Abstract:
A photoplethysmographic sensor and related method for use with a photoplethysmographic instrument such as a pulse oximeter are provided. In accordance with the present invention, the detector output signal from the sensor is digitized prior to communication from the sensor to the instrument and the sensor operates independent of the instrument with respect to controlling the light signal emitters of the sensor. In one embodiment, the digitized detector output signal is communicated to the instrument via a wireless communication link.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 60/691,051 entitled “Digital Photoplethysmographic Signal Sensor” having a filing date of Jun. 16, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to photoplethysmography, and more particularly to a sensor for use with photoplethysmographic instruments that outputs a digital signal to the instrument.  
       BACKGROUND OF THE INVENTION  
       [0003]     Signal attenuation measurements generally involve transmitting a signal towards or through a medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (e.g., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.  
         [0004]     Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (e.g., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze a tissue or a blood sample with regard to oxygen, carbon dioxide or other gas saturation levels, analyte values (e.g., related to certain hemoglobins) or other composition related values. Signal attenuation measurement systems using optical or light signals are often referred to as photoplethysmographic instruments, and one example of a photoplethysmographic instrument is a pulse oximeter.  
         [0005]     Pulse oximeters determine the levels of oxygen and/or other gases in a patient&#39;s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient&#39;s tissue. Pulse oximeters also determine the patient&#39;s pulse rate from information included in one or more of the attenuated light signals. In particular, pulse oximeters generally include a probe or sensor for attaching to a patient tissue site such as, for example, a finger, earlobe, nasal septum, or foot. The probe is used to transmit pulsed light signals of at least two wavelengths, typically red and infrared, to the patient tissue site. The light signals are attenuated by the patient tissue site. The attenuated light signals are also often referred to as the transmitted signals, and the transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal output by the detector and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood gas saturation levels. As may be appreciated, a multiplexing technique (such as time division, frequency division, code division, or a combination of these techniques) may be employed to drive the light signal emitters in order facilitate obtaining information relating to each of the transmitted light signals from the detector output signal.  
         [0006]     Typical sensors include the light signal emitters and the detector in conjunction with a positioner and a cable for connecting the sensor to the photoplethysmographic instrument. As may be appreciated, the cable typically includes a number of conductors for transmitting drive signals from the instrument to the light signal emitters to control their operation in accordance with the employed multiplexing technique, a conductor for communicating the analog detector output signal to the instrument for further processing thereby, and a common conductor. The cable may also have one or more sense wires for use in monitoring the operations of the light signal emitters (e.g., measuring their resistance). The various signals transmitted via the conductors in the cable, and in particular the analog detector output signal, are susceptible to electromagnetic signal interference from various sources, including other electrically powered equipment often present in hospital rooms and other facilities where patients are treated. Furthermore, the relatively narrow gauge conductors in the probe cables can sometimes be fragile resulting in defective probes.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, the present invention is directed to a sensor and related method for use with a photoplethysmographic instrument such as a pulse oximeter wherein the detector output signal is digitized prior to communication from the sensor to the instrument. Additionally, the present invention is directed to a sensor and related method for use with a photoplethysmographic instrument such as a pulse oximeter wherein the sensor operates independent of the instrument with respect to controlling the light signal emitters or the like in generating and multiplexing the necessary light signals. Further, the present invention is also directed to a sensor and related method for use with a photoplethysmographic instrument such as a pulse oximeter wherein the digitized detector output signal is communicated to the instrument via a wireless communication link.  
         [0008]     The present invention achieves a number of advantages. By digitizing the detector output signal, the potential for corruption of the detector output signal during transmission from the sensor to the monitor due to electromagnetic signal interference or the like is reduced. By controlling operation of the light signal emitters onboard the sensor, at least two conductors can be eliminated in embodiments with a cable connecting the sensor to the instrument, and a wireless connection between the sensor and instrument is permitted. By employing a wireless communication link between the sensor and the instrument to communicate the digitized detector output signal, greater patient mobility is allowed and the instrument may be located at greater distance from the patient.  
