U.S. patent application number 11/270241 was filed with the patent office on 2007-05-10 for method and apparatus for processing signals reflecting physiological characteristics.
Invention is credited to Alexander K. Mills, Bernhard B. Sterling.
Application Number | 20070106136 11/270241 |
Document ID | / |
Family ID | 38001096 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070106136 |
Kind Code |
A1 |
Sterling; Bernhard B. ; et
al. |
May 10, 2007 |
METHOD AND APPARATUS FOR PROCESSING SIGNALS REFLECTING
PHYSIOLOGICAL CHARACTERISTICS
Abstract
The invention comprises a method and apparatus for processing
signals reflecting a physiological characteristic by detecting the
intensity of light following tissue absorption at two wavelengths
and subtracting the best estimate of the desired signal from the
difference between the signals. Corrected first and second
intensity signals are determined by applying a residual derived
from a combination of the first and second intensity signals as
multiplied by a residual factor and subtracted from a difference
between the first and second intensity signals to the first and
second intensity signals. In one embodiment, the method and
apparatus are used to determine arterial oxygen saturation.
Inventors: |
Sterling; Bernhard B.;
(Danville, CA) ; Mills; Alexander K.; (San
Antonio, TX) |
Correspondence
Address: |
Ralph C. Francis
Francis Law Group
1942 Embarcadero
Oakland
CA
94606
US
|
Family ID: |
38001096 |
Appl. No.: |
11/270241 |
Filed: |
November 8, 2005 |
Current U.S.
Class: |
600/336 ;
600/322 |
Current CPC
Class: |
A61B 5/14551
20130101 |
Class at
Publication: |
600/336 ;
600/322 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Claims
1. A device for the monitoring of a physiological characteristic of
a patient's blood, comprising: a first radiation emitter that emits
light at a first wavelength; a second radiation emitter that emits
light at a second wavelength; a radiation detector configured to
receive light at said first and second wavelengths after absorbance
through the patient's blood and provide a first intensity signal
and a second intensity signal corresponding to said first and
second received wavelengths; and a controller for computing said
physiological characteristic of said patient's blood from a
corrected first intensity signal and a corrected second intensity
signal determined by applying a residual derived from a combination
of said first and second intensity signals as multiplied by a
residual factor and subtracted from a difference between said first
and second intensity signals to said first and second intensity
signals.
2. The device of claim 1, wherein said physiological characteristic
is arterial oxygen saturation.
3. The device of claim 2, wherein said first wavelength is in the
range of approximately 650-670 nm.
4. The device of claim 2, wherein said second wavelength is in the
range of 800-1000 nm.
5. The device of claim 1, wherein said residual factor is
determined by minimizing the absolute value of the difference
between said residual at a time midpoint and an average of said
residual at a first data minimum and at a first data maximum.
6. The device of claim 1, wherein said residual factor is
determined by minimizing the absolute value of the difference of
said residual at a first data maximum and said residual at a first
data minimum.
7. The device of claim 6, wherein said residual factor is
determined by also minimizing the absolute value of the difference
of said residual at a first data maximum and said residual at a
first data minimum.
8. The device of claim 7, wherein said first data minimum and said
first data maximum are determined by polynomial fitting.
9. The device of claim 1, wherein said residual factor is related
to reference oxygen saturation to determine said physiological
characteristic.
10. The device of claim 1, wherein a ratio of said corrected
intensity signals is related to reference oxygen saturation to
determine said physiological characteristic.
11. The device of claim 1, wherein said residual is substantially
free of signal related to said physiological characteristic.
12. The device of claim 1, wherein said residual substantially
corresponds to undesirable signal components.
13. A method for processing signals reflecting a physiological
characteristic of a patient's blood, comprising the steps of:
coupling an oximeter sensor arrangement to a tissue region of said
patient; passing first and second lights through said patient's
tissue region, wherein said first light is substantially in a red
light range and said second light is substantially in an infrared
light range; detecting said first and second lights absorbed by
said tissue region and providing a first intensity signal and a
second intensity signal corresponding to said absorbed first and
second lights; and computing said physiological characteristic of
said patient's blood from a corrected first intensity signal and a
corrected second intensity signal determined by applying a residual
derived from a combination of said first and second intensity
signals as multiplied by a residual factor and subtracted from a
difference between said first and second intensity signals to said
first and second intensity signals.
14. The method of claim 13, wherein said physiological
characteristic is arterial oxygen saturation.
15. The method of claim 13, wherein said residual factor is
determined by minimizing the absolute value of the difference
between said residual at a time midpoint and an average of said
residual at a first data minimum and at a first data maximum.
16. The method of claim 13, wherein said residual factor is
determined by minimizing the absolute value of the difference of
said residual at a first data maximum and said residual at a first
data minimum.
17. The method of claim 16, wherein said residual factor is
determined by also minimizing the absolute value of the difference
of said residual at a first data maximum and said residual at a
first data minimum.
18. The method of claim 17, wherein said first data minimum and
said first data maximum are determined by polynomial fitting.
19. The method of claim 13, wherein said residual factor is related
to reference oxygen saturation to determine said physiological
characteristic.
20. The method of claim 13, wherein a ratio of said corrected
intensity signals is related to reference oxygen saturation to
determine said physiological characteristic.
