Abstract:
A physiological assessment system comprises a sensor and first and second processors. The sensor is adapted to generate a signal responsive to a living organism. The first processor is configured to derive a measured parameter from the sensor signal. The second processor is configured to analyze nonlinear dynamics of the measured parameter so as to provide a physiological assessment of the living organism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application relates to and claims the benefit of prior U.S. Provisional Application No. 60/558,426 entitled Chaotic Analysis of Pulse Oximetry Signals in the Assessment of Neonatal Illness Severity, filed Mar. 31, 2004 and incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for measuring the oxygen saturation level of arterial blood. Pulse oximeters perform a spectral analysis of the pulsatile component of arterial blood in order to determine the relative concentration of oxygenated hemoglobin, the major oxygen carrying constituent of blood. By providing early detection of decreases in the arterial oxygen supply, pulse oximetry reduces the risk of accidental death and injury. As a result, pulse oximeters have gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care.  
         [0003]     A typical pulse oximeter comprises a sensor and a monitor. The sensor has emitters and a detector and is attached to a patient at a selected tissue site, such as a fingertip or ear lobe. The emitters project light at red and infrared (IR) wavelengths through the blood vessels and capillaries of the tissue site. The detector is positioned so as to detect the emitted light as it emerges from the tissue site. The signal generated by the detector is proportional to the intensity of the detected light. The detector signal has a variable (AC) component due to light absorption by the pulsatile volume of arterial blood. The detector signal also has a constant (DC) component due to light absorption by the non-pulsatile volume of arterial blood, venous blood, bone and other tissue. A signal processor inputs the detector signal and determines oxygen saturation and pulse rate, which are typically shown as a numerical readout on a display. In addition to oxygen saturation and pulse rate (PR), some pulse oximeters measure perfusion index (PI), which is a relative indication of pulse strength at a monitoring site. In particular, PI is the ratio of the IR AC signal to the IR DC signal. A pulse oximetry sensor and monitor are described, respectively, in U.S. Pat. No. 6,088,607 entitled Low Noise Optical Probe and U.S. Pat. No. 5,482,036 entitled Signal Processing Apparatus and Method, both of which are assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.  
       SUMMARY OF THE INVENTION  
       [0004]     Nonlinear dynamics is the study of systems governed by equations in which a small change in one variable can induce a large systematic change. Chaos theory is one aspect of nonlinear dynamics, which attempts to construct deterministic, nonlinear dynamic models that elucidate irregular, unpredictable behavior. Chaos theory teaches that complex non-linear systems may not reach steady state, but will behave chaotically depending on minute changes in environmental conditions. Nonlinear dynamics can be advantageously applied to analyze the variability of physiological parameters for illness assessment. For example, chaos data analysis is applied to real-time pulse oximetry signals, including perfusion index or pulse rate or both, so as to determine or predict the severity of patient illness or, in contrast, the degree of patient wellness.  
         [0005]     A healthy individual has a physiological system with a complex dynamic that enables it to adapt to a constantly changing environment. Many serious diseases have pathologies that reduce the dynamic complexity of the human physiological system. This translates into reduced variability of measurable physiological parameters, such as those derived by pulse oximetry. As such, the variability of measured physiological parameters can be used to assess illness severity. Use of non-invasively-obtained physiological parameters, for example, parameters obtained through the use of pulse-oximetry, has the advantage that system variability may be monitored continuously and the onset of serious disease may therefore be detected sooner than would otherwise be possible. The early detection of serious disease can be especially important with patients that are highly susceptible and for whom the time interval between disease onset and mortality can be very short, such as neonatal patients.  
         [0006]     One aspect of a physiological assessment system is a sensor and first processor and a second processor. The sensor is adapted to generate a signal responsive to a living organism. The first processor is configured to derive a measured parameter from the sensor signal. The second processor is configured to analyze nonlinear dynamics of the measured parameter so as to provide a physiological assessment of the living organism. In one embodiment, the sensor is adapted to transmit optical radiation of at least two wavelengths into a tissue site and generate a detector signal responsive to the optical radiation after absorption by pulsatile blood flow within the tissue site, the first processor is a pulse oximeter and the measured parameter is a perfusion index. The second processor may calculate a statistic responsive to the variability of the perfusion index and the physiological assessment may correspond to illness severity. Further, the statistic may be determined by chaos data analysis, which may be compared to a predetermined normality range. In a particular embodiment, the pulse oximeter derives a second measured parameter and the second processor calculates a second statistic responsive to the variability of the second measured parameter, wherein the second measured parameter may be pulse rate.  
         [0007]     Another aspect of a physiological assessment system is a pulse oximeter, a variability analyzer and a statistics interpreter. The pulse oximeter is adapted to measure a perfusion index. The variability analyzer is configured to provide a statistic or statistics responsive to variability of the perfusion index. The statistics interpreter is configured to indicate illness severity based upon the statistic or statistics. The physiological assessment system may further comprise a data storage for compiling time series data for the perfusion index. The system may also comprise a visual indication perfusion index variability or illness severity or both. In addition, the system may comprise an aural indication perfusion index variability or illness severity or both. In one embodiment, the analyzer is a chaos data analyzer. A predetermined range may be input to the interpreter, wherein the interpreter is capable of comparing statistic or statistics to the range so as to indicate illness severity.  
