Abstract:
A system is disclosed which first identifies a plurality of characteristics of a physiological signal any one of which may represent a physiological parameter. A plurality of different techniques are used to provide respective likelihood factors for each such identified characteristic. The resulting likelihood factors are then analyzed to select the one characteristic of the physiological signal which most likely represents the desired physiological parameter. The physiological parameter is then calculated from the selected characteristic of the physiological signal.

Description:
[0001]    This is a non-provisional application of provisional application serial No. 60/252,229 by J. Huang filed November 21, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to techniques for estimating a physiological parameter from a physiological signal. More specifically, the invention relates to detecting and estimating oximetry signals from physiological signals which include noise, and even more specifically to techniques for accurately determining the pulse rate from noisy physiological signals.  
         BACKGROUND OF THE INVENTION  
         [0003]    It is well known that physiological parameters (e.g. blood oxygen saturation and pulse rate) are represented by physiological signals, and that such signals often contain substantial noise components, often much larger than the physiological signal component. For example, the blood oxygen saturation (SpO 2 ) level in the blood stream may be determined by shining red and infrared (IR) light on a blood perfused part of a patient&#39;s body (e.g. finger or earlobe). The light passing through, or reflected off, the patient is detected and signals representing the received light are generated. These signals are then processed to generate both an indication of the pulse rate and the blood oxygen level of the patient.  
           [0004]    One problem with such systems is a noise component in the light representative signals. This noise component is substantially caused by movement by the patient, however electromagnetic interference from surrounding equipment, and reception of ambient light by the light sensors also contribute to the noise component.  
           [0005]    In some cases, this noise component can be substantially large, compared to the signal component. Systems were designed to detect the signal components in the light representative signals in the presence of a relatively large noise component.  
           [0006]    Recently, techniques using fast Fourier transforms (FFT) of the light representative signals have been developed. In U.S. Pat. No. 5,632,272, issued May 27, 1997 to Diab et al., data from an FFT of the light representative signals is analyzed to determine the arterial blood saturation. In this patent information from all the FFT frequencies above a threshold level is analyzed with equal weight.  
           [0007]    In U.S. Pat. No. 6,094,592, issued Jul. 25, 2000 to Yorkey et al., generates a ratio signal having a value corresponding to each frequency location in the FFT spectrum, then generates a histogram of the values of the ratio signal weighted by the magnitude of the IR FFT at the frequency associated with the ratio value.  
           [0008]    In all of these systems, the FFT signal was processed according to an algorithm and a pulse rate signal and SpO 2  signal generated. However, there are always clinical situations in which a particular algorithm will perform poorly, and conversely other clinical situations in which that algorithm will perform well. A system which can operate optimally over a range of different clinical situations is desirable.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    In accordance with principles of the present invention, a system first identifies a plurality of characteristics of a physiological signal any one of which may represent a physiological parameter. A plurality of different techniques are used to provide respective likelihood factors for each such identified characteristic. The resulting likelihood factors are then analyzed to select the one characteristic of the physiological signal which most likely represents the desired physiological parameter. The physiological parameter is then calculated based on the selected characteristic of the physiological signal.  
           [0010]    More specifically, a system according to principles of the present invention determines the parameter of pulse rate from SpO 2  physiological signals, which include red and IR light representative signals. The frequency locations of peaks in the spectrum of the IR light representative signal are detected as the characteristics. A plurality of different techniques each generate a likelihood factor for each identified peak, respectively. All of the likelihood factors are then analyzed to select one of the identified peaks as the characteristic most likely representing the actual pulse rate. The pulse rate parameter is calculated from the red and IR light representative signals at the frequency location of the selected peak. The red and IR signals at the frequency location of the selected peak may be further processed to generate a second parameter of the SpO 2  value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    In the drawing:  
         [0012]    [0012]FIG. 1 is a functional block diagram illustrating processing in accordance with principles of the present invention;  
         [0013]    [0013]FIG. 2 is a spectrum diagram illustrating an exemplary FFT derived from an IR signal;  
         [0014]    [0014]FIG. 3 a  through  d  are diagrams of probability distribution functions (PDFs) useful in understanding the operation of the spectra probability filtering circuit illustrated in FIG. 1;  
         [0015]    [0015]FIG. 4 a  is a functional block diagram illustrating processing for updating the PDFs as illustrated in FIG. 3, and FIG. 4 b  and FIG. 4 c  are PDFs useful in understanding the operation of the block diagram illustrated in FIG. 4 a;    
         [0016]    [0016]FIG. 5 is a block diagram of a window filter which may be used in the embodiment of the present invention illustrated in FIG. 1  
         [0017]    [0017]FIG. 6 a  is a more detailed block diagram of an IR half period shift circuit, and FIG. 6 b  and c are waveform diagrams useful in understanding the operation of the IR half period shift circuit illustrated in FIG. 6 a;    
         [0018]    [0018]FIG. 7 a  is a more detailed block diagram of an IR full period shift circuit, and FIG. 7 b  and  c  are waveform diagrams useful in understanding the operation of the IR full period shift circuit illustrated in FIG. 7 a ; and  
         [0019]    [0019]FIG. 8 a  is a more detailed block diagram of an IR/Red difference circuit, and FIG. 8 b  is a waveform diagram useful in understanding the operation of the IR/Red difference circuit illustrated in FIG. 8 a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 is a functional block diagram illustrating processing for determining the pulse rate and SpO2 values from physiological signals according to principles of the present invention. One skilled in the art will understand that this processing may be performed by dedicated hardware circuitry, or may be performed within a processor operating under control of a control program, or a combination of these techniques. The particular implementation technique is not germane to the present invention.  
