U.S. patent application number 13/619531 was filed with the patent office on 2014-03-20 for system and method for determining stability of cardiac output.
This patent application is currently assigned to Nellcor Puritan Bennett LLC. The applicant listed for this patent is Matt Clinton. Invention is credited to Matt Clinton.
Application Number | 20140081152 13/619531 |
Document ID | / |
Family ID | 50275182 |
Filed Date | 2014-03-20 |
United States Patent
Application |
20140081152 |
Kind Code |
A1 |
Clinton; Matt |
March 20, 2014 |
SYSTEM AND METHOD FOR DETERMINING STABILITY OF CARDIAC OUTPUT
Abstract
A PPG system for determining cardiac stability of a patient
includes a PPG sensor configured to be secured to an anatomical
portion of the patient, wherein the PPG sensor is configured to
sense a physiological characteristic of the patient. The PPG system
includes a monitor operatively connected to the PPG sensor. The
monitor receives a PPG signal from the PPG sensor. The monitor
includes a cardiac stability analysis module configured to
determine an amplitude variance of the PPG signal over a
predetermined time period and configured to determine a pulse
period variance of the PPG signal over the time period. The cardiac
stability analysis module is configured to determine cardiac
stability as a function of the amplitude variance and the pulse
period variance.
Inventors: |
Clinton; Matt; (Boulder,
CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Clinton; Matt |
Boulder |
CO |
US |
|
|
Assignee: |
Nellcor Puritan Bennett LLC
Boulder
CO
|
Family ID: |
50275182 |
Appl. No.: |
13/619531 |
Filed: |
September 14, 2012 |
Current U.S.
Class: |
600/479 |
Current CPC
Class: |
A61B 5/02028 20130101;
A61B 5/02438 20130101; A61B 5/742 20130101; A61B 7/04 20130101;
A61B 5/7235 20130101; A61B 8/02 20130101; A61B 5/746 20130101; A61B
5/0402 20130101; A61B 5/02416 20130101 |
Class at
Publication: |
600/479 |
International
Class: |
A61B 6/00 20060101
A61B006/00; A61B 5/021 20060101 A61B005/021; A61B 5/024 20060101
A61B005/024; A61B 5/02 20060101 A61B005/02 |
Claims
1. A PPG system configured to determine cardiac stability of a
patient, the PPG system comprising: a PPG sensor configured to be
secured to an anatomical portion of the patient, wherein the PPG
sensor is configured to sense a physiological characteristic of the
patient; and a monitor operatively connected to the PPG sensor, the
monitor receiving a PPG signal from the PPG sensor, the monitor
comprising a cardiac stability analysis module configured to
determine an amplitude variance of the PPG signal over a
predetermined time period and configured to determine a pulse
period variance of the PPG signal over the time period, the cardiac
stability analysis module configured to determine cardiac stability
as a function of the amplitude variance and the pulse period
variance.
2. The PPG system of claim 1, wherein the cardiac stability is
calculated as a function of the amplitude variance and as a
function of an inverse of the pulse period variance.
3. The PPG system of claim 1, wherein the cardiac stability
analysis module determines a cardiac stability ratio as the
amplitude variance over the pulse period variance, the cardiac
stability analysis module calculating the cardiac stability as a
product of the cardiac stability ratio and a scaling factor.
4. The PPG system of claim 1, wherein the cardiac stability
analysis module determines a cardiac stability ratio as a function
of the amplitude variance and the pulse period variance, a
numerator of the cardiac stability ratio is configured to decrease
as the cardiac stability of the patient decreases, a denominator of
the cardiac stability ratio is configured to increase as the
cardiac stability of the patient decreases, wherein the cardiac
stability ratio is configured to decrease as the cardiac stability
of the patient decreases.
5. The PPG system of claim 1, wherein the cardiac stability
analysis module determines the amplitude variance as an average of
the squared differences of each of the amplitudes from the mean
amplitude over the time period.
6. The PPG system of claim 1, wherein the cardiac stability
analysis module determines the pulse period variance as an average
of the squared differences of each of the pulse periods from the
mean pulse period over the time period.
7. The PPG system of claim 1, wherein the PPG signal forms a PPG
waveform, the cardiac stability analysis module analyzing a contour
of the PPG waveform along the primary peak to identify a minimum
amplitude and a maximum amplitude for each pulse, the cardiac
stability analysis module calculating the amplitude variance based
on the absolute amplitude of each pulse.
8. The PPG system of claim 1, wherein the monitor includes an
alarm, the monitor storing a threshold cardiac stability, the
monitor activating the alarm when the determined cardiac stability
crosses the threshold cardiac stability.
9. The PPG system of claim 1, wherein the monitor displays an
output of the cardiac stability on a display.
10. A method of determining cardiac stability of a patient from a
PPG system, the method comprising: securing a PPG sensor to an
anatomical portion of the patient; sensing a physiological
characteristic of the patient with the PPG sensor; receiving a PPG
signal from the sensor at a monitor having a cardiac stability
analysis module; analyzing an amplitude component of the PPG signal
at the cardiac stability analysis module to determine an amplitude
variance of the PPG signal over a predetermined time period;
analyzing a temporal component of the PPG signal at the cardiac
stability analysis module to determine a pulse period variance of
the PPG signal over the time period; and calculating cardiac
stability of the patient at the cardiac stability analysis module
based on the amplitude variance and the pulse period variance.
11. The method of claim 10, wherein the analyzing an amplitude
component operation comprises calculating an inverse variance of
the pulse period variance, the cardiac stability being a function
of the inverse variance of the pulse period.
12. The method of claim 10, wherein the calculating operation
comprises: determining a cardiac stability ratio as the amplitude
variance over the pulse period variance; and calculating the
cardiac stability as a product of the cardiac stability ratio and a
scaling factor.
13. The method of claim 10, wherein the calculating operation
comprises determining a cardiac stability ratio as a function of
the amplitude variance and the pulse period variance with a
numerator of the cardiac stability ratio decreasing as the cardiac
stability of the patient decreases and with a denominator of the
cardiac stability ratio increasing as the cardiac stability of the
patient decreases, wherein the cardiac stability ratio is
configured to decrease as the cardiac stability of the patient
decreases.
14. The method of claim 10, wherein the analyzing an amplitude
component operation comprises determining the amplitude variance as
an average of the squared differences of each of the amplitudes
from the mean amplitude over the time period.
15. The method of claim 10, wherein the analyzing a temporal
component operation comprises determining the pulse period variance
as an average of the squared differences of each of the pulse
periods from the mean pulse period over the time period.
16. A tangible and non-transitory computer readable medium that
includes one or more sets of instructions configured to direct a
computer to: receive a PPG signal from a sensor secured to an
anatomical portion of a patient over a predetermined time period;
determine an amplitude variance of the PPG signal over the time
period; determine a pulse period variance of the PPG signal over
the time period; and calculate cardiac stability of the patient
based on the amplitude variance and the pulse period variance.
17. The tangible and non-transitory computer readable medium of
claim 16, further configured to calculate an inverse variance of
the pulse period variance, the cardiac stability being a function
of the inverse variance of the pulse period.
18. The tangible and non-transitory computer readable medium of
claim 16, further configured to: determine a cardiac stability
ratio as the amplitude variance over the pulse period variance; and
calculate the cardiac stability as a product of the cardiac
stability ratio and a scaling factor.
19. The tangible and non-transitory computer readable medium of
claim 16, further configured to determine a cardiac stability ratio
as a function of the amplitude variance and the pulse period
variance with a numerator of the cardiac stability ratio decreasing
as the cardiac stability of the patient decreases and with a
denominator of the cardiac stability ratio increasing as the
cardiac stability of the patient decreases, wherein the cardiac
stability ratio is configured to decrease as the cardiac stability
of the patient decreases.
20. The tangible and non-transitory computer readable medium of
claim 16, further configured to: determine the amplitude variance
as an average of the squared differences of each of the amplitudes
from the mean amplitude over the time period; and determine the
pulse period variance as an average of the squared differences of
each of the pulse periods from the mean pulse period over the time
period.