21. The method of claim 13, wherein said residual is substantially
free of signal related to said physiological characteristic.
22. The method of claim 13, wherein said residual substantially
corresponds to undesirable signal components.
23. The method of claim 13, wherein said residual factor is
determined by minimizing a qualifier.
24. The method of claim 23, wherein said qualifier is selected to
correspond to said physiological characteristic.
25. A method for determining a patient's arterial oxygen saturation
comprising the steps of: coupling an oximeter sensor arrangement to
a tissue region of said patient; passing first and second lights
through said patient's tissue region, wherein said first light is
substantially in a red light range and said second light is
substantially in an infrared light range; detecting said first and
second lights absorbed by said tissue region and providing a first
intensity signal and a second intensity signal corresponding to
said absorbed first and second lights; and computing said arterial
oxygen saturation of said patient's blood from a corrected first
intensity signal and a corrected second intensity signal determined
by applying a residual derived from a combination of said first and
second intensity signals as multiplied by a residual factor and
subtracted from a difference between said first and second
intensity signals to said first and second intensity signals,
wherein said residual factor is determined by minimizing the
absolute value of the difference between said residual at a first
timepoint of a pulse corresponding to a time midpoint and an
average of said residual at a second timepoint of said pulse
corresponding to a first data minimum and at a third timepoint of
said pulse corresponding to a first data maximum.
Description
FIELD OF THE PRESENT INVENTION
[0001] The present invention relates to the field of signal
processing. More specifically, the invention relates to a method
for processing signals reflecting physiological
characteristics.
BACKGROUND OF THE INVENTION
[0002] Physiological monitoring systems and apparatus, which are
adapted to acquire signals reflecting physiological
characteristics, are well known in the art. The physiological
characteristics include, for example, heart rate, blood pressure,
blood gas saturation (e.g., oxygen saturation) and respiration
rate.
[0003] The signals acquired by the noted physiological monitoring
systems and apparatus are however composite signals, comprising a
desired signal portion that directly reflects the physiological
process that is being monitored and an undesirable signal portion,
typically referred to as interference or noise. The undesirable
signal portions often originate from both AC and DC sources. The DC
component, which is easily removed, results from the transmission
of energy through differing media that are of relatively constant
thickness within the body (e.g., bone, tissue, skin, blood,
etc.).
[0004] Undesirable AC components of the acquired signal correspond
to variable or erratic noise and interference, and thus have been
conventionally quite difficult to characterize and remove.
[0005] One example of a physiological monitoring apparatus, wherein
the measured signal can, and in many instances will, include
undesirable signal components, is a pulse oximeter.
[0006] Pulse oximeters typically measure and display various blood
constituents and blood flow characteristics including, but not
limited to, blood oxygen saturation of hemoglobin in arterial
blood, the volume of individual blood pulsations supplying the
flesh and the rate of blood pulsations corresponding to each
heartbeat of the patient. Illustrative are the apparatus described
in U.S. Pat. Nos. 5,193,543; 5,448,991; 4,407,290; and
3,704,706.
[0007] As is well known in the art, a pulse oximeter passes light
through human or animal body tissue where blood perfuses the
tissue, such as a finger, an ear, the nasal septum or the scalp,
and photoelectrically senses the absorption of light in the tissue.
The amount of light absorbed is then used to calculate the amount
of blood constituent being measured.
[0008] Two lights having discrete frequencies in the range of about
650-670 nanometers in the red range and about 800-1000 nanometers
in the infrared range are typically passed through the tissue. The
light is absorbed by the blood in an amount representative of the
amount of the blood constituent present in the blood. The amount of
transmitted light passed through the tissue varies in accordance
with the changing amount of blood constituent in the tissue and the
related light absorption.
[0009] The output signal from the pulse oximeter, which is
sensitive to the arterial blood flow, contains a component that is
a waveform representative of the patient's blood gas saturation.
This component is referred to as a "plethysmographic wave or
waveform" (see curve P in FIG. 1).
[0010] The plethysmograph signal (and the optically derived pulse
rate) may however be subject to irregular variants that interfere
with the detection of the blood constituents. The noise,
interference and other artifacts can, and in many instances will,
cause spurious pulses that are similar to pulses caused by arterial
blood flow. These spurious pulses, in turn, may cause the oximeter
to process the artifact waveform and provide erroneous data.
[0011] Several signal processing methods (and apparatus) have been
employed to reduce the effects of undesirable signal components on
the measured signal and, hence, the derived plethysmograph
waveform. Illustrative are the methods and apparatus disclosed in
U.S. Pat. No. 4,934,372, which correlate a subject's
electrocardiogram waveform with the acquired signal to identify
desired portions of the signal to more accurately detect blood
constituents.
[0012] Similarly, U.S. Pat. Nos. 5,490,505, 6,036,642, 6,206,830,
and 6,263,222, all disclose signal processors that generate either
a noise reference or a signal reference which is used to drive a
correlation canceler and generate a waveform that approximates
either the desired or undesired component of the acquired signal. A
primary intended application of the noted signal processors is the
measurement of blood oxygen saturation in a manner that minimizes
the effect of motion artifacts. However, a consequence of the
process used to generate the reference is that a third optical
signal must be acquired to provide ratiometric calculation of
saturation.