         [0021]    In FIG. 1, an output terminal of a controller  102  is coupled to a light emitting device  104 . In a preferred embodiment, the light emitting device  104  is a set of light emitting diodes (LEDs), as indicated by the schematic symbol in block  104 . The light emitting device  104  includes at least one LED for emitting red light, and one LED for emitting infrared (IR) light. Other LEDs, emitting other color light, may also be included. Some portion of the light emitted by the light emitting device  104  is received by a light sensor device  106 , as illustrated by the light indicative line between the light emitting device  104  and the light sensor  106 . In a preferred embodiment, the light sensor is a phototransistor, as indicated by the schematic symbol in block  106 . The light sensor  106  receives the portion of the light from the light emitting device  104  and generates an electrical signal at an output terminal representing the intensity of the received light. The combination of the light emitting device  104  and the light sensor  106  form a blood oxygen concentration sensor  100  of known design.  
         [0022]    The output terminal of the light sensor  106  is coupled to an input terminal of a signal processor  108 . An output terminal of the signal processor  108  is coupled to a signal input terminal of a bandpass filter BPF  110 . An output terminal of the BPF  110  is coupled to an input terminal of a fast Fourier transform (FFT) circuit  112  and a first input terminal of a window filtering circuit  118 . An output terminal of the FFT circuit  112  is coupled to an input terminal of an IR spectral peak identification circuit  114 . An output terminal of the IR spectral peak identification circuit  114  is coupled to an input terminal of a spectra probability filtering circuit  116  and a second input terminal of the window filtering circuit  118 .  
         [0023]    An output terminal of the window filtering circuit  118  is coupled to respective input terminals of an IR half period shift circuit  122 , an IR full period shift circuit  124 , and an IR/red difference circuit  126 . An output terminal of the spectra probability filtering circuit  116  is coupled to an input terminal of a first weighting circuit  127 ( 1 ). An output terminal of the IR half period shift circuit  122  is coupled to an input terminal of a second weighting circuit  127 ( 2 ). An output terminal of the IR full period shift circuit  124  is coupled to an input terminal of a third weighting circuit  127 ( 3 ). An output terminal of the IR/red difference circuit  126  is coupled to an input terminal of a fourth weighting circuit  127 ( 4 ). The first, second, third and fourth weighting circuits ( 127 ( 1 ),  127 ( 2 ),  127 ( 3 ),  127 ( 4 )) form a weighting subsystem  127 .  
         [0024]    Respective output terminals of the first, second, third and fourth weighting circuits ( 127 ( 1 ),  127 ( 2 ),  127 ( 3 ),  127 ( 4 ) are coupled to corresponding input terminals of an arbitrator  128 . An output terminal of the arbitrator  128  is coupled to an input terminal of a pulse rate and SpO 2  calculating and display circuit  130 . An output terminal of the pulse rate and SpO 2  calculating and display circuit  130  is coupled to a second input terminal of the spectra probability filtering circuit  116 .  
         [0025]    In operation, the SpO 2  sensor  100 , including the light emitting device  104  and light sensor  106 , is placed adjacent a blood perfused portion of a patient&#39;s body, illustrated in FIG. 1 as  105 , such as a finger or ear lobe, in a known manner. The light representative signal produced by the light sensor  106  is relatively low power. The signal processor  108  receives the low power signal and generates a higher power signal in a known manner. In FIG. 1, the signal processor  108  generates two signals, one representing the intensity of red light received by the light sensor  106 , and one representing the intensity of IR light received by the light sensor  106 . These signals are sampled and converted to digital form by respective digital-to-analog converters (not shown) to generate respective digital signals, all in a known manner. These digital signals are processed in parallel by the remainder of the processing blocks illustrated in FIG. 1, unless explicitly described otherwise below.  
         [0026]    The light representative digital signals are first bandpass filtered by the BPF  110  to remove signal components which are not in the range of frequencies within which it is reasonable to expect a pulse frequency. This reduces out-of-band noise, and improves the signal-to-noise ratio of the filtered signal. One skilled in the art will understand how to adjust the passband of the BPF  110  in an appropriate manner.  
         [0027]    The filtered signals (red and IR) are then transformed to the frequency domain by the FFT circuit  112 . The FFT circuit  112  produces, in a known manner, successive FFT spectra. Each FFT spectrum consists of a set of  1024  complex values each having a magnitude representing the energy at a respective frequency location. FIG. 2 is a spectrum diagram illustrating an exemplary FFT derived from an IR signal including a relatively limited number of frequency locations. The horizontal direction in FIG. 2 represents frequency and the vertical direction represents magnitude. Each frequency location in the FFT is represented by a vertical rectangle having a horizontal location representing the frequency, and a height representing the magnitude of the FFT at that frequency. The numbers along the horizontal axis represent the pulse rate expressed in beats per minute (BPM). In the illustrated embodiment, a  1024  point FFT is calculated every 10 seconds for each of the red and IR digital signals from the signal processor  108 . Consequently, the sampling rate is around 100 samples per second. In the illustrated embodiment, therefore, each FFT consists of a set of  1024  complex values at frequency locations spaced every 0.01 Hz starting from DC to around 100 Hz. However, for the application of determining the pulse rate, only those frequencies from DC to around 5 Hz (300 BPM) are of interest.  
