U.S. patent application number 09/939391 was filed with the patent office on 2002-04-11 for pulse oximeter and method of operation.
This patent application is currently assigned to Cybro Medical Ltd.. Invention is credited to Mendelson, Yitzhak.
Application Number | 20020042558 09/939391 |
Document ID | / |
Family ID | 11074711 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020042558 |
Kind Code |
A1 |
Mendelson, Yitzhak |
April 11, 2002 |
Pulse oximeter and method of operation
Abstract
A sensor for use in an optical measurement device and a method
for non-invasive measurement of a blood parameter. The sensor
includes sensor housing, a source of radiation coupled to the
housing, and a detector assembly coupled to the housing. The source
of radiation is adapted to emit radiation at predetermined
frequencies. The detector assembly is adapted to detect reflected
radiation at least one predetermined frequency and to generate
respective signals. The signals are used to determine the parameter
of the blood.
Inventors: |
Mendelson, Yitzhak;
(Worcester, MA) |
Correspondence
Address: |
James R. Yee
HOWARD & HOWARD
Suite 101
39400 Woodward Avenue
Bloomfield Hills
MI
48304-2856
US
|
Assignee: |
Cybro Medical Ltd.
|
Family ID: |
11074711 |
Appl. No.: |
09/939391 |
Filed: |
August 24, 2001 |
Current U.S.
Class: |
600/323 ;
600/322 |
Current CPC
Class: |
A61B 5/1455 20130101;
A61B 5/14552 20130101 |
Class at
Publication: |
600/323 ;
600/322 |
International
Class: |
A61B 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2000 |
IL |
138884 |
Claims
What is claimed is:
1. A sensor for use in an optical measurement device for
non-invasive measurement of a blood parameter, the sensor
comprising: (a) a light source for illuminating a measurement
location with incident light of at least three wavelengths, the
first wavelength .lambda.1 lying in a red (R) spectrum, and the at
least second and third wavelengths .lambda.2 and .lambda.3 lying
substantially in the infrared (IR) spectrum; and (b) a detector
assembly for detecting light returned from the illuminated
location, the detector assembly being arranged so as to define a
plurality of detection locations along at least one closed path
around the light source.
2. A sensor as set forth in claim 1, for use in a pulse oximeter,
the at least second and third wavelengths .lambda.2 and .lambda.3
being selected to coincide with a spectral region of the optical
absorption curve, where HbO.sub.2 absorbs slightly more light than
Hb, and where the extinction coefficients of Hb and HbO.sub.2 are
nearly equal and remain relatively constant as a function of
wavelength.
3. A sensor, as set forth in claim 2, wherein the second wavelength
.lambda.2 is in the IR spectral region around 940 nm+/-20 nm, and
the third wavelength .lambda.3 is above 700 nm.
4. A sensor, as set forth in claim 1, wherein the detector assembly
comprises at least one array of detector elements arranged in a
spaced-apart relationship along the at least one closed path.
5. A sensor, as set forth in claim 1, wherein the detector assembly
comprises at least one ring-shaped detector element.
6. A sensor according to claim 1, wherein the plurality of the
detection locations are arranged along two concentric closed paths
around the light source.
7. A sensor, as set forth in claim 6, wherein the detector assembly
comprises two arrays of detector elements, the detector elements of
each array being arranged in a spaced apart relationship along the
corresponding one of the closed paths.
8. A sensor, as set forth in claim 6, wherein the detector assembly
comprises two concentric ring-shaped detector elements.
9. A sensor, as set forth in claim 1, manufactured by CMOS
technology, the sensor comprising a package including said light
source, and an integrated circuit plate, which comprises said at
least one closed path of the detector assembly positioned around
the light source, and a plurality of printed contact areas and
electric conductors for mounting the light source thereon,
controlling the light source, and transmitting electric signals
produced by the detector assembly for further processing.
10. A sensor for use in an optical measurement device for
non-invasive measurement of a blood parameter, the sensor
comprising: a light source for illuminating a measurement location
with incident light of at least three wavelengths, the first
wavelength .lambda.1 lying in a red (R) spectrum, and the at least
second and third wavelengths .lambda.2 and .lambda.3 lying
substantially in the infrared (IR) spectrum; and a detector
assembly for detecting light returned from the illuminated
location, the detector assembly being arranged so as to define a
plurality of detection locations along two concentric closed path
around the light source.
11. A pulse oximeter comprising a sensor and a control unit for
operating the sensor and analyzing data generated thereby, the
sensor comprising: (a) a light source for illuminating a
measurement location with incident light of at least three
wavelengths, the first wavelength .lambda.1 lying in a red (R)
spectrum, and the at least second and third wavelengths .lambda.2
and .lambda.3 lying substantially in the infrared (IR) spectrum;
and (b) a detector assembly for detecting light returned from the
illuminated location, the detector assembly being arranged so as to
define a plurality of detection locations along at least one closed
path around the light source.
12. A method for non-invasive determination of a blood parameter,
the method comprising the steps of: (i) illuminating a measurement
location with at least three different wavelengths, a first
wavelength .lambda.1 lying in a red (R) spectrum, and at least
second and third wavelengths .lambda.2 and .lambda.3 lying
substantially in the infrared (IR) spectrum; (ii) detecting light
returned from the measurement location at different detection
locations and generating data indicative of the detected light,
wherein said different detection locations are arranged so as to
define at least one closed path around the measurement location;
and (iii) analyzing the generated data and determining the blood
parameter.
13. The method according to claim 12, wherein the analysis of the
generated data comprises the steps of: calculating data indicative
of an AC/DC ratio in the light detected at each of the detection
locations for the at least three wavelengths: analyzing the
calculated data and determining accepted detection locations to
select corresponding AC/DC ratios for each of the at least three
wavelengths, .lambda.1, .lambda.2 and .lambda.3 ;and utilizing the
selected ratios for determining the blood parameter.
14. The method according to claim 13, wherein the determination of
the blood parameter comprises the steps of: calculating values of
the ratio W.sub.2/W.sub.3 for the accepted detection locations in
at least one closed path; analyzing each of the calculated values
to determine whether it satisfies a first predetermined condition,
so as to generate a signal indicative of that a sensor position is
to be adjusted, if the condition is not satisfied; if the condition
is satisfied, determining whether the quality of a
photoplethysmogram is acceptable; if the quality is acceptable,
analyzing the selected ratios for calculating ratios
W.sub.1/W.sub.2 and W.sub.1/W.sub.3 from the data detected in at
least one closed path, and calculating the differences ABS
(W.sub.1/W.sub.2 -W.sub.1/W.sub.3); and, analyzing the calculated
differences for determining whether each of the differences
satisfies a second predetermined condition for determining the
blood parameter if the condition is satisfied.
15. The method according to claim 14, wherein said first
predetermined condition consists of that the calculated value of
W.sub.2/W.sub.3 is inside a predetermined range around the value
one, said predetermined range being defined by the first threshold
value, and the second predetermined condition consists of that the
calculated difference ABS (W.sub.1/W.sub.2 -W.sub.1/W.sub.3) is
less than certain, second threshold value.
