Abstract:
A pulse oximeter may reduce power consumption in the absence of overriding conditions. Various sampling mechanisms may be used individually or in combination. Various parameters may be monitored to trigger or override a reduced power consumption state. In this manner, a pulse oximeter can lower power consumption without sacrificing performance during, for example, high noise conditions or oxygen desaturations.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation of U.S. application Ser. No. 10/785,573, entitled “Low Power Pulse Oximeter,” filed Feb. 24, 2004, which is a continuation of application Ser. No. 10/184,028, entitled “Low Power Pulse Oximeter,” filed Jun. 26, 2002, now U.S. Pat. No. 6,697,658, which claims priority benefit under 35 U.S.C. §  119 (e) from U.S. Provisional Application No. 60/302,564, entitled “Low Power Pulse Oximeter,” filed Jul. 2, 2001. The present application incorporates each of the foregoing disclosures herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of a person&#39;s arterial blood, an indicator of their oxygen supply. Oxygen saturation monitoring is crucial in critical care and surgical applications, where an insufficient blood supply can quickly lead to injury or death.  FIG. 1  illustrates a conventional pulse oximetry system  100 , which has a sensor  110  and a monitor  150 . The sensor  110 , which can be attached to an adult&#39;s finger or an infant&#39;s foot, has both red and infrared LEDs  112  and a photodiode detector  114 . For a finger, the sensor is configured so that the LEDs  112  project light through the fingernail and into the blood vessels and capillaries underneath. The photodiode  114  is positioned at the finger tip opposite the fingernail so as to detect the LED emitted light as it emerges from the finger tissues. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to the assignee of the present invention and incorporated by reference herein.  
         [0003]     Also shown in  FIG. 1 , the monitor  150  has LED drivers  152 , a signal conditioning and digitization front-end  154 , a signal processor  156 , a display driver  158  and a display  159 . The LED drivers  152  alternately activate the red and IR LEDs  112  and the front-end  154  conditions and digitizes the resulting current generated by the photodiode  114 , which is proportional to the intensity of the detected light. The signal processor  156  inputs the conditioned photodiode signal and determines oxygen saturation based on the differential absorption by arterial blood of the two wavelengths emitted by the LEDs  112 . Specifically, a ratio of detected red and infrared intensities is calculated by the signal processor  156 , and an arterial oxygen saturation value is empirically determined based on the ratio obtained. The display driver  158  and associated display  159  indicate a patient&#39;s oxygen saturation, heart rate and plethysmographic waveform.  
       SUMMARY OF THE INVENTION  
       [0004]     Increasingly, pulse oximeters are being utilized in portable, battery-operated applications. For example, a pulse oximeter may be attached to a patient during emergency transport and remain with the patient as they are moved between hospital wards. Further, pulse oximeters are often implemented as plug-in modules for multiparameter patient monitors having a restricted power budget. These applications and others create an increasing demand for lower power and higher performance pulse oximeters. A conventional approach for reducing power consumption in portable electronics, typically utilized by devices such as calculators and notebook computers, is to have a “sleep mode” where the circuitry is powered-down when the devices are idle.  
         [0005]      FIG. 2  illustrates a sleep-mode pulse oximeter  200  utilizing conventional sleep-mode power reduction. The pulse oximeter  200  has a pulse oximeter processor  210  and a power control  220 . The power control  220  monitors the pulse oximeter output parameters  212 , such as oxygen saturation and pulse rate, and controls the processor power  214  according to measured activity. For example, if there is no significant change in the oxygen saturation value over a certain time period, the power control  220  will power down the processor  210 , except perhaps for a portion of memory. The power control  220  may have a timer that triggers the processor  210  to periodically sample the oxygen saturation value, and the power control  220  determines if any changes in this parameter are occurring. If not, the power control  220  will leave the processor  210  in sleep mode.  
