Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-12-17
2002-03-26
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S336000
Reexamination Certificate
active
06363269
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to spectrophotometric analysis for measuring the concentrations of a plurality of analytes in a given sample, and more particularly, to the use of frequency division multiplexed signals in such analysis. The invention is particular apt for use in photoplethysmographic systems, and even more specifically, in pulse oximetry applications.
BACKGROUND OF THE INVENTION
Spectrophotometric analysis is employed to estimate the concentration of one or more analytes in a given sample and entails the passage of light from one or more light source(s) through the sample. The amount of light transmitted through the sample is measured and typically employed in one or more calibration equation(s) to obtain the analyte concentration estimate(s). The calibration equation(s) is based upon the unique light absorption characteristics of each analyte(s) to be measured.
In the field of photoplethysmography, pulses of light having different center wavelengths are transmitted through a tissue under test to non-invasively determine various blood analyte values. More particularly, pulse oximeters are employed to determine pulse rates and blood oxygen levels, and typically include a probe that is releasably attached to a patient's appendage (e.g., finger, ear lobe or nasal septum). The probe directs light signal pulses generated by a plurality of emitters through the appendage, wherein portions of the light signals are absorbed by the tissue. The intensity of light transmitted by the tissue is monitored by one or more detector(s) which outputs a signal(s) indicative of the light absorbency characteristics of the tissue. Because the blood analytes of interest absorb more light at one wavelength than at another wavelength, the detector output signal(s) may be used to compute the blood analyte concentrations.
By way of primary example, it is known that oxyhemoglobin (O
2
Hb) absorbs light more readily in the infrared region than in the red region, whereas reduced hemoglobin (RHb), or deoxyhemoglobin, more readily absorbs light in the red region than in the infrared region. As such, oxygenated blood with a high concentration of oxyhemoglobin and a low concentration of reduced hemoglobin will tend to have a high ratio of optical transmissivity in the red region to optical transmissivity in the infrared region. The relative transmissivity of blood at red and infrared center wavelengths may be employed as a measure of blood oxygen saturation (SpO2). See, e.g., U.S. Pat. No. 5,503,148, hereby incorporated by reference in its entirety.
It is also recognized that concentrations of other related blood constituents (e.g., carboxyhemoglobin (COHb) and methemoglobin(MetHb)) can be measured with a similar approach since such analytes also have unique light absorbency characteristics at different corresponding center wavelengths. The determination of such additional constituents can serve to enhance the measurement of blood oxygen saturation. See, e.g., U.S. Pat. No. 5,842,979, hereby incorporated by reference it its entirety.
In pulse oximetry applications where a single detector is used, some modulation method must be employed with the different light sources so that tissue light transmission corresponding with each of the sources can be distinguished in the multiplexed detector output signal. One approach, called time-division multiplexing, provides for the pulsing of the light sources at different predetermined or monitored points in time during the modulation cycle so that the multiplexed detector output signal can be demultiplexed based on the monitored transmission times. See., e.g., U.S. Pat. No. 5,954,644, hereby incorporated by reference in its entirety. In frequency-division multiplexing approaches, the different light sources are pulsed at different frequencies so that the frequency of pulsing becomes the basis for demultiplexing the multiplexed detector output signal. That is, the detector output signal may be demodulated at each of the frequencies used to modulate the light sources so as to separate signal portions corresponding with each of the light sources. See, e.g., U.S. Pat. No. 4,800,885, hereby incorporated by reference in its entirety.
As will be appreciated, the detector output signal in pulse oximeters contains non-pulsatile and pulsatile components. The non-pulsatile component is influenced by the absorbency of tissue, venous blood, capillary blood, non-pulsatile arterial blood, the intensity of the light signals and the sensitivity of the detector. The pulsatile component reflects the expansion of the arteriolar bed with arterial blood. The varying amplitude of the pulsatile component depends upon the blood volume change per pulse and the oxygen saturation level of the blood. As such, the pulsatile component provides a basis for monitoring changes in the concentration of the noted blood analytes.
Given the relatively small contribution of the pulsatile component to the output signal of a detector in pulse oximeters, it has been recognized that the quality of analyte and oxygen saturation measurements can be significantly impacted by the presence of system noise. In this regard, any phenomena, whether mechanical, electrical or optical, that causes an artifact in the pulsatile component of a detector output signal can significantly compromise performance. Of primary interest here are artifacts that can arise due to rising/falling light amplitude levels associated with ambient light changes or due to electrical interference.
SUMMARY OF THE INVENTION
A broad objective of the present invention is to provide a spectrophotometric system having improved reliability.
More particularly, a primary objective of the present invention is to provide a photoplethysmographic method and apparatus yielding improved reliability through the reduction of system noise sensitivity. Relatedly, an objective of the present invention is to attenuate artifacts occasioned by rising/falling ambient light signal amplitudes, and by electrical interference.
The above objectives and additional advantages are realized in an inventive photoplethysmographic measurement apparatus that includes a plurality of light sources for emitting light signals at different corresponding wavelengths into a tissue under test and a detector for detecting at least a portion of the light signals transmitted through the tissue under test. Modulation means are included for modulating the light signals at corresponding different carrier frequencies and in accordance with a predetermined phase relationship therebetween. Correspondingly, demodulation means are included for demodulating a composite detection signal (e.g., a multiplexed signal corresponding with an output signal from the detector that indicates the intensity of the detected light signals), based upon the different carrier frequencies and in accordance with the predetermined phase relationship, to obtain signal portions corresponding with each of the light sources. In turn, such signal portions are employable to determine a blood analyte level in the tissue under test.
Of note, the inventive apparatus may include a synchronization means for synchronizing operation of the modulation means and demodulation means during each of one or more analyte measurement periods. In one arrangement, such synchronization means may comprise a master clock for providing clocking signals to the modulation means and demodulation means as embodied in a digital signal processor. As will be appreciated upon further consideration, enhanced measurement reliability may be realized via the maintenance of both a predetermined phase relationship between the modulated light signals and synchronization of the modulation and demodulation processes.
The noted modulation means may define a plurality of different periodic waveforms for modulating a corresponding plurality of light sources, and similarly the referenced demodulation means may define a common plurality of corresponding demodulation waveforms for demodulating the composite detection signal to obtain a pl
Hanna D. Alan
Norris Mark A.
Datex-Ohmeda Inc.
Kremer Matthew
Marsh & Fischmann & Breyfogle LLP
Winakur Eric F.
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