         [0008]     A further aspect of a physiological assessment system comprises time series data derived for at least one physiological parameter and a nonlinear dynamics measure calculated based upon the time series data. The nonlinear dynamics measure is evaluated with respect to a predetermined criterion, and a physiological assessment is provided based upon the evaluation. The time series data may be a perfusion index measured with a pulse oximeter. The nonlinear dynamics measure may be a variability measure obtained for the perfusion index. The variability measure may be compared to a predetermined cutoff value, and an illness severity indication may be outputted. In a particular embodiment, a pulse rate is also measured and a variability measure is calculated for the combination of perfusion index and pulse rate. A chaos data analysis may be performed on the time series data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a general block diagram of a physiological assessment system;  
         [0010]      FIG. 2  is a block diagram of one embodiment of a physiological assessment system; and  
         [0011]      FIG. 3  is a flowchart of a physiological assessment process. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]      FIG. 1  illustrates a physiological assessment system  100  that advantageously monitors the nonlinear dynamics of one or more physiological parameters  122  so as to provide a health assessment  132  of a living organism. The physiological assessment system  100  has one or more sensors  110  capable of responding to a living organism so as to provide sensor signals  112  to one or more physiological parameter processors  120 . The physiological parameter processors  120  provide measured parameters  122  derived from the sensor signals  112 . A nonlinear dynamics processor  130  analyzes the dynamics of the measured parameters  122  so as to provide a physiological assessment  132  of the living organism. A sensor(s)  110  may be a pulse oximetry sensor, a blood pressure transducer, ECG or EEG electrodes or a capnometer, or a combination of these, to name a few. The physiological parameter processor(s)  120 , accordingly, may be a pulse oximeter, a blood pressure monitor, an ECG monitor, an EEG monitor, a CO 2  monitor or a multi-parameter patient monitor, or a combination of these, to name a few. Likewise, the measured parameters  122  may be, as examples, oxygen saturation, pulse rate, perfusion index, blood glucose, blood pressure and ETCO2 among others. The nonlinear dynamics processor  130  can be hardware, software or a combination that analyzes, for example, the variability of the measured parameters  122 . The physiological assessment  132  may be a numerical readout, message or other visual display or an alarm or other audible indication, or a combination of these. The physiological assessment  132  may assist in the determination of illness severity, wellness, or depth of sedation, to name a few; may provide information helpful for triage in emergency, hospital or surgical environments, for example; or may help in the prediction of, for instance, recovery time, length of hospitalization or medical costs.  
         [0013]      FIG. 2  illustrates one embodiment of a physiological assessment system  200  having a pulse oximeter  210 , a data storage  220 , a variability analyzer  230  and a statistics interpreter  240 . The pulse oximeter  210  measures parameters  212  such as perfusion index (PI) or pulse rate (PR) or a combination of PI and PR. The data storage  212  compiles a trend of the parameters  212  so as to create a data record  224 . The data record length (T) is set by a predetermined storage interval input  222 . The variability analyzer  230  processes the data record  224  so as to provide data statistics  232 . In a particular embodiment, the variability analyzer  230  is a Chaos Data Analyzer (CDA-Pro) available from The Academic Software Library, North Carolina State University, Raleigh, N.C. The CDA-Pro performs various tests for detecting hidden determinism in a seemingly random time series, such as the probability distribution, power spectrum, Lyapunov exponent, and various measures of the fractal dimension. The statistics interpreter  240  evaluate the data statistics  232  to determine if variability is in a normal or abnormal range and provides an illness severity indicator  244  accordingly. A predetermined normality range input  242  sets this range. The illness severity indicator  244  may be displayed  250  or may trigger an alarm  260  or both. In an alternative embodiment, the range input  242  may specify multiple ranges, which may include one or more of variability above a normal level indicating illness, variability within a normal range indicating, variability within an indeterminate range, and variability below a normal range indicating illness. The range input  242  may also be a threshold or cutoff between normal and abnormal variability. In another embodiment, PR variability may be used as an early predictor of a return to health, and PI variability may be used as an early predictor of the onset of illness.  
         [0014]     In a particular embodiment, the pulse oximeter  210  incorporates the data storage  220 , variability analyzer  230  and statistics interpreter  240 . The display  250  is a scaled readout on a pulse oximeter  210  showing PI variance on a relative scale having a gauge of maximum and minimum ranges. An alarm  260  is configured to sound when PI variance drops below a predetermined level.  
         [0015]      FIG. 3  illustrates a physiological assessment method  300  wherein physiological parameters are iteratively measured  310  and recorded as a time series  320 . The physiological parameters may be, for example, perfusion index or pulse rate or both. Nonlinear linear dynamics of the time series are calculated  330 . These calculations may be based upon chaos data analysis. The nonlinear dynamics are evaluated with respect to predetermined criteria  340  so as to provide a physiological assessment  350 . In one embodiment, the nonlinear dynamics may provide a measure of variability of the physiological parameter or parameters, which may be compared to a range of normal or abnormal variability or, alternatively, a threshold or cutoff value of variability so as to assess illness severity or depth of sedation, for example.  
         [0016]     A physiological assessment system 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 art will appreciate many variations and modifications.