Abstract:
A detector signal in a pulse oximeter is prefiltered, prior to processing so as to determine a physiological parameter value, so as to reduce the effects of artifact in the signal. An exemplary prefilter multiplies signal amplitude values by a quantitude that is inversely proportional to a substantially current amplitude so as to reduce the amplitude of large amplitude sections typically associated with artifact. In one implementation, the quantity is a ratio of an amplitude associated with a clean section of the signal by some exponential factor of a substantially current amplitude. In this manner, artifact affected portions of the signal are deemphasized in subsequent analyses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 60/694,767 entitled “SYSTEM FOR PREFILTERING A PLETHYSMOGRAPHIC SIGNAL”, filed on Jun. 28, 2005, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to photoplethysmography and, in particular, to pre-filtering a photoplethysmographic signal so as to reduce the effects of artifact in the signal. 
     BACKGROUND OF THE INVENTION 
     Pulse oximeters determine an oxygen saturation level of a patient&#39;s blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from a patient&#39;s tissue. In particular, pulse oximeters generally include a probe for attaching to a patient&#39;s appendage such as a finger, earlobe or nasal septum, or another location, particularly in the case of reflective oximeters. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient&#39;s tissue. 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 and analyzing signal values for each of the wavelengths at different portions of the patient&#39;s pulse cycle, information can be obtained regarding blood oxygen saturation and/or other parameter values such as pulse rate, or blood pressure/blood volume related values. 
     The algorithms for determining blood oxygen saturation related values are normally implemented in a digital processing unit. Accordingly, one or more analog-to-digital (A/D) converters are generally interposed between the detector and the digital processing unit. Additionally, the detector signal is generally demodulated and demultiplexed by signal processing components. Demodulation involves separating the physiological signal of interest (generally including a more rapidly changing AC portion including a plethysmographic waveform and an optically based “DC” offset due to slowly changing absorption values associated with non-pulsatile tissue absorption) from a carrier waveform associated with the flashing optical sources. Demultiplexing involves separating the different wavelength components associated with the different signal sources. That is, because blood oxygen saturation is calculated based on differential absorption values for different transmitted optical signal wavelengths, the detected signal is generally separated, or demultiplexed, into at least two different wavelength components. Typically, demodulation and demultiplexing have been implemented in analog circuitry operatively disposed between the optical signal detector and the analog-to-digital converter(s), but can be digitally implemented. 
     A persistent problem in the field of pulse oximetry is eliminating or otherwise accounting for noise and other artifact that can easily obscure or interfere with the pulsatile signals of interest. Some of the sources of this artifact include power line noise, electrical noise from other medical equipment, and artifact associated with patient motion. In this regard, certain filtering techniques have been employed both on the front end (i.e., in the analog circuitry between the optical signal detector and the analog-to-digital converter or converters) and in the back end (i.e., in the digital domain based on the signal from the analog-to-digital converter or converters) of the signal processing components. 
     Such front end filtering is generally used to filter the modulation signal as opposed to the physiological signal of interest. In this regard, the modulation signal may be approximately in the form of a square wave whereas the physiological signal of interest, which is carried by the modulation signal, may be in the form of a plethysmographic waveform. The front end filtering may include high pass and low pass filtering. For example, a low pass filter may be used to reject certain high frequency electronic noise and a high pass filter may be used to exclude certain low frequency phenomenon. Thus, such front end filtering is generally used to pass a broad frequency range including the modulation frequency or frequencies and is not directed to targeted elimination of interference with respect to the AC portion of the signal. 
     Back end filtering is sometimes used to filter noise from the physiological signal of interest. This often involves frequency dependent filtering such as bandpass filtering. Unfortunately, some sources of artifact can include frequency components within the physiological range of interest. For example, motion artifact may be observed within the physiological range of interest. With regard to motion, a number of different digital filtering or other compensation algorithms have been proposed or implemented with varying degrees of success. However, in some cases, these algorithms may either fail to satisfactorily address the effects of motion artifact or may filter out useful pulsatile information to an undesirable extent. 
