Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-12-21
2002-06-04
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S310000, C600S335000
Reexamination Certificate
active
06400972
ABSTRACT:
FIELD OF THE INVENTION
This invention in is the field of non-invasive optical measuring techniques and relates to a method for determining the concentration of a substance in the patient's blood, such as glucose, hemoglobin, drugs or cholesterol.
BACKGROUND OF THE INVENTION
Optical methods of determining the chemical composition of blood include spectrophotometric measurements which enable the indication of the presence of various blood constituents based on known spectral behaviors of these constituents. These spectrophotometric measurements may be effected either in vitro or in vivo. The measurements in vitro are invasive, i.e. require a blood sample to be physically withdrawn and investigated. At present, these measurements have become unpopular, due to the increasing danger of infection.
The non-invasive optical measurements in vivo may be briefly divided into two main groups based on different methodological concepts. The first group represents a so-called “DC measurement technique”, and the second group is called “AC measurement technique”.
According to the DC measurement technique, any desired location of a blood perfused tissue is illuminated by the light of a predetermined spectral range, and the tissue reflection and/or transmission effect is studied. Notwithstanding the fact that this technique provides a relatively high signal-to-noise ratio, the results of such measurements depend on all the spectrally active components of the tissue (i.e. skin, blood, muscles, fat, etc.), and therefore they need to be further processed to separate the “blood signals” from the detected signals. Moreover, proportions of the known components vary from person to person and from time to time. To resolve this problem, a calibration must periodically be provided, which constitutes an invasive blood test and therefore renders the DC technique of optical measurements to be actually invasive.
The AC measurement technique focuses on measuring only the “blood signal” of a blood perfused tissue illuminated by a predetermined range of wavelengths. To this end, what is actually measured is a time-dependent component only of the total light reflection or light transmission signal obtained from the tissue.
A typical example of the AC measurement technique is a known method of pulse oximetry, wherein a pulsatile component of the optical signal obtained from a blood perfused tissue is utilized for determining the arterial blood oxygen saturation. In other words, the difference in light absorption of the tissue measured during the systole and the diastole is considered to be caused by blood that is pumped into the tissue during the systole phase from arterial vessels and therefore has the same oxygen saturation as in the central arterial vessels. Not only can the oxygen saturation be determined, but in a similar way, concentrations of other chemical elements in the arterial blood can be determined.
The major drawback of such an AC measurement technique is its relatively low signal-to-noise ratio, as compared to that of the DC measurement technique, especially in cases where an individual has a poor cardiac output, insufficient for providing a pulsatile signal suitable for accurate measurements.
Various methods have been suggested to enhance the natural pulsatile signal of an individual for effecting non-invasive optical measurements.
U.S. Pat. No. 4,883,055 discloses a method and device for artificially inducing blood pulse for use with a pulse oximeter. A cuff wrapped around a body member having an artery upstream from a testing site is adapted for applying a squeezing pulse to the body member, the squeezing pulse being synchronized with a normal blood pulse. Oxygen saturation in the arterial blood is determined based on spectrophotometric non-invasive measurements, which are effected according to the general approach of the above-mentioned AC technique.
U.S. Pat. No. 4,927,264 discloses a non-invasive apparatus and a method for measuring blood constituents in venous blood. The venous blood stream is made time-variant by applying pressure with a peak value of the minimum blood pressure to a proximal portion from a measuring part.
U.S. Pat. No. 5,638,816 discloses a blood glucose monitoring system, which provides for inducing an active pulse in the blood volume of a patient according to a predictable cyclic pattern. The induction of an active pulse causes a cyclic change in the flow of arterial blood through a fleshy medium under the test. By actively inducing a change of the blood volume, modulation of the volume of blood can be obtained to provide a greater signal-to-noise ratio. This enables constituents in blood to be detected at concentration levels below those previously detectable in a non-invasive system. Radiation which passes through the fleshy medium is sensed by a detector which generates a signal indicative of the intensity of the detected radiation. Signal processing is performed on the electrical signal to separate those optical characteristics of the electrical signal which are associated with the optical characteristics of the blood.
The techniques disclosed in the above patents use the artificially induced volumetric changes of either arterial or venous blood. Since each of these techniques is specific about the kind of blood under test, they all impose severe restrictions on a value of the artificially applied pressure. This is due to different “disturbing pressure values” allowed for different kinds of blood flow. It means that for each kind of blood flow, there is a pressure value that disturbs specifically this kind of flow much more than any other kind. For example, when the artificial pressure at a value of 60 mmHg is applied to a proximal body part, the venous blood flow will be affected, whereas the arterial blood flow will not be affected, since the individual's diastolic pressure is usually higher than 60 mmHg. The applied artificial pressure definitely should not exceed pressures causing substantial deformation of the tissue, since only blood flow changes are supposed to be detected by optical measurements, and the measurements are to be effected in synchronism with the artificial pulse. However, if such an artificially induced pulse causes uncontrollable changes of the optical properties of the tissue, these changes cannot be distinguished from those caused by the blood flow fluctuations which are the target of the measurements.
SUMMARY OF THE INVENTION
There is accordingly a need in the art to facilitate non-invasive, optical measurements of the chemical composition of blood, by providing a novel method combining advantages of both the DC and AC techniques of optical measurements with a high signal-to-noise ratio.
The main idea of the present invention is based on the fact that the light response characteristics (i.e., absorption and/or scattering) of a blood perfused medium dramatically changes when a character of blood flow changes. It has been found by the inventors, that the optical characteristics of a blood perfused fleshy medium (e.g., the patient's finger) start to change in time, when causing blood flow cessation. In other words, once the blood flow cessation state is established, the optical characteristics start to change dramatically, such that they differ from those of the fleshy medium with a normal blood flow by about 25 to 45%, and sometimes even by 60%.
Hence, the accuracy (i.e., signal-to-noise ratio) of the optical measurements can be substantially improved by taking at least two timely separated measurement sessions each including at least two measurements with different wavelengths of incident radiation. The light response of the medium at these two sessions essentially differ from each other. At least one of the measurement sessions during which the measurement is effected should be chosen either during temporary blood flow cessation, or during the state of transitional blood flow.
In the inventive method, it is therefore suggested to distinguish the normal blood flow from the state of temporary blood cessation by detecting that at
Fitch Even Tabin & Flannery
Kremer Matthew
Orsense Ltd.
Winakur Eric F.
LandOfFree
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