         [0028]    In function block  114 , magnitudes and frequency locations of peaks in the IR spectrum from the FFT circuit  112  are identified, in a known manner. First, the magnitudes in the FFT are normalized to a predetermined value. That is, the maximum magnitude in the FFT spectrum is set to the predetermined value, and the remainder scaled appropriately in a known manner. In FIG. 2, the maximum magnitude (illustrated at the lowest frequency location) is set to 1000, and the rest are scaled appropriately.  
         [0029]    Second, peaks are identified. In the illustrated embodiment, to be identified as a peak, the magnitude of an IR spectral location must (1) be higher than a predetermined threshold magnitude, and (2) be represented by a rise and fall of greater than some predetermined magnitude value on either side of the location. Each FFT spectral location satisfying these criteria is identified as a peak in a known manner. A list of magnitudes and spectral locations of the identified IR peaks is generated. In FIG. 2, peaks are identified at around 35 BPM, 46 BPM, 76 BPM, 140 BPM, and 160 BPM. It is further possible to limit the number of identified peaks for further processing. For example, in the illustrated embodiment only the 20 peaks with the largest magnitudes will be processed. In FIG. 2, identified peaks are illustrated by filled rectangles, while nonpeak frequency locations are illustrated by empty rectangles.  
         [0030]    In general, the FFT information at each spectral location identified as an IR peak, and possibly other information, as described below, is processed to determine a set of four likelihood factors LFs: one from each of the spectra probability filtering circuit  116  (LF( 1 )), the IR half period shift circuit  122  (LF( 2 )), the IR full period shift circuit  124  (LF( 3 )), and the IR/red difference circuit  126  (LF( 4 )). That is, a first set of four LFs (LF 1 ( 1 ), LF 1 ( 2 ), LF 1 ( 3 ), LF 1 ( 4 )) is generated for IR FFT peak 1, a second set of four LFs (LF 2 ( 1 ), LF 2 ( 2 ), LF 2 ( 3 ), LF 2 ( 4 )) is generated for IR FFT peak 2 and so forth. In the remainder of this application, the notation LF i (j) will refer to the likelihood factor LF for the i th  FFT peak from the j th  processing function.  
         [0031]    More specifically, referring again to FIG. 2, a first set of four LFs (LF 1 ) is produced by the spectra probability filtering circuit  116 , the IR half period shift circuit  122 , the IR full period shift circuit  124 , and the IR/red difference circuit  126 , for the 35 BPM spectral location, identified as a peak in the IR FFT, as described above. A second set of four LFs (LF 2 )is produced for the 46 BPM spectral location; a third set (LF 3 ) for the 76 BPM spectral location, and so on for the 140 BPM (LF 4 ), and 160 BPM (LF 5 )spectral locations.  
         [0032]    Each set of four LF i s (LF i ( 1 ), LF i ( 2 ), LF i ( 3 ), and LF i ( 4 )) is then weighted by the corresponding weighting circuit in the weighting subsystem  127 . That is, each LF i ( 1 ) is weighted by the weight W1 in weighting circuit  127 ( 1 ) to generate a corresponding weighted likelihood factor WLF i ( 1 ), each LF i ( 2 ) is weighted by the weight W2 in the weighting circuit  127 ( 2 ) to generate a corresponding weighted likelihood factor WLF i ( 2 ) and so forth. The result is a set of four WLF i s associated with each of the identified IR spectral peak locations (35 BPM, 46 BPM, 76 BPM, 140 BPM and 160 BPM).  
         [0033]    The arbitrator  128  processes all of the sets of WLF i s associated with all of the IR spectral peak locations, and selects one of the IR spectral peak locations as the frequency most likely to represent the pulse rate. One skilled in the art will understand that there are many different ways in which the WLFs may be processed to select the pulse rate representative frequency. For example, the arbitrator  128  may normalize the WLFs, identify the WLF having the highest value, and select the IR FFT peak associated with the identified WLF as representing the pulse rate. Alternatively, in a preferred embodiment, the arbitrator  128  forms a combination of the four WLFs associated with a respective IR FFT peak, then selects the IR FFT peak associated with the combination having the highest value as representing the pulse rate. However, the particular selection process used in the arbitrator  128  is not germane to the present invention.  
         [0034]    The pulse rate and SpO 2  calculating and display circuit  130  processes the FFT information at the frequency selected by the arbitrator  128  in a known manner to calculate the pulse rate, and the SpO 2  value corresponding to the selected pulse rate.  
         [0035]    In block  116  determination of likelihood factors LF i ( 1 ) is based on probabilistic filtering. A probability density function (PDF) of pulse rate frequencies is maintained based on previously identified pulse rates and SpO 2  values, as received from the SpO 2  calculating and display circuit  130 . The respective probabilities, running from zero to one, in this PDF at each of the frequency locations of the IR peaks identified in block  114  are the LFs for that IR peak. The PDF is then updated, all in a manner to be described in more detail below.  
