System for pulse oximetry SpO2 determination

• US 5,934,277 A
• Filed: 03/16/1995
• Issued: 08/10/1999
• Est. Priority Date: 09/03/1991
• Status: Expired due to Term

• Current Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Original Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Inventors: Mortz, Margaret S.
• Primary Examiner(s): Weiss, John G.
• Assistant Examiner(s): Reichle, K. M.
• Application Number:
  US08/405,569
• Time in Patent Office: 1,608 Days
• Field of Search: 128/633, 128/664, 364/413.09, 356/41
• US Class Current: 600/323
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/6843 Monitoring or controlling sensor contact pressure
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

The improved pulse oximeter preprocesses the sets of red and infrared signals received from the probe to remove ambient light, to remove noise, and to de-exponeniate the signals. The linearity of the processed red and infrared signals allows the use of statistical techniques such as linear regression and linear correlation to fit a best fit straight line to the set of pairs of processed red and infrared data points and to measure the goodness of the straight line fit to these data points. The result of this analysis is a linear regression slope and a goodness of fit correlation coefficient. The correlation coefficient is a measure of the linearity of the input data points and, if less than a predetermined threshold, it indicates that a distorted signal has been received from the probe. This permits the pulse oximeter to detect probe off conditions and/or motion in the patient. The computed linear regression slope is converted to a RRatio which is used in a empirical calibration formula to compute the average SpO2 value. The minimum size of the data set required for high confidence calculations using this apparatus is significantly smaller than the a pulse period and permits faster response to changing input data.

Claims

1. Apparatus for determining oxygen saturation of hemoglobin in arterial blood using signals received from a probe, which signals are indicative of the light absorption of arterial blood, which has pulsatile and non-pulsatile components, at each of a respective one of two light wavelengths, said apparatus comprising:

means, responsive to said signals received from said probe, for producing a series of sets of data values, each of said sets including first and second data values, which are indicative of the light absorption of arterial blood at a respective one of said two light wavelengths;

means for storing a plurality of said sets of first and second data values;

means for computing a ratio of an effective optical extinction coefficient of said pulsatile component of said arterial blood at a first one of said two light wavelengths to an effective optical extinction coefficient of said pulsatile component of said arterial blood at a second one of said two light wavelengths, said ratio being determined by a slope of a linear regression fit taken through n data points, each data point comprising said first and second data values from a one of said stored plurality of sets of data values, where n is a positive integer greater than 2; and

means for determining oxygen saturation of said hemoglobin using said ratio.

2. The apparatus of claim 1 further comprising:

means for calculating an error measure indicative of a reliability of said first and second stored data values, taken over m of said sets of data values, where m is a positive integer greater than 1.

3. The apparatus of claim 2 further comprising:

means for generating an error indication, indicative of the presence of data values of questionable validity in said stored first and second data values which are used to compute said ratio, when said error measure exceeds a predetermined boundary.

4. The apparatus of claim 1 wherein said computing means includes:

means for filtering n sets of said stored first and second data values;

means for processing said n sets of first and second filtered data values to compute terms proportional to said effective optical extinction coefficient of said pulsatile component of said arterial blood at said first light wavelength and proportional to said effective optical extinction coefficient of said pulsatile component of said arterial blood at said second light wavelength; and

means for determining a linear regression fit to obtain a best fit line through at least one of said n sets of processed first and second data values to obtain a best fit to said processed values.

5. The apparatus of claim 4 wherein said computing means includes:

means for computing a linear correlation coefficient using said best fit line and said at least one of said n sets of processed first and second data values to indicate a linearity of said at least one of said n sets of processed first and second data values.

6. The apparatus of claim 5 further comprising:

means for generating an error indication, indicative of the presence of data values of questionable validity in at least one of said stored first and second data values which are used to compute said ratio, when said linear correlation coefficient is outside a predetermined boundary.

7. The apparatus of claim 4 wherein said computing means further includes:

means for normalizing said n sets of processed first and second data values.

8. The apparatus of claim 4 wherein said determining means includes:

means for computing a slope of said best fit line; and

means for determining oxygen saturation of said hemoglobin from said computed slope.

9. The apparatus of claim 1 wherein said computing means includes:

means for differentiating said stored sets of first and second sets of data values over time.

10. The apparatus of claim 1 wherein said computing means includes:

means for computing a logarithmic difference between said first data values in successive sets and said second data values in successive sets.

