Patent Application
20110245628
The present disclosure relates generally to non-invasive measurement of physiological parameters and, more particularly, using empirical mode decomposition to process physiological signals.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Pulse oximetry may be defined as a non-invasive technique that facilitates monitoring of a patient's blood flow characteristics. Specifically, these blood flow characteristic measurements may be acquired using a non-invasive sensor that passes light through a portion of a patient's tissue and photo-electrically senses the absorption and scattering of the light through the tissue. One or more physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered, which may be referred to as a plethysmograph waveform or a pulse oximetry signal, may then be used to estimate, for example, blood oxygen saturation of hemoglobin in a patient's arterial blood and/or the patient's heart rate.
However, typical algorithms used to calculate heart rate and/or blood oxygen saturation may not determine other physiological information which may be determinable from the plethysmograph waveform. In fact, as many physiological conditions may affect a patient's blood flow characteristics, the plethysmograph waveform may have signal characteristics which reflect various other physiological conditions. For example, in addition to oscillatory patterns corresponding to heart rate which may be found in the plethysmograph waveform, other oscillatory patterns which provide information on conditions such as respiratory rate, respiratory effort, heart arrhythmia, etc. may also be found in the plethysmograph waveform.
Certain aspects commensurate in scope with the originally disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the embodiments might take and that these aspects are not intended to limit the scope of the presently disclosed subject matter. Indeed, the embodiments may encompass a variety of aspects that may not be set forth below.
Present embodiments relate to systems, methods, and devices for decomposing a physiological signal of a patient using empirical mode decomposition (EMD). In one embodiment, the EMD algorithm may involve identifying a frequency component, referred to as an intrinsic mode function, in the physiological signal. The physiological signal may be decomposed into one or more intrinsic mode functions through multiple iterations of the EMD algorithm. Each subsequent mode function may have a different frequency component of the original physiological signal input into the EMD algorithm. Further, each mode function may be analyzed and/or processed to determine various physiological data corresponding to blood flow in the patient.
Advantages of the presently disclosed subject matter may become apparent upon reading the following detailed description and upon reference to the drawings in which:
One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
Present embodiments relate to systems and methods of processing physiological signals corresponding to blood flow in a patient. Specifically, empirical mode decomposition (“EMD”) techniques may be applied to a physiological signal of the patient to decompose the signal into one or more components. The components decomposed from a signal may be referred to as “intrinsic mode functions,” which may each include a different frequency component of the original signal. Thus, each intrinsic mode function decomposed from a physiological signal may correspond to a physiological condition of the patient, including, for example, a pulse rate, respiratory rate, respiratory effort, sympathetic nervous activity, or any other repetitive variation affecting the patient's blood flow characteristics.
EMD may decompose a physiological signal, such that frequency components of the physiological signal (i.e., the intrinsic mode functions) may be analyzed within the time domain. In particular, the intrinsic mode functions may be analyzed with respect to time, such that the scale and frequency content of each mode function may vary in time. Furthermore, in accordance with the present techniques, the decomposition of the physiological signal is based only on the signal itself, and not on any predetermined frequencies or basis functions. Thus, the intrinsic mode functions obtained from the physiological signal represent the original frequency and scale content of the physiological signal with respect to time.
Using EMD may be particularly useful for physiological signals which may include frequency variations attributable to any number of physiological causes (e.g., pulse variations, respiratory variations, etc.). Such variations in a physiological signal may occur at specific times or in specific intervals, and analyzing the variations in the time domain may enable the determination of different causes for the variations in the physiological signal. Thus, EMD techniques may provide further physiological information not available under other methods of signal processing which transform time-domain signals out of the time domain and into the frequency domain or wavelet domain. For example, time information may not be preserved when using Fourier transforms, and certain physiological information may not be attainable by using only Fourier transforms to analyze a physiological signal.
A physiological signal may include a plethysmographic waveform, a pulse oximetry signal, or any other signal corresponding to blood flow in a patient. Physiological information determined from a physiological signal may include any repetitive variation in the patient which affects blood flow characteristics of the patient. For example, physiological information may include a pulse beat, respiratory rate, respiratory effort, sympathetic nervous activity, etc. Physiological information may also include less predictable variations corresponding to a patient's blood flow, which may be used to indicate heart arrhythmia or other heart irregularities.
In one embodiment, a physiological signal such as a pulse oximetry signal may be obtained from a patient by using a pulse oximetry system.
