds or less, 20 seconds or less, or 10 seconds or less. Further, in some instances, the plurality of relative blood pressure values are preferably measured at a substantially constant sensor height relative to the heart of the user to minimize errors due to hydrostatic pressure effects, as discussed in greater below. The absolute reference blood pressure measurement may be obtained from a variety of sources including volume oscillometry (as described herein), applanation tonometry devices (as described herein), an oscillometric cuff, or an input by the user. In some instances, if the difference between the determined absolute blood pressure and the reference blood pressure is greater than ±5 mmHg mean error or ±8 mmHg sigma error, a second absolute reference blood pressure measurement may be required for accurate calibration of the relative pressure values. In this instance, a blood pressure index of the relative blood pressure values may be displayed or transmitted instead of the absolute blood pressure values. 
         [0016]    Generally, user-dependent calibration of the relative blood pressure values may be periodically carried out at least once a week, monthly, or yearly, wherein active measurement approaches may require more frequent recalibration intervals than passive measurement approaches. Methods of the present invention further include recalibration, wherein the absolute reference blood pressure measurement is obtained at a first time period and a second absolute reference blood pressure measurement is obtained in coordination with a second relative blood pressure at a second time period later than the first time period (e.g., 1 month later). An absolute blood pressure of the second relative blood pressure value may then be determined in response to a difference between the second relative blood pressure and the second absolute reference blood pressure. 
         [0017]    Calibration may be carried out locally by a controller coupled to the wrist-worn device or externally of the wrist-worn device by a mobile device, tablet, computer, or database. Further, the plurality of calibrated relative blood pressure values may be transmitted to a second wrist-worn device, mobile device, tablet, computer, or database for further processing, storage, retrieval, or display as described herein. The wrist-worn device of the present invention may comprise an active band, watch, and/or heart rate monitor. For example, the device may comprise a single integral electronic watch device that includes both a heart rate monitor and blood pressure monitor. Still further, the blood pressure monitor may be incorporated into a separate active band that is connectable to the watch device as described in greater detail below. 
         [0018]    The first sensor may comprise at least one impedance cardiogram (ICG), electrocardiogram (ECG/EKG), ballistocardiogram (BCG), phonocardiogram (PCG), or seismocardiogram (SCG) sensor coupled to the wrist-worn device for sensing the first user signal indicative of ventricular ejection of blood from the heart of the user. For example, the at least one ICG or ECG sensor comprise at least a first pair of dry electrodes non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes contacted 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 or electrical potential measurements respectively. In another example, the at least one ICG or ECG sensor comprise at least a first pair of dry electrodes non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes, wherein the second pair of dry electrodes and/or wrist-worn device non-invasively engage a skin surface of a sternum of the user. In addition or alternatively, the least one BCG sensor comprises an accelerometer non-invasively engaging an anterior surface of the wrist so as to passively measure a relative blood pressure. It will be appreciated that engagement with a glabrous skin surface provides improved electrical contact, but the sensors described herein can also engage the posterior surface of the wrist for measurements. Still further, the at least one PCG sensor comprises a sound sensor and the sound sensor, wrist-worn device and/or hand of the wrist-worn device non-invasively engage a skin surface of a sternum of the user. Optionally, the at least one SCG sensor comprises an accelerometer and the accelerometer, wrist-worn device and/or hand of the wrist-worn device non-invasively engage the sternum. 
         [0019]    The second sensor may comprise at least one PPG sensor or pressure sensor coupled to the wrist-worn device for sensing the second user signal indicative of arrival of the pressure pulse in the wrist. The at least one PPG sensor may 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. The pressure sensor may comprise at least one pressure transducer, accelerometer, or strain gauge configured to be positioned over a radial artery of the wrist of the user. 
         [0020]    It will be appreciated that multiple combinations of sensors may be utilized on the wrist-worn device for measuring the first and/or second user signals. For example, the first sensor may comprise first and second cardiogram sensors coupled to the wrist-worn device for sensing the first user signal indicative of ventricular ejection of blood from the heart of the user, wherein the second cardiogram sensor is different than the first cardiogram sensor. In this example, the first cardiogram may comprise an ICG sensor for a cross body measurement and the second cardiogram sensor may comprise a BCG sensor for comparison to a passive measurement or a SCG/PCG sensor for comparison to an active measurement that has little or no error due to hydrostatic pressure changes as the SCG/PCG measurement is made at the chest which is relatively aligned with a height of the heart. 
         [0021]    It will be appreciated that multiple combinations of sensors may be utilized on both the wrist-worn device and separate non-wrist worn devices (e.g., mobile device, tablet, stand-alone or attached accessory) for measuring the first and/or second user signals. In another example, an accelerometer of a mobile device may be utilized to provide a SCG measurement of the first user signal indicative of ventricular ejection of blood from the heart of the user by having the mobile device held or strapped against the chest or placed in the user&#39;s shirt pocket while the PPG sensor of the wrist-worn device measures the second user signal indicative of arrival of the pressure pulse in the wrist. Still further, non-wrist worn devices may be utilized to provide ECG/ICG measurements nominally across the heart, a pressure pulse over the radial artery (or a carotid or femoral artery), or a PPG measurement over a finger, thumb, neck, thigh, forehead, or earlobe. For multi-device implementations of wrist-worn and non-wrist worn devices, time synchronization between devices may be carried out via a wireless or telemetry interface (e.g., Bluetooth or WiFi) or by conducting a signal through the user&#39;s body (e.g., small electrical pulse) as a reference strobe. 
         [0022]    The present invention further includes a first wrist-worn device for determining a pressure of blood within a cardiovascular system of a user. The device may comprise an elongate band non-invasively engaging the skin on the wrist of the user, wherein the elongate band is releasably coupleable to a second wrist-worn electronic device. At least one PTT or pressure sensor may be coupled to the elongate band, the sensor non-invasively engaging the skin over the wrist of the user for measuring user signals from the cardiovascular system of the user. A controller may be coupled to the elongate band and at least one PTT or pressure sensor for determining relative or absolute blood pressure signals based on the user signals. A power source may be coupled to the elongate band and the controller or the at least one PTT or pressure sensor for providing power to the wrist-worn device. A telemetry/wireless interface (e.g., Bluetooth or WiFi) may be coupled to the elongate band and the controller. 
         [0023]    The second wrist-worn electronic device may comprise a watch or heart rate monitor having a housing encasing a second controller, second power source, and second telemetry interface that are distinct and separate from the first wrist-worn blood pressure monitoring band. Advantageously, providing bands that are releasably coupleable to the second wrist-worn device (e.g., watch) provides for user customization of the watch based on the desired sensor monitoring. For example, a first band may comprise an ICG/PPG sensor combination for measuring relative blood pressure values while a second band may comprise a pressure sensor/actuator combination for measuring absolute blood pressure values. Still further, a third band may monitor an entirely different diagnostic than blood pressure (e.g., heart rate monitor). The user may selectively choose between the first, second, or third bands for the desired sensor monitoring and may further interchange the bands at any time period as desired (e.g., a fourth band comprising a passive BCG/PPG sensor combination for night time blood pressure monitoring and a fifth band comprising an active ECG/PPG sensor combination for day time blood pressure monitoring) via a releasable coupling feature. Still further, the first wrist-worn device may easily communicate (e.g., transmit blood pressure values, receive updated instructions, such as new calibration equations, etc.) with the second wrist-worn device via WiFi or Bluetooth. The elongate band further comprises at least one releasable connection or coupling feature for securing the selected band to the watch or heart rate monitor. For example, the connection or coupling feature may be mechanical (pin/peg connection, clasp, snap fit, set-screw, or slide-in connector) or magnetic. It will be appreciated still further that some embodiments of the present invention may utilize the same controller, power source, or telemetry interface for both the first and second wrist-worn devices. Still further, the first and second wrist-worn devices (e.g., blood pressure monitor and hear rate monitor) may be incorporated into a single integral electronic watch device. 
         [0024]    As described above, the least one PTT sensor may comprise a 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. As described above, 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. 
         [0025]    Absolute blood pressure bands (e.g., applanation tonometry approaches) may incorporate at least one pressure sensor comprising at least one pressure transducer, piezoelectric film, or piezoresistive film 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 elongate band may further comprise at least one actuator configured to apply a constant or variable pressure over a radial artery of the wrist. Still further, at least one height sensor, barometric pressure sensor, gyroscope, or accelerometer may be coupled to the elongate band so as to account for hydrostatic pressure effects. 
         [0026]    The telemetry interface of the elongate band may be configured to transmit the relative or absolute blood pressure signals to the second wrist-worn electronic device, a mobile device, tablet, computer, or database for further processing, storage, retrieval by other devices or programs, and/or display. For example, the telemetry interface of the elongate band may be configured to transmit the relative or absolute blood pressure signals to an electronic health or medical record (e.g., on a database) or health application software (e.g., on a mobile device, tablet, or computer). In another example, the telemetry interface of the elongate band may be configured to transmit the relative or absolute blood pressure signals to a display on the second wrist-worn electronic device or a third non-wrist device (e.g., a mobile device, tablet, computer), the display viewable by the user or a health care professional for use in diagnostic or therapeutic decision making. The telemetry interface of the elongate band may also be configured to transmit trending data (e.g., blood pressure index) for a time period based on the relative blood pressure signals, wherein the time period comprises one or more days, weeks, months, or years. 
         [0027]    Embodiments of the present invention further include methods for providing a plurality of active bands for blood pressure monitoring of a user as described above. In one method, a first wrist-worn band is provided having at least one PIT sensor coupled to the first wrist-worn band and configured to non-invasively engage the skin over the wrist of the user for measuring user signals from the cardiovascular system for determining relative blood pressure signals. A second wrist-worn band is provided having at least one pressure sensor coupled to the second wrist-worn band and configured to non-invasively engage the skin over the wrist of the user for measuring user signals from the cardiovascular system for determining absolute blood pressure signals. The user is able to selectively choose and/or interchange between the first and second wrist-worn bands, wherein the selected first or second band is releasably coupleable to a wrist-worn electronic device. As discussed above, it will be appreciated that several other combinations of bands having various sensing modalities are possible (e.g., first band requiring user interaction for blood pressure measurement while the second band is passive for blood pressure measurement). 
