Abstract:
An alarm processor suppresses alarms when a physiological parameter is below a predetermined value but recovering toward a normal range.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority benefit under 35 U.S.C. § 120 to, and is a continuation of, U.S. patent application Ser. No. 10/351,735, filed Jan. 24, 2003, now U.S. Pat. No. 6,822,564 entitled “ Parallel Measurement Alarm Processor,”  which claims priority benefit under 35 U.S.C. § 119(e) from U.S. Provisional Application No. 60/351,510, filed Jan. 24, 2002, entitled “ Parallel Measurement Alarm Processor.”  The present application also incorporates the foregoing utility disclosure herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Physiological measurement instruments employed in healthcare environments often feature visual and audible alarm mechanisms that alert a caregiver when a patient&#39;s vital signs are outside of predetermined limits. One example is a pulse oximeter, which measures the oxygen saturation level of arterial blood, an indicator of oxygen supply. A typical pulse oximeter displays a numerical readout of the patient&#39;s oxygen saturation, a numerical readout of pulse rate, and a plethysmograph, which is indicative of a patient&#39;s pulse. In addition, a pulse oximeter provides an alarm that warns of a potential desaturation event. 
       FIG. 1  illustrates a prior art pulse oximeter portion  100  having a signal input  101  and generating an oxygen saturation measurement output  103  and an alarm output  105 . The pulse oximeter portion  100  has an oxygen saturation (SpO 2 ) processor  110  and an associated threshold detector  120 . The SpO 2  processor  110  derives an oxygen saturation measurement from the signal input  101 . The signal input  101  is typically an amplified, filtered, digitized and demodulated sensor signal. A sensor emits both red and infrared (IR) wavelength light, which is transmitted through a patient&#39;s tissue, detected and input to the pulse oximeter. The pulse oximeter calculates a normalized ratio (AC/DC) of the detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on a ratio of these normalized ratios, as is well-known in the art. The oxygen saturation measurement output  103  is typically a digital signal that is then communicated to a display. 
       FIG. 2  illustrates the operation of a conventional threshold detector  120  ( FIG. 1 ) utilizing a graph  200  of oxygen saturation  201  versus time  202 . The graph  200  displays a particular oxygen saturation measurement  210  corresponding to the measurement output  103  ( FIG. 1 ) and a predetermined alarm threshold  206 . During an alarm time period  270  when the measured oxygen saturation  210  is below the threshold  206 , an alarm output  105  ( FIG. 1 ) is generated, which triggers a caregiver alert. Adjusting the threshold  206  to a lower value of oxygen saturation  201  reduces the probability of an alarm, i.e. reduces the probability of a false alarm and increases the probability of a missed event. Likewise, adjusting the threshold  206  to a higher value of oxygen saturation  201  increases the probability of an alarm, i.e. increases the probability of a false alarm and decreases the probability of a missed event. 
     SUMMARY OF THE INVENTION 
     One performance measure for a physiological measurement instrument is the probability of a false alarm compared with the probability of a missed event. Missed events, such as an oxygen desaturation when measuring oxygen saturation, may detrimentally effect patient health. False alarms waste caregiver resources and may also result in a true alarm being ignored. It is desirable, therefore, to provide an alarm mechanism to reduce the probability of false alarms without significantly increasing the probability of missed events, and, similarly, to reduce the probability of missed events without significantly increasing the probability of false alarms. 
