Method for calibration of gas measuring sensors for dissolved gasses and method for measuring the concentration of CO.sub.2 in blood with the assistance of such a calibration method

• US 5,763,760 A
• Filed: 02/13/1997
• Issued: 06/09/1998
• Est. Priority Date: 02/13/1996
• Status: Expired due to Term

• Current Assignee: Sphere Medical Limited (Sphere Medical Holding Ltd.)
• Original Assignee: Siemens AG
• Inventors: Stanzel, Manfred, Gumbrecht, Walter
• Primary Examiner(s): RAEVIS, ROBERT R
• Application Number:
  US08/799,532
• Time in Patent Office: 481 Days
• Field of Search: 73/19.1, 73/1.02-1.07, 436/8, 436/9, 436/11, 422/98
• US Class Current: 73/1.06
• CPC Class Codes: A61B 5/14539 for measuring pH
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/1473 invasive, e.g. introduced into the body by a catheter A61B5/1482 takes precedence
• CPC Class Codes: G01N 33/4925 measuring blood gas content, e.g. O2, CO2 , HCO3

Abstract

Method for calibrating gas measuring sensors for dissolved gasses and method for measuring the concentration of CO₂ in blood with the assistance of such a calibration method two or more separate calibration fluids in which a gas to be measured is dissolved with respectively different partial pressures are employed. For simplification, another (i.e., a second) calibration fluid is produced from one (i.e., a first) calibration fluid by modifying the partial pressure of the first calibration fluid to a different value, preferably by material transport between the first fluid and a gas phase. Analysis equipment operating with this method, particularly for CO₂ measurement of blood, become simpler, smaller and less expensive.

Claims

1. A method for calibrating a sensor for measuring a concentration of gas dissolved in a fluid and for generating a measured value corresponding to the measured concentration, comprising the steps of:

providing a predetermined, first calibration fluid having a predetermined, first concentration of a gas dissolved therein;

providing a gas phase containing the same gas as is dissolved in said first calibration fluid, said gas in said gas phase being at a partial pressure in said gas phase differing from said partial pressure of said gas in said first calibration fluid, so that a partial pressure difference for said gas exists between said gas phase and said first calibration fluid;

separating said gas phase from said fist calibration fluid by a gas-permeable wall;

forming a second calibration fluid from said first calibration fluid by modifying the concentration of said gas in said first calibration fluid to a predetermined, second concentration of said gas in said second calibration fluid exclusively by causing a material transport between said first calibration fluid-and said gas phase through said gas-permeable wall by virtue of said partial pressure difference, said second concentration being different from said first concentration;

supplying said first and second calibration fluids to a sensor to be calibrated; and

calibrating said sensor by identifying first and second measured values using said sensor corresponding said first and second concentrations.

2. A method as claimed in claim 1 wherein the step of causing a material transport includes:

adjusting said partial pressure difference until said predetermined, second concentration in said second calibration fluid is present.

3. A method as claimed in claim 1 wherein said gas phase comprises air.

4. A method as claimed in claim 1 wherein the step of causing a material transport comprises:

supplying said first calibration fluid to said sensor through a tube having a gas-permeable tube wall, comprising said gas-permeable wall, surrounded at an exterior by said gas phase and having an interior cross-section which is small relative to a quantity of said first calibration fluid to be delivered to said sensor per unit of time for preventing modification of said first concentration of said gas in said first calibration fluid beyond a prescribed amount due to said material transport during delivery of said first calibration fluid to said sensor; and

producing said second calibration fluid from said first calibration fluid by allowing said first calibration fluid to temporarily dwell in said tube thereby producing a defined material transport between said first calibration fluid in said tube and said gas phase via said tube wall.

5. A method as claimed in claim 4 wherein the step of supplying said first calibration fluid to said sensor via said tube is further defined by supplying said first calibration fluid from a gas-impermeable vessel to said sensor via said tube.

6. A method as claimed in claim 1 wherein the step of calibrating said sensor comprises generating in said sensor respective measured values which are directly proportional to the respective concentrations of said gas in said first and second calibration fluids supplied to said sensor.

7. A method as claimed in claim 1 wherein the step of calibrating said sensor comprises generating in said sensor respective measured values which are proportional to a logarithm of the respective concentrations of said gas in said first and second fluids supplied to said sensor.

