Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
2000-08-03
2002-06-18
Winakur, Eric F. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S336000
Reexamination Certificate
active
06408198
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to photoplethysmographic measurement systems, and, in particular, to a method and system for improving photoplethysmographic analyte measurements by preprocessing measurement data to compensate for measurement artifact such as by de-weighting motion-contaminated data.
BACKGROUND OF THE INVENTION
In the field of photoplethysmography, light pulses from different portions of the electromagnetic spectrum are used to noninvasively determine various blood analyte related values, such as blood oxygen saturation, in test subjects. Typically, photoplethysmographic measurement systems, such as pulse oximeters, include a probe for releasably attaching to the tip of a patient's appendage (e.g., a finger, earlobe or the nasal septum). The probe directs light signals into the appendage where the probe is attached. Some portion of the light is absorbed by the tissue and a remaining portion of the light passes through patient tissue. The intensity of light passing through the tissue is monitored by a sensor typically contained in the probe. The intensity related signals produced by the sensor are used to compute blood analyte related values.
During a medical examination, measurements such as blood oxygen saturation levels computed by the pulse oximeter can be distorted by various factors including movement of the appendage where the probe is attached. For example, the movement of the patient may affect source/detector alignment, ambient light levels, blood volume fluctuation or other factors, thus resulting in potential measurement errors. Some instrument designs have attempted to address such artifact through the use of hardware such as filters to screen the detector signals. However, such approaches have generally had a limited ability to distinguish artifact components of the signal from desired information, resulting in loss of information and/or admission of substantial artifact into the data used for calculations.
SUMMARY OF THE INVENTION
There is a particular need for a photoplethysmographic measurement instrument that is capable of accurately analyzing the quality of output signals produced by an instrument sensor so that appropriate steps may be taken to improve photoplethysmographic analyte related measurements computed by the instrument. In particular, there is a need for a system with improved ability to distinguish artifact from desired information and to preprocess the data based on such distinction so as to enhance overall instrument performance.
The present invention is directed to a system and corresponding method for use in a pulse oximeter to improve the way in which data output by a pulse oximeter sensor, such as data representing a pulse waveform, is preprocessed prior to the computation of blood oxygen saturation levels. The present system quantifies artifactual components (e.g., motion artifact) contained in a set of data aggregated over a certain time interval (e.g., one or more pulse cycles or portions thereof or simply a given predetermined time interval) and preprocesses the set of data such that the effect of artifactual components is reduced. The blood oxygen saturation levels may be computed based on multiple sets of preprocessed data so as to improve the accuracy of the pulse oximetry measurements.
In accordance with one aspect of the present invention, an apparatus for evaluating the reliability of output signals produced by a pulse oximeter sensor is provided. Electrical output signals produced by the sensor may be captured at a defined interval. Each captured set of signals may be analyzed independently to estimate a degree of artifactual component reflected in the set of output signals. Unreliable sets of output signals can be addressed in a variety of ways. In one embodiment, a weight metric may be generated which accurately reflects the degree of artifactual component in the output signals. The weight metric may then be used to relatively emphasize or de-emphasize each set of output signals in proportion to the degree of artifactual component estimated such that the sets of output signals used in the blood analyte computation are dominated by output signals with a relatively small artifactual component. In another embodiment, sets of output signals that are deemed to be unreliable, i.e., the degree of artifactual component associated therewith exceeds a certain threshold, may be excluded from the blood analyte computation.
In accordance with a related aspect of the present invention, motion affected data is identified based on a numerical or mathematical analysis of the data. The electrical output signals generated by the photodetector may be separated into red and infrared data by a hardware or software based demultiplexer. The reliability of output signals may be evaluated on the basis of data points derived from separated red and infrared pulse waveforms. The motion contained in output signals produced by the pulse oximetry sensor may be quantified based on spread of the data points, e.g., from a regression model such as a best-fit linear regression line. In one embodiment, the red and infrared pulse waveforms are used to acquire differential absorption values dA
x
and dA
y
respectively. Each pulse cycle is represented by a set of data points, where each data point represents a pair of corresponding differential absorption values dA
x
and dA
y
obtained at a specific point in time within each pulse cycle. A principal component analysis (“PCS”) may be performed on each set of data points to estimate an amount of motion affecting the data. First and second principal components derived from the PCA account for different amounts of variation among the set of data points. The first principal component may describe the cluster of data points disposed along a longitudinal axis and the second principal component may describe the amount of spread of the data points with respect to the longitudinal axis. An amount of motion associated with a set of data points may be approximated by multiplying the RMS (Root Mean Square) variation along the axis of the first principal component by the RMS variation along the axis of the second principal component.
According to another related aspect of the invention, a method is provided for distinguishing motion artifact from the plethysmographic signal. It has been recognized that the artifact due to motion can be distinguished from other data anomalies because motion affects tend to be relatively evenly distributed over multiple channels (a channel being the electronic or digital data received from any given emitter in a photoplethysmographic system). Accordingly, motion effects can be identified and quantified by mathematically accounting for a corresponding bias or tendency in the data. For example, if two channels of data are plotted against one another over a set of samples, motion effects tend to be reflected by a slant, e.g., an approximately 45-degree bias in the data spread since some common types of motion tend to induce similar amounts of artifact into each channel (actually into the dA's calculated from the channel data), while the plethysmographic signal will tend to spread the data at some other angle determined by blood analyte levels. It will be appreciated that such effects may be identified mathematically rather than graphically. In this regard, each data point generally includes a plethysmographic content and a motion content. The motion content of a data point may be represented by a shift of the data point at a 45-degree angle, the amount of shift indicating the degree of motion affecting the data point. Simultaneously, the data will also be spread along some other angle which is determined by blood analyte levels. The random spreading of points in two directions expands the area occupied by the points. Thus, motion artifact present in a data point may be quantified based on the area filled by the spreading of the points. Typically, a set of data points may be characterized by a parallelogram, the boundary of which encompasses a ma
Hanna D. Alan
Pologe Jonas A.
Datex-Ohmeda Inc.
Kremer Matthew
Marsh & Fischmann & Breyfogle LLP
Winakur Eric F.
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