Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2002-02-22
2004-10-19
Jones, Mary Beth (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S324000
Reexamination Certificate
active
06805673
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to the noninvasive monitoring of low frequency heart rate and blood volume variability based on optical (visible and/or non-visible spectrum) signals and, in particular, to monitoring related parameters based on the processing of received optical signals. The invention can be readily implemented in connection with pulse oximetry instruments so as to expand the utility of such instruments.
BACKGROUND OF THE INVENTION
Photoplethysmography relates to the use of optical signals transmitted through or reflected by a patient's blood, e.g., arterial blood and/or perfused tissue, for monitoring a physiological parameter of the patient. Such monitoring is possible because the optical signal is modulated by interaction with the patient's blood. That is, interaction with the patient's blood, generally involving a wavelength and/or time dependent attenuation due to absorption, reflection and/or diffusion, imparts characteristics to the transmitted signal that can be analyzed to yield information regarding the physiological parameter of interest. Such monitoring of patients is highly desirable because it is noninvasive, typically yields substantially instantaneous and accurate results, and utilizes minimal medical resources, thereby proving to be cost effective.
A common type of photoplethysmographic instrument is the pulse oximeter. Pulse oximeters determine an oxygen saturation level of a patient's blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient's tissue. In particular, pulse oximeters generally include a probe for attaching to a patient's appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, through the patient's appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient's pulse cycle, information can be obtained regarding blood oxygen saturation.
The algorithms for determining blood oxygen saturation related values are normally implemented in a digital processing unit. Accordingly, one or more analog to digital (A/D) converters are generally interposed between the detector and the digital processing unit. Depending on the specific system architecture employed, a single multi-channel digital signal may be received by the digital processing unit or separate digital signals for each channel may be received. In the former case, the digital processing unit may be used to separate the received signal into separate channel components. Thus, in either case, the digital processing unit processes digital information representing each of the channels. Such information, whether in digital or another form, defines input photoplethysmographic signals or “pleths.”
These pleths generally contain two components. The first component is a low frequency or substantially invariant component in relation to the time increments considered for blood oxygen saturation calculations, sometimes termed the “DC component,” which generally corresponds to the attenuation related to the non-pulsatile volume of the perfused tissue and other matter that affects the transmitted plethysmographic signal. The second component, sometimes termed the “AC component,” generally corresponds to the change in attenuation due to the pulsation of the blood. In general, the AC component represents a varying waveform which corresponds in frequency to that of the heartbeat. In contrast, the DC component is a more steady baseline component, since the effective volume of the tissue under investigation varies little or at a low frequency if the variations caused by the pulsation of the heart are excluded from consideration.
Pulse oximeters typically provide as outputs blood oxygen saturation values and, sometimes, a heart rate and a graphical representation of a pulsatile waveform. The information for generating each of these outputs is generally obtained from the AC component of the pleth. In this regard, some pulse oximeters attempt to filter the DC component from the pleth, e.g., in order to provide a better digitized AC component waveform. Other pulse oximeters may measure and use the DC component, e.g., to normalize measured differential values obtained from the AC component or to provide measurements relevant to motion or other noise corrections. Generally, though, conventional pulse oximeters do not monitor variations in the DC component of a pleth or pleths to obtain physiological parameter information in addition to the outputs noted above.
SUMMARY OF THE INVENTION
The present invention is directed to using photoplethysmography to obtain physiological parameter information related to low frequency heart rate and blood volume variability. The invention thus provides important diagnostic or monitoring information noninvasively. Moreover, various aspects of the invention can be implemented using one or more channels and/or other components of a conventional pulse oximeter, thereby providing additional functionality to instruments that are widely available and trusted, as well as providing access to important information for treatment of patients on a cost-effective basis.
It has been recognized that low frequency heart rate and blood volume variability have important diagnostic significance and that such variability can be conveniently monitored through appropriate processing of pleth signals. In the latter regard, the pleth signal includes information regarding the patient's pulsatile waveform and can be processed to provide information regarding waveform variations. Spectral analysis of heart frequency indicates that such spectra characteristically include three peaks: a peak associated with respiration that typically has a frequency around 0.3 to 0.5 Hz, but may have a frequency of 1 Hz or greater in the case of infants; a peak typically in the 0.1 Hz range associated with the autonomic nervous system or vaso motor center, sometimes termed the “Mayer Wave”; and a very low frequency peak, e.g., less than 0.05 Hz, associated with temperature control. Regarding the second of these, the origin and nature of the Mayer Wave is not fully settled. For present purposes, the Mayer Wave relates to a low frequency variation in blood pressure, heart rate, and/or vaso constriction.
The Mayer Wave has particular significance for diagnostic and patient monitoring purposes. In particular, the amplitude and frequency of the Mayer Wave are seen to change in connection with hypertension, sudden cardiac death, ventricular tachycardia, coronary artery disease, myocardial infarction, heart failure, diabetes, and autonomic neuropathy and after heart transplantation. The present invention is based, in part, on the recognition that effects related to the Mayer Wave can be monitored based on analyzing a pleth to obtain physiological parameter information. In particular, it is expected that the Mayer Wave influences heart rate (and related parameters such as variations in blood pressure and blood volume) by direct influence on the vaso motor center. A pleth signal can be processed to monitor heart rate and variations therein, thus yielding diagnostic information related to the Mayer Wave. Alternatively or additionally, the pleth signal can be processed to monitor blood volume variations to obtain similar information related to the Mayer Wave.
A difficulty associated with obtaining physiological parameter information based on the Mayer Wave relates to distinguishing the effects associated with the Mayer Wave from effects associated with the above-noted respiration wave, particularly in view of the fact that each of these waves can occur within overlapping frequency ranges. There are a number of ways in which the Mayer Wave and t
Datex-Ohmeda Inc.
Jones Mary Beth
Kremer Matthew
Marsh & Fischmann & Breyfogle LLP
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