Method and apparatus for calibrating an absolute oxygen saturation sensor

• US 8,781,547 B2
• Filed: 10/28/2011
• Issued: 07/15/2014
• Est. Priority Date: 10/28/2011
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Kuhn, Jonathan L.
• Primary Examiner(s): Chen, Tse
• Assistant Examiner(s): Liu, Chu Chuan (J J)
• Application Number:
  US13/283,930
• Publication Number:
  US 20130109938A1
• Time in Patent Office: 991 Days
• Field of Search: 600/310, 600/316, 600/322, 600/323, 600/324, 600/326, 600/328, 600/331, 600/333, 600/336, 600/341, 600/473, 600/476, 356/41
• US Class Current: 600/331
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

A method for using a medical device comprising an optical sensor to measure calibrated oxygen saturation in a body tissue uses a standard spectral response of blood established for multiple of oxygen saturations and a standard spectral response of a reference material. The standard responses are established using a spectrometer. The spectral power output of the optical sensor is measured using a spectrometer. The optical sensor output signal response to the reference material is obtained. A processor computes a device-specific calibration curve for the medical device using the measured spectral power output and the standard spectral response of blood and computes an optical gain using the standard spectral response of the reference material and the measured spectral power output of the optical sensor. The device-specific calibration curve and optical gain of the optical sensor are stored in a memory of the medical device.

Claims

1. A method for using a medical device comprising an optical sensor to measure calibrated oxygen saturation in a body tissue, the method comprising:

establishing a standard spectral response of blood for a plurality of oxygen saturations by a control processor coupled with the optical sensor;

establishing a standard spectral response of a reference material by a spectrometer;

determining a spectral power output of each wavelength emitted by the optical sensor by the spectrometer;

obtaining an optical sensor output signal response of each wavelength to the reference material by a detector of the optical sensor;

determining a calibration curve for the optical sensor using the measured spectral power output and the standard spectral response of blood by the control processor;

determining an optical gain for each wavelength for converting a voltage signal generated by the optical sensor to a remittance measurement by the control processor, the optical gain being determined by;

using the standard spectral response of the reference material and the measured spectral power output of the optical sensor to generate a remittance measurement value for the reference material that is expected to be measured by the optical sensor, wherein generating the remittance measurement value for the reference material comprises computing a dot product of the standard spectral response of the reference material and the spectral power output of each wavelength emitted by the optical sensor, and

dividing the expected remittance measurement value by an actual voltage signal produced by the optical sensor responsive to the optical sensor detecting remitted light at a corresponding wavelength from the reference material; and

storing the calibration curve and the optical gain.

2. The method of claim 1, wherein the optical sensor comprises a plurality of light sources emitting light at spaced apart wavelengths, and wherein establishing the spectral response of blood comprises establishing the spectral response over a range of light wavelengths encompassing the spaced apart wavelengths.

3. The method of claim 1, further comprising measuring the optical sensor output signal response to the reference material at a plurality of temperatures and determining an optical gain for each of the plurality of temperatures.

4. The method of claim 3, further comprising determining a temperature compensated optical gain curve for each of a plurality of spaced apart wavelengths emitted by the optical sensor, and storing the temperature-compensated optical gain curve for each of the wavelengths.

5. The method of claim 1, wherein computing the calibration curve for the optical sensor comprises computing a weighted average remittance at each of a plurality of wavelengths emitted by the optical sensor for each of the plurality of oxygen saturations using the standard spectral response of blood and the measured spectral power output.

6. The method of claim 5, wherein computing the calibration curve comprises converting the weighted average remittances for each of the plurality of oxygen saturations to an attenuation spectrum.

7. The method of claim 6, wherein computing the calibration curve further comprises determining a scaled second derivative of the attenuation spectra for each of the plurality of oxygen saturations.

8. The method of claim 7, wherein computing the calibration curve comprises determining calibration coefficients for a curve defining the plurality of oxygen saturations as a function of the scaled second derivative.

9. The method of claim 7, further comprising computing a calibration coefficient for computing a total hemoglobin concentration index as a function of a second derivative of the attenuation spectra and the scaled second derivative.

10. The method of claim 1, further comprising computing the oxygen saturation in a tissue by measuring a voltage signal of the optical sensor, applying the stored optical gain to convert the voltage signal to a remittance signal, converting the remittance signal to an attenuation signal, computing a scaled second derivative of the attenuation signal, and computing an absolute oxygen saturation of the tissue using the scaled second derivative and the stored calibration curve.