         [0006]     There are a number of disadvantages to applying consumer electronic sleep mode techniques to pulse oximetry. By definition, the pulse oximeter is not functioning during sleep mode. Unlike consumer electronics, pulse oximetry cannot afford to miss events, such as patient oxygen desaturation. Further, there is a trade-off between shorter but more frequent sleep periods to avoid a missed event and the increased processing overhead to power-up after each sleep period. Also, sleep mode techniques rely only on the output parameters to determine whether the pulse oximeter should be active or in sleep mode. Finally, the caregiver is given no indication of when the pulse oximeter outputs were last updated.  
         [0007]     One aspect of a low power pulse oximeter is a sensor interface adapted to drive a pulse oximetry sensor and receive a corresponding input signal. A processor derives a physiological measurement corresponding to the input signal, and a display driver communicates the measurement to a display. A controller generates a sampling control output to at least one of said sensor interface and said processor so as to reduce the average power consumption of the pulse oximeter consistent with a predetermined power target.  
         [0008]     In one embodiment, a calculator derives a signal status output responsive to the input signal. The signal status output is communicated to the controller to override the sampling control output. The signal status output may indicate the occurrence of a low signal quality or the occurrence of a physiological event. In another embodiment, the sensor interface has an emitter driver adapted to provide a current output to an emitter portion of the sensor. Here, the sampling control output determines a duty cycle of the current output. In a particular embodiment, the duty cycle may be in the range of about 3.125% to about 25%.  
         [0009]     In another embodiment, the sensor interface has a front-end adapted to receive the input signal from a detector portion of the sensor and to provide a corresponding digitized signal. Here, the sampling control output determines a powered-down period of the front-end. A confidence indicator responsive to a duration of the powered-down period may be provided and displayed.  
         [0010]     In yet another embodiment, the pulse oximeter comprises a plurality of data blocks responsive to the input signal, wherein the sampling control output determines a time shift of successive ones of the data blocks. The time shift may vary in the range of about 1.2 seconds to about 4.8 seconds.  
         [0011]     An aspect of a low power pulse oximetry method comprises the steps of setting a power target and receiving an input signal from a pulse oximetry sensor. Further steps include calculating signal status related to the input signal, calculating power status related to the power target, and sampling based upon the result of the calculating signal status and the calculating power status steps.  
         [0012]     In one embodiment, the calculating signal status step comprises the substeps of receiving a signal statistic related to the input signal, receiving a physiological measurement related to the input signal, determining a low signal quality condition from the signal statistic, determining an event occurrence from the physiological measurement, and indicating an override based upon the low signal quality condition or the event occurrence. The calculating power status step may comprise the substeps of estimating an average power consumption for at least a portion of the pulse oximeter, and indicating an above power target condition when the average power consumption is above the power target. The sampling step may comprise the substep of increasing sampling as the result of the override. The sampling step may also comprise the substep of decreasing sampling as the result of the above power target condition, except during the override.  
         [0013]     Another aspect of a low power pulse oximetry method comprises the steps of detecting an override related to a measure of signal quality or a physiological measurement event, increasing the pulse oximeter power to a higher power level when the override exists, and reducing the pulse oximeter power to a lower power level when the override does not exist. The method may comprise the further steps of predetermining a target power level for a pulse oximeter and cycling between the lower power level and the higher power level so that an average pulse oximeter power is consistent with the target power level.  
         [0014]     In one embodiment, the reducing step comprises the substep of decreasing the duty cycle of an emitter driver output to the sensor. In another embodiment, the reducing step comprises the substep of powering-down a detector front-end. A further step may comprise displaying a confidence indicator related to the duration of the powering-down substep. In yet another embodiment, the reducing step comprises the substep of increasing the time-shift of post-processor data blocks.  
         [0015]     Another aspect of a low power pulse oximeter comprises a sensor interface adapted to receive an input signal from a sensor, a signal processor configured to communicate with the sensor interface and to generate an internal parameter responsive to the input signal, and a sampling controller responsive to the internal parameter so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The signal processor may be configured to generate an output parameter and the sampling controller may be responsive to a combination of the internal and output parameters so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The internal parameter may be indicative of the quality of the input signal. The output parameter may be indicative of oxygen saturation.  
