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Description  |
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This invention relates to non-invasive pulse oximetry and specifically to
an improved method and apparatus for calculating arterial saturation
during transient conditions based upon photoelectric determination of a
patient's plethysmograph. This specification is accompanied by a software
appendix.
BACKGROUND OF THE INVENTION
Non-invasive photoelectric pulse oximetry has been previously described in
U.S. Pat. Nos. 4,407,290, 4,266,554, 4,086,915, 3,998,550, 3,704,706,
European Patent Application No. 102,816 published Mar. 13, 1984, European
Patent Application No. 104,772 published Apr. 4, 1984, European Patent
Application No. 104,771 published Apr. 4, 1984, and PCT International
Publication WO86/05674 published October 9, 1986. Pulse oximeters are
commercially available from Nellcor Incorporated, Hayward, Calif., U.S.A.,
and are known as, for example, Pulse Oximeter Model N-100 (herein "N-100
oximeter") and Model N-200 (herein "N-200 oximeter").
Pulse oximeters typically measure and display various blood flow
characteristics including but not limited to blood oxygen saturation of
hemoglobin in arterial blood, volume of individual blood pulsations
supplying the flesh, and the rate of blood pulsations corresponding to
each heartbeat of the patient. The oximeters pass 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 sense the
absorption of light in the tissue. The amount of light adsorbed is then
used to calculate the amount of blood constituent being measured.
The light passed through the tissue is selected to be of one or more
wavelengths that 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 will vary in accordance with
the changing amount of blood constituent in the tissue and the related
light absorption.
For example, the N-100 oximeter is a microprocessor controlled device that
measures oxygen saturation of hemoglobin using light from two light
emitting diodes ("LED's"), one having a discrete frequency of about 660
nanometers in the red light range and the other having a discrete
frequency of about 925 nanometers in the infrared range. The N-100
oximeter microprocessor uses a four-state clock to provide a bipolar drive
current for the two LED's so that a positive current pulse drives the
infrared LED and a negative current pulse drives the red LED to illuminate
alternately the two LED's so that the incident light will pass through,
e.g., a fingertip, and the detected o transmitted light will be detected
by a single photodetector. The clock uses a high strobing rate, e.g., one
thousand five hundred cycles per second, to be easily distinguished from
other light sources. The photodetector current changes in response to the
red and infrared light transmitted in sequence and is converted to a
voltage signal, amplified, and separated by a two-channel synchronous
detector--one channel for processing the red light waveform and the other
channel for processing the infrared light waveform. The separated signals
are filtered to remove the strobing frequency, electrical noise, and
ambient noise and then digitized by an analog to digital converter
("ADC"). As used herein, incident light and transmitted light refers to
light generated by the LED or other light source, as distinguished from
ambient or environmental light.
The light source intensity may be adjusted to accommodate variations among
patients' skin color, flesh thickness, hair, blood, and other variants.
The light transmitted is thus modulated by the absorption of light in the
variants, particularly the arterial blood pulse or pulsatile component,
and is referred to as the plethysmograph waveform, or the optical signal.
The digital representation of the optical signal is referred to as the
digital optical signal. The portion of the digital optical signal that
refers to the pulsatile component is labeled the optical pulse.
The detected digital optical signal is processed by the microprocessor of
the N-100 oximeter to analyze and identify optical pulses corresponding to
arterial pulses and to develop a history as to pulse periodicity, pulse
shape, and determined oxygen saturation. The N-100 oximeter microprocessor
decides whether or not to accept a detected pulse as corresponding to an
arterial pulse by comparing the detected pulse against the pulse history.
To be accepted, a detected pulse must meet certain predetermined criteria,
for example, the expected size of the pulse, when the pulse is expected to
occur, and the expected ratio of the red light to infrared light of the
detected optical pulse in accordance with a desired degree of confidence.
