Condensed oximeter system with noise reduction software

• US 5,351,685 A
• Filed: 08/05/1991
• Issued: 10/04/1994
• Est. Priority Date: 08/05/1991
• Status: Expired due to Term

• Current Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Original Assignee: Nellcor Incorporated (Medtronic Plc)
• Inventors: Potratz, Robert S.
• Primary Examiner(s): Sykes, Angela D.
• Application Number:
  US07/740,362
• Time in Patent Office: 1,156 Days
• Field of Search: 128/633, 128/664, 128/665, 128/666-667, 356/39, 356/41
• US Class Current: 600/330
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

A compact pulse oximetry system which separates the combined signal into its respective AC and DC components. By separating the signal into AC and DC components, a smaller order bit A/D converter may be used while still maintaining signal accuracy. Instead of using the combined signal to calculate the oxygen saturation content, the system microprocessor computes the Ratio of Ratios using the derivative of the separated AC component of the diffused signal to calculate the oxygen saturation of the measured fluid. To calculate the Ratio of Ratios, a ratio of the derivative value of the separated AC component is used. Instead of taking a single sample between the peak and valley of the signal, the oximeter system samples each value. To decrease the effect of system noise, a linear regression is performed over each sample.

Claims

1. A method for determining a parameter of blood, comprising the steps of:

emitting first and second wavelengths of light through a sample;

detecting said first and second wavelengths of light through said sample;

producing first and second analog signals responsive to said detection of said first and second wavelengths, respectively, said first and second analog signals being proportional to the intensity of said first and second wavelengths of light, respectively, said first analog signal having an AC and DC component and said second analog signal having an AC and a DC component;

separating said AC component of said first analog signal from said first analog signal to produce a first separated AC component and separating said AC component of said second analog signal from said second analog signal to produce a second separated AC component;

converting said first and second separated AC components into a first digital signal and a second digital signal, respectively;

computing the derivatives of said first and second separated AC components;

taking a linear regression of the ratio of said derivatives of said first and second separated AC components for a plurality of sample points in a period; and

computing said parameter of blood, said parameter of blood being dependent on the derivatives of said first and second separated AC components.

2. The method of claim 1 wherein an A/D converter having an input voltage range is used for said converting step, the method further comprising the steps of amplifying said first and second analog signals to be in said range, wherein said first analog signal is amplified at a different level than said second analog signal.

3. The method of claim 1 wherein an A/D converter having an input voltage range is used for said converting step, the method further comprising the step of amplifying said first and second separated AC components to be in said range.

4. The method as recited in claim 3, wherein the amount of amplification of said first and second separated AC components is controlled by a pulse width modulator, wherein the length of the pulse of said pulse width modulator is proportional to the amount of amplification of said first and second separated AC components.

5. The method as recited in claim 1, further comprising the step of increasing the intensity of said first wavelength of light such that said first analog signal is in the range of an A/D converter used for said converting step.

6. The method as recited in claim 5, wherein the intensity of said first wavelength of light is controlled by a pulse width modulator.

7. The method as recited in claim 1, wherein the step of separating the AC component from said first and second analog signals includes,

periodically charging first and second capacitors to approximately the value of said DC component of said first and second analog signals, respectively;

and

subtracting the DC component stored on said capacitors from said first and second analog signals, respectively.

8. The method recited in claim 7 wherein charging of said first and second capacitors is controlled by a microprocessor controlled switch.

9. The method as recited in claim 1, wherein said derivatives of said first and second separated AC components are calculated from the change in value between consecutive digital sample points in a period.

10. The method as recited in claim 1, further comprising the step of converting said first and second analog signals into a third and fourth digital signal, respectively.

11. The method as recited in claim 10, wherein said step of computing said parameter of the blood is calculated using the relationship ##EQU17## where I_out λ

₁ is equal to said third digital signal, I_out λ

₂ is equal to said fourth digital signal, ##EQU18## represents the derivative of said first separated AC component, and ##EQU19## represents the derivative of said second separated AC component.

12. The method as recited in claim 1, wherein the parameter of the blood is the oxygen saturation level.

13. The method as recited in claim 1, wherein said first and second wavelengths of light are in the infrared and red regions, respectively.

14. The method as recited in claim 1 further comprising adding a fixed DC offset to said first and second separated AC components, wherein the sum of said fixed DC offset and said first separated AC component and the sum of said fixed DC offset and said second separated AC component are in the middle of an input voltage range of said A/D converter.

15. An apparatus for measuring a parameter of the blood, comprising:

a first and second light emitting means for emitting first and second wavelengths of light, respectively, said first and second light emitting means directing light through a sample;

means responsive to said first and second light emitting means for detecting said first and second wavelengths of light transmitted through said sample, said detection means including a photodetector, said detection means producing first and second analog signals, said first and second analog signals being proportional to the intensity of said first and second wavelengths of light, respectively, said first and second analog signals each having an AC and a DC component;

means for separating said AC component of said first analog signal from said first analog signal to produce a first separated AC component and separating said AC component of said second analog signal from said second analog signal to produce a second separated AC component, wherein said means for separating includes a subtractor circuit;

an analog to digital converter, coupled to said means for separating, said converter converting said first separated AC component into a first digital signal and said second separated AC component into a second digital signal, said analog to digital converter having an input voltage range; and

means for taking a linear regression of the ratio of derivatives of said first and second separated AC components for a plurality of sample points in a period, said means for taking a linear regression including a microprocessor which computes said parameter of blood from said first and second digital signals, wherein the value of said parameter of blood is dependent on the derivative of said first separated AC component and the derivative of said second separated AC component.