[0013] Accordingly, each of the noted prior art references require
the acquisition of additional signals to help measure blood oxygen
saturation. As such, these systems are inherently more complex and
costly. Further, the noted references are primarily concerned with
filtering out motion artifacts. Therefore, these references are not
tailored to the removal of undesired signal components that arise
from other sources.
[0014] It is therefore an object of the present invention to
provide a cost effective, reliable means of determining a
physiological characteristic by detecting a minimum number of
signals.
[0015] It is another object of the invention to provide a method
for processing signals reflecting a physiological characteristic
that does not require correlation canceling.
[0016] Another object of the invention is to provide a method for
processing signals reflecting a physiological characteristic that
minimizes undesirable signal components.
[0017] It is yet another object of the invention to provide a
method and apparatus for correcting signals reflecting a
physiological characteristic that does not require a pulse waveform
model or data from preceding pulse waveforms.
[0018] Yet another object of the invention is to provide a method
and apparatus for correcting signals reflecting a physiological
characteristic using data from a single pulse.
[0019] A further object of the invention is to provide a method and
apparatus for determining arterial oxygen saturation with improved
accuracy.
[0020] It is another object of the invention to provide a method
for processing oximetry signals based on specific time dependent
differences during a single pulse.
[0021] Another object of the invention is to provide a method for
improving an oximetry signal based on analytical, mathematical
steps that are analytically transparent, interpretable and
adjustable on a physiological and physical level, and based on an
understanding of the variables that interfere with oximetry
signals, to optimally minimize specific interferences with the
oximetry signal.
SUMMARY OF THE INVENTION
[0022] In accordance with the above objects and those that will be
mentioned and will become apparent below, the invention includes a
device for the monitoring of a physiological characteristic of a
patient's blood, having first and second radiation emitters that
emit light at first and second wavelengths, a radiation detector
configured to receive light at the first and second wavelengths
after absorbance through the patient's blood and provide first and
second intensity signals corresponding to the first and second
received wavelengths, and a controller for computing the
physiological characteristic of the patient's blood from a
corrected first and second intensity signal determined by applying
a residual derived from a combination of the first and second
intensity signals as multiplied by a residual factor and subtracted
from a difference between the first and second intensity signals to
the first and second intensity signals.
[0023] In one embodiment, the device is configured to determine
arterial oxygen saturation.
[0024] Preferably, the first wavelength is in the range of
approximately 650-670 nm. Also preferably, the second wavelength is
in the range of 800-1000 nm.
[0025] In one aspect of the invention, the residual factor is
determined by minimizing the absolute value of the difference
between the residual at a time midpoint and an average of the
residual at a first data minimum and at a first data maximum. In
another aspect of the invention, the residual factor is determined
by minimizing the absolute value of the difference of the residual
at a first data maximum and the residual at a first data minimum.
Preferably, the residual factor is determined by minimizing both
values. Also preferably, the first data minimum and the first data
maximum are determined by polynomial fitting.
[0026] In another embodiment of the invention, the residual factor
is related to reference oxygen saturation to determine the
physiological characteristic.
[0027] In yet another embodiment of the invention, a ratio of the
corrected intensity signals is related to reference oxygen
saturation to determine the physiological characteristic.
[0028] According to one embodiment of the invention, the residual
is substantially free of signal related to the physiological
characteristic.
[0029] In another embodiment, the residual substantially
corresponds to undesirable signal components.
[0030] The invention also comprises a method for processing signals
reflecting a physiological characteristic of a patient's blood,
comprising the steps of (i)coupling an oximeter sensor arrangement
to a tissue region of the patient; (ii) passing first and second
lights through the patient's tissue region, wherein the first light
is substantially in a red light range and the second light is
substantially in an infrared light range; (iii) detecting the first
and second lights absorbed by the tissue region and providing a
first and second intensity signal corresponding to the absorbed
first and second lights; and (iv) computing the physiological
characteristic of the patient's blood from a corrected first and
second intensity signal determined by applying a residual derived
from a combination of the first and second intensity signals as
multiplied by a residual factor and subtracted from a difference
between the first and second intensity signals to the first and
second intensity signals. Preferably, the physiological
characteristic determined is arterial oxygen saturation.
[0031] In the noted embodiment, the residual factor is preferably
determined by minimizing the absolute value of the difference
between the residual at a time midpoint and an average of the
residual at a first data minimum and at a first data maximum. Also
preferably, the residual factor is determined by minimizing the
absolute value of the difference of the residual at a first data
maximum and the residual at a first data minimum. Most preferably,
the residual factor is determined by minimizing both values.
[0032] According to one aspect of the invention, the residual
factor is related to reference oxygen saturation to determine the
physiological characteristic. In another embodiment, a ratio of the
corrected intensity signals is related to reference oxygen
saturation to determine the physiological characteristic.
[0033] In one embodiment of the invention, the residual factor is
determined by minimizing a qualifier. Preferably, the qualifier is
selected to correspond to the physiological characteristic.
