U.S. patent number RE35,122 [Application Number 07/840,437] was granted by the patent office on 1995-12-19 for method and apparatus for detecting optical pulses.
This patent grant is currently assigned to Nellcor Incorporated. Invention is credited to Andras Boross, Deborah A. Briggs, James E. Corenman, David E. Goodman, Robert T. Stone.
United States Patent |
RE35,122 |
Corenman , et al. |
December 19, 1995 |
Method and apparatus for detecting optical pulses
Abstract
A method and apparatus for improving the calculation of oxygen
saturation and other blood constituents by non-invasive pulse
oximeters. The method and apparatus permit more accurate
determination of blood flow by collecting time-measures of the
absorption signal at two or more wavelengths and processing the
collected time-measure to obtain composite pulsatile flow data from
which artifacts have been filtered. The processing may occur in the
time domain or in the frequency domain. In the preferred time
domain embodiment, successive portions of periodic information are
weighted and added together in synchrony to obtain the composite
pulse information. In the preferred frequency domain embodiment,
the time-measure is Fourier transformed into its spectral
components to form the composite information. A new method and
apparatus for correlating the heartbeat and optical pulse is
provided whereby a product of the ECG R-wave and optical pulse
signals corresponding to the same heartbeat is obtained, and one
signal is time shifted relative to the other until a maximum
waveform product corresponding to the heartbeat is determined.
Inventors: |
Corenman; James E. (Oakland,
CA), Stone; Robert T. (Sunnyvale, CA), Boross; Andras
(Fremont, CA), Briggs; Deborah A. (San Ramon, CA),
Goodman; David E. (San Francisco, CA) |
Assignee: |
Nellcor Incorporated
(Pleasanton, CA)
|
Family
ID: |
27390501 |
Appl.
No.: |
07/840,437 |
Filed: |
February 24, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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742720 |
Jun 7, 1985 |
4802486 |
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718525 |
Apr 1, 1985 |
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Reissue of: |
175152 |
Mar 30, 1988 |
04911167 |
Mar 27, 1990 |
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Current U.S.
Class: |
600/324; 356/41;
600/326; 600/500; 600/521 |
Current CPC
Class: |
A61B
5/14551 (20130101); A61B 5/7203 (20130101); A61B
5/7207 (20130101); A61B 5/7257 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61B 005/00 () |
Field of
Search: |
;128/632-633,637,667-668,670,671,687-690,696,700,706,708 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102816 |
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Mar 1984 |
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EP |
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104722 |
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Apr 1984 |
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EP |
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104771 |
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Apr 1984 |
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EP |
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0104771 |
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Apr 1984 |
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EP |
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2089999 |
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Jun 1982 |
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GB |
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Other References
Goodlin et al., "Systolic Time Intervals in the Fetus and Neonate
Obstetrics and Gynecology", vol. 34, p. 295 (Feb. 1972). .
Goodlin, Care For The Fetus, p. 101 (Masson, 1979). .
Schotz et al., "The Ear Oximeter as a Circulating Monitor,"
Anesthesiology, vol. 19, p. 386 (1958). .
Cohen et al., "Self-Balancing System for Medical and Physiological
Instrumentation," IEEE Trans. Bio-Med. Eng., vol. BME-18, p. 66,
(1971). .
Goodlin, "Interpartum Fetal Heart Rate Responses and
Plethysmographic Pulse", Amer. J. Obstet. Gynec., vol. 110, p. 210
(1971)..
|
Primary Examiner: Cohen; Lee S.
Assistant Examiner: Nasser, Jr.; Robert L.
Attorney, Agent or Firm: Townsend and Townsend and Crew
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application of copending
and commonly assigned U.S. application Ser. No. 742,720, now U.S.
Pat. No. 4,802,486, entitled Improved Method and Apparatus For
Detecting Optical Pulses, filed Jun. 7, 1985 in the names of James
E. Corenman and David E. Goodman, which is a continuation of U.S.
application Ser. No. 718,525, entitled Improved Method and
Apparatus For Detecting Optical Pulses, filed Apr. 1, 1985 in the
names of James E. Corenman and David E. Goodman, now abandoned.
This invention relates to non-invasive pulse oximetry and
specifically to an improvement on the method and apparatus for
photoelectric determination of blood constituents disclosed in U.S.
applications Ser. No. 742,720 and 718,525. This specification is
accompanied by software appendices A and B.
Claims
We claim:
1. A method for calculating .[.the.]. .Iadd.an .Iaddend.amount of a
blood constituent from the blood flow characteristics of a patient
comprising:
detecting an absorption signal corresponding to .[.the.].
absorption of light measured at two or more wavelengths in the
patient's tissue including periodic changes caused by periodic
arterial pulses in the blood flow characteristics and aperiodic
changes unrelated to the patient's heartbeat;
detecting an ECG signal corresponding to the patient's ECG waveform
corresponding to the periodic electrical heart activity of the
patient;
selecting a first time-measure of the absorption signal including a
maximum and minimum amplitude change in absorption;
selecting a second time-measure of the ECG signal including
components corresponding to the occurrence of the heartbeat;
correlating the ECG signal and the absorption signal by multiplying
the first time-measure of the absorption signal and the second
time-measure of the ECG signal together to form a waveform product
and thereafter shifting the time-measure of the absorption signal
backwards in time relative to the time-measure of the ECG signal
and multiplying the shifted and unshifted waveforms together to
identify a maximum waveform product corresponding to the product of
the ECG signal corresponding to the occurrence of the heartbeat and
the maximum amplitude of the absorption signal;
processing the absorption signal and the determined correlation to
identify the periodic changes in the absorption signal associated
with the determined maximum waveform product likely to correspond
to arterial pulses in the patient's blood flow characteristics;
and
calculating the amount of the blood constituent from the identified
periodic changes in the absorption signal.
2. The method of claim 1 wherein the ECG signal further comprises
the R-wave component of the ECG signal so that the maximum waveform
product corresponds to the product of the R-wave component and the
maximum of the absorption signal.
3. The method of claim 2 wherein the first time-measure is shorter
in length than the second time-measure so that as the absorption
signal time-measure is shifted backward relative to the ECG
time-measure the latest maximum waveform product corresponds to the
most recent heartbeat activity in the first and second
time-measures.
4. The method of claim 3 wherein the first time-measure includes a
first plurality of maximum amplitudes and the second time-measure
includes a second plurality of R-wave signals so that the maximum
waveform product further comprises an array of maximum waveform
product.
5. Apparatus for calculating .[.the.]. .Iadd.an .Iaddend.amount of
a blood constituent from the blood flow characteristics of a
patient comprising:
means for photoelectrically detecting an absorption signal
corresponding to .[.the.]. absorption of light measured at two or
more wavelengths in the patient's tissue including periodic changes
caused by periodic arterial pulses in the blood flow
characteristics and aperiodic changes unrelated to the patient's
heartbeat;
means for electrically detecting an ECG signal corresponding to the
patient's ECG waveform corresponding to the periodic electrical
heart activity of the patient;
means for selecting a first time-measure of the absorption signal
including a maximum and minimum amplitude change in absorption;
means for selecting a second time-measuring of the ECG signal
including components corresponding to the occurrence of the
heartbeat;
means for correlating the ECG signal and the absorption signal by
multiplying the first time-measure of the absorption and the second
time-measure of the ECG signal together to form a waveform product
and thereafter shifting the time-measure of the absorption signal
backwards in time relative to the time-measure of the ECG signal
and multiplying the shifted and unshifted waveforms together to
identify a maximum waveform product corresponding to the product of
the ECG signal corresponding to the occurrence of the heartbeat and
the maximum amplitude of the absorption signal;
means for processing the absorption signal and the determined
correlation to identify the periodic changes in the absorption
signal associated with the determined maximum waveform product
likely to correspond to arterial pulses in the patient's blood flow
characteristics; and
means for calculating the amount of the blood constituent from the
identified periodic changes in the absorption signal.
6. The apparatus of claim 5 wherein the means for electrically
detecting an ECG signal further comprises means for detecting the
R-wave component of the ECG signal so that the maximum waveform
product corresponds to the product of the R-wave component and the
maximum of the absorption signal.
7. The apparatus of claim 6 wherein the first time-measure is
shorter in length and later in time than the second time-measure so
that as the absorption signal time-measure is shifted backwards
relative to the ECG time-measure, the latest maximum waveform
product corresponds to the most recent heartbeat activity in the
first and second time-measures.
8. The apparatus of claim 7 wherein the first time-measure includes
a first plurality of maximum amplitudes and the second time-measure
includes a second plurality of R-wave signals so that the maximum
waveform product further comprises an array of maximum waveform
products.
9. A method for calculating .[.the.]. .Iadd.an .Iaddend.amount of a
blood constituent from a patient's blood flow characteristics
including aperiodic information corresponding to artifacts and
periodic information corresponding to arterial pulses,
comprising:
.Iadd.directing light of two or more wavelengths toward the
patient's tissue; .Iaddend.
detecting a set of first signals representing .[.the.]. absorption
of light measured at .Iadd.said .Iaddend.two or more wavelengths
.[.of light.]. in the patient's tissue including detected
information corresponding to changes in absorption as a result of
changes in the blood flow characteristics;
detecting a second signal representing the occurrences of a
selected portion of .[.the.]. .Iadd.an .Iaddend.ECG waveform of the
patient;
processing the first and second signals to enhance .[.the.].
periodic information contained in each individual signal whereby
the first signals are processed in a repetitive manner, one
discrete portion at a time, and the occurrence of a selected
portion of the patient's ECG waveform in the second signal
initiates processing of a discrete portion of the first signals;
and
calculating the amount of .Iadd.said .Iaddend.blood constituent
using the enhanced periodic information in the processed portions
of the first signals.
