Patent Publication Number: US-2015065830-A1

Title: System and method for operating a sensor for determining blood characteristics of a subject

Description:
This invention was made with partial Government support under contract number W81XWH1110833 awarded by U.S. Department of the Army. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     The subject matter disclosed herein generally relates to operating a sensor for determining blood characteristics of a subject. More specifically, the subject matter relates to systems and methods for switching the operation of a pulse oximeter sensor for determining blood characteristics of a subject based on the quality of the subject&#39;s plethysmographic data. 
     Doctors, primary physicians, and the like, often use sensors (e.g., pulse oximeter sensor) to monitor blood characteristics, for example, oxygen saturation level, heart rate, and the like, of their patients. Existing pulse oximeter sensors have numerous problems. For example, the pulse oximeter sensors that continuously operate during a cardiac cycle of a patient consume significant amounts of power to pulse the light emitting diodes of the pulse oximeter sensor. In another example, the pulse oximeter sensors that operate during only the systolic phase of a cardiac cycle face challenges in synchronizing the pulsing of LEDs with the heart rate of the subject. 
     Thus, there is a need for an enhanced system and method for operating a sensor for determining blood characteristics of a subject. 
     BRIEF DESCRIPTION 
     In accordance with one aspect of the present technique, a method includes receiving continuous photoplethysmographic (PPG) data of a subject from a sensor and calculating a continuous blood characteristic (BC) based on the continuous PPG data. The method also includes calculating a first quality metric of the continuous PPG data based on a sequence of the continuous BC. The method further includes determining whether the first quality metric satisfies a stability criterion and sending a first notification to the sensor in response to determining that the first quality metric satisfies the stability criterion. The first notification instructs the sensor to collect compressed PPG data of the subject. 
     In accordance with one aspect of the present systems, a system includes a calculation module configured to receive continuous PPG data of a subject from a sensor, calculate a continuous BC based on the continuous PPG data, and calculate a first quality metric of the continuous PPG data based on a sequence of the continuous BC. The system also includes a determination module configured to determine whether the first quality metric satisfies a stability criterion and send a first notification to the sensor in response to determining that the first quality metric satisfies the stability criterion. The first notification instructs the sensor to collect compressed PPG data of the subject. 
     In accordance with one aspect of the present technique, a computer program product encoding instructions is disclosed. The instructions when executed by a processor, causes the processor to receive continuous PPG data of a subject from a sensor and calculate a continuous BC based on the continuous PPG data. The instructions further cause the processor to calculate a first quality metric of the continuous PPG data based on a sequence of the continuous BC. The instructions further cause the processor to determine whether the first quality metric satisfies a stability criterion and send a first notification to the sensor in response to determining that the first quality metric satisfies the stability criterion. The first notification instructs the sensor to collect compressed PPG data of the subject. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram illustrating a system for determining blood characteristics of a subject according to one embodiment; 
         FIG. 2  is a graphical representation of the operation of a sensor according to one embodiment; 
         FIG. 3  is a graphical representation of the operation of a sensor according to another embodiment; 
         FIG. 4  is a graphical representation of a calibration function according to one embodiment; 
         FIG. 5  is a flow diagram of a method for operating a sensor for determining blood characteristics according to one embodiment; and 
         FIG. 6  is a graphical representation for operating a sensor for determining blood characteristics according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     As used herein, the term “non-transitory computer-readable media” is intended to be representative of any tangible computer-based device implemented in any method or technology for short-term and long-term storage of information, such as computer-readable instructions, data structures, program modules and sub-modules, or other data in any device. Therefore, the methods described herein may be encoded as executable instructions embodied in a tangible, non-transitory, computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. Moreover, as used herein, the term “non-transitory computer-readable media” includes all tangible, computer-readable media, including, without limitation, non-transitory computer storage devices, including, without limitation, volatile and nonvolatile media, and removable and non-removable media such as a firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal. 