Description
BACKGROUND
[0001] Embodiments of the present disclosure generally relate to
physiological signal processing and, more particularly, to
processing physiological signals to determine a cardiac stability
ratio of a patient.
[0002] In cardiovascular physiology, cardiac output (CO) and stroke
volume (SV) are important measurements of cardiac strength and
stability. Because SV and CO decreases in certain conditions and
disease states, SV and CO correlate with cardiac function. CO is
the volume of blood being pumped by the heart and is a product of
SV and the heart rate of the patient.
[0003] SV has traditionally been calculated using measurements of
ventricle volumes from an echocardiogram (ECG) and subtracting the
volume of the blood in the ventricle at the end of a beat
(end-systolic volume) from the volume of blood just prior to the
beat (called end-diastolic volume). Some systems and methods have
been used to measure stroke volume using a photoplethysmogram (PPG)
system in connection with ECG systems to aid in determining SV by
measuring pulse transit times from the pulse measurement by the ECG
system and the pulse measurement by the PPG system. The PPG system
performs a non-invasive, optical measurement that may be used to
detect changes in blood volume within tissue, such as skin, of an
individual.
SUMMARY
[0004] Certain embodiments provide a PPG system for determining
cardiac stability of a patient. The PPG system may include a PPG
sensor configured to be secured to an anatomical portion of the
patient. The PPG sensor is configured to sense a physiological
characteristic of the patient. The PPG system may include a monitor
operatively connected to the PPG sensor. The monitor may receive a
PPG signal from the PPG sensor. The monitor includes a cardiac
stability analysis module configured to determine an amplitude
variance of the PPG signal over a predetermined time period and
configured to determine a pulse period variance of the PPG signal
over the time period. The cardiac stability analysis module is
configured to determine cardiac stability as a function of the
amplitude variance and the pulse period variance.
[0005] Optionally, the cardiac stability may be calculated as a
function of the pulse period variance and as a function of an
inverse of the amplitude variance. The cardiac stability analysis
module may determine a cardiac stability ratio as an inverse
variance of the amplitude over the pulse period variance. The
cardiac stability analysis module may calculate the cardiac
stability as a product of the cardiac stability ratio and a scaling
factor. Optionally, a numerator of the cardiac stability ratio may
be configured to decrease as the cardiac stability of the patient
decreases and the denominator of the cardiac stability ratio may be
configured to increase as the cardiac stability of the patient
decreases. The cardiac stability ratio may be configured to
decrease as the cardiac stability of the patient decreases.
[0006] Optionally, the cardiac stability analysis module may
determine the amplitude variance as an average of the squared
differences of each of the amplitudes from the mean amplitude over
the time period. The cardiac stability analysis module may
determine the pulse period variance as an average of the squared
differences of each of the pulse periods from the mean pulse period
over the time period.
[0007] The PPG signal may form a PPG waveform. The cardiac
stability analysis module may analyze a contour of the PPG waveform
along the primary peak to identify a minimum amplitude and a
maximum amplitude for each pulse. The cardiac stability analysis
module may calculate the amplitude variance based on the absolute
amplitude of each pulse.
[0008] Optionally, the monitor may include an alarm. The monitor
may store a threshold cardiac stability. The monitor may activate
the alarm when the determined cardiac stability crosses the
threshold cardiac stability. The monitor may display an output of
the cardiac stability on a display.
[0009] Certain embodiments provide a method of determining cardiac
stability of a patient from a PPG system. The method includes
securing a PPG sensor to an anatomical portion of the patient and
sensing a physiological characteristic of the patient with the PPG
sensor. The method includes receiving a PPG signal from the sensor
at a monitor that includes a cardiac stability analysis module. The
method includes analyzing an amplitude component of the PPG signal
at the cardiac stability analysis module to determine an amplitude
variance of the PPG signal over a predetermined time period. The
method includes analyzing a temporal component of the PPG signal at
the cardiac stability analysis module to determine a pulse period
variance of the PPG signal over the time period. The method
includes calculating cardiac stability of the patient at the
cardiac stability analysis module based on the amplitude variance
and the pulse period variance.
[0010] Certain embodiments provide a tangible and non-transitory
computer readable medium that includes one or more sets of
instructions configured to direct a computer to receive a PPG
signal from a sensor secured to an anatomical portion of a patient
over a predetermined time period, determine an amplitude variance
of the PPG signal over the time period, determine a pulse period
variance of the PPG signal over the time period, and calculate
cardiac stability of the patient based on the amplitude variance
and the pulse period variance.
[0011] Embodiments of the present disclosure allow for quick and
simple determination of cardiac stability through analysis of a PPG
signal. In contrast to previous systems and methods, embodiments
may not require an ECG system to determine cardiac stability. The
PPG signal may be obtained from a single pleth-only system. The
cardiac stability may be determined quickly, frequently,
inexpensively, with little power, and with high sensitivity.
[0012] Certain embodiments may include some, all, or none of the
above advantages. One or more other technical advantages may be
readily apparent to those skilled in the art from the figures,
descriptions, and claims included herein. Moreover, while specific
advantages have been enumerated above, various embodiments may
include all, some, or none of the enumerated advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A illustrates a simplified block diagram of a system
configured to determine a physiological parameter of a patient,
according to an embodiment.
[0014] FIG. 1B illustrates an electrocardiogram (ECG) waveform of
the patient, according to an embodiment.
[0015] FIG. 1C illustrates a phonocardiogram (PCG) waveform of the
patient, according to an embodiment.
[0016] FIG. 1D illustrates a photoplethysmogram (PPG) waveform of
the patient, according to an embodiment.
[0017] FIG. 2 illustrates an isometric view of a PPG system,
according to an embodiment.
[0018] FIG. 3 illustrates a simplified block diagram of a PPG
system, according to an embodiment.
[0019] FIG. 4 is an illustrative processing system in accordance
with an embodiment.
[0020] FIG. 5 illustrates a pulse waveform of a PPG signal,
according to an embodiment.
[0021] FIG. 6 illustrates a pulse waveform of a PPG signal,
according to an embodiment.
[0022] FIG. 7 illustrates a flow chart of a method of determining
cardiac stability of a patient, according to an embodiment.
[0023] FIG. 8 illustrates a flow chart of a method of operating a
PPG system, according to an embodiment.
DETAILED DESCRIPTION
[0024] FIG. 1A illustrates a simplified block diagram of a system
100 configured to determine a physiological parameter of a patient
102. The system 100 is configured to acquire physiological signals
(or biosignals) from the patient 102 and analyze the physiological
signals to determine cardiac stability of the patient. The cardiac
stability may be based on a physiological parameter determined by
the system 100, such as a heart rate (HR), a blood pressure (BP), a
stroke volume (SV) and/or a cardiac output (CO) of the patient 102.
The physiological signals are indicative of phenomena occurring in
the patient. For example, the physiological signals may describe
cardiac function relating to cardiac strength and patient safety.
The system 100 may provide a sensitive indicator of cardiac
stability and may relate to hypovolemia or other conditions. The
physiological signals may be electrical, optical, and/or acoustical
signals.
[0025] The system 100 may include a sensor 104 that is configured
to detect one or more types of physiological signals. By way of
example, the system 100 may include an electrocardiogram (ECG)
system that detects electrical signals corresponding to muscle
excitation of the heart. In such cases, the sensor 104 may include
a plurality of electrodes that are coupled to different anatomical
locations of the patient 102 (e.g., chest, wrists, and/or ankles).
FIG. 1B illustrates, according to an embodiment, a representative
ECG waveform 110A based on the ECG signals acquired by the
electrode-sensor 104.
[0026] As another example, the system 100 may include a
phonocardiogram (PCG) system that detects sounds that may be caused
by the closing of heart valves. In such cases, the sensor 104 may
include one or more microphones that are coupled to the patient
102. FIG. 1C illustrates, according to an embodiment, a
representative PCG waveform 110B based on the PCG signals acquired
by the microphone-sensor 104. In alternative embodiments, the
system 100 includes an ultrasound system configured to detect heart
beats from the patient 102.
[0027] In certain embodiments, the system 100 includes a
photoplethysmogram (PPG) system, which can measure changes in blood
volume through an anatomical portion or location (e.g., a finger).