     An alarm processor has a signal input responsive to a physiological parameter and a plurality of parameter processors responsive to the signal input so as to provide a plurality of measurements of the parameter having differing characteristics. In addition, the alarm processor has an alarm condition applicable to at least one of the measurements so as to define a limit for the parameter. Further, the alarm processor has an alarm indicator operating on the measurements and the alarm condition so as to provide an alarm output that changes state to indicate that the parameter may have exceeded the limit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art pulse oximeter portion; 
         FIG. 2  is a graph of oxygen saturation versus time illustrating a conventional threshold detector alarm; 
         FIG. 3  is a block diagram of an alarm processor utilizing parallel measurements of a physiological parameter; 
         FIG. 4  is a block diagram of a pulse oximeter processor utilizing dual oxygen saturation measurements; 
         FIG. 5  is a block diagram of a predictive alarm indicator utilizing a threshold detector with a slow oxygen saturation measurement input and a slope detector with a fast oxygen saturation measurement input; 
         FIGS. 6A–B  are graphs of oxygen saturation versus time illustrating operation of the alarm indicator according to  FIG. 5 ; 
         FIG. 7  is a block diagram of a pattern recognition alarm indicator utilizing a threshold detector with a slow oxygen saturation measurement input and a pattern extractor with a fast oxygen saturation measurement input; and 
         FIG. 8  is a graph of oxygen saturation versus time illustrating the pattern recognition alarm indicator according to  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  illustrates a parallel measurement alarm processor  300 . The alarm processor  300  has a sensor signal input  301  responsive to a physiological parameter and provides one or more alarm outputs  303  to indicate that the physiological parameter may have exceeded particular limits. The alarm processor  300  also has multiple parameter processors  310 , which do not necessarily have the same or similar internal configurations. The multiple parameter processors  310  input the sensor signal  301  and provide parallel measurements  312  of the physiological parameter, each measurement having differing characteristics, such as response time or bandwidth to name a few. The alarm processor  300  further has an alarm indicator  320  that inputs the parallel parameter measurements  312  and generates the alarm outputs  303  based upon alarm conditions  305 . The alarm outputs  303  change state to indicate that the parameter may have exceed one or more limits and to trigger an alarm accordingly. The alarm conditions  305  define particular limits with respect to one or more of the measurements  312 . The alarm conditions  305  may be predefined, such as by user input, or determined by a separate process, such as a measurement of sensor signal quality or data confidence as described in U.S. patent application Ser. No. 09/858,114 entitled “Pulse Oximetry Data Confidence Indicator,” assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. The alarm processer  300  may also have a display driver  330  that processes one or more of the parameter measurements  312  and provides one or more display outputs  307 . 
       FIG. 4  illustrates a pulse oximeter embodiment  400  of the alarm processor  300  ( FIG. 3 ) described above. A pulse oximeter sensor (not shown) provides a signal input  301  responsive to arterial oxygen saturation, as described with respect to  FIG. 1 , above. The alarm processor  400  has dual oxygen saturation processors  310 . An integrator oxygen saturation (SpO 2 ) processor  410  outputs a slow SpO 2  measurement  412 , i.e. a measurement having a slow response time to changes in the SpO 2  parameter. A predictor SpO 2  processor  420  outputs a fast SpO 2  measurement  422 , i.e. a measurement having a fast response time that tracks changes in the SpO 2  parameter. The slow SpO 2  measurement  412  is input to a display driver  330 , which provides an oxygen saturation display output  307 . For example, the display output  307  may be input to a digital display that provides a numerical readout of oxygen saturation to a caregiver. Both the slow SpO 2  measurement  412  and the fast SpO 2  measurement  422  are input to an alarm indicator  320  that generates at least one alarm output  303  based upon alarm conditions  305 , as described in further detail with respect to  FIGS. 5-8 , below. 
     The integrator SpO 2  processor  410 , advantageously, provides a smoothed measurement of oxygen saturation suitable for threshold detection. The predictor SpO 2  processor  420 , advantageously, provides a curve-fitting or a predictive measurement of oxygen saturation that detects trends in oxygen saturation, as described in further detail with respect to  FIG. 5  and  FIGS. 6A–B , below. Further, the predictor SpO 2  processor  420  advantageously tracks oxygen saturation details that may signal a critical physiological event, as described in further detail with respect to  FIGS. 7–8 , below. The integrator SpO 2  processor  410  and predictor SpO 2  processor  420  may be a pulse oximeter as described in U.S. patent application Ser. No. 09/586,845 entitled “Variable Mode Averager,” assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. 