11. The apparatus of claim 1 wherein said computing means includes:

means for averaging said first and second data values of each set over adjacent ones of said sets.

12. The apparatus of claim 1 further comprising:

means for filtering said signals received from said probe to remove noise components therefrom.

13. A method for determining oxygen saturation of hemoglobin in arterial blood using signals received from a probe, which signals which are indicative of the light absorption of arterial blood, which has pulsatile and non-pulsatile components, at each of a respective one of two light wavelengths, said method comprising the steps of:

producing, in response to said signals received from said probe, a series of sets of data values, each of said sets including first and second data values, which are indicative of the light absorption of arterial blood at a respective one of said two light wavelengths;

storing a plurality of said sets of first and second data values;

computing a ratio of an effective optical extinction coefficient and said pulsatile component of said arterial blood at a first one of said two light wavelengths to an effective optical extinction coefficient of said pulsatile component of said arterial blood at a second one of said two light wavelengths, said ratio being determined by a slope of a linear regression fit taken through n data points, each data point comprising said first and second data values from a one of said stored plurality of sets of data values, where n is a positive integer greater than 2; and

determining oxygen saturation of said hemoglobin using said ratio.

14. The method of claim 13 further comprising the step of:

calculating an error measure indicative of a reliability of at least one of said first and second stored data values, taken over m of said sets of data values, where m is a positive integer greater than 1.

15. The method of claim 14 further comprising the step of:

generating an error indication, indicative of the presence of data values of questionable validity in at least one of said stored first and second data values which are used to compute said ratio, when said error measure exceeds a predetermined boundary.

16. The method of claim 13 wherein said step of computing includes:

filtering said n sets of stored first and second data values;

processing said n sets of first and second filtered data values to compute terms proportional to said effective optical extinction coefficient of said pulsatile component of said arterial blood at said first light wavelength and proportional to said effective optical extinction coefficient of said pulsatile component of said arterial blood at said second light wavelength; and

determining a linear regression fit to obtain a best fit line through at least one of said n sets of processed first and second data values to obtain a best fit of said processed values.

17. The method of claim 16 wherein said step of computing includes:

computing a linear correlation coefficient using said best fit line and said at least one of said n sets of processed first and second data values to indicate a linearity of said at least one of said n sets of processed first and second data values.

18. The method of claim 17 further comprising the step of:

generating an error indication, indicative of the presence of data values of questionable validity in at least one of said stored first and second data values which are used to compute said ratio, when said linear correlation coefficient is outside a predetermined boundary.

19. The method of claim 16 wherein said step of computing further includes:

normalizing said n sets of processed first and second data values.

20. The method of claim 16 wherein said step of determining includes:

computing a slope of said best fit line; and

determining oxygen saturation of said hemoglobin from said computed slope.

21. The method of claim 13 wherein said step of computing includes:

differentiating said stored sets of first and second sets of data values over time.

22. The method of claim 13 wherein said step of computing includes:

computing a logarithmic difference between said first data values in successive sets and said second data values in successive sets.

23. The method of claim 13 wherein said step of computing includes:

averaging said first and second data values of each set over adjacent ones of said sets.

24. The method of claim 13 further comprising the steps of:

filtering said signals received from said probe to remove noise components therefrom.

25. Apparatus for determining oxygen saturation of hemoglobin in arterial blood using signals received from a probe, which signals are indicative of the light absorption of arterial blood, which has pulsatile and non-pulsatile components, at each of a respective one of two light wavelengths, said apparatus comprising:

a probe interface circuit responsive to said signals received from said probe wherein said probe interface circuit produces a series of sets of data values, each of said sets including first and second data values, which are indicative of the light absorption of arterial blood at a respective one of said two light wavelengths;

a memory wherein a plurality of said sets of first and second data values are stored;

a data processing circuit which computes a ratio of an effective optical extinction coefficient of said pulsatile component of said arterial blood at a first one of said two light wavelengths to an effective optical extinction coefficient of said pulsatile component of said arterial blood at a second one of said two light wavelengths, said ratio being determined by a slope of a linear regression fit taken through n data points, each data point comprising said first and second data values from a one of said stored plurality of sets of data values, where n is a positive integer greater than 2 and which then computes said oxygen saturation of said hemoglobin using said ratio.

26. The apparatus according to claim 25 further comprising a display wherein said oxygen saturation of said hemoglobin is displayed to a user of said apparatus.