Method and apparatus for measuring blood oxygen saturation

• US 8,280,472 B2
• Filed: 12/27/2007
• Issued: 10/02/2012
• Est. Priority Date: 07/19/2007
• Status: Active Grant

• Current Assignee: Shenzhen Mindray Bio-Medical Electronics Co., Ltd. (Excelsior Union Ltd.)
• Original Assignee: Shenzhen Mindray Bio-Medical Electronics Co., Ltd. (Excelsior Union Ltd.)
• Inventors: Li, Xu, Zhang, Xu
• Primary Examiner(s): Winakur, Eric
• Assistant Examiner(s): Fardanesh, Marjan
• Application Number:
  US11/965,634
• Publication Number:
  US 20090024012A1
• Time in Patent Office: 1,741 Days
• Field of Search: 600309-344
• US Class Current: 600/323
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7242 using integration

Abstract

A method and apparatus are disclosed for measuring blood oxygen saturation by using spectrophotometry to improve the accuracy of the measurement under a condition of low perfusion.

Claims

1. A method for measuring blood oxygen saturation, the method comprising:

acquiring intensities of transmitted lights that are obtained by transmitting respectively a light of a first wavelength and a light of a second wavelength through organism tissues, and converting the intensities of the transmitted lights into corresponding signals for the light of the first wavelength and the light of the second wavelength;

defining at least one interval on waveforms of the signals for the light of the first wavelength and the light of the second wavelength, and performing an area integral on the waveforms of the signals for the light of the first wavelength and the light of the second wavelength in the at least one interval to produce a total area integral of the signal for the light of the first wavelength and a total area integral of the signal for the light of the second wavelength;

calculating a ratio of the total area integral of the signal for the light of the first wavelength to the total area integral of the signal for the light of the second wavelength as a blood oxygen content R; and

calculating a blood oxygen saturation according to the following formula;

SpO₂=(A×

R+B)/(C×

R+D),

where SpO₂ is the blood oxygen saturation;

A=ε

₄;

B=−

ε

₂;

C=ε

₄−

ε

₃;

D=ε

₁−

ε

₂;

ε

₁, ε

₂ are respectively an absorption rate of oxyhemoglobin to the light of the first wavelength and an absorption rate of deoxyhemoglobin to the light of the first wavelength; and

ε

₃, ε

₄ are respectively an absorption rate of oxyhemoglobin to the light of the second wavelength and an absorption rate of deoxyhemoglobin to the light of the second wavelength;

wherein the at least one interval is divided into different confidence intervals according to different degrees of noise interference in a pulse fluctuation cycle, wherein a confidence value of one confidence interval with relatively high interference is less than that of another confidence interval with relatively low interference, wherein the total area integral of the signal for the light of the first wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the first wavelength in all the confidence intervals and the corresponding confidence, and wherein the total area integral of the signal for the light of the second wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the second wavelength in all the confidence intervals and the corresponding confidence.

2. The method of claim 1, further comprising:

before defining the at least one interval, performing respectively a natural logarithm operation on the signal for the light of the first wavelength and the signal for the light of the second wavelength.

3. The method of claim 1, wherein the confidence intervals include a rise stage interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at a rise edge, and a fall edge interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at a fall edge, wherein the confidence of the rise edge interval is less than the confidence of the fall edge interval.

4. The method of claim 3, wherein determining the rise edge interval and the fall edge interval comprises:

searching a wave valley in each pulse cycle of the signal for the light of the first wavelength and a wave valley in each pulse cycle of the signal for the light of the second wavelength from the signal over a period of time to obtain positions V_R1, V_R2, V_Rj . . . V_Rn of the wave valleys of the signal for the light of the first wavelength and positions V_I1, V_I2, V_Ij . . . V_In of the wave valleys of the signal for the light of the second wavelength, where j is the jth pulse cycle;

determining positions M_R1, M_R2, M_Rj . . . M_Rn of wave peaks of the signal for the light of the first wavelength and positions M_I1, M_I2, M_Ij . . . M_In of wave peaks of the signal for the light of the second wavelength based on the positions of the wave valleys;

determining, based on the values of the wave peak and the wave valley of each pulse cycle, a length L of the fall edge of the pulse cycle,

L=min(V_Ri,V_Ii)−

max(M_Ri,M_Ii),

where V_Ri, M_Ri are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the first wavelength, V_Ii, M_Ii are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the second wavelength, and i is any value between 1 and n; and

determining the rise interval [V_ri, M_ri] and the fall interval [M_fi, V_fi] in the i-th pulse cycle based on the values of the wave peak and the wave valley and the length L of the fall edge in the i-th pulse cycle,

wherein V_ri=max (V<sub>R(i−

1)</sub>, V<sub>I(i−

1)</sub>), M_ri=min (M_Ri, M_Ii), M_fi=max (M_Ri, M_Ii)+L×

0.1, and V_fi=min (V_Ri, V_Ii)−

L×

0.1.