         [0036]    [0036]FIG. 3 a  is a diagram of a PDF useful in understanding the operation of the spectra probability filtering circuit  116  illustrated in FIG. 1. In FIG. 3 a , a PDF  602  is illustrated by cross hatching. The horizontal axis represents pulse rate in beats per minute, and the vertical axis represents the probability from zero to one. The PDF  602  in FIG. 3 a  is illustrated in continuous form. However, one skilled in the art will understand that in the illustrated embodiment the PDF  602  is discrete, containing a set of N probability representative entries, each having a value ranging from zero to one, corresponding to the set of frequency locations in each of the FFTs from the FFT circuit  112 . As described above, in the illustrated embodiment, there are N=1024 frequency locations in the FFTs from the FFT circuit  112 , and consequently  1024  corresponding frequency locations in the PDF  602  illustrated in FIG. 3 a.    
         [0037]    In FIG. 3 a , three peaks from the IR FFT are illustrated: one  604  at 46 BPM, one  606  at 76 BPM and one  608  at 140 BPM. Each of these peaks is represented by a vertical line. The horizontal location of the line represents the frequency of the peak, and the height of the line is the probability of the PDF  602  at that frequency location. The line  604 , representing the peak at 46 BPM, has the probability P(46); the line  606 , representing the peak at  76  BPM, has the probability P(76); and the line  608 , representing the peak at 140 BPM, has the probability P(140). In FIG. 3, a first likelihood factor, LF i ( 1 ), associated with the 46 BPM frequency has a value of P(46), a second LF 2 ( 1 ), associated with the 76 BPM frequency, has a value of P(76), and a third LF 3 ( 1 ), associated with the 140 BPM frequency, has a value of P(140). As described above, after the LFs have been determined, the PDF  602  is updated.  
         [0038]    [0038]FIG. 4 a  is a functional block diagram illustrating how the PDF  602  of FIG. 3 a  is updated, and FIG. 4 b  and  c  are illustrations of PDFs useful in understanding the operation of FIG. 4 a . In FIG. 4 a , pulse rate values from the pulse rate and SpO 2  calculation and display circuit  130  (of FIG. 1) are entered into a list  610  of such values. The list is processed by a deviation measure calculating block  612 . An output from the deviation measure calculating block  612  is coupled to an input of a PDF parameter calculating circuit  614 . SpO 2  values from the pulse rate and SpO 2  calculation and display circuit  130  are supplied to another input of the parameter calculating circuit  614 . The parameter calculating circuit  614  calculates two parameters, A and B in a manner described in detail below. The parameters A and B are used to produce a new PDF  620 .  
         [0039]    In FIG. 4 a,  the list  610  of pulse rate values, containing M entries, is maintained. These pulse rate values are the final pulse rate values reported to the user and displayed on the display device by the pulse rate and SpO 2  calculation and display circuit  130 . As each new pulse rate value is received from the pulse rate and SpO 2  calculation and display circuit  130 , the oldest value is discarded from the list  610  and replaced with the new value.  
         [0040]    A measure of the deviation Dev, or spread, of these M pulse rate values is calculated in block  612 . To maintain statistical accuracy there is a minimum value for M, which in the illustrated embodiment is around 10. To minimize computation time, there is also a maximum value for M, which in the illustrated embodiment is around 30. The deviation Dev may be represented by the standard deviation in a normal manner. That is, a mean {overscore (p)} is first calculated from the list  610  of pulse rate values, then the standard deviation from that mean value {overscore (p)} is calculated, both in the known manner. Alternatively, the deviation of the pulse rate values in the list  610  from the latest pulse rate value {overscore (p)} from the pulse rate and SpO 2  calculation and display circuit  130  may be calculated as  
         Dev   =       1     M   -   1              ∑     i   =   1     M                     (       p   i     -     p   ^       )           ,                         
 
         [0041]    where p i  represents the i th  entry in the list  610 .  
         [0042]    For example, if the IR peak  606  at 76 BPM was selected by the pulse rate and SpO 2  calculation and display circuit  130  in the previous iteration as representing the pulse rate {circumflex over (p)}, then the deviation of the pulse rate values in the list  610  around the frequency location {circumflex over (p)} representing 76 BPM is calculated in block  612 .  
         [0043]    A low Dev value represents a situation in which the SpO 2  signals are clean and relatively noise free, and in which no external factors, such as patient movement, are degrading pulse rate and SpO 2  readings. The readings in this case are relatively stable, and have a relatively small spread in frequency. A high Dev value represents a situation in which the SpO 2  signals are noisy, or in which patient movement is degrading the pulse rate and SpO 2  readings. The readings in this situation can change rapidly and erratically. This situation can also indicate that the pulse rate is changing relatively rapidly.  
         [0044]    The deviation measure Dev, and the SpO 2  value are then used to produce values A and B necessary to generate a new PDF  620 , as illustrated in FIG. 4 b  and  c.  The new PDF  620  has a height, represented by the parameter A, and a width, represented by the parameter B. To simplify calculations, in the illustrated embodiment the new PDF is triangular in shape, though one skilled in the art will understand that other PDF shapes may be used as well, such as the known Gaussian shape. The parameter A is calculated as  
       A   ∝       SpO   2     Dev                           
 
         [0045]    The parameter B is calculated as B∝Dev. Thus, the more stable the pulse rate signals, indicated by a relatively low Dev value, the higher and narrower the new PDF, as illustrated in PDF  620 . This represents the higher probability that the pulse rate is within a relatively narrow frequency range. This is illustrated in FIG. 4 b  in which the new PDF  620  is relatively tall and narrow. Conversely, the more erratic and unstable the pulse rate signals, indicated by a relatively high Dev value, the lower and wider the new PDF, as illustrated in PDF  620 ′. This represents the lower probability of the pulse rate being at any particular frequency, but instead that the pulse rate is somewhere within a relatively wide range of frequencies. This is illustrated in FIG. 4 c  in which the new PDF  620 ′ is relatively short and wide.  