10. The method of claim 9 wherein processing the first and second
signals further comprises correlating the discrete portions of the
first signals and the occurrences of .[.a.]. .Iadd.said
.Iaddend.selected portion of the patient's ECG waveform in the
second signals.
11. The method of claim 10 wherein detecting the second signal
further comprises detecting a signal representing the occurrences
of the R wave portion of the ECG waveform of the patient.
12. The method of claim 11 wherein processing the first and second
signals further comprises determining whether or not the detected
information in the .Iadd.discrete .Iaddend.portions of the first
signals are likely to be arterial pulses by using the determined
correlation and preferentially processing detected information
determined likely to be arterial pulses for use in calculating
.Iadd.the amount of said .Iaddend.blood .[.constituents.].
.Iadd.constituent.Iaddend..
13. The method of claim 12 wherein preferentially processing
detected information further comprises rejecting detected
information determined not likely to be arterial pulses so that
rejected information is not used in calculating the blood
constituent.
14. The method of claim 12 wherein processing the first and second
signals further comprises determining whether or not detected
information in a discrete portion of the first signals is likely to
be an arterial pulse by using the determined correlation after each
occurrence of an R wave portion in the second signal.
15. The method of claim 11 wherein correlating the first and second
signals further comprises:
determining a period of time after the occurrence of .[.an.].
.Iadd.said .Iaddend.R wave in which detected information in the
.Iadd.discrete .Iaddend.portions of the first signals corresponding
to blood flow characteristic changes caused by arterial pulses are
likely to be detected;
determining that detected signal information in the discrete
portions of the first signals are likely to be arterial pulses when
detected in the determined period of time after the occurrence of
.[.an.]. .Iadd.said .Iaddend.R wave; and
calculating the .Iadd.amount of said .Iaddend.blood
.[.constituents.]. .Iadd.constituent .Iaddend.further comprises
preferentially processing any determined detected information for
use in calculating .Iadd.the amount of said .Iaddend.blood
.[.constituents.]. .Iadd.constituent.Iaddend..
16. The method of claim 15 wherein preferentially processing
detected information further comprises rejecting detected
information detected other than in the determined period of time
after the R wave occurs so that rejected information is not used in
the calculation of .Iadd.the amount of said .Iaddend.blood
.[.constituents.]. .Iadd.constituent.Iaddend..
17. The method of claim 15 wherein determining whether or not
detected information in a discrete portion of the first signals is
acceptable for preferential processing as corresponding to an
arterial pulse by using the determined correlation after each
occurrence of an R wave portion in the second signal.
18. The method of claim 11 wherein calculating the amount of
.Iadd.said .Iaddend.blood constituent further comprises calculating
oxygen saturation of hemoglobin in arterial blood.
19. The method of claim 10 wherein correlating the first and second
signals further comprises:
synchronizing the occurrence of a plurality of discrete portions of
the first signals;
synchronizing the occurrence of a plurality of occurrences of the R
wave portion in the second signal; and
correlating the synchronized discrete portions of the .[.firs.].
.Iadd.first .Iaddend.signals and the R wave portions.
20. Apparatus for calculating .[.the.]. .Iadd.an .Iaddend.amount of
a blood constituent from a patient's blood flow characteristics
including arterial pulses and artifacts comprising:
means for photoelectrically detecting a set of first signals
representing the absorption of light measured at two or more
wavelengths in the patient's tissue including detected information
corresponding to changes in absorption as a result of changes in
the blood flow characteristics;
means for detecting a second signal representing the occurrences of
a selected portion of .[.the patient's.]. .Iadd.an .Iaddend.ECG
waveform .Iadd.of the patient.Iaddend.;
means for processing the first and second signals to enhance the
periodic information contained in each individual signal whereby
each occurrence of .[.a.]. .Iadd.said .Iaddend.selected portion in
the second signal initiates processing of .[.a.]. discrete portions
of the first signals so that the information in the first signals
are processed in a repetitive manner; and
means for calculating the amount of .Iadd.said .Iaddend.blood
constituent using the enhanced information in the processed
portions of the first signals.
21. The apparatus of claim 20 wherein the means for processing the
first and second signals further comprises means for correlating
the discrete portions of the first signals with the occurrences of
.[.a.]. .Iadd.said .Iaddend.selected portion of the patient's ECG
waveform in the second signal.
22. The apparatus of claim 21 wherein the means for detecting a
second signal further comprises means for detecting a signal
corresponding to the occurrences of the R wave portion of the
patient's ECG waveform.
23. The apparatus of claim 21 wherein the processing means further
comprises:
means for determining whether or not the detected information in
the discrete portions of the first signals are likely to be
arterial pulses using the determined correlation; and
means for preferentially processing detected information determined
likely to be arterial pulses for use in calculating .Iadd.the
amount of said .Iaddend.blood .[.constituents.].
.Iadd.constituent.Iaddend..
24. The apparatus of claim 23 wherein the means for preferentially
processing detected information further comprises means for
rejecting detected information determined not likely to be arterial
pulses so that rejected information is not used in calculating
.Iadd.the amount of said .Iaddend.blood .[.constituents.].
.Iadd.constituent.Iaddend..
25. The apparatus of claim 23 wherein the processing means further
determines whether or not detected information in a discrete
portion of the first signals is likely to be an arterial pulse
using the determined correlation after each occurrence of an R wave
portion in the second signal.
26. The apparatus of claim 21 wherein the means for correlating the
first and second signals further comprises:
first determining means for determining a time period after the
occurrence of an R wave in which detected information in the first
signals corresponding to blood flow characteristic changes caused
by arterial pulses are likely to be detected;
second determining means for determining that detected information
in the discrete portions of the first signals are likely to be
arterial pulses when detected in the determined period of time
after the occurrence of .[.an.]. .Iadd.said .Iaddend.R wave;
and
wherein the means for calculating .Iadd.the amount of said
.Iaddend.blood constituent further comprises means for
preferentially processing detected information for use in
calculating .Iadd.the amount of said .Iaddend.blood
.[.constituents.]. .Iadd.constituent.Iaddend..
27. The apparatus of claim 26 wherein the second determining means
further comprises means for rejecting detected information detected
other than in the determined period of time after the R wave occurs
so that rejected information is not used in calculating .Iadd.the
amount of said .Iaddend.blood .[.constituents.].
.Iadd.constituent.Iaddend..
28. The apparatus of claim 26 wherein the second determining means
further comprises means for determining whether or not detected
information in a discrete portion of the first signals is likely to
be an arterial pulse using the determined correlation after each
occurrence of an R wave.
29. The apparatus of claim 21 wherein the means for calculating
further comprises calculating the amount of oxygen saturation of
hemoglobin in arterial blood. .Iadd.
30. A method of calculating an amount of a blood constituent from
the blood flow characteristics of a patient comprising:
subjecting the patient's tissue to electromagnetic energy, thereby
facilitating the transformation of arterial blood flow in the
patient's tissue into a blood flow signal, the blood flow signal
corresponding to the arterial blood flow including periodic changes
caused by periodic arterial pulses in the blood flow
characteristics and changes caused by artifact;
detecting the blood flow signal;
detecting the occurrence of a heartbeat of the patient;
correlating the detected blood flow signal and the occurrence of a
heartbeat;
processing the blood flow signal and the determined correlation to
identify the periodic changes in the blood flow signal likely to
correspond to arterial pulses in the patient's blood flow
characteristics; and
calculating the amount of the blood constituent from the identified
periodic changes in the blood flow signal. .Iaddend. .Iadd.
31. The method of claim 30 wherein said blood constituent is the
oxygen saturation of hemoglobin in arterial blood. .Iaddend.
.Iadd.
32. The method of claim 31 wherein said step of detecting the blood
flow signal comprises detecting an absorption signal corresponding
to the absorption of the electromagnetic energy measured at two or
more wavelengths in the patient's tissue. .Iaddend. .Iadd.
33. The method of claim 31 wherein said correlating step comprises
the steps of:
synchronizing the occurrence of a plurality of changes in the blood
flow signal;
synchronizing the occurrences of a plurality of selected portions
of the heartbeat; and
correlating the synchronized changes in the blood flow signal with
the synchronized portions of the heartbeat. .Iaddend. .Iadd.
34. The method of claim 30 wherein said blood constituent is the
oxygen saturation of hemoglobin in arterial blood;
said step of detecting the blood flow signal comprises detecting an
absorption signal corresponding to the absorption of the
electromagnetic energy measured at two or more wavelengths in the
patient's tissue; and
said correlating step comprises the steps of
synchronizing the occurrence of a plurality of changes in the blood
flow signal,
synchronizing the occurrences of a plurality of selected portions
of the heartbeat, and
correlating the synchronized changes in the blood flow signal with
the synchronized portions of the heartbeat. .Iaddend.
Description
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, and European patent application No. 104,771
published Apr. 4, 1984. 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").