11. The method of claim 1, further comprising:

establishing the standard spectral response of blood and the standard spectral response of the reference material using a common spectrometer a single time;

determining a device-specific spectral power output for each of a plurality of optical sensors;

determining a device-specific calibration curve for each of the plurality of optical sensors using the single standard spectral response of blood and the respective device-specific spectral power outputs; and

computing a device-specific optical gain for each of the plurality of optical sensors by using the single standard spectral response of the reference material and the respective device-specific spectral power outputs to generate an expected remittance curve for the reference material for each of the plurality of optical sensors and dividing the expected remittance curves by respective actual voltage signals produced by respective ones of the plurality of optical sensors responsive to detecting remitted light from the reference material.

12. The method of claim 11, further comprising:

programming the device-specific calibration curve and the device specific optical gain into an implantable sensing device comprising a respective one of the plurality of optical sensors; and

enabling processing circuitry of the implantable sensing device to convert an optical sensor voltage signal to a measured remittance using the programmed device-specific optical gain.

13. The method of claim 1, wherein determining the spectral power output of the optical sensor comprises collecting light emitted by the optical sensor and measuring the collected light using the spectrometer.

14. A medical device system for measuring absolute oxygen saturation in a body tissue, the system comprising:

an optical sensor;

a memory storing a standard spectral response of blood for a plurality of oxygen saturations using a spectrometer and a standard spectral response of a reference material;

a control processor coupled with the optical sensor and the memory, the processor configured to;

cause to the optical sensor to emit light at spaced apart wavelengths by a plurality of light sources to enable measuring the spectral power output of the optical sensor, acquire the optical sensor output signal response of each wavelength to the reference material by a detector of the optical sensor, compute a calibration curve for the optical sensor using the measured spectral power output and the standard spectral response of blood, and

compute an optical gain for each wavelength by;

using the standard spectral response of the reference material and the measured spectral power output of the optical sensor to generate a remittance measurement value for the reference material that is expected to be measured by the optical sensor, wherein the remittance measurement value is generated by computing a dot product of the standard spectral response of the reference material and the spectral power output of each wavelength emitted by the optical sensor, and

dividing the expected remittance measurement value by an actual voltage signal produced by the optical sensor responsive to the optical sensor detecting remitted light at a corresponding wavelength from the reference material; and

a sensor memory coupled to the optical sensor to store the computed calibration curve and the optical gain.

15. The system of claim 14, wherein the standard spectral response of blood comprises the spectral response over a range of light wavelengths encompassing the spaced apart wavelengths.

16. The system of claim 14, wherein the processor receives the optical sensor output signal response to the reference material at a plurality of temperatures and is configured to determine an optical gain for each of the plurality of temperatures.

17. The system of claim 16, wherein the processor is further configured to determine a temperature compensated optical gain curve for each of a plurality of spaced apart wavelengths emitted by the optical sensor, and the sensor memory is programmed to store the temperature-compensated optical gain curve for each of the wavelengths.

18. The system of claim 14, wherein computing the calibration curve for the optical sensor comprises computing a weighted average remittance at each of a plurality of wavelengths emitted by the optical sensor for each of the plurality of oxygen saturations using the standard spectral response of blood and the measured spectral power output.

19. The system of claim 18, wherein computing the calibration curve comprises converting the weighted average remittances for each of the plurality of oxygen saturations to an attenuation spectrum.

20. The system of claim 19, wherein computing the calibration curve further comprises determining a scaled second derivative of the attenuation spectra for each of the plurality of oxygen saturations.

21. The system of claim 20, wherein computing the calibration curve comprises determining calibration coefficients for a curve defining the plurality of oxygen saturations as a function of the scaled second derivative.

22. The system of claim 20, wherein the processor is further configured to compute a calibration coefficient for computing a total hemoglobin concentration index as a function of a second derivative of the attenuation spectra and the scaled second derivative.

23. The system of claim 14, wherein the oxygen saturation in a tissue is determined by obtaining a voltage signal of the optical sensor, applying the stored optical gain to convert the voltage signal to a remittance signal, converting the remittance signal to an attenuation signal, computing a scaled second derivative of the attenuation signal, and computing an absolute oxygen saturation of the tissue using the scaled second derivative and the stored calibration curve.