Calibration technique for non-invasive medical devices

• US 20040010186A1
• Filed: 07/11/2002
• Published: 01/15/2004
• Est. Priority Date: 07/11/2002
• Status: Active Grant

• Current Assignee: Optical Sensors Incorporated
• Original Assignee: Optical Sensors Incorporated
• Inventors: Furlong, Steven C., Kimball, Victor E., Pierskalla, Irvin
• Granted Patent: US 6,865,407 B2
• Time in Patent Office: Days
• US Class Current: 600/310
• CPC Class Codes: A61B 5/02133 by using induced vibration of the blood vessel
• CPC Class Codes: A61B 5/14539 for measuring pH
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

There is a need for a non-invasive method of calibrating medical devices at the point of care, where the calibration is performed without the removal of blood or bodily fluids. The invention is directed to an approach for calibrating a first non-invasive sensor in which the tissue being measured is modulated in some way so as to after the value of the parameter being measured by the first optical sensor. A second sensor detects another parameter that also changes with the modulation. The second sensor is absolutely calibrated. Where there is a known relationship between the first and second parameters, a calibration may be derived for the first sensor. Such a technique is applicable to calibrating non-invasive sensors for monitoring a wide variety of physiologic parameters including, inter alia, glucose, blood gases, blood electrolytes and blood pH.

Claims

1. A method of calibrating a non-invasive sensor for a first physiologic parameter in tissue, comprising:

(a) non-invasively measuring first values of the first physiologic parameter and of a second physiologic parameter in the tissue, a known relationship existing between the first and second physiologic parameters;

(b) acting on the tissue so as to change the first and second physiologic parameters;

(c) non-invasively measuring second values of the first physiologic parameter and of the second physiologic parameter in the tissue, the measurements of the first and second values of the second physiologic parameter being absolutely calibrated; and

(d) determining a calibrated value for at least one of the first and second values of the first physiologic parameter from the first and second values of both the first and second physiologic parameters.

2. A method as recited in claim 1, wherein non-invasively measuring the first and second values of the first parameter include measuring light received from the tissue.

3. A method as recited in claim 1, wherein non-invasively measuring the first and second values of the second parameter include measuring light received from the tissue.

4. A method as recited in claim 1, wherein the values for the first and second physiologic parameters are measured in the same region of the tissue.

5. A method as recited in claim 1, wherein non-invasively measuring the first values of the first physiologic parameter includes exciting the tissue with excitation light and detecting a resulting fluorescent signal.

6. A method as recited in claim 1, wherein acting on the tissue includes applying a systemic stimulus the tissue.

7. A method as recited in claim 1, wherein acting on the tissue includes applying a stimulus to the tissue localized to a region of the tissue where the first and second physiologic parameters are non-invasively measured.

8. A method as recited in claim 1, wherein acting on the tissue includes administering a drug to the patient whose tissue is being examined.

9. A method as recited in claim 1, wherein applying a systemic stimulus to the tissue includes changing a metabolic rate of the tissue.

10. A method as recited in claim 1, wherein acting on the tissue includes applying pressure to a region of the tissue where the first and second physiologic parameters are measured.

11. A method as recited in claim 1, wherein acting on the body includes changing temperature of a region of the body where the first and second physiologic parameters are measured.

12. A method as recited in claim 1, wherein non-invasively measuring the second values of the first physiologic parameter and of the second physiologic parameter includes measuring the second values of the first physiologic parameter and of the second physiologic parameter simultaneously.

13. A method as recited in claim 1, wherein the first physiologic parameter is pH.

14. A method as recited in claim 13, wherein measuring the pH includes making a fluorescence measurement of NADH in the region of the tissue being measured.

15. A method as recited in claim 13, wherein the second physiologic parameter is concentration of CO₂.

16. A method as recited in claim 1, wherein the known relationship between the first and second physiologic parameters includes a third physiologic parameter.

17. A method as recited in claim 16, wherein the third physiologic parameter is substantially unaffected by the acting on the body.

18. A method as recited in claim 16, wherein determining the calibrated value for at least one of the first and second values of the first physiologic parameter includes determining a value of the third physiologic parameter from a ratio of i) the difference between the first and second values of the second physiologic parameter over ii) the difference between the first and second values of the first physiologic parameter.

19. A method as recited in claim 18, wherein determining the calibrated value for at least one of the first and second values of the first physiologic parameter further includes calculating the calibrated value using the value of the third physiologic parameter and at least the respective calibrated first or second value of the second physiologic parameter.

