Three wavelength pulse oximetry

An apparatus is disclosed for determining validity of a measured in-blood percentage of oxygenated hemoglobin. The apparatus has multiple pulse oximetry channels having at least three light sources of at least three distinct wavelengths, which are detected and converted to digital signals. The light sources are selectively activated, and two or more estimated in-blood percentages of oxygenated hemoglobin are calculated. It is determined whether a signal quality associated with the calculated in-blood percentages exceeds a predetermined accuracy threshold, and an associated validity indication is provided.

BACKGROUND

Pulse oximetry refers to processes for measuring and monitoring the percentage of oxygenated hemoglobin in blood. Arterial oxygen saturation (SaO2) is a medical parameter that provides a highly accurate measurement of oxygenated hemoglobin however, it requires access to arterial blood, the obtaining of which is typically painful and invasive. Measuring peripheral oxygen saturation (SpO2) through pulse oximetry, by contrast, is painless and non-invasive. Transmissive peripheral pulse oximetry involves transmitting two or more wavelengths of light through a peripheral part of the body, such as a foot, earlobe, or fingertip and measuring how much of the light is absorbed by pulsing arterial blood in peripheral capillaries. Typically, light sources of red and infrared or near-infrared wavelengths are selected because they are typically not absorbed by other non-vascular tissues and water that make up peripheral parts of the body. Reflective peripheral pulse oximetry involves reflecting two or more wavelengths of light through a peripheral part of the body and measuring a ratio of oxygenated hemoglobin absorption of the reflected light. Reflective peripheral pulse oximetry is convenient because reflective techniques can be employed at a surface of part of the body, such as the feet, forehead, chest, or wrist, in connection with a wearable device such as a smart watch. Unfortunately, reflective peripheral pulse oximetry tends to provide unstable readings resulting in difficulty in taking accurate, predictable, and repeatable readings. Accordingly, there is a need for more accurate and reliable reflective peripheral pulse oximetry technology and for systems to determine an associated level of validity of SpO2 measurements.

SUMMARY

Embodiments of the disclosure address the above-identified need by providing apparatuses for determining validity of a measured in-blood percentage of oxygenated hemoglobin. In particular, a first embodiment of the disclosure is broadly directed to an apparatus for determining validity of a measured in-blood percentage of oxygenated hemoglobin, the apparatus comprising: a first pulse oximetry channel comprising: a first light source producing light having a first wavelength and a first intensity, a second light source producing light having a second wavelength and a second intensity, a third light source producing light having a third wavelength and a third intensity, a first light detector configured to selectively detect light from the first pulse oximetry channel light sources, an analog to digital converter coupled with the first light detector, the analog to digital converter outputting a digital value corresponding to a measured intensity of light detected by the first light detector, a processor coupled with the display, the first pulse oximetry channel and the analog to digital converter, the processor programmed to execute steps comprising: selectively activating the first light source with a first activation intensity, receiving the first measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the first light source, selectively activating the second light source with a second activation intensity, receiving the second measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the second light source, calculating a first estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the first and second digital values, selectively activating the third light source with a third activation intensity, receiving the third measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the third light source, calculating a second estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the second and third digital values, based on a comparison of the first and second estimated in-blood percentages of oxygenated hemoglobin, determining that a signal quality of the first estimated in-blood percentage of oxygenated hemoglobin exceeds a predetermined accuracy threshold, and based on the determination, providing a validity indication that the first estimated in-blood percentage is sufficiently accurate.

