Measuring process for blood gas analysis sensors

• US 6,226,540 B1
• Filed: 06/04/1999
• Issued: 05/01/2001
• Est. Priority Date: 12/13/1995
• Status: Expired due to Term

• Current Assignee: Bernreuter, Peter
• Original Assignee: Bernreuter, Peter
• Inventors: Bernreuter, Peter
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US09/325,841
• Time in Patent Office: 697 Days
• Field of Search: 600/310, 600/322, 600/323, 600/330, 600/333, 600/336
• US Class Current: 600/323
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: G01N 21/3151 using two sources of radiation of different wavelengths G01N21/33 - G01N21/39 take precedence

Abstract

The invention relates to a measuring process, the purpose of which is to increase the measuring accuracy of pulse oxymeters and comparable optical devices which are used in vivo to ascertain oxygen saturation of arterial blood. The measuring process according to the invention is provided for the purpose of ascertaining oxygenation of arterial blood in tissue by evaluating the differential light attenuation at several wavelengths. It is characterized by the fact that light attentuation of at least one wavelength is determined in order to choose those calibration curves of several variables (Ω1, Ω2, . . . ) produced by different wavelength pairings with a minimized error in order to generate an output signal for the arterial blood oxygenation.

Claims

1. A method of determining the level of oxygenation of arterial blood SaO₂ in tissue in vivo considering optical tissue properties by pulse oximetry, comprising the steps of:

a) emitting light through said tissue at a plurality of wavelengths at different instants of time, and different blood flow;

b) measuring said light received passing through said tissue at said plurality of wavelengths at said instants of time;

c) calculating a plurality of ratios of differential attenuation vs. time, Ω

_i corresponding to ${\mathrm{\Omega <br><br>}}_i=\frac{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_a}{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_b}$

for each wavelength pair i=a,b of different frequencies, wherein at least one wavelength pairing is selected based on optical tissue parameters, sufficient wavelength pairings and calibration curves of SaO2 v. Ω

_i have been empirically made such that the calibration curves are chosen for Ω

_i depending on optical tissue parameters to minimize error in producing an output signal indicative of level of arterial oxygenation;

d) calculating at least one attenuation ratio LA_j through said tissue for at least one wavelength, where LA_j corresponds to;

LA_j=ln (light intensity received/light intensity emitted);

using calibration data of SaO₂ v. Ω

_i for at least one said LA_j; and

f) computing the level of oxygenation by comparing measured data with the calibration data.

2. The method of claim 1, wherein one of the wavelength pairings is at isobestic points at 560 and at 805 nm.

3. The method of claim 1, wherein one of the wavelength pairings is at about 660 and about 740 nm.

6. The method of claim 1 or 4, wherein at least two emitter/detector pairs are utilized, comprising the steps of:

g) measuring light emitted in a first optical unit and detected in a first optical unit;

h) measuring light emitted in said first optical unit and detected in a second optical unit;

i) measuring light emitted in said second optical unit and detected in said first optical unit;

j) measuring light emitted in said second optical unit and detected in said second optical unit;

k) weighting light attenuations between detectors and emitters to generate LA;

l) repeating steps g)-k) for more than two optical units and weighting generated values of LA in order to generate a resulting light attenuation; and

m) using the determined light attenuation to choose an associated calibration curve.

7. The method of claim 1 or 4, wherein at least two emitter/detector optical units are utilized, comprising the steps of:

n) measuring light emitted in a first optical unit and detected in said first optical unit A1;

o) measuring light emitted in said first optical unit and detected in a second optical unit A2;

p) measuring light emitted in said second optical unit and detected in said first optical unit A3;

q) measuring light emitted in said second optical unit and detected in said second optical unit A4;

r) computing the optical constitution of said tissue alone relating to the measured light attenuation by weighting and accumulating the light attenuations between the optical units (A2, A3) and subtracting therefrom the weighted and accumulated light attenuations within the optical units (A1, A4); and

s) repeating steps n)-r) for any other sets of optical unit pairs, and an appropriate weighting in accordance with step r); and

t) generating an output signal for the arterial oxygen saturation depending on LA or a set of LA values.