     Other approaches to addressing artifact involve deemphasizing or excluding parameter calculations deemed to be based on data that is significantly affected by motion or other artifact. For example, presumed high artifact conditions have been identified based on an analysis of a spectrum of the detector signal to identify spectral characteristics indicative of artifact or the absence of spectral characteristics indicative of a well-defined pulsatile signal. In other cases, presumed high artifact conditions have been identified based on a result of calculations deemed unlikely to have a physiological basis, e.g., calculated values corresponding to an unlikely value of arterial oxygen saturation, an abrupt change thereof, or an unlikely variance from a trend in data related to oxygen saturation with respect to a time window under consideration. 
     Upon identification of such artifact conditions, associated calculated values may be ignored for purposes of determining a result or may be deweighted, for example, by increasing the size of a time window of data used for calculations (thereby presumably reducing the impact the motion affected data on the result) or by applying confidence or weighting factors to each of a series of calculated values used in obtaining a resulting value, so as to achieve a kind of weighted average wherein motion affected data is deemphasized. However, such approaches have had limited success in addressing a variety of motion conditions. Moreover, in some cases, such approaches have required difficult or questionable judgments in distinguishing different motion conditions, have required complicated processing and/or have limited the methodologies available for physiological parameter calculations. 
     SUMMARY OF THE INVENTION 
     The present invention relates to prefiltering a detector signal in a medical instrument, such as a pulse oximeter, so as to deemphasize artifact, such as motion artifact, in the signal prior to performing calculations to determine physiological information based on the signal. In this manner, the calculations can proceed, across a range of artifact conditions, without requiring selection between alternative calculations or the results thereof, without associated judgments as to characterization of the motion environment, and without variations or dynamic corrections to the algorithms. Moreover, short intervals of well-defined physiological signals can be effectively utilized to provide meaningful results. Such prefiltering also allows for effective implementation of a variety of calculation processes, including processes executed with respect to time or frequency domain data, over a range of artifact conditions. Such calculations can be implemented substantially free of additional processing for motion correction or compensation. 
     The present inventor has recognized, in the context of pulse oximetry, that motion is often episodic and has the transient effect of increasing the amplitude of the AC signal, often to the point that the signal of interest is practically obscured. That is, during motion episodes, the AC signal may be viewed as being composed of the superimposition of an artifact signal on the pulsatile signal, resulting in an integrated signal tending to have peaks of greater amplitude (e.g., associated with intervals of constructive interference between the components) than the peaks associated with intervals of substantially clean pulsatile signals having little or no artifact. In connection with certain signal processing techniques, e.g., certain spectral domain analyses, the noisy portions of the signal tend to be emphasized in relation to cleaner portions, which is typically undesirable. This effect is reversed, in accordance with the present invention, by implementing a prefiltering process to deemphasize signal portions believed to be affected by artifact prior to processing to obtain physiological information regarding a patient. 
     In accordance with one aspect of the present invention, a method and apparatus (collectively “utility”) is provided for reducing the amplitude of a portion of a pulse oximeter signal deemed to be affected by artifact. Specifically, the utility involves receiving an electronic signal reflective of one or more optical signals incident on a detector of a pulse oximeter where the electronic signal includes a first temporal portion that is potentially corrupted by artifact. For example, the electronic signal may be an AC signal corresponding to one of the red and infrared channels of a pulse oximeter. The signal is processed such that an amplitude associated with the first temporal portion of the signal is reduced in relation to that of a second temporal portion of the signal. The resulting processed signal can then be used to obtain physiological information regarding the patient, such as a pulse rate or SpO 2  value. In one implementation, the noted processing is implemented by a prefilter which operates substantially continuously on the electronic signal such that it is unnecessary to explicitly identify the artifact affected portion of the signal. Alternatively, such a filter may operate intermittently based on identification of specified artifact conditions, such as signal portions exceeding an amplitude threshold or other threshold believed to be related to artifact. The noted utility thus provides a prefiltered signal where the effects of artifact are reduced. This signal can be processed in conventional fashion to obtain the desired physiological information with reduced impact due to artifact. 