A second embodiment of the disclosure is broadly directed to an apparatus for determining validity of a measured in-blood percentage of oxygenated hemoglobin, the apparatus comprising: a first pulse oximetry channel comprising: a first light emitting diode producing light having a first wavelength and a first current, a second light emitting diode producing light having a second wavelength and a second current, a third light emitting diode producing light having a third wavelength and a third current, a first photodiode configured to selectively detect light from the first pulse oximetry channel light emitting diodes, an analog to digital converter coupled with the first light detector, the analog to digital converter outputting a digital value corresponding to a voltage derived from a measured intensity of light detected by the first light detector, a processor coupled with the display, the first pulse oximetry channel and the analog to digital converter, the processor programmed to execute steps comprising: selectively activating the first light emitting diode with a first activation current, receiving the first measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the first light emitting diode, selectively activating the second light emitting diode with a second activation current, receiving the second measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the second light emitting diode, calculating a first estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the first and second digital values, selectively activating the third light emitting diode with a third activation current, receiving the third measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the third light emitting diode, calculating a second estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the second and third digital values, based on a comparison of the first and second estimated in-blood percentages of oxygenated hemoglobin, determining that a signal quality of the first estimated in-blood percentage of oxygenated hemoglobin exceeds a predetermined validity threshold, and based on the determination, providing an indication that the first estimated in-blood percentage is valid.

A third embodiment of the disclosure is broadly directed to an apparatus for determining validity of a measured in-blood percentage of oxygenated hemoglobin, the apparatus comprising: a first pulse oximetry channel comprising: a first light emitting diode producing light having a first wavelength and a first current, a second light emitting diode producing light having a second wavelength and a second current, a third light emitting diode producing light having a third wavelength and a third current, a first photodiode configured to selectively detect light from the first pulse oximetry channel light emitting diodes, a second pulse oximetry channel comprising: a fourth light source producing light having a fourth wavelength and a fourth intensity, a fifth light source producing light having a fifth wavelength and a fifth intensity; and a sixth light source producing light having a sixth wavelength and a sixth intensity, a second photodiode configured to selectively detect light from the second pulse oximetry channel light emitting diodes, an analog to digital converter coupled with the first light detector, the analog to digital converter outputting a digital value corresponding to a voltage derived from a measured intensity of light detected by the first light detector, a processor coupled with the display, the first pulse oximetry channel and the analog to digital converter, the processor programmed to execute steps comprising: selectively activating the first light emitting diode with a first activation current, receiving the first measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the first light emitting diode, selectively activating the second light emitting diode with a second activation current, receiving the second measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the second light emitting diode, calculating a first estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the first and second digital values, selectively activating the third light emitting diode with a third activation current, receiving the third measured digital value corresponding to the intensity of light detected by the first light detector based on activation of the third light emitting diode, calculating a second estimated in-blood percentage of oxygenated hemoglobin based on a comparison between the second and third digital values, based on a comparison of the first and second estimated in-blood percentages of oxygenated hemoglobin, determining that a signal quality of the first estimated in-blood percentage of oxygenated hemoglobin exceeds a predetermined validity threshold, and based on the determination, providing an indication that the first estimated in-blood percentage is valid.

DETAILED DESCRIPTION

The disclosure describes various embodiments of a system for determining validity of a measured in-blood percentage of oxygenated hemoglobin using light sources having three or more wavelengths. The subject matter of embodiments of the disclosure is described in detail below to meet statutory requirements; however, the description itself is not intended to limit the scope of claims. Rather, the claimed subject matter might be embodied in other ways to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Minor variations from the description below will be obvious to one skilled in the art and are intended to be captured within the scope of the claims. Terms should not be interpreted as implying any particular ordering of various steps described unless the order of individual steps is explicitly described.

The following detailed description of embodiments of the disclosure references the accompanying drawings that illustrate specific embodiments in which the disclosure can be practiced. The embodiments are intended to describe aspects of the disclosure in sufficient detail to enable those skilled in the art to practice the disclosure. Other embodiments can be utilized, and changes can be made without departing from the scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of embodiments of the disclosure is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

Turning first toFIG.1, an exemplary a view of one embodiment of a wearable device for performing three-wavelength pulse oximetry is depicted. The device100may be configured in a variety of ways to determine and provide pulse oximetry information, as well as user-performance information and navigation functionality to the user of the device100. The device100includes a housing102or a case configured to substantially enclose various components of the device100. The housing102may be formed from a lightweight and impact-resistant material such as plastic, nylon, or combinations thereof, for example. The housing102may be formed from a conductive material, a non-conductive material, and combinations thereof. The housing102may include one or more gaskets, e.g., a seal, to make it substantially waterproof and/or water resistant. The housing102may include a location for a battery and/or another power source for powering one or more components of the device100. The housing102may be a singular piece or may include multiple sections.