[0034] In a further embodiment, the invention comprises a method
for determining a patient's arterial oxygen saturation comprising
the steps of i) coupling an oximeter sensor arrangement to a tissue
region of the patient, ii) passing first and second lights through
the patient's tissue region, wherein the first light is
substantially in a red light range and the second light is
substantially in an infrared light range, iii) detecting the first
and second lights absorbed by the tissue region and providing a
first intensity signal and a second intensity signal corresponding
to the absorbed first and second lights, and iv) computing the
arterial oxygen saturation of the patient's blood from a corrected
first intensity signal and a corrected second intensity signal
determined by applying a residual derived from a combination of the
first and second intensity signals as multiplied by a residual
factor and subtracted from a difference between the first and
second intensity signals to the first and second intensity signals,
wherein the residual factor is determined by minimizing the
absolute value of the difference between the residual at a first
timepoint of a pulse corresponding to a time midpoint and an
average of the residual at a second timepoint of the pulse
corresponding to a first data minimum and at a third timepoint of
the pulse corresponding to a first data maximum.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Further features and advantages will become apparent from
the following and more particular description of the preferred
embodiments of the invention, as illustrated in the accompanying
drawings, and in which like referenced characters generally refer
to the same parts or elements throughout the views, and in
which:
[0036] FIG. 1 is a graphical illustration of an r-wave portion of
an electrocardiogram waveform and the related plethysmographic
waveform;
[0037] FIG. 2 is a schematic illustration of a pulse oximeter
apparatus, according to the invention;
[0038] FIGS. 3 and 4 are graphical illustrations of red and
infrared optical signals taken from independent sensors, according
to the invention;
[0039] FIGS. 5 and 6 are graphical illustrations of amplitude
averages and differences of the red and infrared optical signals
taken from the independent sensors, according to the invention;
[0040] FIGS. 7 and 8 are graphical illustrations of amplitude
differences and corrected amplitude differences of the red and
infrared optical signals taken from the independent sensors,
according to the invention;
[0041] FIGS. 9 and 10 are graphical illustrations of the corrected
amplitudes of the red and infrared optical signals taken from the
independent sensors, according to the invention;
[0042] FIG. 11 is a graphical illustration of the residual factor f
used to correct the red and infrared optical signals as a function
of oxygen saturation, according to the invention; and
[0043] FIG. 12 is a graphical illustration comparing the corrected
data of the red and infrared optical signals taken from the
independent sensors to the uncorrected data, according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0044] Before describing the present invention in detail, it is to
be understood that this invention is not limited to particularly
exemplified materials, methods or structures as such may, of
course, vary. Thus, although a number of materials and methods
similar or equivalent to those described herein can be used in the
practice of the present invention, the preferred materials and
methods are described herein.
[0045] It is also to be understood that the terminology used herein
is for the purpose of describing particular embodiments of the
invention only and is not intended to be limiting.
[0046] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as commonly understood by one
having ordinary skill in the art to which the invention
pertains.
[0047] Further, all publications, patents and patent applications
cited herein, whether supra or infra, are hereby incorporated by
reference in their entirety.
[0048] Finally, as used in this specification and the appended
claims, the singular forms "a, "an" and "the" include plural
referents unless the content clearly dictates otherwise.
Definitions
[0049] The term "signal", as used herein, is meant to mean and
include an analog electrical waveform or a digital representation
thereof, which is collected from a biological or physiological
sensor.
[0050] The term "desired signal component", as used herein, is
meant to mean and include the portion of a signal that directly
corresponds to the biological or physiological function being
monitored.
[0051] The term "motion artifact", as used herein, is meant to mean
and include variability in a signal due to changes in the tissue
being monitored that are caused by muscle movement proximate to the
oximeter sensor.
[0052] The term "undesirable signal component", as used herein, is
meant to mean and include any portion of a signal that does not
correspond to the biological or physiological function being
monitored. As such, the term includes, without limitation, noise,
interference, and other variables that hinder the measurement of
the biological or physiological function. Generally, motion
artifacts are not the subject of this invention.
[0053] The terms "patient" and "subject", as used herein, is meant
to mean and include humans and animals.
[0054] The present invention substantially reduces or eliminates
the disadvantages and drawbacks associated with convention signal
processing systems, apparatus and techniques. As discussed in
detail below, a desired signal component is separated deliberately,
analytically and specifically from an undesirable signal component
using time dependent differences in signals derived from a single
pulse. A combination of the amplitudes of two optical signals is
used to correct the divergence between the signals, and thus
estimate a residual signal that corresponds to the undesirable
signal component. In turn, the estimated residual signal is used to
correct the optical signals. Thus, the invention provides a method
for improving an oximetry signal based on analytical, mathematical
steps that are analytically transparent, interpretable and
adjustable on a physiological and physical level, and based on an
understanding of the variables that interfere with oximetry
signals, to optimally minimize specific interferences with the
oximetry signal
[0055] Preferably, the residual signal is estimated from data
reflecting the highest signal to noise ratio. For oximetry data,
the highest signal to noise ratio can be obtained by selecting data
from time points corresponding to the maximal and minimal
amplitudes of the plethysmographic waveform and from a time point
corresponding to an analytical derivative of the maximum and
minimum. Accordingly, the inventive analysis is based on specific
time dependent differences in signal amplitude and can be performed
using information from only three distinct time points of a single
pulse wave. Further, by using values from the maximal and minimal
amplitudes, the relative magnitude of the total oximetry signal is
maximized, providing the best estimation of the undesirable signal
component.