         [0016]     In another embodiment, the sampling controller is responsive to a predetermined power target in combination with the internal parameter so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The signal processor may be configured to generate an output parameter and the sampling controller may be responsive to a combination of the internal and output parameters and the power target so as to generate a sampling control to alter the power consumption of at least one of the sensor interface and the signal processor. The sensor interface may comprise an emitter driver and the sampling control may modify a duty cycle of the emitter driver. The sensor interface may comprise a detector front-end and the sampling control may intermittently power-down the detector front-end. The processor may generate a plurality of data blocks corresponding to the input signal, where each of the data blocks have a time shift from a preceding one of the data blocks, and where the sampling control may determine the amount of the time shift.  
         [0017]     A further aspect of a low power pulse oximeter comprises an interface means for communicating with a sensor, a processor means for generating an internal parameter and an output parameter, and a controller means for selectively reducing the power consumption of at least one of the interface means and the processor means based upon the parameters. In one embodiment, the interface means comprises a driver means for determining the duty cycle of emitter current to the sensor, the driver means being responsive to the controller means. In another embodiment, the interface means comprises a detector front-end means for receiving an input signal from the sensor, the power for the detector front-end means being responsive to the controller means. In yet another embodiment, the processor means comprises a post-processor means for determining a time shift between data blocks, the post-processor means being responsive to the controller means. In a further embodiment, the controller means comprises a signal status calculator means for generating an indication of a low signal quality or a physiological event based upon at least one of an internal signal statistic and an output physiological measurement, and a control engine means in communications with the signal status calculator means for generating a sampling control responsive to the indication. In yet a further embodiment, the controller means comprises a power status calculator means for generating a power indication of power consumption relative to a power target, and a control engine means in communications with the power status calculator means for generating a sampling control responsive to the power indication. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram of a conventional pulse oximeter sensor and monitor;  
         [0019]      FIG. 2  is a block diagram of a pulse oximeter having a conventional sleep mode;  
         [0020]      FIG. 3  is a top-level block diagram of a low power pulse oximeter;  
         [0021]      FIG. 4  is a detailed block diagram of a low power pulse oximeter illustrating a sensor interface, a signal processor and a sampling controller;  
         [0022]      FIG. 5  is a graph of emitter drive current versus time illustrating variable duty cycle processing;  
         [0023]      FIG. 6  is a graph of oxygen saturation versus time illustrating intermittent sample processing;  
         [0024]     FIGS.  7 A-B are graphs of data buffer content versus time illustrating variable data block overlap processing;  
         [0025]      FIG. 8  is a graph of power versus time illustrating power dissipation conformance to an average power target using variable duty cycle and intermittent sample processing;  
         [0026]      FIG. 9  is a state diagram of the sampling controller for variable duty cycle and intermittent sample processing;  
         [0027]      FIG. 10  is a graph of power versus time illustrating power dissipation using variable data block overlap processing; and  
         [0028]      FIG. 11  is a state diagram of the sampling controller for variable data block overlap processing. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]      FIG. 3  illustrates one embodiment of a low power pulse oximeter. The pulse oximeter  300  has a sensor interface  320 , a signal processor  340 , a sampling controller  360  and a display driver  380 . The pulse oximeter  300  also has a sensor port  302  and a display port  304 . The sensor port  302  connects to an external sensor, e.g. sensor  110  ( FIG. 1 ). The sensor interface  320  drives the sensor port  302 , receives a corresponding input signal from the sensor port  302 , and provides a conditioned and digitized sensor signal  322  accordingly. Physiological measurements  342  are input to a display driver  380  that outputs to the display port  304 . The display port  304  connects to a display device, such as a CRT or LCD, which a healthcare provider typically uses for monitoring a patient&#39;s oxygen saturation, pulse rate and plethysmograph.  
         [0030]     As shown in  FIG. 3 , the signal processor  340  derives the physiological measurements  342 , including oxygen saturation, pulse rate and plethysmograph, from the input signal  322 . The signal processor  340  also derives signal statistics  344 , such as signal strength, noise and motion artifact. The physiological measurements  342  and signal statistics  344  are input to the sampling controller  360 , which outputs sampling controls  362 ,  364 ,  366  accordingly. The sampling controls  362 ,  364 ,  366  regulate pulse oximeter power dissipation by causing the sensor interface  320  to vary the sampling characteristics of the sensor port  302  and by causing the signal processor  340  to vary its sample processing characteristics, as described in further detail with respect to  FIG. 4 , below. Advantageously, power dissipation is responsive not only to output parameters, such as the physiological measurements  342 , but also to internal parameters, such as the signal statistics  344 .  