16. A pulse oximeter for detecting a value of a parameter of blood,
comprising: a sensor housing; a source of radiation coupled to the
housing and being adapted to emit radiation at predetermined
frequencies; a detector assembly coupled to the housing and being
adapted to detect reflected radiation at first, second, and third
frequencies and to generate respective first, second, and third
signals, wherein the first, second, and third signals are
indicative of a value of the reflected radiation at the respective
first, second, and third frequencies; and, a control unit coupled
to the detector assembly and adapted to receive the first, second,
and third signals, to calculate first, second and third ratios of
the first, second, and third signals and to responsively determine
the parameter of the blood as a function of the first, second and
third ratios.
17. A pulse oximeter, as set forth in claim 16, wherein the control
unit is adapted to determine the parameter of the blood as a
function of the first and second ratios and a calibration
curve.
18. A pulse oximeter, as set forth in claim 17, wherein the
calibration curve is adjusted as a function of the third ratio.
19. A pulse oximeter, as set forth in claim 16, wherein the first
ratio is defined by the first signal divided by the second
signal.
20. A pulse oximeter, as set forth in claim 16, wherein the second
ratio is defined by the first signal divided by the third
signal.
21. A pulse oximeter, as set forth in claim 16, wherein the third
ratio is defined by the second signal divided by the third
signal.
22. A pulse oximeter, as set forth in claim 16, wherein the first
frequency is in a red frequency range, the second frequency is in a
near-infrared frequency range, and the third frequency is in an
infrared frequency range.
23. A pulse oximeter, as set forth in claim 22, wherein the first
ratio is defined by the first signal divided by the second signal,
the second ratio is defined by the first signal divided by the
third signal, and the third ratio is defined by the second signal
divided by the third signal.
24. A pulse oximeter, as set forth in claim 16, wherein the control
unit is adapted to determine the parameter of the blood as a
function of a more stable one of the first and second ratios.
25. A pulse oximeter for detecting a value of a arameter of blood,
comprising: a sensor housing; a source of radiation coupled to the
housing and being adapted to emit radiation at predetermined
frequencies; a detector assembly coupled to the housing and being
adapted to detect reflected radiation at first, second, and third
frequencies and to generate respective first, second, and third
signals, wherein the first, second, and third signals are
indicative of a value of the reflected radiation at the respective
first, second, and third frequencies; and, a control unit coupled
to the detector assembly and being adapted to calculate first and
second ratios of the first, second, and third signals, wherein the
first ratio is defined by the first signal divided by the second
signal and the second ratio is defined by the first signal divided
by the third signal, and wherein the control unit is adapted to
determine the parameter of the blood as a function of a more stable
one of the first and second ratios.
26. A pulse oximeter, as set forth in claim 25, wherein the control
unit is adapted to determine the parameter of the blood as a
function of the more stable one of the first and second ratios and
a calibration curve.
27. A pulse oximeter, as set forth in claim 26, wherein the
calibration curve is adjusted as a function of a third ratio.
28. A pulse oximeter, as set forth in claim 27, wherein the third
ratio is defined by the second signal divided by the third
signal.
29. A pulse oximeter, as set forth in claim 25, wherein the first
frequency is in a red frequency range, the second frequency is in a
near-infrared frequency range, and the third frequency is in an
infrared frequency range.
30. A pulse oximeter, as set forth in claim 25, wherein the control
unit is adapted to track the first and second ratios and determine
which one of the first and second ratios is more stable in
real-time.
31. A pulse oximeter for detecting a value of a parameter of blood,
comprising: a sensor housing; a source of radiation coupled to the
housing and being adapted to emit radiation at predetermined
frequencies; and, a plurality of detectors coupled to the housing
and being adapted to detect reflected radiation at first, second,
and third frequencies and to responsively generate a plurality of
first sensor signals indicative of the reflected radiation at the
first frequency, a plurality of second sensor signals indicative of
the reflected radiation at the second frequency, and a plurality of
third sensor signals indicative of the reflected radiation at the
third frequency; a control unit being coupled to the plurality of
detectors and adapted to receive the plurality of first, second and
third sensor signals, to analyze the first, second and third sensor
signals and determine which of the first, second and third sensor
signals are valid and to generate first, second, and third
frequency signals as a function of valid first sensor signals,
valid second sensor signals, and valid third sensor signals,
respectively and to determine the parameter of the blood as a
function of the valid first, second, and third sensor signals.
32. A pulse oximeter, as set forth in claim 31, wherein the control
unit is adapted to calculate first, second and third ratios of the
valid first, second, and third signals and to responsively
determine the parameter of the blood as a function of the first,
second and third ratios.
33. A pulse oximeter, as set forth in claim 32, wherein the control
unit is adapted to determine the parameter of the blood as a
function of the first and second ratios and a calibration
curve.
34. A pulse oximeter, as set forth in claim 33, wherein the
calibration curve is adjusted as a function of the third ratio.
35. A pulse oximeter, as set forth in claim 32, wherein the first
ratio is defined by the valid first signals divided by the valid
second signals.
36. A pulse oximeter, as set forth in claim 32, wherein the second
ratio is defined by the valid first signals divided by the valid
third signals.
37. A pulse oximeter, as set forth in claim 32, wherein the third
ratio is defined by the valid second signals divided by the valid
third signals.
38. A pulse oximeter, as set forth in claim 31, wherein the first
frequency is in a red frequency range, the second frequency is in a
near-infrared frequency range, and the third frequency is in an
infrared frequency range.
39. A pulse oximeter, as set forth in claim 32, wherein the first
ratio is defined by the valid first signals divided by the valid
second signals, the second ratio is defined by the valid first
signals divided by the valid third signals, and the third ratio is
defined by the valid second signals divided by the valid third
signals.
40. A pulse oximeter, as set forth in claim 32, wherein the control
unit is adapted to determine the parameter of the blood as a
function of a more stable one of the first and second ratios.
41. A pulse oximeter, as set forth in claim 31, wherein the
plurality of first, second, and third sensor signals having an AC
portion and a DC portion.
42. A pulse oximeter, as set forth in claim 41, wherein a sensor
signal is valid if it a ratio of the AC portion to the DC portion
is within a predetermined range.
43. A pulse oximeter, as set forth in claim 42, wherein the
predetermined range is 0.05 to 2.0 percent.
44. A sensor for use in an optical measurement device for
non-invasive measurement of a blood parameter, comprising: a sensor
housing; a source of radiation coupled to the housing and being
adapted to emit radiation at predetermined frequencies; a detector
assembly coupled to the housing and being adapted to detect
reflected radiation at least one predetermined frequency and to
generate respective signals, wherein the detector assembly is ring
shaped.
45. A sensor, as set forth in claim 44, wherein the detector
assembly includes a plurality of detectors arranged along a closed
loop path.
46. A sensor, as set forth in claim 45, wherein the closed loop
path has a circular shape.
47. A sensor, as set forth in claim 45, wherein the closed loop
path has an elliptical shape.
48. A sensor, as set forth in claim 45, wherein the closed loop
path has a polygonal shape.
49. A sensor, as set forth in claim 44, wherein the detector
assembly includes a continuous photodetector ring.
50. A sensor, as set forth in claim 49, wherein the continuous
photodetector ring has a circular shape.
51. A sensor, as set forth in claim 49, wherein the continuous
photo detector ring has an elliptical shape.
52. A sensor, as set forth in claim 49, wherein the continuous
photo detector ring has a polygonal shape.