     As used herein, the terms “software” and “firmware” are interchangeable, and may include any computer program stored in memory for execution by devices that include, without limitation, mobile devices, clusters, personal computers, workstations, clients, and servers. 
     As used herein, the term “computer” and related terms, e.g., “computing device”, are not limited to integrated circuits referred to in the art as a computer, but broadly refers to at least one microcontroller, microcomputer, programmable logic controller (PLC), application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. 
     Approximating language, as used herein throughout the description and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or inter-changed, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     A system and method for operating a sensor for determining blood characteristics (e.g., oxygen saturation, heart rate, and the like) of a subject (e.g., a patient in a hospital and the like) is described herein.  FIG. 1  illustrates a block diagram of a system  100  configured to determine blood characteristics of a subject according to one embodiment. The system  100  includes a sensor  105  and a switching unit  150  that are communicatively coupled via a network  140 . 
     The network  140  may be a wired or wireless communication type, and may have any number of configurations such as a star configuration, token ring configuration, or other known configurations. Furthermore, the network  140  may include a local area network (LAN), a wide area network (WAN) (e.g., the Internet), and/or any other interconnected data path across which multiple devices may communicate. In one embodiment, the network  140  may be a peer-to-peer network. The network  140  may also be coupled to or include portions of a telecommunication network for transmitting data in a variety of different communication protocols. In another embodiment, the network  140  includes Bluetooth communication networks or a cellular communications network for transmitting and receiving data such as via a short messaging service (SMS), a multimedia messaging service (MMS), a hypertext transfer protocol (HTTP), a direct data connection, WAP, email, and the like. While only one network  140  is shown coupled to the sensor  105  and the switching unit  150 , multiple networks  140  may be coupled to the entities. 
     The sensor  105  may be any type of device for collecting PPG data of a subject. Typically, the PPG data indicates the change in volume within a body part of the subject due to fluctuations in the amount of blood, air, and the like, within the body part. In the illustrated embodiment, the sensor  105  is a pulse oximeter sensor including an optoelectronic unit  110  and a control unit  115  configured to collect photoplethysmographic (PPG) data of the subject. The sensor  105  is further configured to send the collected PPG data of the subject to the switching unit  150  via the network  140 . The sensor  105  is coupled to the network  140  via a signal line  135 . The signal line  135  is provided for illustrative purposes and represents the sensor  105  communicating by wires or wirelessly over the network  140 . 
     The optoelectronic unit  110  includes a plurality of light emitting elements (not shown), for example, light emitting diodes (LED) for emitting light through a body part (e.g., finger, ear lobe, and the like) of a subject. In one embodiment, the optoelectronic unit  110  includes two LEDs for emitting light at wavelengths of 660 nm (red) and 940 nm (infrared). Although, the optoelectronic unit  110  is described herein as including two LEDs emitting red light and infrared light, in other embodiments, the optoelectronic unit  110  may include LEDs emitting light at any wavelength. The optoelectronic unit  110  further includes at least one photo detector (not shown) for receiving the light emitted by the LEDs after passing through a body part of the subject and converting them into electrical signals, i.e., PPG data. 
     The control unit  115  includes a continuous module  120  and a compressed module  130  configured to control the operation of the optoelectronic unit  110  (i.e., switching on and switching off of the plurality of LEDs) to collect the PPG data during one or more cardiac cycles of a subject. A cardiac cycle refers to a sequence of events related to the flow of blood that occurs from the beginning of one heartbeat to the beginning of the next heartbeat of a subject. The cardiac cycle includes a systolic phase and a subsequent diastolic phase. During the systolic phase, the heart ventricles contract and pump blood into the arteries of the subject. During the diastolic phase, the ventricles of the heart relax and get filled with blood. In one embodiment, the control unit  115  may include a memory (not shown) and a processor (not shown) for storing and executing the codes and routines of the continuous module  120  and the compressed module  130 . Although, the control unit  115  is described above according to one embodiment as a part of the sensor  105 , in other embodiments, the control unit  105  may be included in the switching unit  150 . 