A simplified block diagram of a pulse oximeter system 10 is illustrated in
In one embodiment, the detector 24 may be capable of detecting light at various intensities and wavelengths. In operation, light enters the detector 24 after propagating through the tissue of the patient 26. The detector 24 may convert the light at a given intensity, which may be directly related to the absorbance and/or reflectance of light in the tissue of the patient 26, into an electrical signal. That is, when more light at a certain wavelength is absorbed or reflected, less light of that wavelength is typically received from the tissue by the detector 24. After converting the received light to an electrical signal, the detector 24 may send the signal to the monitor 12, where physiological characteristics may be calculated based at least in part on the absorption of light in the tissue of the patient 26. In some embodiments, physiological characteristics may also be calculated based in part on the scattering of light in the tissue of the patient 26. Furthermore, physiological characteristics may be determined based on one or more signal characteristics (oscillatory patterns) of the signal. The electrical signal converted by the detector 24 may also be referred to as a physiological signal, and may be in the form of a plethysmogram or any other representation corresponding to the light received from the patient 26 at the detector 24.
The sensor 14 may also include an encoder 28, which may contain information about the sensor 14, such as what type of sensor it is (e.g., whether the sensor is intended for placement on a forehead or digit) and the wavelengths of light emitted by the emitter 22. This information may allow the monitor 12 to select appropriate algorithms and/or calibration coefficients or to derive a filter for estimating the patient's physiological characteristics. The encoder 28 may, for instance, be a memory on which one or more of the following information may be stored for communication to the monitor 102. In some embodiments, the data or signal from the encoder 28 may be decoded by a detector/decoder 30 in the monitor 12.
Signals from the detector 24 and the encoder 28 may be transmitted to the monitor 12. The monitor 12 may include one or more processors 32 coupled to an internal bus 34. Also connected to the bus may be a RAM memory 36, ROM memory 56, and a display 38. A time processing unit (TPU) 40 may provide timing control signals to light drive circuitry 42, which controls when the emitter 22 is activated, and if multiple light sources are used, the multiplexed timing for the different light sources. TPU 40 may also control the gating-in of signals from detector 24 through a switching circuit 44. These signals are sampled at the proper time, depending at least in part upon which of multiple light sources is activated, if multiple light sources are used. The received signal from the detector 24 may be passed through an amplifier 46, an analog filter 48, and an analog-to-digital (A/D) converter 50, and/or a digital filter 52 for amplifying, filtering, digitizing, and/or processing the electrical signals from the sensor 14. After amplifying, filtering, digitizing, and/or processing, the digital data may then be stored in a queued serial module (QSM) 54, for later downloading to RAM 36 as QSM 54 fills up. In an embodiment, there may be multiple parallel paths for separate amplifiers, filters, and A/D converters for multiple light wavelengths or spectra received.
In some embodiments, based at least in part upon the physiological signal corresponding to the light provided by the detector 24, the processor 32 may use various algorithms to determine physiological information. The processor 32 may also access memory (e.g., RAM 36 or ROM 56) to access stored algorithms. In one or more embodiments, the processor 32 may apply algorithms such as empirical mode decomposition (EMD) algorithms, to extract frequency components from the physiological signal. The frequency components, also referred to as intrinsic mode functions or mode functions, may be analyzed to determine physiological information including, for example, pulse beat, respiration rate, respiratory effort, sympathetic nervous activity, or any other repetitive variation in heart rhythm.
One embodiment of a process 70 for applying an empirical mode decomposition (EMD) algorithm to obtain intrinsic mode functions from a physiological signal is provided as a flow chart in
The process 70 may determine (block 74) the local maxima and minima of the input signal X(t) 72. The determination (block 74) of the local maxima and minima may be based on the type of intrinsic mode function to be extracted. For example, if an intrinsic mode function corresponding to a pulse rate is to be extracted from the physiological signal X(t) 72, the determination of the local maxima and minima may be designed to ignore artifacts substantially smaller than a typical or recent pulse amplitude. The physiological signal X(t) 72 may also include physiological signal characteristics which may not be useful in determining the pulse rate. For example, the dicrotic notch may not be a relevant signal characteristic for determining pulse rate. Thus, when the process 70 is extracting an intrinsic mode function corresponding to pulse rate, the determination (block 74) of the local maxima and minima of the physiological signal X(t) 72 may also be designed to ignore the dicrotic notch. Accounting for and ignoring artifacts and/or non-relevant physiological signal characteristics may be performed by the processor 32 using filters or any other suitable signal processing techniques. For embodiments involving multiple signals (e.g., multiple wavelength signals and/or signals from multiple detectors), timing information and clock cycles for the samples from each signal may be used to differentiate the multiple signals, such that the local maxima and minima of each of the multiple signals may be identified. As will be discussed, the process 70 may have more than one iteration using the output of the process 70 as a new input, and criteria for determining (block 74) the local maxima and minima may be modified at each subsequent iteration.