20. A method as recited in claim 1, wherein the known relationship between the first and second physiologic parameters is

pH=pK−

log([CO₂]/[HCO₃⁻

])

where pH is the first physiologic parameter and [CO₂] is the second physiologic parameter.

21. A method as recited in claim 1, wherein the known relationship relates N physiologic parameters that change upon acting on the tissue so as to change the first and second physiologic parameters, and acting on the tissue N−

1 times so as to take N measurements of each of the N physiologic parameters at different levels of stimulus arising from the acting on the tissue.

22. A method as recited in claim 1, further comprising non-invasively measuring the second values of the first physiologic parameter and of the second physiologic parameter in the tissue while acting on the tissue.

23. A method as recited in claim 1, wherein the tissue is epithelial tissue.

24. A method as recited in claim 1, wherein the tissue is selected from skin tissue, muscle tissue and organ tissue.

25. A method as recited in claim 1, further comprising performing successive non-invasive measurements of the first physiologic parameter of the tissue and determining respective absolute calibrated values of the first physiologic parameter from the first and second values of both the first and second physiologic parameters.

26. A method as recited in claim 25, further comprising repeating steps (a)-(c) after performing the successive non-invasive measurements and determining whether a calibration of an instrument for non-invasively measuring the first physiologic parameter has changed.

27. A system for determining a value of a first physiologic parameter within tissue, comprising:

first measuring means for non-invasively measuring first and second values of the first physiologic parameter in the tissue;

second measuring means for non-invasively measuring first and second values of a second physiologic parameter in the tissue, a known relationship existing between the first and second physiologic parameters, the second measuring means being absolutely calibrated;

means for acting on the body so as to change the first and second physiologic parameters; and

means for determining a calibrated value for at least one of the first and second values of the first physiologic parameter from the first and second values of both the first and second physiologic parameters.

28. An apparatus for determining a value of a first physiologic parameter within tissue, comprising:

an uncalibrated, first non-invasive sensor for measuring the first physiologic parameter of the tissue;

a calibrated, second non-invasive sensor for measuring a second physiologic parameter of the tissue; and

a processor coupled to receive first and second signals from each of the first and second non-invasive sensors, and to calculate calibrated values for the first physiologic parameter from the first and second signals from each of the first and second non-invasive sensors when a characteristic of the tissue is modulated between the first and second signals.

29. An apparatus as recited in claim 28, wherein the first non-invasive sensor is an optically based sensor.

30. An apparatus as recited in claim 28, wherein the second non-invasive sensor is an optically based sensor.

31. An apparatus as recited in claim 28, wherein the first non-invasive sensor is a pH sensor.

32. An apparatus as recited in claim 28, wherein the second non-invasive sensor is a calibrated, non-invasive CO₂ sensor.

36. An apparatus as recited in claim 28, further comprising a patient interface module to house the first and second non-invasive sensors.

37. An apparatus as recited in claim 28, further comprising a modulator to modulate the characteristic of the tissue at the region of the tissue where the first and second physiologic parameters are measured.

38. An apparatus as recited in claim 37, wherein the modulator includes an extensible member to apply pressure of the region of the tissue where the first and second physiologic parameters are measured.

39. An apparatus as recited in claim 37, wherein the modulator includes a thermally responsive member to change temperature of the region of the tissue where the first and second physiologic parameters are measured.

40. An apparatus as recited in claim 37, further comprising a patient interface module, the first and second non-invasive sensors and the modulator being housed within the patient interface module.

41. An apparatus as recited in claim 40, wherein the modulator is removable from the patient interface module, the first and second non-invasive sensors being enabled to perform measurements on the first and second physiologic parameters respectfully after the modulator is removed from the patient interface.

42. An apparatus as recited in claim 28, wherein the processor calculates calibration constants for the first physiologic parameter from the first and second signals from each of the first and second non-invasive sensors.

43. An apparatus as recited in claim 42, wherein the processor produces calibrated values for the first physiologic parameter from measurement signals received from the first non-invasive sensor and the calibration constants.

44. An apparatus as recited in claim 43, wherein the processor performs a check on the calibration constants by measuring the first and second physiologic parameters before and after application of a stimulus to the tissue.

33. An apparatus as recited in claim 33, wherein the pH sensor emits excitation light in the wavelength range 300 nm-400 nm.

34. An apparatus as recited in claim 33, wherein the pH sensor detects light emitted for the tissue in a wavelength range of 400-600 nm.

35. An apparatus as recited in claim 33, wherein the pH sensor detects fluorescence from NADH in the tissue.