         [0031]      FIG. 4  illustrates further detail regarding the sensor interface  320 , the signal processor  340  and the sampling controller  360 . The sensor interface  320  has emitter drivers  480  and a detector front-end  490 . The emitter drivers  480  are responsive to a sampling control  362 , described below, and provide emitter drive outputs  482 . The emitter drive outputs  482  activate the LEDs of a sensor attached to the sensor port  302  ( FIG. 3 ). The detector front-end  490  receives an input signal  492  from a sensor attached to the sensor port  302  ( FIG. 3 ) and provides a corresponding conditioned and digitized input signal  322  to the signal processor  340 . A sampling control  364  controls power to the detector front-end  490 , as described below.  
         [0032]     As shown in  FIG. 4 , the signal processor  340  has a pre-processor  410  and a post processor  430 . The pre-processor  410  demodulates red and IR signals from the digitized signal  322 , performs filtering, and reduces the sample rate. The pre-processor provides a demodulated output, having a red channel  412  and an IR channel  414 , which is input into the post-processor  430 . The post processor  430  calculates the physiological measurements  342  and the signal statistics  344 , which are output to a signal status calculator  450 . The physiological measurements  342  are also output to a display driver  380  ( FIG. 3 ) as described above. A pulse oximetry signal processor is described in U.S. Pat. No. 6,081,735 entitled “Signal Processing Apparatus,” which is assigned to the assignee of the present invention and incorporated by reference herein.  
         [0033]     Also shown in  FIG. 4 , the sampling controller  360  has a control engine  440 , a signal status calculator  450  and a power status calculator  460 . The control engine  440  outputs sampling controls  362 ,  364 ,  366  to reduce the power consumption of the pulse oximeter  300 . In one embodiment, the control engine  440  advantageously utilizes multiple sampling mechanisms to alter power consumption. One sampling mechanism is an emitter duty cycle control  362  that is an input to the emitter drivers  480 . The emitter duty cycle control  362  determines the duty cycle of the current supplied by the emitter drive outputs  482  to both red and IR sensor emitters, as described with respect to  FIG. 5 , below. Another sampling mechanism is a front-end control  364  that intermittently removes power to the detector front-end  490 , as described with respected to  FIG. 6 , below. Yet another sampling mechanism is a data block overlap control  366  that varies the number of data blocks processed by the post processor  430 . These various sampling mechanisms provide the flexibility to reduce power without sacrificing performance during, for example, high noise conditions or oxygen desaturation events, as described below in further detail.  
         [0034]     The sampling controls  362 ,  364 ,  366  modify power consumption by, in effect, increasing or decreasing the number of input samples received and processed. Sampling, including acquiring input signal samples and subsequent sample processing, can be reduced during high signal quality periods and increased during low signal quality periods or when critical measurements are necessary. In this manner, the control engine  440  regulates power consumption to satisfy a predetermined power target, to minimize power consumption, or to simply reduce power consumption, as described with respect to  FIGS. 8 and 10 , below. The current state of the control engine is provided as a control state output  442  to the power status calculator  460 . The control engine  440  utilizes the power status output  462  and the signal status output  452  to determine its next control state, as described with respect to  FIGS. 9 and 11 , below.  
         [0035]     Further shown in  FIG. 4 , the signal status calculator  450  receives physiological measurements and signal statistics from the post processor  430  and determines the occurrence of an event or a low signal quality condition. An event determination is based upon the physiological measurements output  342  and may be any physiological-related indication that justifies the processing of more sensor samples and an associated higher power consumption level, such as an oxygen desaturation, a fast or irregular pulse rate or an unusual plethysmograph waveform to name a few. A low signal quality condition is based upon the signal statistics output  344  and may be any signal-related indication that justifies the processing of more sensor samples and an associated higher power consumption level, such as a low signal level, a high noise level or motion artifact to name a few. The signal status calculator  450  provides the signal status output  452  that is input to the control engine  440 .  