A typical example of a PPG system is a pulse oximetry system
although other PPG systems exist and may be used with embodiments
described herein. The PPG-sensor 104 may include a probe having one
or more light sources and one or more light detectors that are
coupled to the patient 102. The light source(s) provide an incident
light that is scattered, absorbed, reflected, and/or transmitted by
the blood. The light detector(s) detect an amount of light that may
correspond to blood volume. For example, as the volume of blood
increases at the anatomical location, the light is attenuated more
and, as such, a smaller amount of light is detected. FIG. 1C
illustrates, according to an embodiment, a representative PPG
waveform 110C based on the PPG signals acquired by the PPG-sensor
104.
[0028] As shown in FIG. 1A, the system 100 may include a monitor
(or computing system) 106 that includes one or more components for
analyzing and/or processing the physiological signals. For example,
the monitor 106 may include a pre-processing module 112, a
validation module 113, a rate-determining module 114, an analysis
module 115, and a graphical user interface (GUI) module 116. As
used herein, a "module" may include hardware components (e.g.,
processor, controller), software components, or a combination
thereof including any associated circuitry.
[0029] The pre-processing module 112 is configured to remove
unwanted signal data (e.g., noise) from raw physiological signal
data obtained from the individual 102. For example, raw PPG signals
may include artifacts caused by motion of the patient relative to
the light detector, instrumentation bias (e.g., bias by amplifiers
used in the PPG system), powerline interference, low amplitude PPG
signals, etc. Raw physiological signals from other types of
monitoring systems, such as ECG and PCG systems, may also include
unwanted noise. The pre-processing module 112 is configured to
remove the noise to provide clearer and/or cleaner physiological
signals to the other components of the system 100.
[0030] The validation module 113 is configured to analyze the
physiological signals to identify valid heart beats and waveforms
from the physiological signals. In some embodiments, the validation
module 113 is part of the pre-processing module 112 or another
module. The validation module 113 may analyze the physiological
signals after the physiological signals have been processed. In
some embodiments, the validation module 113 examines the
physiological signals to identify one or more reference features in
the physiological signals. For instance, a series of data points
over time may provide waveforms, such as the waveforms 110A-110C. A
reference feature may be an identifiable point, segment, or
characteristic of the waveform (e.g., peak, trough (or foot),
notch, amplitude, width, area, slope of a designated segment,
threshold, etc.) that may be relied upon in analysis of the
physiological signals. In many cases, a reference feature of a
waveform corresponds to a known physiological activity (e.g.,
excitation of heart muscles, closure or opening of valves, maximum
volume of blood at an anatomical location, etc.). The validation
module 113 may examine the data points, or a select number of data
points (e.g., a segment of the waveform), to confirm that the data
points are caused by a designated event of a cardiac cycle and are
not a result of noise or other unwanted event, such as when the
sensor 104 is being adjusted. The data points associated with valid
heart beats may then be used by a rate-determining module 114 to
determine a heart rate signal. In some embodiments, the data points
that are not identified as corresponding to heart beats may not be
considered in subsequent analysis.
[0031] The rate-determining module 114 is configured to analyze the
heart beats or, more specifically, the data points corresponding to
the valid heart beats identified by the validation module 113 and
determine a heart rate (HR) of the individual at a designated
moment of time. For example, the HR may be calculated by analyzing
time intervals or pulse periods between two or more heart beats or
by analyzing portions of a waveform that correspond to a single
heart beat. By way of example only, when analyzing the
physiological signals, the rate-determining module 114 may identify
one or more reference features (e.g., points, segments, and/or
characteristics that correspond to a waveform) that may be used to
calculate HR. For example, in the ECG waveform 110A, the
rate-determining module 114 may identify an R-wave peak 118 in each
heart beat. A pulse period 120 between the R-wave peaks 118A, 118B
may be determined and divided by a unit of time to calculate the
HR. For example, if the time interval is 0.90 seconds between the
two R-wave peaks 118A, 118B, then the HR is 67 beats/minute. The
system may calculate a variance of each heart beat or a series of
heart beats to determine one or more physiological parameters of
the patient. For example, a variance of one or more pulse periods
120 may be analyzed.
[0032] Corresponding to each heart beat, the PPG waveform 110C may
include a systolic peak 122, a diastolic peak 124, and a dichrotic
notch 126 that exists therebetween. In some cases, the diastolic
peak is not a peak but instead a change in slope. To determine HR,
the rate-determining module 114 may identify for each heart beat a
reference point that exists at a foot 128 of the wave before the
systolic peak 122. The HR may be determined in a similar manner as
described above with respect to the ECG waveform by identifying a
time interval or pulse period 129 between the foot 128A and the
foot 128B. The system may calculate a variance of each heart beat
or a series of heart beats to determine one or more physiological
parameters of the patient. For example, a variance of one or more
pulse periods 129 may be analyzed.
[0033] However, it should be noted that the above description is
just exemplary and that many reference points and/or waveform
segments may be analyzed and used in calculating a HR or other
physiological parameter of an individual. Furthermore, the
physiological signals may be processed in various manners to
determine a HR. For example, a first derivative or second
derivative of the PPG waveform may be used to locate certain
reference data points in the PPG waveform. Such reference data
points may be used for determining the pulse period, heart rate or
for determining other physiological parameters.
[0034] As will be described in greater detail below, the analysis
module 115 is configured to identify data points from the
physiological signals. The signal data points may be a limited
number of data points from a series of data points. For example,
the signal data points may correspond to a peak data point, a
trough data point, an amplitude of the waveform or a portion of the
waveform, a change in amplitude, a variance in amplitude compared
to other pulses, a segment of data points that correspond to a
slope of the waveform, and the like. To calculate a physiological
parameter, such as a cardiac stability ratio, the analysis module
115 may use one or more of the data points to calculate the
physiological parameter.
[0035] The system 100 may also include a user interface 130 that
includes a display 132. The user interface 130 may include
hardware, firmware, software, or a combination thereof that enables
a user to directly or indirectly control operation of the system
100 and the various components thereof. The display 132 is
configured to display one or more images, such as one or more of
the waveforms 110A-110C. The display 132 may also be configured to
show a representation of the physiological parameter, for example,
a number representing cardiac stability of the patient.
[0036] In some embodiments, the user interface 130 may also include
one or more input devices (not shown), such as a physical keyboard,
mouse, touchpad, and/or touch-sensitive display. The user interface
130 may be operatively connected to the GUI module 116 and receive
instructions from the GUI module 116 to display designated images
on the display 132. The user interface 130 may also include a
printer or other device for providing (e.g. printing) a report. The
user interface 130 may also include an alarm or alert system.
[0037] There are many medical conditions in which cardiac stability
is relevant. For example, cardiac stability may be an indicator of
blood loss. Hypovolemia may be identified by analyzing cardiac
stability. Hypovolemia may be identified more quickly using the
system 100, such as by providing a representation of cardiac
stability on the display 132.
[0038] Detecting a change in the cardiac stability or other
physiological parameter, such as SV, CO, HR and/or BP, can alert
medical providers to potentially dangerous patient conditions.
Analyzing and/or processing the physiological signals to provide a
representation of cardiac stability on a display for a care
provider may be more meaningful than merely monitoring BP and HR
readings. A monitoring system that tracks and provides information
relating to cardiac stability for a care provider and indicates a
patient status in response to a cardiac stability ratio provides a
tool in patient diagnosis and treatment. The present disclosure
relates to systems and methods for determining a cardiac stability
ratio, and more particularly, relates to analyzing a trending
nature of a PPG waveform to determine cardiac stability to alert a
care provider to a patient condition. For example, the present
disclosure relates to systems and methods that analyze a cardiac
stability ratio of amplitude variance over time variance to
determine if a heart function is steady or unsteady.
[0039] FIG. 2 illustrates an isometric view of a PPG system 210,
according to an embodiment. While the system 210 is shown and
described as a PPG system 210, the system may be various other
types of physiological detection systems, such as an
electrocardiogram system, a phonocardiogram system, and the like.
