Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-03
2001-12-11
Casier, Brian L. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06330468
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to medical diagnostic instruments and, more specifically, to a pulse oximeter using two green light sources to detect the oxygen saturation of hemoglobin in a volume of intravascular blood.
BACKGROUND OF THE INVENTION
The degree of oxygen saturation of hemoglobin, SpO
2
, in arterial blood is often a vital index of the condition of a patient. As blood is pulsed through the lungs by the heart action, a certain percentage of the deoxyhemoglobin, RHb, picks up oxygen so as to become oxyhemoglobin, HbO
2
. From the lungs, the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO
2
gives up its oxygen to support the life processes in adjacent cells.
By medical definition, the oxygen saturation level is the percentage of HbO
2
divided by the total hemoglobin; therefore, SpO
2
=HbO
2
/(RHb+HbO
2
). The saturation value is a very important physiological value. A healthy, conscious person will have an oxygen saturation of approximately 96 to 98%. A person can lose consciousness or suffer permanent brain damage if that person's oxygen saturation value falls to very low levels for extended periods of time. Because of the importance of the oxygen saturation value, “Pulse oximetry has been recommended as a standard of care for every general anesthetic.” Kevin K. Tremper & Steven J. Barker,
Pulse Oximetry
, Anesthesiology, January 1989, at 98.
An oximeter determines the saturation value by analyzing the change in color of the blood. When radiant energy interacts with a liquid, certain wavelengths may be selectively absorbed by particles which are dissolved therein. For a given path length that the light traverses through the liquid, Beer's law (the Beer-Lambert or Bouguer-Beer relation) indicates that the relative reduction in radiation power (P/Po) at a given wavelength is an inverse logarithmic function of the concentration of the solute in the liquid that absorbs that wavelength.
For a solution of oxygenated human hemoglobin, the extinction coefficient maximum is at a wavelength of about 577 nm (green) O. W. Van Assendelft,
Spectrophotometry of Haemoglobin Derivatives,
Charles C. Thomas, Publisher, 1970, Royal Vangorcum LTD., Publisher, Assen, The Netherlands. Instruments that measure this wavelength are capable of delivering clinically useful information as to oxyhemoglobin levels. In addition, the reflectance pulsation spectrum shows a peak at 577 nm as well. Weijia Cui, Lee L. Ostrander, Bok Y. Lee, “In Vivo Reflectance of Blood and Tissue as a Function of Light Wavelength”, IEEE Trans. Biom. Eng. 37:6:1990, 632-639.
In general, methods for noninvasively measuring oxygen saturation in arterial blood utilize the relative difference between the electromagnetic radiation absorption coefficient of deoxyhemoglobin, RHb, and that of oxyhemoglobin, HbO
2
. The electromagnetic radiation absorption coefficients of RHb and HbO
2
are characteristically tied to the wavelength of the electromagnetic radiation traveling through them.
In practice of the transmittance pulse oximetry technique, the oxygen saturation of hemoglobin in intravascular blood is determined by (1) alternatively illuminating a volume of intravascular blood with electromagnetic radiation of two or more selected wavelengths, e.g., a red (600-700 nm) wavelength and an infrared (800-940 nm) wavelength, (2) detecting the time-varying electromagnetic radiation intensity transmitted through by the intravascular blood for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood by applying the Lambert-Beer's transmittance law to the transmitted electromagnetic radiation intensities at the selected wavelengths.
Whereas apparatus is available for making accurate measurements on a sample of blood in a cuvette, it is not always possible or desirable to withdraw blood from a patient, and it obviously impracticable to do so when continuous monitoring is required, such as while the patient is in surgery. Therefore, much effort has been expanded in devising an instrument for making the measurement by noninvasive means.
A critical limitation in prior art noninvasive pulse oximeters is the few number of acceptable sites where a pulse oximeter probe may be placed. Transmittance probes must be placed in an area of the body thin enough to pass the red/infrared frequencies of light from one side of the body part to the other, e.g., ear lobe, finger nail bed, and toe nail bed. Although red/infrared reflectance oximetry probes are known to those skilled in the art, they do not function well because red and infrared wavelengths transmit through the tissue rather than reflect back to the sensor. Therefore, red/infrared reflectance sensor probes are not typically used for many potentially important clinical applications including: use at central body sites (e.g., thigh, abdomen, and back), enhancing poor signals during hypoperfusion, decreasing motion artifact, etc.
SUMMARY OF THE INVENTION
According to the present invention, a reflectance oximeter is provided using two green light sources to detect the oxygen saturation of hemoglobin in a volume of intravascular blood. Preferably the two light sources emit green light centered at 560 nm and 577 nm, respectively, which gives the biggest difference in absorption between deoxyhemoglobin, RHb, and oxyhemoglobin, Hbo
2
. The green reflectance oximeter is a significant improvement compared to the red/infrared state of the art because the reflectance pulsation spectrum peaks at 577 nm. Practically, several combinations of two green light sources can be used. Ideally, these light sources comprise very narrow band (e.g., 1.0 nm wide) sources such as laser diodes at the desired frequencies. However, the benefits of the present invention can be realized using other green light sources, such as narrow band (e.g., 10 nm wide) light emitting diodes (LEDs) at two green frequencies (e.g., 562 nm and 574 nm) with optional ultra-narrowband (e.g., 0.5-4.0 nm wide) filters at two green frequencies (e.g., 560 nm and 577 nm).
In one embodiment of the present invention, two filtered green LEDs alternatively illuminate an intravascular blood sample with two green wavelengths of electromagnetic radiation. The electromagnetic radiation interacts with the blood and a residual optical signal is reflected by the blood. Preferably a photodiode in a light-to-frequency converter (LFC) detects the oximetry optical signals from the intravascular blood sample illuminated by the two LEDs. The LFC produces a periodic electrical signal in the form of a pulse train having a frequency proportional to the light intensity. The data becomes an input to a high-speed digital counter, either discrete or internal to a processor (e.g., digital signal processor, microprocessor, or microcontroller), which converts the pulsatile signal into a digital word suitable to be analyzed by the processor. In the alternative, a separate silicon photodiode, a current-to-voltage converter (a transimpedance amplifier), a preamplifier, a filter, a sample and hold, and an analog-to-digital (A/D) converter can be used to capture the oximetry signal.
Once inside the processor, the time-domain data is converted into the frequency domain by, for example, performing the well-known Fast Fourier Transform (FFT). The frequency domain data is then processed to determine the oxygen saturation value using any of a number of methods known to those skilled in the art.
It is therefore an advantage of the present invention to provide a green-light reflectance-type pulse oximeter capable of measuring oxygen saturation at central body surfaces.
It is a further object of this invention to provide a reflectance-type pulse oximeter using only green wavelengths of light to measure oxygen saturation.
These and other advantages of the present invention shall become more apparent from a detailed description of the invention.
REFERENCES:
patent: 3802776 (1974-04-01), Tchang
patent: 3815583 (1974-06
Calfee Halter & Griswold LLP
Casier Brian L.
University of South Florida
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