         [0036]     In addition,  FIG. 4  shows that the power status calculator  460  has a control state input  442  and a power status output  462 . The control state input  442  indicates the current state of the control engine  440 . The power status calculator  460  utilizes an internal time base, such as a counter, timer or real-time clock, in conjunction with the control engine state to estimate the average power consumption of at least a portion of the pulse oximeter  300 . The power status calculator  460  also stores a predetermined power target and compares its power consumption estimate to this target. The power status calculator  460  generates the power status output  462  as an indication that the current average power estimate is above or below the power target and provides this output  462  to the control engine  440 .  
         [0037]      FIG. 5  illustrates emitter driver output current versus time. The graph  500  depicts the combination of a red LED drive current  510  and an IR drive current  560 . The solid line graph  502  illustrates drive currents having a high duty cycle. The dashed line graph  504  illustrates drive currents having a low duty cycle. In a typical pulse oximeter, the duty cycle of the drive signals is constant and provides sufficient dark bands  508  to demodulate the detector response into red and IR channels. The emitter drivers  480  ( FIG. 4 ), however, require a significant portion of the overall pulse oximeter power budget. Intermittently reducing the drive current duty cycle can advantageously reduce power dissipation without compromising signal integrity. As an example, a low power pulse oximeter implementation nominally consuming 500 mw may be able to reduce power consumption on the order of 70 mw by such drive current duty cycle reductions. In a preferred embodiment, the drive current duty cycle is varied within a range from about 25% to about 3.125%. In a more preferred embodiment, the drive current duty cycle is intermittently reduced from about 25% to about 3.125%. In conjunction with an intermittently reduced duty cycle or as an independent sampling mechanism, there may be a “data off” time period longer than one drive current cycle where the emitter drivers  480  ( FIG. 4 ) are turned off. The detector front-end  490  ( FIG. 4 ) may also be powered down during such a data off period, as described with respect to  FIGS. 8 and 9 , below.  
         [0038]      FIG. 6  is a graph  600  of a pre-processor output signal  610  over time depicting the result of intermittent sampling at the detector front-end  490  ( FIG. 4 ). The output signal  610  is a red channel  412  ( FIG. 4 ) or an IR channel  414  ( FIG. 4 ) output from the pre-processor  410  ( FIG. 4 ), which is input to the post processor  430  ( FIG. 4 ), as described above. The output signal  610  has “on” periods  612 , during which time the detector front-end  490  ( FIG. 4 ) is powered-up and “off” periods  614 , during which time the detector front-end  490  ( FIG. 4 ) is powered-down. The location and duration of the on periods  612  and off periods  614  are determined by the front-end control  364  ( FIG. 4 ).  
         [0039]     Also shown in  FIG. 6  is a corresponding timeline  601  of overlapping data blocks  700 , which are “snap-shots” of the pre-processor output signal  610  over specific time intervals. Specifically, the post processor  430  ( FIG. 4 ) processes a sliding window of samples of the pre-processor output signal  610 , as described with respect to FIGS.  7 A-B, below. Advantageously, the post processor  430  ( FIG. 4 ) continues to function during off portions  614 , marking as invalid those data blocks  640  that incorporate off portions  614 . A freshness counter can be used to measure the time period  660  between valid data blocks  630 , which can be displayed on a pulse oximeter monitor as an indication of confidence in the current measurements.  
         [0040]     FIGS.  7 A-B illustrate data blocks  700 , which are processed by the post processor  430  ( FIG. 4 ). Each data block  700  has n samples  702  of the pre-processor output and corresponds to a time interval  704  of n/f, where f s  is the sample frequency. For example, in one embodiment n=600 and f=62.5 Hz. Hence, each data block time interval  704  is nominally 9.6 sec.  