Furthermore, for embodiments using overlapping time windows, determining (block 74) the local maxima and minima of the physiological signal X(t) 72 may also involve using the maxima and minima information already determined in a previous time window. The previously determined maxima and minima may be compared with the new samples in the non-overlapping portion of the new window. Such a technique may save time in searching a previously analyzed window for local maxima and minima.
Once the local maxima and minima of the physiological signal X(t) 72 are identified, the process 70 may estimate (block 76) upper and lower envelopes based on the local maxima and minima. In one embodiment, upper and lower envelopes may be constructed by fitting cubic splines to the identified maxima and minima of the physiological signal X(t) 72. In estimating (block 76) the upper and lower envelopes, the process 70 may account for local maxima and minima not occurring at the beginning and/or end of the window of physiological signal X(t) 72. For example, estimating (block 76) the upper and lower envelopes may duplicate the nearest identified maxima and minima at the beginning and/or end of the data window. In estimating (block 76) the upper and lower envelopes, the process may also compensate for changes in the physiological signal X(t) 72 which may be due to non-physiological causes, such as adjustments of the internal gain of the pulse oximetry system 10, adjustments in the source intensity (e.g., from the emitter 22 and/or light drive 42 of the system 10), and/or periods of interruption in the physiological signal, such as during sensor 14 adjustments or during periods when the sensor 14 is disconnected. Furthermore, in embodiments involving multiple signals (e.g., multiple wavelength signals and/or signals from multiple detectors), timing information and clock cycles for the samples from each signal may be used, such that cubic splines may be fitted for the appropriate data values of each respective signal.
Once the upper and lower envelopes have been estimated (block 76), the process 70 may then calculate (block 78) the mean mk 80 of the upper and lower envelopes. By subtracting (block 82) the mean mk 80 of the upper and lower envelopes from the original physiological signal X(t) 72, the process 70 produces an intrinsic mode function hk 84. This relationship is represented in equation (1), below:
X(t)−mk=hk equation (1)
By definition, the intrinsic mode function hk 84 may have the same number of extrema (i.e., maxima and minima) as the physiological signal X(t) 72, and may represent an oscillatory mode of the physiological signal X(t) 72. As discussed, the physiological signal X(t) 72 may be a representation of blood flow in a patient 26, which may include one or more oscillatory patterns (e.g., oscillatory concentrations of blood cells, oscillatory ratios of oxygenated to deoxygenated hemoglobin, etc.) resulting from certain physiological conditions of the patient 26. The empirical mode decomposition process 70 may identify such repeating signal characteristics by decomposing the physiological signal X(t) 72 into intrinsic mode functions hk 84. As a intrinsic mode function hk 84 is decomposed from the original physiological signal X(t) 72 without leaving the time domain, the original scale of the intrinsic mode function hk 84 may be preserved in time. Thus, in some embodiments, the intrinsic mode function hk 84 may be further analyzed and/or processed with respect to time. Retaining time information may be valuable when analyzing physiological signals, as the timing of physiological causes may be important in identifying certain conditions of the patient 26.
In some embodiments, the process 70 may include iterative refinement of each intrinsic mode function hk 84, which may involve repeating the steps 74, 76, 78, and 82 until the process determines (block 86) that the intrinsic mode function hk 84 is refined. If the intrinsic mode function hk 84 is determined to be not sufficiently refined, the intrinsic mode function hk 84 may be subtracted from the input signal, and steps 74, 76, 78, and 82 may be performed on the residual of this subtraction. Iterations of this portion of the process 70 may be performed until the intrinsic mode function hk 84 is sufficiently refined.
Determining (block 86) whether the intrinsic mode function hk 84 is sufficiently refined may involve comparing a statistical measure of an intrinsic mode function hk 84 to a predetermined threshold and/or to a statistical measure of an intrinsic mode function hk 84 from a previous iteration (e.g., comparing statistical measures of h2 and h3). Such statistical measures may include calculating the kurtosis of an intrinsic mode function hk 84, which should asymptotically decrease as lower-frequency modes are decomposed from the signal. For example, a higher kurtosis may indicate that more of the variance of an intrinsic mode function hk 84 is a result of relatively infrequent and extreme deviations (which may be more indicative of noise or other non-physiological conditions), as opposed to a more frequent and less extreme deviation (which may be more indicative of a physiological characteristic). Some embodiments may also involve statistical measures such as quantifying the variability in the amplitude, maxima, or minima of the input signal. Furthermore, some embodiments may include determining the number of minima, maxima, zero crossings, or any other indication of the number of cycles expressed by an intrinsic mode function hk 84.