8. A method as claimed in claim 1 wherein the step of providing said predetermined, first calibration fluid comprises providing a predetermined, first calibration fluid having a first concentration of said gas dissolved therein which is a defined, non-constant function of a variable pH value of said first calibration fluid, and wherein the step of producing said predetermined, second calibration fluid comprises producing a second calibration fluid from said first calibration fluid having a pH value precisely determined from said pH value of said first calibration fluid.

9. A method as claimed in claim 8 wherein the step of providing said predetermined, first calibration fluid comprises providing a predetermined, first calibration fluid having a logarithm of the concentration of said gas in said first calibration fluid which is a non-constant linear function of said variable pH value of said first calibration fluid.

10. A method as claimed in claim 9 wherein the step of calibrating said sensor comprises:

producing, using said sensor to be calibrated, measured values comprising a concentration U_pG (C) of said gas in said first calibration fluid and a concentration U_pG (C'"'"') of said gas in said second calibration fluid;

supplying said first and second calibration fluids to a pH sensor and, using said pH sensor, producing a measured value U_pH (C) proportional to the pH value of said first calibration fluid and a measured value U_pH (C'"'"') of said second calibration fluid; and

generating a calibration quantity

space="preserve" listing-type="equation">-K.sub.pG =(U.sub.pG (C)-U.sub.pG (C'"'"'))/(U.sub.pH (C)-U.sub.pH (C'"'"')).

11. A method as claimed in claim 1 wherein said gas in each of said first and second calibration fluids comprises CO₂, and wherein sensor to be calibrated comprises a CO₂ sensor for measuring the respective partial pressures of CO₂ in said first and second calibration fluids as respective indicators for the respective concentrations of CO₂ in said first and second calibration fluids.

12. A method as claimed in claim 11 wherein said predetermined, first calibration fluid comprises a fluid having HCO₃^- dissolved therein.

13. A method as claimed in claim 1 wherein said first and second calibration fluids comprise a physiological body fluid having said gas dissolved therein in respectively different concentrations, and wherein said sensor to be calibrated comprises a sensor for measuring the respective partial pressures of said gas in said physiological body fluid.

14. A method as claimed in claim 13 wherein said first and second calibration fluids comprise blood with CO₂ dissolved therein respectively in said first and second concentrations, and wherein said sensor to be calibrated comprises a sensor for measuring a partial pressure of CO₂ in blood.

15. A method for measuring a partial pressure of CO₂ dissolved in blood using a CO₂ sensor for measuring said CO₂ partial pressure and for generating a corresponding measured value proportional to a logarithm of the CO₂ partial pressure comprising the steps of:

providing a predetermined, first calibration fluid having CO₂ dissolved therein with a defined physiological, first partial pressure and having HCO₃^- dissolved therein with a defined physiological concentration having a defined physiological, first pH value;

providing a gas phase containing the same gas as is dissolved in said first calibration fluid, said gas in said gas phase being at a partial pressure in said gas phase differing from said partial pressure of said gas in said first calibration fluid, so that a partial pressure difference for said gas exists between said gas phase and said first calibration fluid;

separating said gas phase from said first calibration fluid by a gas-permeable wall;

producing a second calibration fluid from said first calibration fluid by material transport between said first calibration fluid and said gas phase exclusively by causing a material transport between said first calibration fluid and said gas phase through said gas-permeable wall by virtue of said partial pressure difference, said second calibration fluid differing from said first calibration fluid only by having a physiological, second CO₂ partial pressure therein differing from said first CO₂ partial pressure and a physiological, second pH value differing from said first pH value;

supplying said first and second calibration fluids successively, in an arbitrary sequence, to a CO₂ sensor and to a pH sensor;

providing a base line fluid having CO₂ dissolved therein with a concentration corresponding to a partial pressure of less than 10 Torr and having a defined, third physiological pH value differing from said first pH value;

additionally supplying said pH sensor with said base line fluid;

using said CO₂ sensor and said pH sensor, producing a measured value U_pCO2 (c) corresponding to the first CO₂ partial pressure in said first calibration fluid, a measured value U_pCO2 (C'"'"') corresponding to the second CO₂ partial pressure of said second calibration fluid, a measured value U_pH (C) corresponding to the first pH value of said first calibration fluid, a measured value U_pH (C'"'"') corresponding to the second pH value of said second calibration fluid, and a measured value U_pH (BF) corresponding to the third pH value of said base line fluid, and a measured value U_pCO2 (BF) corresponding to a CO₂ partial pressure of said base line fluid;

generating a quantity

space="preserve" listing-type="equation">S.sub.pCO2 =K.sub.pCO2 ·

S.sub.pH

space="preserve" listing-type="equation">wherein K.sub.pCO2 =- (U.sub.pCO2 (C)-U.sub.pCO2 (C'"'"'))/(U.sub.pH (C)-U.sub.pH (C'"'"'))!