         [0041]     As shown in  FIG. 7A , each data block  700  also has a relative time shift  706  from the preceding data block, where is an integral number of sample periods. That is, =m/f s , where m is an integer representing the number of samples dropped from the preceding data block and added to the succeeding data block. In the embodiment described above, m=75 and =1.2 sec, nominally. The corresponding overlap  708  of two adjacent data blocks  710 ,  720  is (n−m)/f s . In the embodiment described above, the overlap  708  is nominally 9.6 sec−1.2 sec=8.4 sec. The greater the overlap  708 , i.e. the smaller the time shift  706 , the more data blocks there are to process in the post-processor  430  ( FIG. 4 ), with a corresponding greater power consumption. The overlap  708  between successive data blocks  710 ,  720  may vary from n−1 samples to no samples, i.e. no overlap. Also, as shown in  FIG. 7B , there may be a sample gap  756  or negative overlap, i.e. samples between data blocks that are not processed by the post-processor, allowing further post-processor power savings. Sample gaps  756  may correspond to detector front-end off periods  614  ( FIG. 6 ).  
         [0042]      FIG. 8  illustrates an exemplar power consumption versus time profile  800  for the pulse oximeter  300  ( FIG. 3 ) during various control engine states. In one embodiment, the control engine  440  ( FIG. 4 ) has three states related to the sampling control outputs  362 ,  364  that affect pulse oximeter power consumption accordingly. One of ordinary skill in the art will recognize that the control engine  440  ( FIG. 4 ) may have greater or fewer states and associated power consumption levels. The profile  800  shows the three control engine states  810  and the associated power consumption levels  820 . These three states are high duty cycle  812 , low duty cycle  814  and data off  818 .  
         [0043]     In the high duty cycle state  812 , the control engine  440  ( FIG. 4 ) causes the emitter drivers  480  ( FIG. 4 ) to turn on sensor emitters for a relatively long time period, such as 25% on time for each of the red  510  and IR  560  drive currents. In the low duty cycle state  814 , the control engine  440  ( FIG. 4 ) causes the emitter drivers  480  ( FIG. 4 ) to turn on sensor emitters for a relatively short time period, such as 3.125% of the time for each of the red  510  and IR  560  drive currents. In the data off state  818 , the control engine  440  ( FIG. 4 ) turns off the emitter drivers  480  ( FIG. 4 ) and powers down the detector front-end  490  ( FIG. 4 ). Also shown is a predetermined target power consumption level  830 . The control engine  440  ( FIG. 4 ) alters the sensor sampling of the pulse oximeter  300  ( FIG. 3 ) so that the average power consumption matches the target level  830 , as indicated by the power status output  462  ( FIG. 4 ), except when overridden by the signal status output  452  ( FIG. 4 ).  
         [0044]     As shown in  FIG. 8 , power consumption changes according to the control states  810  during each of the time intervals  850 . In a first time interval  851 , the pulse oximeter is in a low duty cycle state  814  and transitions to a high duty cycle state  812  during a second time interval  852  due to an event or low quality signal. During a third time interval  853 , the pulse oximeter is able to enter the data off state  818 , during which time no sensor samples are processed. In a forth time interval  854 , sensor samples are again taken, but at a low duty cycle  814 . During the fifth and sixth time intervals  855 ,  856 , sensor samples are shut off and turned on again as the pulse oximeter  300  ( FIG. 3 ) alternates between the data off state  818  and the low duty cycle state  814  so as to maintain an average power consumption at the target level  830 .  
         [0045]      FIG. 9  illustrates a state diagram  900  for one embodiment of the control engine  440  ( FIG. 4 ). In this embodiment, there are three control states, high duty cycle  910 , low duty cycle  940  and data off  970 , as described with respect to  FIG. 8 , above. If the control state is data off  970 , an event triggers a data-off to high-duty-cycle transition  972 . If the control state is low duty cycle  940 , an event similarly triggers a low-duty cycle to high-duty-cycle transition  942 . In this manner, the occurrence of an event initiates high duty sensor sampling, allowing high fidelity monitoring of the event. Similarly, if the control state is low duty cycle  940 , low signal quality triggers a low-duty cycle to high-duty-cycle transition  942 . In this manner, low signal quality initiates higher duty sensor sampling, providing, for example, a larger signal-to-noise ratio.  