Identified individual optical pulses accepted for processing are used to
compute the oxygen saturation from the ratio of maximum and minimum pulse
levels as seen by the red wavelength compared to the maximum and minimum
pulse levels as seen by the infrared wavelength, in accordance with the
following equation:
##EQU1##
wherein BO1 is the extinction coefficient for oxygenated hemoglobin at
light wavelength 1 (Infrared)
BO2 is the extinction coefficient for oxygenated hemoglobin at light
wavelength 2 (red)
BR1 is the extinction coefficient for reduced hemoglobin at light
wavelength 1
BR2 is the extinction coefficient for reduced hemoglobin at light
wavelength 2
light wavelength 1 is infrared light
light wavelength 2 is red light
and R is the ratio of the optical density of wavelength 2 to wavelength 1
and is calculated as:
##EQU2##
wherein I.sub.max2 is the maximum light transmitted at light wavelength 2
I.sub.min2 is the minimum light transmitted at light wavelength 2
I.sub.max1 is the maximum light transmitted at light wavelength 1
I.sub.min1 is the minimum light transmitted at light wavelength 1
The various extinction coefficients are determinable by empirical study as
are well known to those of skill in the art. For convenience of
calculation, the natural log of the ratios may be calculated by use of the
Taylor expansion series for the natural log.
Several alternate methods of processing and interpreting optical signal
data have been disclosed in the patents and references cited above.
Normally, the relative oxygen content of the patient's arterial pulses
remains about the same from pulse to pulse and the average background
absorption between pulses remains about the same. Consequently, the red
and infrared light that is transmitted through the pulsatile flow produces
a regularly modulated plethysmograph waveform having periodic optical
pulses of comparable shape and amplitude and a steady state background
transmittance. This regular pulse provides for an accurate determination
of the oxygen saturation of the blood based on the detected relative
maximum and minimum transmittance of the red and infrared light.
Changes in the patient's local blood volume at the optical detection site
affect the absorption of light. These localized changes often result from
motion artifact or respiratory artifact which introduce artificial pulses
into the blood flow. For example, on each inhalation, the venus return is
occluded slightly, which results in the background intensity component of
transmittance being decreased due to the relatively larger volume of blood
at the optical detection site. Exhalation allows the venus return to
expand, thereby decreasing the volume of blood and increasing the
background intensity component of transmittance. Consequently, the
periodic optical pulses ride on a background intensity component of
transmittance that rises and falls with blood volume change. This
background intensity component variation, which is not necessarily related
to changes in saturation, affects the pulse to pulse uniformity of shape,
amplitude and expected ratio of the maximum to minimum transmittance, and
can affect the reliability and accuracy of the saturation determination.
In addition, there are times when the patient's background level of oxygen
saturation undergoes transient changes, for example, when the patient
loses or reacquires oxygen exchange in the lungs while under gaseous
anesthesia. Consequently, the detected red and infrared light
transmittance changes and the detected plethysmograph waveform rises or
falls over time with changes in the average oxygen saturation level in the
patient's blood. The transient waveform distorts the pulse shape,
amplitude, and the expected ratio of the pulses, which in turn affects the
reliability and accuracy of the saturation determination.
Heretofore, with the foregoing known techniques for calculating arterial
oxygen saturation, it was known that, during changes in the background
intensity absorption component due to artifacts from changes in the
patient's blood volume or transient saturation changes, the determined
saturation value was not accurate and that it would not become accurate
again until the average absorption (or transmittance) level stabilized at
the end of the artifact or the saturation transient.
It also was known that saturation calculations based upon transient optical
signals provided an over-estimation or under-estimation of the actual
saturation value, depending upon the trend. The transmittance of red light
near the 660 nanometer wavelength increases as oxygen saturation
increases. This results in the detected optical signal value having a
smaller pulsatile amplitude, i.e., a smaller relative difference between
the maximum and minimum of the pulse. In contrast, the transmittance of
the infrared light near the 910 nanometer wavelength decreases as
saturation increases, which causes the infrared pulsatile
amplitude--relative maximum to minimum--to increase. For both wavelengths,
the transmittance changes with changing saturated are substantially linear
and continuous in the range of clinical interest, i.e., oxygen saturations
between 50% and 100%.
The accuracy of the estimation is of particular concern during rapid
desaturation, where average oxygen saturation drops rapidly, but the
saturation determination based on the detected optical signals indicates a
greater drop than has actually occurred. The determined saturation thus
may actuate low limit saturation alarms on an oximeter device that can
result in unnecessary and wasteful efforts to resuscitate a patient not in
danger.
Applicants believe that the change in transmittance that occurs between the
maximum transmittance time and minimum transmittance time is due to the
difference in arterial pulsatile length of pulse that has the same oxygen
saturation. Because the pulsatile amplitude is quite small, typically less
than 5% of the overall intensity change, any small change in overall or
background transmittance, such as slight changes in average blood
saturation, can have a relatively large effect in the difference in
maximum and minimum intensity of the light levels. Because the
transmittance effect of changing oxygen saturation is opposite in
direction for the red light at 660 nanometers than for infrared light at
910 nanometers, this can result in over-estimation of the pulsatile ratio
during periods when saturation is decreasing, and under-estimation during
periods when saturation is increasing.
