Abstract:
First and second carrier signals, distinguishable by phase, are respectively applied to infrared and red energy emitters. A detector receives the sum of the energy after modulation at the infrared and red wavelengths. The signal received by the detector is then demultiplexed into its original first and second components, thereby allowing determining of both the infrared and red modulation components. The first and second carrier signals may comprise time-varying periodic signals with identical frequency and frequency spectra, such as a pair of sine waves which are indistinguishable except by phase and amplitude. A 90° phase difference is preferred, but any phase other than 0 or an integer multiple of 180° is workable. A carrier frequency which avoids excessive interference from ambient light is preferred.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to photoplethysmographics. More specifically, this invention relates to phase-division multiplexing and demultiplexing of signals for infrared and red absorption of blood. 
     2. Description of Related Art 
     It is well known in the art to collect photoplethysmographic data simultaneously at a plurality of energy wavelengths. For example, blood oxygen concentration may be measured by determining absorption by a patient&#39;s tissues on infrared and red light; the degree of absorption is typically different for these two wavelengths. Infrared and red light are emitted into the patient&#39;s tissues (e.g., by infrared and red LEDs) and the total energy received to be detected by a single detector (e.g., a photodiode). However, one problem is that the signal produced by the detector must be processed to separate the infrared and red portions from each other. 
     One method of the prior art is shown in U.S. Pat. No. 4,407,290. Time-division multiplexing is used to alternately switch on the infrared and red emitters, at a frequency greater than the patient&#39;s pulse rate. The detector signal is then separated into infrared and red portions by sampling in synchrony with the on/off switching of the infrared and red emitters. 
     While this method successfully separates the infrared and red portions, it generally requires that sampling the detector signal must be synchronized with the on/off switching of the infrared and red emitters. It is also difficult while using this method to compensate for noise sources such as ambient light and electromagnetic interference. 
     A second method of the prior art is shown in U.S. Pat. No. 4,800,885. The infrared and red emitters are driven at two different frequencies. The detector signal is then separated into infrared and red portions by filtering at those two different frequencies. 
     While this method successfully separates the infrared and red portions, the method described in the patent requires demultiplexing signals which are phase-synchronized with the multiplexing frequencies, and produces a higher power output than the time-division multiplexing method. Also, while this method may avoid noise sources at predetermined and known frequencies, it is difficult to compensate for noise sources which were not known before the multiplexing frequencies were chosen, particularly because two separate frequencies which are free of interference must be chosen. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of photoplethysmo-graphics by phase-division multiplexing (as defined herein) and demultiplexing of signals for infrared and red absorption of blood. First and second carrier signals, distinguishable by phase, are respectively applied to infrared and red energy emitters. A detector receives the sum of the energy after modulation at the infrared and red wavelengths. The signal received by the detector is then demultiplexed into its original first and second components, thereby allowing determining of both the infrared and red modulation components. 
     In a preferred embodiment, the first and second carrier signals may comprise time-varying periodic signals with identical frequency and frequency spectra, such as a pair of sine waves which are indistinguishable except by phase and amplitude. A 90° phase difference is preferred, but any phase other than 0 or an integer multiple of 180° is workable. Also, a carrier frequency which avoids excessive interference from ambient light is preferred, such as 30 Hz. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a photoplethysmographic system comprising an embodiment of the invention. 
     FIG. 2 shows a block diagram of the phase-division multiplexer and demultiplexer of an embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of this invention may be used together with inventions which are disclosed in a copending application titled &#34;PHOTOPLETHYSMOGRAPHICS USING ENERGY-REDUCING WAVEFORM SHAPING&#34;, U.S. patent application Ser. No. 07/664,782, filed the same day in the name of the same inventors, hereby incorporated by reference as if fully set forth herein. 
     FIG. 1 shows a block diagram of a photoplethysmographic system comprising an embodiment of the invention. 
     A plurality of energy emitters 101 may each be tuned to a separate wavelength. In a preferred embodiment for measuring blood oxygen, one of the emitters 101 may comprise an infrared light emitter and may operate at a wavelength of about 880  nanometers; another one of the emitters 101 may comprise a red light emitter and may operate at a wavelength of about 656 nanometers. (As used herein, &#34;light&#34; refers to electromagnetic energy of any wavelength, whether visible or not.) However, it may occur that other wavelengths may be useful, such as for measuring blood carbon dioxide, blood carbon monoxide, other blood gas concentrations, blood glucose, or more generally, other chemical and/or physical concentrations. 
     In a preferred embodiment, each of the emitters 101 may comprise an LED (such as part number OPC-8803 made by Marktech International Corp. for the infrared LED and part number MT1500-PUR made by Marktech International Corp. for the red LED), as is well known in the art, and may be coupled by means of an LED driver 102, as is well known in the art, to a carrier output 103 of a mux/demux circuit 104 (see FIG. 2). 
     Energy from the emitters 101 is applied to a tissue section 105 of a patient. In a preferred embodiment for measuring blood oxygen, the tissue section 105 is preferably chosen such that energy from the emitters 101 passes through the patient&#39;s blood vessels, such as an end of the patient&#39;s finger, the patient&#39;s earlobe, or (for neonates) the patient&#39;s hand or foot. The tissue section 105 may modulate the energy from the emitters 101, as is well known in the art, e.g., by absorbing some of the energy at each wavelength. Typically, energy may be modulated by transmission through the tissue section 105, but it may occur that energy may be modulated by reflection or by other means. 
     A detector 106 receives energy after modulation by the tissue section 105 and generates an output signal which indicates the total energy received. In a preferred embodiment, the detector 106 may comprise a photodiode (such as part number OSI-1140 made by Opto Sensors, Inc.) as is well known in the art. An output of the detector 106 is amplified by an amplifier 107 and coupled by means of a filter 108 to a detector input 109 of the mux/demux circuit 104. 
     The mux/demux circuit 104 generates a data output signal 110 at a data output 111, for each energy wavelength, which indicates the modulation which the tissue section 105 applied to that energy wavelength. In a preferred embodiment for measuring blood oxygen, information such as blood oxygen concentration may be calculated from the output signal, as is well known in the art. 
     Phase-division Multiplexing 
     Phase-division multiplexing, as used herein, is defined as follows. In phase-division multiplexing, a plurality of carrier signals are constructed, each of which may comprise a mixture of carrier components, and which are distinguishable by phase. (In a preferred embodiment, the carrier signals are identical except for phase.) Each carrier signal may be separately modulated, and the resultants summed. Thereafter, the separate modulations may be recovered from the sum, as disclosed herein. 
     In a preferred embodiment, a first carrier α may comprise a sine wave, e.g., cos (2π f1 t), and a second carrier β may comprise a sine wave which is phase-shifted with respect to the first carrier, e.g., sin (2π f1 t). Alternatively, the first carrier e may comprise a sum of two or more carrier components, e.g., cos (2π f1 t)+cos (2π f2 t), and the second carrier β may comprise a sum of two or more carrier components which is distinguishable from the first carrier by phase, e.g., cos (2π f1 t+φ1)+cos (2π f2 t+φ 2). Possibly, f2 may comprise a harmonic of f1, but this is not required. 
     The following relations describe separate modulation of each carrier signal, with a 90° phase difference: 
     