         [0028]    Embodiments of the present invention further include methods for obtaining and transmitting relative blood pressure measurements of a user. One method comprising sensing, with a first sensor of a wrist-worn device non-invasively engaging the skin on the wrist of the user, first user signals indicative of ventricular ejections of blood from the heart of the user, the first sensed ventricular ejection signals each having an associated ventricular ejection time. A second sensor of the wrist-worn device non-invasively engaging the skin on the wrist of the user, measures second user signals indicative of pressure pulse arrivals in the wrist, the second sensed pressure pulse signals associated with the first sensed ventricular ejection signals, each of the second sensed pressure signals having an associated pulse arrival time. FIT measurements are identified from a difference between the first sensed ventricular ejection signals and the second sensed pressure signals and the PTT measurements are transmitted directly to a second electronic device or database in a non-calibrated (e.g., non-manipulated) format. For example, the second electronic device may comprise a watch, phone, tablet, or a computer. 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. In some instances, the PTT measurements may be transmitted to a phone or tablet, and then re-transmitted to a cloud database for further processing. In other instances, the PTT measurements may be transmitted specifically to an electronic health or medical record or health application software. Still further, trending data may be transmitted for a specified time period based on the PTT measurements, wherein the time period comprises one or more days, weeks, months, or years. 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. 
         [0029]    Embodiments of the present invention further include methods for filtering non-invasive blood pressure measurements from a wrist-worn device. One method comprises receiving a plurality of relative or absolute blood pressure signals from at least one pulse transit time (PTT) or pressure sensor coupled to a wrist of a user, filtering the relative or absolute blood pressure signals based on contextual information associated with the user, and discarding or masking the filtered relative or absolute blood pressure signals. Contextual filtering may be based on a variety of information that may provide context for any measured blood pressure changes or artifacts. The contextual information associated with the user may comprise at least one of the following: (a) input from the user, (b) health application software information associated with the user, (c) an electronic medical record information associated with the user, (d) location information associated with the user (e.g., GPS), (e) calendar information associated with the user, (f) time information, (g) temperature information, (h) current activity as entered by the user or detected by the device (e.g. sitting, standing, walking, sleeping, driving), or (i) medication usage/dosage. For example, location information may allow filtering of blood pressure signals when the user is driving, calendar information may allow filtering of blood pressure signals when the user is at an exercise class, and temperature information may allow filtering of blood pressure signals when the user is in an extremely cold environment. Filtering relative or absolute blood pressure signals may also reduce power consumption of the wrist-worn device as only non-filtered relative or absolute blood pressure signals are transmitted to a second wrist worn device, mobile device, tablet, computer, or database. In addition to filtering to remove certain measurements, the contextual information can also be used to annotate blood pressure information over time, in order to discern trends that affect blood pressure (e.g. blood pressure reduction after walking, blood pressure increase during driving). 
         [0030]    Embodiments of the present invention further include methods for accounting for hydrostatic effects, particularly for non-invasive blood pressure measurements from a wrist-worn device having ICG/ECG sensors for cross body measurements (e.g., finger to opposite wrist-worn device) or a BCG sensor for passive measurements. For example, pressure differentials as large as 30 mmHg can be due to a 40 cm variation in the height of the sensor relative to the heart during a measurement. Methods are provided herein for addressing pressure differentials due to taking a measurement when the user&#39;s wrist is at a various heights (e.g., down by their side, up in the air, folded across, etc.) relative to the heart. One method comprises receiving relative blood pressure signals from PIT measurements from a wrist-worn device, wherein each PTT measurement comprises a time period from ventricular ejection of a heart to pulse arrival at a wrist and the PTT ventricular ejection of the heart is determined from at least one ICG, ECG, or BCG sensor. A signal is received indicative of a height of the sensor relative to the heart associated with each PTT measurement and the relative blood pressure signals adjusted based on the height of the sensor relative to the heart signal associated with each PTT measurement so as to account for hydrostatic pressure differentials. For example, the height signal may be received and/or calculated from at least a height sensor, accelerometer, gyroscope, and/or a barometric pressure sensor coupled to the wrist-worn device. Still further, user input or anthropometric data may also be utilized in combination with the height sensor, accelerometer, gyroscope, and/or a barometric pressure sensor signals to determine the height measurement. It will be appreciated however that hydrostatic effects may also be negated by taking measurements while the user is lying down (e.g., BCG passive monitoring while the user is asleep) so that there is little to no variation between the height of the wrist sensor relative to the heart or by aligning the wrist sensor relative to the height of the heart during a measurement (e.g., ICG contact with the sternum). 
         [0031]    The details of one or more implementations are set forth in the accompanying drawings and the description below. A better understanding of the features and advantages of the present invention will be obtained by reference to the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  illustrates a propagation path of a blood pressure pulse from ejection from the left ventricle of the heart to a wrist on which a wrist-worn blood pressure measurement device is worn according to embodiments of the present invention. 
           [0033]      FIG. 2  illustrates EKG, ICG, and PPG signals relative to a PIT for a blood pressure pulse propagating from the left ventricle to a wrist on which a wrist-worn blood pressure measurement device is worn according to embodiments of the present invention. 
           [0034]      FIG. 3  schematically illustrates a four-electrode configuration used to measure impedance of a subject according to embodiments of the present invention. 
           [0035]      FIGS. 4-5  are schematic side views of wrist-worn blood-pressure measurement devices according to embodiments of the present invention 
           [0036]      FIG. 6  schematically illustrates electrode locations and related body impedances in an approach for measuring chest-cavity impedance variations according to embodiments of the present invention. 
           [0037]      FIG. 6A  is a cross-sectional view of another wrist-worn blood-pressure measurement device having exterior electrodes shown engaged with skin of a user&#39;s thorax according to embodiments of the present invention. 
           [0038]      FIG. 7  is a schematic diagram of a wrist-worn blood-pressure measurement device main unit according to embodiments of the present invention. 
           [0039]      FIG. 8  shows typical EKG and ICG data traces according to embodiments of the present invention. 
           [0040]      FIG. 9  illustrates accelerometer and PPG signals relative to a PTT for a blood pressure pulse propagating from the left ventricle to a wrist on which a blood pressure measurement device is worn according to embodiments of the present invention. 
           [0041]      FIG. 10  is a schematic side view of a wrist-worn blood pressure measurement device held in contact with a user&#39;s chest according to embodiments of the present invention. 
           [0042]      FIG. 11  is a typical time-domain trace of a measured Seismo-Cardiogram acceleration oriented normal to a user&#39;s chest surface according to embodiments of the present invention. 
           [0043]      FIG. 12  is a typical frequency-domain Seismo-Cardiogram according to embodiments of the present invention. 
           [0044]      FIG. 13  is a typical spectrogram Seismo-Cardiogram according to embodiments of the present invention. 
           [0045]      FIG. 14  shows x-axis acceleration, y-axis acceleration, z-axis acceleration, and vector-sum acceleration Seismo-Cardiogram plots according to embodiments of the present invention. 
           [0046]      FIG. 15  shows x-axis acceleration, y-axis acceleration, z-axis acceleration, and vector-sum acceleration Ballisto-Cardiogram plots according to embodiments of the present invention. 
           [0047]      FIG. 16  is a schematic diagram of a wrist-worn blood-pressure measurement device according to embodiments of the present invention. 
           [0048]      FIG. 17  is a schematic diagram of an approach for processing recorded acceleration data to identify when blood is ejected from the left ventricle of a user&#39;s heart according to embodiments of the present invention. 
           [0049]      FIG. 18  illustrates a cross-section of tissue layers between a wrist skin surface and an underlying artery of a subject. 
           [0050]      FIGS. 19-21  illustrate 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 according to embodiments of the present invention. 
           [0051]      FIGS. 22-23  show relative contribution by subsurface layer to returning light detected by the light detectors disposed at different distances for two different light source wavelengths according to embodiments of the present invention. 
           [0052]      FIG. 24  illustrates variation of mean penetration depth as a function of source-detector separation for two different source light wavelengths according to embodiments of the present invention. 
           [0053]      FIG. 25  illustrates variation of the ratio of photons from the deep blood plexus (DBP) layer as a function of source-detector separation for two different source light wavelengths according to embodiments of the present invention. 
           [0054]      FIG. 26  illustrates a propagation path of a blood pressure pulse from ejection from the left ventricle past an auxiliary PPG sensor to a wrist on which a wrist-worn blood-pressure measurement device is worn according to embodiments of the present invention. 
           [0055]      FIG. 27  is a schematic side view of an arm-worn auxiliary PPG sensor for a wrist-worn blood-pressure measurement device according to embodiments of the present invention. 
           [0056]      FIG. 28  is a cross-sectional view of another wrist-worn blood-pressure measurement device that can be used with the auxiliary PPG sensor of  FIG. 27  according to embodiments of the present invention. 
           [0057]      FIG. 29  illustrates a method for calculating a mean arterial pressure of a user according to embodiments of the present invention. 
           [0058]      FIG. 29A  shows a piezoelectric film sensor according to embodiments of the present invention. 
           [0059]      FIG. 29B  shows a piezoelectric pressure sensor according to embodiments of the present invention. 
           [0060]      FIG. 30  illustrates a method for determining a hydrostatic pressure acting on the wrist of a user according to embodiments of the present invention. 
           [0061]      FIGS. 31A-31C  illustrate a method of changing the hydrostatic pressure at the wrist of the user according to embodiments of the present invention. 
           [0062]      FIGS. 32-35  illustrate various applanation tonometry devices for measuring pressure pulses at the wrist of the user according to embodiments of the present invention. 
           [0063]      FIG. 36  illustrates a fluid bladder according to embodiments of the present invention. 
           [0064]      FIGS. 37-39  illustrate various pressure sensor arrays that may be used with embodiments of the present invention. 
           [0065]      FIG. 40  illustrates a method of selectively actuating subsets of the plurality of pressure sensors against a wrist of a user according to embodiments of the present invention. 
           [0066]      FIG. 41  illustrates the coupling of a device having a plurality of sensors and a plurality of actuators to a wrist of a user according to embodiments of the present invention. 
           [0067]      FIGS. 42-45  illustrate selective actuation of a skin interface against a wrist of a user according to embodiments of the present invention. 
           [0068]      FIG. 46A-46C  show pressure sensor data obtained from an array of pressure sensors applied to a user according to embodiments of the present invention. 
           [0069]      FIG. 47  illustrates a method of calibrating relative blood pressure signals according to embodiments of the present invention. 
           [0070]      FIG. 48  illustrates a schematic diagram of an overall system including a wrist-worn band, wrist-worn electronic device, and a mobile phone according to embodiments of the present invention. 
           [0071]      FIGS. 49A-49C  schematically illustrate a plurality of wrist-worn bands for coupling to a wrist-worn electronic device according to embodiments of the present invention. 
           [0072]      FIG. 50  schematically illustrates an active band releasably coupleable to a wrist-worn electronic device according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0073]      FIG. 1  illustrates a propagation path of a blood pressure pulse from ejection from the left ventricle of a subject&#39;s heart to a wrist on which a wrist-worn blood-pressure measurement device  10  is worn, in accordance with many embodiments. The wrist-worn device  10  is 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 device  10 . The wrist-worn device  10  is 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 device  10 . The determined PTT is then used to determine one or more blood-pressure values for the subject. 
         [0074]    In general, a PTT is the time it takes for a pulse pressure wave to propagate through a length of a subject&#39;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). 
         [0000]    
       