     In accordance with another aspect of the present invention, a utility is provided for identifying an artifact condition based on the amplitude of an oximeter signal. The utility involves receiving an electronic signal reflective of one or more optical signals incident on a detector of a pulse oximeter and identifying, based on an amplitude related analysis of the received signal, an artifact affected portion of the signal. In this regard, an artifact condition may be identified based on an increased amplitude of an AC portion of the signal. For example, the artifact condition may be identified based on an amplitude exceeding a predetermined threshold, a change in the amplitude over a time period exceeding a predetermined threshold or another amplitude-related analysis. Upon identification of the artifact condition, a correction is applied to reduce an effect of the artifact. In this regard, various processes have been developed for reducing the effect of artifact including bandpass filtering around the pulsatile fundamental frequency, motion cancellation, and motion correction factors applied to physiological parameter calculation. Any such correction or combinations thereof may be applied in accordance with the present invention. Alternatively, the noted identification and correction may be implemented by a prefilter, as described above, that operates to reduce the amplitude of the artifact affected portion of the signal. 
     Any process that has the effect of reducing an amplitude of the artifact affected portion of the signal in relation to a clean portion of the signal may be used in this regard. For example, only the amplitude of the clean portion of the signal may be increased, only the amplitude of the artifact affected portion of the signal may be reduced, or the amplitude of the clean portion may be increased while that of the artifact affected portion is reduced. Alternatively, the amplitude of the artifact affected portion may be increased by a lesser factor than that of the clean portion or the amplitude of the artifact affected portion may be reduced by a greater factor than that of the clean portion. The amplitude of the artifact affected portion may be processed to achieve the effect of multiplying the amplitude by a factor of less than 1.0 (e.g., multiplying by a value less than 1.0 or dividing by a value greater than 1.0). In this regard, an amplitude value of the artifact affected portion may be processed to achieve the effect multiplying the amplitude value by a factor that is inversely proportional to an amplitude related value of the artifact affected signal portion or a multiple or exponential value thereof. As a further alternative, the clean and artifact affected portions of the signal may be processed to achieve the effect of multiplying by a quantity including a ratio of some multiple or exponential factor of a reference amplitude (e.g., associated with a clean signal portion) by some multiple or exponential factor of a substantially current amplitude. 
     In accordance with a still further aspect of the present invention, a utility is provided for prefiltering an oximeter signal prior to spectral processing. The utility involves receiving a time-based signal reflective of one or more optical signals incident on a detector of a pulse oximeter and performing a correction with respect to a portion of the received time-based signal to reduce an effect of artifact. For example, the signal may be prefiltered as discussed above to reduce the amplitude of an artifact affected portion of the signal in relation to relatively clean signal portions. A transform is then performed on the processed time-based signal to obtain transformed information relative to a second domain. In this regard, the processed, e.g., prefiltered, signal may be subjected to a Fourier transform such as an FFT to yield a frequency domain signal. The transformed information may include the frequency domain signal or parameters thereof such as peak frequency and amplitude values. The transformed information is then processed to obtain physiological information regarding a patient. For example, in the case of frequency domain processing, a fundamental frequency may correspond to pulse rate or a ratio of amplitudes of the fundamental or other peaks of the red and infrared channel signals may be used to obtain an indication of SpO 2 . It will be appreciated that prefiltering in this regard may assist in identification of the desired spectral peaks relating to pulsatile phenomena, thereby enhancing such conventional spectral domain processing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a block diagram of a pulse oximeter in accordance with the present invention; 
         FIGS. 2A-2C  illustrate a prefiltering process in accordance with the present invention; 
         FIG. 3  is a schematic diagram illustrating a system implementing a prefilter in accordance with the present invention; and 
         FIG. 4  is a flow chart illustrating a method for processing a detector signal including prefiltering in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , there is shown a block diagram of one embodiment of a pulse oximeter  10  in which a prefilter in accordance with the present invention may be implemented. The pulse oximeter  10  is configured for use in determining the pulse rate of a patient as well as one or more blood analyte levels in the patient, such as an SpO 2  level. It should be appreciated that a prefilter in accordance with the present invention may be implemented in pulse oximeters that are configured differently from the pulse oximeter depicted in  FIG. 1  as well as in other environments wherein plethysmographic signals are processed in order to obtain desired information relating to patient physiological conditions from the plethysmographic signals. 