It is therefore an object of this invention to provide a method and
apparatus for compensating for the effects of transient conditions in the
actual optically detected signal, thereby providing a more accurate
estimation of the actual oxygen saturation value.
It is another object of this invention to compensate for the effects of
distortion in the detected oxygen saturation signal caused by artifacts
due to localized blood volume changes.
It is another object of this invention to compensate for the effects of
distortion in the detected oxygen saturation signal caused by transient
saturation or blood volume artifact by using a determined rate of change
from pulse to pulse, including using interpolation techniques.
It is another object of this invention to compensate for the effects of
distortion in the detected oxygen saturation signal caused by transient
saturation or blood volume artifact by using the low frequency
characteristics of the detected signal values.
SUMMARY OF THE INVENTION
This invention provides a method and apparatus for compensating for the
artifactual errors in light transmittance during blood volume changes or
transient saturation changes (hereinafter collectively referred to as
"transient conditions"), thereby providing for improved accuracy of oxygen
saturation calculations during transient conditions. The invention
provides apparatus for processing the detected optical signals during
transient conditions so that the distortion in transmittance caused by the
transient can be compensated. In one embodiment, the compensation is made
by converting a transient plethysmograph waveform into a steady state
waveform whereby the ratio of the maximum and minimum transmittance can be
determined based on the converted waveform and used in making the
saturation determination. In an alternate embodiment, the compensation is
made by dividing the detected optical signal by its low frequency
components, i.e., the background and transient frequencies below the heart
beat frequency, from which quotient signal the compensated maximum and
minimum transmittance values can be detected and used in making the
saturation determination. Throughout this application, the words
compensate, correct and adjust are intended to have the same meaning in
that the actual detected value is converted to an artificial value that
results in a more accurate estimation of the actual oxygen saturation of
the patient.
In the preferred embodiment, the detected optical signals are obtained
conventionally by passing red and infrared light through a patient's blood
perfused tissue, detecting the transmitted light which is modulated by the
blood flow, and providing red and infrared detected optical signals that
are preferably separately processed and optionally converted from analog
to digital signals. The corresponding red and infrared digital optical
signals are then processed in accordance with the present invention and
the light modulation ratios are determined based on the resulting
corrected transmittance pulse and used to calculate oxygen saturation.
In one embodiment, the transient error is corrected by linear interpolation
whereby the determined maxima and minima for a first and second optical
pulses are obtained, the second pulse following the first and preferably
immediately following the first pulse, and the respective rates of change
in the transmittance of that wavelength is determined from the maximum
transmittance point of the first detected pulse to the second detected
pulse. The determined rates of change are then used to compensate any
distortion in the detected transmittance of the first detected pulse
introduced by the transient in accordance with the following algorithm:
##EQU3##
where tmax(n) is the time of occurrence of the detected maximum
transmittance at the n maximum; tmin(n) is the time of occurrence of the
detected minimum transmittance of the wavelength at the n minimum; Vmax(n)
is the detected optical signal maximum value at the maximum transmittance
of the wavelength at the n maximum; Vmax(n)* is the corrected value, for n
being the first optical pulse, and n+1 being the second optical pulse of
that wavelength.
By application of the foregoing linear interpolation routine, the detected
maximum transmittance value at t=n can be corrected, using the detected
values detected at the next coming pulse t=n+1, to correspond to the
transmittance value that would be detected as if the pulse were detected
at steady state conditions. The corrected maximum value and the detected
(uncorrected) minimum value thus provide an adjusted optical pulse maximum
and minimum that correspond more closely to the actual oxygen saturation
in the patient's blood at that time, notwithstanding the transient
condition. Thus, using the adjusted pulse values in place of the detected
pulse values in the modulation ratio for calculating oxygen saturation
provides a more accurate measure of oxygen saturation than would otherwise
be obtained during transient operation.