         [0009]     The aforementioned features and advantages of the present invention are achieved by a number of aspects of the present invention. According to one aspect of the present invention a photoplethysmographic sensor for use with a photoplethysmographic instrument such as, for example, a pulse oximeter, includes at least first and second light signal emitters, a detector and a signal processing device. The light signal emitters, detector, and signal processing device (and other components of the sensor) may all be incorporated into a positioner configured for attachment to a patient tissue site that positions the first and second light signal emitters and the detector in an appropriate relation with one another and the patient tissue site.  
         [0010]     The first and second light signal emitters are operable to transmit at least first and second light signals centered at first and second wavelengths (e.g., Red and Infrared), respectively, into a tissue site of a patient. The patient tissue site attenuates the first and second light signals resulting in first and second attenuated light signals. The detector is operable to detect the first and second attenuated light signals and to output an analog detector output signal corresponding to the first and second attenuated signals. The signal processing device is operable to receive the analog signal from the detector and to generate a digital signal corresponding to the analog detector output signal. The digital signal is communicable to the photoplethysmographic instrument whereby the photoplethysmographic instrument may obtain information from the digital signal relating to a physiological condition of the patient (e.g., the patient&#39;s blood oxygen level and/or pulse rate).  
         [0011]     The signal processing device may comprise an electronic device such as a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC), with the FPGA or ASIC configured to incorporate an amplifier and an analog-to-digital converter. The sensor may also include a light signal emitter drive unit operable to control the emission of light signals from the light signal emitters. In this regard, the light signal emitter drive unit may be an FPGA or an ASIC separate from the signal processing device or it may be incorporated as part of an FPGA or ASIC comprising the signal processing device.  
         [0012]     The sensor may be configured to communicate the digital signal to the photoplethysmographic instrument via a wired communication link. In this regard, the sensor may include a cable connectable with an input of the photoplethysmographic instrument. Where the photoplethysmographic instrument is configured to receive an analog input signal, the sensor may be accompanied by an adaptor unit connectable with the input of the instrument and the cable of the sensor that converts the digital signal from the sensor to an analog signal.  
         [0013]     The sensor may also be configured to communicate the digital signal to the photoplethysmographic instrument via a wireless communication link. In this regard, the sensor may also include a wireless transmitter operable to communicate the digital signal to the photoplethysmographic instrument via the wireless communication link (e.g., radio-frequency or free-space optical). In order to accommodate wireless communication of the digital signal, the photoplethysmographic instrument needs to be configured to receive the digital signal via the wireless communication link by, for example, including a wireless receiver within the instrument. Alternatively, the sensor may be accompanied by a separate wireless receiver unit that is configured to connect to an input of the photoplethysmographic instrument. The wireless receiver unit adapts the photoplethysmographic instrument to receive the digital signal via the wireless communication link. Where a wireless communication link is employed between the sensor and the instrument, it may be desirable to encode the digital signal prior to communication of the digital signal via the wireless communication link in order to facilitate association of the digital signal with the particular sensor, particularly in environments where other digital photoplethysmographic sensors may be present. Further, the sensor may include a power source such as, for example, a battery, in order to provide electrical power to the various electronic components of the sensor.  
         [0014]     According to another aspect of the present invention, a system for obtaining information relating to a physiological condition (e.g., blood oxygen level and/or pulse rate) of a patient based on information derived from light signals attenuated by a tissue site of the patient includes a sensor and a monitor. The sensor is operable to generate and direct at least two light signals at the patient tissue site, with the light signals being centered at different wavelengths (e.g., Red and Infrared). The sensor is also operable to detect the light signals after being attenuated by the patient tissue site and to digitize the detected attenuated light signals. The monitor includes a digital signal processor that is operable to receive the digitized detected attenuated light signals and to process the digitized detected attenuated light signals to obtain the patient physiological condition therefrom.  
         [0015]     The sensor may include at least two light signal emitters operable to emit the light signals, a light signal emitter drive unit coupled to the light signal emitters and operable to control the emission of light signals from the light signal emitters, a detector operable to the detect attenuated light signals and to output an analog detector output signal corresponding to the attenuated signals, and an analog-to-digital converter coupled to the detector and operable to digitize the analog detector signal. The sensor may also include an amplifier coupled between the detector and the analog-to-digital converter. The analog-to-digital converter and the amplifier may be implemented within a first electronic component (e.g., an FPGA or ASIC), and the light signal emitter drive unit may be implemented within a second electronic component (e.g. another FPGA or ASIC). The light signal emitter drive unit, analog-to-digital converter, and amplifier may instead all be implemented within a single electronic component (e.g., FPGA or ASIC).  