     The illustrated pulse oximeter  10  includes a pair of optical signal sources  20   a ,  20   b  for emitting a corresponding pair of light signals  30   a ,  30   b  centered at different predetermined center wavelengths λ 1 , λ 2  through a suitable tissue site of a patient and on to a detector  40  (e.g., a photo-sensitive diode). It will be appreciated that the signals may be reflected from the tissue rather than transmitted through the tissue in accordance with the present invention. 
     The optical signal sources  20   a ,  20   b  and detector  40  may be included in a positioning device  50 , or probe structure, to facilitate alignment of the light signals  30   a ,  30   b  with the detector  40 . For example, the positioning device  50  may be of clip-type or flexible strip configuration adapted for selective attachment to a suitable patient tissue site (e.g., a finger, an ear lobe, a foot, or the nose of the patient). The center wavelengths λ 1 , λ 2  required depend upon the blood analyte level to be determined. For example, in order to determine an SpO 2  level, λ 1  may be in the red wavelength range and λ 2  may be in the infrared wavelength range. It should be appreciated that the pulse oximeter  10  may be readily implemented with more optical signal sources (e.g., four) depending, for example, upon the number of different blood analyte levels to be measured. 
     The optical signal sources  20   a ,  20   b  are activated by a corresponding plurality of drive signals  60   a ,  60   b  to emit the light signals  30   a ,  30   b . The drive signals  60   a ,  60   b  are supplied to the optical signal sources  20   a ,  20   b  by a corresponding plurality of drive signal sources  70   a ,  70   b . The drive signal sources  70   a ,  70   b  may be connected with a digital processor  80 , which is driven with a clock signal  90  from a master clock  100 . The digital processor  80  may be programmed to define modulation waveforms, or drive patterns, for each of the optical signal sources  20   a ,  20   b . More particularly, the digital processor  80  may provide separate digital trigger signals  110   a ,  110   b  to the drive signal sources  70   a - b , which in turn generate the drive signals  60   a ,  60   b . In this regard, the digital trigger signals  110   a ,  110   b  may be configured to provide for multiplexing of the drive signals  60   a ,  60   b , and in turn the light signals  30   a ,  30   b , in accordance with a multiplexing scheme (e.g., time division, frequency division, and/or code division multiplexing). 
     The drive signal sources  70   a ,  70   b , processor  80  and clock  100  may all be housed in a monitor unit  120 . While the illustrated embodiment shows the optical signal sources  20   a ,  20   b  physically interconnected with the positioning device  50  (e.g., mounted within the positioning device  50  or mounted within a connector end of a cable that is selectively connectable with the positioning device  50 ), it should be appreciated that the optical signal sources  20   a ,  20   b  may also be disposed within the monitor unit  120 . In the latter case, the light signals  30   a ,  30   b  emitted from the optical signal sources  20   a ,  20   b  may be directed from the monitor unit  120  via one or more optical fibers to the positioning device  50  for transmission through the tissue site. Furthermore, the drive signal sources  70   a ,  70   b  may comprise a single drive signal generator unit that supplies each of the drive signals  60   a ,  60   b  to the optical signal sources  20   a ,  20   b.    
     Transmitted light signals  130   a ,  130   b  (i.e., the portions of light signals  30   a ,  30   b  exiting the tissue) are detected by the detector  40 . The detector  40  detects the intensities of the transmitted signals  130   a ,  130   b  and outputs a current signal  140  wherein the current level is indicative of the intensities of the transmitted signals  130   a ,  130   b . As may be appreciated, the current signal  140  output by the detector  40  comprises a multiplexed signal in the sense that it is a composite signal including information about the intensity of each of the transmitted signals  130   a ,  130   b . Depending upon the nature of the drive signals  60   a ,  60   b , the current signal  140  may, for example, be time division multiplexed, wavelength division multiplexed, and/or code division multiplexed. 