       FIG. 5  illustrates a trend embodiment of an alarm indicator  320 , which has a threshold detector  510 , a slope detector  520  and alarm detector  530 . The threshold detector  510  has a slow SpO 2  measurement  412  and a threshold alarm condition  305  as inputs and a logic output BELOW  512 . The slope detector  520  has a fast SpO 2  measurement  422  input and a logic output POSITIVE/ 522 . The alarm detector  530  has BELOW  512  and POSITIVE/ 522  logic inputs and generates an alarm output  303 . The threshold detector  510  is a comparator that asserts BELOW  512  while the slow SpO 2  measurement  412  is less in value than the value of the threshold  305 . The slope detector  520  is a differentiator and comparator that asserts POSITIVE/ 522  while the slope of the fast SpO 2  measurement  422  is non-positive, i.e. while the derivative of the fast SpO 2  measurement  422  is zero or less than zero. The alarm detector  530  performs a logical AND function, asserting the alarm output  303  and indicating an alarm when BELOW  512  and POSITIVE/ 522  are both asserted. In this manner, an alarm output  303  only changes state when the slow SpO 2  measurement  412  is below a threshold  305  and the fast SpO 2  measurement  422  has not begun to increase in value. Advantageously, the trend recognition alarm indicator  320  reduces false alarms by suppressing a threshold-based alarm on the slow SpO 2  measurement  412  when the fast SpO 2  measurement  422  determines that a patient&#39;s oxygen saturation is in recovery, as described in further detail with respect to  FIGS. 6A–B , below. 
       FIGS. 6A–B  illustrate operation of the trend recognition alarm indicator  320  ( FIG. 5 ). In  FIG. 6A , a graph  600  has an SpO 2  axis  601  and a time axis  602 . Shown along the SpO 2  axis  601  is a constant SpO 2  value  606  corresponding to a threshold  305  ( FIG. 5 ). The graph  600  shows a first plot of SpO 2  versus time  610  corresponding to a fast SpO 2  measurement  422  ( FIG. 5 ). The graph  600  also shows a second plot of SpO 2  versus time  620  corresponding to a slow SpO 2  measurement  412  ( FIG. 5 ). A suppressed alarm interval  640  along the time axis  602  corresponds to an alarm that would be indicated by the threshold detector  510  ( FIG. 5 ) but is suppressed as occurring during a positive slope portion  630  of a fast SpO 2  measurement  610 . The alarm detector  530  ( FIG. 5 ) would not assert an alarm output  303  ( FIG. 5 ) during this interval. 
     In  FIG. 6B , a graph  650  shows a first plot of SpO 2  versus time  660  corresponding to a fast SpO 2  measurement  422  ( FIG. 5 ). The graph  650  also shows a second plot of SpO 2  versus time  670  corresponding to a slow SpO 2  measurement  412  ( FIG. 5 ). An alarm interval  690  along the time axis  602  corresponds to an alarm period triggered by the alarm output  303  ( FIG. 5 ). This alarm interval  640  occurs while a slow SpO 2  measurement  670  is below the threshold  606  and before a positive slope portion  680  of a fast SpO 2  measurement  660 . 
       FIG. 7  illustrates a pattern recognition embodiment of an alarm indicator  320 , having a threshold detector  710 , a pattern extractor  720 , a pattern memory  730  and a pattern comparator  740 . Further, the alarm indicator  320  has slow SpO 2    412  and fast SpO 2    422  measurement inputs in addition to threshold  701  and reference pattern  732  alarm condition inputs  305 . The threshold detector  710  has a slow SpO 2  measurement  412  and a SpO 2  threshold  701  as inputs and a first alarm output  712 . The threshold detector  710  changes the state of the first alarm output  712  when the value of the slow SpO 2  measurement  412  crosses the SpO 2  threshold  701 . For example, the first alarm output  712  changes state to trigger an alarm when the slow SpO 2  measurement  412  becomes less than the SpO 2  threshold  701 . 
     As shown in  FIG. 7 , the pattern extractor  720  has a fast SpO 2  measurement  422  and a pattern threshold  734  as inputs and an extracted pattern output  722 . The pattern extractor  720  identifies features of the fast SpO 2  measurement  422  that may be used for pattern matching. Features may be, for example, the number of times the fast SpO 2  measurement  422  crosses the pattern threshold  734  within a certain time period, or the duration of each time period that the fast SpO 2  measurement  422  is less than the pattern threshold  734 , to name a few. The pattern memory  730  has a pattern selection input  705  and a reference pattern output  732 . The pattern memory  730  stores values for particular features that are identified by the pattern extractor  720 . The reference pattern output  732  transfers these stored values to the pattern comparator  740 . The pattern memory  730  may be nonvolatile and one or more patterns may be stored at the time of manufacture or downloaded subsequently via a data input (not shown). One of multiple patterns may be determined via the pattern selection input  705 , by a user or by a separate process, for example. The pattern threshold  734  may be generated in response to the pattern selection input  705  or in conjunction with a selected reference pattern  732 . 