The device100includes a display device104with a user interface. The display device104may include a liquid crystal display (LCD), a thin film transistor (TFT), a light-emitting diode (LED), a light-emitting polymer (LEP), and/or a polymer light-emitting diode (PLED). The display device104may be capable of presenting text, graphical, and/or pictorial information. The display device104may be backlit such that it may be viewed in the dark or other low-light environments. One example embodiment of the display device104is a 100-pixel by 64-pixel film compensated super-twisted nematic display (FSTN) including a bright white light-emitting diode (LED) backlight. The display device104may include a transparent lens that covers and/or protects components of the device100. The display device104may be provided with a touch screen to receive input (e.g., data, commands, etc.) from a user. For example, a user may operate the device100by touching the touch screen and/or by performing gestures on the screen. In some embodiments, the touch screen may be a capacitive touch screen, a resistive touch screen, an infrared touch screen, combinations thereof, and the like. The device100may further include one or more input/output (I/O) devices (e.g., a keypad, buttons, a wireless input device, a thumbwheel input device, etc.). The I/O devices may include one or more audio I/O devices, such as a microphone, speakers, and the like. Additionally, user input may be provided from movement of the housing102, for example, an inertial sensor(s), e.g., accelerometer, may be used to identify vertical, horizontal, angular movement and/or tapping of the housing102or the lens.

In accordance with one or more embodiments of the present disclosure, the user interface may include one or more control buttons106. As illustrated inFIG.1, four control buttons106are associated with, e.g., adjacent, the housing102. WhileFIG.1illustrates four control buttons106associated with the housing102, it is understood that the device100may include a greater or lesser number of control buttons106. In one embodiment, each control button106is configured to generally control a function of the device100. Functions of the device100may be associated with a location determining component and/or a performance monitoring component as further described below in connection withFIG.2B. Functions of the device100may include, but are not limited to, displaying a current geographic location of the device100, mapping a location on the display104, locating a desired location and displaying the desired location on the display104, and presenting information based on a physiological characteristic (e.g., heart-rate, heart-rate variability, blood pressure, or SpO2 percentage, for example) or a physiological response (e.g., stress level, body energy level, etc.) of the individual.

FIG.2Adepicts a bottom view of one embodiment of a wearable device for performing three-wavelength pulse oximetry. The device100also includes a pulse oximetry signal assembly, including one or more emitters (e.g., LEDs112) of visible and/or non-visible light and one or more receivers (e.g., photodiodes114) of visible and/or non-visible light that generate a light intensity signal based on the received reflection of light.

The device100includes a means for attaching108, e.g., a strap, that enables the device100to be worn by a user. In particular, when the device is worn by the user, one or more LEDs and one or more photodiodes may be securely placed against the skin of a user. The strap108is coupled to and/or integrated with the housing102and may be removably secured to the housing102via attachment of securing elements to corresponding connecting elements. Some examples of securing elements and/or connecting elements include, but are not limited to, hooks, latches, clamps, snaps, and the like. The strap108may be made of a lightweight and resilient thermoplastic elastomer and/or a fabric, for example, such that the strap108may encircle a portion of a user without discomfort while securing the device100to the user. The strap108may be configured to attach to various portions of a user, such as a user's leg, waist, wrist, forearm, upper arm, and/or torso.