53. A sensor, as set forth in claim 44, wherein the detector
assembly includes a first plurality of detectors arranged along an
inner closed loop path and a second plurality of detectors arranged
along an outer closed loop path.
54. A sensor, as set forth in claim 53, wherein the inner and outer
closed loop paths have a circular shape.
55. A sensor, as set forth in claim 49, wherein the inner and outer
closed loop paths have an elliptical shape.
56. A sensor, as set forth in claim 49, wherein the inner and outer
closed loop paths have a polygonal shape.
57. A sensor for use in an optical measurement device for
non-invasive measurement of a blood parameter, comprising: a sensor
housing; a source of radiation coupled to the housing and being
adapted to emit radiation at predetermined frequencies; a detector
assembly coupled to the housing and being adapted to detect
reflected radiation at least one predetermined frequency and to
generate respective signals, wherein the detector assembly includes
a plurality of pairs of detectors, each pair of detectors including
a near detector and a far detector.
58. A sensor, as set forth in claim 57, wherein the near detectors
are arranged along an inner closed loop path and the far detectors
are arranged along an outer closed loop paths.
59. A sensor, as set forth in claim 58, wherein the inner and outer
closed loop paths have a circular shape.
60. A sensor, as set forth in claim 58, wherein the inner and outer
closed loop paths have an elliptical shape.
61. A sensor, as set forth in claim 58, wherein the inner and outer
closed loop paths have a polygonal shape.
62. A method for detecting a value of a parameter of blood using a
sensor adapted to emit radiation at predetermined frequencies, to
detect reflected radiation at first, second, and third frequencies
and to generate respective first, second, and third signals,
wherein the first, second, and third signals are indicative of a
value of the reflected radiation at the respective first, second,
and third frequencies, the method including the steps of: receiving
the first, second, and third signals; calculating first, second and
third ratios of the first, second, and third signals; and,
responsively determining the parameter of the blood as a function
of the first, second and third ratios.
63. A method, as set forth in claim 62, wherein the parameter of
the blood is determined as a function of the first and second
ratios and a calibration curve.
64. A method, as set forth in claim 63, including the step of
adjusting the calibration curve as a function of the third
ratio.
65. A method, as set forth in claim 62, wherein the first ratio is
defined by the first signal divided by the second signal.
66. A method, as set forth in claim 62, wherein the second ratio is
defined by the first signal divided by the third signal.
67. A method, as set forth in claim 62, wherein the third ratio is
defined by the second signal divided by the third signal.
68. A method, as set forth in claim 62, wherein the first frequency
is in a red frequency range, the second frequency is in a
near-infrared frequency range, and the third frequency is in an
infrared frequency range.
69. A method, as set forth in claim 62, wherein the first ratio is
defined by the first signal divided by the second signal, the
second ratio is defined by the first signal divided by the third
signal, and the third ratio is defined by the second signal divided
by the third signal.
70. A method, as set forth in claim 62, including the step of
determining a more stable of the first and second ratios, wherein
the parameter of the blood is determined using the more stable one
of the first and second ratios.
71. A method for detecting a value of a parameter of blood using a
sensor adapted to emit radiation at predetermined frequencies, to
detect reflected radiation at first, second, and third frequencies
and to generate respective first, second, and third signals,
wherein the first, second, and third signals are indicative of a
value of the reflected radiation at the respective first, second,
and third frequencies, the method including the steps of: receiving
the first, second and third signals; calculate first and second
ratios of the first, second and third signals, wherein the first
ratio is defined by the first signal divided by the second signal
and the second ratio is defined by the first signal divided by the
third signal; and, determining the parameter of the blood as a
function of a more stable one of the first and second ratios.
72. A method, as set forth in claim 71, wherein the parameter of
the blood as a function of the more stable one of the first and
second ratios and a calibration curve.
73. A method, as set forth in claim 72, including the step of
adjusted the calibration curve as a function of a third ratio.
74. A method, as set forth in claim 73, wherein the third ratio is
defined by the second signal divided by the third signal.
75. A method, as set forth in claim 71, wherein the first frequency
is in a red frequency range, the second frequency is in an infrared
frequency range, and the third frequency is in a near-infrared
frequency range.
76. A method, as set forth in claim 71, including the step of
tracking the first and second ratios and determining which one of
the first and second ratios is more stable in real-time.
77. A method for detecting a value of a parameter of blood using a
sensor adapted to emit radiation at predetermined frequencies, to
detect reflected radiation at first, second, and third frequencies
and to responsively generate a plurality of first sensor signals
indicative of the reflected radiation at the first frequency, a
plurality of second sensor signals indicative of the reflected
radiation at the second frequency, and a plurality of third sensor
signals indicative of the reflected radiation at the third
frequency, the method comprising: receiving the plurality of first,
second and third sensor signals; analyzing the first, second and
third sensor signals and determining which of the first, second and
third sensor signals are valid; generating first, second, and third
frequency signals as a function of valid first sensor signals,
valid second sensor signals, and valid third sensor signals,
respectively; and, determining the parameter of the blood as a
function of the valid first, second, and third sensor signals.
78. A method, as set forth in claim 77, including the step of
calculating first, second and third ratios of the first, second,
and third valid signals and responsively determining the parameter
of the blood as a function of the first, second and third
ratios.
79. A method, as set forth in claim 78, wherein the parameter of
the blood is determined as a function of the first and second
ratios and a calibration curve.
80. A method, as set forth in claim 79, including the step of
adjusting the calibration curve as a function of the third
ratio.
81. A method, as set forth in claim 78, wherein the first ratio is
defined by the valid first signals divided by the valid second
signals.
82. A method, as set forth in claim 78, wherein the second ratio is
defined by the valid first signals divided by the valid third
signals.
83. A method, as set forth in claim 78, wherein the third ratio is
defined by the valid second signals divided by the valid third
signals.
84. A method, as set forth in claim 78, wherein the first frequency
is in a red frequency range, the second frequency is in an infrared
frequency range, and the third frequency is in a near-infrared
frequency range.
85. A method, as set forth in claim 78, wherein the first ratio is
defined by the valid first signals divided by the valid second
signals, the second ratio is defined by the valid first signals
divided by the valid third signals, and the third ratio is defined
by the valid second signals divided by the valid third signals.
86. A method, as set forth in claim 78, including the step of
determining the parameter of the blood as a function of a more
stable one of the first and second ratios.
87. A method, as set forth in claim 77, wherein the plurality of
first, second, and third sensor signals have an AC portion and a DC
portion.
88. A method, as set forth in claim 87, wherein a sensor signal is
valid if a ratio of the AC portion to the DC portion is within a
predetermined range.
89. A method, as set forth in claim 88, wherein the predetermined
range is 0.05 to 2.0 percent.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention is generally in the field of pulse oximetry,
and relates to a sensor for use in a pulse oximeter, and a method
for the pulse oximeter operation.
[0003] 2. Background of the Invention
[0004] Oximetry is based on spectrophotometric measurements of
changes in the color of blood, enabling the non-invasive
determination of oxygen saturation in the patient's blood.
Generally, oximetry is based on the fact that the optical property
of blood in the visible (between 500 and 700 nm) and near-infrared
(between 700 and 1000 nm) spectra depends strongly on the amount of
oxygen in blood.