     The current signal  140  is directed to an amplifier  150 , which may be housed in the monitor unit  120  as is shown. As an alternative, the amplifier  150  may instead be included in a probe/cable unit that is selectively connectable with the monitor unit  120 . The amplifier  150  converts the current signal  140  to a voltage signal  160  wherein a voltage level is indicative of the intensities of the transmitted signals  130   a ,  130   b . The amplifier  150  may also be configured to filter the current signal  140  from the detector  40  to reduce noise and aliasing. By way of example, the amplifier  150  may include a bandpass filter to attenuate signal components outside of a predetermined frequency range encompassing modulation frequencies of the drive signals  60   a ,  60   b.    
     Since the current signal  140  output by the detector  40  is a multiplexed signal, the voltage signal  160  is also a multiplexed signal, and thus, the voltage signal  160  is demultiplexed in order to obtain signal portions corresponding with the intensities of the transmitted light signals  130   a ,  130   b . In this regard, the digital processor  80  may be provided with demodulation software for demultiplexing the voltage signal  160 . In order for the digital processor  80  to demodulate the voltage signal  160 , it is converted from analog to digital. Conversion of the analog voltage signal  160  is accomplished with an analog-to-digital (A/D) converter  170 , which may also be included in the monitor unit  120 . The A/D converter  170  receives the analog voltage signal  160  from the amplifier  150 , samples the voltage signal  160 , and converts the samples into a series of digital words  180  (e.g., eight, sixteen or thirty-two bit words), wherein each digital word is representative of the level of the voltage signal  160  (and hence the intensities of the transmitted light signals  130   a ,  130   b ) at a particular sample instance. In this regard, the A/D converter  170  preferably provides for sampling of the voltage signal  160  at a rate sufficient to provide for accurate tracking of the shape of the various signal portions comprising the analog voltage signal  160  being converted. For example, the A/D converter  170  may provide for a sampling frequency at least twice the frequency of the highest frequency drive signal  60   a ,  60   b , and more preferably at an even greater sampling rate in order to more accurately represent the analog voltage signal. 
     The series of digital words  180  is provided by the A/D converter  170  to the processor  80  to be demultiplexed. More particularly, the processor  80  may periodically send an interrupt signal  190  (e.g., once per every eight, sixteen or thirty-two clock cycles) to the A/D converter  170  that causes the A/D converter  170  to transmit one digital word  180  to the processor  80 . The demodulation software may then demultiplex the series of digital words  180  in accordance with an appropriate method (e.g., time, frequency and/or code) to obtain digital signal portions indicative of the intensities of each of the transmitted light signals  130   a ,  130   b . In this regard, the demultiplexed digital signal portions comprise time domain plethysmographic signals corresponding to the center wavelengths λ 1 , λ 2  (e.g., red and infrared) of the optical signal sources  20   a ,  20   b . The red and infrared time domain plethysmographic signals may then be processed by the processor  80  to obtain desired patient physiological condition related information therefrom such as the patient&#39;s pulse rate and SpO 2  level. 
     In accordance with the present invention, a pulse oximeter as described above includes a prefilter for prefiltering the detector signal to reduce the effects of artifact in the signal. As noted above, the detector signal generally includes a physiological signal carried by a modulation waveform which generally implements a multiplexing scheme. The physiological signal generally includes a larger, more slowly varying portion commonly referred to as the DC portion, and a smaller, higher frequency portion generally referred to as the AC portion. The AC portion includes the pulsatile signal which is generally of interest for determining pulse rate, SpO 2  and the like. However, it has been observed that in periods of high artifact such as certain motion episodes that this AC component can become significantly distorted. This distortion is characterized by a substantial increase in AC amplitude. 
     This is generally shown in  FIG. 2A .  FIG. 2A  illustrates an exemplary time interval of an AC signal  200 . The illustrated time interval includes an initial relatively clean time period generally indicated at  202  followed by a high artifact interval  204  and concludes with another interval  206  characterized by a relatively clean signal. That is, in intervals  202  and  206 , the AC signal  200  is characterized by a relatively well-defined plethysmographic waveform (“pleth”). By contrast, during interval  204 , the pleth is substantially distorted by artifact. 
       FIG. 2B  illustrates a theoretical decomposition of the pulsatile signal  200  into a pulsatile component  200   a  and an artifact signal  200   b  during the time interval  204 . As shown, it is generally theoretically expected that the pulsatile signal is present during periods of motion. However, the signal is affected by interference by the superimposed artifact signal  200   b . One effect of this superimposition of signal components  200   a  and  200   b  is that the signal  200   a  is substantially obscured. Another effect is that the combination of these signals results in an increased amplitude of the composite signal  200  during time interval  204  as shown in  FIG. 2A . Unfortunately, when a Fourier transform is performed extending across the integrated time period  202 ,  204  and  206 , the high amplitude portion of the integrated time period, that is, time interval  204 , may dominate the resulting spectrum. Consequently, the contributions due to time intervals  202  and  206  are largely overwhelmed. 
     It will be appreciated that these intervals  202  and  206  of well defined pulsatile signals include useful information which may provide an accurate indication of SpO 2 , pulse rate and other parameters. Moreover, in many cases, motion is episodic and transient. For example, in connection with monitoring neonates, short intervals of well defined pulsatile signals may be present within an otherwise noisy waveform. It would be desirable to isolate or emphasize these useful signal portions in relation to the surrounding artifact affected portions. However, as previously noted, conventional processing techniques often allow such useful signal portions to be overwhelmed by the artifact affected portions. 
     In one implementation of the present invention, the AC signal  200  is prefiltered so as to reduce the amplitude of large amplitude signal portions in relation to smaller amplitude signal portions. Because the low amplitude signal portions are believed to generally correspond to clean, well defined pulsatile signals, whereas the larger amplitude portions are believed to correspond to artifact affected signals, such deemphasizing of the large amplitude portions results in a prefiltered signal where the clean signal portions are relatively emphasized. These prefiltered signals can then be processed using conventional techniques to obtain desired physiological information such as SpO 2  or pulse rate with reduced impact due to artifact. 
     The functionality of this prefilter may be understood by reference to  FIG. 2A . Specifically, in one implementation, the prefilter utilizes two signal parameters, a and b, to execute a transfer function for generating the prefiltered signal. The parameter a is a measure of the amplitude of pulsatile signal. This value may be obtained in a variety of ways. For example, the value a may be determined by a peak to trough measurement of the AC signal  200  during a time interval deemed to represent a clean waveform, e.g., interval  202  or  206 . Alternatively, the value a may be based on the amplitude of a pulsatile spectral peak after performing a transform on the AC signal  200 . Many other techniques are available to obtain an indication of the parameter a. In this regard, it should be noted that pulse oximeters generally use a value corresponding to a in determining SpO 2 . Accordingly, such information may be readily available from calculations performed on preceding time intervals of the AC signal  200 . Moreover, as will be understood from the description below, it is not essential that the value of a be determined accurately so long as it provides a reasonable reference value for use in the transfer function as described below. 
     The second parameter used by the prefilter, b, is a substantially instantaneous measure of the amplitude of the AC signal  200 . As shown in  FIG. 2A , this value may indicate the instantaneous width of an envelope described by the AC signal  200 . Again, this value may be determined in various ways. For example, the value may be determined based on a peak to trough measurement of the time-based AC signal  200 . Alternatively, a curve may be fitted to the local extrema of the AC signal  200  to define the upper and lower boundaries of the envelope  208 . The instantaneous distance between these boundaries will then define the parameter b. It will be appreciated that many other techniques are possible to determine the parameter b in accordance with the present invention. 