The PPG system 210 may be used as part of the system 100 (shown in
FIG. 1). The PPG system 210 may be a pulse oximetry system, for
example. The system 210 may include a PPG sensor 212 and a PPG
monitor 214. The PPG sensor 212 may include an emitter 216
configured to emit light into tissue of a patient. For example, the
emitter 216 may be configured to emit light at two or more
wavelengths into the tissue of the patient. The PPG sensor 212 may
also include a detector 218 that is configured to detect the
emitted light from the emitter 216 that emanates from the tissue
after passing through the tissue.
[0040] The system 210 may include a plurality of sensors forming a
sensor array in place of the PPG sensor 212. Each of the sensors of
the sensor array may be a complementary metal oxide semiconductor
(CMOS) sensor, for example. Alternatively, each sensor of the array
may be a charged coupled device (CCD) sensor. In another
embodiment, the sensor array may include a combination of CMOS and
CCD sensors. The CCD sensor may include a photoactive region and a
transmission region configured to receive and transmit, while the
CMOS sensor may include an integrated circuit having an array of
pixel sensors. Each pixel may include a photodetector and an active
amplifier.
[0041] The emitter 216 and the detector 218 may be configured to be
located at opposite sides of a digit, such as a finger or toe, in
which case the light that is emanating from the tissue passes
completely through the digit. The emitter 216 and the detector 218
may be arranged so that light from the emitter 216 penetrates the
tissue and is reflected by the tissue into the detector 218, such
as a sensor designed to obtain pulse oximetry data.
[0042] The sensor 212 or sensor array may be operatively connected
to and draw power from the monitor 214. Optionally, the sensor 212
may be wirelessly connected to the monitor 214 and include a
battery or similar power supply (not shown). The monitor 214 may be
configured to calculate physiological parameters based at least in
part on data received from the sensor 212 relating to light
emission and detection. Alternatively, the calculations may be
performed by and within the sensor 212 and the result of the
oximetry reading may be passed to the monitor 214. Additionally,
the monitor 214 may include a display 220 configured to display the
physiological parameters or other information about the system 210
and the patient. The monitor 214 may also include a speaker 222
configured to provide an audible sound that may be used in various
other embodiments, such as for example, sounding an audible alarm
in the event that physiological parameters are outside a predefined
normal range.
[0043] The sensor 212, or the sensor array, may be communicatively
coupled to the monitor 214 via a cable 224. Alternatively, a
wireless transmission device (not shown) or the like may be used
instead of, or in addition to, the cable 224.
[0044] The system 210 may also include a multi-parameter
workstation 226 operatively connected to the monitor 214. The
workstation 226 may be or include a computing sub-system 230, such
as standard computer hardware. The computing sub-system 230 may
include one or more modules and control units, such as processing
devices that may include one or more microprocessors,
microcontrollers, integrated circuits, memory, such as read-only
and/or random access memory, and the like. The workstation 226 may
include a display 228, such as a cathode ray tube display, a flat
panel display, such as a liquid crystal display (LCD),
light-emitting diode (LED) display, a plasma display, or any other
type of monitor. The computing sub-system 230 of the workstation
226 may be configured to calculate physiological parameters and to
show information from the monitor 214 and from other medical
monitoring devices or systems (not shown) on the display 228. For
example, the workstation 226 may be configured to display cardiac
stability of the patient, SV information, CO information, an
estimate of a patient's blood oxygen saturation generated by the
monitor 214 (referred to as an SpO2 measurement), pulse rate
information from the monitor 214 and blood pressure from a blood
pressure monitor (not shown) on the display 228.
[0045] The monitor 214 may be communicatively coupled to the
workstation 226 via a cable 232 and/or 234 that is coupled to a
sensor input port or a digital communications port, respectively
and/or may communicate wirelessly with the workstation 226.
Alternatively, the monitor 214 and the workstation 226 may be
integrated as part of a common device. Additionally, the monitor
214 and/or workstation 226 may be coupled to a network to enable
the sharing of information with servers or other workstations. The
monitor 214 may be powered by a battery or by a conventional power
source such as a wall outlet.
[0046] FIG. 3 illustrates a simplified block diagram of the PPG
system 210, according to an embodiment. When the PPG system 210 is
a pulse oximetry system, the emitter 216 may be configured to emit
at least two wavelengths of light (for example, red and infrared)
into tissue 240 of a patient. Accordingly, the emitter 216 may
include a red light-emitting light source such as a red
light-emitting diode (LED) 244 and an infrared light-emitting light
source such as an infrared LED 246 for emitting light into the
tissue 240 at the wavelengths used to calculate the patient's
physiological parameters. For example, the red wavelength may be
between about 600 nm and about 700 nm, and the infrared wavelength
may be between about 800 nm and about 1000 nm. In embodiments where
a sensor array is used in place of single sensor, each sensor may
be configured to emit a single wavelength. For example, a first
sensor may emit a red light while a second sensor may emit an
infrared light.
[0047] As discussed above, the PPG system 210 is described in terms
of a pulse oximetry system. However, the PPG system 210 may be
various other types of systems. For example, the PPG system 210 may
be configured to emit more or less than two wavelengths of light
into the tissue 240 of the patient. Further, the PPG system 210 may
be configured to emit wavelengths of light other than red and
infrared into the tissue 240. As used herein, the term "light" may
refer to energy produced by radiative sources and may include one
or more of ultrasound, radio, microwave, millimeter wave, infrared,
visible, ultraviolet, gamma ray or X-ray electromagnetic radiation.
The light may also include any wavelength within the radio,
microwave, infrared, visible, ultraviolet, or X-ray spectra, and
that any suitable wavelength of electromagnetic radiation may be
used with the system 210. The detector 218 may be configured to be
specifically sensitive to the chosen targeted energy spectrum of
the emitter 216.
[0048] The detector 218 may be configured to detect the intensity
of light at the red and infrared wavelengths. Alternatively, each
sensor in the array may be configured to detect an intensity of a
single wavelength. In operation, light may enter the detector 218
after passing through the tissue 240. The detector 218 may convert
the intensity of the received light into an electrical signal. The
light intensity may be directly related to the absorbance and/or
reflectance of light in the tissue 240. For example, when more
light at a certain wavelength is absorbed or reflected, less light
of that wavelength is received from the tissue by the detector 218.
After converting the received light to an electrical signal, the
detector 218 may send the signal to the monitor 214, which
calculates physiological parameters based on the absorption of the
red and infrared wavelengths in the tissue 240.
[0049] In an embodiment, an encoder 242 may store information about
the sensor 212, such as sensor type (for example, whether the
sensor is intended for placement on a forehead or digit) and the
wavelengths of light emitted by the emitter 216. The stored
information may be used by the monitor 214 to select appropriate
algorithms, lookup tables and/or calibration coefficients stored in
the monitor 214 for calculating physiological parameters of a
patient. The encoder 242 may store or otherwise contain information
specific to a patient, such as, for example, the patient's age,
weight, and diagnosis. The information may allow the monitor 214 to
determine, for example, patient-specific threshold ranges related
to the patient's physiological parameter measurements, and to
enable or disable additional physiological parameter algorithms.
The encoder 242 may, for instance, be a coded resistor that stores
values corresponding to the type of sensor 212 or the types of each
sensor in the sensor array, the wavelengths of light emitted by
emitter 216 on each sensor of the sensor array, and/or the
patient's characteristics. Optionally, the encoder 242 may include
a memory in which one or more of the following may be stored for
communication to the monitor 214: the type of the sensor 212, the
wavelengths of light emitted by emitter 216, the particular
wavelength each sensor in the sensor array is monitoring, a signal
threshold for each sensor in the sensor array, any other suitable
information, or any combination thereof.
[0050] Signals from the detector 218 and the encoder 242 may be
transmitted to the monitor 214. The monitor 214 may include a
general-purpose control unit, such as a microprocessor 248
connected to an internal bus 250. The microprocessor 248 may be
configured to execute software, which may include an operating
system and one or more applications, as part of performing the
functions described herein. A read-only memory (ROM) 252, a random
access memory (RAM) 254, user inputs 256, the display 220, and the
speaker 222 may also be operatively connected to the bus 250. The
control unit and/or the microprocessor 248 may include a cardiac
stability analysis module 249 that is configured to determine a
trending nature, index or value of the PPG signals or waveform to
determine cardiac stability of the patient. In an embodiment, the
cardiac stability analysis module 249 analyzes the PPG signal to
determine a variance in amplitude of the PPG signal of one or more
pulses over a time period as a basis for determining the cardiac
stability of the patient. The cardiac stability analysis module 249
is configured to determine a cardiac stability ratio of the
amplitude variance with respect to a pulse period variance based on
calculations, measurements or other information, data or signals
received from the PPG sensor 212 or other components of the system
200.
[0051] The RAM 254 and the ROM 252 are illustrated by way of
example, and not limitation. Any suitable computer-readable media
may be used in the system for data storage. Computer-readable media
are configured to store information that may be interpreted by the
microprocessor 248. The information may be data or may take the
form of computer-executable instructions, such as software
applications, that cause the microprocessor to perform certain
functions and/or computer-implemented methods. The
computer-readable media may include computer storage media and
communication media. The computer storage media may include
volatile and non-volatile media, removable and non-removable media
implemented in any method or technology for storage of information
such as computer-readable instructions, data structures, program
modules or other data. The computer storage media may include, but
are not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other
solid state memory technology, CD-ROM, DVD, or other optical
storage, magnetic cassettes, magnetic tape, magnetic disk storage
or other magnetic storage devices, or any other medium which may be
used to store desired information and that may be accessed by
components of the system.
[0052] The monitor 214 may also include a time processing unit
(TPU) 258 configured to provide timing control signals to a light
drive circuitry 260, which may control when the emitter 216 is
illuminated and multiplexed timing for the red LED 244 and the
infrared LED 246. The TPU 258 may also control the gating-in of
signals from the detector 218 through an amplifier 262 and a
switching circuit 264. The signals are sampled at the proper time,
depending upon which light source is illuminated. The received
signal from the detector 218 may be passed through an amplifier
266, a low pass filter 268, and an analog-to-digital converter 270.
The digital data may then be stored in a queued serial module (QSM)
272 (or buffer) for later downloading to RAM 254 as QSM 272 fills
up. In an embodiment, there may be multiple separate parallel paths
having amplifier 266, filter 268, and A/D converter 270 for
multiple light wavelengths or spectra received.
[0053] The microprocessor 248 may be configured to determine the
patient's physiological parameters, such as cardiac stability,
amplitude, variance in amplitude, pulse period, variance in pulse
period, SV, CO, SpO2, pulse rate, and the like, using various
algorithms and/or look-up tables based on the value(s) of the
received signals and/or data corresponding to the light received by
the detector 218. The signals corresponding to information about a
patient, and regarding the intensity of light emanating from the
tissue 240 over time, may be transmitted from the encoder 242 to a
decoder 274. The transmitted signals may include, for example,
encoded information relating to patient characteristics. The
decoder 274 may translate the signals to enable the microprocessor
248 to determine the thresholds based on algorithms or look-up
tables stored in the ROM 252. The user inputs 256 may be used to
enter information about the patient, such as age, weight, height,
diagnosis, medications, treatments, and so forth. The display 220
may show a list of values that may generally apply to the patient,
such as, for example, age ranges or medication families, which the
user may select using the user inputs 256.
[0054] As noted, the PPG system 210 may be a pulse oximetry system.
A pulse oximeter is a medical device that may determine oxygen
saturation of blood. The pulse oximeter may indirectly measure the
oxygen saturation of a patient's blood (as opposed to measuring
oxygen saturation directly by analyzing a blood sample taken from
the patient) and changes in blood volume in the skin. Ancillary to
the blood oxygen saturation measurement, pulse oximeters may also
be used to measure the pulse rate of a patient. Pulse oximeters
typically measure and display various blood flow characteristics
including, but not limited to, the oxygen saturation of hemoglobin
in arterial blood.
[0055] A pulse oximeter may include a light sensor, similar to the
sensor 212, which is placed at a site on a patient, typically a
fingertip, toe, forehead or earlobe, or in the case of a neonate,
across a foot. The pulse oximeter may pass light using a light
source through blood perfused tissue and photoelectrically sense
the absorption of light in the tissue. For example, the pulse
oximeter may measure the intensity of light that is received at the
light sensor as a function of time. A signal representing light
intensity versus time or a mathematical manipulation of this signal
(for example, a scaled version thereof, a log taken thereof, a
scaled version of a log taken thereof, and/or the like) may be
referred to as the photoplethysmogram (PPG) signal. In addition,
the term "PPG signal," as used herein, may also refer to an
absorption signal (for example, representing the amount of light
absorbed by the tissue) or any suitable mathematical manipulation
thereof. The light intensity or the amount of light absorbed may
then be used to calculate the cardiac stability, amplitude,
variance in amplitude, pulse period, variance in pulse period as
well as other physiological parameter when each individual pulse
occurs.
[0056] The light passed through the tissue is selected to be of one
or more wavelengths that are absorbed by the blood in an amount
representative of the amount of the blood constituent present in
the blood. The amount of light passed through the tissue varies in
accordance with the changing amount of blood constituent in the
tissue and the related light absorption. Red and infrared
wavelengths may be used because it has been observed that highly
oxygenated blood will absorb relatively less red light and more
infrared light than blood with lower oxygen saturation. By
comparing the intensities of two wavelengths at different points in
the pulse cycle, it is possible to estimate the blood oxygen
saturation of hemoglobin in arterial blood.
[0057] Cardiac stability may correlate with cardiac health and
other physiological parameters important in patient care, such as
hypovolemia, fluid responsiveness, and the like. Hypovolemia
relates to a decrease in blood volume and may correspond to
hemorrhaging of the patient. Fluid responsiveness relates to the
volume of fluid, such as blood, in the arteries, veins, and
vasculature of an individual. Fluid responsiveness also relates to
hemorrhaging. In general, fluid responsiveness may include a
measurement of the response of stroke volume, the volume of blood
passing out of the heart with each heartbeat, to venous return, the
volume of blood entering the heart with each heartbeat, caused by
the clinical administration of fluid into the vasculature, such as
through an intravenous injection. With each heartbeat, a certain
amount of blood is pumped out of the heart. The more blood that
fills the heart, the more blood the heart can pump out with each
heartbeat. If blood volume is too low, the heart may not fully fill
with blood. Therefore, the heart may not pump out as much blood
with each heartbeat. Consequently, low blood volume may lead to low
blood pressure, and organs and tissues may not receive enough blood
to optimally and/or properly function. Monitoring cardiac stability
may allow a physician to determine whether a patient is
hemorrhaging or otherwise requires additional fluid more quickly
than noticing a decrease in blood pressure. In short, cardiac
stability represents a prediction of whether or not a decrease in
blood pressure is occurring.
[0058] Cardiac stability may be monitored in, for example,
critically-ill patients or trauma patients because fluid
administration plays an important role in optimizing cardiac output
and stability for proper oxygen delivery to organs and tissues.
Trauma patients are generally at greater risk of hemorrhaging, and
the hemorrhage may occur internally or at sites that are
unnoticeable to the physician. Therefore, obtaining reliable
information and parameters that aid clinicians in early detection
of hemorrhaging may help improve patient outcomes.
[0059] FIG. 4 is an illustrative processing system 400 in
accordance with an embodiment. In an embodiment, an input signal
generator 410 generates an input signal 416. The input signal
generator 410 includes a pre-processor 420 coupled to a sensing
device 418. It will be understood that the input signal generator
410 may include any suitable signal source, signal generating data,
signal generating equipment, or any combination thereof to produce
the signal 416. The signal 416 may be a single signal, or may be
multiple signals transmitted over a single pathway or multiple
pathways.
[0060] The pre-processor 420 may apply one or more signal
processing techniques to the signal generated by the sensing device
418. For example, the pre-processor 420 may apply a pre-determined
transformation to the signal provided by the sensing device 418 to
produce an input signal 416 that can be appropriately interpreted
by the processor 412. The pre-processor 420 may also perform any of
the following operations to the signal provided by the sensing
device 418: reshaping the signal for transmission; multiplexing the
signal; modulating the signal onto carrier signals; compressing the
signal; encoding the signal; and filtering the signal.