5. The method of claim 4, wherein in determining the length L of the fall edge, when min (V_Ri, V_Ii)<

=max (M_Ri, M_Ii), the corresponding time calculation of the blood oxygen is cancelled, and when min (V_Ri, V_Ii)>

=max (M_Ri, M_Ii), the length L of the fall edge is determined as;

L=min(V_Ri,V_Ii)−

max(M_Ri,M_Ii).

6. The method of claim 4, wherein the confidence of the rise interval edge is σ

_r, and the confidence of the fall edge interval is σ

_f, wherein

σ

_r=0.25×

(0.8−

γ

<sub>v−

1</sub>−

γ

_m),

σ

_f=0.8−

γ

_v−

γ

_m,

where γ

_m is a fluctuation coefficient of the wave peak in the i-th pulse cycle and γ

_v is a fluctuation coefficient of the wave valley in the i-th pulse cycle,

7. The method of claim 6, wherein the blood oxygen content of the i-th pulse cycle is:

8. The method of claim 7, wherein when the fluctuation coefficient of the wave peak or the wave valley is more than 0.4, the confidence of the blood oxygen content in the corresponding pulse cycle is zero.

9. An apparatus for measuring blood oxygen saturation, the apparatus comprising:

a signal generation component to acquire intensities of transmitted lights that are obtained by transmitting a light of a first wavelength and a light of a second wavelength through organism tissues and to convert the acquired intensities of the transmitted lights into corresponding signals for the light of the first wavelength and the light of the second wavelength;

an interval determination component to define at least one interval on waveforms of the signals for the light of the first wavelength and the light of the second wavelength;

an integral component to perform a natural logarithm operation on the signals for the light of the first wavelength and the light of the second wavelength in the at least one interval respectively;

a first calculating component to calculate a ratio of the total area integral of the signal for the light of the first wavelength to the total area integral of the signal for the light of the second wavelength as the blood oxygen content R; and

a second calculating component to calculate blood oxygen saturation according to the following formula;

SpO₂=(A×

R+B)/(C×

R+D),

where SpO₂ is the blood oxygen saturation;

A=ε

₄;

B=−

ε

₂;

C=ε

₄−

ε

₃;

D=ε

₁−

ε

₂;

ε

₁, ε

₂ are respectively an absorption rate of oxyhemoglobin to the red light and an absorption rate of deoxyhemoglobin to the red light; and

ε

₃, ε

₄ are respectively an absorption rate of oxyhemoglobin to the infrared light and an absorption rate of deoxyhemoglobin to the infrared light;

wherein the interval determination component is used to divide the at least one interval into different confidence intervals according to the different degrees of the noise interference within the pulse fluctuation cycle, and the confidence for the interval with greater interference is less than the confidence for the interval with lower interference, and the integral component is further used to calculate the total area integral of the signal for the light of the first wavelength and the total area integral of the signal for the light of the second wavelength according to the confidence of each confidence interval.

10. The apparatus of claim 9, further comprising:

a logarithm operation component to perform a natural logarithm operation on the signals for the light of the first wavelength and the light of the second wavelength respectively and to output the result to the integral component.

11. The apparatus of claim 9, wherein the confidence intervals include a rise edge interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at rise stage and a fall edge interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at fall stage, wherein the confidence of the rise edge interval is less than the confidence of the fall edge interval.