FIG.2Bdepicts a system diagram showing the components of a device100for carrying out embodiments of the disclosure. The device100includes a user interface module116, a location determining component118(e.g., a global positioning system (GPS) receiver, assisted-GPS, etc.), a communication module120, an inertial sensor122(e.g., accelerometer, gyroscope, etc.), and a controller124. The device100may be a general-use wearable and mobile computing device (e.g., a watch, activity band, etc.), a cellular phone, a smartphone, a tablet computer, or a mobile personal computer, capable of monitoring a physiological characteristic and/or response of an individual as described herein. The device100may be a thin-client device or terminal that sends processing functions to a server device136via a network138. Communication via the network138may include any combination of wired and wireless technology. For example, the network138may include a USB cable between the device100and a computing device140(e.g., smartphone, tablet, laptop, etc.) to facilitate the bi-directional transfer of data between the device100and the computing device140.

The controller124may include a memory device126, a microprocessor (MP)128, a random-access memory (RAM)130, and an input/output (I/O) circuitry132, all of which may be communicatively interconnected via an address/data bus134. Although the I/O circuitry132is depicted inFIG.2Bas a single block, the I/O circuitry132may include a number of different types of I/O circuits. The memory device126may include an operating system142, a data storage device144, a plurality of software applications146, and/or a plurality of software routines150. The operating system142of memory device126may include any of a plurality of mobile platforms, such as the iOS®, Android™, Palm® webOS, Windows® Mobile/Phone, BlackBerry® OS, or Symbian® OS mobile technology platforms, developed by Apple Inc., Google Inc., Palm Inc. (now Hewlett-Packard Company), Microsoft Corporation, Research in Motion (RIM), and Nokia, respectively. The data storage device144of memory device126may include application data for the plurality of applications146, routine data for the plurality of routines150, and other data necessary to interact with the server136through the network138. In particular, the data storage device144may include cardiac component data associated with one or more individuals. The cardiac component data may include one or more compilations of recorded physiological characteristics of the user, including, but not limited to, a hemoglobin saturation values, a heart rate (HR), a heart-rate variability (HRV), a blood pressure, motion data, a determined distance traveled, a speed of movement, calculated calories burned, body temperature, and the like. In some embodiments, the controller124may also include or otherwise be operatively coupled for communication with other data storage mechanisms (e.g., one or more hard disk drives, optical storage drives, solid state storage devices, etc.) that may reside within the device100and/or operatively coupled to the network138and/or server device136.

The device100also includes a pulse oximetry signal assembly including one or more light sources, such as LEDs112. The pulse oximetry signal assembly also includes one or more light detectors such as photodiodes114. In some embodiments, the LEDs112output visible and/or non-visible light and the one or more photodiodes114receive transmissions or reflections of the visible and/or non-visible light and convert the received light into electrical current, which, in some embodiments, is converted into a digital value by an analog to digital converter. Each LED112generates light based on an intensity determined by the processor. For example, LEDs112may include any combination of green light-emitting diodes (LEDs), red LEDs, and/or infrared or near-infrared LEDs that may be configured by the processor to emit light into the user's skin. In some embodiments, the red LEDs operate at a wavelength between approximately 610 and 700 nm. In some embodiments, a first LED produces light at approximately 630 nm, a second LED operates at approximately 940 nm, and a third LED operates at approximately 660 nm. The device100also includes display device104as described in connection withFIG.1above.

The device100also includes one or more photodiodes114capable of receiving transmissions or reflections of visible-light and/or infrared (IR) light output by the LEDs112into the user's skin and generating a SpO2 signal based on the intensity of the reflected light received by each photodiode114. The light intensity signals generated by the one or more photodiodes114may be communicated to the processor. In embodiments, the processor includes an integrated a photometric front end for signal processing and digitization. In other embodiments, the processor is coupled with a photometric front end. The photometric front end may include filters for the light intensity signals and analog-to-digital converters to digitize the light intensity signals into SpO2 signals including a cardiac signal component associated with the user's heartbeat.

Typically, when the device100is worn against the user's body (e.g., wrist, fingertip, ear, etc.), the one or more LEDs112are positioned against the user's skin to emit light into the user's skin and the one or more photodiodes344are positioned near the LEDs112to receive light emitted by the one or more emitters after transmission through or reflection from the user's skin. The processor128of device100may receive a SpO2 signal based on a light intensity signal output by one or more photodiodes114based on an intensity of light after transmission of the light through or reflection from the user's skin that has been received by the photodiodes114.