[0061] In the embodiment of FIG. 4, the signal 416 is coupled to
the processor 412. The processor 412 may be any suitable software,
firmware, and/or hardware, and/or combinations thereof for
processing the signal 416. For example, the processor 412 may
include one or more hardware processors (e.g., integrated
circuits), one or more software modules, computer-readable media
such as memory, firmware, or any combination thereof. The processor
412 may, for example, be a computer or may be one or more chips
(i.e., integrated circuits). The processor 412 may, for example, be
configured of analog electronic components. The processor 412 may
perform some or all of the calculations associated with the
monitoring methods of the present disclosure. For example, the
processor 412 may analyze the physiological signals, waveforms, and
the like and compute pulse trending characteristics thereof to
determine a cardiac stability ratio and associated cardiac
stability of the patient, as discussed further below. The processor
412 may also perform any suitable signal processing to filter the
signal 416, such as any suitable band-pass filtering, adaptive
filtering, closed-loop filtering, and/or any other suitable
filtering, and/or any combination thereof. The processor 412 may
also receive input signals from additional sources (not shown). For
example, the processor 412 may receive an input signal containing
information about the patient or treatments provided to the
patient. These additional input signals may be used by the
processor 412 in any of the calculations or operations it performs
in accordance with the processing system 400.
[0062] The processor 412 may be coupled to one or more memory
devices (not shown) or incorporate one or more memory devices such
as any suitable volatile memory device (e.g., RAM, registers,
etc.), non-volatile memory device (e.g., ROM, EPROM, magnetic
storage device, optical storage device, flash memory, etc.), or
both. In an embodiment, the processor 412 may store physiological
measurements or previously received data from the signal 416 in a
memory device for later retrieval. The processor 412 may be coupled
to a calibration device (not shown) that may generate or receive as
input reference measurements for use in calibrating
calculations.
[0063] The processor 412 is coupled to an output 414 through a
patient status indicator signal 419, and may be coupled through
additional signal pathways not shown. The output 414 may be any
suitable output device such as, for example, one or more medical
devices (e.g., a medical monitor that displays various
physiological parameters, a medical alarm, or any other suitable
medical device that either displays physiological parameters or
uses the output of the processor 412 as an input), one or more
display devices (e.g., monitor, PDA, mobile phone, any other
suitable display device, or any combination thereof), one or more
audio devices, one or more memory devices (e.g., hard disk drive,
flash memory, RAM, optical disk, any other suitable memory device,
or any combination thereof), one or more printing devices, any
other suitable output device, or any combination thereof. In an
embodiment, the patient status indicator signal 419 includes at
least one of an identification of a medical condition of the
patient; an alert; a current cardiac stability of the patient; a
current stroke volume measurement; a current cardiac output
measurement; a current HR measurement; a current BP measurement;
another current physiological measurement; an estimated patient
status; and an estimated patient outcome. In some embodiments, the
patient status indicator signal 419 will be stored in a memory
device or recorded in another physical form for future, further
analysis.
[0064] It will be understood that the system 400 may be
incorporated into the system 100 (shown in FIG. 1) and/or the
system 210 (shown in FIGS. 2 and 3) in which, for example, the
input signal generator 410 may be implemented as parts of the
sensor 212 and/or the monitor 214 and the processor 412 may be
implemented as part of the monitor 214. In some embodiments,
portions of the system 400 may be configured to be portable. For
example, all or a part of the system 400 may be embedded in a
small, compact object carried with or attached to the patient
(e.g., a watch, other piece of jewelry, or cellular telephone). In
such embodiments, a wireless transceiver (not shown) may also be
included in the system 400 to enable wireless communication with
other components of system 210. As such, the system 210 may be part
of a fully portable and continuous monitoring solution.
[0065] FIG. 5 illustrates a PPG signal 500 over time, according to
an embodiment. The PPG signal 500 is an example of a physiological
signal. However, embodiments may be used in relation to various
other physiological signals, such as electrocardiogram signals,
phonocardiogram signals, ultrasound signals, and the like. The PPG
signal 500 may be determined, formed, and displayed as a waveform
by the monitor 214 (shown in FIG. 2) that receives signal data from
the PPG sensor 212 (shown in FIG. 2). For example, the monitor 214
may receive signals from the PPG sensor 212 positioned on a finger
of a patient. The monitor 214 processes the received signals, and
displays the resulting PPG signal 500 on the display 228 (shown in
FIG. 2).
[0066] The PPG signal 500 may include a plurality of pulses
502a-502n over a predetermined time period. The time period may be
a fixed time period, or the time period may be variable. Moreover,
the time period may be a rolling time period, such as a 5 second
rolling timeframe.
[0067] Each pulse 502a-502n may represent a single heartbeat and
may include a pulse-transmitted or primary peak 504 separated from
a pulse-reflected or trailing peak 506 by a dichrotic notch 508.
The primary peak 504 represents a pressure wave generated from the
heart to the point of detection, such as in a finger where the PPG
sensor 212 is positioned. The trailing peak 506 represents a
pressure wave that is reflected from the location proximate where
the PPG sensor 212 is positioned back toward the heart.
Characteristics of the primary peak 504 and/or the trailing peak
506 may be analyzed by the cardiac stability analysis module 249
(shown in FIG. 3) to calculate the cardiac stability ratio or other
physiological parameters of the patient.
[0068] As shown in FIG. 5, each pulse 502a-502n has a particular
amplitude. For example, the pulse 502a has an amplitude 510. The
amplitudes 510 may differ with respect to one another. In general,
the overall amplitude 510 of the PPG signal 500 over time t may
modulate. The cardiac stability analysis module 249 (shown in FIG.
3) of the monitor 214 may track and store the magnitude of the
amplitude modulation of the PPG signal 500 over time t. Optionally,
the cardiac stability analysis module 249 of the monitor 214 may
track and store the magnitude of the amplitude 510 of any number of
the pulses 502a-502n for use in determining a cardiac stability
ratio or other physiological parameter of the patient. For example,
the cardiac stability analysis module 249 of the monitor 214 may
use a single pulse 502a, analyze the single pulse to determine a
trending nature of the waveform thereof relative to previous
pulses, and calculate the cardiac stability ratio or other
physiological parameter of the patient based on the single pulse
502a. Alternatively, the cardiac stability analysis module 249 may
use multiple pulses 502a-502n, analyze the trending nature of the
waveforms thereof relative to previous or baseline waveforms, and
calculate the cardiac stability ratio or other physiological
parameter of the patient based upon a comparison of the amplitudes,
pulse periods or other aspects of the waveforms of the pulses
502a-502n to calculate the cardiac stability ratio or other
physiological parameter of the patient. Optionally, the cardiac
stability analysis module 249 of the monitor 214 may determine an
average modulation of the pulses 502a-502n over a time period t and
use the average modulation to calculate the cardiac stability ratio
or other physiological parameter of the patient.
[0069] The frequency of the pulses 502a-502n may vary. For example,
the frequency of the pulses over a first period of time may vary
from a frequency over a later period of time. The monitor 214
(shown in FIG. 2) may monitor and determine the frequencies. The
frequency variation may be based upon respiration, blood pressure,
heart rate, or other factors. The cardiac stability analysis module
249 of the monitor 214 may detect a magnitude of frequency
modulation over a time period t. The cardiac stability analysis
module 249 of the monitor 214 may use the frequency of the pulses,
or any other temporal element of the pulses, to analyze the
trending nature of the waveforms thereof, and calculate the cardiac
stability ratio or other physiological parameter of the
patient.
[0070] Various waveform characteristics may be measured and/or
calculated from the pulse waveform of the PPG signal 500. The
waveform characteristics may be used by the cardiac stability
analysis module 249 (shown in FIG. 3) to calculate the cardiac
stability ratio or other physiological parameter of the patient.
For example, as described in further detail below, the cardiac
stability analysis module 249 may utilize a pulse period, a transit
time, an amplitude, a peak, a temporal element, a change in any of
the waveform characteristics, and the like to calculate the cardiac
stability ratio or other physiological parameter of the
patient.