         [0046]     Also shown in  FIG. 9 , if the control state is high duty cycle  910  and either an event is occurring or signal quality is low, then a null transition  918  maintains the high duty cycle state  910 . If the pulse oximeter is not above the power target for more than a particular time interval, a null transition  948  maintains the low duty cycle state  940 , so that sampling is turned-off only when necessary to track the power target. Further, if the control state is data off  970  and no time-out has occurred, a null transition  978  maintains the data off state  970 , providing a minimum power consumption.  
         [0047]     In addition,  FIG. 9  shows that when the control state is in a high duty cycle state  910 , if neither an event nor low signal quality are occurring, then a high-duty-cycle to low-duty-cycle transition  912  occurs by default. Also, if the control state is low duty cycle  940 , if neither an event nor low signal quality are occurring and the power consumption is above the target level for longer than a particular time interval, a low-duty-cycle to data-off transition  944  occurs by default, allowing power consumption to come down to the target level. Further, if the control state is data off  970 , if no event occurs and a timeout does occur, a data-off to low-duty-cycle transition  974  occurs by default, preventing excessively long periods of no sensor sampling.  
         [0048]      FIG. 10  illustrates an exemplar power consumption versus time profile  1000  for the post processor  430  ( FIG. 4 ) during various control engine states. In one embodiment, the control engine  440  ( FIG. 4 ) has three states related to the sampling control output  366  ( FIG. 4 ) that affect post processor power consumption accordingly. One of ordinary skill in the art will recognize that the control engine may have greater or fewer states and associated power consumption levels. The profile  1000  shows the three control engine states  1010  and the associated post processor power consumption levels  1020 . These three states are large overlap  1012 , medium overlap  1014  and small overlap  1018 .  
         [0049]     As shown in  FIG. 10 , in the large overlap state  1012 , the control engine  440  ( FIG. 4 ) causes the post processor to process data blocks that have a comparatively small time shift  706  ( FIG. 7A ), and the post processor exhibits relatively high power consumption under these conditions, say 300 mw. In the medium overlap state  1014 , the control engine  440  ( FIG. 4 ) causes the post processor to process data blocks that have a comparatively larger time shift  706  ( FIG. 7A ). For example, the data blocks may be time shifted twice as much as for the large overlap state  1012 , and, as such, the post processor performs only half as many computations and consumes half the nominal power, say 150 mw. In the small overlap state  1018 , the control engine  440  ( FIG. 4 ) causes the post processor to process data blocks that have a comparatively large time shift. For example, the data blocks may be time shifted twice as much as for the medium overlap state  1014 . As such, the post processor performs only a quarter as many computations and consumes a quarter of the nominal power, say 75 mw, as for the large overlap state  1012 . In one embodiment, the control engine  440  ( FIG. 4 ) alters the data block overlap of the post processor in conjunction with the duty cycle of the emitter drivers described with respect to  FIG. 5 , above, and the front-end sampling described with respect to  FIG. 6 , above, so that the average power consumption of the pulse oximeter matches a target level indicated by the power status output  462  ( FIG. 4 ) or so that the power consumption is otherwise reduced or minimized.  
         [0050]     In a preferred embodiment, data blocks are time shifted by either about 0.4 sec or about 1.2 sec, depending on the overlap state of the control engine  440  ( FIG. 4 ). In a more preferred embodiment, the data blocks are varied between about 1.2 sec and about 4.8 sec. In a most preferred embodiment, the data blocks are time shifted by either about 1.2 sec, about 2.4 sec or about 4.8 sec, depending on the overlap state of the control engine  440  ( FIG. 4 ). Although the post-processing of data blocks is described above with respect to only a few overlap states and a corresponding number of particular data block time shifts, there may be many overlap states and a corresponding range of data block time shifts.  
         [0051]     Further shown in  FIG. 10 , power consumption  1020  changes according to the control states  1010  during each of the time intervals  1050 . In a first time interval  1052 , the post processor is in a large overlap state  1012  and transitions to a medium overlap state  1014  during a second time interval  1054 , so as to meet a power target during a high signal quality period, for example. During a third time interval  1055 , the post processor enters a small overlap state  1018 , for example to meet a power target by further reducing power consumption. In a forth time interval  1056 , the post processor transitions back to a large overlap state  1012 , such as during an event or low signal quality conditions.  