In both the transmitted and reflected uses, the intensity of measured light may be modulated by the cardiac cycle due to variation in tissue blood perfusion during the cardiac cycle. In activity environments, the intensity of measured light may also be strongly influenced by many other factors, including, but not limited to, static and/or variable ambient light intensity, body motion at measurement location, static and/or variable sensor pressure on the skin, motion of the sensor relative to the body at the measurement location, breathing, and/or light barriers (e.g., hair, opaque skin layers, sweat, etc.). Relative to these sources, the cardiac cycle component of the SpO2 signal can be very weak, for example, by one or more orders of magnitude.

The location determining component118generally determines a current geolocation of the device100and may process a first electronic signal, such as radio frequency (RF) electronic signals, from a global navigation satellite system (GNSS) such as the global positioning system (GPS) primarily used in the United States, the GLONASS system primarily used in the Soviet Union, or the Galileo system primarily used in Europe. The location determining component118may include satellite navigation receivers, processors, controllers, other computing devices, or combinations thereof, and memory. The location determining component118may be in electronic communication with an antenna (not shown) that may wirelessly receive an electronic signal from one or more of the previously-mentioned satellite systems and provide the first electronic signal to location determining component118. The location determining component118may process the electronic signal, which includes data and information, from which geographic information such as the current geolocation is determined. The current geolocation may include geographic coordinates, such as the latitude and longitude, of the current geographic location of the device100. The location determining component118may communicate the current geolocation to the processor128. Generally, the location determining component118is capable of determining continuous position, velocity, time, and direction (heading) information.

In some embodiments, the inertial sensor122may incorporate one or more accelerometers positioned to determine the acceleration and direction of movement of the device100. The accelerometer may determine magnitudes of acceleration in an X-axis, a Y-axis, and a Z-axis to measure the acceleration and direction of movement of the device100in each respective direction (or plane). It will be appreciated by those of ordinary skill in the art that a three-dimensional vector describing a movement of the device100through three-dimensional space can be established by combining the outputs of the X-axis, Y-axis, and Z-axis accelerometers using known methods. Single and multiple axis models of the inertial sensor308are capable of detecting magnitude and direction of acceleration as a vector quantity and may be used to sense orientation and/or coordinate acceleration of the user.

The SpO2 signal assembly (including LEDs112and photodiodes114), location determining component118, and the inertial sensor122may be referred to collectively as the “sensors” of the device100. It is also to be appreciated that additional location determining components118and/or inertial sensor(s)122may be operatively coupled to the device100. The device100may also include or be coupled to a microphone incorporated with the user interface module116and used to receive voice inputs from the user while the device100monitors a physiological characteristic and/or response of the user determines physiological information based on the cardiac signal.

Communication module120may enable device100to communicate with the computing device140and/or the server device136via any suitable wired or wireless communication protocol independently or using I/O circuitry132. The wired or wireless network138may include a wireless telephony network (e.g., GSM, CDMA, LTE, etc.), one or more standard of the Institute of Electrical and Electronics Engineers (IEEE), such as 802.11 or 802.16 (Wi-Max) standards, Wi-Fi standards promulgated by the Wi-Fi Alliance, Bluetooth standards promulgated by the Bluetooth Special Interest Group, a near field communication standard (e.g., ISO/IEC 18092, standards provided by the NFC Forum, etc.), and so on. Wired communications are also contemplated such as through universal serial bus (USB), Ethernet, serial connections, and so forth.

The device100may be configured to communicate via one or more networks138with a cellular provider and an Internet provider to receive mobile phone service and various content, respectively. Content may represent a variety of different content, examples of which include, but are not limited to: map data, which may include route information; web pages; services; music; photographs; video; email service; instant messaging; device drivers; real-time and/or historical weather data; instruction updates; and so forth.

