Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-06-04
2001-05-01
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S333000
Reexamination Certificate
active
06226540
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a measuring process, the purpose of which is to increase the measuring accuracy of pulse oxymeters and comparable optical devices which are used in vivo to ascertain oxygen saturation of arterial blood.
According to the current prior art pulse oxymeters function on the basis that differing wavelengths blood attenuates light very differently depending upon the level of oxygenation. Pulse waves starting from the heart cause in the arterial blood vessel system a periodic fluctuation in the arterial blood content in the tissue. As a consequence, a periodic change in the light absorption (
FIG. 1
) can be registered between the light transmitter, whose radiation passes through the tissue, and the receivers, which are integrated in a pulse oxymetry sensor. The evaluation of the sensor signals is normally carried out at light wavelength of 660 and 940 nm by calculating the differential change of light absorption. It is possible to create a measured variable &OHgr; (sometimes also referred to as R) which is obtained in the following manner or in a similar manner:
Ω
=
ln
I
minλ1
I
maxλ1
ln
I
minλ2
I
maxλ2
The light intensities described in the formula represent the light intensities received in the receiver of the sensors used in pulse oxymetry. The measured variable &OHgr; serves as a measurement for the oxygen saturation. The formation of a quotient in order to form the measured variable is intended to compensate any possible influences the haemoglobin content of the tissue, the pigmentation of the skin or the pilosity may have on the measurement of the oxygen saturation of arterial blood. (See also “Biomedizinische Technik” [Biomedical Technology] Volume 33, Supplementary volume 3, page 6 ff.:“Pulse oxymetrie: Stand und Entwicklung der Technik” [Pulse oxymtery: Status and developement of the technology”; Volume 35, Supplementary volume 1, page 38 ff. “Pulsoxymetrie” [Pulse oxymetry] by K Fortsner Institute for Biomedical Technology, Stuttgart). The influences of blood perfusion in the tissue, the pigmentation and pilosity are not taken into consideration in this measuring process.
When measuring oxygen saturation of arterial blood in the tissue in a range of 70 to 100% using light of wavelength 940 nm and 660 nm this also produces sufficiently accurate measured values. However, in order to measure lower oxygen saturation of arterial blood it is necessary to assume a strong influence on the measured variable &OHgr; in particular caused by perfusion (i.e. blood content) (see: IEEE Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry by Joseph M. Schmitt) and other optical parameters of tissue.
The dependency of the oxygen saturation of arterial blood SaO
2
on the variable &OHgr; and the perfusion p can be written as follows (see also FIG.
2
):
SaO
2
=f(&OHgr;,p)
Similar influences can be caused by pigmentation and pilosity of the skin or scattering and inhomogeneous tissue.
The technical problem resides in the fact that oxygen saturation of arterial blood must be determined in vivo using the process of pulse oxymetry without the perfusion, scattering and inhomogeneity in the tissue or pigmentation and pilosity of the skin influencing the measured result. For this reason, it is necessary to locate from the number of possible calibration curves, those curves which render it possible to determine in the most precise manner the oxygen saturation of arterial blood.
SUMMARY OF THE INVENTION
The invention relates to a method for determining the oxygenation level of blood in living tissue by evaluating the differential attenuation of light at several wavelengths. Light attenuation at at least one wavelength is determined in order to choose those calibration curves of several variables (&OHgr;
1
, &OHgr;
2
. . . ) produced by different wavelength pairings with a minimized error in order to generate an output signal for the arterial blood oxygenation.
By measuring the light attenuation LA in the tissue, which can be determined by relating the intensity I registered in the receiver of the sensor to the initial intensity I
0
generated by the emitter for at least one wavelength, it is possible to select from a number of possible calibration curves those curves with which the oxygen saturation of the arterial blood can be determined in the most precise manner possible to improve considerably the accuracy of the measured values using pulse oxymeters particularly in cases where the oxygen N saturation of the blood is low. (Note: In this context “attenuation” means, that there is a defined relation between I(t) and I
0
(t)).
The invention utilized calibration data SaO
2
vs. (&OHgr;
1
, &OHgr;
2
, . . . ) in order to produce calibration curves. These calibration curves are used in the method of the invention to obtain an output signal of SaO
2
.
In a further embodiment, the measured value error can be minimized by measuring variables which are obtained from different wavelength pairings. Special choice of wavelength pairings allows to measure additional optical parameters if the calibration curve of this wavelength pairing is very sensitive to changes of the said parameter. Thus the best fitting calibration curves of wavelength pairings which are more sensitive to arterial oxygen changes can be chosen in order to determine the oxygen saturation.
In order to exclude the influence of pilosity and pigmentation, these parameters can be measured by determination of the attenuation between a receiver and emitter of light, which are close to one another (optical unit) (FIG.
4
). By measuring the attenuation within further optical units and between these optical units optical parameters of the tissue can be determined without the influence of these parameters.
According to the above embodiments, measuring arterial oxygenation is accomplished by concurrently measuring the electrical activity of the subject whose blood oxygenation is determined in order to detect artefacts while registering the differential attenuation of blood.
A further method to reduce the error of SaO
2
is by characterizing the tissue inhomogeneity. This can be done by comparing the differential light absorptions
I
minλ1
I
maxλ1
at different distances of the light receivers and emitters.
REFERENCES:
patent: 4807631 (1989-02-01), Hersh et al.
patent: 5595176 (1997-01-01), Yamaura
patent: 5922607 (1999-07-01), Bernreuter
Dennison, Scheiner Schultz & Wakeman
Winakur Eric F.
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