[0056] In one embodiment of the invention, the method and apparatus
for processing signals reflecting a physiological characteristic
generally comprises detecting the intensity of light following
tissue absorption at two wavelengths and subtracting the best
estimate of the desired signal from the difference between the
signals.
[0057] As noted above, conventional pulse oximeters generate a
composite oximetry signal that includes the desired signal
component corresponding to the true oximetry component and an
undesirable signal component reflecting noise, interference and
variability. The undesirable signal components are estimated to
comprise in the range of approximately 5 to 15% of the total
signal. This invention provides a means for eliminating a
substantial portion of the undesirable signal components in an
easily implemented process that does not require the acquisition of
additional signals. For the purposes of providing an improved
oximetry measurement, it relatively unimportant to understand the
exact sources of the undesired signal components so long as their
effect can be removed or minimized.
[0058] Non-invasive pulse oximetry is based on a comparison of the
absorption of red and infrared wavelengths. There is however no
fundamental physiological reason for the red and infrared signals
to vary from each other in their timing with respect to any given
portion of a cardiac cycle. Accordingly, any measured difference
can be attributed to interfering, undesirable signal
components.
[0059] A conventional pulse oximeter generates a signal that is
estimated to comprise as high as 90% desirable signal component. As
demonstrated below, the invention involves subtracting the best
estimate portion of the total signal from the difference between
the red and infrared signals at every time and wavelength point.
This reduces the undesirable signal components from approximately
10% to approximately 1%. Thus, the resulting corrected oximetry
signal approaches 99% accuracy, improving the quality of the data
several fold.
[0060] Referring first to FIG. 1, there is shown a graphical
illustration of an "r-wave" portion of an electrocardiogram (ECG)
waveform (designated "r") and the related plethysmographic waveform
(designated "p"). As will be appreciated by one having ordinary
skill in the art, the ECG waveform comprises a complex waveform
having several components that correspond to electrical heart
activity. The QRS component relates to ventricular heart
contraction.
[0061] The r-wave portion of the QRS component is typically the
steepest wave therein, having the largest amplitude and slope, and
can be used for indicating the onset of cardiovascular activity.
The arterial blood pulse flows mechanically and its appearance in
any part of the body typically follows the R wave of the electrical
heart activity by a determinable period of time that remains
essentially constant for a given patient. See, e.g., Goodlin et
al., Systolic Time Intervals in the Fetus and Neonate, Obstetrics
and Gynecology, Vol. 39, No. 2, (February 1972) and U.S. Pat. No.
3,734,086.
[0062] Referring now to FIG. 2, there is shown a schematic
illustration of one embodiment of a pulse oximeter apparatus 5 that
can be employed within the scope of the invention. As discussed
above, conventional pulse oximetry methods and apparatus typically
employ two lights; a first light having a discrete wavelength in
the range of approximately 650-670 nanometers in the red range and
a second light having a discrete wavelength in the range of
approximately 800-1000 nanometers. For example, a suitable red LED
emits light at approximately 660 nanometers and a suitable infrared
LED emits light at approximately 880 nanometers.
[0063] The lights are typically directed through a finger 4 via
emitters 12, 14 and detected by a photo detector 16, such as a
square photodiode with an area of 49 mm.sup.2. Emitters 12 and 14
are driven by drive circuitry 18, which is in turn governed by
control signal circuitry 20. Detector 16 is in communication with
amplifier 22. In one embodiment, the LEDs are activated at a rate
of 8,000 times per second (8 kHz) per cycle, with a cycle
comprising red on, quiescent, IR on, quiescent. In the noted
embodiment, the total cycle time is 125 microseconds and the LEDs
are active for approximately 41.25 microseconds at a time.
[0064] The photo detector 16 provides an output signal that is
transmitted to an amplifier 22. The amplified signal from amplifier
22 is then transmitted to demodulator 24, which is also synched to
control signal circuitry 20. As will be appreciated by one having
skill in the art, the output signal from the demodulator 24 would
be a time multiplexed signal comprising (i) a background signal,
(ii) the red light range signal and (iii) the infrared light range
signal.
[0065] The demodulator 24, which is employed in most pulse oximeter
systems, removes any common mode signals present and splits the
time multiplexed signal into two (2) channels, one representing the
red voltage (or optical) signal and the other representing the
infrared voltage (or optical) signal.
[0066] As illustrated in FIG. 2, the signal from the demodulator 24
is transmitted to analog-digital converter (ADC) 26. The desired
computations are performed on the output from the converter 26 by
signal processor (DSP) 28 and the results transmitted to display
30. In one embodiment, ADC 26 converts the analog signals into
16-bit signed digital signals at a rate of 8 kHz. Further, DSP 28
preferably notch filters the data at 40 Hz to eliminate power line
frequency noise limit high frequency noise from other sources. Also
preferably, the DSP then parses each data stream by a factor of 4
to give two digital data streams at a rate of 2 kHz.
[0067] Further details of the conventional pulse oximeter
components, and related functions, are set forth in U.S. Pat. No.
4,934,372, which is incorporated by reference herein.