         [0046]    The PDF  602  is then updated using this new PDF  620 . In the illustrated embodiment the relatively tall and narrow PDF  620 , as illustrated in FIG. 4 b , is used. Referring back to FIG. 3 b , the new PDF  620  is inserted into the preexisting PDF  602  of FIG. 3 a . The new PDF  620  is centered at the frequency selected by the pulse rate and SpO 2  calculation and display circuit  130 . In the illustrated embodiment this is the frequency location corresponding to a pulse rate of 76 BPM. The resulting PDF  602 ′ is illustrated in FIG. 3 c.    
         [0047]    The magnitude of the PDF  602 ′ of FIG. 3 c  is then reduced by a factor  
       f   ∝       1   Dev     .                           
 
         [0048]    As described above, a low Dev value indicates relatively clean signals and stable pulse rate, while a high Dev value indicates relatively noisy signals, or erratic and/or rapidly changing pulse rate. The lower the Dev value, indicating clean signals and stable pulse rate, the higher the factor f and the more the PDF is reduced. This results in a narrow PDF which loses much of its information with each iteration, enabling the spectra probability filter  116  to follow changes in the pulse rate more easily. The higher the Dev value, indicating noisy signals or erratic pulse rate, the lower the factor f, and the less the PDF is reduced. This results in a wider PDF which retains most of its information from iteration to iteration. FIG. 3 d  illustrates the resulting PDF  602 ″, which is then used to determine LFs for the next set of IR peak values from the IR peak identification circuit  114 , as illustrated in FIG. 3 a.    
         [0049]    In function block  118 , for each IR peak identified, the red and IR signals are filtered by a window filter having a relatively narrow passband surrounding the frequency location F of that peak. Any of the number of known implementations of window filters may be used. For example, referring to FIG. 2, the FFT value at the frequency location F, and FFT values for a predetermined number of surrounding frequency locations are selected, while the FFT values for all the other frequency locations are ignored. An inverse Fourier transform of the selected FFT frequency values is then performed to generate respective ten second, window filtered, time domain signals representing the red and IR signals in the frequency neighborhood of the selected peak.  
         [0050]    [0050]FIG. 5 illustrates an alternative, preferred implementation of a window filter  118 . The illustrated window filter  118  may be used to filter both the red and IR light representative signals. In FIG. 5, the bandpass filtered light representative signal from the bandpass filter  110  is coupled to respective input terminals of a plurality  702  of n bandpass filters. Respective output terminals of the plurality  702  of bandpass filters are coupled to corresponding input terminals of a multiplexer  704 . An output terminal of the multiplexer  704  is coupled to the processing circuitry: IR half period shift circuit  122 , the IR full period shift circuit  124  and the IR/red difference circuit  126 . A signal representing the frequency of the IR peak currently being processed from the IR spectral peak identification circuit  114  is coupled to an input terminal of a control circuit  706 . An output terminal of the control circuit  706  is coupled to a control input terminal of the multiplexer  704 .  
         [0051]    In operation, each one of the plurality  702  of bandpass filters has a relatively narrow passband, and the plurality  702  have respective center frequencies selected so that the entire frequency range of interest (i.e. DC to around 5 Hz) is covered. In the illustrated embodiment, there are nine bandpass filters  702 ( 1 ) to  702 ( 9 ). All of the bandpass filters  702  have a passband of around 1 Hz. The first bandpass filter  702 ( 1 ) has a center frequency of around 0.5 Hz and a passband of DC to around 1 Hz. The second bandpass filter  702 ( 2 ) has a center frequency of around 1 Hz and a passband of from around 0.5 Hz to around 1.5 Hz, and so forth. The ninth bandpass filter  702 ( 9 ) has a center frequency of around 4.5 Hz and a passband of from around 4 Hz to around 5 Hz. The respective passbands of the plurality  702  of bandpass filters, therefore, overlap.  
         [0052]    The control circuit  706  receives a signal representing the frequency of the IR peak currently being processed. The control circuit  706  then selects the bandpass filter  702  having a center frequency which is closest to the IR peak frequency, and conditions the multiplexer  704  to couple the selected bandpass filter  702  to its output terminal, and, thus, to the processing circuitry IR half period shift circuit  122 , the IR full period shift circuit  124  and the IR/red difference circuit  126 .  
         [0053]    [0053]FIG. 6 a  is a more detailed block diagram of an IR half period shift circuit  122 , and FIG. 6 b  and  c  are waveform diagrams useful in understanding the operation of the IR half period shift circuit illustrated in FIG. 6 a . In FIG. 6 a , the window filtered IR signal from the window filtering circuit  118  (of FIG. 1) is coupled to an input terminal of a delay circuit  302  and a first input terminal of an adder  304 . An output terminal of the delay circuit  302  is coupled to a second input terminal of the adder  304 . An output terminal of the adder  304  is coupled to the second weighting circuit  127 ( 2 ) (of FIG. 1).  
