Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Chemical analysis
Reexamination Certificate
2002-02-15
2003-09-16
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Chemical analysis
C702S030000, C702S019000, C600S322000
Reexamination Certificate
active
06622095
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
(NOT APPLICABLE)
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to measurements of oxygen saturation and concentrations of hemoglobins in arterial blood by using a pulse oximeter, and more particularly to measurement of a concentration of methemoglobin (MetHb).
2. Related Art
A conventional pulse oximeter is constructed such that near-infrared rays of light and red rays of light are irradiated onto a living tissue, ratios of the pulsating components of attenuations of these lights having passed through the living tissue are processed, and an arterial oxygen saturation is noninvasively measured from the result of the processing.
The measuring principle of the pulse oxometer is known as disclosed in JP-A-53-26437, proposed by the applicant of the present patent application. The measuring principle of the pulse oximeter will be described in brief hereunder.
For example, suppose a living tissue R is divided into a blood layer R
1
and a layer R
2
of a tissue other than blood this tissue will be referred to as a pure tissue), and it is assumed that a thickness of the blood layer R
1
is pulsated, but a thickness of the pure tissue layer R
2
is not pulsated, viz., it is constant. Where the living tissue R is irradiated with light, an incident light amount IO is reduced by the living tissue R, and an amount of light passing through the living tissue R is I. when a thickness of the blood layer R
1
is pulsated to be increased by &Dgr;Db, the amount of the transmitted light is reduced to be (I−&Dgr;I). In this case, an attenuation &Dgr;A of the light, which is produced by a thickness change &Dgr;Db of the blood layer R
1
, is given by
&Dgr;
A
=log [
I
/(
I−&Dgr;I
)]
When lights of different wavelengths &lgr;
1
and &lgr;
2
are irradiated onto the living tissue R, a ratio &PHgr; of attenuations &Dgr;A
1
and &Dgr;A
2
of lights of the wavelengths &lgr;
1
and &lgr;
2
, which are produced by the pulsation of the tissue thickness is mathematically approximated by
&PHgr;=&Dgr;
A
1
/&Dgr;A
2
={square root over ( )}{
E
1
(
E
1
+
F
1
)}/{square root over ( )}{
E
2
(
E
2
+
F
2
)} (1)
This is theoretically and empirically confirmed.
In the above expression, Ei is an absorption coefficient of hemoglobin, Fi is a scattering coefficient of light in blood, and i=1, 2, which represent the wavelengths &lgr;
1
and &lgr;
2
. Assuming that light absorbing materials in blood are only oxyhemoglobin and deoxyhemoglobin, then the absorption coefficient Ei of the hemoglobin is given by the following expression.
Ei=SEOi
+(1
−S
)
Eri
(2)
In the expression, S is an oxygen saturation, and Eoi is an absorption coefficient of oxyhemoglobin and Eri is an absorption coefficient of deoxyhemoglobin. Substituting the expression (2) for the expression (1), then we have the following expression
&PHgr;=
&Dgr;A
1
/&Dgr;A
2
={square root over ( )}[{
SEo
1
+(1
−S
)
Er
1
}[{
SEo
1
+(1
−S
)
Er
1
}+
F
1
]]/{square root over ( )}[{
SEo
2
+(1
−S
)
Er
2
}[{
SEo
2
+(1
−S
)
Er
2
}+
F
2
]] (3)
In the expression (3), Eol, Er
1
, Eo
2
, Er
2
F
1
and F
2
are known values. Therefore, an oxygen saturation S can be obtained in a manner that &PHgr;=&Dgr;A
1
/&Dgr;A
2
is measured, substituted for the expression (3), and the expression is solved for the S.
If methemoglobin MetHb is present in blood, a drop arises in a reading of a degree of oxygen saturation measured by a related-art pulse oximeter using two wavelengths; that is, near-infrared rays of light and red rays of light. Since the oximeter cannot determine a concentration of methemoglobin MetHb, the presence/absence or concentration of methemoglobin MetHb in blood (also called a “blood methemoglobin MetHb concentration”) remains uncertain until blood of interest is sampled and subjected to measurement performed by a carbon monoxide oximeter (CO-Oximeter).
Meanwhile, where the arterial blood pulsates, the theory teaches that concentration ratios of “n” number of light absorbing materials in the blood can be measured by using “n” number of wavelengths of lights. Accordingly, the theory also teaches that it is impossible to measure concentration ratios of three hemoglobins, oxyhemoglobin O2Hb, deoxyheoglobin RHband methemoglobin MetHb by using two wavelengths of lights, and at least three wavelengths must be used for the measurement.
Actually, however, the influence by pure tissues other than the blood will produce measuring errors. Accordingly, to accurately measure concentrations of “n” number of light absorbing materials in the blood, it is preferable to use (n+1) number of wavelengths, this fact was found and confirmed by us. The applicant of the present patent application developed an apparatus for determining concentrations of materials in blood based on the above fact, and filed the patent application on the apparatus (JP-B-5-88609). Other light absorbing materials, such as carboxyhemoglobin (COHb) and bilirubin, are also contained in the blood. To remove the influence by those materials is attempted, the number of wavelengths used is further increased, and further cost to manufacture the apparatus is also increased.
In adding a third wavelength for measuring the methemoglobin Metub to the pulse oximeter (JP-A-5-228129), the ratio of absorption coefficients of oxyhemoglobin O
2
Hb and methemoglobin MetHb at the wavelengths of lights, which are longer than the red wavelengths, as shown in
FIG. 11
, is almost constant. For this reason, where the third wavelength is selected from those wavelengths longer than the red wavelengths it is very difficult to determine the methemoglobin MetHb concentration sensitively.
Scharf proposed in his patent (U.S. Pat. No. 5,830,137) the use of the green wavelength region for the third wavelength. The absorption coefficient of every kind of hemoglobin, as shown in
FIG. 11
, is considerably large in the yellow and green wavelength regions. The absorption coefficients of the oxyhemoglobin O2Hb in the wavelength region of 500 nm to 620 nm are at least 10 times as large as those at 660 nm. Light having passed through the blood is very weak, and the measurement at good S/N ratio is very difficult.
SUMMARY OF INVENTION
Accordingly, an object of the present invention is to provide an apparatus for determining concentrations of hemoglobins which, using an orange or red orange wavelength region for the third wavelength in addition to the near-infrared and red wavelength regions, which are conventionally used, can detect a change of the transmitted light by a change of the methemoglobin MetHb at good S/N ratio, and can easily discriminate between the methemoglobin Metab and the deoxyhemoglobin RHb, and hence can perform a proper measurement of methemoglobin MetEb.
The present invention provides an apparatus for determining concentrations of hemoglobins according to the present invention comprises:
a light source which emits at least three different light rays: that is, light in a near-infrared wavelength region as a first wavelength, light in a red wavelength region as a second wavelength, and light in a red orange wavelength region as a third wavelength;
light-receiving means for receiving light that has originated from the light source and has passed through or has been reflected by a living tissue;
attenuation ratio processing means which processes an attenuation ratio &PHgr; between the light rays of the wavelengths in accordance with a change in a received-light output signal in each wavelength output from the light-receiving means as a result of pulsation of blood; and
concentration ratio processing means which processes at least concentration ratios of oxyhemoglobin and that of methemoglobin.
In this case, the concentration ratio processing means can be
Kanemoto Michio
Kobayashi Naoki
Takamura Yoshiaki
Ukawa Teiji
Usuda Takashi
Nihon Kohden Corporation
Shah Kamini
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