[0005] Referring to FIG. 1, there is illustrated a hemoglobin
spectra measured by oximetry based techniques. Graphs G1 and G2
correspond, respectively, to reduced hemoglobin, or deoxyhemoglobin
(Hb), and oxygenated hemoglobin, or oxyhemoglobin (HbO.sub.2),
spectra. As shown, deoxyhemoglobin (Hb) has a higher optical
extinction (i.e., absorbs more light) in the red region of spectrum
around 660 nm, as compared to that of oxyhemoglobin (HbO.sub.2). On
the other hand, in the near-infrared region of the spectrum around
940 nm, the optical absorption by deoxyhemoglobin (Hb) is lower
than the optical absorption of oxyhemoglobin (HbO.sub.2).
[0006] Prior art non-invasive optical sensors for measuring
arterial oxyhemoglobin saturation (SaO.sub.2) by a pulse oximeter
(termed SpO.sub.2) are typically comprised of a pair of small and
inexpensive light emitting diodes (LEDs), and a single highly
sensitive silicon photodetector. A red (R) LED centered on a peak
emission wavelength around 660 nm and an infrared (IR) LED centered
on a peak emission wavelength around 940 nm are used as light
sources.
[0007] Pulse oximetry relies on the detection of a
photoplethysmographic signal caused by variations in the quantity
of arterial blood associated with periodic contraction and
relaxation of a patient's heart. The magnitude of this signal
depends on the amount of blood ejected from the heart into the
peripheral vascular bed with each systolic cycle, the optical
absorption of the blood, absorption by skin and tissue components,
and the specific wavelengths that are used to illuminate the
tissue. SaO.sub.2 is determined by computing the relative
magnitudes of the R and IR photoplethysmograms. Electronic circuits
inside the pulse oximeter separate the R and IR photoplethysmograms
into their respective pulsatile (AC) and non-pulsatile (DC) signal
components. An algorithm inside the pulse oximeter performs a
mathematical normalization by which the time-varying AC signal at
each wavelength is divided by the corresponding time-invariant DC
component which results mainly from the light absorbed and
scattered by the bloodless tissue, residual arterial blood when the
heart is in diastole, venous blood and skin pigmentation.
[0008] Since it is assumed that the AC portion results only from
the arterial blood component, this scaling process provides a
normalized R/IR ratio (i.e., the ratio of AC/DC values
corresponding to R- and IR-spectrum wavelengths, respectively),
which is highly dependent on SaO.sub.2, but is largely independent
of the volume of arterial blood entering the tissue during systole,
skin pigmentation, skin thickness and vascular structure. Hence,
the instrument does not need to be re-calibrated for measurements
on different patients. Typical calibration of a pulse oximeter is
illustrated in FIG. 2 by presenting the empirical relationship
between SaO.sub.2 and the normalized R/IR ratio, which is
programmed by the pulse oximeters' manufacturers.
[0009] Pulse oximeters are of two kinds operating, respectively, in
transmission and reflection modes. In transmission-mode pulse
oximetry, an optical sensor for measuring SaO.sub.2 is usually
attached across a fingertip, foot or earlobe, such that the tissue
is sandwiched between the light source and the photodetector.
[0010] In reflection-mode or backscatter type pulse oximetry, as
shown in FIG. 3, the LEDs and photodetector are both mounted
side-by-side next to each other on the same planar substrate. This
arrangement allows for measuring SaO.sub.2 from multiple convenient
locations on the body (e.g. the head, torso, or upper limbs), where
conventional transmission-mode measurements are not feasible. For
this reason, non-invasive reflectance pulse oximetry has recently
become an important new clinical technique with potential benefits
in fetal and neonatal monitoring. Using reflectance oximetry to
monitor SaO.sub.2 in the fetus during labor, where the only
accessible location is the fetal scalp or cheeks, or on the chest
in infants with low peripheral perfusion, provides several more
convenient locations for sensor attachment.
[0011] Reflection pulse oximetry, while being based on similar
spectrophotometric principles as the transmission one, is more
challenging to perform and has unique problems that can not always
be solved by solutions suitable for solving the problems associated
with the transmission-mode pulse oximetry. Generally, comparing
transmission and reflection pulse oximetry, the problems associated
with reflection pulse oximetry consist of the following:
[0012] In reflection pulse oximetry, the pulsatile AC signals are
generally very small and, depending on sensor configuration and
placement, have larger DC components as compared to those of
transmission pulse oximetry. As illustrated in FIG. 4, in addition
to the optical absorption and reflection due to blood, the DC
signal of the R and IR photoplethysmograms in reflection pulse
oximetry can be adversely affected by strong reflections from a
bone. This problem becomes more apparent when applying measurements
at such body locations as the forehead and the scalp, or when the
sensor is mounted on the chest over the ribcage. Similarly,
variations in contact pressure between the sensor and the skin can
cause larger errors in reflection pulse oximetry (as compared to
transmission pulse oximetry) since some of the blood near the
superficial layers of the skin may be normally displaced away from
the sensor housing towards deeper subcutaneous structures.
Consequently, the highly reflective bloodless tissue compartment
near the surface of the skin can cause large errors even at body
locations where the bone is located too far away to influence the
incident light generated by the sensor.
[0013] Another problem with currently available reflectance sensors
is the potential for specular reflection caused by the superficial
layers of the skin, when an air gap exists between the sensor and
the skin, or by direct shunting of light between the LEDs and the
photodetector through a thin layer of fluid which may be due to
excessive sweating or from amniotic fluid present during
delivery.
[0014] It is important to keep in mind the two fundamental
assumptions underlying the conventional dual-wavelength pulse
oximetry, which are as follows:
[0015] (1) the path of light rays with different illuminating
wavelengths in tissue are substantially equal and, therefore,
cancel each other, and (2) each light source illuminates the same
pulsatile change in arterial blood volume.
[0016] Furthermore, the correlation between optical measurements
and tissue absorptions in pulse oximetry are based on the
fundamental assumption that light propagation is determined
primarily by absorbance due to Lambert-Beer's law neglecting
multiple scattering effects in biological tissues. In practice,
however, the optical paths of different wavelengths in biological
tissues is known to vary more in reflectance oximetry compared to
transmission oximetry, since it strongly depends on the light
scattering properties of the illuminated tissue and sensor
mounting.
[0017] Several human validation studies, backed by animal
investigations, have suggested that uncontrollable physiological
and physical parameters can cause large variations in the
calibration curve of reflectance pulse oximeters primarily at low
oxygen saturation values below 70%. It was observed that the
accuracy of pulse oximeters in clinical use might be adversely
affected by a number of physiological parameters when measurements
are made from sensors attached to the forehead, chest, or the
buttock area. While the exact sources of these variations are not
fully understood, it is generally believed that there are a few
physiological and anatomical factors that may be the major source
of these errors. It is also well known for example that changes in
the ratio of blood to bloodless tissue volumes may occur through
venous congestion, vasoconstriction/vasodilatation, or through
mechanical pressure exerted by the sensor on the skin.