16. The apparatus of claim 15, further comprising an amplifier which amplifies said first and second analog signals to be in the range of said analog to digital converter, wherein said first analog signal is amplified at a different level than said second analog signal.

17. The apparatus of claim 15, further comprising an amplifier which amplifies said first and second separated AC components to be in the range of said analog to digital converter.

18. The apparatus of claim 15, further comprising a pulse width modulator, wherein said pulse width modulator controls an amount of amplification of said first and second separated AC components, wherein the pulse length of said pulse width modulator is proportional to the amount of amplification of said first and second separated AC components.

19. The apparatus recited in claim 15, further comprising a means for increasing the intensity of said first wavelength of light such that said first analog signal is in the range of said analog to digital converter.

20. The apparatus recited in claim 15, wherein the means for separating said AC component from said first and second analog signals includes,

first and second capacitors, said first and second capacitors periodically charging to approximately the value of the DC component of said first and second analog signals, respectively.

21. The apparatus recited in claim 20 further comprising a microprocessor controlled switch, wherein charging of said first and second capacitors is controlled by said microprocessor controlled switch.

22. The apparatus recited in claim 15, wherein said means for taking a linear regression calculates said derivatives of said first and second separated AC components from a change in value between two consecutive digital sample points in the period.

23. The apparatus recited in claim 15, wherein said analog to digital converter converts said first and second analog signals into a third and fourth digital signal, respectively.

24. The apparatus as recited in claim 22, wherein said means for taking a linear regression calculates said parameter of the blood in said microprocessor according to the relationship ##EQU20## where I_out λ

₁ is equal to said third digital signal, I_out λ

₂ is equal to said fourth digital signal, ##EQU21## represents the derivative of said first separated AC component, and ##EQU22## represents the derivative of said second separated AC component.

25. The apparatus recited in claim 15, wherein said apparatus measures the oxygen saturation level.

26. The apparatus recited in claim 15, wherein said first and second light emitting means emit first and second wavelengths of light in the infrared and red regions, respectively.

27. The apparatus recited in claim 15 wherein said analog to digital converter has a range, further comprising means for adding a fixed DC offset to said first and second separated AC components, wherein the sum of said fixed DC offset and said first separated AC component and the sum of said fixed DC offset and said second separated AC component are in the middle of an said range.

28. A method for determining a parameter of blood, comprising the steps of:

emitting first and second wavelengths of light through a sample;

detecting said first and second wavelengths of light through said sample;

producing first and second analog signals responsive to said detection of said first and second wavelengths, respectively, said first and second analog signals being proportional to the intensity of said first and second wavelengths of light, respectively, said first analog signal having an AC and DC component and said second analog signal having an AC and a DC component;

separating said AC component of said first analog signal from said first analog signal to produce a first separated AC component and separating said AC component of said second analog signal from said second analog signal to produce a second separated AC component;

converting said first and second separated AC components into a first digital signal and a second digital signal, respectively, using an A/D converter having an input voltage range;

amplifying said first and second separated AC components to be in the range of the A/D converter used for the converting step, wherein the amount of amplification of said first and second separated AC components is controlled by a pulse width modulator, wherein the pulse length of said pulse width modulator is proportional to the amount of amplification of said first and second separated AC components;

computing the derivative of said first and second separated AC components; and

computing said parameter of blood, said parameter of blood being dependent on the value of said first and second digital signals.

29. A method for determining a parameter of blood, comprising the steps of:

emitting first and second wavelengths of light through a sample;

detecting said first and second wavelengths of light through said sample;

producing first and second analog signals responsive to said detection of said first and second wavelengths, respectively, said first and second analog signals being proportional to the intensity of said first and second wavelengths of light, respectively, said first analog signal having an AC and DC component and said second analog signal having an AC and a DC component;

separating said AC component of said first analog signal from said first analog signal to produce a first separated AC component and separating said AC component of said second analog signal from said second analog signal to produce a second separated AC component;

converting said first and second separated AC components into a first digital signal and a second digital signal, respectively, using an A/D converter having an input voltage range wherein the intensity of said first wavelength of light is increased such that said first analog signal is in the range of the A/D converter;

computing the derivative of said first and second separated AC components;

taking a linear regression of the ratio of said derivatives of said first and second separated AC components for a plurality of sample points in a period; and

computing said parameter of blood, said parameter of blood being dependent on the value of said first and second digital signals.

30. A method for determining a parameter of blood, comprising the steps of:

emitting first and second wavelengths of light through a sample;

detecting said first and second wavelengths of light through said sample;

producing first and second analog signals responsive to said detection of said first and second wavelengths, respectively, said first and second analog signals being proportional to the intensity of said first and second wavelengths of light, respectively, said first analog signal having an AC and DC component and said second analog signal having an AC and a DC component;

periodically charging first and second capacitors to approximately the value of said DC component of said first and second analog signals, respectively;

subtracting the DC component values to which said capacitors are charged from said first and second analog signals, respectively, to produce a first separated AC component and a second separated AC component;

converting said first and second separated AC components into a first digital signal and a second digital signal, respectively;

computing the derivative of said first and second separated AC components; and

computing said parameter of blood, said parameter of blood being dependent on the value of said first and second digital signals.