In some embodiments, the skewness of the derivative of an intrinsic mode function hk 84 may also be used to determine whether substantially all of the oscillatory content of the physiological signal X(t) 72 has been decomposed. For example, the skewness of the derivative of a mode should decrease as the physiological signal X(t) 72 waveform is refined and as mode estimates are decomposed from the signal X(t) 72. Once the skewness of a mode estimate does not decrease when compared to a previous mode estimate, then the intrinsic mode function hk 84 may be refined, as indicated by the refined intrinsic mode function hk 88. It should be noted that in some iterations of the process 70, the intrinsic mode function hk 84 may be determined (block 86) to be sufficiently refined. Thus, in some iterations of the process 70, the intrinsic mode function hk 84 may be the same as the refined intrinsic mode function hk 88, and the refined intrinsic mode function hk 88 may simply be referred to henceforth as the intrinsic mode function hk 88.
The process 70 may involve finding more than one intrinsic mode function hk 88, as a patient's 26 blood flow may be affected by more than one system (e.g., circulatory system and respiratory system), and the physiological signal X(t) 72 may include more than one oscillatory mode. For example, the first intrinsic mode function hk 88 found from the physiological signal X(t) 72 may be referred to as an intrinsic mode function h0. To find a subsequent intrinsic mode function h1, the intrinsic mode function h0 may be subtracted (block 90) from the original physiological signal X(t) 72, resulting in the residual rn+1 92, as represented in the equation below:
X(t)−hk=rn+1 equation (2)
The residual rn+1 92 may then be used as the input signal for each subsequent iteration (where the k of hk represents the iteration number) of the process 70, and the maxima and minima of the residual rn+1 92 may be identified (block 74). As discussed, the maxima and minima identification for each subsequent residual rn+1 92 may be modified according to typical characteristics of the physiological signal X(t) 72, the number of iterations k, the number of intrinsic mode functions hk 88 already calculated, and/or the type of intrinsic mode function hk 88 to be extracted from the physiological signal X(t) 72.
As the intrinsic mode function h0 has already been subtracted (block 90) from the physiological signal X(t) 72 to produce the residual rn+1 92, the remaining features in the residual may be less extreme than the features of the physiological signal X(t) 72. For example, the maxima identified in the residual rn+1 92 may be smaller than the previously identified maxima of the physiological signal X(t) 72, and the minima identified in the residual rn+1 92 may be larger than the previously identified minima of the physiological signal X(t) 72. Thus, subsequent iteration of the process 70 (iteration k=1) on the residual rn+1 92 may produce an intrinsic mode function h1 having a lower order frequency compared to the first intrinsic mode function h0. Each subsequent intrinsic mode function hk 88 may represent a progressively lower order frequency component of the physiological signal X(t) 72, and the sum of all identified intrinsic mode functions hk 88 of a physiological signal X(t) 72 may be approximately equal to the total oscillatory content of the physiological signal X(t) 72. In other words, each subsequent iteration of the EMD algorithm may produce an intrinsic mode function hk 88 having the next most distinguishing features of the physiological signal X(t) 72, and when the physiological signal X(t) 72 has been refined (i.e., decomposed), all of the distinguishing features may be extracted in the form of intrinsic mode functions hk 88.
In some embodiments, the process 70 may continue until substantially all of the oscillatory content of the physiological signal X(t) 72 is decomposed into intrinsic mode functions hk 88. For example, methods of determining whether substantially all of the oscillatory content in the physiological signal X(t) 72 waveform has been sufficiently decomposed may involve analyzing each residual rn+1 92 and determining whether the residual rn+1 92 is smaller than a predetermined value, or whether it is a monotonic function. If the residual rn+1 92 is smaller than a predetermined value and/or if the residual rn+1 92 is a monotonic function, then the process 70 may have identified substantially all the intrinsic mode functions hk 84 of the physiological signal X(t) 72.