         [0054]    A pulse oxymetry IR signal component (i.e. without a noise component) of the window filtered oximetry signal at or near the actual frequency of the pulse rate is symmetrical in each period. That is, each pulse waveform is similar in form to a sine wave, with equal height peaks and troughs spaced equally in time. Consequently, provided the heart rate remains constant, a pulse oxymetry signal component waveform shifted one half cycle will appear to be an inverted version of the unshifted waveform. Further, this remains true for any shift of n+½ cycles, where n is an integer. The sum of the shifted and unshifted signal, therefore, will be substantially close to zero.  
         [0055]    In the presence of noise, the cancellation will be incomplete. However, only the signal represented by the FFT peak at or near the frequency of the actual pulse rate contains a pulse oxymetry signal component in addition to the noise component. For this peak, the pulse rate component will be substantially canceled. For all other peaks, there is no such component to be canceled and the resulting signals from those peaks will tend to remain greater than those from the peak containing the signal component.  
         [0056]    In FIG. 6, in function block  122  operates separately on each peak identified in the IR FFT spectrum by the peak identification block  114  (of FIG. 1). For each peak, a cycle period T is calculated as 1/F, where F is the frequency location of the peak in Hertz. A version of the window filtered time domain signal for that peak, delayed by T/2, is then generated by the delay circuit  302 . This delayed signal is added to the original window filtered time domain signal by the adder  304 , which operates as an accumulator. The result of the accumulation by adder  304  is the LF i ( 2 ) for this frequency, and is supplied to the weighting circuit  127 ( 2 ). The weight W2 applied by the weighting circuit  127 ( 2 ) is a negative weight, consequently giving a higher weight (less negative) to the signal with the lowest accumulation, on the assumption that that signal is most likely to have had a signal component canceled, as described above.  
         [0057]    [0057]FIG. 6 b  illustrates an IR waveform for the FFT spectral peak at the frequency location corresponding to 76 BPM (F≈1.25 Hz). This waveform has been window filtered with a passband from 1 Hz to 2 Hz. The cycle period for this waveform is T=1/F≈0.8 seconds. The window filtered waveform is illustrated as a darker line  202 , and represents the signal from the window filter  118 . The delayed signal is illustrated by a lighter line  204  and is the waveform  202  delayed by a time T/2. When the delayed signal  204  is added to the undelayed signal  202 , the result will be relatively close to zero, especially in the time areas  210  and  212 , where the noise component is low and the delayed signal  204  more closely approximates the inverse of the undelayed signal  202 . An accumulated summation of these two signals, representing the LF for this IR peak frequency, will be relatively small.  
         [0058]    [0058]FIG. 6 c  illustrates an IR waveform for the FFT spectral peak at the frequency location corresponding to 46 BPM (F′≈0.75 Hz). This waveform has been window filtered having a passband from 0.5 Hz to 1.5 Hz. The cycle period for this waveform is T′=1/F′≈1.3 seconds. The window filtered waveform is illustrated as a darker line  206 , and the delayed signal is illustrated by a lighter line  208  delayed by a time T′/2. When the delayed signal  208  is added to the undelayed signal  206 , the result is not as close to zero because the delayed signal  208  does not approximate the inverse of the undelayed signal  206 . The accumulated summation of these two signals will be relatively large, or at least larger than that for the signals illustrated in FIG. 6 b.    
         [0059]    An accumulation is performed for each IR FFT spectral peak identified by block  114  (of FIG. 1) and the LF( 2 ) for each peak is saved along with the frequency and magnitude of that peak. The LF( 2 )s for all the identified IR FFT peaks are then supplied to the second weighting circuit  127 ( 2 ). As described above, the cancellation effect of the pulse oxymetric signal will occur at all half period shift times. That is for (n+{fraction (1/2)})T, where n is an integer. Thus, it is possible to repeat the accumulation process for more than one half period shift, e.g. for T/2, 3T/2, 5T/2 etc. The total accumulation for all such half period shift times then forms the LF( 2 ) for this frequency peak, and is weighted as described above.  
         [0060]    [0060]FIG. 7 a  is a more detailed block diagram of an IR full period shift circuit  124 , and FIG. 7 b  and  c  are waveform diagrams useful in understanding the operation of the IR full period shift circuit illustrated in FIG. 7 a . In FIG. 7 a , the window filtered IR signal from the window filtering circuit  118  (of FIG. 1) is coupled to an input terminal of a delay circuit  402  and a first input terminal of an subtractor  404 . An output terminal of the delay circuit  402  is coupled to a second input terminal of the subtractor  404 . An output terminal of the subtractor  404  is coupled to the third weighting circuit  127 ( 3 ) (of FIG. 1).  
         [0061]    A pulse oxymetry signal component of the window filtered oximetry IR signal at or near the actual frequency of the pulse rate is cyclical with a periodicity related to the actual pulse rate. As described above, each pulse waveform of the signal component is similar in form to a sine wave, with equal height peaks and troughs spaced equally in time. This signal repeats with every heart beat. Consequently, provided the pulse rate remains constant, a pulse oxymetry signal component waveform shifted one cycle will appear to be the same as the unshifted waveform. Further, this remains true for any shift of n cycles, where n is an integer. The difference between the shifted and unshifted signal, therefore, will be substantially close to zero.  
         [0062]    As before, in the presence of noise the cancellation will be incomplete. However, only the signal representing the IR FFT peak at or near the frequency of the actual pulse rate contains a pulse oxymetry signal component in addition to the noise component. For this peak, the pulse rate component will be substantially canceled. For all other peaks, no such component will be canceled and the resulting signals from those peaks will tend to remain greater than those from the peak containing the signal component.  
