Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Chemical analysis
Reexamination Certificate
2000-11-30
2002-07-02
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Chemical analysis
C702S031000, C702S019000, C600S322000
Reexamination Certificate
active
06415236
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to the measurements of oxygen saturations and concentrations of hemoglobins in arterial blood by using a pulse oximeter, and more particularly to the measurement of a concentration of carboxyhemoglobin.
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 computation.
The measuring principle of the pulse oximeter 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.
As shown in FIGS.
9
(A) and
9
(B), a living tissue R is divided into a blood layer R1 and a layer R2 of a tissue from which blood has removed (this tissue will be referred to as a pure tissue), and it is assumed that a thickness of the blood layer R1 is pulsated, but a thickness of the pure tissue layer R2 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 R1 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 R1, 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;A1 and &Dgr;A2 of lights of the wavelengths &lgr;1 and &lgr;2, which are produced by the pulsation of the tissue thickness is mathematically approximated by
[Expression 1]
&PHgr;=&Dgr;
A
1/&Dgr;
A
2={{square root over (])}(
E
1(
E
1+
F
)}]/[{square root over ( )}{
E
2(
E
2+
F
)}] (1)
This is theoretically and empirically confirmed.
In the above expression, E1,2(Ei) are absorption coefficients, F is a scattering coefficient of light in blood and has no wavelength dependency, and suffixes 2 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 and Eri are an absorption coefficient Eoi of oxyhemoglobin and absorption coefficient Eri of deoxyhemoglobin. Substituting the expression (2) for the expression (1), then we have the following expression
[Expression 3]
Φ
=
Δ
A1
/
Δ
A2
=
[
{
[
SEo1
+
(
1
-
S
)
Er1
)
{
SEo1
+
(
1
-
S
)
Er1
+
F
)
}
]
/
[
{
(
SEo2
+
(
1
-
S
)
Er2
)
(
SEo2
+
(
1
-
S
)
Er2
+
F
)
}
]
(
3
)
In the expression (3), Eo1, Er1, Eo2, Er2 and F are known values. Therefore, an oxygen saturation S can be obtained in a manner that &PHgr;=&Dgr;A1/&Dgr;A
2
is measured, substituted for the expression (3), and the expression is solved for the S.
The conventional pulse oximeter using two wavelengths of near-infrared rays of light and red rays of light cannot detect an increase of a concentration of carboxyhemoglobin COHb in blood. Accordingly, it has a disadvantage that an arterial oxygen saturation displayed is higher than an actual one. When the pulse oximeter is coupled to a patient suffering from carbon monoxide poisoning and operated for the monitoring, the result of the measurement by the pulse oximeter will lead to such misunderstanding by the medical staff that the sufficient amount of oxygen is present even though an amount of transporting oxygen is actually reduced. This is tremendously dangerous for the patient. In diagnosing a patient showing the carbon monoxide poisoning, it is very difficult to judge the illness as the carbon monoxide poisoning from only the symptoms of the patient. Accordingly, the carbon monoxide poisoning has frequently been missed in the diagnosis of the patient, though it is dangerous.
It is reported that in the operation under anesthesia, a patient shows the carbon monoxide poisoning in which a concentration of the carbon monoxide in blood reaches 10 to 30%. The cause of it is estimated that inhalative anesthetic and dried CO2 absorbent generate carbon monoxide. However, the conventional pulse oximeter cannot find the generated carbon monoxide. Accordingly, there is a danger of missing the generation of the carboxyhemogrobin.
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 carboxyhemoglobin COHb by using two wavelengths of lights, and at least three wavelengths must be used for the measurement.
Actually, however, the influence by puretissues 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 methemoglobin 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 carboxyhemoglobin COHb to the pulse oximeter (JP-A-5-228129), the absorption coefficients of it at the wavelengths of lights, which are longer than the red wavelengths, as shown in
FIG. 10
, are extremely small. Accordingly, it is very difficult to detect it. The absorption coefficient of the carboxyhemoglobin COHb at the wavelength of 700 nm is about {fraction (1/10)} as large as that of oxyhemoglobin O2Hb. Accordingly, in this case, a change of the transmitted light which results from a change of the carboxyhemoglobin COHb, is about {fraction (1/10)} as large as a change of the same which results from a change of the oxyhemoglobin O2Hb, and is extremely small. For this reason, where the third wavelength is selected from those wavelengths ranging from the red wavelengths to near-infrared wavelengths, a sensitivity of the apparatus is too small to discriminate the carboxyhemoglobin COHb from other hemoglobins Hb., and it is very difficult to measure the carboxyhemoglobin COHb.
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. 10
, is considerably large in the yellow and green wavelength regions. The absorption coefficients of the carboxyhemoglobin COHb and the oxyhemoglobin O2Hb in the wavelength region of 500 nm to 590 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 orangey red wavelength region for the third wavelength in addition to the near-i
Kanemoto Michio
Kobayashi Naoki
Ukawa Teiji
Usuda Takashi
Nihon Kohden Corporation
Shah Kamini
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