[0018] Additionally, the empirically derived calibration curve of a
pulse oximeter can be altered by the effects of contact pressure
exerted by the probe on the skin. This is associated with the
following. The light paths in reflectance oximetry are not well
defined (as compared to transmission oximetry), and thus may differ
between the red and infrared wavelengths. Furthermore, the forehead
and scalp areas consist of a relatively thin subcutaneous layer
with the cranium bone underneath, while the tissue of other
anatomical structures, such as the buttock and limbs, consists of a
much thicker layer of skin and subcutaneous tissues without a
nearby bony support that acts as a strong light reflector.
[0019] Several in vivo and in vitro studies have confirmed that
uncontrollable physiological and physical parameters (e.g.,
different amounts of contact pressure applied by the sensor on the
skin, variation in the ratio of bloodless tissue-to-blood content,
or site-to-site variations) can often cause large errors in the
oxygen saturation readings of a pulse oximeter, which are normally
derived based on a single internally-programmed calibration curve.
The relevant in vivo studies are disclosed in the following
publications:
[0020] 1. Dassel, et al., "Effect of location of the sensor on
reflectance pulse oximetry", British Journal of Obstetrics and
Gynecology, vol. 104, pp. 910-916, (1997);
[0021] 2. Dassel, et al., "Reflectance pulse oximetry at the
forehead of newborns: The influence of varying pressure on the
probe", Journal of Clinical Monitoring, vol. 12, pp. 421-428,
(1996).]
[0022] The relevant in vitro studies are disclosed, for example in
the following publication:
[0023] 3. Edrich et al., "Fetal pulse oximetry: influence of tissue
blood content and hemoglobin concentration in a new in-vitro
model", European Journal of Obstetrics and Gynecology and
Reproductive Biology, vol. 72, suppl. 1, pp. S29-S34, (1997).
[0024] Improved sensors for application in dual-wavelength
reflectance pulse oximetry have been developed. As disclosed in the
following publication: Mendelson, et al., "Noninvasive pulse
oximetry utilizing skin reflectance photoplethysmography", IEEE
Transactions on Biomedical Engineering, vol. 35, no. 10, pp.
798-805 (1988), the total amount of backscattered light that can be
detected by a reflectance sensor is directly proportional to the
number of photodetectors placed around the LEDs. Additional
improvements in signal-to-noise ratio were achieved by increasing
the active area of the photodetector and optimizing the separation
distance between the light sources and photodetectors.
[0025] Another approach is based on the use of a sensor having six
photodiodes arranged symmetrically around the LEDs that is
disclosed in the following publications:
[0026] 4. Mendelson, et al., "Design and evaluation of a new
reflectance pulse oximeter sensor", Medical Instrumentation, vol.
22, no. 4, pp. 167-173 (1988); and
[0027] 5. Mendelson, et al., "Skin reflectance pulse oximetry: in
vivo measurements from the forearm and calf", Journal of Clinical
Monitoring, vol. 7, pp. 7-12, (1991).
[0028] According to this approach, in order to maximize the
fraction of backscattered light collected by the sensor, the
currents from all six photodiodes are summed electronically by
internal circuitry in the pulse oximeter. This configuration
essentially creates a large area photodetector made of six discrete
photodiodes connected in parallel to produce a single current that
is proportional to the amount of light backscattered from the skin.
Several studies showed that this sensor configuration could be used
successfully to accurately measure SaO.sub.2 from the forehead,
forearm and the calf on humans. However, this sensor requires a
means for heating the skin in order to increase local blood flow,
which has practical limitations since it could cause skin
burns.
[0029] Yet another prototype reflectance sensor is based on eight
dual-wavelength LEDs and a single photodiode, and is disclosed in
the following publication: Takatani et al., "Experimental and
clinical evaluation of a noninvasive reflectance pulse oximeter
sensor", Journal of Clinical Monitoring, vol. 8, pp. 257-266
(1992). Here, four R and four IR LEDs are spaced at 90-degree
intervals around the substrate and at an equal radial distance from
the photodiode.
[0030] A similar sensor configuration based on six photodetectors
mounted in the center of the sensor around the LEDs is disclosed in
the following publication: Konig, et al., "Reflectance pulse
oximetry--principles and obstetric application in the Zurich
system", Journal of Clinical Monitoring, vol. 14, pp. 403-412
(1998).
[0031] According to the techniques disclosed in all of the above
publications, only LEDs of two wavelengths, R and IR, are used as
light sources, and the computation of SaO.sub.2 is based on
reflection photoplethysmograms measured by a single photodetector,
regardless of whether one or multiple photodiodes chips are used to
construct the sensor. This is because of the fact that the
individual signals from the photodetector elements are all summed
together electronically inside the pulse oximeter. Furthermore,
while a radially-symmetric photodetector array can help to maximize
the detection of backscattered light from the skin and minimize
differences from local tissue inhomogeneity, human and animal
studies confirmed that this configuration can not completely
eliminate errors caused by pressure differences and site-to-site
variations.
[0032] The use of a nominal dual-wavelength pair of 735/890 nm was
suggested as providing the best choice for optimizing accuracy, as
well as sensitivity in dual-wavelength reflectance pulse oximetry,
in U.S. Pat. Nos. 5,782,237 and 5,421,329. This approach minimizes
the effects of tissue heterogeneity and enables to obtain a balance
in path length changes arising from perturbations in tissue
absorbance. This is disclosed in the following publications.
[0033] 6. Mannheimer at al., "Physio-optical considerations in the
design of fetal pulse oximetry sensors", European Journal of
Obstetrics and Gynecology and Reproductive Biology, vol. 72, suppl.
1, pp. S9-S19, (1997); and
[0034] 7. Mannheimer at al., "Wavelength selection for
low-saturation pulse oximetry", IEEE Transactions on Biomedical
Engineering, vol. 44, no. 3, pp. 48-158 (1997)].
[0035] However, replacing the conventional R wavelength at 660 nm,
which coincides with the region of the spectrum where the
difference between the extinction coefficient of Hb and HbO.sub.2
is maximal, with a wavelength emitting at 735 nm, not only lowers
considerably the overall sensitivity of a pulse oximeter, but does
not completely eliminate errors due to sensor placement and varying
contact pressures.
[0036] Pulse oximeter probes of a type comprising three or more
LEDs for filtering noise and monitoring other functions, such as
carboxyhemoglobin or various indicator dyes injected into the blood
stream, have been developed and are disclosed, for example, in WO
00/32099 and U.S. Pat. No. 5,842,981. The techniques disclosed in
these publications are aimed at providing an improved method for
direct digital signal formation from input signals produced by the
sensor and for filtering noise.
[0037] None of the above prior art techniques provides a solution
to overcome the most essential limitation in reflectance pulse
oximetry, which requires the automatic correction of the internal
calibration curve from which accurate and reproducible oxygen
saturation values are derived, despite variations in contact
pressure or site-to-site tissue heterogeneity.
[0038] In practice, most sensors used in reflection pulse oximetry
rely on closely spaced LED wavelengths in order to minimize the
differences in the optical path lengths of the different
wavelengths. Nevertheless, within the wavelength range required for
oximetry, even closely spaced LEDs with closely spaced wavelengths
mounted on the same substrate can lead to large random error in the
final determination of SaO.sub.2.
SUMMARY OF THE INVENTION AND ADVANTAGES
[0039] The object of the invention is to provide a novel sensor
design and method that functions to correct the calibration
relationship of a reflectance pulse oximeter, and reduce
measurement inaccuracies in general. Another object of the
invention is to provide a novel sensor and method that functions to
correct the calibration relationship of a reflectance pulse
oximeter, and reduce measurement inaccuracies in the lower range of
oxygen saturation values (typically below 70%), which is the
predominant range in neonatal and fetal applications.