8. The method of claim 7, wherein light attenuation is defined as LA_k=ln (light intensity received).

9. The method of claim 1 or 4, wherein said attenuation ratio LA_j is calculated as:

${\mathrm{LA}}_j=\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{light}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{received}}{\mathrm{light}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{emitted}}.$

10. The method of claim 1 or 4, wherein LA_j depends on change of light intensity versus time emitted by at least one emitter I_n(t) and on change of light intensity received by at least one receiver I(t) which receives the light of said at least one emitter.

14. The method of claim 1, 4, 11 or 13, wherein one of the wavelength pairings is at isobestic points 560 and 805 nm.

15. The method of claim 1, 4, 11 or 13, wherein one of the wavelength pairings is at about 660 and about 740 nm.

16. The method of claim 1, 4, 11 or 13, additionally comprising calculating Ω

_i as;

${\mathrm{\Omega <br><br>}}_1=\frac{\frac{\mathrm{change}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{at}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{freq}.\text{\hspace{1em}<br><br>}<br><br>a}{\mathrm{total}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{light}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{at}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{freq}.\text{\hspace{1em}<br><br>}<br><br>a}}{\frac{\mathrm{change}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{of}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{at}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{freq}.\text{\hspace{1em}<br><br>}<br><br>b}{\mathrm{total}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{light}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{intensity}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{at}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{freq}.\text{\hspace{1em}<br><br>}<br><br>b}}.$

18. The method of claim 1, 4, 11 or 13, wherein tissue inhomogeneity LAT_m is evaluated by measuring:

u) differential attenuation, dLA_j, at a first distance between emitter and detector;

v) differential attenuation, dLA_k, at a second distance between emitter and detector;

w) calculating LAT_m=dLA_j/dLA_k;

x) repeating steps u)-x) for further pairs of emitter/detector distances;

y) generating an output signal for the arterial oxygen saturation depending on LAT_m.

19. The method of claim 18, wherein the differential attenuation is defined as:

dLA=change in light absorption at a distance j,k within a time interval Δ

t.

20. The method of claim 18, additionally comprising using said calibration data to prepare calibration curves, and computing the level of oxygenation by obtaining the data from the calibration curves.

21. The method of claim 1, 4 , 11 or 13, wherein in step c), an output signal indicative of level of arterial oxygenation is generated by an additional step comprising selecting a function which depends on at least one of the parameters Ω

values based on optical tissue parameters, light attenuations and differential light absorptions, and approximates empirical data of SaO₂ vs. Ω

_i and said parameters.

4. A method of determining the level of oxygenation of arterial blood SaO₂ in tissue in vivo considering optical tissue properties by pulse oximetry, comprising the steps of:

a) emitting light through said tissue at a plurality of wavelengths at different instants of time, and different blood flow;

b) measuring said light received passing through said tissue at said plurality of wavelengths at said instants of time;

c) calculating one ratio of differential attenuation vs. time, Ω

_i corresponding to ${\mathrm{\Omega <br><br>}}_i=\frac{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_a}{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_b}$

for each wavelength pair i=a,b of different frequencies;

d) calculating at least one attenuation ratio LA_j through said tissue for at least one wavelength, where LA_j corresponds to;

LA_j=ln (light intensity received/light intensity emitted);

e) using stored calibration data of SaO₂ v. Ω

_i at least one said LA_j; and

f) computing the level of oxygenation by comparing measured data with the calibration data.