[0068] In one embodiment, the system electronics are configured
such that emitters 12 and 14 are driven with a variable gain to
produce an AC signal (corresponding to the photoplethysmograph
pulse waveform) riding on a larger DC signal. The current supplied
to the emitters is feedback driven to produce a constant DC signal
of approximately 1.25 V, for both the red and infrared signals. The
actual DC value is reported continuously. The magnitude of the AC
signals is computed relative to the DC signal. The AC component is
the signal that is given to the ADC 26 and converted to digital,
with the DC signal treated as the "zero point". This creates a
factor of the voltage range of the ADC 26 divided by the dynamic
(digital) range of the DSP 28. As one having skill in the art will
recognize, actual AC voltage level is computed by multiplying the
digital AC counts are multiplied by the voltage conversion factor
times the DC voltage.
[0069] As discussed above, a significant portion of optical pulse
signals that are collected with conventional oximetry probes
comprises a desired signal component, which is related to blood
oxygen saturation. Indeed, it is estimated that approximately 85 to
95% of the entire optical pulse signal corresponds to desired
signal component. The remaining approximately 5 to 15% of the
composite signal comprises an undesirable signal component from
highly variable combinations of sources.
[0070] The undesirable signal component chiefly corresponds to at
least five categories. First, there is instrument-related
electronic noise and drift. Second, there are low frequency
components, in the range of approximately 0.1 to 10 Hz of variable
amplitude, likely induced by variations in vasomotor control.
Third, there is a variable excursion that appears to coincide with
maximal blood pressure changes during the rise. This may represent
a blood pressure change induced pressure wave propagating through
tissue, which likely that such alters the ratio of absorbed to
scattered radiation. Fourth, there are subject and site related
irregularities. These include features such as skin thickness, scar
tissue, lesions, bone density that could effect sensor output due
to different time-dependent compression and relaxation effects.
Finally, there are motion artifacts, which are not the focus of
this invention.
[0071] As one having skill in the art will appreciate, a
measurement of a subject's oxygen saturation would be significantly
enhanced by removing the undesirable signal component before
calculating the blood oxygen saturation. The resulting corrected
oximetry data can be used in a conventional way, such as by
relating the log ratio of optical maxima at the systole and optical
minima at a diastole to reference oxygen saturation.
[0072] According to the invention, the average of the amplitude
that is multiplied by a variable, "residual factor f," and
subtracted from the difference in amplitude between the red and
infrared signals corresponds to a residual signal reflecting the
undesirable signal component. By correcting the amplitude of the
red and infrared signal at each time point with this residual, a
signal is generated that can be used to more accurately determine
blood oxygen saturation.
[0073] The specifics of this process are discussed below with
respect to exemplary signal data obtained from pulse oximeter 5
using two independent sensors A and B, for example, one attached to
the index finger of each hand of a subject. FIGS. 3 and 4 show data
collected during a single pulse, from independent sensors A and B.
In one embodiment, maximal and minimal amplitudes of the data
streams are determined using a comparator on a continuous moving
average of 50 samples. Depending upon the application, different
sample rates can be used.
[0074] According to the invention, both outputs of sensor A are
converted to amplitudes in mV, A.sub.IR and A.sub.Red. The
difference between these two amplitudes A.sub.Diff is
A.sub.Diff=A.sub.IR-A.sub.Red and the average A.sub.Avg of these
two absorbances is A.sub.Avg=(A.sub.IR+A.sub.Red)/2
[0075] Preferably, all calculations are done at all time points of
the chosen pulse. The corresponding generated signals A.sub.Diff
and A.sub.Avg from sensor A and sensor B are shown in FIGS. 5 and
6, respectively.
[0076] The desired signal component is then estimated and
subtracted from the total signal until there is no detectable
oximetry signal detectable in the residual. At this point, the
residual corresponds to the undesirable signal components.
Therefore, the variable, "residual factor f," is used as a
multiplier for A.sub.Avg and the product is subtracted from
A.sub.Diff to obtain corrected absorbance difference, or
A.sub.DiffCorr=A.sub.Diff-(f*A.sub.Avg) which is free of the
optical oximetry signal.
[0077] In the noted embodiment, an average of the infrared and red
signals is used because there is no specific basis for determining
which signal is affected more by undesirable signal components. In
alternative embodiments, different combinations of the two signals
can be used. For example, if it were expected that the red signal
was more affected than the infrared signal a combination that
weighted the red signal more heavily would be desirable.
[0078] The residual factor f is preferably advanced in small
increments such as 0.01 from -1 to +1. As the residual factor is
advanced, at a value that depends on saturation, one can visually
observe the typical oximetry pulse wave signal as a component of
the residual become smaller, go through the remaining residual at
every time point of the A.sub.Diffco, and become the negative
mirror image as it grows in the opposite direction. Thus, there is
a value of f at which the entire measured oximetry component is
removed and the residual is by default the sum of the undesirably
signal components.
[0079] According to the invention, the residual factor f can thus
not only be used for correction of the original waveform, but also
for calibration purposes of the oximeter signal after removal of
undesirable signal component. The residual factor f can
additionally be calibrated against reference CO-oximeter data and
serve as the operating parameter for pulse oximetry.