[0040] Yet another object of the present invention is to provide
automatic correction of the internal calibration curve from which
oxygen saturation is derived inside the oximeter in situations
where variations in contact pressure or site-to-site tissue
heterogeneity may cause large measurement inaccuracies.
[0041] Another object of the invention is to eliminate or reduce
the effect of variations in the calibration of a reflectance pulse
oximeter between subjects, since perturbations caused by contact
pressure remain one of the major sources of errors in reflectance
pulse oximetry. In fetal pulse oximetry, there are additional
factors, which must be properly compensated for in order to produce
an accurate and reliable measurement of oxygen saturation. For
example, the fetal head is usually the presenting part, and is a
rather easily accessible location for application of reflectance
pulse oximetry. However, uterine contractions can cause large and
unpredictable variations in the pressure exerted on the head and by
the sensor on the skin, which can lead to large errors in the
measurement of oxygen saturation by a dual-wavelength reflectance
pulse oximeter. Another object of the invention is to provide
accurate measurement of oxygen saturation in the fetus during
delivery.
[0042] The basis for the errors in the oxygen saturation readings
of a dual-wavelength pulse oximeter is the fact that, in practical
situations, the reflectance sensor applications affect the
distribution of blood in the superficial layers of the skin. This
is different from an ideal situation, when a reflectance sensor
measures light backscattered from a homogenous mixture of blood and
bloodless tissue components. Therefore, the R and IR DC signals
practically measured by photodetectors contain a relatively larger
proportion of light absorbed by and reflected from the bloodless
tissue compartments. In these uncontrollable practical situations,
the changes caused are normally not compensated for automatically
by calculating the normalized R/IR ratio since the AC portions of
each photoplethysmogram, and the corresponding DC components, are
affected differently by pressure or site-to-site variations.
Furthermore, these changes depend not only on wavelength, but
depend also on the sensor geometry, and thus cannot be eliminated
completely by computing the normalized R/IR ratio, as is typically
the case in dualwavelength pulse oximeters.
[0043] The inventor has found that the net result of this nonlinear
effect is to cause large variations in the slope of the calibration
curves. Consequently, if these variations are not compensated
automatically, they will cause large errors in the final
computation of SpO.sub.2, particularly at low oxygen saturation
levels normally found in fetal applications.
[0044] Another object of the present invention is to compensate for
these variations and to provide accurate measurement of oxygen
saturation. The invention consists of, in addition to two
measurement sessions typically carried out in pulse oximetry based
on measurements with two wavelengths centered around the peak
emission values of 660 nm (red spectrum) and 940 nm.+-.20 nm (IR
spectrum), one additional measurement session is carried out with
an additional wavelength. At least one additional wavelength is
preferably chosen to be substantially in the IR region of the
electromagnetic spectrum, i.e., in the NIR-IR spectrum (having the
peak emission value above 700 nm). In a preferred embodiment the
use of at least three wavelengths enables the calculation of an at
least one additional ratio formed by the combination of the two IR
wavelengths, which is mostly dependent on changes in contact
pressure or site-to-site variations. In a preferred embodiment,
slight dependence of the ratio on variations in arterial oxygen
saturation that may occur, is easily minimized or eliminated
completely, by the proper selection and matching of the peak
emission wavelengths and spectral characteristics of the at least
two IR-light sources.
[0045] Preferably, the selection of the IR wavelengths is based on
certain criteria. The IR wavelengths are selected to coincide with
the region of the optical absorption curve where HbO.sub.2 absorbs
slightly more light than Hb. The IR wavelengths are in the spectral
regions where the extinction coefficients of both Hb and HbO.sub.2
are nearly equal and remain relatively constant as a function of
wavelength, respectively.
[0046] In a preferred embodiment, tracking changes in the ratio
formed by the two IR wavelengths, in real-time, permits automatic
correction of errors in the normalized ratio obtained from the
R-wavelength and each of the IR-wavelengths. The term "ratio"
signifies the ratio of two values of AC/DC corresponding to two
different wavelengths. This is similar to adding another equation
to solve a problem with at least three unknowns (i.e., the relative
concentrations of HbO.sub.2 and Hb, which are used to calculate
SaO.sub.2, and the unknown variable fraction of blood-to-tissue
volumes that effects the accurate determination of SaO.sub.2),
which otherwise must rely on only two equations in the case of only
two wavelengths used in conventional dual-wavelength pulse
oximetry. In a preferred embodiment, a third wavelength provides
the added ability to compute SaO.sub.2 based on the ratio formed
from the R-wavelength and either of the IR-wavelengths. In a
preferred embodiment, changes in these ratios are tracked and
compared in real-time to determine which ratio produces a more
stable or less noisy signal. That ratio is used predominantly for
calculating SaO.sub.2.
[0047] The present invention utilizes collection of light reflected
from the measurement location at different detection locations
arranged along a closed path around light emitting elements, which
can be LEDs or laser sources. Preferably, these detection locations
are arranged in two concentric rings, the so-called "near" and
"far" rings, around the light emitting elements. This arrangement
enables optimal positioning of the detectors for high quality
measurements, and enables discrimination between photodetectors
receiving "good" information (i.e., AC and DC values which would
result in accurate calculations of SpO.sub.2) and "bad" information
(i.e., AC and DC values which would result in inaccurate
calculations of Sp0.sub.2).
[0048] There is thus provided according to one aspect of the
present invention, a sensor for use in an optical measurement
device for non-invasive measurements of blood parameters, the
sensor comprising:
[0049] (1) a light source for illuminating a measurement location
with incident light of at least three wavelengths, the first
wavelength lying in a red (R) spectrum, and the at least second and
third wavelengths lying substantially in the infrared (IR) spectrum
and
[0050] (2) a detector assembly for detecting light returned from
the illuminated location, the detector assembly being arranged so
as to define a plurality of detection locations along at least one
closed path around the light source.
[0051] The term "closed path" used herein signifies a closed curve,
like a ring, ellipse, or polygon, and the like.
[0052] The detector assembly is comprised of at least one array of
discrete detectors (e.g., photodiodes) accommodated along at least
one closed path, or at least one continuous photodetector defining
the closed path.
[0053] The term "substantially IR spectrum" used herein signifies a
spectrum range including near infrared and infrared regions.
[0054] According to another aspect of the present invention, there
is provided a pulse oximeter utilizing a sensor constructed as
defined above, and a control unit for operating the sensor and
analyzing data generated thereby.
