Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2002-12-17
2004-06-22
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S336000
Reexamination Certificate
active
06754515
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to a method and apparatus for eliminating the effects of noise inherent in certain optical sources used in photoplethysmographic measurements.
BACKGROUND OF THE INVENTION
In the science of photoplethysmography, light is used to illuminate or trans-illuminate living tissue for the purpose of measuring blood analytes or other hemodynamic or tissue properties. In this monitoring modality light is injected into living tissue and a portion of the light which is not absorbed by the tissues, or scattered in some other direction, is detected a short distance from the entry point. The detected light is converted into an electronic signal that is indicative of the received light signal from the tissue. This electronic signal is then used to calculate physiologic parameters such as arterial blood oxygen saturation and hemodynamic variables such as heart rate, cardiac output, or tissue perfusion. Among the blood analytes that may be measured by photoplethysmography are the various species of hemoglobin, including the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood. A device which detects and processes photoplethysmographic signals to measure the levels of various blood analytes and hemodynamic parameters is referred to as a photoplethysmographic apparatus, device, or instrument. Typically these instruments also include, and control, the light sources or emitters used to generate the light that illuminates the tissue.
The first widespread commercial use of photoplethysmography in medicine was in the pulse oximeter, a device designed to measure arterial blood oxygen saturation. To make these measurements, two different bands of light must be used, with each light band possessing a unique spectral content. Each spectral band, or light band, is typically referred to by the center wavelength, or sometimes by the peak wavelength, of the given band. In pulse oximetry two different light emitting diodes (LEDs) are typically used to generate the sensing light, one with a center, or peak, wavelength near 660 nanometers (nm) and a second with a center, or peak, wavelength near 940 nm.
Light from each LED light source, or emitter, is passed into the tissue-under-test, usually a finger, earlobe, or other relatively thin, well-perfused tissue sample. After passing some distance through the tissue-under-test, a portion of the light not absorbed by the tissue or scattered in some other direction is collected by a photodetector and converted into electronic signals that are directly proportional to the received light signals. The channels, or electronic signals from each of the different light sources, are kept separated through the use of any one of a number of different well-published techniques, including but not limited to, time-division multiplexing or frequency-division multiplexing.
The signals received from the tissue are referred to as photoplethysmographic signals. These signals consist of a small pulsatile component and a rather large constant component that changes slowly over time when compared with the pulsatile component of the signal. The pulsatile component of the photoplethysmographic signal is created by the pulsation of the blood in the tissue-under-test. When the heart contracts, it pushes blood out of the heart and into the peripheral tissues. This increases the optical density of the tissue located between the emitter and detector elements of the sensor, which decreases the amplitude of the received optical signals. As the heart relaxes and refills with blood, the optical density of the tissue-under-test decreases, and the received signal amplitude increases. The comparatively constant component of the photoplethysmographic signal is often referred to as the DC component of the signal, and the pulsatile component of the photoplethysmographic signal is often referred to as the AC component of the signal.
The photoplethysmographic signals are processed to obtain a measurement of the oxygen saturation in the arterial blood. This can be done in a number of different ways but all require mathematically relating the amplitude of the photoplethysmographic signals from each of the two channels to the arterial oxygen saturation.
In conventional pulse oximetry the instrument has only two channels, one associated with each emitter or light source used. With only two channels, only two blood analytes can be measured. Conventional pulse oximetry makes the mathematical assumption that there are primarily only two types of blood analytes in the arterial blood, oxyhemoglobin and reduced hemoglobin.
In order to measure only the arterial oxygen saturation, the pulse oximeter makes use of both the pulsatile component and the DC component of the photoplethysmographic signals. Because any pulsation of the venous system or capillaries is small by comparison to the arterial pulsation, changes in the amplitude of the photoplethysmographic signals will be dominated by the arterial pulsation. Note that the photoplethysmographic signals can be severely distorted by artifacts such as patient motion or electrocautery but elimination of these sources of artifacts is not the focus of this patent and will not be specifically addressed herein.
The amplitude of the pulsatile component of these photoplethysmographic signals can be extremely small. It is not uncommon for the percent modulation, or the peak-to-peak amplitude of the pulsatile portion divided by the constant portion, to be less than one part in one thousand, or 0.1%. Thus it is crucial that extremely quiet light sources are used to generate the signals for probing the tissue-under-test. This is necessary because any intensity noise in the light source that is within the frequency range of the passband of the photoplethysmographic device, or which can alias into its passband, will show up in the received photoplethysmographic signals and corrupt the desired measurements. Thus to allow for accurate and precise photoplethysmographic measurements, the light sources should be extremely quiet (also referred to as “noise free”) or a means must be found to eliminate the light noise from the received signals.
Since the inception of photoplethysmography, this monitoring modality has been used to detect more and more different parameters. For example, a device was disclosed in Jarman et al U.S. Pat. No. 5,983,122 that is capable of measuring the percentages of four different analytes in the arterial blood, including oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin.
As the number of different parameters measured by photoplethysmography increases, so too does the number of different bands of light required to make the measurements. Further, because a fairly high intensity of light over a fairly narrow spectral range is needed for these measurements, it has been found that the most successful sources of light for these measurements have been discrete, narrow-band emitters such as LEDs or laser diodes. These types of light sources are typically used because broadband sources (in conjunction with filters or a diffraction grating to obtain the required spectral bands) produce too little energy over the desired narrow spectral bandwidths to provide sufficient signal amplitude for photoplethysmographic measurements.
LEDs are inherently very quiet light sources but do not have a sufficiently narrow spectral bandwidth for use at all of the required center wavelengths. Conventional edge-emitting diode lasers provide the necessary narrow bandwidth but can be quiet noisy. For example, in a time-division multiplexed system, the intensity of the light emitted by a laser diode can vary from pulse to pulse, or can even jump almost instantaneously during any given pulse. These intensity variations can easily be large enough to prevent the measurement of the desired blood analytes to clinically-acceptable accuracy and precision levels. While not all types of laser diodes are noisy, often the photoplethysmographic instrument designer must
Kestrel Labs, Inc.
Winakur Eric F.
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