         α=cos (w t)                                          (112) 
    
     
         β=sin (w t)                                           (113) 
    
     
         σ=m1α+m2 β                                (114) 
    
     where w is a carrier frequency; ml is a first modulating effect (e.g., at an infrared wavelength); m2 is a second modulating effect (e.g., at a red wavelength); and σ is a detected sum of the modulated carrier signals α, β 
     The detected sum σ is separately multiplied by twice the first carrier α and by twice the second carrier β: 
     
         2ασ=m1+m1 cos (2w t)+m2sin (2w t)              (115) 
    
     
         2βσ=m2-m2 cos (2w t)+m1sin (2w t)               (116) 
    
     These products 2 α σ and 2 β σ are filtered to recover m1 and m2. 
     The following relations describe separate modulation of each carrier signal, with a phase difference other than 90°: 
     
         α=cos (w t)                                          (117) 
    
     
         β=cos (w t+φ)                                     (118) 
    
     
         σ=m1α+m2 β                                (119) 
    
     where w is the carrier frequency; ml is the first modulating effect (e.g., at an infrared wavelength); m2 is the second modulating effect (e.g., at a red wavelength); and σ is the detected sum of the modulated carrier signals α, β 
     The detected sum σ is separately multiplied by twice the first carrier α and by twice the second carrier β: 
     
         2 ασ=m1+m1 cos (2w t)+m2cos(φ)+m2cos (2w t+φ) (120) 
    
     
         2βσ=m2+m2 cos (2w t+2φ)+m1cos(φ)+m1cos (2w t+φ) (121) 
    