[0080] According to the invention, it is desirable to determine
residual factor f using data when the oximetry signal has a high
signal to noise ratio. This can be achieved by selecting time
points that correspond to maximal and minimal amplitudes of the
plythesmographic waveform. Thus, averages of amplitude signals of
A.sub.DiffCorr can be calculated at the minimum, shown in FIGS. 5
and 6 between time points 124 and 126, and at the maximum, shown in
FIGS. 5 and 6 between time points 158 and 160. As described above,
the maximal and minimal amplitudes are readily determined using a
comparator. The time mid-point is the arithmetic mean between the
time point at the maximum and the time point at the minimum, ((time
point at max+time point at min)/2).
[0081] Preferably, the first data minimum in a selected pulse is
used, because this minimum occurs more closely to the maximal
amplitude and is therefore less affected by any drift in the
instrument. As shown in FIGS. 5 and 6, the time midpoint occurs at
time point 142. The average signal amplitude is calculated from the
values at time points 141 to 143. In this manner, the amplitudes of
A.sub.DiffCorr are known at the first data minimum, the first data
maximum (later maxima are ignored) and at the data midpoint.
[0082] In other embodiments of the invention, an average of the
first data minimum and a second data minimum can be used. The
second minimum at the end of the current pulse is the lowest
amplitude occurring between the current pulse and the succeeding
pulse. Alternatively, the plethysmographic waveform can have a
relatively long flat portion that corresponds to a minimum value.
In these circumstances, averaging the value of the curve in that
portion can provide a suitable value for the analysis.
[0083] Additionally, baseline adjustment can be used to remove
specific noise prior to the steps described here. Baseline
adjustment is well known in many applications of spectroscopy and
hence, is not further described herein. The beneficial effect of
this additional step is small in most cases.
[0084] According to the invention, it can be more accurate to
identify the time points by curve fitting the maximum and minimum
sections, especially with noisy data. In these alternate
embodiments, polynomial fits over a time-variable section of the
red and infrared data are preferred for determining the exact time
points. As the heart rate is highly variable, a more accurate
approach for finding the optimal time section for fitting can be to
define the fitted section by the time it takes for the amplitude to
change by a significant percentage, e.g., two percent (2%). In such
embodiments, a curve fit through the midpoint can also include more
time points.
[0085] In a preferred embodiment, two qualifiers are defined to
determine residual factor f for any given pulse. The qualifiers are
selected to provide an optimal estimation of the residual based
upon the physiological and physical characteristics of the oximetry
measurement. Preferably, these qualifiers are equal to at least
four decimals, and more preferably, to six or better, such that the
current undesirable signal components dominate the residual
A.sub.DiffCorr.
[0086] Qualifier 1 (Q1) is the difference of the absolute value of
the amplitude of the residual at time midpoint, also referred to as
A.sub.DiffCorrMid and the average of the amplitudes of the
residuals at the first data minimum (A.sub.DiffCorrMin) and the
residual at the first data maximum(A.sub.DiffCorrMax):
Q1=|A.sub.DiffCorrMid-((A.sub.DiffCorrMin+A.sub.DiffCorrMax)/2)|
[0087] Qualifier 2 (Q2) is the absolute difference of the amplitude
at the first data maximum and the amplitude at the first data
minimum: Q2=|A.sub.DiffCorrMin-A.sub.DiffCorrMax|
[0088] In one embodiment, Q1 is employed such that the residual
factor f can be determined from data collected at a minimum of
three discrete timepoints, i.e., the first data maximum, the first
data minimum and the data midpoint. The derived correction can then
be applied to all timepoints of the pulse. In an alternate
embodiment, Q2 is employed.
[0089] Preferably, both Q1 and Q2 are minimized simultaneously to
reflect maximum suppression of the pulse oximetry content in the
residual. Residual factor f is varied until the difference between
the qualifiers approaches zero. For example, f can be set to 1 and
then decreased towards -1 in increments. Depending upon the
computation power of the instruments used, the increment can range
from approximately 0.1 to 0.000001. Preferably, the value for f
that produces the smallest difference between the two qualifiers is
then used to determine the residual at all time points.
[0090] Turning to FIGS. 7 and 8, A.sub.Diff and A.sub.DiffCorr are
shown for the data obtained from sensors A and B, respectively. As
can be seen, common features in the optical signal that correlate
to changes in blood pressure appear in A.sub.Diff, but not in
A.sub.DiffCorr. For example, the pulse maximum occurring around
time point 150 is no longer recognizable in the residual,
A.sub.DiffCorr. Accordingly, the residual corresponds to
undesirable signal components and does not reflect the pulse
oximetry signal.
[0091] As discussed above, in a preferred embodiment of the
invention, residual factor f is chosen by minimizing Q1 and Q2. As
can be seen in FIGS. 7 and 8, A.sub.DiffCorr is not necessarily
flat between the first data minimum and maximum. Thus, minimizing
the difference of the residuals between these points does not
represent an optimal solution. Since there is curvature and slope
in the residual, the deviation of the mean value of the actual
residual from the arithmetic mean needs to be minimized to provide
the best estimation of residual factor f.