         
           
             
               
                 
                   MAP 
                   = 
                   
                     
                       1 
                       α 
                     
                      
                     
                       ln 
                        
                       
                         [ 
                         
                           
                             ρ 
                              
                             
                                 
                             
                              
                             
                               
                                 D 
                                  
                                 
                                   ( 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     d 
                                   
                                   ) 
                                 
                               
                               2 
                             
                           
                           
                             
                               
                                 hE 
                                 0 
                               
                                
                               
                                 ( 
                                 PTT 
                                 ) 
                               
                             
                             2 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0075]    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;   E 0  is the Young&#39;s modulus of the artery at zero pressure;   α is a subject dependent physiological constant; and   Δd is the arterial distance between the subjects left ventricle and the wrist.       
 
         [0083]    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&#39;s modulus of the artery at zero pressure (E 0 ), 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&#39;s modulus of the artery at zero pressure (E 0 ) 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). 
         [0000]    
       
         
           
             
               
                 
                   MAP 
                   = 
                   
                     
                       1 
                       α 
                     
                      
                     
                       ln 
                        
                       
                         [ 
                         
                           K 
                           
                             
                               ( 
                               PTT 
                               ) 
                             
                             2 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where 
         [0000]    
       
         
           
             K 
             = 
             
               
                 ρ 
                  
                 
                     
                 
                  
                 
                   
                     D 
                      
                     
                       ( 
                       
                         Δ 
                          
                         
                             
                         
                          
                         d 
                       
                       ) 
                     
                   
                   2 
                 
               
               
                 hE 
                 0 
               
             
           
         
       
     
         [0000]    is suitable for the subject and the arterial tree segment over which PIT is being measured. 
         [0084]    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 device  10 . 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). 
         [0085]    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. 
         [0000]        MAP=K   MAP ×[log( PTT )−log( PTT   0 )]+ MAP   BASELINE   (3)
 
         [0086]    where: MAP is predicted mean arterial blood pressure;
       MAP BASELINE  is a baseline measured MAP;   K MAP  is a subject dependent constant for MAP;   PTT is the measured pulse transit time; and   PTT 0  is the measured pulse transit time for MAP BASELINE .       
 
         [0000]        SBP=K   SBP ×[log( PTT )−log( PTT   0 )]+ SBP   BASELINE   (4)
 
         [0091]    where: SBP is predicted systolic blood pressure;
       SBP BASELINE  is a baseline measured systolic blood pressure;   K SBP  is a subject dependent constant for systolic blood pressure;   PTT is the measured pulse transit time; and   PTT 0  is the measured pulse transit time for SBP BASELINE .       
 
         [0000]        DBP=K   DBP ×[log( PTT )−log( PTT   0 )]+ DBP   BASELINE   (5)
 
         [0096]    where: DBP is predicted diastolic blood pressure;
       DBP BASELINE  is a baseline measured diastolic blood pressure;   K DBP  is a subject dependent constant for diastolic blood pressure;   PTT is the measured pulse transit time; and   PTT 0  is the measured pulse transit time for DBP BASELINE .       
 