         [0052]      FIG. 11  illustrates a state diagram  1100  for one embodiment of the control engine  440  ( FIG. 4 ). These states may function in parallel with, or in combination with, the sampling states described with respect to  FIG. 9 , above. In the illustrated embodiment, there are three control states, large overlap  1110 , medium overlap  1140  and small overlap  1170 , as described with respect to  FIG. 10 , above. If the control state is small overlap  1170 , an event triggers a small overlap to large overlap transition  1172 . If the control state is medium overlap  1140 , an event similarly triggers a medium overlap to large-overlap transition  1142 . In this manner, the occurrence of an event initiates the processing of more data blocks, allowing more robust signal statistics and higher fidelity monitoring of the event. Similarly, if the control state is medium overlap  1140 , low signal quality triggers a medium overlap to large overlap transition  1142 . In this manner, low signal quality initiates the processing of more data blocks, providing more robust signal statistics during lower signal-to-noise ratio periods.  
         [0053]     Also shown in  FIG. 11 , if the control state is large overlap  1110  and either an event is occurring or signal quality is low, then a null transition  1118  maintains the large overlap state  1110 . If the pulse oximeter is not above the power target for more than a particular time interval, a null transition  1148  maintains the medium overlap state  1140 , so that reduced data processing occurs only when necessary to track the power target. Further, if the control state is small overlap  1170 , a null transition  1178  maintains this power saving state until the power target is reached or an event or low signal quality condition occurs.  
         [0054]     In addition,  FIG. 11  shows that when the control state is in a large overlap state  1110 , if neither an event nor low signal quality are occurring, then a large overlap to medium overlap transition  1112  occurs by default. Also, if the control state is medium overlap  1140 , if the power consumption is above the target level for longer than a particular time interval and no low signal quality condition or event is occurring, a medium overlap to small overlap transition  1174  occurs, allowing power consumption to come down to the target level. Further, if the control state is small overlap  1170 , if no event occurs but the power target has been met, a small overlap to medium overlap transition  1174  occurs.  
         [0055]     A low power pulse oximeter embodiment is described above as having a power status calculator  460  ( FIG. 4 ) and an associated power target. Another embodiment of a low power pulse oximeter, however, functions without either a power status calculator or a power target, utilizing the sampling controls  362 ,  364 ,  366  ( FIG. 3 ) in response to internal parameters and/or output parameters, such as signal statistics  344  ( FIG. 3 ) and/or physiological measurements  342  ( FIG. 3 ) to reduce power consumption except during, say, periods of low signal quality and physiological events.  
         [0056]     One of ordinary skill in the art will recognize that various state diagrams are possible representing control of the emitter drivers, the detector front-end and the post-processor. Such state diagrams may have fewer or greater states with differing transitional characteristics and with differing relationships between sampling mechanisms than the particular embodiments described above. In relatively simple embodiments of the control engine  440  ( FIG. 4 ), only a single sampling mechanism is used, such as the sampling mechanism used to vary the duty cycle of the emitter drivers. The single sampling mechanism may be based only upon internal parameters, such as signal quality, only upon output parameters, such as those that indicate the occurrence of physiological events, or upon a combination of internal and output parameters, with or without a power target.  
         [0057]     In relatively more complex embodiments of the control engine  440  ( FIG. 4 ), sampling mechanisms are used in combination. These sampling mechanisms may be based only upon internal parameters, only upon output parameters, or upon a combination of internal and output parameters, with or without a power target. In a particular embodiment, the emitter duty-cycle, front-end duty-cycle and data block overlap sampling mechanisms described above are combined. A “reduced overlap” state relating to the post-processing of data blocks is added to the diagram of  FIG. 9  between the “low duty cycle” state and the “data off” state. That is, sampling is varied between a high duty cycle state, a low duty cycle state, a reduced overlap state and a data off state in response to signal quality and physiological events, with or without a power target.  
         [0058]     The low power pulse oximeter has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not to limit the scope of the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications.