Temperature Compensation of a Respiratory Gas Sensor

• US 20080161711A1
• Filed: 12/18/2007
• Published: 07/03/2008
• Est. Priority Date: 12/21/2006
• Status: Active Grant

• Current Assignee: Philips RS North America LLC (Koninklijke Philips N.V.)
• Original Assignee: RIC INVESTMENTS LLC (Respironics Incorporated)
• Inventors: Orr, Joseph A., Gunneson, Paul B., Pierry, Anthony T.
• Granted Patent: US 8,273,033 B2
• Time in Patent Office: Days
• US Class Current: 600/532
• CPC Class Codes: A61B 5/0833 Measuring rate of oxygen consumption
• CPC Class Codes: A61B 5/097 Devices for facilitating collection of breath or for directing breath into or through measuring devices
• CPC Class Codes: Y10T 436/207497 Molecular oxygen

Abstract

A mainstream gas monitoring system and method that includes a mainstream airway adapter, and a gas sensing assembly associated with the mainstream airway adapter to measure an analyte of a gas flow through the adapter. A gas sensing portion outputs a signal indicative of the analyte in a gas flow in the mainstream airway adapter. A processing portion receives the signal from the gas sensing portion and determines an amount of the analyte in the gas flow based on the signal from the gas sensing portion. The gas sensing portion is subject to temperature variations associated with variations in flow rate and direction of respiratory gases. Methods are described that utilize the measurement of instantaneous respiratory flow rate combined with estimates of gas temperature and composition to estimate the sensor cooling effects from which a flow based time varying compensation factor is derived.

Claims

1. A system for measuring an amount of a gas component within a gas flow, comprising:

a gas sensor constructed and arranged to measure an amount of a component of gas within a flow of gas, wherein the gas sensor has a sensing element constructed and arranged to contact the flow of gas, and wherein an output the sensing element is sensitive to temperature fluctuations of the sensing element;

a second sensor constructed and arranged to measure a characteristic associated with the flow of gas; and

a processor that receives first signals as a function of the amount of the component of gas measured by the gas sensor and second signals as a function of the characteristic measured by the second sensor, wherein the processor adjusts the measurement of the amount of the component of gas measured by the gas sensor based upon the characteristic associated with the flow of gas.

2. The system of claim 1, wherein the second sensor is a flow sensor that measures a rate of the glow of gas as the characteristic, or a pressure sensor that measures a pressure of the flow of gas as the characteristic.

3. The system of claim 1, wherein the second sensor is a pressure sensor, and further comprising a flow sensor, and wherein the processor adjusts the measurement of the amount of the component of gas measured by the gas sensor based upon a pressure measured by the pressure sensor and a flow measured by the flow sensor.

4. The system of claim 3, wherein the gas sensor comprises an oxygen sensor, and wherein the processor adjusts the amount of oxygen measured by the oxygen sensor according to the equation:

% oxygen<sub>—

flow—

compensated</sub>[n]=% oxygen<sub>—

pressure—

compensated</sub>[n]*(1.0+K_X*flow_fitered[n]),

where “

% oxygen<sub>—

flow—

compensated</sub>[n]”

is a flow compensated percent oxygen for a current (nth sample), K_X=K_I during inspiration, K_X=K_E during expiration, “

K_I”

is a compensation factor applied for inspiratory flow, “

K_E”

is a compensation factor applied for expiratory flow, and “

flow_filtered[n]”

is a flow signal delayed and low-pass filtered to match a time response of the oxygen signal, and where % oxygen<sub>—

pressure—

comp</sub>[n]=% oxygen<sub>—

raw</sub>[n] (total airway pressure[n]/barometric pressure).

5. The system of claim 4, wherein K_I is between 0.00006 and 0.00018, and K_E is between −

0.00020 and −

0.00040.

6. The system of claim 1, wherein the sensing element comprises a film coated with a photosensitive chemical.

7. The system of claim 6, wherein the gas sensor further comprises a photodetector that measures radiation emitted by the photosensitive chemical upon exposure to oxygen.

8. The system of claim 1, further comprising a temperature sensor and a thermal regulator to regulate a temperature of the sensing element based on an output of the temperature sensor.

9. The system of claim 1, wherein the processor maintains compensation factors, each of the compensation factors identifying an amount of adjustment of the measurement of the amount of the component of gas corresponding to a measured characteristic associate with the flow of gas.

10. The system of claim 9, wherein the compensation factors are obtained empirically.

11. A system for measuring an amount of oxygen in a gas flow, comprising:

a conduit through which a gas flow passes;

an oxygen sensor having a sensing element exposed to the gas flow within the conduit, the oxygen sensor sensing an amount of oxygen in the gas flow;

a second sensor constructed and arranged to measure a characteristic associated with the gas flow in the conduit; and

a processor that receives a signal based on the amount of oxygen sensed by the oxygen sensor and that receives a signal based on the characteristic measured by the sensor, the processor adjusting the oxygen measured by the oxygen sensor based on the characteristic.

12. The system of claim 11, wherein the second sensor is a flow sensor that measures a rate of the glow of gas as the characteristic, or a pressure sensor that measures a pressure of the flow of gas as the characteristic.

13. The system of claim 12, wherein the sensing element is sensitive to variations in temperature, and wherein the processor adjusts the oxygen measured by the oxygen sensor by an amount calculated to compensate for temperature changes related to the characteristic.

14. The system of claim 13, wherein the amount of adjustment is calculated using a compensation factor, the compensation factor identifying amounts of adjustment for a plurality of different values of the characteristic.

15. The system of claim 14, wherein the compensation factors are (a) obtained empirically, (b) based on a history of prior measurements provided by the oxygen sensor, (c) provided during calibration of the system, or any combination of (a), (b) and (c).

16. The system of claim 11, further comprising a temperature sensor and a thermal regulator to regulate a temperature of the sensing element based on an output of the temperature sensor.

17. A method of measuring an amount of oxygen in a flow of gas, comprising:

measuring an amount of oxygen in a gas flow;

measuring a characteristic associated with the gas flow; and

adjusting the measurement of the amount of oxygen in the gas flow based upon the characteristic associated with the gas flow.

18. The method of claim 17, wherein the step of adjusting includes the steps of

determining a compensation factor based on an instantaneous measurement of the characteristic, and

modifying the measurement of the amount of oxygen based on the compensation factor.

19. The method of claim 18, wherein the step of modifying includes (a) scaling the measurement of the amount of oxygen using the compensation factor, (b) adding the compensation factor to the measurement of the amount of oxygen, or both (a) and (b).

20. The method of claim 18, wherein the compensation factors are obtained empirically.

21. The method of claim 17, wherein the characteristic is a flow rate measured by a flow sensor, or a pressure measured by a pressure sensor.

22. The method of claim 17, wherein the amount of oxygen is measured by a sensing element comprising a film coated with a photosensitive chemical.

23. The method of claim 22, wherein the sensing element further comprises a photodetector that measures radiation emitted by the photosensitive chemical upon exposure to oxygen.

24. The method of claim 22, further comprising:

measuring a temperature indicative of a temperature of the sensing element; and

controlling the temperature of the sending element based on the measured temperature.

26. The system of claim 24, wherein the characteristic associated with the gas flow is a rate of the gas flow, and wherein the means for measuring the characteristic associated with the gas flow is a flow sensor.

27. The system of claim 24, wherein the characteristic associated with the gas flow is a pressure of the gas flow, and wherein the means for measuring the characteristic associated with the gas flow is a pressure sensor.

25. A system for measuring an amount of a gas component within a gas flow, comprising:

means for measuring an amount of a component of gas within a gas flow;

means for measuring a characteristic associated with the gas flow; and

means for adjusting the amount of the component of gas based upon the characteristic associated with the gas flow.

28. The system of claim 25, wherein the means for measuring an amount of a component of gas includes means for measuring radiation emitted by a photosensitive element, and wherein the radiation is emitted responsive to exposure of the photosensitive element to oxygen.

29. The system of claim 28, further comprising means for regulating a temperature of the photosensitive element.

30. The system of claim 25, wherein the component of gas is oxygen, and wherein the amount of oxygen is measured as a partial pressure of oxygen within the gas flow.

31. The system of claim 25, wherein the means for adjusting the measured amount includes means for identifying a compensation factor for calculating an amount of adjustment of the measurement of the amount of the component of gas corresponding to the measured characteristic.