[0071] A pulse transit time (PTT) is a measure of a temporal
element of the pulse. For example, the PTT may be a transit time of
a given pulse from the heart to the location proximate where the
PPG sensor 212 is positioned. The PTT may be calculated by using an
ECG system to detect the pulse at the heart and a PPG system to
detect the pulse at the finger, and the time difference between the
detection at the heart and the detection at the finger corresponds
to the PPT. The PTT may be calculated by using a dual-pleth system
where two PPG sensors are attached at two different locations of
the patient and measuring a differential time between detection of
the pulse at the first PPG sensor and detection of the pulse at the
second PPG sensor (e.g. at a finger and at an ear). The time
difference between the pulse detections correspond to the PPT.
Other methods of detecting and/or calculating the PPT may be used
in other embodiments. The PPT may be affected by other
physiological conditions of the patient, such as blood pressure,
heart rate, respiration, and the like. The PTT is variable
depending on the physiological status of the patient.
[0072] A pulse period 512, defined by the heart rate (HR) of the
patient, may be calculated by measuring the time difference between
the pulses. For example, the pulse period 512 may be a measurement
of the time difference from the initiation of one pulse 502a to the
initiation of the second pulse 502b. Alternatively, the pulse
period 512 may be a measurement of the time difference from the
peak 504 of the pulse 502a to the peak 504 of the second pulse
502b. The pulse period 512 may be affected by other physiological
conditions of the patient, such as blood pressure, heart rate,
respiration, and the like. The pulse period 512 is variable
depending on the physiological status of the patient.
[0073] The PPG signal 500 shown in FIG. 5 corresponds to a steady
heart. The cardiac stability and function are generally normal and
healthy. The steady heart is characterized as being regular (e.g.
the pulse periods 512 are similar over time) and consistent (e.g.
the amplitudes 510 have high variance from the mean amplitude). The
peaks and troughs characteristic of the steady heart are generally
far apart (e.g. have high variance from the mean amplitude). An
unsteady heart is generally the opposite of a steady heart. The
unsteady heart is characterized as being irregular (e.g. the pulse
periods 512 tend to change form beat to beat) and inconsistent
(e.g. the amplitudes 510 tend to have a smaller variance from the
mean amplitude over time, such as when the peaks and troughs are
closer to the mean amplitude).
[0074] FIG. 6 illustrates a PPG signal 600 over time, according to
an embodiment. The PPG signal 600 corresponds to an unsteady heart.
Features of the PPG signal 600 that correspond to like features of
the PPG signal 500 are identified with like reference numerals
(e.g. amplitude 510 and pulse period 512). A comparison of the PPG
signal 500 showing an embodiment of a steady heart can be seen with
additional reference to FIG. 5. The steady heart shows strong
statistical variance around the average amplitude between the peak
absorptions and the minimum absorptions associated with each
pulse.
[0075] In an embodiment, characteristics of the PPG signals 500,
600 may be useful in determining a cardiac stability ratio to
determine cardiac stability of the patient. For example, the
cardiac stability analysis module 249 may use the amplitudes 510 of
the primary peaks 504 to calculate cardiac stability or other
physiological parameters of the patient. The cardiac stability
analysis module 249 may use the pulse periods 512 of the pulses to
calculate cardiac stability or other physiological parameters of
the patient. The cardiac stability analysis module 249 may
determine a cardiac stability ratio based on variance in amplitude
510 and variance in pulse period 512 over time. For example, the
cardiac stability analysis module 249 may determine a cardiac
stability ratio (CSR) based on the following:
CSR=(V(A))N(PP) Equation (1)
[0076] where V(A) is the variance of the amplitude of the pulse(s)
and V(PP) is the variance of the pulse period of the pulse. The
cardiac stability analysis module 249 may determine the CSR for
each individual pulse and compare the pulse to previous pulses to
determine the amplitude and pulse period variances thereof.
Alternatively, the cardiac stability analysis module 249 may
determine the CSR for multiple pulses over a predetermined time.
Optionally, the cardiac stability analysis module 249 may
continuously determine the CSR over a predetermined time
period.
[0077] The variance of the amplitude V(A) may be the average of the
squares of difference between the amplitudes of the pulses and the
mean amplitude of the pulses. For example, the variance of the
amplitude V(A) may be determined by the cardiac stability analysis
module 249 based on the following:
.SIGMA. ( each pulse amplitude - average pulse amplitude ) 2 number
of pulses in time t Equation ( 2 ) ##EQU00001##
[0078] The variance of the pulse period V(PP) may be the average of
the squares of difference between the pulse period of each pulses
and the mean pulse period of the pulses. For example, the variance
of the pulse period V(PP) may be determined by the cardiac
stability analysis module 249 based on the following:
.SIGMA. ( each pulse period - average pulse period ) 2 number of
pulses in time t Equation ( 3 ) ##EQU00002##
[0079] Alternatively, the cardiac stability analysis module 249 may
determine the CSR based on a range (e.g. maximum amplitude-minimum
amplitude) over a period of time and/or a range (e.g. maximum pulse
period-minimum pulse period) over a period of time rather than the
variances thereof. Using the ranges may provide a less sensitive
tool, however the CSR may be determined more quickly, more
frequently and/or with less computing power.
[0080] The CSR determined by the cardiac stability analysis module
249 provides an index that decreases quickly as cardiac capability
decreases. For example, as the heart becomes unsteady, the variance
of the amplitude V(A) will decrease. Because the CSR varies
proportionally with respect to the variance of the amplitude V(A),
as the variance of the amplitude V(A) decreases, the numerator of
Equation 1 will decrease. Similarly, as the heart becomes unsteady,
the variance of the pulse period V(PP) will increase. Because the
CSR varies inversely with respect to the variance of the pulse
period V(PP), as the variance of the pulse period V(PP) increases,
the denominator of Equation 1 will increase, causing the CSR to
decrease. Having both the numerator and the denominator cause the
CSR to decrease as the heart becomes unsteady, the CSR is sensitive
to changes in the cardiac stability.
[0081] Conversely, CSR determined by the cardiac stability analysis
module 249 provides an index that increases as heart becomes
steady. For example, as the heart becomes steady, the variance of
the amplitude V(A) will increase causing the numerator of Equation
1 to increase. Similarly, as the heart becomes steady, the variance
of the pulse period V(PP) will increase causing the denominator of
Equation 1 to decrease.
[0082] The cardiac stability analysis module 249 may calculate
cardiac stability (CS) of the patient based on the CSR. For
example, the CSR may be determined based on the following:
CS=(CSR)K Equation (4)
[0083] where K is a scaling factor based on empirically-determined
constants that may be determined through clinical examinations of
patients, a calibration constant, the nature of the subject and/or
the nature of the signal detecting devices. The scaling factor K
may be computed from relationships derived from observed historical
data (e.g., relationships with patient demographic data such as
body mass index (BMI), height, weight, and the like) and/or
measured signal characteristics (e.g., heart rate, PTT, amplitude,
pulse period, and the like).
[0084] In some embodiments, the PPG signal may be corrected or
normalized to account for changes in vascular tone and/or motion
artifacts through analysis of the PPG signal. Normalizing may be
performed prior to calculating K.
[0085] The CS determined by the cardiac stability analysis module
249 is displayed on the monitor 214 of the system 200 for use by
the physician in analyzing the health of the patient and/or
treating the patient. The scaling factor K may be selected to
correlate the cardiac stability CS with a healthy or nominal
number, such as 100. Having the CS decrease from the nominal number
as the heart becomes unsteady provides a logical indicator for a
physician to monitor. The displayed CS may be treated as a
percentage of the nominal number where a CS of 70, for example, is
an indication to the physician that the patient is having problems
with cardiac functionality. Optionally, the cardiac stability
analysis module 249 may store a threshold CS level, below which the
system 200 may provide an alarm condition, such as an audible
alarm, a visual alarm, or another type of alarm.