space="preserve" listing-type="equation">and S.sub.pH =(Δ

U.sub.pH (C)/(pH(C)-pH(BF))

space="preserve" listing-type="equation">wherein Δ

U.sub.pH (C)=U.sub.pH (C)-U.sub.pH (BF);

supplying blood to said CO₂ sensor and obtaining a measured value U_pCO2 (B) corresponding to a CO₂ partial pressure of said blood;

calculating a logarithm of the CO₂ partial pressure of said blood log(pCO₂ (B)) according to

space="preserve" listing-type="equation">log(pCO.sub.2 (B))=(Δ

U.sub.pCO2 (B)-Δ

U.sub.pCO2 (C))/S.sub.pCO2 +log(pCO2(C))

wherein

space="preserve" listing-type="equation">Δ

U.sub.pCO2 (B)=U.sub.pCO2 (B)-U.sub.pCO2 (BF) and

space="preserve" listing-type="equation">Δ

U.sub.PCO2 (C)=U.sub.pCO2 (C)-U.sub.pCO2 (BF); and

identifying the CO₂ partial pressure in said blood from log(pCO2(B)).

16. A method as claimed in claim 15 wherein the CO₂ partial pressure of said base line fluid changes over time, and comprising the additional steps of:

forming the quantity Δ

U_pCO2 (C) by

space="preserve" listing-type="equation">U.sub.pCO2 (C)=U.sub.pCO2 (C)-U.sub.pCO2 (BF(t.sub.c))

with a measured value U_pCO2 (BF(t_c) of the CO₂ partial pressure of the base line fluid measured close in time to the measurement of the first CO₂ partial pressure of the first calibration fluid at time t_c ; and

forming the quantity Δ

U_pCO2 (B) by

space="preserve" listing-type="equation">Δ

U.sub.pCO2 (B)=U.sub.pCO2 (B)-U.sub.pCO2 (BF(t.sub.b))

using a measured value U_pCO2 (BF(t_b) of the CO₂ partial pressure of the base line fluid measured close in time to the measurement of the CO₂ partial pressure of said blood.

17. A method as claimed in claim 15 wherein the step of providing a predetermined, first calibration fluid comprises providing a predetermined, first calibration fluid having NaHCO₃ dissolved therein in a physiological concentration.

18. A method as claimed in claim 15 wherein the step of producing said second calibration fluid from said first calibration fluid comprises producing said second calibration fluid by allowing CO₂ to outgas from said first calibration fluid.

19. A method as claimed in claim 15 wherein the step of causing a material transport comprises:

supplying said first calibration fluid to said sensor through a tube having a gas-permeable tube wall comprising said gas-permeable wall, surrounded at an exterior by said gas phase and having an interior cross-section which is small relative to a quantity of said first calibration fluid to be delivered to said sensor per unit of time for preventing modification of said first concentration of said gas in said first calibration fluid beyond a prescribed amount due to said material transport during delivery of said first calibration fluid to said sensor; and

producing said second calibration fluid from said first calibration fluid by allowing said first calibration fluid to temporarily dwell in said tube thereby producing a defined material transport between said first calibration fluid in said tube and said gas phase via said tube wall.

20. A method as claimed in claim 15 wherein the step of supplying a base line fluid comprises a base line fluid which is maintained air-saturated before delivery to said pH sensor.

21. A method as claimed in claim 20 wherein the step of supplying said base line fluid to said pH sensor comprises supplying said base line fluid to said pH sensor through a gas-permeable tube having an interior cross-section dimensioned large relative to a quantity of said base line fluid to be delivered to said pH sensor in a defined unit of time for causing said base line fluid to dwell in said tube during delivery to said pH sensor for a time for adapting said base line fluid to an ambient pressure.

22. A method as claimed in claim 21 wherein the step of supplying said base line fluid to said pH sensor comprises supplying said base line fluid from a gas-impermeable vessel to said pH sensor via said tube.

23. A method as claimed in claim 15 comprising the additional step of:

repeatedly using said quantity S_pCO2 for multiple identifications of CO₂ partial pressure of different blood samples by repeated, updated determination of the measured value U_pCO2 (B) and the measured value U_pCO2 (BF).