Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Patent
1996-11-19
1998-06-30
Kamm, William E.
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
600336, A61N 500
Patent
active
057725896
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a measuring process, the purpose of which is to increase the measuring accuracy of pulse oxymeters which are used in vive to ascertain oxygen saturation of arterial blood.
According to the current prior art pulse oxymeters function on the basis that at 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. 4) 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 wavelengths of 660 and 940 nm. It is possible to create a measured variable .OMEGA. (sometimes also referred to as R) which is obtained in the following manner or in a similar manner: ##EQU1##
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 .OMEGA. 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 33, Supplementary volume 3, page 6 ff.: "Pulse oxymetrie: Stand und technology"; Volume 35, Supplementary volume 1, page 38 ff. 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 a 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 .OMEGA. in particular caused by perfusion, as is shown as follows (or according to the article: IEEE Transactions on biomedical engineering, vol, 38 No. 12 December 1991: Simple Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry by Joseph M. Schmitt):
If light beams are emitted by one transmitter (Page -1-; FIG. 1), wherein in this example 3 we have chosen light path representatives which together with the weight factors kn to be established later should characterise the light distribution in the tissue, then these light beams are attenuated when perfusion occurs through additional blood layers of different density (arterial blood which flows in additionally at a density .delta.), if one assumes that the arterial pulsation is equal to zero, then the light intensity obtained in the receiver is:
If the arterial vessels now expand owing to new blood flowing in from the heart (the same naturally also applies for venous blood), then the density of the blood layer (and the light attenuation) increases for the individual beam paths and the following is obtained for the intensity received: =I.sub.2 .multidot.e.sup.-.alpha..multidot..delta.2, I'.sub.3 =I.sub.3
If the ratio is formed from I.sub.min and I.sub.max, then the following is obtained: ##EQU2## The coefficients kn characterise the scatter and light distribution in the tissue.
In order to determine the oxygen saturation it is necessary to use at least two wavelengths, whose coefficients of absorption differ from each other in dependence upon the level of oxygenation. In the case of increased absorption, the light portions which travel longer light path distances in the tissue are attenuated to a comparatively greater extent than the light portions which travel the shorter distances.
A similar situation is to be observed as the blood content in the tissue varies. If the blood content in the tissue varies, t
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