         [0101]      FIG. 2  shows an EKG trace segment  12 , an ICG trace segment  14 , and a PPG signal  16  relative to a pulse transit time (PTT)  18  for a blood pressure pulse between the left ventricle of the subject to the wrist-worn device  10 . In many embodiments, the wrist-worn device  10  includes 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 segment  12  has 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)  20  is the time between the peak (R) of the EKG trace and arrival of the blood pressure pulse at the wrist-worn device  10 . 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)  22  is the time period between the peak (R) of the EKG trace and the opening of the aortic valve. The PEP  22  correlates poorly with blood pressure. The ICG trace  14  provides 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 trace  14  is processes to identify a start  24  of 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 signal  16 , which includes an inflection point  26  that occurs upon arrival of the blood pressure pulse to the wrist-worn device  10 . 
         [0102]      FIG. 3  schematically illustrates a four-electrode configuration  30  used to measure impedance of a subject, in accordance with many embodiments. The four-electrode configuration  30  includes a drive current generator  32  electrically coupled with a first drive current electrode  34  and a second drive current electrode  36 . In many embodiments, the drive current generator  32  imparts an alternating current to a subject  38  via the electrodes  34 ,  36 . The four-electrode configuration  30  also includes a voltage sensor  40  electrically coupled with a first sense electrode  42  and a second sense electrode  44 . The use of the sense electrodes  42 ,  44 , which are separated from the drive current electrodes  34 ,  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 electrodes  34 ,  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. 
         [0103]      FIG. 4  shows a side view of a wrist-worn blood-pressure measurement device  50 , in accordance with many embodiments. The wrist-worn device  50  includes a main unit  52 , a wrist-worn elongate band  54 , a first drive current electrode  56 , a first sense electrode  58 , a second drive current electrode  60 , a second sense electrode  62 , and a PPG sensor  64 . The first drive current electrode  56 , the first sense electrode  58 , and the PPG sensor  64  are: 1) supported on the wrist-worn elongate band  54 ,  2 ) positioned and oriented to interface with a subject&#39;s wrist upon which the wrist-worn device  50  is worn, and 3) operatively connected with the main unit  52 . The second drive current electrode  60  and the second sense electrode  62  are: 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 device  50  is worn), and 3) operatively connected with the main unit  52 . The main unit  52  includes circuitry and/or software for imparting drive current through the subject via the first and second drive current electrodes  56 ,  60  and for processing signals from the PPG sensor  64  and the first and second sense electrodes  58 ,  62  so as to measure a PTT and calculate one or more blood pressure values for the subject based on the PTT. 
         [0104]      FIG. 5  shows a side view of another wrist-worn blood-pressure measurement device  70 , in accordance with many embodiments. The wrist-worn device  70  includes the same components as for the wrist-worn device  50 , but has the first drive current electrode  56  and the first sense electrode  58  located to enhance contact pressure with a wrist  72  of the subject. In the illustrated embodiment, the first drive current electrode  56  is disposed on a directly opposite inside surface of the wrist-worn band  54  relative to the second drive current electrode  60  such that contact pressure between, for example, a finger of the subject and the second drive current electrode  60  transfers compression through the wrist-worn band  54  to the first drive current electrode  56 , thereby increasing contact pressure between the first drive current electrode  56  and the wrist  72 . In a similar fashion, the first sense electrode  58  is disposed on a directly opposite inside surface of the wrist-worn band  54  relative to the second sense electrode  62  such that contact pressure between, for example, a finger of the subject and the second sense electrode  62  transfers compression through the wrist-worn band  54  to the first sense electrode  58 , thereby increasing contact pressure between the first sense electrode  58  and the wrist  72 . Any suitable variation can be used. For example, the locations of the first drive current electrode  56  and the first sense electrode  58  can be exchanged. As another example, the electrodes  56 ,  58 ,  60 ,  62  can be located at any other suitable locations on the wrist-worn band  54 . As another example, any suitable number of the electrodes  56 ,  58 ,  60 ,  62  can be disposed on the main unit  52 . 
         [0105]    In the illustrated embodiment, the PPG sensor  64  is located on the wrist-worn band  54  so as to be disposed to sense the arrival of the blood-pressure pulse within a radial artery  74  of the subject. Cross sections of the ulna bone  76  and the radius bone  78  of the subject are shown for reference. 
         [0106]      FIG. 6  schematically 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 electrode  56  and the first sense electrode  58  are held in contact with the left wrist of the subject. The second drive current electrode  60  is contacted by the right index finger of the subject. The second sense electrode  62  is contacted by the right thumb of the subject. The first and second drive current electrodes  56 ,  60  impart a cross-body alternating drive current  80  between the drive current electrodes  56 ,  60 . The cross-body drive current  80  propagates 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 electrode  56  and the contact impedance of the first drive current electrode  56  and 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 electrode  60  and the contact impedance of the second drive current electrode  60  and the right index finger is schematically represented as an impedance (Z3). The net cross-body impedance between the impedances (Z1 and Z3) is schematically represented as an impedance (Z5). The combined impedance of the left wrist local to the first sense electrode  58  and the contact impedance of the first sense electrode  58  and the left wrist is schematically represented as an impedance (Z2). The combined impedance of the right thumb in contact with the second sense electrode  62  and the contact impedance of the second sense electrode  62  and the right thumb is schematically represented as an impedance (Z4). In many embodiments, because the first and second sense electrodes  58 ,  62  are configured to measure a voltage difference without transferring any significant amount of current, the resulting voltage drops across the impedances (Z2 and Z4) are small so that the voltage difference sensed by the first and second sense electrodes  58 ,  62  matches the voltage difference across the impedance (Z5). 
         [0107]      FIG. 6A  shows a side view of another wrist-worn blood-pressure measurement device  71 , in accordance with many embodiments. The wrist-worn device  71  includes the same components as for the wrist-worn device  70 , but has the second drive current electrode  60  and the second sense electrode  62  located so that they can be engaged with another portion of the user via the user positioning the arm on which the wrist-worn device  71  is worn so as to press the electrodes  60 ,  62  into contact with any suitable skin portion of the user. For example,  FIG. 6A  illustrates the electrodes  60 ,  62  being pressed against a skin location on the user&#39;s thorax  73  (e.g., lower breast skin opposite to the arm on which the device  71  is worn). As another example, the electrodes  60 ,  62  can be pressed against skin on the user&#39;s arm opposite to the arm on which the device  71  is worn. 
         [0108]      FIG. 7  schematically represents an embodiment of a wrist-worn device for measuring blood pressure. In the illustrated embodiment, the wrist-worn device includes one or more processors  82 , memory  84 , a display  86 , one or more input/output devices  88 , a data bus  90 , an ICG/EKG unit  92 , the PPG sensor  64 , and a PPG sensor control unit  94 . In many embodiments, the memory  84  includes read only memory (ROM)  96 , and random access memory (RAM)  98 . The one or more processors  82  can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA). 
         [0109]    The ICG/EKG unit  92  includes an ICG/EKG signal processing unit  100 , an ICG/EKG digital to analog unit  102 , an ICG/EKG analog front end unit  104 , and an ICG/EKG analog to digital unit  106 . The signal processing unit  100  generates 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 unit  102 . The digital to analog unit  102  generates a sinusoidal drive current matching the digital alternating drive signal and supplies the sinusoidal drive current to the analog front end unit  104 . The analog front end  100  supplies the sinusoidal drive current to the first and second drive current electrodes  56 ,  60  for propagation through the subject (e.g., as the cross-body alternating drive current  80  illustrated in  FIG. 6 ). Resulting voltage levels are sensed via the first and second sense electrodes  58 ,  62 . Signals from the sense electrodes  58 ,  62  are processed by the analog front end  104  to generate an analog voltage signal supplied to the analog to digital unit  106 . The analog to digital unit  106  converts analog voltage signal to a corresponding digital signal that is supplied to the signal processing unit  100 . The signal processing unit  100  then generates corresponding ICG/EKG digital data that can be processed by the one or more processors  82  to 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. 
         [0110]    The PPG sensor unit  64  includes a PPG illumination unit  108  and detector line array  110 . The PPG illumination unit  108  includes two light sources  112 ,  114  which transmit light having different wavelengths onto the wrist. While any suitable wavelengths can be used, the first light source  112  generates a beam of light having a wavelength of 525 nm. The second light source  114  generates 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 array  110  can include any suitable number of light detectors. In many embodiments, the light detectors are disposed at a plurality of different distances from the light sources  112 ,  114  so 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 array  110  includes four light detectors  116 ,  118 ,  120 ,  122 , with each of the light detectors  116 ,  118 ,  120 ,  122  being disposed at a different distance from the light sources  112 ,  114 . For example, the light detectors  116 ,  118 ,  120 ,  122  can be disposed at 2 mm, 3 mm, 4 mm, 6 mm, or 10 mm respectively, from each of the light sources  112 ,  114 . Signals generated by the light detectors  116 ,  118 ,  120 ,  122  are supplied to the PPG control unit  94 , which includes an analog to digital converter to generate PPG sensor digital data that can be processed by the one or more processors  82  to determine the arrival of the blood pressure pulse to the wrist-worn device. The PPG control unit  94  controls activation of the light sources  112 ,  114 , and can alternately illuminate the light sources  112 ,  114  at 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 sources  112 ,  114 . 
         [0111]    The generated ICG/EKG digital data and the PPG sensor digital data can be transferred to, and stored in, the RAM  98  for any suitable subsequent use. For example, the data can be: 1) processed by the one or more processors  82  to determine PTTs and corresponding blood pressure values for the subject, 2) displayed on the display  86 , and/or 3) output via the input/output devices  88  for any suitable purpose such as to a health care professional and/or a monitoring service. In many embodiments, the one or more processors  82  processes 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 RAM  98 , 2) displayed on the display  86 , and/or 3) output via the input/output devices  88  for any suitable purpose such as to a health care professional and/or a monitoring service. 
         [0112]      FIG. 8  shows 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). 
         [0000]        Z ( t )= V ( t )/ I ( t )  (6)
 
         [0113]    The ICG signal is then generated by calculating the negative time differential of Z(t) as shown in equation (7). 
         [0000]        ICG  Signal=− dZ/dt   (7)
 
