Abstract:
A plurality of carrier signals, distinguishable by amplitudes of signal components (e.g., frequency components), are respectively applied to a plurality of energy emitters (e.g., infrared and red light emitters). A detector receives the sum of the energy after modulation at each emitter wavelength, e.g. by blood tissue of a patient. An output of the detector is then demultiplexed, whereby a component of modulation at each emitter wavelength may be determined. The carrier signals may comprise time-varying periodic signals with identical frequency and frequency components, such as mixtures of identical sets of pure sine waves. When the number of signal components exceeds the number of emitter wavelengths, sufficient information is provided during demultiplexing to detect and correct errors introduced by ambient light sources and other interference.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to photoplethysmographics. More specifically, this invention relates to component-amplitude-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 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. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of multiplexing and demultiplexing of signals, called &#34;component-amplitude-division&#34; herein, which may be applied to measuring blood tissue absorption at infrared and red wavelengths. A plurality of carrier signals, distinguishable by amplitudes of signal components (e.g., frequency components), are respectively applied to a plurality of energy emitters (e.g., infrared and red emitters). A detector receives the sum of the energy after modulation at each emitter wavelength, e.g. by blood tissue of a patient. An output of the detector is then demultiplexed, whereby a component of modulation at each emitter wavelength may be determined. 
     In a preferred embodiment, the carrier signals may comprise time-varying periodic signals with identical frequency and frequency components, such as mixtures of identical sets of pure sine waves. For example, in a preferred embodiment, a first carrier α may comprise a mixture of two sine waves α1w1+α2w2, while a second carrier may comprise a different mixture of the same two sine waves β1w1+β2w2. Alternatively, β may comprise a mixture of three sine waves α1w1+α2w2+α3w3, while β may comprise a different mixture of the same three sine waves β1w1+β2w2+β3w3. When the number of signal components exceeds the number of emitter wavelengths, sufficient information is provided during demultiplexing to detect and correct errors introduced by ambient light sources and other interference. 
    
    
     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 component-amplitude-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;, application Ser. No. 07/664,782, Lyon &amp; Lyon, filed the same day in the name of the same inventors, hereby incorporated by reference as if fully set forth herein. 
     Photoplethysmographic System 
     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 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 MT1500PUR 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. 
     Component-Amplitude-Division Multiplexing 
     Component-amplitude-division multiplexing (&#34;CADM&#34;), as used herein, is defined as follows. In CADM, a plurality of carrier signals are constructed, each of which may comprise a mixture of carrier components. Each carrier signal may be separately modulated, and the resultants summed. Thereafter, the separate modulations may be recovered from the sum, as disclosed herein. 
     Thus, a first carrier α may comprise a mixture of two carrier components α1 w1+α2 w2, while a second carrier β may comprise a different mixture of the same two carrier components β1 w1+β2 w2. Alternatively, α may comprise a mixture of three components α1 w1+β2 w2+α3 w3, while β may comprise a different mixture of the same three components β1 w1+β2 w2+β3 w3. 
     The following relations describe construction of each carrier when the number of carrier components (m) and the number of carrier signals (n) both equal 2, i.e. m=n=2: ##EQU1## or 
     
         KΩ=C                                                 (113) 
    
     where K is a matrix of mixing factors α1, α2, β1 β2; Ω is a vector of carrier components w1, w2; and C is a vector of carrier signals α, β 
     Applying these relations to the case where m=n&gt;2 would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein. 
     The following relation describes separate modulation of each carrier signal: 
     
         m1α+m2β=σ                                 (114) 
    
     where m1 is a first modulating effect (e.g., at an infrared wavelength); m2 is a second modulating effect (e.g., at a red wavelength); and is a detected sum of the modulated carrier signals 
     The detected sum σ may be decomposed into separate parts for each carrier component w1, w2: 
     
         σ=m1(α1w1+α2w2)+m2(β1w1+β2 w2) (115) 
    
     or 
     
         σ=t1 w1+t2 w2                                        (116) 
    
     
         t1=m1 α1+m2 β1                                  (117) 
    
     
         t2=m1 α2+m2 β2                                  (118) 
    
     or ##EQU2## or 
     
         KM=T                                                       (120) 
    
     where K is the matrix of mixing factors α1, α2, β1, β2; M is a vector of modulation effects m1, m2; and T is a vector of modulated carrier component parts t1, t2 
     Separate components may be demultiplexed by multiplying by the left multiplicative inverse of the mixing matrix K: 
     
         M=K.sup.-1 T                                               (121) 
    
     or 
     
         M=K.sup.-1 KM                                              (122) 
    