     The prefilter in the illustrated embodiment then uses the parameters a and b to deemphasize larger amplitude portions of the AC signal  200 . The result is to substantially invert the shape of the envelope  208 . This can be simply accomplished by implementing the following transfer function.
 
 f ( n )= a/bx 
 
     where x is preferably greater than or equal to 1. 
     Thus, each value of the AC signal  200  is multiplied by the transfer function above to yield a resulting value of the prefiltered signal. In periods of substantially clean pulsatile signal, the value of a will be substantially equal to the value of b. In this case, if the value of a is normalized to be equal to 1.0, the amplitude of the pulsatile signal in the prefiltered waveform will be equal to the amplitude in the unfiltered detector signal. Moreover, in this case, where the amplitude of the AC signal  200  increases relative to the reference value a, the corresponding amplitude of the prefiltered signal will be reduced. The degree of such reduction will depend on the value of x. Currently, it is believed that good results regarding reducing the effects of artifact can be achieved by setting x to be on the order of 2 to 3, but x need not be an integer value. 
     The resulting prefiltered signal is generally illustrated in  FIG. 2C . As shown, the relatively clean sections  202  and  206  are substantially unaffected by application of the prefilter. By contrast, the artifact affected section  204 , rather than having an increased amplitude as in  FIG. 2A , now has a substantially reduced amplitude. As a result, various algorithms applied with respect to the integrated time period  202 ,  204  and  206  will have a reduced impact due to the artifact. For example, a Fourier transform executed with respect to the integrated time period  202 ,  204  and  206  will likely reflect larger peaks associated with the pulsatile signal and only smaller peaks corresponding to the artifact signal. This will substantially improve associated calculations to determine values such as pulse rate and SpO 2  even without any other modification of the conventional algorithms in this regard. 
     The prefilter can be implemented in hardware or software. In one implementation, the prefilter is implemented in the processor  80  of  FIG. 1 . Specifically, the processor receives the AC signal  200  after digitization, and demultiplexing of the detector signal. In addition, the carrier wave and DC portion may be removed from the detector signal. The values of a and b can then be determined as described above to define the prefilter transfer function. The transfer function may be implemented by setting appropriate filter coefficients. 
     An associated system  300  including the prefilter is illustrated in  FIG. 3 . The illustrated system  300  includes an analog to digital converter  302 , as generally described above, that receives the analog red and infrared photoplethysmographic signals or pleths, from the detector via certain front end circuitry. The resulting digitized, multiplexed signal  303  is then processed by demultiplexer  304  to provide separate AC signals  305  corresponding to the separate channels of the pulse oximeter. Each of the signals  305  is processed by a prefilter  306  as described above to yield prefiltered signals  307 . In the illustrated implementation, these signals  307  are processed by a fast Fourier transform module  308  to yield frequency spectra  309 . These spectra  309  are then processed in conventional fashion by SpO 2  and pulse rate module  310  to provide pulse rate and oxygen saturation information that can be output to a display  311  of the pulse oximeter. For example, a fundamental peak of one of the spectra may be used to determine pulserate. The fundamental (and/or other) peaks of both spectra may be used to determine SpO 2 . 
     A corresponding process  400  can be summarized by reference to the flow chart of  FIG. 4 . The process  400  is initiated by receiving ( 402 ) the analog detector signal. The analog detector signal is then digitized ( 404 ) by an A/D converter such as a fast A to D converter as discussed above. The resulting signals are demultiplexed ( 406 ) to obtain channel signals corresponding to the red and infrared optical signals of the pulse oximeter. The pleth amplitude and AC envelope amplitude are then measured ( 408 ) to provide the parameters used by the prefilter. The signal is then prefiltered ( 410 ) so as to deemphasize artifact affected portions of the signal and transformed ( 412 ) to obtain frequency domain information, e.g., red and infrared spectra. The frequency domain information is then processed to compute and display ( 414 ) the desired physiological parameters such as pulse rate and SpO 2 . 
     While various embodiments of the present invention have been described in detail, it is apparent that further modifications and adaptations of the invention will 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.