         [0114]    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 electrodes  58 ,  62  can 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. 
         [0115]      FIG. 9  shows an electrocardiogram (EKG) trace segment  212 , a Ballisto-Cardiogram (BCG) or Seismo-Cardiogram (SCG) trace segment  214 , and a PPG signal  216  relative to a pulse transit time (PTT)  218  for a blood pressure pulse between the left ventricle of the subject to the wrist-worn device  210 . In many embodiments, the wrist-worn device  210  includes 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&#39;s left ventricle. A PPG sensor is used to generate a PPG signal for the subject. The EKG trace segment  212  is shown for reference in describing the operation of the heart. The EKG trace segment  212  has 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)  220  is the time between the peak (R) of the EKG trace and arrival of the blood pressure pulse at the wrist-worn device  210 . 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)  222  is the time period between the peak (R) of the EKG trace and the opening of the aortic valve. The PEP  222  correlates poorly with blood pressure. The BCG/SCG trace  214  can 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 device  210 . In many embodiments, the arrival of the blood pressure pulse is detected via the PPG signal  216 , which includes an inflection point  224  that occurs upon arrival of the blood pressure pulse to the wrist-worn device  210 . 
         [0116]      FIG. 10  shows a schematic side view of the wrist-worn device  210  held in contact with a user&#39;s chest  225 , in accordance with many embodiments. When the wrist-worn device  210  is held in contact with a user&#39;s chest, SCG data is generated. When the wrist-worn device  210  is not held in contact with a user&#39;s chest, BCG data is generated. The wrist-worn device  210  includes a main unit  226 , a wrist-worn elongate band  228 , an accelerometer  230 , and a PPG sensor  232 . The accelerometer  230  and the PPG sensor  232  are supported on the wrist-worn elongate band  228  and operatively connected with the main unit  226 . The PPG sensor  232  is positioned and oriented to interface with a wrist  234  of the user when the device  210  is worn on the wrist  234 . The main unit  226  includes circuitry and/or software for processing output from the accelerometer  230  and the PPG sensor  232  so as to measure a PIT and calculate one or more blood pressure values for the subject based on the PTT. In the illustrated embodiment, the PPG sensor  232  is located on the wrist-worn band  228  so as to be disposed to sense the arrival of the blood-pressure pulse within a radial artery  236  of the subject. Cross sections of the ulna bone  238  and the radius bone  240  of the subject are shown for reference. In described embodiments, the accelerometer  230  is oriented to measure accelerations in each of axes Ax and Ay (in the plane of the user&#39;s chest  225 ) and axis Az (which is perpendicular to the user&#39;s chest  225 ). 
         [0117]      FIG. 11  shows a typical time-domain SCG trace  242  for acceleration measured in a direction normal to a user&#39;s chest surface, in accordance with many embodiments. The SCG trace  242  has localized peaks  244 , which correspond to the opening of the aortic valve and associated ejection of blood into the aorta from the user&#39;s left ventricle. The SCG trace  242  can be processed to identify the localized peaks  244  and 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&#39;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 sensor  232  or 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&#39;s wrist. The calculated PTT can then be used to generate one or more blood pressure values for the user as described herein. 
         [0118]      FIGS. 12 and 13  show additional plots that can be generated from output of the accelerometer  230 .  FIG. 12  shows a typical frequency-domain SCG  246  generated from the output of an accelerometer held in contact with a user&#39;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 peaks  244  are separated by a time interval consistent with the identified heart rate.  FIG. 13  shows a typical spectrogram SCG, which can also be used to identify heart rate for the user. 
         [0119]      FIG. 14  shows 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&#39;s chest. Each of the z-axis acceleration (normal to the subject&#39;s chest) and the vector-sum acceleration (Atotal) exhibits clear acceleration peaks corresponding to respective ejections of blood from the subject&#39;s left ventricle. The y-axis acceleration (in plane of the subject&#39;s chest) is relatively less clear with respect to having acceleration peaks corresponding to respective ejections of blood from the subject&#39;s left ventricle. And the x-axis acceleration (also in plane with the subject&#39;s chest) is the least clear with respect to having acceleration peaks corresponding to respective ejections of blood from the subject&#39;s left ventricle. 
         [0120]      FIG. 15  shows 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&#39;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&#39;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&#39;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&#39;s left ventricle. As is described herein with reference to  FIG. 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&#39;s left ventricle. 
         [0121]      FIG. 16  schematically represents an embodiment of the wrist-worn device  210 . In the illustrated embodiment, the wrist-worn device  210  includes one or more processors  282 , memory  284 , a display  286 , one or more input/output devices  288 , a data bus  290 , the accelerometer  230 , the PPG sensor  232 , and a PPG sensor control unit  294 . In many embodiments, the memory  284  includes read only memory (ROM)  296 , and random access memory (RAM)  298 . The one or more processors  282  can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA) or integrated circuits. The accelerometer  230  can be any suitable accelerometer (e.g., a three-axis low noise accelerometer). 
         [0122]    The PPG sensor unit  232  includes a PPG illumination unit  308  and detector line array  310 . The PPG illumination unit  308  includes two light sources  312 ,  314  which transmit light having different wavelengths onto the wrist. While any suitable wavelengths can be used, the first light source  312  generates a beam of light having a wavelength of 525 nm. The second light source  314  generates 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 array  310  can include any suitable number of light detectors. In many embodiments, the light detectors are disposed at a plurality of different distances from the light sources  312 ,  314  so 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 array  310  includes four light detectors  316 ,  318 ,  320 ,  322 , with each of the light detectors  316 ,  318 ,  320 ,  322  being disposed at a different distance from the light sources  312 ,  314 . For example, the light detectors  316 ,  318 ,  320 ,  322  can be disposed at 2 mm, 3 mm, 4 mm, and 6 mm, respectively, from each of the light sources  312 ,  314 . Signals generated by the light detectors  316 ,  318 ,  320 ,  322  are supplied to the PPG control unit  294 , which includes an analog to digital converter to generate PPG sensor digital data that can be processed by the one or more processors  282  to determine the arrival of the blood pressure pulse to the wrist-worn device. The PPG control unit  294  controls activation of the light sources  312 ,  314 , and can alternately illuminate the light sources  312 ,  314  at 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 sources  312 ,  314 . 
         [0123]    Measured acceleration data and the PPG sensor digital data can be transferred to, and stored in, the RAM  298  for any suitable subsequent use. For example, the data can be: 1) processed by the one or more processors  282  to determine PTTs and corresponding blood pressure values for the subject, 2) displayed on the display  286 , and/or 3) output via the input/output devices  288  for any suitable purpose such as to a health care professional and/or a monitoring service. In many embodiments, the one or more processors  282  processes 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 RAM  298 , 2) displayed on the display  286 , and/or 3) output via the input/output devices  288  for any suitable purpose such as to a health care professional and/or a monitoring service. 
         [0124]      FIG. 17  illustrates an approach  350  for processing recorded acceleration data to identify when blood is ejected from the left ventricle of a user&#39;s heart, in accordance with many embodiments. In the approach  350 , output from the PPG sensor  232  is processed with a suitable bandpass filter  352  (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 (act  354 ) 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 accelerometer  230  (three component acceleration vector data, which varies over time) is also processed with a suitable bandpass filter  356  (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&#39;s left ventricle, thereby exhibiting clearer acceleration peaks corresponding to respective ejections of blood from the subject&#39;s left ventricle. In one variation of the approach  350 , a magnitude trace is calculated from the three component acceleration vector data (act  358 ). As illustrated in  FIGS. 14 and 15  for 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&#39;s left ventricle. In another variation of the approach  350 , a principal component analysis (PCA) can be performed (act  358 ) 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&#39;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&#39;s left ventricle. The PCA procedure is repeated for subsequent time windows of interest that contain measured acceleration data. In act  360 , 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&#39;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. 
         [0125]      FIG. 18  illustrates 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 1.10/), 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 PIT 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. 
         [0126]      FIGS. 19 through 21  illustrate 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. 19  illustrates 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. 20  illustrates 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. 21  illustrates 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. 22 and 23  show contribution of the total detected returned light for each layer for each wavelength and source-detector separation.  FIGS. 24 and 25  show combined graphs corresponding to the graphs of  FIGS. 20 and 21 . 
         [0127]    Using the data illustrated in  FIGS. 19 through 25 , the signals from the detectors  116 ,  118 ,  120 ,  122 ,  316 ,  318 ,  320 ,  322  generated for each of the light wavelengths generated by the light sources  112 ,  114 ,  312 ,  314  can 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 detectors  116 ,  118 ,  120 ,  122 ,  316 ,  318 ,  320 ,  322  that 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. 
         [0128]    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. 
         [0129]      FIG. 26  illustrates 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 device  430  to arrival at a wrist-worn device  432 . The auxiliary device  430  and the wrist-worn device  432  can use any suitable approach for detecting the arrival of the blood-pressure pulse, such as via a PPG sensor as described herein. 
         [0130]      FIGS. 27 and 28  show side views of the auxiliary device  430  and the wrist-worn device  432 . The auxiliary device  430  includes an arm-worn elongate band  434  and an auxiliary PPG sensor  436  coupled to the band  434 . The auxiliary device  430  can include one or more reference features or marks to as to enable reliable positioning and/or orientation of the auxiliary PPG sensor  436  relative to a selected underlying artery so as to detect arrival of the blood pressure pulse within the selected underlying artery. The wrist-worn device  432  can be configured similar to any of the wrist-worn devices described herein with respect to the PPG sensor  464  and can have a main unit  438  that is configured similar to any of the main units described herein with respect to all relevant functionality thereof. 
         [0131]      FIG. 29  illustrates an exemplary method  510  for calculating a mean arterial pressure with a wrist-worn pressure sensor. At step  510 , after the wrist-worn device is coupled with a user&#39;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 wrist  514 . The user may then be instructed to sweep their arm between a first height and a second height  516  to 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 signal  520 . A hydrostatic pressure associated with the maximum pressure pulse is obtained  522  after identifying the maximum pressure pulse. A mean arterial pressure may then be calculated  524  based on the obtained hydrostatic pressure and the constant pressure applied to the wrist. An indication may then be outputted  526  to 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 to  FIG. 29 . 
         [0132]    The exemplary method  510  utilizes the changes in hydrostatic pressure for applanation of an artery of the user. In many embodiments, the method  510  may 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 method  510  for 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&#39;s blood pressure. Further, some methods of monitoring or measuring blood pressure may require wrist harnesses that lock the user&#39;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. 
         [0133]    In many embodiments, after the user has coupled the device to their wrist, a constant pressure may be applied  512  by 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. 
         [0134]    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&#39;s arm is at a resting position (e.g., by the user&#39;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&#39;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&#39;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). 
         [0135]    Once the pressure sensor is coupled with the wrist of the user, a pressure signal/measurement may be received from the pressure sensor  514 . 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. 29A  shows an exemplary piezoelectric film sensor that may be used with embodiments of the present invention described herein.  FIG. 29B  shows an exemplary piezoresistive pressure sensor that may be used with embodiments of the present invention described herein. 
         [0136]    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&#39;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. 
         [0137]    In some embodiments a piezoresistive may be preferable as the film may also conform to the contours of the user&#39;s wrist and may further measure a static and dynamic pressure. 
         [0138]    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. 
         [0139]    The user may be instructed to sweep their arm between a first height and a second height  516 . The first height and second heights may be, for example, a resting position where the user&#39;s arm rests against their side when standing and a raised position where the user&#39;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&#39;s shoulder, may be calculated using an angle of the arm and a shoulder-to-wrist length. 
         [0140]    As the user moves their arm to different heights, a swept pressure signal may be received  518 . 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. 
         [0141]    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&#39;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&#39;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 signal  518 , a maximum pressure pulse in the swept pressure signal is identified  520  as 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. 
         [0142]    To calculate a mean arterial pressure  524 , 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: 
         [0000]        MAP=P   applied   −P   hydrostatic ,  (8)
 
         [0143]    where: P applied  is the constant pressure applied at the wrist and P hydrostatic  is the hydrostatic pressure acting on the wrist during the measurement of the maximum pressure pulse. 
         [0144]    P hydrostatic  may be calculated by: 
         [0000]        P   hydrostatic   =μgh,   (8a)
 
         [0145]    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/m 3 . The gravitational constant is approximately 9.8 m/s 2 . The height difference, h, may be defined as: 
         [0000]        h =Height heart −Height wrist ,  (9)
 
         [0146]    where h is obtained in centimeters (cm) and where MAP is outputted in mmHg, equation (1) may be rewritten to: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       MAP 
                        
                       
                         ( 
                         mmHg 
                         ) 
                       
                     
                     = 
                     
                       
                         Pressure 
                         applied 
                       
                       - 
                       
                         0.78 
                          
                         
                           ( 
                           
                             mmHg 
                             cm 
                           
                           ) 
                         