     The mixing matrix K should have a left multiplicative inverse. It would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that K=I, the identity matrix, and even K≈I, would be workable, and are within the scope and spirit of the invention. However, as used herein, a mixing matrix K differs from I. Also, it is generally preferable that I differs substantially from I. 
     Error Detection and Correction 
     The following relations describe construction of each carrier when the number of carrier components (m) &gt;the number of carrier signals (n), which equals 2, i.e. m&gt;n=2: ##EQU3## or 
     
         KΩ=C                                                 (124) 
    
     where K is the matrix of mixing factors α1, α2, α3, β1, ⊕2, β3, z1, z2, z3; is the vector of carrier components w1, w2, w3; and C is the vector of carrier signals α, ⊕, z 
     An additional row z1, z2, z3 has been added to K to preserve its invertability, and an additional element z has been added to C as a result. Because no carrier signal z is actually used, the row z1, z2, z3 may be chosen arbitrarily, so long as K remains invertible. Of course, the value of K -1  depends upon the selection of the row z1, z2, z3. 
     Because the row z1, z2, z3 may be chosen arbitrarily, K -1  may be computed more than once, using more than one row z1, z2, z3. Thus, there will be Ka, using z1a, z2a, z3a, with K -1  a Kb, using z1b, z2b, z3b, with K -1  b, and Kc, using z1c, z2c, z3c, with K -1  c. Ka, Kb and Kc may each be used to compute M. By comparing the resultant elements of M generated using Ka, Kb, and Kc, interference in one or more carrier components w1, w2, w3 may be detected. Errors may be corrected by majority voting the resultant elements of M. 
     Applying these relations to the case where m&gt;n&gt;2 would be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein. 
     Multiplexer/Demultiplexer Circuit 
     FIG. 2 shows a block diagram of the component-amplitude-division multiplexer and demultiplexer of an embodiment of the invention. 
     A carrier component generator 201 generates a plurality of carrier components 202 w1, w2. In a preferred embodiment part of each carrier component 202 w1, w2 is allocated to each emitter wavelength. Also, in a preferred embodiment, each carrier component 202 w1, w2 may comprise a sine wave, as follows: 
     
         w1=cos (2π f1 t)                                        (203) 
    
     
         w2=cos (2π f2 t)                                        (204) 
    
     where f1, f2 are frequencies 
     In a preferred embodiment, f1 and f2 are chosen such that interference from noise sources, such as ambient light and electromagnetic interference, is minimized. In a preferred embodiment, f1 and f2 are also chosen such that a bandwidth of about 4 Hz for the modulating effects m1, m2 is allowed. Frequencies in the range of about 10-50 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 w1, w2 must be sine waves. Other types of carrier components 202, such as square waves or other waveforms, 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. 
     It would also be clear to one of ordinary skill in the art, after perusal of the specification, drawings and claims herein, that the choice of m=n=2 is particular to measurement of blood oxygen, and that other choices of m, n would be workable, and are within the scope and spirit of the invention. For example, it may occur that other choices of m, n 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. 
     Each carrier component 202 w1, w2 is coupled by means of a phase delay 205 to a coefficient amplifier 206 for multiplying by a coefficient of the mixing matrix K, to produce a mixing product 207. The mixing products 207 are summed by a plurality of mixing summing circuits 208 to produce a plurality of carrier signals 209 α, β. This is the matrix multiplication shown in equation 113, 120. 
     Each carrier signal 209 α, β is coupled by means of a brightness amplifier 210, 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 hetrodyned, as is well known in the art, with the complex carrier components 202 w1, w2 to restore each of the modulated carrier components 202 w1, w2 to baseband. The detector input 109 is coupled to an input of each of a plurality of hetrodyne elements 211. A second input of each of the hetrodyne elements 211 is coupled to one of the carrier components 202 w1, w2, phase-shifted for a real or an imaginary part, as is well known in the art. The phase-shifted carrier components 202 w1, w2 are multiplied to produce a set of complex (real and imaginary) components 212 of each of the carrier components 202 w1, w2, as is well known in the art. 
     The complex components 212 are coupled to a baseband filter 213, which removes all components except complex baseband components 214. The complex baseband components 214 are coupled to a vector magnitude computer 215, which computes a vector magnitude 216 of the complex baseband components 214. 
     The vector magnitude 216 is coupled to an inverse coefficient amplifier 217 for multiplying by coefficients of the inverse mixing matrix K -1 , to produce an inverse mixing product 218. The inverse mixing products 218 are summed by a plurality of inverse mixing summing circuits 219 to produce the data output signals 110. This is the matrix multiplication shown in equation 121. 
     The data output signals 110 each indicate the product of the modulation effect for the corresponding carrier signal 209 w1, w2, as multiplied by a correction by the corresponding brightness amplifier 210. 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. 
     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.