12. The apparatus of claim 11, wherein the interval determination component includes:

a peak and valley determination unit to respectively search a wave valley in each pulse cycle of the signal for the light of the first wavelength and a wave valley in each pulse cycle of the signal for the light of the second wavelength from the signal over a period of time to obtain positions V_R1, V_R2, V_Rj . . . V_Rn of the wave valleys of the signal for the light of the first wavelength and positions V_I1, V_I2, V_I1 . . . V_In of the wave valleys of the signal for the light of the second wavelength, where j is the jth pulse cycle, and to determine positions M_R1, M_R2, M_Rj . . . M_Rn of wave peaks of the signal for the light of the first wavelength and positions M_I1, M_I2, M_Ij . . . M_In of wave peaks of the signal for the light of the second wavelength based on the positions of the wave valleys;

a fall edge length determination unit to determine, based on the values of the wave peak and the wave valley of each pulse cycle, a length L of the fall edge of the pulse cycle,

L=min(V_Ri,V_Ii)−

max(M_Ri,M_Ii,

where V_Ri, M_Ri are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the first wavelength, V_Ii, M_Ii, are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the second wavelength, and i is any value between 1 and n; and

a rise edge and fall edge intervals determination unit to determine the rise interval and the fall interval in the i-th pulse cycle based on the values of the wave peak and the wave valley and the length L of the fall edge in the i-th pulse cycle, wherein the rise edge interval is [V_ri, M_ri] and the fall edge interval is [M_fi, V_fi], wherein V_ri=max (V<sub>R(i−

1)</sub>, V<sub>I(i−

1)</sub>), M_ri=min (M_Ri, M_Ii), M_fi=max (M_Ri, M_Ii)+L×

0.1, and V_fi=min (V_Ri, V_Ii)−

L×

0.1.

13. The apparatus of claim 12, wherein the interval determination component further includes a confidence determination unit to determine confidences of the rise edge interval and the fall edge interval, wherein the confidence of the rise edge interval is σ

_r, the confidence of the fall edge interval is σ

_f, and

σ

_r=0.25×

(0.8−

γ

<sub>v−

1</sub>−

γ

_m),

σ

_f=0.8−

γ

_v−

γ

_m,

where γ

_m is a fluctuation coefficient of the wave peak in the i-th pulse cycle and γ

_v is a fluctuation coefficient of the wave valley at the i-th pulse cycle,

14. The apparatus of claim 13, wherein the blood oxygen content in the i-th pulse cycle is:

15. A non-transitory, computer-readable medium comprising program code for performing a method for measuring blood oxygen saturation, the method comprising:

acquiring intensities of transmitted lights that are obtained by transmitting respectively a light of a first wavelength and a light of a second wavelength through organism tissues, and converting the intensities of the transmitted lights into corresponding signals for the light of the first wavelength and the light of the second wavelength;

defining at least one interval on waveforms of the signals for the light of the first wavelength and the light of the second wavelength, and performing an area integral on the waveforms of the signals for the light of the first wavelength and the light of the second wavelength in the at least one interval to produce a total area integral of the signal for the light of the first wavelength and a total area integral of the signal for the light of the second wavelength;

calculating a ratio of the total area integral of the signal for the light of the first wavelength to the total area integral of the signal for the light of the second wavelength as a blood oxygen content R; and

calculating a blood oxygen saturation according to the following formula;

SpO₂=(A×

R+B)/(C×

R+D),

where SpO₂ is the blood oxygen saturation;

A=ε

₄;

B=−

ε

₂;

C=ε

₄−

ε

₃;

D=ε

₁−

ε

₂;

ε

₁, ε

₂ are respectively an absorption rate of oxyhemoglobin to the light of the first wavelength and an absorption rate of deoxyhemoglobin to the light of the first wavelength; and

ε

₃, ε

₄ are respectively an absorption rate of oxyhemoglobin to the light of the second wavelength and an absorption rate of deoxyhemoglobin to the light of the second wavelength;

wherein the at least one interval is divided into different confidence intervals according to different degrees of noise interference in a pulse fluctuation cycle, wherein a confidence value of one confidence interval with relatively high interference is less than that of another confidence interval with relatively low interference, wherein the total area integral of the signal for the light of the first wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the first wavelength in all the confidence intervals and the corresponding confidence, and wherein the total area integral of the signal for the light of the second wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the second wavelength in all the confidence intervals and the corresponding confidence.

16. The computer-readable medium of claim 15, wherein the confidence intervals include a rise stage interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at a rise edge, and a fall edge interval in which the signal for the light of the first wavelength and the signal for the light of the second wavelength are at a fall edge, wherein the confidence of the rise edge interval is less than the confidence of the fall edge interval.