                         * 
                         
                           h 
                            
                           
                             ( 
                             cm 
                             ) 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0147]    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. 
         [0148]      FIG. 30  illustrates an exemplary method  528  of calculating the hydrostatic pressure at the wrist  522 . At step  530 , 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 obtained  532 . A height of the sensor relative to the user&#39;s shoulder may be calculated  534  using the signal indicative of the angle of the pressure sensor and the obtained shoulder-to-wrist length. A height of the user&#39;s shoulder may then be obtained  536  for use in calculating a wrist height  38  based on the shoulder height and the sensor height relative to the shoulder. A user&#39;s heart height may then be obtained  540 . A height difference between the pressure sensor/wrist and the heart may then be calculated  542  based on the obtained user heart height  540  and the calculated wrist height  538 . Using the calculated height difference, a hydrostatic pressure acting on the wrist at the height of the sensor may be calculated  544  and used to calculate the MAP  524  (e.g., using equation 10). 
         [0149]    In some embodiments, an accelerometer may be coupled with the wrist-worn device and may output an angle of the pressure sensor  530  while receiving the swept pressure signal. The received angle information  530  may be used with an obtained shoulder-to-wrist height  532  to 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 (Height shoulder-to-wrist ) may be calculated with the following: 
         [0000]      Height shoulder-to-wrist   =l   shoulder-to-wrist *sin θ wrist ;  (11)
 
         [0150]    where: l shoulder-to-wrist  is 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. 
         [0151]    Optionally, if the accelerometer returned an angle, φ, of the pressure sensor  530  relative 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: 
         [0000]      Height shoulder-to-wrist   =l   shoulder-to-wrist *cos φ wrist .  (12)
 
         [0152]    The length of the shoulder to the wrist of the user may be obtained  532  directly from a user input  546  for 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). 
         [0153]    In some embodiments of the invention, the user may input anthropometric data  548  and 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&#39;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&#39;s inputted height, and in some embodiments preferably about 330%-34% of the user&#39;s inputted height and in further embodiments, even more preferably about 33.4%-33.5% of the user&#39;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&#39;s inputted height, and in some embodiments, even more preferably about 33%-35% of the user&#39;s inputted height, and in further embodiments, even more preferably about 33.3%-34.5% of the user&#39;s inputted height. 
         [0154]    Thereafter, a user&#39;s wrist height (Height wrist ) may be calculated  538  by obtaining a user shoulder height  536  with the following: 
         [0000]      Height wrist =Height shoulder +Height shoulder-to-wrist .  (13)
 
         [0155]    Optionally, equation (13) may be substituted into equation (9) to provide: 
         [0000]        h =Height heart-wrist =Height heart −(Height shoulder +Height shoulder-to-wrist ).  (14)
 