[0086] FIG. 7 illustrates a flow chart of a method of determining
cardiac stability of a patient, according to an embodiment. The
method may be performed by various systems, such as the system 100
(shown in FIG. 1), the system 210 (shown in FIGS. 2 and 3), or
other capable systems. The method may include acquiring at 700 PPG
signals. The PPG signals may be acquired by securing a PPG sensor
to an anatomical portion of the patient and sensing a physiological
characteristic of the patient with the PPG sensor. While the
embodiment of the method described herein references acquiring PPG
signals, such as using the PPG sensor 212 (shown in FIG. 2), the
method may include acquiring other types of physiological signals,
such as ECG signals, PCG signals, and/or ultrasound signals that
characterize or describe cardiac activity. The physiological
signals may be obtained from the individual for at least a
designated period of time.
[0087] In an embodiment, the PPG signal is analyzed by the cardiac
stability analysis module 249 (shown in FIG. 3). The cardiac
stability analysis module 249 analyzes the PPG signal to determine
waveform characteristics of the PPG signal, such as an amplitude of
the PPG signal and a pulse period of the PPG signal for one or more
pulses. For example, at 702, the PPG signal is analyzed to
determine an amplitude of each primary peak of the PPG signal.
Then, at 704, the PPG signal is analyzed to determine a pulse
period of each pulse of the PPG signal.
[0088] At 706, the system calculates the variance of the amplitudes
of the pulses for a certain time period, such as over 60 seconds.
For example, the system may calculate the average of the squared
differences of the amplitudes from the mean amplitude. The system
may calculate the variance of the amplitudes according to equation
2. At 708, the system calculates the inverse variance of the
amplitudes. The inverse variance is used to determine the cardiac
stability ratio (CSR).
[0089] At 710, the system calculates the variance of the pulse
periods of the pulses for a certain time period, such as over 60
seconds. For example, the system may calculate the average of the
squared differences of the pulse periods from the mean pulse
period. The system may calculate the variance of the pulse periods
according to equation 3. The variance of the pulse periods is used
to determine the cardiac stability ratio (CSR).
[0090] At 712, the system calculates the CSR based on the
amplitudes and the pulse periods of the pulses. For example, the
system may calculate the CSR using the inverse variance of the
amplitudes of the pulses and the variance of the pulse periods of
the pulses over a certain time period to calculate the CSR. The CSR
may be calculated according to equation 1. The CSR may be
calculated with a pleth-only system. For example, the system may be
operated without the need for an invasive monitoring system, an ECG
or any other monitoring system. The system may calculate the CSR
with the use of a single PPG sensor. Further, the system, at 714,
may calculate the cardiac stability (CS) based on the CSR. For
example, a scaling factor may be used to adjust the CSR to
calculate a meaningful cardiac stability. The CS may be calculated
according to equation 4.
[0091] FIG. 8 illustrates a flow chart of a method of operating a
PPG system, according to an embodiment. The method may be performed
by various systems, such as the system 100 (shown in FIG. 1), the
system 210 (shown in FIGS. 2 and 3), or other capable systems. The
method may include acquiring at 800 PPG signals. In an embodiment,
the PPG signal is analyzed by the cardiac stability analysis module
249 (shown in FIG. 3). The cardiac stability analysis module 249
analyzes the PPG signal to determine waveform characteristics of
the PPG signal.
[0092] At 802, the PPG signal is analyzed to determine an amplitude
of the primary peak of each pulse. At 804, the PPG signal is
analyzed to determine a pulse period of each pulse. Then, at 806, a
cardiac stability ratio (CSR) is calculated based on the amplitudes
and pulse periods of the pulses over a period of time. The CSR is
calculated as a function of the inverse variance of the amplitudes.
The CSR is calculated as a function of the variance of the pulse
periods. The CSR may be calculated according to equation 1.
[0093] At 808, the system calculates a cardiac stability (CS) of
the patient. The CS may be based on the CSR. For example, a scaling
factor may be used to adjust the CSR to calculate a meaningful
cardiac stability. The CS may be calculated according to equation
4.
[0094] At 810, the system displays the CS on a monitor, such as the
monitor 214 (shown in FIG. 2). The CS may be displayed as a number,
a grade, a graphical representation, and the like.
[0095] At 812, the system determines if the CS is below a
threshold. The threshold may be based on physiological conditions
of the patient, such as age, weight, height, diagnosis,
medications, treatments, and so forth. If the CS falls below the
threshold, the system at 814 provides an alarm condition. The alarm
may be a visual alarm, an audible alarm, or another type of alarm.
The alarm may be triggered on the monitor 214 and/or may be
transmitted to another location, such as a central monitoring
station to alert medical professionals.
[0096] Various embodiments described herein provide a tangible and
non-transitory (for example, not an electric signal)
machine-readable medium or media having instructions recorded
thereon for a processor or computer to operate a system to perform
one or more embodiments of methods described herein. The medium or
media may be any type of CD-ROM, DVD, floppy disk, hard disk,
optical disk, flash RAM drive, or other type of computer-readable
medium or a combination thereof.
[0097] The various embodiments and/or components, for example, the
control units, modules, or components and controllers therein, also
may be implemented as part of one or more computers or processors.
The computer or processor may include a computing device, an input
device, a display unit and an interface, for example, for accessing
the Internet. The computer or processor may include a
microprocessor. The microprocessor may be connected to a
communication bus. The computer or processor may also include a
memory. The memory may include Random Access Memory (RAM) and Read
Only Memory (ROM). The computer or processor further may include a
storage device, which may be a hard disk drive or a removable
storage drive such as a floppy disk drive, optical disk drive, and
the like. The storage device may also be other similar means for
loading computer programs or other instructions into the computer
or processor.
[0098] As used herein, the term "computer," "computing system," or
"module" may include any processor-based or microprocessor-based
system including systems using microcontrollers, reduced
instruction set computers (RISC), application specific integrated
circuits (ASICs), logic circuits, and any other circuit or
processor capable of executing the functions described herein. The
above examples are exemplary only, and are thus not intended to
limit in any way the definition and/or meaning of the term
"computer" or "computing system".
[0099] The computer, computing system, or processor executes a set
of instructions that are stored in one or more storage elements, in
order to process input data. The storage elements may also store
data or other information as desired or needed. The storage element
may be in the form of an information source or a physical memory
element within a processing machine.
[0100] The set of instructions may include various commands that
instruct the computer or processor as a processing machine to
perform specific operations such as the methods and processes of
the various embodiments of the subject matter described herein. The
set of instructions may be in the form of a software program. The
software may be in various forms such as system software or
application software. Further, the software may be in the form of a
collection of separate programs or modules, a program module within
a larger program or a portion of a program module. The software
also may include modular programming in the form of object-oriented
programming. The processing of input data by the processing machine
may be in response to user commands, or in response to results of
previous processing, or in response to a request made by another
processing machine.
[0101] As used herein, the terms "software" and "firmware" are
interchangeable, and include any computer program stored in memory
for execution by a computer, including RAM memory, ROM memory,
EPROM memory, EEPROM memory, and non-volatile RAM (NVRAM) memory.
The above memory types are exemplary only, and are thus not
limiting as to the types of memory usable for storage of a computer
program.
[0102] As discussed above, embodiments may provide a system and
method of determining cardiac stability of a patient through
analysis of physiological signals, such as PPG signals, by
analyzing waveform characteristics of the PPG signal and
calculating an amplitude variance and a pulse period variance of
the PPG signal over time.
[0103] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. In addition, many modifications may be
made to adapt a particular situation or material to the teachings
without departing from its scope. While the dimensions, types of
materials, and the like described herein are intended to define the
parameters of the disclosure, they are by no means limiting and are
exemplary embodiments. Many other embodiments will be apparent to
those of skill in the art upon reviewing the above description. The
scope of the disclosure should, therefore, be determined with
reference to the appended claims, along with the full scope of
equivalents to which such claims are entitled. In the appended
claims, the terms "including" and "in which" are used as the
plain-English equivalents of the respective terms "comprising" and
"wherein." Moreover, in the following claims, the terms "first,"
"second," and "third," etc. are used merely as labels, and are not
intended to impose numerical requirements on their objects.
Further, the limitations of the following claims are not written in
means--plus-function format and are not intended to be interpreted
based on 35 U.S.C. .sctn.112, sixth paragraph, unless and until
such claim limitations expressly use the phrase "means for"
followed by a statement of function void of further structure.
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