Patent Application
20150313486
The present invention is directed to systems and methods for determining pulse transit time for a subject using a sensor to acquire an electrocardiogram (ECG) signal and a photodetector to obtain a photoplethysmographic (PPG) signal by continuously sensing light reflected off a region of exposed skin where a PPG signal can be detected by that photodetector.
The ability to capture physiological signals is highly desirable in the healthcare industry. One important physiological signal is pulse transit time (PTT). PTT correlates well with blood pressure and can provide healthcare professionals with vital information relating to blood velocity and blood vessel dilation over time. Localized PTT can be used to assess blood vessel blockage between two points and thus can be used as an indirect marker for assessing peripheral vascular disease.
What is disclosed is a system and method for determining a pulse transit time for a subject. In one embodiment, a handheld wireless cellular device is configured with a sensor to acquire an electrocardiogram (ECG) signal from the subject and a photodetector generating a time-series signal in response to continuously sensing a reflection of source light off a region of exposed skin of the subject where a photoplethysmographic (PPG) signal can be detected by that photodetector. The acquired ECG and PPG signals are received. Thereafter, a temporally overlapping segment of the PPG and ECG signals is then analyzed to obtain a pulse transit time between a reference point on the PPG signal and a reference point on the ECG signal. The pulse transit time can be used to assess pathologic conditions such as peripheral vascular disease. Advantages of the above-described system and method will become apparent from the following detailed description and accompanying drawings.
The foregoing and other features and advantages of the subject matter disclosed herein will be made apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:
What is disclosed is a system and method for determining a pulse transit time for a subject.
A “subject” refers to a living being. One example subject 100 is shown in
“Plethysmography” is the study of signals containing relative blood volume changes in vessels which are close to the skin surface.
A “photoplethysmographic signal”, or simply “PPG signal” is a signal obtained using a photodetector.
A “photodetector” is a device which can continuously measure light intensities and generates, as output, a time-series signal.
A “time-series signal” is a signal which contains the sum total of relative blood volume changes in the blood vessels close to the skin surface in the region of exposed skin being sensed. The time-series signal may be filtered with a cutoff frequency defined as a function of a frequency of the subject's cardiac pulse. Filtered time-series signal segments can be upsampled to a pre-selected sampling frequency to increase a total number of data points in order to enhance an accuracy of peak-to-peak pulse point detection. Upsampling may involve, for example, an interpolation technique using a cubic spline function and a pre-selected sampling frequency. The upsampled signals can be smoothed using any of a variety of smoothing techniques which are well established in the arts. In one embodiment, the time-series signal comprises a PPG signal. In another embodiment, the PPG signal is extracted from the time-series signal.
“Extracting a PPG signal” from the time-series signal can be effectuated by performing signal separation on the time-series signals. The obtained signal is converted to a zero-mean unit variance. One method for determining PTT is further disclosed in: “System And Method For Determining Arterial Pulse Wave Transit Time”, U.S. patent application Ser. No. 14/204,397, by Mestha et al. and in “Determining Arterial Pulse Transit Time From Time-Series Signals Obtained At Proximal And Distal Arterial Sites”, U.S. patent application Ser. No. 14/515,618, by Mestha et al., both of which are incorporated herein in their entirety by reference. A filtering step can be performed to improve peak detection accuracy. Additional corrections may be necessary based on estimating the average of the amplitudes obtained from previous peaks. The extracted PPG signal can be filtered using any of: an FFT-based phase preservation filter, a zero-phase digital filter, a linear time invariant (LTI) filter, a linear time varying (LTV) filter, a finite impulse response (FIR) filter, an infinite impulse response (IIR) filter, or a non-linear filter such as a median filter.
A “sensor”, as used herein, is a device which senses the electrical activity of a beating heart.
An “electrocardiogram (ECG) signal” (alternatively, EKG, from the Greek “kardia”, meaning heart) are well understood by cardiac specialists. For a further discussion of ECG/EKG signals, the reader is directed to the introductory text: “EKGs for the Nurse Practitioner and Physician Assistant”, Springer Publishing Co. (2013), ISBN-13: 978-0826199560.