[0055] According to yet another aspect of the present invention,
there is provided a method for non-invasive determination of a
blood parameter, the method comprising the steps of:
[0056] illuminating a measurement location with at least three
different wavelengths .lambda.1, .lambda.2 and .lambda.3, the first
wavelength .lambda.1 lying in a red (R) spectrum, and the at least
second and at least third wavelengths .lambda.2 and .lambda.3 lying
substantially in the infrared (IR) spectrum;
[0057] detecting light returned from the measurement location at
different detection locations and generating data indicative of the
detected light, wherein said different detection locations are
arranged so as to define at least one closed path around the
measurement location; and
[0058] analyzing the generated data and determining the blood
parameter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] Other advantages of the present invention will be readily
appreciated as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings wherein:
[0060] FIG. 1 illustrates hemoglobin spectra as measured by
oximetry based techniques;
[0061] FIG. 2 illustrates a calibration curve used in pulse
oximetry as typically programmed by the pulse oximeters
manufacturers;
[0062] FIG. 3 illustrates the relative disposition of light source
and detector in reflectionmode or backscatter type pulse
oximetry;
[0063] FIG. 4 illustrates light propagation in reflection pulse
oximetry;
[0064] FIGS. 5A and 5B illustrate a pulse oximeter reflectance
sensor operating under ideal and practical conditions,
respectively;
[0065] FIG. 6 illustrates variations of the slopes of calibration
curves in reflectance pulse oximetry measurements;
[0066] FIG. 7 illustrates an optical sensor according to the
invention;
[0067] FIG. 8 is a block diagram of the main components of a pulse
oximeter utilizing the sensor of FIG. 7;
[0068] FIG. 9 is a flow chart of a selection process used in the
signal processing technique according to the invention; and
[0069] FIGS. 10A to 10C are flow charts of three main steps,
respectively, of the signal processing method according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0070] Referring to the Figures, wherein like numerals indicate
like or corresponding parts throughout the several views, FIGS. 1
and 2 illustrate typical hemoglobin spectra and calibrations curve
utilized in the pulse oximetry measurements.
[0071] The present invention provides a sensor for use in a
reflection-mode or backscatter type pulse oximeter. The relative
disposition of light source and detector in the reflection-mode
pulse oximeter are illustrated in FIG. 3.
[0072] FIG. 4 shows light propagation in the reflection-mode pulse
oximeter where, in addition to the optical absorption and
reflection due to blood, the DC signal of the R and IR
photoplethysmograms can be adversely affected by strong reflections
from the bone.
[0073] FIGS. 5A and 5B illustrate a pulse oximeter reflectance
sensor operating under, respectively, ideal and practical
conditions. Referring now to FIG. 5A, it is shown that, under ideal
conditions, reflectance sensor measures light backscattered from a
homogenous mixture of blood and bloodless tissue components.
Accordingly, the normalized R/IR ratio in dual-wavelength
reflection type pulse oximeters, which relies on proportional
changes in the AC and DC components in the photoplethysmograms,
only reflect changes in arterial oxygen saturation.
[0074] Referring now to FIG. 5B, in practical situations, the
sensor applications affect the distribution of blood in the
superficial layers of the skin. Accordingly, the R and IR DC
signals measured by photodetectors contain a relatively larger
proportion of light absorbed by and reflected from the bloodless
tissue compartments. As such, the changes in DC signals depend not
only on wavelength but also sensor geometry and thus cannot be
eliminated completely by computing the normalized R/IR ratio, as is
typically the case in dual-wavelength pulse oximeters. The result
is large variations in the slope of the calibration curves, as
illustrated in FIG. 6. Referring now to FIG. 6, graphs C1, C2 and
C3 show three calibration curves, presenting the variation of the
slope for oxygen saturation values between 50% and 100%.
[0075] Referring to FIG. 7, there is illustrated an optical sensor
10 designed according to the invention aimed at minimizing some of
the measurement inaccuracies in a reflectance pulse oximeter. The
sensor 10 comprises such main constructional parts as a light
source 12 composed of three closely spaced light emitting elements
(e.g., LEDs or laser sources) 12a, 12b and 12c generating light of
three different wavelengths, respectively, an array of discrete
detectors (e.g., photodiodes), a "far" detector 16 and a "near"
detector 18, arranged in two concentric ring-like arrangements
(constituting closed paths) surrounding the light emitting
elements, and a light shield 14. In the present example, six
photodiodes form each ring. All these elements are accommodated in
a sensor housing 17. The light shield 14 is positioned between the
photodiodes and the light emitting elements, and prevents direct
optical coupling between them, thereby maximizing the fraction of
backscattered light passing through the arterially perfused
vascular tissue in the detected light.
[0076] It should be noted that more than three wavelengths can be
utilized in the sensor. The actual numbers of wavelengths used as a
light source and the number of photodetectors in each ring are not
limited and depend only on the electronic circuitry inside the
oximeter. The array of discrete photodiodes can be replaced by one
or more continuous photodetector rings.
[0077] In addition to the R and IR light emitting elements 12a and
12b as used in the conventional pulse oximeter sensors, the sensor
10 incorporates the third, reference, light emitting element 12c,
which emits light in the NIR-IR spectrum. Wavelength .lambda.1 and
.lambda.2 of the R and IR light emitting elements 12a and 12b are
centered, respectively, around the peak emission values of 660 nm
and 940 nm, and wavelength .lambda.3 of the third light emitting
element 12c has the peak emission value above 700 nm (typically
ranging between 800 nm and 900 nm). In the description below, the
light emitting elements 12b and 12c are referred to as two IR light
emitting elements, and wavelengths .lambda.2 and .lambda.3 are
referred to as two IR wavelengths.
[0078] During the operation of the sensor 10, different light
emitting elements are selectively operated for illuminating a
measurement location (not shown) with different wavelengths. Each
of the photodetectors detects reflected light of different
wavelengths and generates data indicative of the intensity I of the
detected light of different wavelengths.
[0079] It should be noted that the sensor can be of a compact
design utilizing an integrated circuit manufactured by CMOS
technology. This technique is disclosed in a copending application
assigned to the assignee of the present application. According to
this technique, the sensor comprises a package including the light
source, a block of two tubular optical waveguides of different
diameters concentrically dislocated one inside the other and
surrounding the light source, and an integrated circuit plate
comprising two ring-like areas of photodiodes positioned
concentrically one inside the other. The integrated circuit is also
provided with a plurality of printed contact areas and electric
conductors intended for mounting the light source thereon,
controlling the light source, and transmitting electric signals
produced by the photodiodes areas for further processing.
[0080] FIG. 8 illustrates a block diagram of a pulse oximeter 20
utilizing the above-described sensor 10. The pulse oximeter
typically includes a control unit 21, which is composed of an
electronic block 22 including A/D and D/A converters connectable to
the sensor 10, a microprocessor 24 for analyzing measured data, and
a display 26 for presenting measurement results. The measured data
(i.e., electrical output of the sensor 10 indicative of the
detected light) is directly processed in the block 22, and the
converted signal is further processed by the microprocessor 24. The
microprocessor 24 is operated by a suitable software model for
analyzing the measured data and utilizing reference data (i.e.,
calibration curve stored in a memory) to compute the oxygen
saturation value, which is then presented on the display 26. The
analysis of the measured data utilizes the determination of AC- and
DC-components in the detected light for each wavelength, .lambda.1,
.lambda.2, and .lambda.3, respectively, i.e., I.sub.1.sup.(AC),
I.sub.1.sup.(DC), I.sub.2.sup.(AC), I.sub.2.sup.(DC),
I.sub.3.sup.(AC), and I.sub.3.sup.(DC), and the calculation of
AC/DC ratio for each wavelength, namely,
W.sub.1=I.sub.1.sup.(AC)/I.sub.1.sup.(DC),
W.sub.2=I.sub.2.sup.(AC)/I.sub- .2.sup.(DC), and
W.sub.3=I.sub.3.sup.(AC)/I.sub.3.sup.(DC), as will be described
more specifically further below with reference to FIGS. 9 and
10A-10C.
