Method and apparatus for eliminating interference in pulse oxygen measurement

• US 20070149872A1
• Filed: 09/18/2006
• Published: 06/28/2007
• Est. Priority Date: 12/23/2005
• Status: Active Grant

• Current Assignee: Shenzhen Mindray Bio-Medical Electronics Co., Ltd. (Excelsior Union Ltd.)
• Original Assignee: Shenzhen Mindray Bio-Medical Electronics Co., Ltd. (Excelsior Union Ltd.)
• Inventors: Zhang, Xu, Li, Xu, Li, Shunan
• Granted Patent: US 8,050,730 B2
• Time in Patent Office: Days
• US Class Current: 600/336
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

The present invention discloses a method and apparatus for eliminating interference in pulse oxygen measurement. The method comprises the steps of: collecting a first wavelength light and a second wavelength light transmitting through the object to be measured and converting collected optic signals into electric signals to form a plethysmogram; processing the plethysmogram so as to normalize it, in order to decompose the normalized plethysmogram into a combination of an ideal plethysmogram and noise, and expand the ideal plethysmogram by using functions that can make up a complete orthonormal system; eliminating the noise in the plethysmogram through differential operation; and restoring the plethysmogram free of noise through integral operation for calculating oxygen saturation. The apparatus comprises a collecting module, a processing module, a noise eliminating module, and a restoring module. The method and apparatus suitable for the measurement of oxygen saturation under weak perfusion and movement conditions.

Claims

1. A method for eliminating interference in pulse oxygen measurement, comprising the following steps of:

collecting a first wavelength light and a second wavelength light transmitting through an object to be measured, and converting collected optic signals into electric signals so as to form a plethysmogram;

processing the plethysmogram so as to normalize it, in order to decompose the normalized plethysmogram into a combination of an ideal plethysmogram and a noise, and expand the ideal plethysmogram by using functions that can make up a complete orthonormal system;

eliminating the noise in the plethysmogram through differential operation; and

restoring the plethysmogram free of the noise through integral operation for calculating oxygen saturation.

2. The method of claim 1, further comprising the step of:

conducting an analog-to-digital conversion to the electric signals so as to form a digitalized plethysmogram for the subsequent processing.

3. The method of claim 1, further comprising the step of:

fitting, for elimination of a nonlinear slow baseline drift and subsequent calculation of oxygen saturation;

which step further comprises;

computing respective drift baseline fitting curve coefficient matrixes according to sampling frequencies and sampling sequences of the transmitted light intensity of the first wavelength light and the second wavelength light; and

subtracting the corresponding fitting curves from the plethysmogram waveform curves of the transmitted first wavelength light and second wavelength light.

4. The method of claim 1, wherein in the processing step, the ideal plethysmogram may be expanded by sine functions or cosine functions that make up a complete orthonormal system.

5. The method of claim 1, wherein the noise eliminating step is specified as:

differentiating the plethysmogram having been processed, and then normalizing it to eliminate a baseline drift noise caused by movement.

6. The method of claim 1, wherein the noise eliminating step is specified as:

differentiating the plethysmogram having been processed to transform a step noise caused by abrupt extrusion of the object to be measured into an impluse function similar to the δ

function, and subsequently conducting a three-point or five-point median filtering to eliminate the step noise.

7. The method of claim 1, wherein the noise eliminating step is specified as:

differentiating the plethysmogram having been processed to transform an impulsive noise caused by mutation of sampled value into positive and negative double impulse functions, and subsequently conducting a five-point median filtering to eliminate the impulse noise.

8. The method of claim 1, wherein the oxygen saturation calculation is carried out by an area integration recursive algorithm, comprising:

integrating the plethysmogram having been restored in a period of time to eliminate a white noise occurring in the same period and obtain a ratio between an AC component of the first wavelength light and that of the second wavelength light;

introducing a forgetting factor λ

, to obtain the ratio between the AC component of the first wavelength light and that of the second wavelength light according to the following formula after iterating for a number of times;

9. The method of claim 8, wherein the period of time ranges from 2 to 3 seconds, and the forgetting factor satisfies:

0<

λ

<

1.

10. The method of claim 9, wherein the forgetting factor is 0.8.

11. The method of claim 1, wherein the first wavelength light and the second wavelength light are the red light and the infrared light respectively.

12. An apparatus for eliminating interference in pulse oxygen measurement, comprising:

a collecting module including a luminotron and a corresponding phototube, for collection of a first wavelength light and a second wavelength light transmitting through an object to be measured and for converting collected optic signals into electric signals so as to form a plethysmogram;

a processing module, for normalization of the plethysmogram so as to decompose the normalized plethysmogram into a combination of an ideal plethysmogram and a noise, and for expansion of the ideal plethysmogram by using functions that can make up a complete orthonormal system;

a noise eliminating module, for elimination of the noise in the plethysmogram through differential operation; and

a restoring module, for restoration of the plethysmogram free of noise through integral operation for calculating oxygen saturation.

13. The apparatus of claim 12, further comprising:

a converting module, for analog-to-digital conversion of the electric signals so as to form a digitalized plethysmogram for the subsequent processing.

14. The apparatus of claim 12, further comprising:

a fitting module, for computing respective drift baseline fitting curve coefficient matrixes according to sampling frequencies and sampling sequences of the transmitted light intensity of the first wavelength light and the second wavelength light and subtracting the corresponding fitting curves from the plethysmogram waveform curves of the transmitted first wavelength light and second wavelength light so as to eliminate a nonlinear slow baseline drift.

15. The apparatus of claim 14, wherein the fitting module, processing module, noise eliminating module and restoring module may be either a physical module, or a computer-executable software module.

16. The apparatus of claim 12, wherein the functions that make up a complete orthonormal system are sine functions or cosine functions.

17. The apparatus of claim 12, wherein the noise eliminating module executes the following functions:

differentiating the plethysmogram having been processed by the processing module, and then normalizing it to eliminate a baseline drift noise caused by movement.

18. The apparatus of claim 12, wherein the noise eliminating module executes the following functions:

differentiating the plethysmogram having been processed by the processing module to transform a step noise caused by abrupt extrusion of the object to be measured into an impluse function similar to the δ

function, and subsequently conducting a three-point or five-point median filtering to eliminate the step noise.

19. The apparatus of claim 12, wherein the noise eliminating module executes the following functions:

differentiating the plethysmogram having been processed by the processing module to transform an impulsive noise caused by mutation of sampled value into positive and negative double impulse functions, and subsequently conducting a five-point median filtering to eliminate the impluse noise.

20. The apparatus of claim 12, wherein the first wavelength light and the second wavelength light are the red light and the infrared light respectively.