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 absorbed 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 or 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 process by the
microprocessor the N-100 oximeter to analyze and identify 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.
Several alternate methods of processing and interpreting optical
signal data have been disclosed in the patents and references cited
above.
A problem with non-invasive pulse oximeters is that the
plethysmograph signal and the optically derived pulse rate may be
subject to irregular variants in the blood flow, including but not
limited to motion artifact, that interfere with the detection of
the blood flow characteristics. Motion artifact is caused by the
patient's muscle movement proximate to the oximeter sensor, for
example, the patient's finger, ear or other body part to which the
oximeter sensor is attached, and may 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. This problem is particularly
significant with infants, fetuses, or patients that do not remain
still during monitoring.
A second problem exists in circumstances where the patient is in
poor condition and the pulse strength is very weak. In continuously
processing the optical data, it can be difficult to separate the
true pulsatile component from artifact pulses and noise because of
a low signal to noise ratio. Inability to reliably detect the
pulsatile component in the optical signal may result in a lack of
the information needed to calculate blood constituents.
It is well known that electrical heart activity occurs
simultaneously with the heartbeat and can be monitored externally
and characterized by the electrocardiogram ("ECG") waveform. The
ECG waveform, as is known to one skilled in the art, comprises a
complex waveform having several components that correspond to
electrical heart activity. The QRS component relates to ventricular
heart contraction. The R wave portion of the QRS component is
typically the steepest wave therein, having the largest amplitude
and slope, and may 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, where it is shown that the scalp pulse of fetuses
lag behind the ECG "R" wave by 0.03-0.04 second, and U.S. Pat. NO.
3,734,086.
In prior U.S. application Ser. No. 742,720, copending and commonly
assigned, the disclosure (including the software appendix, of which
is hereby expressly incorporated by reference, and in corresponding
International PCT Application publication No. WO 86/05674 published
Oct. 9, 1986, also commonly assigned, there is disclosed an
invention for measuring the patient's heart activity and
correlating it with the patient's detected blood flow signal to
calculate more accurately the patient's oxygen saturation and pulse
rate. The correlation includes auto- and cross correlation
techniques to enhance the periodic information contained in each
individual waveform as well as determine that time relationship of
one waveform to another.
Correlating the occurrence of cardiovascular activity with the
detection of arterial pulses occurs by measuring an ECG signal,
detecting the occurrence of the R-wave portion of the ECG signal,
determining the time delay by which an optical pulse in the
detected optical signal follows the R-wave, and using the
determined time delay between an R-wave, and using the determined
time delay between an R-wave and the following optical pulse so as
to evaluate arterial blood flow only when it is likely to present a
true blood pulse for waveform analysis. The measured time delay is
used to determine a time window when, following the occurrence of
an R-wave, the probability of finding an optical pulse
corresponding to a true arterial pulse is high. The time widow
provides an additional criterion to be used in accepting or
rejecting a detected pulse as an optical pulse. Any spurious pulses
caused by motion artifact or noise occurring outside of that tie
window are typically rejected and are not used to calculate the
amount of blood constituent. Correlating the ECG with the detected
optical pulses thereby provided for more reliable measurement of
oxygen saturation.
That application and publication refers to a modified N-100
oximeter (the "enhanced N-100 oximeter") whereby the device is
provided with an additional heart activity parameter in the form of
a detected R-wave from the patient's ECG waveform, in addition to
the N-100 pulse oximeter functions, and the microprocessor is
modified to include software and memory for controlling and
processing the optical signal and heart activity information.
The additional heart activity parameter is independent of the
detection of peripheral arterial pulses, e.g., ECG signals,
ultrasound, ballistocardiogram, and maybe, accelerometers, nuclear
magnetic resonators, electrical impedance techniques, and the like,
and provides an identifiable and detectable signal to response to
each heartbeat for use by the signal processing of the
oximeter.
It is an object of this invention to provide for improved
processing of the detected optical signal containing periodic
information corresponding to arterial pulsatile blood flow and
aperiodic information corresponding to noise, spurious signals, and
motion artifact unrelated to the beating heart and arterial
pulsatile blood flow, to improve further the reliability and
accuracy of the determination of blood constituent, particularly
oxygen saturation of hemoglobin by a non-invasive oximeter
device.
It is another object of this invention to provide an improved
method and apparatus for collecting successive portions of detected
optical signals encompassing periodic information for more than one
heartbeat and processing the collected portions to attenuate and
filter therefrom aperiodic signal waveforms to provide enhanced
periodic information from which the patient's blood constituent can
be accurately determined.
It is another object to maintain the enhanced periodic information
updated by continuing to add new portions of detected optical
signals as they are obtained. It is another object of this
invention to create enhanced periodic information by collecting and
processing successive portions of detected optical signals wherein
the periodic information corresponding to the optical pulses have
been added together in phase, synchronized to the occurrence of the
patient's ECG and preferably the R-wave signal.
It is another object of this invention to add synchronized periodic
information in a weighted fashion so that the most recent portion
of detected optical signal is accorded a greater weight in the
collected sum than any one prior portion of periodic information
data.
It is another object of this invention to create the enhanced
periodic information by adding together a predetermined number of
the most recent successive portions of detected optical signal,
whereby each portion corresponds to a heartbeat event and is given
a weight according to its relative age so as to emphasise the
newest information in the resultant weighted collective sum.
It is another object of this invention to correlate the periodic
information with the ECG R-wave by using a waveform product
technique to identify the occurrence of the heartbeat and the
optical pulse corresponding to that heartbeat.
It is another object of this invention to evaluate the collect
periodic information for a predetermined number of successive
portions of the detected optical signal corresponding to a
predetermined number of heartbeats in the frequency domain to
obtain enhanced periodic information.
It is another object of this invention to Fourier transform a
time-measure of detected optical signals including periodic
information for N heartbeats to determine the relative maxima at
the fundamental frequency N and the minima at the zero frequency
for use in determining the light modulation ratio for the amount of
blood constituents.
It is another object of this invention to correlate the Fourier
Transform of the time-measure of detected optical signals with the
Fourier Transform of a time-measure of the ECG signal, and more
particularly the R-wave events of the ECG signal, to determine the
maxima at the fundamental heart frequency.
It is another object of this invention to correlate the periodic
information in a time-measure of the detected optical signal with a
time-measure of the detected heart activity, preferably in the form
of the ECG signal and more preferably in the form of the R-wave of
the ECG signal, to define a predetermined number of samples in a
data set and use frequency domain analysis techniques to evaluate
the collected predetermined number of sample data sets to determine
the relative maxima at the fundamental frequency.
SUMMARY OF THE INVENTION
This invention provides enhanced periodic information with improved
rejection of noise, spurious pulses, motion artifact, and other
undesired aperiodic waveforms and thereby improves the ability of
oximeters to accurately determine amounts of blood
constituents.
The present invention provides methods and apparatus for collecting
a time-measure of the detected optical signal waveform containing a
plurality of periodic information corresponding to arterial pulses
caused by the patient's heartbeat and aperiodic information
unrelated to pulsatile flow, and processing the collected
time-measure of information to obtain enhanced periodic information
that is closely related to the most recent arterial pulsatile blood
flow. The time-measure may comprise a continuous portion of
detected optical signals including a plurality of periodic
information from successive heartbeats, or a plurality of discrete
portions of detected optical signals including a corresponding
plurality of periodic information.
By updating the time-measure of information to include the most
recently detected periodic information, and processing the updated
measure collectively, an updated enhanced periodic information is
obtained (including the new and historical data) from which
aperiodic information (including any new aperiodic information is
attenuated. In some embodiments, the updating process includes
subtracting detected signals older than a certain relative time
from the collected time-measure. Applicants have discovered that by
collectively processing a time-measure including successive
periodic information to obtain the enhanced periodic information,
and using the enhanced periodic information as the basis for making
oxygen saturation calculations, the accuracy and reliability of
oxygen saturation determinations can be significantly increased.
Applicants also have discovered that the time-measures may be
collectively processed in either the time domain or the frequency
domain.
The amount of a blood constituent, for example, oxygen saturation,
can be then determined from this enhanced periodic information
(also referred to as composite signal information) by determining
the relative maxima and minima in the enhanced periodic information
for the respective wavelengths for use in determining the
modulation ratios of the known Lambert-Beers equations.
In the preferred embodiment, the detected optical signals are
conventionally obtained by passing red (660 nanometers) and
infrared (910 nanometers) 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, for example, as described above for
the Nellcor N-100 oximeter. Portions of the corresponding red and
infrared digital signals are then collectively processed in
accordance with the present invention and the light modulation
ratios are determined based on the resulting enhanced periodic
information and used to calculate oxygen saturation.
In the time domain analysis embodiment, the invention provides a
method and apparatus for adding together a plurality of successive
portions of the detected optical signal waveform whereby one
portion of the detected optical signal waveform is added to the
following selected portion so that their respective periodic
information is added in synchrony, i.e., in phase. The synchronized
sum thus forms a composite portion of detected optical signal
information having enhanced periodic information. The following
portion is then added to the composite portion so that the new
periodic information is added to the prior composite periodic
information in synchrony, forming an updated composite portion with
update enhanced periodic information. Thereafter, subsequent
successive portions of detected optical signal are added to the
prior updated composite portion, one at a time, so that the
composite and enhanced periodic information are update with each
new portion and corresponding heartbeat event.