         [0063]    In FIG. 7 a , in a similar manner to function block  122  in FIG. 6 a , the function block  124  also operates separately on each peak identified in the IR FFT spectrum by the peak identification block  114  (of FIG. 1). A version of the window filtered signal for that peak, delayed by the previously calculated cycle period T, is generated by the delay circuit  402 . This delayed signal is subtracted from the original window filtered time domain signal by the subtractor  404 , operating as an accumulator. The result of the accumulation by subtractor  404  is the LF( 3 ) associated with that IR FFT peak, and is supplied to the weighting circuit  127 ( 3 ). The weight W3 applied by the weighting circuit  127 ( 3 ) is also a negative weight, giving a higher weight (less negative) to the signal with the lowest accumulation, on the assumption that that signal is most likely to have had a signal component canceled, as described above.  
         [0064]    [0064]FIG. 7 b  corresponds to FIG. 6 b  and illustrates the IR waveform for the FFT spectral peak at the frequency location corresponding to 76 BPM (F≈1.25 Hz). As described above, this waveform is illustrated as a darker line  202 , has been window filtered having a passband from 1 Hz to 2 Hz, and has a cycle period of T=1/F≈0.8 seconds. The delayed signal, delayed by a time T, is illustrated by a lighter line  220 . When the delayed signal  220  is subtracted from the undelayed signal  202 , the result will be relatively close to zero, especially in the time area  224 , where the noise component is relatively low and the delayed signal  220  more closely approximates the undelayed signal  202 . An accumulated difference of these two signals will be relatively small.  
         [0065]    [0065]FIG. 7 c  corresponds to FIG. 6 c  and illustrates the waveform for the IR FFT spectral peak at the frequency location corresponding to 46 BPM. As described above, this waveform has been window filtered having a passband from 0.5 Hz to 1.5 Hz, has a cycle period of T′=1/F′ 1.3 seconds, and is illustrated as a darker line  206 . The delayed signal, delayed by a time T′, is illustrated by a lighter line  222 . When the delayed signal  222  is subtracted from the undelayed signal  206 , the result is not close to zero because the delayed signal does not approximate the undelayed signal. The accumulated difference of these two signals will be relatively large, or at least larger than that for the signals illustrated in FIG. 7 b.    
         [0066]    As described above, the cancellation effect of the pulse oxymetric signal will occur at all full period shift times. That is for nT, where n is an integer. Thus, it is possible to repeat the accumulation process for more than one full period shift, e.g. for T, 2T, 3T etc. The total difference accumulation for all such full period shift times is the LF( 3 ) for this frequency, and is weighted as described above.  
         [0067]    [0067]FIG. 8 a  is a more detailed block diagram of an IR/Red difference circuit  126 , and FIG. 8 b  and FIG. 8 c  are waveform diagrams useful in understanding the operation of the IR/Red difference circuit illustrated in FIG. 8 a . In FIG. 8 a , a window filtered red signal (R) and the window filtered IR signal from the window filtering circuit are coupled to respective input terminals of a subtractor  504  via a scaling circuit  506 . An output terminal of the subtractor  504  is coupled to the fourth weighting circuit  127 ( 4 ).  
         [0068]    The pulse oxymetry signal component of the window filtered IR oximetry signal at or near the actual frequency of the pulse rate is correlated with the signal component of the window filtered red oximetry signal at the same frequency. That is, the IR and red window filtered pulse oxymetry signal components have substantially congruent waveforms. Thus, the difference between the window filtered red oximetry signal component and the window filtered IR oximetry signal will be relatively close to zero.  
         [0069]    As before, in the presence of noise the correlation will be corrupted. However, only the red and IR signals represented by the respective FFT peaks at or near the frequency of the actual pulse rate contain a pulse oxymetry signal component in addition to the noise component. For these peaks, the pulse rate components will be substantially canceled from the difference. For all other peaks, no such component will be canceled and the resulting difference from those peaks will tend to remain greater than that from the peak containing the signal component.  
         [0070]    In FIG. 8, in a similar manner to function blocks  122  of FIG. 6 and  124  of FIG. 7, the function block  126  also operates separately on each peak identified in the IR FFT spectrum by the peak identification block  114  (of FIG. 1). The window filtered red signal R is subtracted from the window filtered IR signal by the subtractor  504 , operating as an accumulator. One skilled in the art will understand that to minimize skew, and to maximize cancellation, the window filtered red and IR signals must be scaled so that they are approximately of equal magnitudes. The scaling circuit  506  is, thus, conditioned to scale the red and IR signals so that they have equal peak magnitudes. The result of the accumulation by subtractor  504  is the LF( 4 ) for the IR FFT peak, and is supplied to the fourth weighting circuit  127 ( 4 ). The weight W4 applied by the weighting circuit  127 ( 4 ) is also a negative weight, giving a higher weight (less negative) to the signal with the lowest accumulation, on the assumption that that signal is most likely to represent the FFT spectral peak having the highest correlation between the red R signal component and the IR signal component.  
         [0071]    [0071]FIG. 8 b  corresponds to FIG. 6 b  and FIG. 7 b , and illustrates respective IR and red waveforms for the FFT spectral peak at the frequency location corresponding to 76 BPM. The waveform representing the IR signal is illustrated as a darker line  202 , and the waveform representing the red signal is illustrated by a lighter line  230 . Because the IR  202  and red  230  signal waveforms are relatively congruent, indicative of a low noise component level and a high level of correlation, when the red signal  230  is subtracted from the IR signal  202 , the result will be relatively close to zero. An accumulated difference of these two signals will be relatively small.  