In the preferred embodiment, the transient error is corrected by linear
interpolation whereby the determined maxima and minima for a first and
second optical pulses are obtained, the second pulse following the first
and preferably immediately following the first pulse, and the respective
rates of change in the transmittance of that wavelength is determined from
the minimum transmittance point of the first detected pulse to the minimum
of the second detected pulse. The determined rates of change are then used
to compensate for any distortion in the detected minimum transmittance of
the second detected pulse introduced by the transient in accordance with
the following algorithm:
##EQU4##
where tmax(n) is the time of occurence of the detected maximum
transmittance at the n maximum; tmin(n) is the time of occurrence of the
detected minimum transmittance of the wavelength at the n minimum; Vmin(n)
is the detected optical signal minimum value at the minimum transmittance
of the wavelength at the n minimum; Vmin(n)* is the corrected value, for n
being the second optical pulse, and n-1 being the first optical pulse of
that wavelength.
By application of the foregoing linear interpolation routine, the detected
minimum transmittance value at t=n can be compensated, using the detected
values detected at the preceding pulse t=n-1, to correspond to the
transmittance value that would be detected as if the pulse were detected
at steady state conditions. The compensated minimum value and the detected
(uncompensated) maximum value thus provide an adjusted optical pulse
maximum and minimum that correspond more closely to the actual oxygen
saturation in the patient's blood at that time, notwithstanding the
transient condition. Thus, using the adjusted pulse values in place of the
detected pulse values in the modulation ratio for calculating oxygen
saturation provides a more accurate measure of oxygen saturation than
would otherwise be obtained during transient operation.
As is apparent from the algorithms, during steady state conditions the
compensated value is equal to the detected value. Therefore, the linear
interpolation routine may be applied to the detected signal at all times,
rather than only when transient conditions are detected. Also, the
algorithm may be applied to compensate the detected other minimum or
maximum transmittance values by appropriate adjustment of the algorithm
terms.
The amount of oxygen saturation can be then determined from this adjusted
optical pulse signal by determining the relative maxima and minima as
compensated for the respective wavelengths and using that information in
determining the modulation ratios of the known Lambert-Beers equations.
Indeed, the present invention may be applied to any pulsatile flow
detected by light absorption or transmittance corresponding to the flow
having transient changes or conditions, whether based on the occurrence of
individual pulses or averaged pulses.
Applicants also have discovered that the detected optical signals can be
processed and corrected in accordance with the present invention by using
the frequency characteristics of the detected optical signal. The optical
signals for a given wavelength corresponding to the pulsatile arterial
blood flow have spectral components including a zero frequency at the
background transmittance intensity level, a fundamental frequency at the
frequency of the beating heart, and additional harmonic frequencies at
multiples of the fundamental frequency. Noise, spurious signals, and
motion artifact that appear in the detected optical signal have
frequencies that spread across the spectrum. Transient changes to the
background transmittance intensity appear as low frequency signals that
are below the heart rate frequency.
In accordance with an alternate embodiment of the invention, for each of
the wavelengths of the light transmitted, the detected optical signal is
split into two portions. For one of the portions, the frequency domain
corresponding to the frequency components below the range of the measured
heart rate, including the background and any transient frequency
components, is separated from the higher frequency components. Applicants
have discovered that if the first domain is separated so that no phase
shifting occurs relative to the other portion of the unfiltered detected
signal, the first domain signal can be divided into the unfiltered signal,
thereby to correct for changes in the pulsatile amplitude in the
unfiltered signal portion on a continuous basis, for the background
transmittance during steady state conditions, during artifactual blood
volume changes and transient saturation transmittance changes. It may be
appropriate to amplify the separated or filtered signal, the unfiltered
signal, or the resulting quotient signal to obtain an adjusted signal
having an appropriate amplitude and resolution for making the saturation
determination.
Separation of the low frequency components may be realized in either the
time domain or the frequency domain. In the time domain, the separation
may occur by passing one portion of the analog detected optical signal
through conventional electronic circuits such as low pass filters
configured to avoid any phase shifting to obtain a filtered signal having
only the background and low frequency components, and then passing the
filtered signal and a portion of the unfiltered analog detected signal
into dividing amplifiers to divide the low passed signal into the
unfiltered signal in phase. This process results in a compensated optical
signal that can be processed as if it were the actual detected optical
signal to determine the relative maxima and minima of the detected pulses
for the saturation calculations. Alternately, the detected optical signal
may be digitized and processed using digital signal processing techniques
to filter the detected signal and divide the filtered signal into the
unfiltered detected signal.