Weighting functions are applied to the two portions before they are
added each other. This provides a scaled or weighted sum that can
be adjusted, by selection of the respective weighting functions, to
more closely reflect the patient's current condition, rather than
the historical condition. In the preferred embodiment, the
weighting functions are fractional multipliers which sum to one to
provide a stable filter, and are discussed in greater detail
below.
The periodic information (optical pulse) generally has the same
pulse shape, height, and duration from heartbeat to heartbeat and,
as is described in U.S. Ser. No. 742,720, follows heart activity by
a determinable period of time.
Applicants have discovered that by synchronizing the occurrence of
successive R-waves, it becomes possible to add the corresponding
successive portions of the detected optical signal together so that
the periodic information (optical pulses) corresponding to the
arterial pulse in each portion will add in phase. The weighted
magnitude of the new periodic information is reinforced by the
existence of the weighted enhanced periodic information at the same
time location in accordance with the degree of synchrony. If the
new optical pulse is identical to the composite pulse, then the
updated result is a composite optical pulse having the same
magnitude. If the magnitudes differ, the additive result will
differ according to the relative weights.
As a result of the collected, synchronized additive process, any
aperiodic information that may be present in the portions of the
detected optical signals also are weighted and added to the
weighted composite portion waveform. However, because aperiodic
signals differ in pulse shape, duration, height-, and relative time
of occurrence within each portion, and are not synchronous with
heart activity, they do not add in phase. Rather, they add in a
cancelling manner whereby their weighted sum is spread across the
relative time frame of the composite portion.
Applicants have discovered that by processing portions including
the periodic information collectively, aperiodic information is
attenuated by the absence of any corresponding historical aperiodic
signal in the prior composite portion or any subsequent aperiodic
at that relative time following heart activity. Further, because
the new information can be given a small weight when compared to
the absolute weight given the prior composite (as distinguished
from the effective lesser weight given to any single prior portion
of optical signals as explained below) new aperiodic information is
quickly and effectively attenuated, and thus filtered out of the
resultant additive portions.
To the extent that any aperiodic information would overlap and
thereby obscure some periodic information in a portion, then that
aperiodic information would be reinforced by the existing periodic
information in the prior composite portion; but only to the extent
there was overlap. Thus, the collective processing does not lose
optical pulse information hidden by an artifact. Subsequent
periodic information lacking "identical" aperiodic information
would attentuate any overlapping aperiodic pulse over time.
The collective additive sum having synchronized periodic
information waveform thus presents enhanced periodic information
that is a composite data set that corresponds to a composite
optical pulse from which noise, spurious signals, and motion
artifact, have been filtered out. By weighting the collective
additive process to favor the most recent information and
processing this weighted composite portion as it is update, an
accurate estimated optical pulse (enhanced periodic information)
that closely reflects the actual conditions is maintained. Basing
oxygen saturation determinations on this enhanced optical pulse as
it is updated thus provides a more accurate measure than was
available by conventional and prior processing techniques.
As discussed in application Ser. No. 742,720, the determinable time
period between the R-wave and the optical pulse makes it possible
to determine a time window whose time length is long enough to
include any likely periodic information, and short enough to
exclude detected optical signals that are not of any significant or
clinical use in making the determination of the selected blood
constituent. A time window can be used in the present invention,
following the occurrence of heart activity, to select a portion of
detected optical signals for processing in accordance with this
invention to reduce the amount of detected optical signal
information that must be processed, to improve the rejection of
aperiodic signals not proximate to the optical pulse, and to
improve the resolution of the oximeter. The timing of the portion
can be selected empirically, by considering the time length of the
heartbeat pulse and how long it takes for the pulse to travel to
the optiCal detection site so that the window is opened before the
optiCal pulse maximum occurs at the optical detection site. In the
preferred embodiment, the portion of signal is portion that begins
40 ms after the detection of an R-wave event, based on
experimentation, and ends after the relative minimum of the optical
pulse is detected, which ending time can vary from portion to
portion, and may be, for example, about 230 ms after the R-wave
event.
The time domain processing of collective weighted portions of the
detected optical signal waveforms synchronized by the R-wave of the
ECG waveform provides the equivalent of an optimal filter in the
frequency domain, whose band-pass elements are those of an ideal
heartbeat for the patient under examination. All frequencies which
are found in a normal heartbeat are passed with weights of one, and
all nonsynchronous frequencies are rejected with attenuation
depending on the degree of asynchrony, and the time length of the
filter (the effective number of portions processed collectively).
As the weight of the periodic information corresponding to the
current heartbeat is decreased, greater rejection of the
low-frequency aperiodic artifacts occurs, but the delay in
reporting the most accurate arterial pulsatile flow increases.
The weighting functions also assure that the new periodic
information is not absorbed into the time and amplitude average of
the old data. Using fractional weights provides scaling of the new
and old composite information sum, and when the fractional weights
add to one, stable performance of the filter is assured. Repeated
multiplication of the old data by weights less than one accomplish
the effective removal of older data, thus limiting in effect the
number of periods processed collectively.
In the preferred embodiment, the detected optical signal
information is processed in digitized form. Because the successive
digitized information is weighted and added, the amount of digital
computer memory required to contain the historical and updated
composite periodic information only need be as long as the time
period for a relative typical heartbeat, so that it can contain the
entire time for a selected portion including an optical pulse. This
simplified oximeter operation.
In the preferred embodiment, applicants have found that optimal
performance occurs when the most recent information is accorded a
weight of 1/6 and the historical weight-averaged composite
information is accorded a weight of 5/6. Weights which are in
powers of 2, e.g., 1/2, 1/4, 1/8, etc., are attractive to use with
binary digital computers because they require simpler mathmatical
operations, however, they do not necessarily provide the optimal
time and noise attenuation tradeoff in selecting weighting
functions.
The resultant enhanced periodic information is a weighted composite
optiCal pulse that is evaluated in the same manner that prior
oximeters evaluated individual pulses they determined were
appropriate optical pulses for determining blood constituents,
whether or not the criteria included use of a time window. The
relative maxima and minima for each of the red and infrared
composite optical pulses are separately determined and used in the
modulations ratios for determining amounts of blood constituents,
e.g., in the modulation ratio R of the Lambert-Beers equations that
are commonly used to determine oxygen saturation of arterial
hemoglobins as described below. As additional data sets are taken,
the collective set of periodic information is updated.
Consequently, the most recent waveform data representing the actual
amount of blood constituent is included in the updated composite
optical pulse form which the updated oxygen saturation can be
determined and displayed. Although the foregoing and following
discussions generally discuss only a detected optical signal, if
should be understood that both the red and infrared signal are
separately obtained and processed by these techniques, except as
indicated.
In another embodiment of the time domain embodiment, the
time-measure of the detected optical signal is collected in a
different manner. The digitized portions of information that are to
be weighted, synchronously added together, and processed
collectively are accumulated in a memory device having sufficient
memory locations for storing separately the raw data for a
predetermined number of portions of the detected optical signal.
The time of occurrence of the R-wave also may be stored in memory
as a pointer for the raw data. This filter embodiment permits
assigning a different weighting function to each raw data set
corresponding to a different heartbeat in the memory, to improve
the attenuation of artifacts and reduce the time needed to estimate
the actual arterial pulsatile flow in the detected optical
signal.
In this embodiment, for N predetermined heartbeats, the average
value of the detected optical signal for those N heartbeats is
computed by assigning a weight to each data set and adding the
weighted data for each heartbeat synchronously into a buffer with a
weight of 1, then dividing by N. After each computation, the data
set from the oldest stored heartbeat is subtracted from the buffer.
As a new R-wave is detected, the incoming data is added to the
buffer, and the result is divided by N for computation of relative
amplitudes of the two wavelength (red and infrared) periodic
information. Thus, the equivalent delay in determining the arterial
oxygen saturaton is N/2 times the heartbeat interval. The stored
R-wave pointer may be used to correlate the weighting function with
the raw data so that the oldest data is given the smallest weight
and the most recent data is given the greatest weight, and after
each composite heartbeat computation, the oldest data set can be
subtracted from the buffer before the following newest data is
added.
In an alternate embodiment of the time domain analysis techniques,
the ECG and periodic information can be correlated by using a
waveform product of the ECG signal and the detected optical signal
to determine the location of the optical pulse from which oxygen
saturation and heart rate values may be computed. The R-wave of the
ECG has the largest slope component within the ECG waveform. In the
optical pulse waveform in the portion of detected optical signal,
the largest slope is created when the heart contacts to expell
blood and thereby produce the arterial pulse. Thus, because of the
determinable time interval between the ECG R-wave and the
appearance of the optical pulse at the detection site, the detected
optical signal can be moved backwards in time, relative to the ECG
waveform, an amount equal to the determined time interval so that
the portions of maximum slope in the two signals will be aligned,
and their product will be at a maximum.
A periodic signals, such as motion artifact, having high slopes
will not occur synchronously on the ECG and the detected optical
signals. Therefore, once the two periodic waveforms are aligned,
the largest slope product of the two will occur at the heart rate
interval. Detection of the maximum slope product can be used to
pinpoint the occurrence of a heartbeat, and the portion of the
detected optical signal that is associated with that maximum slope
product can be sued for calculating oxygen saturation.
In this embodiment, the time interval between the R-wave and the
optical pulse can be determined by collecting a predetermined time
measure history of optical and ECG waveform data comprising n
seconds. The time interval must be long enough for the samples to
include at least one R-wave and one pule respectively, given that
the heartbeat may vary from 20-30 beats per minute at the slowest
rates. A measure of six seconds is acceptable. The samples are
conventionally digitized and stored in memory. An array of
sample-to-sample slopes is obtained for each n second sample of the
ECG waveform and for the second half of each optical pulse waveform
sample. The first half of the optical pulse sample is discarded so
that when the first optical pulse in the second half is slid
backwards, the first R-wave peak it will come upon will be its
corresponding R-wave, and also so that the most recent heartbeat
data is detected. An optical pulse in the first half of the sample
could miss its R-wave.
The number of slope values in the second half of the optical
waveform, i.e. the number of data points minus 1 at the given
sampling rate of 57 samples per second (every 17.5 msec), is taken
as m, which corresponds to n/2 seconds of data.
A slope product is obtained by multiplying each element of the
optical slope array by its corresponding three and one-half points
in the ECG slope array (the ECG signal is sampled every 5 msec) and
summing the products. This process is repeated for each of the m
optical sample points as the optical waveform slope array is moved
backwards relative to the ECG slope array, one optical waveform
sample at a time. The backwards slide terminates when the first
sample of the ECG waveform is aligned with the first sample of the
second half of the optical waveform.
The maximum slope product is found to occur after the optical
waveform slop array has been slid x optical sample points
backwards. This establishes the time interval t between the
detection of the ECG R-wave and the detection of the optical pulse
produced by the same heart contraction. This time interval t is
expressed in terms of a number of waveform samples, and is used in
the determination of heartbeat occurrence.
Computation of the aligned waveform slope product will yield a
slope product value for each optical waveform sample. A percentage
of the maximum slope product produced during the establishment of
the time interval component can be used to compute a maximum
product threshold. For example, a percentage of 75% of the maximum
slope product may be used. Thus, when the ECG and optical signal
waveform slope product exceeds the maximum product threshold, it is
likely that a true pulse has been located.
The "true" pulse, as it appears the detected optical signal, can
then be validated and processed using known techniques for
calculating oxygen saturation from detected optical signals. For
example, the slope product could be used to synchronize the R-wave
events so that the corresponding periodic information can be added
in phase in accordance with the preferred embodiment of this
invention, or the slope product could be used as an additional
criterion for accepting the corresponding optical pulse as valid,
and the oxygen saturation determination could be based only on the
corresponding optical pulse. Alternately, the waveform product for
the maximum and minimum values for the red and infrared waveforms
could be used as the maximum and minimum values in calculating
saturation. Also, one could integrate some portion of the selected
waveform product waveform and compare the area of the change to the
area of the total signal to obtain the relative transmittance for
use in determining saturation. For example one could integrate the
portion above a selected threshold and compare that area to the
integral of the entire pulse.
Qualified "true" pulses are then used to update the slope product
threshold value so that it will change as the patient's condition
changes or as the quality of the received signals changes.
Applicants also have discovered that a time-measure of detected
optical signals containing a plurality of periodic information
corresponding to successive heartbeats can be collectively
processed and analyzed using frequency domain techniques. These
frequency domain techniques utilize the synchronous nature of the
heartbeat and the asynchronous characteristics of noise, spurious
signals, and motion artifacts.
In the frequency domain, the optical signals for a given wavelength
corresponding to the pulsatile arterial blood flow have spectral
components including a zero frequency at the background 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 in the average
background intensity level have frequencies that appear spread out
between the zero frequency and the fundamental frequency.
The frequency domain embodiment of the present invention provides a
method and apparatus for collecting a time-measure of detected
optical signals including a predetermined number of optical pulses,
converting the collected detected optical signals into the
frequency domain, and analyzing the spectral components of the
frequency spectrum thereby to determine the red and infrared
relative maxima intensity at the fundamental frequency, and
relative minima at the background intensity zero frequency, for use
as maxima and minima in the percentage modulation ratio for
calculating oxygen saturation.
Applicants have discovered that if the digitized values of the time
domain detected optical signals are stored in memory for a period
of N heartbeats, and the stored data set is transformed into the
frequency domain using Fourier Transforms, the amplitude of the
fundamental heartrate is summed for the N heartbeats and appears in
the frequency spectrum at a location of N cycles. In contrast, the
amplitude of asynchronous signals is 1/m where m is the number of
data pints in the digitized stored data set, and appear spread
across the frequency domain spectrum. The average intensity of the
detected optical signal background intensity appears at the
spectral line corresponding to zero cycles and corresponds to the
average background intensity for that wavelength.
If the detected optical signal for the red and infrared signals is
considered as a single complex data set, i.e., having real and
imaginary components, only a single Fourier transform is required
to analyze the spectral contents of the collective time-measure of
the two signals. If F(s) represents the Fourier Transform of the
complex data set f(t)=Red(t)+jIR(t)(for Red(t) being the red
detected optical signal and IR(t) being the infrared detected
optical signal), the Fourier Transform of the real component of
f(t) is found by
Similarly, the Fourier transform of the imaginary component of f(t)
is found by
F*(-s) is the complex conjugate of F(s) with the indexes
reversed.
The relative amplitudes of the red and infrared fundamentals at the
heartrate has been found to be equivalent to the foregoing time
domain techniques for computation of arterial oxygen saturation.
The amplitude data may be found by searching the frequency spectrum
in the region of expected heart rates for a relative maximum and
insuring that this is the fundamental by determining the existence
of another relative maximum at twice this rate. This provides a
technique for obtaining relative modulation data to calculate
arterial oxygen saturation without the need to identify the heart
rate independently, e.g., by detecting the ECG. Alternately, the
amplitude data at the fundamental may be found by the use of
independent heart rate determining mechanism such as ECG or
phonoplethysmography or the like to determine a heart rate.
However, unlike the time domain techniques, the precise time of
occurrence of each heartbeat need not be determined and the optical
signal and a heart rate parameter need no be correlated to obtain
accurate saturation values. Rather, it is sufficient to obtain an
approximate indicator of heart rate, which will facilitate
identification of the fundamental frequency and improve saturation
reliability.
The number of spectral lines computed is preferably optimized to
include the expected range of clinically applicable heartbeats
(from 20-250 beats per minute), while the length of the data set is
selected by the allowable equivalent delay in displaying measured
arterial oxygen saturaton. A time-measure of data of, for example,
9-10 seconds represent delays of only 4-5 seconds in the display of
computed saturations, and, depending upon the computational speed
of the oximeter microprocessor, the time-measure can be updated in
a timely fashion every 1 to 2 seconds.
In the preferred embodiment, the optical signal is digitized at 57
samples per second for each red and infrared signal. When 512 data
points are accumulated, the data is Fourier transformed, and the
red and infrared fundamental maxima are located. The percentage
modulation ratio (red/infrared) is computed by dividing the energy
at each maxima by the zero cycle background intensity for that
wavelength, then dividing the red modulation by the infrared
modulation. The resultant ratio, R, is the used in the manner set
forth in the Lamber-Beers equations for calculating arterial
saturation of hemoglobin. The collective data can be updated so
that new data points replace the oldest data points by using a push
down stack memory or equivalent so that the transform, evaluation
and saturation calculation could be made after each new data set
was obtained.
An alternative embodiment of the frequency domain analysis
technique includes sampling the real tim ECG waveform and the real
time detected optical signal at high rates, e.g., 1000 samples per
second. By examining the ECG wave, the time of occurrence for each
heartbeat and the appropriate sample rate to obtain m samples
during that heartbeat could be determined. Thus, the data set for
each heartbeat can be selected to contain the same number of m
samples, where each sample is a fraction of the heartbeat period
and N heartbeats contains mxN samples. Taking the Fourier transform
of this mxN data set and processing the spectral components of the
transform in the same manner as described previously, results in a
spectral analysis having several additional advantages. First, the
fundamental maximum would always occur at the spectral line for N
cycles in "heartbeat" space. Second, any signal present in the data
set which did not remain synchronous with the heart, including
noise, artifact and transient background intensity changes, would
be spread over the heartbeat spectrum. Third, the enhancement in
signal-to-noise would be the same for all heart rates. Fourth,
because only two spectral lines are of interest, the zero spectral
line corresponding to the zero frequency background intensity and
the N spectral line corresponding to the number of heartbeats for
the data set, the Fourier Transform need only be made at the two
frequency components and not of the entire spectrum, and the
computation efforts required by the microprocessor are
significantly diminished.
The apparatus of the present invention can be used for either time
domain or frequency domain analyses, and includes inputs for the
plethysmographic detected optical signals and ECG signals of a
patient, an analog to digital converter for converting the analog
plethysmographic signal to the digital optical signals and for
converting the analog ECG signals into digital ECG signals (unless
the plethysmographic or ECG 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 microprocessor, memory devices,
buffers, software 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, and receive at ECG signal from the
patient. In the preferred embodiment, the apparatus of this
invention is a part of the Nellcor N-200 Pulse Oximeter (herein the
"N-200 oximeter"), a commercially available noninvasive pulse
oximeter device manufactured and sold by Nellcor, Incorporated,
Hayward, Calif. U.S.A.
The N-200 oximeter is an improved version of the enhanced N-100
oximeter described above and in the prior application Ser. No.
742,720. The N-200 includes circuits that perform many of the same
functions as in the N-100 device, but includes some changes, for
example, to expand the dynamic range of the device over the N-100
device and to include a 16 bit microprocessor manufactured by Intel
Corporation, Model No. 8088. The N-100 oximeter uses an 8 bit
microprocessor manufactured by Intel Corporation, Model 8085. The
N-200 oximeter includes 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 some structure and
processing methods that are unrelated to the present invention, and
therefore are not discussed herein. The software could be modified
to perform any of the other time domain or frequency domain
analysis techniques of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are a block diagram of the apparatus of this
invention and the apparatus associated with the present
invention.
FIG. 2A is a detailed circuit schematic of the saturation
preamplifier in the patient module of FIG. 1.
FIG. 2B is a detailed circuit schematic of the ECG preamplifier and
input protection circuit in the patient module of FIG. 1.
FIGS. 3A and 3B are a 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.
FIG. 5 is a detailed circuit schematic of the ECG analog front end
circuit of FIG. 1.
FIGS. 6A and 6B are a detailed circuit schematic of the analog to
digital converter section of FIG. 1.
FIGS. 7A, 7B, and 7C are a detailed circuit schematic of the
digital signal processing section of FIG. 1.
FIG. 8 is a detailed circuit schematic of the external ECG
circuitry of FIG. 1.
FIGS. 9A, 9B, 9C, 9D, 9E and 9F are flow charts for the time domain
ECG and optical signal processing of this invention.
FIG. 10 is a flow chart for the frequency domain optical pulse
processing of this invention.
FIGS. 10A, 10B, 10C, 10D and 10E are a series of waveforms
corresponding to the flow chart of FIG. 10.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1A and 1B, the preferred embodiment of the
present invention relates to the apparatus for processing the
detected analog optical signal and the analog ECG 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 and ECG signals 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 and the analog ECG 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 and the conventional ECG electrodes with saturation analog
front end circuit 300 and ECG analog front end circuit 400
respectively, saturation analog circuit 300 for processing the
detected optical signals into separate red and infrared channels
that can be digitized, and ECG analog front end circuit 400 for
processing the ECG signal so that it can be digitized. The N-200
oximeter also includes external ECG input circuit 500 for receiving
an external ECG signal and processing the signal so that it is
compatible with the N-200 processing techniques (as explained
below), 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 for using the present invention in
its operating context in the preferred embodiment.
In the preferred embodiment, the invention requires three input
signals, the two plethysmograph or detected optical signals (e.g.,
red and infrared) and the ECG signal of the patient. If analog
signals are provided, they must be within or be adjusted by, for
example, offset amplifiers, to be within the voltage input range
for the ADC. In circumstances where the signals have been digitized
already, they must be bit compatible with the digital signal
processing devices, DSP.
The plethysmograph signal is obtained in a conventional manner for
a non-invasive oximeter, typically by illuminating the patients
tissue with red and infrared light in an alternating fashion, in
the manner described above for the N-100 oximeter. Referring to
FIGS. 1A and 1B, sensor circuit 100 has red LED 110 and infrared
LED 120 connected in parallel, anode to cathode, so that the LED
drive current alternately illuminates one LED and then the other
LED. Circuit 100 also includes photodetector 130, preferably a
photodiode, which detects the level of light transmitted through
the patient's tissue, e.g., finger 140, as a single, analog optical
signal containing both the red and infrared light plethysmographic,
detected optical signal waveforms.
Referring to FIGS. 1A, 1B, 2A, and 2B, patient module 200 includes
preamplifier 210 for preamplifying the analog detected optical
signal of photodetector 130, ECG preamplifer 220 for preamplifying
the analog ECG signal detected from the ECG electrodes that would
be attached to the patient in a conventional manner, and protection
circuitry 250 interposed between instrumentation amplifier 220 and
inverter 230 and the three ECG signal leads, to prevent high
voltage transients from damaging the ECG preamplifier
electronics.
Preamplifier 210 may be an operational amplifier configured as a
current to voltage converter, biased by a positive voltage to
extend the dynamic range of the system, thereby converting the
photocurrent of photo-diode 130 into a usable voltage signal. ECG
preamplifer 220 is preferably a high quality instrumentation
amplifier which amplifies the differential signal present on the
two ECG signal electrodes. The common-mode signal present on the
two signal electrodes is inverted by inverter 230 and returned to
the patient by the third ECG lead, effectively nulling the
common-mode signals. A biasing network on tee two ECG signal leads
is provided to aid in the detection of when an ECG electrode lead
becomes disconnected from patient module 200 or the patient.
Patient module 200 also includes leads for passing the LED drive
voltages to LEDs 110 and 120.
Referring to FIGS. 1A, 1B, 3A and 3B, saturation analog front end
circuit 300 receives the analog optical signal from patient module
200 and filters and processes the detected signal to provide
separate red and infrared analog voltage signals corresponding to
the detected red add infrared optical pulses. The voltage signal is
passed through low pass filter 310 to remove unwanted high
frequency components above, for example, 100 khz, AC coupled
through capacitor 325 to remove the DC component, passed through
high pass filter 320 to remove any unwanted low frequencies below,
for example, 20 hertz, and passed through programmable gain stage
330 to amplify and optimize the signal level presented to
synchronous detector 340.
Synchronous detector 340 removes any common mode signals present
and splits the time multiplexed optical signal into two channels,
one representing the red voltage signals and the other representing
the infrared voltage signals. Each signal is then passed through
respective filter chains having two 2-pole 20 hertz low pass
filters 350 and 360, and offset amplifier 370 and 380. The filtered
voltage signals now contain the signal information corresponding to
the red and infrared detected optical signals. Additionally,
circuits for use in preventing overdriving the amplifiers in
saturation circuit 300 may be applied, for example, level-sensing
circuits 312 and 314 (located after low pass filter 310) for
indicating unacceptable LED drive current, and level sensing
circuit 315 (located after programmable gain amplifier 330) for
indicating unacceptable input amplifier gain setting.
Referring to FIGS. 1A, 1B, and 8, ECG analog front end circuit 4000
receives the preamplified ECG signal from patient module 200 and
processes it for use with the present invention. The analog ECG
signal is passed through 2-pole 40 hertz low pass filter 410 for
removing unwanted frequencies above 40 hertz, and programmable
notch filter 420 for removing unwanted line frequency components.
Optionally, circuitry may be provided to measure the line frequency
and to select an appropriate clock frequency for the notch filter.
The ECG signal is then passed through low pass filter 430,
preferably configured to remove further unwanted components above
about 40 hertz, and in particular any frequency components that may
have been generated by notch filter 420. Thereafter, the ECG signal
is passed through 2-pole 0.5 hertz high pass filter 440 to remove
any low-frequency baseline shifts present in tee original ECG
signal, and then passed through offset amplifier 450 to add an
offset voltage that the voltage is within the input signal
specifications of the analog to digital converter device and the
complete waveform will be properly digitized.
It also is desirable to pass the signal output from low pass filter
410 into a circuit that detects whether or not the ECG signal is
being detected to identify a "leads-off" condition. The signal
voltage is passed through absolute value circuit 480 to take the
absolute value of the low pass filter output voltage and sends the
value to comparator 490. Comparator 490 compares the absolute value
voltage to a reference threshold or range and, when the absolute
value voltage is not within the acceptable range, comparator 490
changes state which change is input to latch 495, to indicate this
condition to, for example, the microprocessor.
Referring to FIGS. 1A, 1B and 8, the Nellcor N-200 device also is
equipped with external ECG circuit 500 for receiving the ECG signal
of a stand alone ECG detector device and processing the ECG signal
so that it can be used with the N-200 oximeter and the present
invention. Circuit 500 receives the external analog ECG signal,
passes it across capacitor 510 to remove any DC offset voltage and
then passes the signal through peak detection circuit 530. A
portion of the AC coupled signal also is passed through buffer
amplifier 520 and input to comparator 570. The held peak voltage is
used as the reference threshold voltage that is fed to the other
input of comparator 570 so that subsequent QRS complexes in the ECG
signal that rise above the threshold generate a trigger signal that
is transferred to DPS 2000 by an electrically isolated optical
serial communication link comprising serial driving opto-isolator
580, electrically isolated optical link 590, and corresponding
serial driving opto-isolator 2590 in DSP 2000.
Referring to FIGS. 1A, 1B, 6A and 6B, ADC 1000 provides the analog
to digital conversions required by the N-200 oximeter. The
aforementioned three voltage signals, the red detected optical
signal, the infrared detected optical signal, and the ECG signal
(preferably the ECG signal from patient module 200), are input to
ADC 1000. These three signals are conventionally multiplexed and
digitized by an expanded range 12-bit analog to digital conversion
technique, yielding 16-bit resolution. The input signals are passed
through multiplexor 1010 and buffer amplifier 1020. The converter
stage includes offset amplifier 1030, programmable gain circuitry
1040 which allows a portion of the signal to be removed and the
remainder to be further amplified for greater resolution, sample
and hold circuit 1050, comparator 1060, and 12-bit digital to
analog converter 1080. The buffered signal is passed through offset
amplifier 1030 to add a DC bias to the signal wherein a portion of
the signal is removed and the balance is amplified by being passed
through programmable gain circuitry 1040 to improve the resolution.
The amplified signal is then passed through sample and hold circuit
1050, the output of which is fed to one input of comparator 1060.
The other input of comparator 1060 is the output of digital to
analog ("DAC") converter 1080 so that when the inputs to comparator
1060 are the same, the analog voltage at the sample and hold
circuit is given the corresponding digital word in DAC converter
1080 which is then stored in an appropriate memory device as the
digitized data for the sample, and the next sample is sent to
sample and hold circuit 1050 to be digitized.
Referring to FIGS. 1A, 1B, 4, 6A, 6B, 7A, 7B, and 7C, DAC 1080 also
generates the sensor LED drive voltages, under the control of
microprocessor 2040, using analog multiplexor 610, which separates
the incoming analog signal into one of two channels for
respectively driving the red and infrared LEDs, having respective
sample and hold circuits 620 and 630, and LED driver circuit 640
for converting the respective analog voltage signal into the
respective positive and negative bipolar current signals for
driving LEDs 110 and 120.
Alternate techniques of converting the analog signals to digital
signals could be used, for example, a 16-bit analog to digital
converter.
Referring to FIGS. 1, 7A, 7B and 7C, DSP 2000 controls all aspects
of the signal processing operation including the signal input and
output and intermediate processing. The apparatus includes 16-bit
microprocessor 2040 and its associated support circuitry including
data bus 10, random access memory (RAM) 2020, read only memory
(ROM) 2030, a conventional LED display- device 2010 (not shown in
detail), system timing circuit 2050 for providing the necessary
clock synchronizing and notch filter frequency signals. In the
preferred embodiment, microprocessor 2040 is a model 8088
microprocessor, manufactured by Intel Corporation, Santa Clara,
Calif. Alternate microprocessors may be used, such as any of model
nos. 8086, 80186, and 80286, also made by Intel Corporation.
Referring to FIGS. 9A, 9B, 9C, 9D, 9E, and 9F and software Appendix
A, the flowcharts for the software operation of the preferred
embodiment are shown and described. Software appendix A is written
in the standard programming language for Intel Model 8088
microprocessor devices.
Similar to the enhanced N-100 oximeter described in U.S.
application Ser. No. 742,720, the N-200 oximeter incorporating the
present invention is designed to determine the oxygen saturation in
one of two modes, an unintegrated mode wherein the oxygen
saturation determination is made on the basis of pulses detected in
the optical pulse signal that are determined to be optical pulses
in accordance with conventional pulse detection techniques, and in
an ECG synchronization mode wherein the determination is based on
the synchronized additive, composite optical signal information is
accordance with the preferred embodiment of the present invention.
In an alternate embodiment of the present invention, the
determination of saturation in the unintegrated mode may be based
on the frequency domain analysis techniques in accordance with this
invention with or without the ECG synchronization feature of the
time domain analysis techniques.
Referring to FIG. 9F, interrupt programs control the collection and
digitization of incoming optical and ECG data. As particular events
occur, various software flags are raised which transfer operation
to various routines that are called from the Main Loop Processing
routine. For example, Main Loop Processing calls the ECG routine at
3600, calls a routine that checks the LED levels at 3610 to make
sure that there is enough and not too much light being transmitted,
looks for the presence of new data at 3615, and if there is new
data, calls the MUNCH routine at 3620, looks for processed pulse
data at 3635 and passes such data to the Leve13 routine that
calculates saturation at 3640, and also runs various maintenance
routines related to the oximeter functions which are not pertinant
to the present invention, e.g., at 3625, 3630, 3645, 3650, 3655,
and 3660 and are not discussed herein. The routines pertinent to
the present invention are discussed here. Examples of similar and
peripheral other routines may be found in the software appendix to
application Ser. No. 742,720.
The detected optical signal waveform is sampled at a rate of 57
samples per second. When the digitized red and infrared signals for
a given portion of detected optical signals are obtained, they are
stored in a buffer called DATBUF and a software flag indicating the
presence of data is set at 3615. This set flag calls a routine
referred to as MUNCH as 3620, which processes each new digitized
optical signal waveform sample. The MUNCH routine is called once
per data point and determines pairs of maximum and minimum
amplitudes in the detected signal data and presents the pairs to
the Leve13 routine. The Leve13 routine evaluates the pair of
maximum and minimum amplitudes determined by MUNCH, preferably
utilizing conventional techniques for evaluating whether a detected
pulse is acceptable for processing as an arterial pulse and
performs the saturation calculation based on accepted pulses. The
MUNCH routine first queries whether or not there is ECG
synchronization. If there is ECG synchronization, then the MUNCH
routine obtains from the SLIDER routine the enhanced pulse data on
which the ECG synchronized saturation calculation will be mad. If
there is not synchronization, MUNCH obtains the sample stored in
DATBUF on which the unintegrated saturation calculation will be
made.
Referring to FIG. 9A, the SLIDER routine processes each new
digitized sample portion of detected optical signal containing the
optical pulse to create and maintain the enhanced composite red and
infrared optical pulse waveforms, synchronized with the occurrence
of successive ECG R-wave events.
The SLIDER routine first inquires whether there is an ECG signal at
3100. If there is not, then the routine aborts to exit at 3160 to
main line operation. If there is an ECG signal, then the SLIDER
routine continues and checks the validity of the optical signal at
3110. If the digitized sample in the buffer DATBUF for either of
the red or infrared channels contains an invalid datapoint, the
full content of the slider buffer (SLIDEBUF) is erased and the
routine exited. The validity of the data is checked by looking for
zeros placed in the buffer. Zeros are placed in the buffer when the
signal level of the LEDs changes to permit the 20Hz filters to
settle, or if the signal exceeds the voltage limits of the
processing electronics. This prevents processing of data known to
be bad.
If the data are determined to be valid, then the SLIDER routine
queries whether or not the data should be skipped at 3120. The
optical signal sampling and data collection and processing of the
sampled data are asynchronous processes. On occasion, the data
buffer will have several unprocessed samples of data by the time
the ECG R-wave event trigger occurs (described below). The R-wave
event resets the slider buffer pointer to the beginning of the
slider buffer and marks the R-wave data sample in DATBUF SLIDER
will not process a data point if the slider buffer pointer is reset
already to the beginning of the slider buffer and if the incoming
data point was digitized in DATBUF before the data point marked by
the R-wave event. Data in the DATBUF buffer prior to the R-wave
event are to be skipped. If the data are to be skipped, SLIDER is
exited.
If the data are to be processed, SLIDER calculates the updated
value for the composite portion waveform sample as "slider data"
using the following formula: ##EQU1## wherein "WEIGHT" is the
aforementioned fractional weighting fraction; "new data" is the
data point taken from the incoming sample in DATBUF, and "slider
data" is the pre-existing data point in the composite waveform in
the slider buffer (SLIDEBUF) before the new data point is added and
becomes the updated data point after the computation.
The computation is performed for each data point in DATBUF and any
corresponding pre-existing data in the slider buffer. The
occurrence of an R-wave event indicates the beginning of the
heartbeat sample.
Before making the computation, SLIDER checks the slider buffer to
see if there is any existing data at 3130. If there are data, then
at 3150 SLIDER calculates the new value for the composite optical
signal. If, however, the slider buffer is empty, then the WEIGHT
value is assigned a numerial value of 1 at 3140, and subsequent new
data points will be weighted 100% and the routine continues to
calculate a new value for the composite optical signal at 3150
until the occurrence of the next R-wave event corresponding to the
following heartbeat and portion of detected optical signal.
The SLIDER routine also performs other housekeeping chores for the
processing associated with the slider buffer. First, in the
preferred embodiment, the slider buffer is given a specific length
and is able to store about three seconds worth of data. If, for
whatever reason, the microprocessor does not receive or recognize
an R-wave for more than three seconds, the pointer of the slider
buffer is set to point to the last location and does not get
incremented beyond that location. Subsequently processed samples
are each placed in the last location of the buffer until the next
accepted R-wave occurs or a time-out condition occurs. Time-out
occurs when no further R-wave events are accepted for a
predetermined time of, e.g., five seconds. After time out has
occurred, MUNCH is notified that ECG synchronization is lost so
that saturation calculations will be based only on the optical
signals in DATBUF in the unintegrated mode.
Second, SLIDER continuously compares the updated composite waveform
in the slider buffer to the previous composite waveform. If there
is a large discrepancy, for example, during electromechanical
disassociation, SLIDER takes immediate action to disregard the
slider buffer data.
Third, to avoid corrupting the integrity of the waveform data in
the slider buffer whenever the apparatus hardware or software
triggers a change that influences the signal level of the detected
optical signal or the optical pulse waveform, the content of the
slider buffer is erased.
Referring to FIG. 9B, the ECG BOX routine processes the ECG signal
obtained through patient module 200 and analog ECG front end
circuit to detect ECG R-wave events. The ECG signal is digitized
every 5 msec and the digitized values are maintained in a circular
buffer. The R-wave event is detected by observing the derivative of
the ECG signal. The derivative is obtained at 3200 by application
of the following algorithm: ##EQU2## where "ecg data[n]" is the
digitized value for the ECG signal at sample location n and "abs "
is the absolute value of the bracketed quantity.
The largest magnitude spike in the derivative buffer marks the
R-wave. Because the algorithm generates the absolute value of the
derivative, the derivative buffer contains two spikes very close to
each other, one for the positive-going portion and the other for
the negative-going portion of the R-wave. After the first spike is
recognized, a timer ecg block is set at 3250 to case ECG BOX to
ignore the second spike.
Once the derivative value is obtained, and if the ecg block timer
is not active, then the derivative value is compared to the ECG
threshold at 3240. The ECG threshold is a value that is set at
about 75% of the previous maximum derivative value. If the
derivative is greater than the threshold, ECG BOX starts the ecg
block timer by setting ecg block equal to true at 3250, and it
replaces the maximum derivative value with the current derivative
value at 3260, and calls the R-WAVE CHECKING routine at 3270. After
the R-WAVE CHECKING routine is completed (as discussed below), ECG
BOX is exited at 3280.
If the derivative is not greater than the threshold, then ECG BOX
is exited at 3280. Once the ecg bock timer is activated, ECG BOX
will continue to calculate the derivative and compare the
derivative to the prior maximum derivative value ay 3220. If the
calculated derivative is greater, then the maximum derivative value
is set equal to the current derivative ecg data[n]at 3230 and the
routine is exited. Otherwise the routine is exited.
Referring to FIG. 9E, the R-WAVE CHECKING routine receives the
detected R-wave event at 3500 and checks the elapsed time since the
last R-wave at 3510. If the elapsed time is less than the minimum
internal time limit, preferably set at about 200 msec, the R-wave
event is marked as a false R-wave event at 3520. If the elapsed
time is greater than the minimum limit, then the routine starts a
phasedelay timer/counter at 3530. The purpose of the phase-delay
counter is to ensure that the optical data is placed into the
beginning of the slider buffer after the optical signal minimum
from the preceding pulse, but before the signal maximum of the next
pulse. The preferred phase-delay period is 40 msec, based on the
results of experimentation, and corresponds to the opening of the
time window. It may be desirable to have a phase delay period that
can be adjusted to accommodate varying optical signal detection
conditions for different patients.
The Nellcor N-200 device is equipped with an external ECG input
circuit as described above. The main line operating system
controlling the operation of the N-200 device receives an interrupt
when the external circuit 500 detects an R-wave. On receipt of the
interrupt, a message is sent across isolated optical data
transmission path 580-590-2590 (FIGS. 1A and 1B) to microprocessor
2040. The microprocessor then indicates to the ECG processing
routines that an externally detected R-wave event has occurred, and
the R-wave event is passed to the R-WAVE CHECKING routine. The
external ECG analog circuit 500 thus performs the same function as
the ECG BOX routine, i.e., determination of an R-wave event
followed by the R-WAVE CHECKING routine. The ECG BOX routine is
given priority over external ECG circuit 500 in passing signals to
R-WAVE CHECKING.
Referring to FIG. 9C, the ECG routine provides for ECG
synchronization, the initialization for slider buffer use, and
various other tasks associated with ECG enhancement of the detected
optical signal. The ECG routine is entered from the Main Loop
Processing system (FIG. 9F, at 3600). Its first task is to maintain
the ECG related counters/timers, such as ecg block and phase-delay,
at 3300. Next, at 3310, it checks whether or not the ECG leads from
patient module 200 are present, and if not, it checks at 3320 for
the presence of an external R-wave event trigger from external ECG
circuit 500. If no R-wave event is detected, then the ECG routine
is exited at 3370.
At this point in the processing, the main line processing system is
receiving R-wave events, either from external circuits 500 or from
patient module 200 and ECG BOX. Regardless of the source of the
R-wave event, the subsequent processing of the R-wave event is the
same.
When an external R-wave event is detected or the ECG leads are
present, the ECG routine calls the ECG LV3 routine, shown in FIG.
9D. ECG LV3 runs through a similar patient module 200 lead checking
at 3410 or external circuit 500 trigger at 3420 at the ECG routine
and if no R-wave event has occurred the routine is exited at 3480.
If an R-wave event is detected, it is first checked at 3425 to
determine whether or not it is a new R-wave event, and if it is
not, the ECG LV3 routine is exited. If it is a new R-wave event,
3430 uses the false R-wave flag (set by the R-WAVE CHECKING
routine) to determine whether or not it was a true or false R-wave
event. False R-waves will cause the routine to be exited at this
point.
If the R-wave event is determined not to be a false R-wave, then
the ECG-LV3 routine builds up a history of R-wave events based on
the R-wave to R-wave interval at 3435. The criteria for accepting
an R-wave includes the R-R period and the amplitude of the R-wave.
For external ECG circuit 500 triggers, the R-wave even is a uniform
pulse resulting from a comparison of the R-wave amplitude to a
determined threshold signal.
After computing the R-R interval (or R-R delta) and history, the
ECG LV3 routine checks to see if the ECG is synchronized at 3440.
The ECG is synchronized after receiving the predetermined number,
preferably five, acceptable R-wave triggers. For example, the ECG
synch counter is initialized at five. The routine tests the ECG
synch counter 3440 so that if it is greater than zero, the ECG is
determined to be not synched, and then the ECG synch counter is
decreased by one at 3455. Thus, when the ECG synch counter is at
zero at 3440, indicating that the required prior five acceptable
R-wave event have successively occurred, then it is determined that
there is ECG synchronization and the device will proceed through
MUNCH to calculate oxygen saturation based on the enhanced
composite slider buffer calculations. Whether or not there is EC
synchronization, any R-wave event is checked again at 3450 against
the history and R-R interval, if any, to determine whether there is
an error in synchronization. If there is an error, the ECG LV3
routine is exited. If there is no synchronization error, a routine
is called at 3460 to compute the maximum length of time after which
data in the slider buffer (SLIDEBUF) is disregarded. For example,
if there is no prior R-R interval or history, then there will be no
error for the first R-wave event. Subsequent true R-wave events
will be compared to the prior R-R interval and history and if it
appears to be a valid true pulse, then a routine is called to reset
slider buffer pointers. However, the saturation calculation will be
based upon the slider buffer data only after five R-waves have
passed in synch and the synchronization flag is raised. Loss of
synchronization resets the ECG synch counter to five.
The ECG LV3 routine also calculates the maximum length of the
slider buffer based on the heart rate, which length is preferably 3
seconds or 2.5 times the determined R-R interval, whichever is the
smaller. The ECG LV3 routine also maintains the slider pointers and
counters, resetting them or clearing them as necessary, resets the
ecg timeout and bad R-wave counter, computes and displays heart
rate based on the R-R interval at 3465, updates the history buffers
and sets the trigger for the MUNCH routine to calculate pulse data
for determining oxygen saturation based on the updated slider
buffer data at 3470, sets and computes windows for selecting the
portion of detected optical signal to be processed for each
heartbeat, based on the history and the most recent data at 3475.
In the preferred embodiment, the windows are set to open by the
R-WAVE CHECKING routine phase-delay counter/timer 40 ms after the
R-wave occurs and before the maximum optical pulse wave has
occurred at the detection site, and set to close by the MUNCH
routine after a maximum and minimum pair has been found.
Upon exiting ECG LV3, the program returns to the ECG routine and
checks the threshold of the derivative buffer of the ECG BOX. If
the maximum derivative value is changes substantially, which
indicates that the R-wave slope is changing, then the threshold is
adjusted.
Referring to FIGS. 10, 10A, 10B, 10C, 10D, 10E and the software
appendix B, the flow chart for the software operation of the
frequency domain embodiment of the present invention are shown.
Software appendix B is written is the Asyst computer language which
is a commercially available language.
The routine begins at 4000 with the acquisition of 512 data points
for each of the digitized red and infrared optical signals, which
are shown graphically at FIG. 10A. At 4010, the complex data set,
f(t)=Red(t)+jIR(t), is formed. At 4020, the "D.C." component is
formed by summing all of the data points, and the "D.C." component
is then removed from the complex data set by subtraction at 4030,
which is graphically shown at FIG. 10B. The resulting data is then
decimated in time to 64 samples at 4040, which is illustrated in
FIG. 10C, and the time decimated data is then processed by the
Hamming Window function at 4050, which result is illustrated in
FIG. 10D. Thereafter, the Fourier Transform is taken at 4060. The
spectral components of the transform are shown in FIG. 10E. The
Fourier Transforms of the red and infrared components are then
calculated at 4070 in accordance with the aforementioned equations,
and at 4080 the maximum value at the fundamental heart rate and the
minimum value at the zero frequency are determined for each of the
red and infrared transforms. The saturation ratio R is calculated
as: ##EQU3## The minimum values for the red and infrared waveforms
are taken from the respective real and imaginary components of the
"D.C." component. Thereafter, the pulse data is declared ready and
saturation is calculated in accordance with the foregoing
saturation formula. With each occurrence of the heartbeat, new data
is acquired, the 512 data point set is updated and the routine
operates to determine the saturation ratio R.
In the preferred embodiment, the blood constituent measured is the
oxygen saturaton of the blood of a patient. The calculation of the
oxygen saturation is made based on the ratio of the pulse seen by
the red light compared to the pulse seen by the infrared light in
accordance with the following equation: ##EQU4## wherein BO1 is the
extinction coefficient for oxygenated hemoglobin at light
wavelength 1 (Infrared)
BO2 is the extinction efficient for oxygenated hemoglobin at light
wavelengths 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: ##EQU5## 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 waveguide
1
The various extinction coefficients are determinable by empirical
study as is 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. ##SPC1##
* * * * *