The user interface116of the device100may include a “soft” keyboard that is presented on the display device104of the device100, an external hardware keyboard communicating via a wired or a wireless connection (e.g., a Bluetooth keyboard), and/or an external mouse, or any other suitable user-input device or component. As described earlier, the user interface116may also include or communicate with a microphone capable of receiving voice input from a vehicle operator as well as a display device104having a touch input.

With reference to the controller124, it should be understood that controller124may include multiple microprocessors128, multiple RAMs130and multiple memory devices126. The controller124may implement the RAM130and the memory devices126as semiconductor memories, magnetically readable memories, and/or optically readable memories, for example. The one or more processors128may be adapted and configured to execute any of the plurality of software applications146and/or any of the plurality of software routines150residing in the memory device126, in addition to other software applications. One of the plurality of applications146may be a client application152that may be implemented as a series of machine-readable instructions for performing the various functions associated with implementing the performance monitoring system as well as receiving information at, displaying information on, and transmitting information from the device100. The client application152may function to implement a system wherein the front-end components communicate and cooperate with back-end components as described above. The client application152may include machine-readable instructions for implementing the user interface116to allow a user to input commands to, and receive information from, the device100. One of the plurality of applications146may be a native web browser148, such as Apple's Safari®, Google Android™ mobile web browser, Microsoft Internet Explorer® for Mobile, Opera Mobile™, that may be implemented as a series of machine-readable instructions for receiving, interpreting, and displaying web page information from the server device136or other back-end components while also receiving inputs from the device100. Another application of the plurality of applications146may include an embedded web browser148that may be implemented as a series of machine-readable instructions for receiving, interpreting, and displaying web page information from the server device136or other back-end components within the client application152.

The client applications146or routines154may include an accelerometer routine154that determines the acceleration and direction of movements of the device100, which correlate to the acceleration, direction, and movement of the user. The accelerometer routine154may receive and process data from the inertial sensor122to determine one or more vectors describing the motion of the user for use with the client application152. In some embodiments where the inertial sensor122includes an accelerometer having X-axis, Y-axis, and Z-axis accelerometers, the accelerometer routine154may combine the data from each accelerometer to establish the vectors describing the motion of the user through three-dimensional space. In some embodiments, the accelerometer routine154may use data pertaining to less than three axes.

The client applications146or routines150may further include a velocity routine156that coordinates with the location determining component118to determine or obtain velocity and direction information for use with one or more of the plurality of applications, such as the client application152, or for use with other routines.

The user may also launch or initiate any other suitable user interface application (e.g., the native web browser148, or any other one of the plurality of software applications146) to access the server device136to implement the monitoring process. Additionally, the user may launch the client application152from the device100to access the server device136to implement the monitoring process.

After the above-described data has been gathered or determined by the sensors of the device100and stored in memory device126, the device100may transmit information associated with measured blood oxygen saturation percentage (pulse oximetry signal), peak-to-peak interval (PPI), heart rate (HR), heart-rate variability (HRV), motion data (acceleration information), location information, stress intensity level, and body energy level of the user to computing device140and server device136for storage and additional processing. For example, in embodiments where the device100is a thin-client device, the computing device140or the server136may perform one or more processing functions remotely that may otherwise be performed by the device100. In such embodiments, the computing device140or server136may include a number of software applications capable of receiving user information gathered by the sensors to be used in determining a physiological response (e.g., a stress level, an energy level, etc.) of the user. For example, the device100may gather information from its sensors as described herein, but instead of using the information locally, the device100may send the information to the computing device140or the server136for remote processing. The computing device140or the server136may perform the analysis of the gathered user information to determine a stress level or a body energy level of the user as described herein. The server136may also transmit information associated with the physiological response, such as a stress level, an energy level, of the user. For example, the information may be sent to computing device140or the server device136and include a request for analysis, where the information determined by the computing device140or the server device136is returned to device100.