Digital processing techniques also may be applied to process the detected
optical signal in the frequency domain by the application of well-known
Fourier Transforms. In this embodiment, a time-measure of the detected
optical signal for a predetermined number of heartbeats is collected and
transformed into its spectral components. The frequency components are
then separated into two domains, the first domain including spectral
components below the measured heart rate so that it includes the zero
frequency spectral components of the background intensity and any gradual
changes in the background intensity corresponding to the transient
condition, and the second domain being above the first so that it includes
the spectral components of the fundamental and higher order harmonics of
the fundamental for the number of heartbeats in the sample. The separation
must occur so that no phase shifting occurs in the first domain. Then, the
filtered first domain spectral components can be transformed back into the
time domain, into the background and changing background intensity, and
divided into the unfiltered detected pulsatile waveform in phase thereby
compensating for transient conditions in the unfiltered waveform. As the
time-measure is updated to include the patient's current condition, the
division of the unfiltered waveform by its low frequency components thus
corrects the pulsatile amplitude for changes in the background
transmittance on a continuous basis. Thereafter, the oxygen saturation
calculation can be based upon the compensated quotient waveform.
Similar to the preferred embodiment, this frequency compensation embodiment
may be used all the time.
The apparatus of the preferred embodiment present invention can be used for
either time domain or frequency domain transient correction, and includes
inputs for the detected optical signals, an analog to digital converter
for converting the analog plethysmograph signal to the digital optical
signals (unless the plethysmograph signals are provided in digital form),
and a digital signal processing section for receiving the digital signals
and processing the digital detected optical signal in accordance with one
of the foregoing analysis techniques of the present invention, including a
for controlling the microprocessor, and display devices.
In its context, the apparatus of the present invention is a part of an
oximeter device which has the capability to detect the red and infrared
light absorption. In the preferred embodiment, the apparatus of this
invention is a part of the Nellcor N-200 oximeter which includes a 16 bit
microprocessor manufactured by Intel Corporation, Model No. 8088, software
for controlling the microprocessor to perform the operations of the
preferred embodiment of the time domain analysis techniques of present
invention (in addition to the conventional oximeter functions), and has
structure and processing methods that are unrelated to the present
invention, and therefore are not discussed herein. The software could be
modified to perform the frequency domain analysis techniques of the
present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the apparatus of this invention and the
apparatus associated with the present invention.
FIG. 2 is a detailed circuit schematic of the saturation preamplifier in
the patient module of FIG. 1.
FIGS. 3A and 3B are detailed circuit schematic of the saturation analog
front end circuit of FIG. 1.
FIG. 4 is a detailed circuit schematic of the LED drive circuit of FIG. 1.
FIGS. 5A and 5B are a detailed circuit schematic of the analog to digital
converter section of FIG. 1.
FIGS. 6A, 6B and 6C are a detailed circuit schematic of the digital signal
processing section of FIG. 1.
FIGS. 7a, 7b, 7c, 7d, 7e, and 7f are graphical representations of detected
optical signals during steady state and transient conditions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the preferred embodiment of the present invention
relates to the apparatus for processing the detected analog optical
plethysmograph signal and comprises portions of analog to digital
conversion section ("ADC converter") 1000 and digital signal processing
section ("DSP") 2000, including the software for driving microprocessor
2040, which processes the digitized optical signals in accordance with the
present invention to determine the oxygen saturation of hemoglobin in
arterial blood. Associated with the invention, but not forming a part of
the invention, is the apparatus for obtaining the detected analog optical
signals from the patient that is part of or is associated with the
commercially available Nellcor N-200 Pulse Oximeter. Such apparatus
include plethysmograph sensor 100 for detecting optical signals including
periodic optical pulses, patient module 200 for interfacing plethysmograph
sensor 100 with saturation analog front end circuit 300, and saturation
analog circuit 300 for processing the detected optical signals into
separate red and infrared channels that can be digitized. The N-200
oximeter also includes LED drive circuit 600 for strobing the red and
infrared LEDs in plethysmograph sensor 100 at the proper intensity to
obtain a detected optical signal that is acceptable for processing, and
various regulated power supplies (not shown) for driving or biasing the
associated circuits, as well as ADC 1000 and DSP 2000, from line current
or storage batteries.
The associated elements are straightforward circuits providing specified
functions which are within the skill of the ordinary engineer to design
and build. The associated elements are briefly described here, and
reference is made to the corresponding detailed schematics in the Figures
and circuit element tables set forth below, to place the apparatus of the
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