         [0156]    In a similar manner to receiving a shoulder to wrist length, a shoulder height may be requested and received through a user input  546  or may be estimated using received anthropometric data  548 . For example, in some embodiments, a shoulder height of a male user may be estimated as approximately between 80%-84% of the user&#39;s height, and in further embodiments, preferably between about 81.5%-82.5% of the user&#39;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&#39;s inputted height, and in further embodiments, preferably between 82%-83% of the user&#39;s inputted height, and even more preferably about 82.4%-82.6% of the user&#39;s inputted height. 
         [0157]    To calculate for Height heart-wrist  using equation (13) or equation (14), a user heart height  540  may be obtained directly through user input  542  (user inputted and stored for subsequent use) or may be estimated based on anthropometric data inputted by the user  548  (e.g., gender, height, or the like). In some embodiments, a height of the user&#39;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. 
         [0158]    Once Height heart-wrist  is obtained, a hydrostatic pressure acting on the wrist may be calculated  544  using equation (8a) and a MAP may be calculated  524  using equation (10). 
         [0159]    After calculating an MAP for a user, the method  510  may then proceed to output an indication to the user that is indicative of the calculated MAP  526 . 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. 
         [0160]    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. 
         [0161]    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. 
         [0162]    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. 
         [0163]      FIGS. 31A-31C  illustrate a user  550  sweeping his arm for producing the swept pressure signal for the exemplary method  510 .  FIG. 31A  illustrates the user  550  with a wrist-worn device  552  at a first height  554  relative to his heart  556  where the wrist/wrist-worn device  552  is below the user&#39;s heart  556 .  FIG. 31B  illustrates the user  550  with the wrist-worn device  552  at an height  558  where the wrist/wrist-worn device  552  is approximately equal to a height of his heart  556 .  FIG. 31C  illustrates the user  550  with the wrist-worn device  552  at a second height  560  relative to his heart  556  where the wrist/wrist-worn device  552  is above the user&#39;s heart  556 . 
         [0164]    In  FIG. 31A , Height heart-wrist  has a positive value as the heart height is greater than the wrist height. Accordingly, per equation (8a), the user  550  experiences a positive hydrostatic pressure at the wrist when the wrist is below the heart  556  of 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 heart  556 . 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 height  556 , the calculated MAP is less than the applied pressure. 
         [0165]    In  FIG. 31B , Height heart-wrist  is approximately zero. Accordingly, per equation (8a), at this height, no hydrostatic pressure acts on the wrist relative to the heart  556 . 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. 
         [0166]    In  FIG. 31C , Height heart-wrist  has a negative value as the heart height is less than the wrist height, (see equation (9)). Accordingly, per equation (8a), the user  550  experiences a negative hydrostatic pressure at the wrist relative to the heart when the wrist is above the heart  556  of the user  550 . 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 heart  556 . If the desired applanation of the target artery (or a measurement of the maximum pressure pulse) occurs when the wrist is above the heart height  556 , the calculated MAP is greater than the applied pressure. 
         [0167]    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. 
         [0168]      FIG. 32  shows an exemplary device  562  for monitoring and/or measuring blood pressure of a user. The device  562  may include a wrist strap  564  and an actuator system  566  supported by the wrist strap  564 . The actuator system  566  may include a tip  567  for 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. 
         [0169]    The wrist strap  564  may be provided for coupling with a wrist of the user. While illustrated as configured to partially wrap around a user&#39;s wrists, other embodiments may fully wrap around a user&#39;s wrist. As discussed above, wrist strap  564  may be tightened around the wrist of a user to apply the constant pressure during an MAP measurement. The wrist strap  564  may include clasps, ratcheting mechanisms, or other engagement/tightening features for coupling and/or tightening the device  562  with a wrist of the user. 
         [0170]    In some embodiments, the wrist strap  564  may be configured to couple with/modify a separate wearable device with a strap. For example, the wrist strap  564  may 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. 
         [0171]    Actuator system  566  may be supported relative to a wrist of the user via wrist strap  564 . The actuator system  566  may provide a number of degrees of freedom to a pressure sensor coupled a tip  567  of the actuator system  566  relative 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 system  566  includes a first rail  568  for positioning a coupled pressure sensor perpendicular or transverse to a coupled wrist of a user. Actuator system  566  may further include a second rail  570  for positioning the tip  567  along the length of a target artery. Further, actuator system  566  may include a linear actuator  572  for 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 actuator  572  can be replaced with a voice coil actuator (VCA) or a piezoelectric stack actuator. 
         [0172]    The exemplary device  562  may be configured to carry out the exemplary method  510 . In some embodiments, the exemplary device  562  may be used to monitor blood pressure using applanation tonometry where the actuator  572  is 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. 
         [0173]      FIG. 33  illustrates another exemplary) device  574  for monitoring and/or measuring blood pressure of a user. The device  574  may include a housing  576  with a curved configuration with an inner surface  578  configured to match the curvature of the underside of the wrist of a user. Housing  576  may include slots or engagement features  580  for coupling with a wrist strap (not shown). The housing  576  may include recessed surfaces/slots  582  for receiving a sensor array and corresponding recessed surfaces/slots  584  for receiving sensor leads of a received sensor array. Further, in some embodiments, housing  576  may include a recessed surface/slot  586  for receiving a pressure actuator for urging a received sensor array against a wrist of a user. 
         [0174]    Slots  580  may be configured to receive a wrist strap for coupling the device  574  to 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 device  574  to the wrist. 
         [0175]    The recessed surface  582  may 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 surface  582  may 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 surfaces  582  are provided for two 12×1 sensor arrays. While illustrated with two recessed surfaces  582  for receiving 12×1 sensor arrays, it should be understood that other embodiments may include single recessed surface  582  or may include three or more recessed surfaces  582  for receiving sensor arrays. Further, while the recessed surfaces  582  are 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. 
         [0176]    Recessed surface  586  may be further recessed than recessed surface  582  so that the received pressure actuator may urge the received pressure sensors against the wrist of the user. In some embodiments, the recessed surface  586  may 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 surface  586  and the received fluid bladder may extend transverse to the recessed surfaces  582  so 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 surface  586  may also be configured as an array of bladders to actuate the pressure sensor or sensor array. 
         [0177]    The device  574  may be configured to carry out the exemplary method  510 . In some embodiments, the exemplary device  574  may be used to monitor blood pressure using applanation tonometry where a received pressure actuator in recess  586  is 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. 
         [0178]      FIG. 34  illustrates another exemplary device  588  for monitoring and/or measuring blood pressure of a user. Exemplary device  588  may include an enclosure  590  having slots  592  for receiving a wrist strap for coupling the device  588  to a wrist of a user. Enclosure  590  may include a slot  594  for receiving a pressure bladder or other type of actuator. Enclosure  590  may further house a driver  596  and disposed between the received pressure actuator and pressure sensor. The device  588  may further include a pressure sensor (not shown) coupled to a surface of the driver  596  that is opposite a surface that couples with the received pressure actuator. The pressure sensor or pressure sensor array can be attached to the moving part  596 , then be urged against artery. 
         [0179]    Similar to the embodiment  574  illustrated in  FIG. 33 , device  588  may receive straps through slots  592  for coupling the device  588  with a wrist of the user. Further, the received straps may be used to tighten or to urge the device  588  and a pressure sensor of the device  588  against the wrist of the user. The enclosure  590  may position a driver  596  between a pressure actuator (e.g., a fluid bladder) and a pressure sensor. The driver  596  may be configured to evenly distribute forces from the pressure actuator across the pressure sensor. This may be preferred when device  588  couples 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 driver  596  disposed between a fluid bladder and one or more pressure sensors of device  588  may alleviate these issues by evenly distributing pressure from the fluid bladder across the pressure sensor array. 
         [0180]    In the illustrated embodiment, the driver  596  may have a cross section that generally resembles a “T,” however other configurations are possible. The enclosure  590  may include a T opening  598  in a sidewall  600  of the enclosure  590 . The opening  598  may be dimensioned to receive driver  596  during assembly of enclosure  590 . Once the driver  596  is inserted within the enclosure  590 , an insert  602  may be positioned between the driver  596  and the opening  598  to secure the driver  596  within the enclosure  590 . 
         [0181]    Device  588  may 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. 
         [0182]    Similar to the embodiments described above, the device  588  may be used to carry out method  510 . Further in some embodiments, the exemplary device  588  may be used to monitor blood pressure using applanation tonometry where a received pressure actuator (e.g., fluid bladder) in slot  584  is configured to perform a pressure sweep in the Z direction by urging driver  596  and 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. 
         [0183]      FIG. 35  illustrates yet another exemplary device  604  for measuring or monitoring blood pressure of a user. The exemplary device  604  includes an elastic housing band  606  configured to couple with a wrist of a user. The elastic housing band  606  may include engagement features  608  for coupling to a wrist strap. The elastic housing band  606  may further define a housing for receiving a fluid bladder  610 . An inflation port  612  may extend from the fluid bladder housing  610  to an outer surface of the elastic housing band  606 . 
         [0184]    Elastic housing band  606  may generally have a curved configuration with an inner surface  614  configured to match the curvature of a user&#39;s wrist. The outer surface of the elastic housing band  606  may include ribs  618  and grooves  620  that run transverse to a length of the elastic housing band  606 . The ribs  618  and grooves  620  may be configured to provide additional flexibility in elastic housing band  606 , thereby allowing elastic housing band  606  to better conform to the curvature of a user&#39;s wrists. 
         [0185]    Fluid bladder housing  610  may be configured to receive a fluid bladder. In many embodiments the device  604  may 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. 
         [0186]      FIG. 36  illustrates an exemplary accordion bladder  622 . Accordion bladder  622  may have side walls  624  that generally define a volume for receiving fluid for expanding accordion bladder  622  a desired amount. The defined volume may be in fluid communication with inflation port  622 . The side walls  624  may be generally defined by a plurality of pleats or bellows that expand and contract with the filling and removal of fluid from the bladder  622 . Accordion bladder  622  may further include a generally flat distal face  626  for coupling with a pressure sensor or pressure sensor array. Due to the accordion configuration of the bladder  622 , fluid filling of the bladder  622  projects the distal face  626  of the bladder  622  linearly and evenly, thus increasing surface contact between the bladder  622  and 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 surface  626  and 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 bladder  622 . The accordion type bladder can be made of thermoplastics (e.g. nylon, polyethylene, Teflon, etc.). 
         [0187]    Device  604  may 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. 
         [0188]    Similar to the embodiments described above, the device  604  may be used to carry out method  510 . Further in some embodiments, the exemplary device  604  may be used to monitor blood pressure using applanation tonometry where a received pressure actuator (e.g., accordion fluid bladder) in fluid bladder housing  610  is 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. 
         [0189]      FIG. 37  shows an exemplary pressure sensor array  628  that may be used with the devices and methods described above. Pressure sensor array  628  may be 46 mm×46 mm in dimension and may comprises a plurality of capacitive pressure sensors  630  arranged in a 16×16 array. The pressure sensor array  628  may include a cable  632  to couple the pressure sensor array to a processing device (controller). 
         [0190]    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. 
         [0191]      FIG. 38  illustrates another exemplary pressure sensor array  634 . The array  634  comprises a first array  636  and a second array  638 . The first array  636  may comprise a 4×3 capacitive pressure sensor array and the second array  638  may similarly comprise a 4×3 capacitive pressure sensor array. Each pressure sensor may be 2×2 mm. Accordingly the array  634  may have an active area size of 16 mm×6 mm. The wiring  640  associated with the first array  636  may be routed to a first side of the pressure sensor array  634  and the wiring  642  associated with the second array  638  may be routed to a second side of the pressure sensor array  634 . Wiring  640 ,  642  may each comprise twelve wires that correspond to each of the pressure sensors in the respective arrays. 
         [0192]    The first array  636  and the second array  638  may be symmetric so that the application of this sensor array  634  against the user&#39;s wrist may also symmetric. This type of array  634  may 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 array  634 . 
         [0193]    The wiring  640 ,  642  for the sensor array  634  may be backed by a fabric material  644  (e.g., a cloth material). A fabric backing material  644  may facilitate installation within a monitoring device and may also reduce undesired bending or stretching loads being applied to the sensor array  634 . 
         [0194]      FIG. 39  illustrates an exemplary pressure actuator-pressure sensor assembly  646  that may be used with the devices and methods disclosed herein. Assembly  646  may include an actuator array  648  coupled with a sensor array  650 . Each actuator  652  of the actuator array  648  may be coupled to a pressure sensor  654  in the pressure sensor array  650 . Each of the actuators  652  in the pressure actuator array  648  may be individually controlled to urge each of the pressure sensors  654  of the pressure sensor array  650  against 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. 
         [0195]    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. 
         [0196]    While the array of actuators  648  is illustrated as a 5×9 array and the array of sensors  650  similarly illustrated as a 5×9 array, other array sizes are possible (e.g., smaller or larger). Further, the actuators  652  are 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 sensors  654  may 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. 