The disclosed techniques and described embodiments may be implemented in a wearable monitoring device having a housing implemented as a watch, a mobile phone, a hand-held portable computer, a tablet computer, a personal digital assistant, a multimedia device, a media player, a game device, or any combination thereof. The wearable monitoring device may include a processor configured for performing other activities.

FIGS.3A-3Cdepict various configurations for placement of photodiodes for carrying out embodiments of the disclosure. In some embodiments, the wearable monitoring device includes a plurality of photodiodes114and a plurality of LEDs112.FIGS.3A-3Cdepict different configurations of two photodiodes114for positioning on a portion of a user's extremity or limb, such as the user's neck, lower arm, wrist, ankle, or torso. In accordance with the present disclosure, two or more photodiodes114are positioned on the user's skin tissue along an arterial path that is substantially parallel with a longitudinal axis of the user's extremity. The two photodiodes114are horizontally aligned and separated by a lateral distance that is substantially parallel with the longitudinal axis of the extremity, e.g., forearm, when the photodiodes114are attached to a user. Each photodiode114independently samples the adjacent skin tissue to detect a pulse wave as it travels from the heart to the end of the extremity. Although the two photodiodes114are horizontally positioned, the two photodiodes may be vertically offset with respect to each other, which may not adversely affect detection of the pulse wave as it travels along the limb and subsequent calculation of the physiological characteristic. That is, one photodiode114may be positioned closer to the ulna bone and the other photodiode114may be positioned closer to the radius bone (seeFIGS.3B and3C), and vice-versa.

The wearable monitoring device includes at least one LED112positioned sufficiently near the two photodiodes114to enable the photodiodes114to operatively receive reflected light that was emitted from the at least one LED112and reflected from the user's skin tissue or transmitted through the user's soft tissue. In some embodiments, a plurality of LEDs112may be positioned around each and/or both photodiodes114such that the photodiodes114receive reflected or transmitted light emitted from the plurality of LEDs112.

FIG.4depicts an exemplary placement of photodiodes and LEDs for carrying out embodiments of the disclosure. In some embodiments, the wearable monitoring device may include two photodiodes400,402aligned horizontally and a plurality of LEDs406,408,410vertically positioned between the two photodiodes400,402. The plurality of LEDs406,408,410may extend between the user's ulna and radius bones such that the light sensed by each photodiode400,402is output by the LEDs406,408,410. In another embodiment, the wearable monitoring device may include two or more photodiodes400,402and a combination of one or more shared LEDs406,408,410positioned between the two photodiodes400,402and producing light sensed by the two or more photodiodes400,402and/or one or more unshared LEDs406,408,410that produce light that may be concentrated at the side of each photodiode400farther from the other photodiode402.

When the two or more photodiodes400,406are positioned close to each other, a higher sampling rate may be beneficial for each photodiode to generate the pulse oximetry signals to enable the processor to differentiate the peak of the pulse wave at the first photodiode400from the peak of the pulse wave at the second photodiode406, which will occur shortly after the pulse wave passes by the first photodiode400. At a sufficiently close distance, the second photodiode406may begin to detect (sense) the rise of the pulse wave before it has completely passed the first photodiode.

FIG.5depicts an exemplary process for determining validity of a measured in-blood percentage of oxygenated hemoglobin. The process begins at step502by selectively activating first, second, and third light sources. In some embodiments the first, second, and third light sources are LEDs. In one embodiment, the first light source is a red LED producing light having a wavelength of approximately 630 nm, the second light source is an infra-red LED producing light having a wavelength of approximately 940 nm, and the third light source is a red LED producing light having a wavelength of approximately 660 nm. In some embodiments, the range of wavelengths for the first LED varies from 620 nm to 640 nm. In other embodiments the range of wavelengths for the first LED varies from 610 nm to 650 nm. In some embodiments, the wavelength for the second LED is greater than 750 nm. In other embodiments the range of wavelengths for the second LED varies from 880 nm to 980 nm. In some embodiments the range of wavelengths for the third LED varies from 650 nm to 700 nm. In other embodiments the range of wavelengths for the first LED varies from 650 nm to 750 nm. In some embodiments, the light sources are selectively activated using a drive circuit that is selectively activated by a processor such as processor128. In some embodiments, the light sources are selectively activated by the processor to produce light having a particular intensity based on an amount of activation current produced by the drive circuit.

