Condensed oximeter system with noise reduction software

• US 5,577,500 A
• Filed: 05/23/1995
• Issued: 11/26/1996
• Est. Priority Date: 08/05/1991
• Status: Expired due to Term

• Current Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Potratz, Robert S.
• Primary Examiner(s): Sykes, Angela D.
• Assistant Examiner(s): YARNELL, BRYAN KEITH
• Application Number:
  US08/447,665
• Time in Patent Office: 553 Days
• Field of Search: 128/632, 128/633, 128/637, 128/664, 128/665, 128/668, 128/666, 128/667, 607/88, 364/413.03, 364/413.09, 356/39-41
• 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:

transmitting at least one wavelength of electromagnetic energy through a sample;

detecting the at least one wavelength of electromagnetic energy through the sample;

generating at least one analog signal corresponding to the at least wavelength, the at least one analog signal having an AC and a DC component;

separating the AC component of the at least one analog signal from the at least one analog signal to produce a separated AC component;

computing the derivative of the separated AC component; and

computing the parameter of blood, the parameter of blood corresponding to the derivative of the separated AC component.

2. The method of claim 1 wherein the at least one wavelength of electromagnetic energy comprises first and second wavelengths of light, and wherein the at least one analog signal comprises first and second analog signals, the first analog signal corresponding to the first wavelength of light and the second analog signal corresponding to the second wavelength of light.

3. The method of claim 2, further comprising the step of taking a linear regression of a ratio of the derivatives of the separated AC components of the first and second analog signals for a plurality of sample points in a period.

4. The method of claim 2, further comprising the step of converting the separated AC components of the first and second analog signals into a first digital signal and a second digital signal, respectively.

5. The method of claim 4, further comprising the step of amplifying the first and second analog signals to be in a range of an A/D converter used for the converting step, wherein the first analog signal is amplified at a different level then the second analog signal.

6. The method of claim 4, further comprising the step of amplifying the separated AC components of the first and second analog signals to be in a range of an A/D converter used for the converting step.

7. The method as recited in claim 6, wherein the amount of amplification of the separated AC components of the first and second analog signals is controlled by a pulse width modulator, wherein the length of the pulse of the pulse width modulator is proportional to the amount of amplification of the separated AC components.

8. The method as recited in claim 4, further comprising the step of increasing the intensity of the first wavelength of light such that the first analog signal is in a range of an A/D converter used for the converting step.

9. The method as recited in claim 8, wherein the intensity of the first wavelength of light is controlled by a pulse width modulator.

10. The method as recited in claim 4, further comprising the step of converting the first and second analog signals into third and fourth digital signals, respectively.

11. The method as recited in claim 10, wherein the step of computing the parameter of the blood is calculated using the relationship ##EQU13## where I_out λ

₁ is equal to the third digital signal, I_out λ

₂ is equal to the fourth digital signal, dI/dt^out λ

₁ represents the derivative of the separated AC component of the first analog signal, and dI/dt^out λ

₂ represents the derivative of the separated AC component of the second signal.

12. The method as recited in claim 2, wherein the step of separating the AC components from the first and second analog signals includes,

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

and

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

13. The method as recited in claim 2, wherein the derivatives of the AC components of the first and second analog signals are calculated from the change in value between two consecutive digital sample points in a period.

14. The method as recited in claim 2, wherein the first and second wavelengths of light are in the infrared and red regions, respectively.

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

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

at least two emitting means for emitting first and second wavelengths of electromagnetic energy through a sample;

means responsive to the emitting means for detecting the first and second wavelengths of electromagnetic energy through the sample, the detection means producing first and second analog signals, each of said first and second analog signals having an AC and a DC component, the first analog signal corresponding to the first wavelength of electromagnetic energy and the second analog signal corresponding to the second wavelength of electromagnetic energy;

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

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

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