In one embodiment, the number of iterations in the process 70 (and the number of intrinsic mode functions hk 88 extracted) may also be based on the type of physiological information to be determined from the intrinsic mode functions hk 88. For example, the process 70 may have substantially refined the physiological signal X(t) 72 once all relevant intrinsic mode functions hk 88 have been extracted. In some embodiments, the process 70 may still continue to provide further estimations of intrinsic mode functions hk 88 to account for changes in the signal detected by the detector 24 (e.g., signal interruptions, system 10 changes, etc.), and/or to provide more accurate estimates of the intrinsic mode functions hk 88.
Performing the process 70 on a physiological signal may decompose the signal into multiple intrinsic mode functions hk 88. In some embodiments, the multiple intrinsic mode functions hk 88 may each be further processed (block 94) to determine various physiological information, if any, indicated by each extracted mode function. In accordance with the present techniques, any suitable signal processing techniques may be combined with the EMD algorithm to further enhance the utilization of the intrinsic mode functions and/or aid in the determination of physiological parameters and indications. Signal processing may be performed on any extracted mode function, and may include comparisons of any mode function with a pre-decomposed physiological signal.
Signal processing may be performed by any suitable processor (e.g., microprocessor 32) in the system 10, and may include other elements in the system 10 (
In some embodiments, signal processing techniques (block 94) may also involve linear and/or non-linear filters which may be adjustable or adaptable based on one or more metrics, trends, patterns, and/or distributions of the inputs or outputs of the filters. Such filters may include, for example, Kalman filters, adaptive comb filters, adaptive noise cancellers, joint process filters, and lattice filters. Furthermore, a physiological signal and/or a mode function of the physiological signal may be normalized, resealed, and/or transformed in the frequency and/or wavelet domains. Various techniques may also be used for computing ratios or other combinations of the components (e.g. from multiple wavelengths or detectors) of the physiological signal or intrinsic mode functions extracted from the physiological signal. For example, such techniques may include linear regression, linear combination, multivariate analysis, principal component analysis (PCA), other suitable matrix techniques, or independent component analysis (ICA). Furthermore, signal processing techniques may include use of neural nets, fuzzy logic, genetic-based algorithms, or any other learning-based algorithms. Analysis of parallel or alternative estimates or algorithms, such as a Hidden Markov Model, may also be used.
Signal processing techniques (block 94) may include the combination of a physiological signal with additional sensors, including motion, pressure, temperature, or ultrasound sensors. The additional sensors may provide data to be used with the physiological signal which may aid in distinguishing physiological signals from artifacts or other non-physiological components. Furthermore, the empirical mode decomposition algorithm used herein may be used along with Hilbert Spectral Analysis in the Hilbert-Huang Transform, but is not limited to this combination of techniques.
Turning now to
The second mode function 108 illustrated in the graph 100 may represent another repetitive variation in heart rhythm. For example, the second mode function 108 may contain indications of arrhythmia, and could contain a waveform at approximately half the frequency of a pulse rate. Analyzing the waveform of the extracted mode function may also enable a health practitioner to determine clinical conditions, such as, for example, bi-Gemini, which may appear as alternating large and small pulses. Furthermore, in the absence of waveform characteristics indicative of heart rhythm, the second mode function 108 may also indicate the patient's 26 respiration, as respiratory related changes in intra-thoracic pressure may also impact the rate at which venous blood flows from peripheral to central venous circulation. The third mode function 110 may contain a waveform indicative of the patient's 26 respiration, if respiration is not already contained in a previous mode function. Alternatively, the third mode function 110 could reflect sympathetic nervous activity, such as Mayer waves.
The physiological information determined based on each mode function may not always follow a particular order, and may follow a different order from the examples given above. Further, not all extracted mode functions may provide physiological information. For example, in some situations, the decomposition of any of the mode functions may sometimes be affected by artifacts which may be mistaken for maxima and minima. Such motion artifacts may appear in any mode, depending on their frequency content and the relationship of their frequency to that of physiological signals. For example, high-frequency artifacts may appear in a first mode function. Such artifacts may be identified and/or reduced by using signal processing techniques as discussed with respect to
Furthermore, physiological parameters and indications may not be limited to the examples provided, and may include any physiological condition capable of affecting a patient's blood flow characteristics. For example, a physiological parameter or indication which may be determined using the present techniques may include arterial or venous oxygen saturation, pulse rate, continuous non-invasive blood pressure, pulse transit time, respiratory rate or effort, pulse amplitude, tissue perfusion, hypoxia, hyperoxia, bradycardia, tachycardia, arrhythmia, central or obstructive apnea, hypopnea, Cheyne-Stokes syndrome, hypovolemia, or sympathetic nervous activity (e.g., Mayer waves).
While the embodiments set forth in the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. The disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims.