A “reference point on the ECG signal” can be any characteristic point on the ECG signal of
A “reference point on the PPG signal” can be any characteristic point on the PPG signal. In various embodiments hereof, the reference point on the PPG signal is any of: a maximum or a minimum point on the PPG signal, an average point between a maximum and a minimum on the PPG signal, a maximum of a first derivative of the PPG signal, or a maximum of a second derivative of the PPG signal. A pulse transit time is determined between the reference point on the PPG signal and the reference point on the ECG signal.
“Pulse transit time (PTT)” is the time it takes an arterial pulse pressure wave to travel between two points. An arterial pulse pressure wave is generated when the left ventricle of the heart contracts and pushes a volume of blood out the ascending aorta into the systemic arteries. The repeated push of this blood volume generates a pulsating wave. PTT can be used to determine blood pressure, blood vessel dilation over time, blood vessel blockage, and blood flow velocity. Furthermore, PTT can be used as an indirect marker for assessing the occurrence of cardiac arrhythmia, cardiac stress, heart disease, and peripheral vascular disease. PTT can be determined by averaging an instantaneous PTT over a user defined number of cardiac cycles. In one embodiment, the instantaneous PTT is:
PTT=d/η
where ηHR is the frequency of the subject's cardiac pulse (in beats per minute) and d is the phase difference (in radians per second) between two reference points on the ECG and PPG signals.
A “phase difference” is obtained as follows:
dφ=φPPG(t)−φECG(t)
where φPPG is the phase of the PPG signal at time t and φECG and is the phase of the
ECG signal at time t. Methods for determining a phase of a signal at a given point in time are well understood. The reader is directed to “Handbook of Formulas and Tables for Signal Processing”, CRC Press, 1st Ed. (1998), ISBN-13: 978-0849385797, which is incorporated herein in its entirety by reference. In certain applications, phase unwrapping may be required for one or both of the φPPG(t) and φECG(t) signals.
“Receiving signals” is intended to be widely construed and includes: retrieving, capturing, acquiring, or otherwise obtaining signals for processing in accordance with the methods disclosed herein. Signals can be retrieved from a memory or storage device, retrieved from a media such as a CDROM or DVD, obtained from a remote device over a network, or downloaded from a web-based system or application which makes such signals available.
It should be appreciated that the steps of “determining”, “analyzing”, “obtaining”, “receiving”, “processing”, “detrending”, “filtering”, “performing” and the like, as used herein, include the application of various signal processing and mathematical operations applied to data and signals, according to any specific context or for any specific purpose. It should be appreciated that such steps may be facilitated or otherwise effectuated by a microprocessor executing machine readable program instructions retrieved from a memory or storage device.
Reference is now being made to the flow diagram of
At step 302, receive a time-series signal captured by at least one photodetector in response to sensing light intensities off a surface of a region of exposed skin of a subject where a photoplethysmographic (PPG) signal can be sensed.
At step 304, receive an electrocardiogram (ECG) signal captured by a sensor placed on the subject's body where the electrical activity of the subject's heart can be sensed.
At step 306, process a temporally overlapping segment of the PPG and ECG signals to obtain a pulse transit time (PTT) between a reference point on the PPG signal and a reference point on the ECG signal. A user may use the workstation or the smartphone of
At step 308, communicate the subject's PTT to a display device. One example display device is shown in conjunction with the workstation of
It should be appreciated that the flow diagrams depicted herein are illustrative. One or more of the operations may be performed in a differing order. Other operations may be added, modified, enhanced, or consolidated. Variations thereof are intended to fall within the scope of the appended claims.