     Also shown in  FIG. 7 , the pattern comparator  740  has the extracted pattern  722  and the reference pattern  732  as inputs and generates a second alarm output  742 . That is, the pattern comparator  740  matches extracted measurement features provided by the pattern extractor  720  with selected features retrieved from pattern memory  730 , changing the state of the second alarm output  742  accordingly. For example, the second alarm output  742  changes state to trigger an alarm when features of the fast SpO 2  measurement  422  match the reference pattern output  732 . Advantageously, the pattern recognition alarm indicator  320  reduces missed events by supplementing the threshold-based first alarm output  712  responsive to the slow SpO 2  measurement  412  with a pattern-based second alarm output  742  responsive to detail in the fast SpO 2  measurement  422 . In this manner, if a patient&#39;s oxygen saturation is, for example, irregular or intermittent, the second alarm output  742  may trigger a caregiver alert when the first alarm output  712  does not, as described in further detail with respect to  FIG. 8 , below. 
       FIG. 8  illustrates operation of a pattern recognition alarm indicator  320  ( FIG. 7 ), as described above. A graph  800  has an SpO 2  axis  801  and a time axis  802 . The graph  800  shows a SpO 2  plot versus time  810  corresponding to the slow SpO 2  measurement  412  ( FIG. 7 ). Shown along the time axis  802  is a constant SpO 2  value  812  corresponding to the SpO 2  threshold  701  ( FIG. 7 ). Due to the short duration of irregular and intermittent drops in SpO 2 , the slow SpO 2  measurement  810  does not fall below the SpO 2  threshold  812 . Thus, the first alarm output  712  ( FIG. 7 ) does not trigger an alarm in this example. 
     Also shown in  FIG. 8 , the graph  800  shows a SpO 2  plot versus time  820  corresponding to the fast SpO 2  measurement  422  ( FIG. 7 ). Shown along the time axis  802  is a constant SpO 2  value  822  corresponding to the pattern threshold  734  ( FIG. 7 ). A corresponding graph  805  has a logic level axis  806  and a time axis  807 . The graph  805  shows a logic level plot versus time  830  corresponding to the extracted pattern output  722  ( FIG. 7 ). The logic level plot  830  has a “1” level when the fast SpO 2  plot  820  is above the pattern threshold  822  and a “0” level when the fast SpO 2  plot  820  is below the pattern threshold  822 . In this manner, the logic level plot  830  indicates the number and duration of times the fast SpO 2  plot  820  falls below a threshold value  822 . 
     Further shown in  FIG. 8 , an alarm interval  870  along the time axis  802  corresponds to an alarm period indicated by the pattern comparator  740  ( FIG. 7 ). This alarm interval  870  occurs after a reference pattern  732  ( FIG. 7 ) is detected as matching an extracted pattern  722  ( FIG. 7 ) and ends, correspondingly, when there is no longer a match. For example, assume that the reference pattern output  732  ( FIG. 7 ) has the alarm criteria that at least three below threshold periods of minimum duration τ 1  must occur during a maximum period τ 2 , where the value of τ 1  and τ 2  are illustrated along the time axis  807 . The below threshold time periods  831 – 834  are each greater in duration than τ 2  and a first set of three, below-threshold time periods  831 – 833  occurs within a time period T 1 =τ 2 , as illustrated. Thus, the alarm interval beginning  872  is triggered by the second alarm output  742  ( FIG. 7 ). A second set of three, below-threshold time periods  832 – 834  also occurs within a time period T 2 =τ 2 , as illustrated. Thus, the alarm interval  870  continues. There is no third set of three, below-threshold time periods. Thus, after the end of the time interval T 3 =τ 2 , the alarm interval end  874  is triggered. This example illustrates how the pattern recognition alarm indicator  320  ( FIG. 7 ) can trigger an alarm on an event, such as a period of irregular heartbeats, that might be missed by a threshold-based alarm responsive to the slow SpO 2  measurement  412 . 
     Although some alarm processor embodiments were described above in terms of pulse oximetry and oxygen saturation measurements, one of ordinary skill in the art will recognize that an alarm processor as disclosed herein is also applicable to the measurement and monitoring of other blood constituents, for example blood glucose and total hemoglobin to name a few, and other physiological parameters such as blood pressure, pulse rate, respiration rate, and EKG to name a few. 
     A parallel measurement alarm processor 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.