         [0016]     The sensor may also include a wireless transmitter operable to communicate the digitized detected attenuated light signals to the photoplethysmographic instrument via a wireless communication link (e.g., radio-frequency or free-space optical). In this regard, the monitor may include a wireless receiver operable to receive the digitized detected attenuated light signals via the wireless communication link or the system may further include a wireless receiver unit configured to connect to an input of the monitor to adapt the monitor to receive the digitized detected attenuated light signals via the wireless communication link. The sensor may also include a cable configured to communicate the digitized detected attenuated light signals to the monitor. In this regard, the system may further include an adaptor unit connectable with an input of the monitor and with the cable of the sensor that is operable to convert the digitized detected attenuated light signals receivable from the cable of the sensor to an analog signal transmittable from the adaptor unit to the input of the monitor.  
         [0017]     According to yet another aspect of the present invention, a method for use in obtaining information relating to a physiological condition of a patient from light signals attenuated by a tissue site of the patient includes the step of operating a sensor located at a patient tissue site to direct at least two light signals (e.g., Red and Infrared light signals) into the patient tissue site. Operating the sensor also involves detecting the light signals after the light signals are attenuated by the patient tissue site and generating a digital signal corresponding to the attenuated light signals. In accordance with the method, the digital signal is communicated to a monitor separate from the sensor. In this regard, the digital signal may be communicated using a wired communication link or a wireless communication link between the sensor and the monitor. The method may also include adapting the monitor to receive the digital signal via the wireless communication link or adapting the monitor to receive the digital signal via the wired communication link. However received, the digital signal is processed at the monitor to obtain information relating to the patient physiological condition (e.g., blood oxygen level and/or pulse rate).  
         [0018]     These and other aspects and advantages of the present invention will be apparent upon review of the following Detailed Description when taken in conjunction with the accompanying figures. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0019]     For a more complete understanding of the present invention and further advantages thereof, reference is now made to the following Detailed Description, taken in conjunction with the drawings, in which:  
         [0020]      FIG. 1  is a block diagram of a pulse oximetry system incorporating one embodiment of a digital photoplethysmographic sensor in accordance present invention;  
         [0021]      FIG. 2  is a block diagram showing the digital photoplethysmographic sensor of  FIG. 1  in greater detail;  
         [0022]      FIG. 3  is a block diagram of another pulse oximetry system incorporating a wireless embodiment of a digital photoplethysmographic sensor in accordance present invention;  
         [0023]      FIG. 4  is a block diagram showing the digital photoplethysmographic sensor of  FIG. 3  in greater detail;  
         [0024]      FIG. 5  is a block diagram of another pulse oximetry system incorporating a wireless embodiment of a digital photoplethysmographic sensor and having a wireless receiver adaptor unit in accordance present invention; and  
         [0025]      FIG. 6  is a block diagram of another pulse oximetry system incorporating a wired embodiment of a digital photoplethysmographic sensor and having a digital-to-analog adaptor unit in accordance present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Referring to  FIG. 1 , one embodiment of a pulse oximetry system  10  incorporating a digital photoplethysmographic sensor  30  is shown. The pulse oximetry system  10  includes a pulse oximeter monitor  20  including an input connector  22 , a processor  24 , a display  26 , and a printer  28 . The sensor  30  includes a positioner  32  and a cable  34  shown connected with the input  22  of the monitor  20 . The positioner  32  is configured for attachment to a patient tissue site  12 . In this regard, the positioner  32  may, for example, be a clip-type positioner such as shown, although other configurations may be utilized as well. The input  22  of the monitor  20  receives a digital signal  54  from the positioner  32  via cable  34 . The digital signal  54  is processed by the processor  24  of the monitor  20  to obtain information regarding physiological conditions of the patient such as the patient&#39;s blood gas saturation levels as well as the patient&#39;s pulse rate. Such physiological conditions may be output on the display  26  and/or printed by the printer  28  on a paper roll or the like.  
         [0027]     Referring now to  FIG. 2 , the sensor  30  includes two light signal emitters  36 A and  36 B, although in other embodiments there may be fewer or more than two light signal emitters. The light signal emitters  36 A,  36 B may, for example comprise light-emitting diodes (LEDs), laser diodes, or the like. When excited the light signal emitters  36 A,  36 B emit light centered around different respective first and second wavelengths, such as Red and Infrared, although other wavelength emitters may be employed depending on the intended use of the photoplethysmographic sensor  30 . The light signal emitters  36 A,  36 B are also referred to herein and the Red and Infrared LEDs  36 A,  36 B.  
         [0028]     A light signal emitter drive unit  38  is coupled to the light signal emitters  36 A,  36 B. The drive unit  38  generates and sends drive signals  40 A,  40 B to the Red and Infrared LEDs  36 A,  36 B to cause the LEDs  36 A,  36 B to emit light signals  42 A,  42 B in the direction of the patient tissue site  12 . In this regard, the drive signals  40 A,  40 B may be generated in accordance with an appropriate multiplexing scheme in order to multiplex light signals  42 A,  42 B. The light signals  42 A,  42 B are, in this embodiment, transmitted through the patient tissue site  12  and attenuated thereby producing attenuated or transmitted light signals  44 A,  44 B.  
         [0029]     The sensor  30  also includes a light signal detector  46  such as, for example, a photodiode or the like. In other embodiments there may be more than one detector, with each detector being tuned to receive only particular light frequencies thereby obviating the need the multiplex the light signals  42 A,  42 B. The detector  46  receives both transmitted light signals  44 A,  44 B, and generates an analog composite detector output signal  48 . The output signal  48  includes information relating to both of the transmitted light signals  44 A,  44 B.  
         [0030]     The sensor  30  further includes an amplifier  50  and an analog-to-digital (A/D) converter  52 . The analog composite detector output signal  48  is directed to the amplifier  50  which amplifies the detector output signal  48 . The amplifier  50  may also be configured to filter (e.g., high-pass, low-pass, or bandwidth filter) the detector output signal  48 . After amplification/filtering, the detector output signal  48  is directed to the A/D converter  52 . The A/D converter  52  converts the amplified/filtered detector output signal  48  to a digital output signal  54 . In this regard, the A/D converter should sample the detector output signal  48  at a sufficiently high sample rate (e.g., 30 to 50 Hz) in order to accurately digitize the detector output signal  48  without losing significant information relating to the levels of the transmitted light signals  44 A,  44 B.  
         [0031]     As illustrated, the amplifier  50  and the A/D converter  52  may be implemented within a first signal processing device or electronic component  56 , such as, for example, a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC). Likewise, the light signal emitter drive unit  38  may be implemented using a second signal processing device or electronic component  58  such as, for example, another FPGA or ASIC. In other embodiments, the light signal emitter drive unit  38 , amplifier  50  and A/D converter  52  may be implemented within a single electronic component such as, for example, an FPGA or an ASIC. Still in other embodiments, other electronic components such as an appropriately programmed general purpose microprocessor might be utilized to implement some or all functionality of the light signal emitter drive unit  38 , amplifier  50  and A/D converter  52 .  
         [0032]     As may be appreciated, various components included in the sensor  30  (e.g., the LEDs  36 A,  36 B and the FPGAs (or ASICs)  56 ,  58  comprising the light signal emitter drive unit  38 , the amplifier  50  and the A/D converter  52 ) need electrical power in order to operate. In this regard, the cable  34  may include a conductor for supplying such power from, for example, the monitor unit  20 . In addition to a power supply conductor, the cable  34  may also include a conductor for transmitting the digital output signal  54  as well as a common conductor. Since the light signal emitter drive signals  40 A,  40 B are generated at the sensor  30  by the light signal emitter drive unit  38 , conductors for conducting drive signals from the monitor  20  to the sensor  30  are not required.  
         [0033]     Referring now to  FIGS. 3 and 4 , another embodiment of a pulse oximetry system  110  incorporating a wireless digital photoplethysmographic sensor  130  is shown. The pulse oximetry system  110  and wireless sensor  130  are configured similar to the pulse oximetry system  10  and sensor  30  illustrated in  FIGS. 1 and 2 , and similar components are referenced by the same numbers. The pulse oximetry system  110  includes a pulse oximeter monitor unit  20  including a wireless data receiver  122 , a processor  24 , a display  26 , and a printer  28 . The sensor  130  includes a positioner  32  and a wireless transmitter  134 . The positioner  32  is configured for attachment to a patient tissue site  12 , and may, for example, be a clip-type positioner such as shown, although other configurations may be utilized as well. The wireless receiver  122  of the monitor  20  receives a wireless digital signal transmitted by the wireless transmitter  134  from the positioner  32 . In this regard, the wireless transmitter  134  and wireless receiver  122  may, for example, comprise radio-frequency (RF) components with the wireless digital signal being an RF signal, or where sufficient line-of-sight conditions can be maintained between the sensor  130  and monitor  20 , the wireless transmitter  134  and wireless receiver  122  may, for example, comprise optical components with the wireless digital signal being an optical signal. Regardless of its form, the wireless digital signal received by the wireless receiver  122  is processed by the processor  24  of the monitor  20  to obtain information regarding physiological conditions of the patient such as the patient&#39;s blood gas saturation levels as well as the patient&#39;s pulse rate. Such physiological conditions may be output on the display  26  and/or printed by the printer  28  on a paper roll or the like.  
         [0034]     In addition the various components included in the sensor  30  shown in  FIG. 2  (with the exception of a cable), the wireless sensor  130  includes the wireless transmitter  134  and an electrical power source  136  (e.g., a battery) that supplies power for operating the various electronic components of the wireless sensor  130 . As is shown, the wireless transmitter may be incorporated within the first electronic component  56  along with the amplifier  50  and A/D converter  52 . In other embodiments, the wireless transmitter may be a separate electronic component.  
         [0035]     The digital output signal  54  from the A/D converter  52  is directed to the wireless transmitter  134  for transmission to the wireless receiver  122  of the monitor  20 . In this regard, the wireless transmitter  134  modulates the digital output signal  54  onto a carrier signal (e.g., RF or optical) to obtain a wireless digital output signal  154  that is transmitted to the wireless receiver  122 . The wireless receiver  122  of the monitor  20  receives the wireless digital output signal  154  and demodulates the wireless digital output signal  154  to obtain the digital output signal  54  for further processing by the processor  24  of the monitor  20 . By transmitting the digital output signal  54  wirelessly to the monitor  20  without the use of a cable, the patient is permitted greater mobility and monitor  20  does not need to be within a cable&#39;s length distance of the patient tissue site  12 . In fact, in the case of a RF wireless transmitter  134  and receiver  122 , the monitor  20  may not even need to be within the same room as the patient.  
         [0036]     Since there may be additional wireless (RF or optical) devices in the same room or area as the patient (e.g., other wireless photoplethysmographic sensors being used with other patients), the sensor  130  may be operable to encode the digital output signal  54  prior to it being modulated onto the carrier signal for transmission. In this regard, the digital output signal  54  may be encoded in a manner that identifies it as being associated with the particular wireless digital photoplethysmographic sensor  130  from which the wireless digital output signal  154  is transmitted. Such functionality may, for example, be incorporated within the first electronic component  56 . Upon receipt, the monitor  20  is operable to decode the encoded digital output signal  54 . Such functionality may, for example, be included as part of the wireless receiver  122 . In order to facilitate decoding, information about the digital photoplethysmographic sensor  130 , and in particular the encoding methodology employed, may be provided manually (e.g., by a user) or automatically (e.g., as part of a sensor/monitor initiation sequence) to the monitor  20 .  
         [0037]     Referring now to  FIG. 5 , another embodiment of a pulse oximetry system  210  incorporating a wireless digital photoplethysmographic sensor  230  and a wireless adaptor unit  212  is shown. The monitor  20  of the pulse oximetry system  210  is configured similar to the monitor  20  in the pulse oximetry system  10  shown in  FIG. 1  and the wireless sensor  130  is configured similar to the wireless sensor  130  of the pulse oximetry system  110  illustrated in  FIGS. 3 and 4 , and similar components are referenced by the same numbers. The primary difference between the pulse oximetry system  210  shown in  FIG. 5  and that shown in  FIG. 3  is that the monitor  20  does not include a wireless receiver. Instead, the wireless adaptor unit  212  is connected (via, for example a short cable  214  as shown) with the input connector  22  of the monitor  20 . The wireless adaptor unit  212  adapts a monitor  20  which lacks a wireless receiver for receiving the wirelessly transmitted (e.g., RF or optical) digital output signal  154 . The adapter unit  212  demodulates the digital output signal  54  from the carrier signal of the wireless digital output signal  154 . Where the digital output signal  54  has been encoded, the wireless adaptor unit  212  also may decode the digital output signal  54 . The digital output signal  54  is directed to the input  22  of the monitor  20  where after it may be further processed by the processor  24  of the monitor  20 . In instances where the monitor  20  is not configured to receive a digital signal, the wireless adaptor unit  212  may also convert the digital output signal  54  to an analog signal for input to the monitor  20  via the input  22  of the monitor  20 . This would allow the wireless photoplethysmographic sensor  130  to be utilized with monitors that are configured to receive an analog input signal and include an A/D converter between their input connector and processor.  
         [0038]     Referring now to  FIG. 6 , in some instances it may be desirable to utilize a digital photoplethysmographic sensor  30  such as illustrated in  FIG. 2  with a pulse oximeter monitor unit configured to receive an analog input signal at the input connector thereof. In this regard,  FIG. 6  shows another embodiment of a pulse oximetry system  310  that includes a digital photoplethysmographic sensor  30  having a cable  34  for connecting it to a pulse oximeter monitor unit  320 . The monitor unit  320  of the pulse oximetry system  310  shown in  FIG. 6  is configured similar to the monitor unit  20  in the pulse oximetry system  10  shown in  FIG. 1  and the sensor  30  is configured similar to the sensor  30  illustrated in  FIG. 2 , and similar components are referenced by the same numbers. One of the differences between the pulse oximetry system  310  shown in  FIG. 6  and that shown in  FIG. 1  is that the monitor  320  is enabled to receive an analog input signal at the input connector  22  thereof. In this regard, the monitor unit  320  includes an analog-to-digital (A/D) converter  360  between the processor  24  and input connector  22 . Further, the pulse oximetry system  310  includes a cabled sensor adaptor unit  312  connected (via, for example a short cable  314  as shown) with the input connector  22  of the monitor  320 . The cabled sensor adaptor unit  312  adapts the monitor  320  for receiving the digital output signal  54  from the cable  34  of the sensor  30 . The adapter unit  312  converts the digital output signal  54  to an analog signal  362  (e.g., using a digital-to-analog converter included therein) for input to the monitor  320  via the input  22  of the monitor  320 . This allows the digital photoplethysmographic sensor  30  to be utilized with monitors that are configured to receive an analog input signal.  
         [0039]     In each of the previously described embodiments, since the light signal emitter drive signals  40 A,  40 B are generated by the sensor ( 30  or  130 ), it may be necessary to inform the monitor  20  as to the multiplexing technique being employed so that the processor  24  of the monitor  20  can appropriately demodulate the digital output signal  54  in order to obtain the Red and Infrared transmitted light signals  44 A,  44 B. One manner of doing so is to add information concerning the multiplexing technique to the digital output signal  54  (e.g., an additional two bits might be added to each word in the digital output signal with the values of the bits providing a code identifying the multiplexing technique utilized). Another possibility is to provide an input informing the monitor  20  of the multiplexing technique (either manually or automatically) to the monitor  20  as part of a monitor/sensor initiation procedure. As an alternative to informing the monitor  20  of the multiplexing technique, separate digital output signals corresponding to each transmitted light signal  44 A,  44 B may be generated by the sensor  30  or  130  and transmitted to the monitor. In this regard, the sensor  30  or  130  may further include a demodulation unit (not shown) (e.g., as part of the first electronic component  56 ) that demodulates the digital output signal  54  prior to its transmission to generate separate Red and Infrared digital output signals for transmission to the monitor  20 .  
         [0040]     While various embodiments of the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.