METHOD AND APPARATUS FOR DETERMINING SpO2 OF A SUBJECT FROM AN OPTICAL MEASUREMENT

• US 20160066863A1
• Filed: 04/05/2013
• Published: 03/10/2016
• Est. Priority Date: 04/05/2013
• Status: Abandoned Application

• Current Assignee: Nitto Denko Corporation
• Original Assignee: Nitto Denko Corporation
• Inventors: THAVEEPRUNGSRIPORN, Visit, KASAMSOOK, Kittipong
• Application Number:
  US14/782,305
• Publication Number:
  US 20160066863A1
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7235 Details of waveform analysis A61B5/349 takes precedence
• CPC Class Codes: A61B 5/7246 using correlation, e.g. template matching or determination of similarity
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence
• CPC Class Codes: A61B 5/7289 Retrospective gating, i.e. associating measured signals or images with a physiological event after the actual measurement or image acquisition, e.g. by simultaneously recording an additional physiological signal during the measurement or image acquisition

Abstract

A method 100 and apparatus for determining SpO2, of a subject from an optical measurement is disclosed herein. In a described embodiment, the method 100 includes obtaining a PPG(red) signal and a PPG(IR) signal at steps 102 and 104, and pairing the PPG(red) and PPG(IR) signals at step 106 in which an amplitude of each cardiac rhythm cycle of the first signal is aligned to an amplitude of a respective cardiac rhythm cycle of the second signal to form a plurality of paired windows. At step 108, the method further includes, for each paired window, calculating values of a ratio R from the paired first and second signals and based on the calculated R values, binning the calculated R values into predetermined frequency bins at step 110. At steps 112 and 1 14, at least one of the frequency bins is selected to derive a revised ratio, R_rev and at step 1 16, Sp02 is derived from the revised ratio R_rev. A zoning schema for deriving SpO2 is also disclosed.

Claims

1. A method of determining SpO2 of a subject from an optical measurement, the method comprising:

(i) obtaining a first signal from a first light illumination;

(ii) obtaining a second signal from a second light illumination, the second light illumination having a wavelength which is different from that of the first light illumination;

(iii) pairing the first and second signals in which an amplitude of each cardiac rhythm cycle of the first signal is aligned to an amplitude of a respective cardiac rhythm cycle of the second signal to form a plurality of paired windows;

(iv) for each paired window, calculating values of a ratio R from the paired first and second signals;

(v) based on the calculated R values, binning the calculated R values into predetermined frequency bins; and

(vi) selecting at least one of the frequency bins to derive a revised ratio, R_rev; and

(vii) determining SpO2 from the revised ratio R_rev;

wherein

the method further comprises

(viii) obtaining AC_amb and DC_amb;

(viiii) calculating a ratio, R_amb-signal, based on AC_amb and AC_signal;

(x) providing a zoning schema divided into a plurality of reference zones with each reference zone defined by respective values of first, second and third reference variables which are associated respectively with R_rev, R_amb-signal and DC_amb;

each reference zone being associated with a corresponding method of computing SpO2;

(xi) comparing R_rev, R_amb-signal and DC_amb with respective reference variables of the zoning schema to determine which one of the plurality of reference zones is a selected zone; and

(xii) applying the method of computing SpO2 corresponding to the selected zone for determining SpO2;

where

AC_amb is an AC component value of an ambient signal with the first and second light illuminations switched off;

DC_amb is a DC component value of the ambient signal; and

AC_signal is an AC component value of the first or second signal.

5. A method according to claim 1, wherein selecting at least one of the frequency bins includes applying heuristic rules to the binned R values of the frequency bins.

6. A method according to claim 5, wherein the heuristic rules includes checking the frequency bins to determine a number of maximum bins based on number of R values.

7. A method according to claim 6, wherein there is one maximum bin, and the method further includes checking if frequency of the maximum bin exceeds a threshold, and if the threshold is exceeded, selecting the one maximum bin for deriving R_rev.

8. A method according to claim 7, wherein the one maximum bin includes a plurality of binned R values which are averaged to derive R_rev.

9. A method according to claim 6, wherein there are two maximum bins, and the method further includes

checking if the two maximum bins are directly adjacent to each other;

and

if the two maximum bins are directly adjacent to each other, checking if frequency of the two maximum bins exceeds a threshold, and if the threshold is exceeded,

selecting the two maximum bins for deriving R_rev.

10. A method according to claim 9, wherein if the two maximum bins are not directly adjacent to each other, the method further includes

checking if there is an additional bin between the two maximum bins;

and

if there is, selecting the additional bin and the two maximum bins for deriving R_rev.

11. A method according to claim 10, wherein if there are more than one additional bins between the two maximum bins, the method further includes rejecting the first and second signal waveforms.

12. A method according to claim 6, wherein there are three maximum bins, and the method further includes

checking if the three maximum bins are directly adjacent to each other;

and

if the three maximum bins are directly adjacent to each other, selecting the three maximum bins for deriving R_rev.

13. A method according to claim 12, wherein if the three maximum bins are not directly adjacent to each other, the method further includes

checking if there are two maximum bins which are directly adjacent to each other;

and

if there are, checking if frequency of the two directly adjacent maximum bins exceeds a threshold, and

if the threshold is exceeded, selecting the two directly adjacent maximum bins for deriving R_rev.

14. A method according to claim 13, wherein if there are no two maximum bins which are directly adjacent to each other, the method includes rejecting the first and second signal waveforms.

15. A method according to claim 7, wherein if the threshold is not exceeded, the method includes

selecting at least one peripheral bin directly adjacent to the maximum bin or bins;

and

calculating if the frequency of the at least one peripheral bin and the maximum bin or bins exceeds a further threshold to determine if only the maximum bin or bins are selected or the at least one peripheral bin is selected together with the maximum bin or bins for deriving R_rev.

16. A method according to claim 1, wherein R is calculated based on

17. A method according to claim 1, wherein SpO2 is derived from the revised ratio R_rev based on:

2. -4. (canceled)

18. (canceled)

19. A method of determining SpO2 of a subject from an optical measurement, the method comprising:

(i) in a first interval, obtaining a first signal from a first light illumination and a second signal from second light illumination, the second light illumination having a wavelength which is different from that of the first light illumination;

(ii) calculating values of a ratio R from the first and second signals;

(iii) in a second interval, obtaining an ambient signal with the first and second light illuminations switched off;

(iv) obtaining AC_amb and DC_amb;

(v) calculating a ratio, R_amb-signal, based on AC_amb and AC_signal;

where

AC_amb is an AC component value of the ambient signal;

DC_amb is a DC component value of the ambient signal;

AC_signal is an AC component value of the first or second signal;

wherein, the method further comprises

(vi) providing a zoning schema divided into a plurality of reference zones with each reference zone defined by respective values of first, second and third reference variables which are associated respectively with R, R_amb-signal and DC_amb;

each reference zone being associated with a corresponding method of computing SpO2;

(vii) comparing R, R_amb-signal and DC_amb with respective reference variables of the zoning schema to determine which one of the plurality of reference zones is a selected zone; and

(viii) applying the method of computing SpO2 corresponding to the selected zone for determining SpO2.

20. A method according to claim 19, further comprising obtaining a revised ratio, R_rev based on R and using R_rev in place of R at step (vii).

21. A method according to claim 20, further comprising pairing the first and second signals in which an amplitude of each cardiac rhythm cycle of the first signal is aligned to an amplitude of a respective cardiac rhythm cycle of the second signal to form a plurality of paired windows.

22. A method according to claim 21, further comprising,

for each paired window, calculating values of the ratio R from the paired first and second signals;

based on the calculated R values, binning the calculated R values into predetermined frequency bins; and

selecting at least one of the frequency bins to derive the revised ratio, R_rev.

23. (canceled)

24. A method of determining SpO2 of a subject from an optical measurement, the method comprising:

(i) in a first interval, obtaining a first signal from a first light illumination and obtaining a second signal from a second light illumination, the second light illumination having a wavelength which is different from that of the first light illumination;

(ii) pairing the first and second signals in which an amplitude of each cardiac rhythm cycle of the first signal is aligned to an amplitude of a respective cardiac rhythm cycle of the second signal to form a plurality of paired windows;

(iii) for each paired window, calculating values of a ratio R from the paired first and second signals;

(iv) based on the calculated R values, binning the calculated R values into predetermined frequency bins; and

(v) selecting at least one of the frequency bins to derive a revised ratio, R_rev;

(vi) in a second interval, obtaining an ambient signal with the first and second light illuminations switched off;

(vii) obtaining AC_amb and DC_amb;

(viii) calculating a ratio, R_amb-signal, based on AC_amb and AC_signal;

where

AC_amb is an AC component value of the ambient signal;

DC_amb is a DC component value of the ambient signal;

AC_signal is an AC component value of the first or second signal;

wherein, the method further comprises

(viiii) providing a zoning schema divided into a plurality of reference zones with each reference zone defined by respective values of first, second and third reference variables which are associated respectively with R_rev, R_amb-signal and DC_amb;

each reference zone being associated with a corresponding method of computing SpO2;

(x) comparing R_rev, R_amb-signal and DC_amb with respective reference variables of the zoning schema to determine which one of the plurality of reference zones is a selected zone; and

(xi) applying the method of computing SpO2 corresponding to the selected zone for determining SpO2.

25. (canceled)

26. Optical measurement apparatus for determining SpO2 of a subject, the apparatus comprising:

(a) a photodetector configured to

(i) in a first interval, obtain a first signal from a first light illumination and a second signal from second light illumination, the second light illumination having a wavelength which is different from that of the first light illumination;

(ii) in a second interval, obtain an ambient signal with the first and second light illuminations switched off;

(b) a processor configured to;

(iii) calculate values of a ratio R from the first and second signals;

(iv) obtain AC_amb and DC_amb;

(v) calculate a ratio, R_amb-signal, based on AC_amb and AC_signal;

where

AC_amb is an AC component value of the ambient signal;

DC_amb is a DC component value of the ambient signal;

AC_signal is an AC component value of the first or second signal; and

(c) a zoning schema divided into a plurality of reference zones with each reference zone defined by respective values of first, second and third reference variables which are associated respectively with R, R_amb-signal and DC_amb;

each reference zone being associated with a corresponding method of computing SpO2;

wherein the processor is further configured to;

(vi) compare R, R_amb-signal and DC_amb with respective reference variables of the zoning schema to determine which one of the plurality of reference zones is a selected zone; and

(vii) apply the method of computing SpO2 corresponding to the selected zone for determining SpO2.