In some embodiments, multiple channels of sets of light sources and light detectors are employed, operating independently. In some embodiments, a second channel is provided including fourth, fifth, and sixth light sources. In some embodiments, the second channel light sources are LEDs. In some embodiments, one or more additional light detectors, such as photodiodes are included to detect light from the fourth, fifth and sixth light sources. In various embodiments, the ranges of wavelengths employed are consistent with the wavelengths employed in a single channel. In some embodiments, wavelengths are within the same ranges, having different specific wavelength values between the first and second channels.

Next the process continues on to step504where the processor receives detected light intensity values. In some embodiments, the detected light intensity values are digital values obtained from an analog-to-digital converter that measures a digital value corresponding to current in a photodiode. The current in the photodiode is based on the intensity of light at a particular wavelength that is received at the photodiode. In some embodiments, at least three light sources are employed to generate light having three different wavelengths. In one embodiment, the first light source produces light having a wavelength of approximately 630 nm, the second light source produces light having a wavelength of approximately 940 nm, and the third light sources produces light having a wavelength of approximately 660 nm. In this embodiment, two different SpO2 values can be calculated, one by way of the ratio of the digital value corresponding to the intensity of 630 nm light to 940 nm light.

At step506, first and second in-blood percentages of oxygenated hemoglobin percentages are calculated. For example, a first digital value is calculated by measuring a ratio of intensity of two different red lights to infrared light. In some embodiments, if two values are inconsistent, both values are removed from the samples. In some embodiments, measurements will result in accurate SpO2 calculations only when noise is absent from the subject measurement, i.e. when the wearable device is substantially motionless. In another embodiment, noise is subtracted to generate a more accurate measurement of SpO2. Given a range of noisy values, the noise is subtracted and if the two values are consistent, both measurements are determined to be accurate. In this embodiment, two alternative calculations may be performed. Because valid measurements taken with light sources of differing wavelength should generate the same value for SpO2, if the calculated results are sufficiently close, there is a high certainty that the measured SpO2 is valid. Accordingly, at step508, the first and second percentages are compared. Next, at test510if the percentages are sufficiently close, a validity indication (step512) is provided. If the percentages are not sufficiently close, the process continues back to step502. In some embodiments, a SpO2 measurement is valid if a running root-mean-square deviation of the second percentage of oxygenated hemoglobin to the first percentage is within a sufficiently small deviation. In alternative embodiments, a SpO2 measurement is valid if the first and second percentages of oxygenated hemoglobin are within a sufficiently small absolute deviation of each other in a single measurement. The permitted deviation may be less than 1%, between 1-5%, between 5%-10%, more than 10%, more than 25%, or the like.

FIG.6Adepicts an exemplary graph confirming validity of a measured in-blood percentage of oxygenated hemoglobin. Profile602corresponds to a ratio of ratios of the received 660 nm light as compared to the 940 nm light, and profile604corresponds to a ratio of ratios of the received 630 nm light as compared to the 940 nm light. In this example, ratios of ratios606and608correspond to measured intensities of each of the respective wavelengths and result in a consistent SpO2 calculation610. Accordingly, inFIG.6Aa valid SpO2 calculation was performed.

FIG.6Bdepicts an exemplary graph indicating invalidity of a measured in-blood percentage of oxygenated hemoglobin. In this figure, profile602corresponds to a ratio of ratios of the received 660 nm light as compared to the 940 nm light, and profile604corresponds to a ratio of ratios of the received 630 nm light as compared to the 940 nm light. In this example, ratios of ratios606and608correspond to measured intensities of each of the respective wavelengths and result in inconsistent SpO2 calculations612and614. Accordingly, inFIG.6Ba valid SpO2 calculation was not performed.