     These products 2 σ α and 2 σ β are filtered to recover m1* and m2*. 
     
         m1*=m1+m2 cos(φ)                                       (122) 
    
     
         m2*=m1cosφ)+m2                                         (123) 
    
     
         or ##EQU1## 
    
     
         or 
    
     
         K M=M*                                                     (125) 
    
     where K is a phase-dependent matrix as shown; M is a vector of modulation effects m1, m2; and M* is a vector of modulated carrier component parts m1*, m2* 
     Separate components m1, m2 may be demultiplexed by multiplying by the left multiplicative inverse of the phase-dependent matrix K: 
     
         M=K.sup.-1 M*                                              (126) 
    
     or 
     
         M=K.sup.-1 K M                                             (127) 
    
     Multiplexer/Demultiplexer Circuit 
     FIG. 2 shows a block diagram of the phase-division multiplexer and demultiplexer of an embodiment of the invention. 
     The disclosure herein shows a case where both the first carrier and the second carrier each comprise pure sine waves which differ in phase by exactly 90°. However, applying this disclosure to cases where either the first or the second carrier is not a pure sine wave, or where a component of the first and second carriers differs in phase by other than exactly 90° would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein. 
     A carrier generator 201 generates a plurality of carrier signals 202. In a preferred embodiment, each carrier signal 202 is allocated to one emitter wavelength. Thus, there is a first carrier signal 202 allocated to infrared and a second carrier signal 202 allocated to red. Also, in a preferred embodiment, each carrier signal 202 may comprise a sine wave with frequency fl, as disclosed herein, and the two carrier signals 202 may differ in phase by exactly 90°. 
     In a preferred embodiment, f1 is chosen such that interference from noise sources, such as ambient light and electromagnetic interference, is minimized. In a preferred embodiment, f1 is also chosen such that a bandwidth of about 4 Hz for the modulating effects of the tissue section 105 is allowed. Frequencies in the range of about 30-40 Hz, such as 31.5 Hz, are preferred, but it would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that other frequencies would be workable, and are within the scope and spirit of the invention. 
     It would also be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that there is no requirement that the components of the carrier signal 202 must be sine waves. Other types of carrier components, such as square waves or other waveforms, would be workable, and are within the scope and spirit of the invention. 
     It would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that there is no requirement that the first carrier and the second carrier must differ in phase by exactly 90°. Other phase differences other than 0 or an integer multiple of 180° would be workable, and are within the scope and spirit of the invention. 
     It would also be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that the invention may be adapted to measurement of other constituents, such as blood carbon dioxide, blood carbon monoxide, other blood gas concentrations, blood glucose, or more generally, other chemical and/or physical concentrations. 
     Each carrier signal 202 is coupled by means of a brightness amplifier 203, for adjusting the brightness of a corresponding emitter 101, to the corresponding carrier output 103 of the mux/demux circuit 104. 
     The detector input 109 is coupled, by means of a first filter 204, for removing components at frequencies other than the carrier frequency, to a plurality of demultiplexer elements 205 for demultiplexing the modulated first carrier signal 202 from the modulated second carrier signal 202. A second input of each of the demultiplexer elements 205 is coupled to one of the carrier signals 202. The carrier signals 202 are multiplied, and the products are coupled, by means of a second filter 206, for removing components other than baseband, which shows the modulating effects of the tissue section 105, to produce the data output signals 110. 
     The data output signals 110 each indicate the modulation effect for the corresponding carrier signal 202, as multiplied by a correction by the corresponding brightness amplifier 203. Each data output signal 110 is coupled to the corresponding data output 111 of the mux/demux circuit 104. 
     In a preferred embodiment, signal generation and signal manipulation as described herein are preferably performed by a digital microprocessor (such as part number DSP56001 made by Motorola) operating under software control. It would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that programming a standard digital microprocessor to perform signal generation and signal manipulation as described herein would be a straightforward task and would not require undue experimentation. 
     It would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that the invention may be combined with known methods of computing blood oxygen concentration and other blood gas values from the data output signals 110 which are produced. Providing a system which combines the invention with such known methods would be a straightforward task, after perusal of the specification, drawings and claims herein, and would not require undue experimentation. 
     In a preferred embodiment, the first filter 204 and the second filter 206 should each exhibit a known phase response. Otherwise phase errors might introduce crosstalk between the infrared and red data output signals 110. 
     Alternative Embodiments 
     While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention, and these variations would become clear to one of ordinary skill in the art after perusal of the specification, drawings and claims herein.