5. The method of claim 4, wherein the wavelength pairings are about 660 and about 940 nm and about 730 and about 940 nm.

11. A method of determining the level of oxygenation of arterial blood SaO₂ in tissue in vivo considering optical tissue properties by pulse oximetry, comprising the steps of:

a) emitting light through said tissue at a plurality of wavelengths at different instants of time, and different blood flow;

b) measuring said light received passing through said tissue at said plurality of wavelengths at said instants of time;

c) calculating a plurality of ratios of differential attenuation vs. time, Ω

_i corresponding to ${\mathrm{\Omega <br><br>}}_i=\frac{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_a}{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_b}$

for each wavelength pair i=a,b of different frequencies; and

d) computing oxygenation level depending on multidimensional calibration data of SaO₂ vs. Ω

_i for different ratios of differential attenuation vs. time calculated in step c).

12. The method of claim 11 , additionally comprising using said calibration data to prepare calibration curves, and computing the level of oxygenation by obtaining the data from the calibration curves.

17. The method of claim 11 or 13, wherein the step of determining differential attenuation vs. time is accompanied by a concurrent measurement of electrical activity of the heart.

13. A method of determining the level of oxygenation of arterial blood SaO₂ in tissue in vivo considering optical tissue properties by pulse oximetry, comprising the steps of:

a) emitting light through said tissue at a plurality of wavelengths at different instants of time, and different blood flow;

b) measuring said light received passing through said tissue at said plurality of wavelengths at said instants of time;

c) calculating a plurality of ratios of differential attenuation vs. time, Ω

_i corresponding to ${\mathrm{\Omega <br><br>}}_i=\frac{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_a}{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_b}$

for each wavelength pair i=a,b of different frequencies;

d) selecting at least one wavelength pairing based on optical tissue parameters Ω

_i;

e) selecting sufficient wavelength pairings and calibration curves of SaO₂ vs. Ω

_i which were empirically made and dependent on optical tissue parameters which correspond to Ω

_i;

f) computing the level of oxygenation depending by comparing measured data with the calibration data; and

g) minimizing error in producing an output signal indicative of level of arterial oxygenation by using calibration curves of SaO₂ v. Ω

_j.

22. A method of determining the level of oxygenation of arterial blood SaO₂ in tissue in vivo considering optical tissue properties by pulse oximetry, comprising the steps of:

a) emitting light through said tissue at a plurality of wavelengths at different instants of time, and different blood flow;

b) measuring said light received passing through said tissue at said plurality of wavelengths at said instants of time;

c) calculating a plurality of ratios of differential attenuation vs. time, Ω

_i corresponding to ${\mathrm{\Omega <br><br>}}_i=\frac{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_a}{\ln <br><br>\text{\hspace{1em}<br><br>}<br><br>{\left(i_{\min }/i_{\max }\right)}_b}$

for each wavelength pair i=a,b of different frequencies;

d) approximating a function depending on Ω

values based on optical tissue parameters and arterial blood oxygenation obtained from empirical data in order to minimize error on the output signal for the arterial blood oxygenation; and

e) generating and output signal based on the function obtained by step d) and the measured Ω

values.

23. Apparatus for determining oxygen saturation of arterial blood in living tissue by pulse oximetry, comprising:

means for emitting light through living tissue at a plurality of wavelengths at different instants of time with different blood flows through the tissue;

means for measuring the light emitted and passing through the tissue at said plurality of wavelengths at said instants of time; and

means for computing the level of oxygen saturation depending on data measured and previously obtained calibration data;

wherein said means for emitting and means for measuring comprise at least two optical units, each of said optical units comprising a light emitter and light receiver.

24. Apparatus for determining oxygen saturation of arterial blood in living tissue by pulse oximetry, comprising:

means for emitting light through living tissue at a plurality of wavelengths at different instants of time and different blood flows;

means for measuring light passing through the tissue at said plurality of wavelengths and instants of time; and

means for computing oxygen saturation based on data obtained by said measuring and previously obtained calibration data;

wherein said means for measuring comprises a sensor which measures at least three wavelengths, thereby enabling said means for computing to use at least two values of differential attenuation vs. time Ω

.