Reference is now being made to
Smartphone 101 receives a time-series signal from a photodetector 402 and an ECG signal from a sensor 104 physically connected to the smartphone. In another embodiment, the sensor 104 is in communication with the smartphone 101 via a wireless protocol. In another embodiment, the smartphone itself is the ECG sensor and it obtains ECG signals by the smartphone being placed in physical contact with a surface of the subject's skin. Both the photodetector 102 and the sensor 104 may be configurable by a software application executed by a processor internal to the smartphone. Such an application displays an icon widget in the form of a button which, when pressed by a user touching the smartphone's touchscreen display, activates both the photodetector and the sensor to begin capture of the time-series signal 402 and the ECG signal 403, respectively. Touching the button to switch to the OFF position stops signal acquisition and initiates signal processing.
In the embodiment of
Signal Processing System 404 is shown having been placed in communication with a workstation, generally at 410. A computer case 411 houses various components such as a motherboard with a processor and memory, a network card, a video card, a hard drive capable of reading/writing to machine readable media 412 such as a floppy disk, optical disk, CD-ROM, DVD, magnetic tape, and the like, and other software and hardware needed to perform the functionality of a computer workstation. The workstation further includes a display device 413, such as a CRT, LCD, or touchscreen device, for displaying information and various user-selectable menu options, PPG signals, ECG signals, computed values, patient medical information, results, and the like. A user can view any of that information and make a selection from menu options displayed thereon or directly from the screen of the smartphone 102. Keyboard 414 and mouse 415 effectuate a user input or user selection. The workstation implements a database in storage device 416 wherein records are stored, manipulated, and retrieved in response to a query. Such records, in various embodiments, take the form of patient medical history (collectively at 417) stored in association with information identifying the patient. Although the database is shown as an external device, the database may be internal to the workstation mounted, for example, on a hard disk mounted internally to computer case 411.
The workstation 410 has an operating system and other specialized software configured to display alphanumeric values, menus, scroll bars, dials, slideable bars, pull-down options, selectable buttons, and the like, for entering, selecting, modifying, and accepting information needed for processing image frames, time-series signals, PPG signals, and ECG signals, in accordance with the teachings hereof. The workstation is further enabled to display the various signals for a user to review and to select references points on those signals. In other embodiments, a user or technician uses the user interface of the workstation (or the interface of the smartphone) to set parameters, view and analyze signals, and the like. User selections may be stored/retrieved in any of the storages devices 412 and 416. Default settings and initial parameter values can be stored/retrieved from any of the storage devices.
Although shown as a desktop computer, it should be appreciated that the workstation 410 can be a laptop, mainframe, or a special purpose computer such as an ASIC, circuit, or the like. The workstation of
It should be appreciated that some or all of the functionality performed by any of the modules or processing units of system 404 can be performed, in whole or in part, by the workstation 410 placed in communication with the smartphone 101 over network 418. It should also be understood that any of the functionality performed by the system 404 and/or the workstation 410 may be performed, in whole or in part, by the smartphone 101. The embodiment shown should not be viewed as limiting the scope of the appended claims strictly to that configuration. Various modules may designate one or more components which may, in turn, comprise software and/or hardware designed to perform the intended function.
The above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into other different systems or applications. The teachings hereof can be implemented in hardware or software using any known or later developed systems, structures, devices, and/or software by those skilled in the applicable art without undue experimentation from the functional description provided herein with a general knowledge of the relevant arts. One or more aspects of the methods described herein are intended to be incorporated in an article of manufacture which may be shipped, sold, leased, or otherwise provided separately either alone or as part of a product suite or a service. Presently unforeseen or unanticipated alternatives, modifications, variations, or improvements may become apparent and/or subsequently made by those skilled in this art which are also intended to be encompassed by the following claims. The teachings of any publications referenced herein are each hereby incorporated by reference in their entirety.
This patent application is a continuation-in-part of commonly owned and co-pending U.S. patent application Ser. No. 14/268,656, entitled: “Determining Arterial Pulse Wave Transit Time From VPG And ECG/EKG Signals”, by Mestha et al. which is incorporated herein in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14268656 | May 2014 | US |
| Child | 14596344 | US |