         [0072]    [0072]FIG. 8 c  corresponds to FIG. 6 c  and FIG. 7 c  and illustrates a respective red and IR waveforms for the IR FFT spectral peak at the frequency location corresponding to 46 BPM. The waveform representing the IR signal is illustrated as a darker line  206 , and the waveform representing the red signal is illustrated by a lighter line  232 . Because the IR  206  and red  232  signal waveforms are not relatively congruent, indicative of a high noise component level and a low level of correlation, when the red signal  232  is subtracted from the IR signal  206 , the result will not be relatively close to zero. The accumulated difference of these two signals will be relatively large, or at least larger than that for the signals illustrated in FIG. 8 b.    
         [0073]    One skilled in the art will understand that, as described above with reference to the full period shift circuit  124 , the red and IR pulse oximetry signal components are repetitive over the full period. The correlation effect of the red and IR pulse oxymetric signal components, thus, will also occur when the red and IR signals are time shifted by full period time intervals relative to each other (not shown to simplify the figure). Thus, it is possible to repeat the accumulation process for one or more such full period shifts, e.g. for T, 2T, 3T etc. The total difference accumulation for all such full period shift times is the LF( 4 ) for that IR FFT peak, and is weighted as described above.  
         [0074]    It is known that LFs from some of the processing circuitry, (i.e. some of the spectra probability filtering circuit  116 , the IR half period shift circuit  122 , the full period shift circuit  124  and the IR/red difference circuit  126 ) are more accurate than those from others. The weighting subsystem  127  gives more weight to that processing which is more accurate and less to that processing which is less accurate. This weighting may be different for different clinical situations.  
                                       TABLE 1                                   Weight   W1   W2   W3   W4                           Adult   1   −2       0   −1           Neonate (low noise)   1   −1   −1   −1           Neonate (high noise)   2   −1   −2   −1                      
 
         [0075]    For example, Table 1 above illustrates a set of weighting functions for three clinical situations: an adult; a neonate in which noise (due, for example, to movement of the patient) is low, and a neonate in which noise is high. As described above, the weights W2, W3 and W4 are all negative to make up for corresponding likelihood factors for which lower values represent higher likelihoods. Though the weights in Table 1 are illustrated as integers, one skilled in the art will understand that the weights may be expressed in real numbers as well. One skilled in the art will also understand that different sets of weights may exist for more that just the clinical situations illustrated in Table 1. For example, further classifications may be made by sex, weight, age, health, etc. One skilled in the art will further understand that the weights could be varied dynamically. The weighting subsystem  127  produces one set of six weighted likelihood factors WLF i s (WLF i ( 1 ), WLF i ( 2 ), WLF i ( 3 ) and WLF i ( 4 )) for each peak i identified by the IR spectral peak identification circuit  114  (of FIG. 1).  
                                                                 TABLE 2                                   WLF (1)   WLF (2)   WLF (3)   WLF (4)   FLF                                    P1-35 BPM   WLF 1  (1)   WLF 1  (2)   WLF 1  (3)   WLF 1  (4)   ΣWLF (l)       P2-46 BPM   WLF 2  (1)   WLF 2  (2)   WLF 2  (3)   WLF 2  (4)   ΣWLF (2)       P3-76 BPM   WLF 3  (1)   WLF 3  (2)   WLF 3  (3)   WLF 3  (4)   ΣWLF (3)       P4-140 BPM   WLF 4  (1)   WLF 4  (2)   WLF 4  (3)   WLF 4  (4)   ΣWLF (4)       P5-160 BPM   WLF 5  (1)   WLF 5  (2)   WLF 5  (3)   WLF 5  (4)   ΣWLF (5)                  
 
         [0076]    The arbitrator  128  receives all of the weighted likelihood factors WLFs from the weighting subsystem  127  and selects one of the IR FFT peak frequencies in response. Table 2 illustrates all of the WLFs generated for the IR FFT peaks illustrated in FIG. 2. In the illustrated embodiment, the WLF i s for each peak i are summed, and the resulting sum is the final likelihood factor FLF i  for that peak i. The peak having the highest FLF is selected at the peak representing the actual pulse rate by the arbitrator  128 . One skilled in the art will understand that other methods of selecting one of the identified IR FFT peaks may be used.  
         [0077]    The pulse rate and SpO 2  calculating and display circuit  130  calculates the pulse rate PLS from the frequency of the IR FFT peak selected by the arbitrator  128  as  
       PLS   =         freq        (     IR                 FFT                 peak     )       60     .                           
 
         [0078]    The FFT magnitude values of the red and IR FFTs at the frequency location of the peak selected by the arbitrator  128 , and at the dc frequency location, are then used to calculate the SpO 2  value in a known manner. These calculated values are then displayed on the display device.  
         [0079]    Although the SpO 2  system has been described above generally in terms of hardware circuitry, one skilled in the art would understand that once multibit digital signals representing the red and IR light signals are generated by the signal processor  108 , a processor, such as a microprocessor or digital signal processor, could be programmed to perform the functions illustrated in FIG. 1 and described in detail above. Such a processor could then control a display device to display the results of this processing.