     The continuous module  120  includes codes and routines to operate the optoelectronic unit  110  to collect PPG data throughout one or more cardiac cycles, i.e., during the systolic and the diastolic phases. The continuous module  120  periodically switches on and switches off the plurality of LEDs and receives corresponding PPG waveforms (i.e., PPG data) recorded by the photo detector. 
     Referring now to  FIG. 2 , a graphical representation  200  of the operation of the optoelectronic unit is illustrated according to one embodiment. The graph  220  illustrates a PPG waveform  225  (i.e., PPG data) received by the continuous module by operating an LED emitting, for example, red light over two successive cardiac cycles. Each cardiac cycle includes a systolic phase (represented by the portion of the PPG waveform  225  between time instants T o  and T 1 ) and a diastolic phase (represented by the portion of the PPG waveform  225  between time instants T 1  and T 2 ). 
     The graph  240  illustrates the time instants at which the continuous module switches on (T-on) and switches off (T-off) the LED to record the PPG waveform  225 . In the illustrated embodiment, the continuous module switches on the LED during both systolic and diastolic phases of each cardiac cycle for recording the PPG waveform  225 . Although,  FIG. 2  illustrates only one PPG waveform  225  recorded by operating an LED emitting red light, the continuous module receives another PPG waveform by similarly operating another LED emitting, for example, infrared light. For the purpose of clarity and convenience, the two PPG waveforms received by the continuous module are collectively referred to herein as “continuous PPG data”. 
     Referring back to  FIG. 1 , the compressed module  130  includes codes and routines configured to operate the optoelectronic unit  110  to collect PPG data during the systolic phase of a cardiac cycle. 
       FIG. 3  illustrates a graphical representation  300  of the operation of the optoelectronic unit according to another embodiment. The graph  320  illustrates a PPG waveform  325  received by the compressed module by operating an LED emitting, for example, red light over two successive cardiac cycles. The graph  350  illustrates the time instants at which the compressed module switches on (T-on) and switches off (T-off) for recording the PPG waveform  325 . In the illustrated embodiment, the compressed module switches on the LED only during the systolic phase of each cardiac cycle. Although,  FIG. 3  illustrates only one PPG waveform  325  recorded by operating an LED emitting red light, the compressed module receives another PPG waveform by similarly operating another LED emitting, for example, infrared light. For the purpose of clarity and convenience, the two PPG waveforms received by the compressed module are collectively referred to herein as “compressed PPG data”. 
     Referring again to  FIG. 1 , the switching unit  150  may be any device configured to switch the operation of the sensor  105  based on the quality of a subject&#39;s PPG data for determining blood characteristics of the subject. The switching unit  150  receives the subject&#39;s PPG data from the sensor  105  via the network  140 . The switching unit  150  is communicatively coupled to the network  140  via a signal line  145 . The signal line  145  is provided for illustrative purposes and represents the switching unit  150  communicating by wires or wirelessly over the network  140 . In the illustrated embodiment, the switching unit  150  includes a switching application  160 , a processor  185 , and a memory  190 . The switching application  160  includes a communication module  170 , a calculation module  175 , and a determination module  180 . The plurality of modules of the switching application  160 , the processor  185 , and the memory  190  are coupled to a bus (not shown) for communication with each other. 
     The processor  185  may include at least one arithmetic logic unit, microprocessor, general purpose controller or other processor arrays to perform computations, and/or retrieve data stored on the memory  190 . In another embodiment, the processor  185  is a multiple core processor. The processor  185  processes data signals and may include various computing architectures including a complex instruction set computer (CISC) architecture, a reduced instruction set computer (RISC) architecture, or an architecture implementing a combination of instruction sets. The processing capability of the processor  185  in one embodiment may be limited to supporting the retrieval of data and transmission of data. The processing capability of the processor  185  in another embodiment may also perform more complex tasks, including various types of feature extraction, modulating, encoding, multiplexing, and the like. In other embodiments, other type of processors, operating systems, and physical configurations are also envisioned. 
     The memory  190  may be a non-transitory storage medium. For example, the memory  190  may be a dynamic random access memory (DRAM) device, a static random access memory (SRAM) device, flash memory or other memory devices. In one embodiment, the memory  190  also includes a non-volatile memory or similar permanent storage device, and media such as a hard disk drive, a floppy disk drive, a compact disc read only memory (CD-ROM) device, a digital versatile disc read only memory (DVD-ROM) device, a digital versatile disc random access memory (DVD-RAM) device, a digital versatile disc rewritable (DVD-RW) device, a flash memory device, or other non-volatile storage devices. 
     The memory  190  stores data that is required for the switching application  160  to perform associated functions. In one embodiment, the memory  190  stores the modules (e.g., communication module  170 , calculation module  175 , and the like) of the switching application  160 . In another embodiment, the memory  190  stores stability criteria (e.g., standard deviation threshold value, environmental threshold value, time threshold value, and the like) that are defined, for example, by an administrator of the switching unit  150 . The stability criteria are described below in further detail with reference to the determination module  180 . 
     The communication module  170  includes codes and routines configured to handle communications between the sensor  105  and other modules of the switching application  160 . In one embodiment, the communication module  170  includes a set of instructions executable by the processor  185  to provide the functionality for handling communications between the sensor  105  and other modules of the switching application  160 . In another embodiment, the communication module  170  is stored in the memory  190  and is accessible and executable by the processor  185 . In either embodiment, the communication module  170  is adapted for communication and cooperation with the processor  185  and other modules of the switching application  160 . 
     In one embodiment, the communication module  170  receives PPG data from the control unit  115  of the sensor  105  via the network  140 . For example, the communication module  170  receives the PPG data in real-time corresponding to each cardiac cycle of the subject. The received PPG data includes either continuous PPG data recorded by the continuous module  120  or compressed PPG data recorded by the compressed module  130 . In such an embodiment, the communication module  170  sends the PPG data to the calculation module  175 . In another embodiment, the communication module  170  receives a notification instruction for switching the operation of the sensor  105  from the determination module  180 . In such an embodiment, the communication module  170  sends the notification to the control unit  115  of the sensor  105  via the network  140 . 
     The calculation module  175  includes codes and routines configured to calculate one or more blood characteristics (BCs) and calculating one or more quality metrics of the PPG data. In one embodiment, the calculation module  175  includes a set of instructions executable by the processor  185  to provide the functionality for calculating one or more BCs and one or more quality metrics of the PPG data. In another embodiment, the calculation module  175  is stored in the memory  190  and accessible and executable by the processor  185 . In either embodiment, the calculation module  175  is adapted for communication and cooperation with the processor  185  and other modules of the switching application  160 . 
     The calculation module  175  receives PPG data from the communication module  170  and calculates one or more BCs (e.g., percentage modulation, oxygen saturation, heart rate, and the like) from the received PPG data. In one embodiment, the calculation module  175  receives continuous PPG data that includes a continuous PPG waveform recorded by operating an LED emitting red light and another continuous PPG waveform recorded by operating an LED emitting infrared light. For the purpose of clarity and convenience, a BC calculated from the continuous PPG data is referred to herein as a continuous BC. 
     In one embodiment, the calculation module  175  calculates a percentage modulation (i.e., a perfusion index) of each continuous PPG waveform for each cardiac cycle as a continuous BC. The calculation module  175  calculates a percentage modulation based on the equation: 
     
       
         
           
             
               % 
                
               
                   
               
                
               mod 
             
             = 
             
               
                 
                   AC 
                   max 
                 
                 - 
                 
                   AC 
                   min 
                 
               
               DC 
             
           
         
       
     
     Where: 
     % mod is the percentage modulation of the continuous PPG waveform; 
     AC max  is the maximum value of the continuous PPG waveform; 
     AC min  is the minimum value of the continuous PPG waveform; and 
     DC is the offset level of the continuous PPG waveform. 
     Referring again to  FIG. 2 , the calculation module calculates the % mod R  of the continuous PPG waveform  225  (i.e., received by operating the LED emitting red light) for the second cardiac cycle using the AC max  value  234  and the AC min  value  236 . Similarly, the calculation module  175  calculates the % mod IR  for each cardiac cycle of the continuous PPG waveform received by operating the LED emitting infrared light. 
     In another embodiment, the calculation module  175  calculates a ratio between the % mod R  and the % mod IR  for each cardiac cycle as a continuous BC. In a further embodiment, the calculation module  175  calculates an oxygen saturation (Spo 2  value) of a subject based on the ratio between % mod R  and the % mod IR  using a calibration function. 
       FIG. 4  illustrates a graph  400  depicting the calibration function  430  according to one embodiment. In the illustrated embodiment, if the ratio between the % mod R  and the % mod IR  is 0.5, the calculation module calculates the oxygen saturation as 98 based on the calibration function  430 . 
     Referring again to  FIG. 1 , in one embodiment, the calculation module  175  calculates a heart rate of the subject based on the continuous PPG data as a continuous BC. In such an embodiment, the calculation module  175  determines the time duration between each cardiac cycle of the continuous PPG data for determining the heart rate. 
     In one embodiment, the calculation module  175  receives compressed PPG data from the communication module  170 . The calculation module  175  calculates one or more BCs from the compressed PPG data similar to the aforementioned calculation of the one or more continuous BCs. For the purpose of clarity and convenience, a BC calculated from the compressed PPG data is referred to herein as a “compressed BC”. For example, the calculation module  175  calculates the % mod R  as a compressed BC based on the compressed PPG waveform  325  (See  FIG. 3 ). 
     The calculation module  175  further calculates one or more quality metrics of the received PPG data. In one embodiment, the calculation module  175  calculates a quality metric of the continuous PPG data based on a sequence of continuous BCs corresponding to a sequence of cardiac cycles of the subject. In such an embodiment, the calculation module  175  determines an average BC (e.g., arithmetic mean, a weighted arithmetic mean, geometric mean, median, mode, etc.) of the sequence of continuous BCs. The calculation module  175  then calculates a standard deviation of each continuous BC in the sequence using the average BC as the quality metric. For example, the calculation module  175  determines an average Spo 2  value for a sequence of three Spo 2  values. The calculation module  175  then calculates a standard deviation of each of the three Spo 2  values as the quality metric of the received continuous PPG data. Although, the quality metric based on the sequence of continuous BCs is described above using Spo 2  values according to one example, in other examples, the calculation module  175  can calculate the quality metric using % mod values, ratio between % mod R  and the % mod IR , heart rate, and the like. 
     In another embodiment, the calculation module  175  calculates a quality metric of the continuous PPG data by determining a presence of environmental signals in the received continuous PPG data. The environmental signals include, for example, noise signals caused due to electrical circuitry of the optoelectronic unit  110 , motion artifacts caused due to the movement of the subject, and the like. In such an embodiment, the calculation module  175  transforms the two continuous PPG waveforms (i.e., continuous PPG data) into a Fourier domain to determine the presence of the environmental signals. The calculation module  175  calculates the amplitude of the environmental signal as a quality metric of the received continuous PPG data. 
     In one embodiment, the calculation module  175  receives compressed PPG data from the communication module  170 . In such an embodiment, the calculation module  175  calculates one or more quality metrics for the compressed PPG data similar to the aforementioned calculation of one or more quality metrics for the continuous PPG data. For example, the calculation module  175  determines an average heart rate value (i.e., compressed BC) for a sequence of five heart rate values. The calculation module  175  then calculates a standard deviation of each of the five heart rate values as the quality metric of the received compressed PPG data. In another example, the calculation module  175  determines a presence of an environmental signal in the compressed PPG data and calculates the amplitude of the environmental signal as a quality metric of the compressed PPG data. 
     The calculation module  175  sends the one or more quality metrics of the PPG data to the determination module  180 . In one embodiment, the calculation module  175  generates graphical data for displaying the one or more BCs to, for example, a doctor. In such an embodiment, the calculation module  175  sends the graphical data to a display device (not shown) coupled to either the switching unit  150  or the sensor  105 . 
     The determination module  180  includes codes and routines configured to determine whether one or more quality metrics of the PPG data satisfies a stability criteria and switch the operation of the sensor  105 . In one embodiment, the determination module  180  includes a set of instructions executable by the processor  185  to provide the functionality for determining whether one or more quality metrics of the received PPG data satisfies a stability criteria and for switching the operation of the sensor  105 . In another embodiment, the determination module  180  is stored in the memory  190  and accessible and executable by the processor  185 . In either embodiment, the determination module  180  is adapted for communication and cooperation with the processor  185  and other modules of the switching application  160 . 
     In one embodiment, the determination module  180  receives one or more quality metrics of continuous PPG data. The determination module  180  determines whether the one or more quality metrics of the continuous PPG data satisfy one or more stability criteria. The stability criteria (e.g., standard deviation threshold value, environmental threshold value, time threshold value, and the like) are defined by, for example, an administrator of the switching unit  160 . The determination module  180  sends a first notification to the sensor  105  in response to determining that the one or more quality metrics satisfy the stability criteria. In such an embodiment, the first notification instructs the sensor  105  to operate the compressed module of the optoelectronic unit  115  and collect compressed PPG data of the subject. 
     For example, the determination module  180  receives the standard deviations of the heart rate of a subject over three cardiac cycles as 1, 0, and 1. In such an example, the determination module  180  determines that the standard deviation of each heart rate is within the standard deviation threshold value of 3. The determination module  180  then sends the first notification to the sensor  105  to stop the collection of continuous PPG data and start the collection of compressed PPG data. In another example, the determination module  180  receives the amplitude of an environmental signal present in the received continuous PPG data. If the determination module  180  determines that the amplitude of the environmental signal is lesser than the environmental threshold value, the determination module  180  sends the first notification to the sensor  105 . 
     In another embodiment, the determination module  180  receives one or more quality metrics of compressed PPG data. The determination module  180  determines whether the one or more quality metrics of the compressed PPG data satisfy one or more stability criteria. The determination module  180  sends a second notification to the sensor  105  in response to determining that the one or more quality metrics fail to satisfy the stability criteria. In such an embodiment, the second notification instructs the sensor  105  to operate the continuous module  120  of the optoelectronic unit  115  and collect continuous PPG data of the subject. 
     For example, the determination module  180  receives the standard deviations of the heart rate of a subject over four cardiac cycles as 1, 0, 1 and 5. In such an example, the determination module  180  determines that the standard deviation of the heart rate during the fourth cardiac cycle exceeds the standard deviation threshold value of 3. The determination module  180  then sends the second notification to the sensor  105  to stop the collection of compressed PPG data and start the collection of continuous PPG data. In another example, the determination module  180  receives the amplitude of an environmental signal present in the received compressed PPG data. If the determination module  180  determines that the amplitude of the environmental signal exceeds the environmental threshold value, the determination module  180  sends the second notification to the sensor  105 . 
     In one embodiment, the determination module  180  sends the second notification to the sensor  105  based on an elapsed time. The elapsed time indicates the time duration for which the sensor  105  has been collecting compressed PPG data of the subject. The determination module  180  calculates the elapsed time in response to sending the first notification to the sensor  105 . In such an embodiment, the determination module  180  determines whether the elapsed time is within a time threshold value. The determination module  180  sends the second notification to the sensor  105  in response to determining that the elapsed time has exceeded the time duration value. In another embodiment, the determination module  180  receives a user input, for example, from a doctor, for collecting continuous PPG of a subject. In such an embodiment, the determination module  180  sends the second notification to the sensor  105 . 
     In yet another embodiment, the determination module  180  determines whether the switching unit  150  receives the PPG data continuously from the sensor  105 . For example, the determination module  180  determines whether the communication module  170  receives PPG data in real-time corresponding to every cardiac cycle of the subject. The determination module  180  sends the first notification in response to determining that that the communication module  170  fails to receive the PPG data in real-time. The communication module  170  may fail to receive the PPG data continuously due to, for example, a failure in the functioning of the network  140 , signal lines,  135 ,  145  and the like. The first notification instructs the sensor  105  to collect compressed PPG data of the subject. Such an embodiment is advantageous as temporarily storing compressed PPG data in the memory (not shown) of the sensor  105  requires lesser storage space than storing continuous PPG data. 
       FIG. 5  illustrates a flow diagram  500  of a method for operating a sensor for determining BCs according to one embodiment. The communication module receives continuous PPG data of a subject from a sensor  502 . The calculation module calculates a continuous BC based on the continuous PPG data  504 . For example, the calculation module calculates the Spo 2  value from the continuous PPG data as the continuous BC. The calculation module further calculates a first quality metric of the continuous PPG data based on a sequence of the continuous BCs  506 . In the above example, calculation module calculates the standard deviation for a sequence of five continuous Spo 2  values as the first quality metric. The determination module determines whether the first quality metric satisfies a stability criterion  508 . In the above example, the determination module determines whether the standard deviation of each continuous Spo2 value is within a standard deviation threshold value. 
     If the first quality metric fails to satisfy the stability criterion, the communication module continues to receive continuous PPG data of the subject from the sensor  502 . If the first quality metric satisfies the stability criterion, the determination module sends a first notification instructing the sensor to collect compressed PPG data of the subject  510 . The communication module then receives compressed PPG data of the subject from the sensor  512 . The calculation module calculates a compressed BC based on the compressed PPG data  514 . For example, the calculation module calculates the Spo 2  value from the compressed PPG data as the compressed BC. The calculation module further calculates a second quality metric of the compressed PPG data based on a sequence of the compressed BCs  516 . In the above example, calculation module calculates the standard deviation for a sequence of five compressed Spo 2  values as the second quality metric. 
     The determination module then determines whether the second quality metric satisfies a stability criterion  518 . In the above example, the determination module determines whether the standard deviation of each compressed Spo 2  value is within the standard deviation threshold value. If the second quality metric satisfies the stability criterion, the communication module continues to receive compressed PPG data of the subject from the sensor  512 . If the second quality metric fails to satisfy the stability criterion, the determination module sends a second notification instructing the sensor to collect continuous PPG data of the subject  520 . 
     Although the method  500  is described as switching the operation of the sensor based on single type of quality metric (i.e., standard deviation of continuous Spo 2  and compressed Spo 2 ) according to one embodiment, in other embodiments, the sensor operation may be switched based on a combination of multiple quality metrics. For example, the calculation module calculates the Spo 2  value and the heart rate value from the received continuous PPG data as continuous BCs. The calculation module then calculates the standard deviation for a sequence of Spo 2  and heart rate values as the first quality metric. In such an example, the determination module sends the first notification if the standard deviation of each Spo 2  value and each heart rate value is within the standard deviation threshold value. Although, in this example, determination module compares the standard deviation of the Spo 2  values and the heart rate values with the same standard deviation threshold value, in other examples, the determination module may compare them with different standard deviation threshold values. 
     In another example, the calculation module calculates the Spo 2  value from the received compressed PPG data as a compressed BC. The calculation module calculates the standard deviation for a sequence of Spo 2  values and the amplitude of an environmental signal in the compressed PPG data as the second quality metric. In such an example, the determination module sends the second notification if either the standard deviations of the Spo 2  values exceed the standard deviation threshold value or if the amplitude of the environmental signal exceeds the environmental threshold value. 
     Referring now to  FIG. 6 , a graphical representation  600  for operating a sensor for determining BCs is illustrated according to one embodiment. The graph  620  illustrates a continuous PPG waveform  625  received by the switching unit over three successive cardiac cycles (i.e., cardiac cycles 1-3) of a subject. The continuous PPG waveform  625  is recorded by operating the LED emitting, for example, infrared light using the continuous module. The calculation module determines the % mod IR  of the continuous PPG waveform  625  as the continuous BC. The table  630  illustrates the sequence of % mod IR  values of the continuous PPG waveform  625 . The calculation module further calculates the standard deviation for each of the % mod IR  values shown in the table  630 , as the first quality metric. The determination module determines that the first quality metric is within the standard deviation threshold value and hence satisfies the stability criterion. The determination module then sends a first notification instructing the sensor to collect compressed PPG data of the subject. 
     The graph  650  illustrates the compressed PPG waveform  665  received by the switching unit over three successive cardiac cycles of the subject (i.e., cardiac cycles 4-6). The compressed PPG waveform  665  is recorded by operating the LED emitting infrared light using the compressed module. The control unit of the sensor switches the operation of the LED from the continuous module to the compressed module in response to receiving the first notification from the switching unit. The calculation module then calculates % mod IR  of the compressed PPG waveform  665  as the compressed BC. The table  670  illustrates the sequence of % mod IR  values of the compressed PPG waveform  665 . The calculation module further calculates the standard deviation for each of the % mod IR  values shown in the table  670 , as the second quality metric. The determination module determines that the standard deviation of the % mod IR  value during the sixth cardiac cycle exceeds the standard deviation threshold value and hence fails to satisfy the stability criterion. The determination module then sends a second notification instructing the sensor to collect continuous PPG data of the subject. 
     The above described method for switching the operation of a sensor based on the quality of the PPG data is advantageous compared to existing methods for determining BCs due to lesser power consumption by the LEDs and higher reliability and accuracy of the determined BCs. 
     It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     While the subject matter has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the inventions are not limited to such disclosed embodiments. Rather, the subject matter can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the inventions. Additionally, while various embodiments of the subject matter have been described, it is to be understood that aspects of the inventions may include only some of the described embodiments. Accordingly, the inventions are not to be seen as limited by the foregoing description, but are only limited by the scope of the appended claims.