         [0197]      FIG. 40  illustrates an exemplary method  660  of operating the exemplary assembly  646  of  FIG. 39 . At step  662 , 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 received  664 . One or more swept pressure signals may be received by varying an applied pressure with the first subset of actuators  666 . Thereafter, a second subset of the actuators may be activated to urge a second subset of the sensors against the wrist  668 . One or more pressure signals from the second subset of sensors may then be received  670 . One or more swept pressure signals may be generated by varying the applied pressure with the second subset of actuators  672 . A maximum pressure pulse may then be identified in each of the swept pressure signals  674 . A maximum pressure pulse with the largest amplitude out of the identified maximum pressure pulses may then be identified  676 . In some embodiments, the method may include identifying the pressure sensor that recorded the maximum pressure pulse with the largest amplitude  678  and identifying a location of the identified sensor relative to the wrist of the user  680 . 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. 
         [0198]    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 method  660  may be repeated for additional subsets of actuator-sensor assemblies that remain. 
         [0199]    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. 
         [0200]    Further, in some embodiments, prior to receiving the one or more pressure signals from the second subset of sensors  670 , the first subset of sensors may be retracted away from the wrist. 
         [0201]    Additionally, while method  660  is 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. 
         [0202]      FIG. 41  illustrates the coupling of a device  682  having a plurality of sensor-actuator assemblies  684  to a wrist  686  of a user according to embodiments of the present invention. The device  682  may be configured to measure the blood pressure of a user through applanation of the radial artery  688 . 
         [0203]    The device  682  includes a strap  690  extends around the wrist  686  and supports each of the plurality sensor-actuator assemblies  684  against the wrist  686 . The sensor-actuator assemblies  684  may comprise an actuator  692  coupled with a pressure sensor  694 . The plurality of sensor-actuator assemblies  684  may couple with the wrist  686  at a device skin interface  696 . 
         [0204]    The actuators  692  may be configured to selectively and/or sequentially urge regions of the skin interface  696  adjacent the respective actuators  692  and disposed between the actuators  692  and the wrist against the wrist  686  of the user. The coupled pressure sensor  694  may measure pressure experienced between the actuators  692  and the wrist  686  and provide a respective pressure signal to a processor (not shown). Accordingly, the skin interface  696  may comprise a plurality of regions along the wrist  686 . While illustrated as a cross-section, it should be understood that skin interface  696  may comprise an array of regions that correspond to an array of actuators  692 . 
         [0205]    As illustrated, the skin interface  696  of the device  682  is generally disposed over the radial artery  688 . While the radial artery  688  has 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 artery  688  in a desired manner. In some embodiments, given that not all sensors  694  of the device  682  are 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 sensor  694  and a preferred region for applanation of the radial artery  688 . This may be carried out by analyzing and comparing the signals from the plurality of sensors  694 . For example, the sensors  694  disposed further from the radial artery  688  may provide weaker pressure signals that are not as meaningful for determining a blood pressure of a user. 
         [0206]    In the illustrated embodiment with a plurality of sensors  694 , the actuators  692  may be selectively and/or sequentially activated to urge different regions of the skin interface  696  against the wrist  686  in order to identify a preferred region for applanation of the radial artery  688 . The preferred region for applanation of the radial artery  688  may be identified based on pressure signals received from the one or more sensors  694  of the device  682 . For example, the skin interface region disposed between sensor-actuator assembly  698  may be urged against the wrist  686  and a signal may be received from the corresponding sensor  694  of sensor-actuator assembly  698 . Additionally, the skin interface region disposed between the sensor-actuator assembly  700  may be urged against the wrist  686  and a signal may be received from the corresponding sensor  694  of the sensor-actuator assembly  700 . The signals from the sensor of assembly  698  and the sensor of assembly  700  may then be compared to determine which signal is stronger and/or preferred. Given that the sensor-actuator assembly  700  is positioned closer to radial artery  688  and that the surface face of the sensor of assembly  700  is more perpendicular to pressure pulses from the radial artery  688 , the signal from the sensor of assembly  700  may be stronger and preferred in comparison to the signal of the sensor of assembly  698  as it is further from the radial artery  688  and oriented at an angle relative to pressure pulses from the artery  688  and may suffer from increased signal loss. 
         [0207]    The regions of the skin interface  696  may be selectively urged such that subsets of the regions of the skin interface  696  are urged against the wrist  686  at a time. The subsets of regions may be urged by multiple actuators  692  where a subset of the actuators  692  are 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 actuator  692  for identifying a preferred region and sensor  694 . 
         [0208]      FIG. 42  illustrates the selective actuation of a single region of a skin interface  710  against a wrist of a user according to embodiments of the present invention. Device  701  may include pressure sensors  702  that may be coupled with one of a plurality of actuators  704 . The actuators  704  may be supported adjacent the wrist by a strap  706 . The sensors  702  may couple with the skin  708  of the user via skin interface  710 . As illustrated in  FIG. 42 , in some embodiments, a single region of the skin interface  710  disposed between an actuator  704  and the wrist may be urged against the wrist for applanation of the artery  712  using a single actuator  704 . While applanating the artery  712  with the single actuator  704 , the remaining actuators  704  may not be actively urging respective regions of the skin interface  710  against the wrist. This manner of actuation of regions of the skin interface  710  against the wrist may be performed selectively and sequentially in order to identify a preferred region for applanation of the artery  712  and a preferred sensor signal from one of the sensors  702 . 
         [0209]      FIG. 43  illustrates device  701  selectively actuating more than one region of a skin interface  710  against a wrist of the user according to embodiments of the present invention. As illustrated in  FIG. 43 , a subset of regions (e.g., the right half the regions) of the skin interface  710  positioned between actuators  704  and the wrist are urged against a wrist of a user by activating two of the actuators  704  while the other two actuators  704  may not be actively urging respective regions of the skin interface  710  against the wrist. In some embodiments, pressure signals may only be processed from the advanced pressure sensors  702 . In some embodiments, pressure signals may only be received from the advanced pressure sensors  702 . 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 sensor  702  between the two advanced pressure sensors  702  and 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. 
         [0210]    While  FIG. 41 - FIG. 43  illustrate devices with a plurality of individual sensors  702 , other embodiments may utilize a sensor system comprising a pressure film sensor. For example,  FIG. 44  illustrates a device  800  that includes a pressure film sensor  802  that may be coupled with a plurality of actuators  804 . The actuators  804  may be supported adjacent the wrist by a strap  806 . The sensor  802  may couple with the skin  808  of the user via skin interface  810 . As illustrated in  FIG. 44 , in some embodiments, a single region of pressure film sensor  802  and a single region of the skin interface  810  may be urged against the wrist for applanation of the artery  812  using a single actuator  804 . While applanating the artery  812  with the single actuator  804 , the remaining actuators  804  may not be actively urging respective regions of the pressure film sensor  802  and the skin interface  810  against the wrist. This selective actuation of regions of the pressure film sensor  802  against the wrist may be performed selectively and sequentially in order to identify a preferred region of the pressure film sensor  802  and skin interface  810  for applanation of the artery  812 . 
         [0211]      FIG. 45  illustrates device  800  selectively actuating a subset of regions of a skin interface  810  and pressure film sensor  802  against a wrist of the user according to embodiments of the present invention. As illustrated in  FIG. 45 , a subset of regions (e.g., the right half the regions) of the skin interface  810  are urged against a wrist of a user by activating two of the actuators  804  on the right while the other two actuators  804  on the left may not be actively urging the respective regions of the pressure film sensor  802  against the wrist. Regions of the pressure film sensor  802  may be selectively and/or sequentially urged against the wrist to identify a preferred region of the skin interface  810  for applanation of the target artery  812  and a preferred region of the pressure film sensor  802  for receiving pressure signals. 
         [0212]      FIG. 46A-46C  show 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&#39;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 in  FIG. 46A . The AC pressure waveform versus time for the same sensor element is illustrated in  FIG. 46B .  FIG. 46C  shows 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. 
         [0213]      FIG. 47  illustrates 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 step  910 , 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 step  912 , 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 step  914 . 
         [0214]    At step  916 , 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 step  918 , 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 step  920 . 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 step  926 . 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 step  922 . 
         [0215]    The blood pressure signals may be filtered based on contextual information associated with the user per step  924 . 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 step  926 . It will be appreciated that in some situations, PTT measurements from step  914  may be directly filtered per step  924  and/or transmitted per step  926  directly 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. 
         [0216]      FIG. 48  illustrates a schematic example of an overall system including a first wrist-worn band  928 , a second wrist-worn electronic device (e.g., watch  930 ), and a third non-wrist device (e.g., a mobile device  932 ) according to embodiments of the present invention. The first wrist-worn band  928  may 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 band  928  is releasably coupleable to a second wrist-worn watch  930  as described in greater detail below. At least one PTT or pressure sensor  934  may be coupled to the elongate band  928 , 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 sensor  944  may be coupled to the elongate band  928  so as to account for any hydrostatic pressure effects associated with the measured cardiovascular user signals. One or more processors  936  may be coupled to the elongate band  928  and the at least one PTT or pressure sensor  934  for determining relative or absolute blood pressure signals based on the user signals. The one or more processors  936  can be implemented in any suitable form, including one or more field-programmable gate arrays (FPGA). The elongate band  928  may further include memory  938 , such as read only memory (ROM) and/or random access memory (RAM). A power source  940  may also be coupled to the elongate band  928  and the processor  936  or the at least one PTT or pressure sensor  934  for providing power to the wrist-worn band  928 . A telemetry/wireless interface  942  (e.g., Bluetooth or WiFi) may also be coupled to the elongate band  928  and the processor  936 . 
         [0217]    The second wrist-worn watch  930  may comprise one or more heart rate monitor sensor(s)  946 , a second processor  948 , a second power source  950 , a second memory  952 , a second telemetry interface  954 , and/or a user display  956  that are enclosed within a distinct and separate housing from the first wrist-worn blood pressure monitoring band  928 . The first wrist-worn band  928  may easily communicate (e.g., transmit blood pressure values, receive updated instructions, such as new calibration equations, etc.) with the second wrist-worn watch  930  via WiFi or Bluetooth. Still further, the telemetry interface  942  of the elongate band  928  may be configured to communicate not only with the second wrist-worn watch  930 , but also with the third non-wrist device (mobile device  932 ). For example, the telemetry interface  942  of the elongate band  928  may be configured to transmit the relative or absolute blood pressure signals to a health application software  958  on the mobile device  932 . The mobile device  932  may in turn display the relative blood pressure signals  960  and/or the absolute blood pressure signals  962  in a graphical format dictated by the health application software  958  for a time period of a day, week, month, or year. The blood pressure graphs  960 ,  962  may then be viewable by the user or a health care professional for use in diagnostic or therapeutic decision making. Still further, the mobile device  932  may be configured to receive the blood pressure signals from the wrist-worn band  928  and/or wrist-worn watch  930  and in turn re-transmit this data to a cloud database  964  for 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 database  966 . 
         [0218]    Referring now to  FIGS. 49A-49C , providing bands  928  that are releasably coupleable to the watch  930  provides for user customization of the watch  930  based on the desired sensor monitoring (e.g., absolute, relative, passive, active, etc.). For example, a first applanation tonometry band  968  as illustrated in  FIG. 49A  may comprise a plurality of pressure sensors  970  and actuators  972  for measuring absolute blood pressure values. The pressure sensors  970  may comprise pressure transducers as illustrated or still further a piezoelectric film or piezoresistive film for sensing. The pressure sensors  970  are 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 actuators  972  urge each of the pressure sensors  970  against the wrist of the user by applying a constant or variable pressure thereto. 
         [0219]      FIGS. 49B and 49C  illustrate bands  974 ,  982  for 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. 
         [0220]    With reference to  FIG. 49B , a second band  974  may 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 electrodes  976  non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes  978  contacted 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 sensor  980  may 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 to  FIG. 49C , a third band  982  may comprise a BCG/PPG sensor arrangement for passive monitoring of relative blood pressure values. The BCG sensor  984  may 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 sensor  996  may be coupled to the elongate band  982  so as to account for hydrostatic pressure effects. The user may selectively choose between the first  968 , second  974 , or third bands  982  for the desired sensor monitoring and may further interchange the bands at any time period as desired via a releasable coupling feature  994 . The at least one releasable connection or coupling feature  994  of the elongate bands  968 ,  974 , or  982  may help secure the selected band  986  to the heart rate monitor watch device  930 . 
         [0221]    As shown in  FIG. 50 , a fourth selected band  986  is releasably coupleable to the watch device  930  and 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 electrodes  988  non-invasively engaging glabrous skin on an anterior surface of the wrist of the user and a second pair of dry electrodes  990  contacted 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 sensor  992  comprises an accelerometer and the accelerometer  992 , wrist-worn band  986  and/or hand of the wrist-worn device non-invasively engage the sternum. 
         [0222]    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&#39;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. 
         [0223]    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. 
         [0224]    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.