[0092] By deriving this residual value, the sum of components that
are unrelated to pulse oximetry are quantitatively determined
within the limits of instrument noise in the data. Corrected
amplitudes can thus be determined by calculating, at all time
points during the pulse, the best estimate of the true oximetry
components as A.sub.IRCorr=A.sub.IR-(A.sub.DiffCorr/2) and
A.sub.RedCorr=A.sub.Red+(A.sub.DiffCorr/2)
[0093] Turning now to FIGS. 9 and 10, the signals A.sub.IRCorr and
A.sub.RedCorr are shown for the data collected from sensors A and
B, respectively. As one having skill in the art will readily
recognize, there may still be low frequency drift of the corrected
red and infrared oximetry signals. Different first and second
minima of different pulses demonstrate this effect. However, this
invention provides predictable oximetry information at every time
point during every pulse.
[0094] The above calculations are performed on data derived from
three discrete timepoints during a specific pulse. The residual
calculated from these timepoints is then applied to each timepoint
of the pulse. In this manner, all the required information is
obtained during a single pulse. In contrast to prior art methods of
improving pulse oximetry signals, there is no requirement to fit
the data to preselected pulse waveforms or to derive a pulse
waveform based on preceding pulses.
[0095] According to the invention, the corrected amplitudes
A.sub.IRCorr and A.sub.RedCorr can be used to calculate a ratio of
logarithms, the principal measurement parameter related to
reference saturation percent for calibrating pulse oximeters.
First, the amplitudes of A.sub.RedCorrMin and A.sub.IRCorrMin,
which represent the equivalent of optical transmittance, are
brought to zero. In one embodiment, this step is performed at the
first data minimum. More preferably, this step is performed at the
average of the first and the second data minimum. The ratio R is
then calculated as the absolute logarithm of the zeroed red
amplitude over the absolute logarithm of the zeroed infrared
amplitude: R=|(log (A.sub.RedCorr-A.sub.RedCorrMin))|/|(log
A.sub.IRCorr-A.sub.IRCorrMin))| The resulting ratio R is then
related to the reference oxygen saturation conventionally.
[0096] Preferably, Q1 and Q2 are tailored for the specific
conditions associated with pulse oximetry and the nature of the
variables that interfere with the acquisition of pure oximetry
signals. Other qualifiers can be developed, modified and/or
adjusted to optimize the determination of residual factor f
depending upon the biological or physiological characteristic being
measured and the nature of the interfering variables. For example,
other analytically derived time points between the maximum and
minimum amplitudes can be used.
EXAMPLES
[0097] The following examples are given to enable those skilled in
the art to more clearly understand and practice the present
invention. They should not be considered as limiting the scope of
the invention, but merely as being illustrated as representative
thereof.
Example 1
[0098] Desaturation studies of seven human subjects were performed
and 53 independent data points corresponding to single pulses were
collected. Values for residual factor f were derived using the
steps described above and reference oxygen saturation was measured
independently by a CO-oximeter instrument. As shown in FIG. 11,
there is a close correlation between residual factor f and oxygen
saturation. Correspondingly, residual factor f can serve as a basis
for calibrating pulse oximeters in place of conventional ratios of
logarithms.
Example 2
[0099] A study comparing oximetry determinations using data
corrected as described above to reference oxygen saturation was
performed with 8 adult volunteers. For the study, a catheter was
placed into a radial artery of each subject. A Nellcor N-200 pulse
oximeter was used as a reference device, and also for clinically
monitoring the subject. Each subject was given varying inspired
concentrations of oxygen in order to produce arterial hemoglobin
oxygen saturations in the approximate range of 70-100%. Blood
samples were drawn from the arterial catheter simultaneously with
readings of oxygen saturation, and immediately analyzed. Data were
collected of both the waveform being analyzed, as well as computed
intermediate steps. The arterial blood sample was analyzed on two
separate blood-gas analyzers by Radiometer. The functional
saturation of hemoglobin was computed as oxyhemoglobin/(total
hemoglobin). That is, all non-oxyhemoglobin species were included
in total hemoglobin. At all saturations and for all human study
subjects, the reference values for the algorithmically computed
values were the average readings from two CO-oximeters.
[0100] FIG. 12 shows the results obtained from 26 independent
measurements of the same pulse with two sensors, for a total of 52
measurements. The corrected and uncorrected data points are plotted
on a graph relating the ratio of logarithms to oxygen saturation.
As can be seen, the corrected signals represent much greater
accuracy. Specifically, the process described above provided a 10.6
fold improvement as compared to the uncorrected values. Even when
discarding the two highest improvement values, a 6.5 fold
improvement is still obtained. Accordingly, in this experiment, the
present invention offers a minimum of a six-fold improvement over
conventional pulse oximetry without the requirement of acquiring
any additional data.
[0101] In additional embodiments, the principles represented by the
present invention can also be applied to a wide variety of other
biological and physiological determinations. For example, U.S. Pat.
Nos. 6,480,729, 6,537,225, 6,594,511, 6,719,705, 6,819,950, and
6,921,367 and U.S. application Ser. No. 10/912,721, filed 04 Aug.
2004, all of which are incorporated in their entirety by reference,
each relate to the acquisition of signals for determining
physiological characteristics and can be practiced with the methods
and apparatus of the present invention.
[0102] Without departing from the spirit and scope of this
invention, one having ordinary skill in the art can make various
changes and modifications to the invention to adapt it to various
usages and conditions. As such, these changes and modifications are
properly, equitably, and intended to be, within the full range of
equivalence of the following claims.
* * * * *