17. The computer-readable medium of claim 16, wherein determining the rise edge interval and the fall edge interval comprises:

searching a wave valley in each pulse cycle of the signal for the light of the first wavelength and a wave valley in each pulse cycle of the signal for the light of the second wavelength from the signal over a period of time to obtain positions V_R1, V_R2, V_Rj . . . V_Rn of the wave valleys of the signal for the light of the first wavelength and positions V_I1, V_I2, V_Ij . . . V_In of the wave valleys of the signal for the light of the second wavelength, where j is the jth pulse cycle;

determining positions M_R1, M_R2, M_Rj . . . M_Rn of wave peaks of the signal for the light of the first wavelength and positions M_I1, M_I2, M_Ij . . . M_In of wave peaks of the signal for the light of the second wavelength based on the positions of the wave valleys;

determining, based on the values of the wave peak and the wave valley of each pulse cycle, a length L of the fall edge of the pulse cycle,

L=min(V_Ri,V_Ii)−

max(M_Ri,M_Ij),

where V_Ri, M_Ri are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the first wavelength, V_Ii, M_Ii, are respectively positions of the wave peak and the wave valley in the i-th pulse cycle of the signal for the light of the second wavelength, and i is any value between 1 and n; and

determining the rise interval [V_ri, M_ri]_i and the fall interval [M_fi, V_fi] in the i-th pulse cycle based on the values of the wave peak and the wave valley and the length L of the fall edge in the i-th pulse cycle,

wherein V_ri=max (V<sub>R(i−

1)</sub>, V<sub>I(i−

1)</sub>), M_ri=min (M_Ri, M_Ii), M_fi=max (M_Ri, M_Ii)+L×

0.1, and V_fi=min (V_Ri, V_Ii)−

L×

0.1.

18. The computer-readable medium of claim 17, wherein in determining the length L of the fall edge, when min (V_Ri, V_Ii)<

=max (M_Ri, M_Ii), the corresponding time calculation of the blood oxygen is cancelled, and when min (V_Ri, V_Ii)>

max (M_Ri, M_Ii), the length L of the fall edge is determined as;

L=min(V_Ri,V_Ii)−

max(M_Ri,M_Ii).

19. The computer-readable medium of claim 17, wherein the confidence of the rise interval edge is σ

_r, and the confidence of the fall edge interval is σ

_f, wherein

σ

_r=0.25×

(0.8−

γ

_v-1−

γ

_m),

σ

_f=0.8−

γ

_v−

γ

_m,

where γ

_m is a fluctuation coefficient of the wave peak in the i-th pulse cycle and γ

_v is a fluctuation coefficient of the wave valley in the i-th pulse cycle,

20. The computer-readable medium of claim 19, wherein the blood oxygen content of the i-th pulse cycle is:

21. The computer-readable medium of claim 20, wherein when the fluctuation coefficient of the wave peak or the wave valley is more than 0.4, the confidence of the blood oxygen content in the corresponding pulse cycle is zero.

22. A system for measuring blood oxygen saturation, the system comprising:

means for acquiring intensities of a first light at a first wavelength and a second light at a second wavelength transmitted through organism tissues and for converting the acquired intensities of the transmitted lights into signals corresponding to the light of the first wavelength and the light of the second wavelength;

means for defining at least one interval on waveforms of the signals for the light of the first wavelength and the light of the second wavelength;

means for performing a natural logarithm operation on the signals for the light of the first wavelength and the light of the second wavelength in the at least one interval;

means for calculating a ratio of the total area integral of the signal for the light of the first wavelength to the total area integral of the signal for the light of the second wavelength as the blood oxygen content R; and

means for calculating blood oxygen saturation according to the following formula;

SpO₂=(A×

R+B)/(C×

R+D),

where SpO₂ is the blood oxygen saturation;

A=ε

₄;

B=−

ε

₂;

C=ε

₄−

ε

₃;

D=ε

₁−

ε

₂;

ε

₁, ε

₂ are respectively an absorption rate of oxyhemoglobin to the red light and an absorption rate of deoxyhemoglobin to the red light; and

ε

₃, ε

₄ are respectively an absorption rate of oxyhemoglobin to the infrared light and an absorption rate of deoxyhemoglobin to the infrared light;

wherein the at least one interval is divided into different confidence intervals according to different degrees of noise interference in a pulse fluctuation cycle, wherein a confidence value of one confidence interval with relatively high interference is less than that of another confidence interval with relatively low interference, wherein the total area integral of the signal for the light of the first wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the first wavelength in all the confidence intervals and the corresponding confidence, and wherein the total area integral of the signal for the light of the second wavelength comprises a sum of products of the area integral of the waveform of the signal for the light of the second wavelength in all the confidence intervals and the corresponding confidence.