[0081] The pulse oximeter 20 with the sensor arrangement shown in
FIG. 7 provides the following three possible ratio values:
W.sub.1/W.sub.2, W.sub.1/W.sub.3 and W.sub.2/W.sub.3. It should be
noted that W.sub.1/W.sub.2 and W.sub.1/W.sub.3 are the ratios that
typically have the highest sensitivity to oxygen saturation. This
is due to the fact that .lambda.1 is chosen in the red region of
the electromagnetic spectrum, where the changes in the absorption
between Hb and HbO.sub.2 are the largest, as described above with
reference to FIG. 1. Therefore, in principle, the absorption ratios
formed by either wavelength pair .lambda.1 and .lambda.2 or
wavelength pair .lambda.1 and .lambda.3 can be used to compute the
value of SaO.sub.2.
[0082] The inventor conducted extensive human and animal studies,
and confirmed that either of the two ratios W.sub.1/W.sub.2 and
W.sub.1/W.sub.3 can be affected not only by changes in arterial
oxygen saturation, but also by sensor placement and by the amount
of pressure applied by the sensor on the skin. Any calculation of
SaO.sub.2 based on either of the two ratios W.sub.1/W.sub.2 and
W.sub.1/W.sub.3 alone (as normally done in commercially available
dual-wavelength pulse oximeters) could result in significant
errors. Furthermore, since at least two wavelengths are necessary
for the calculation of arterial oxygen saturation, it is not
feasible to self-correct the calibration curve for variations due
to contact pressure or site-to-site variations utilizing the same
two wavelengths used already to compute SaO.sub.2.
[0083] The inventor has found that the third ratio W.sub.2/W.sub.3
formed by the combination of the two IR wavelengths is mostly
dependent on changes in contact pressure or site-to-site
variations. Furthermore, this ratio can depend, but to a much
lesser degree, on variations in arterial oxygen saturation. The
dependency on arterial oxygen saturation, however, is easily
minimized or eliminated completely, for example by selection and
matching of the peak emission wavelengths and spectral
characteristics of the two IR light emitting elements 12b and
12c.
[0084] Generally, the two IR wavelengths .lambda.2 and .lambda.3
are selected to coincide with the region of the optical absorption
curve where HbO.sub.2 absorbs slightly more light than Hb, but in
the spectral region, respectively, where the extinction
coefficients of both Hb and HbO.sub.2 are nearly equal and remain
relatively constant as a function of wavelength. For example, at
940 nm and 880 nm, the optical extinction coefficients of Hb and
HbO.sub.2 are approximately equal to 0.29 and 0.21, respectively.
Therefore, ideally, the ratio of W2/W3 should be close to 1, except
for situations when the AC/DC signals measured from .lambda.2 and
.lambda.3 are affected unequally causing the ratio W2/W3 to deviate
from 1.
[0085] Fortunately, variations in the ratio W2/W3 mimic changes in
the ratios W.sub.1/W.sub.2 and W.sub.1/W.sub.3 since these ratios
are all affected by similar variations in sensor positioning or
other uncontrollable factors that normally can cause large errors
in the calibration curve from which oxygen saturation is typically
derived. Thus, by tracking in real-time changes in the ratio formed
by wavelengths .lambda.2 and .lambda.3, it is possible to
automatically correct for errors in the normalized ratios obtained
from wavelengths .lambda.1 and .lambda.2, or from .lambda.1 and
.lambda.3.
[0086] The use of an additional third wavelength in the sensor
serves another important function (not available in conventional
dual-wavelength pulse oximeters), which is associated with the
following. Reflectance pulse oximeters have to be capable of
detecting and relying on the processing of relatively low quality
photoplethysmographic signals. Accordingly, electronic or optical
noise can cause large inaccuracies in the final computation of
SaO.sub.2. Although the amount of electronic or optical noise
pickup from the sensor can be minimized to some extent, it is
impossible to render the signals measured by the pulse oximeter
completely noise free. Therefore, pulse oximeters rely on the
assumption that any noise picked up during the measurement would be
cancelled by calculating the ratio between the R- and IR-light
intensities measured by the photodetector. Practically, however,
the amount of noise that is superimposed on the R-and
IR-photoplethysmograms cannot be cancelled completely and, thus,
can lead to significant errors in the final computation of
SaO.sub.2 which, in dual-wavelength pulse oximeters, is based only
on the ratio between two wavelengths.
[0087] By utilizing a third wavelength, the invention has the added
ability to compute SaO.sub.2 based on the ratio formed from either
W.sub.1/W.sub.2 or W.sub.1/W.sub.3. An algorithm utilized in the
pulse oximeter according to the invention has the ability to track
and compare in real-time changes between W.sub.1/W.sub.2 and
W.sub.1/W.sub.3to determine which ratio produces a more stable or
less noisy signal and selectively choose the best ratio for
calculating SaO.sub.2.
[0088] The method according to the invention utilizes the so-called
"selection process" as part of the signal processing technique
based on the measured data obtained with the multiple
photodetectors. The main steps of the selection process are shown
in FIG. 9 in a self-explanatory manner. Here, the symbol i
corresponds to a single photodetector element in the array of
multiple discrete photodetector elements, the term "1st" signifies
the last photodetector element in the array, and the term "DATA"
signify three ratios (AC/DC) computed separately for each of the
three wavelengths, namely, W.sub.1, W.sub.2 and W.sub.3.
[0089] The selection process is associated with the following:
Practically, each time one of the light emitting elements is in its
operative position (i.e., switched on), all of the photodetectors
in the sensor receiving backscattered light from the skin. However,
the intensity of the backscattered light measured by each
photodetector may be different from that measured by the other
photodetectors, depending on the anatomical structures underneath
the sensor and its orientation relative to these structures.
[0090] Thus, the selection process is used to discriminate between
photodetectors receiving "good" signals (i.e., "good" signal
meaning that the calculation of SpO.sub.2 from the pulsating
portion of the electro-optic signal (AC) and the constant portion
(DC) would result in accurate value) and "bad" signals (i.e.,
having AC and DC values which would result in inaccurate
calculations of SpO.sub.2). Accordingly, each data point (i.e.,
ratio W.sub.1i, W.sub.2i or W.sub.3i detected at the corresponding
i.sup.th detector) is either accepted, if it meets a certain
criteria based for example on a certain ratio of AC to DC values
(e.g., such that the intensity of AC signal is about 0.05-2.0% of
the intensity of DC signal), or rejected. All of the accepted data
points (data from accepted detection locations) are then used to
calculate the ratios W.sub.1/W.sub.2, W.sub.1/W.sub.3 and
W.sub.2/W.sub.3, and to calculate the SpO.sub.2 value, in
conjunction with the signal processing technique, as will be
described further below with reference to FIGS. 10A-10C.
[0091] Besides the use of the third IR-wavelength to compensate for
changes in the internal calibration curve of the pulse oximeter,
the pulse oximeter utilizing the sensor according to the invention
provides a unique new method to compensate for errors due to sensor
positioning and pressure variability. This method is based on
multiple photodetector elements, instead of the conventional
approach that relies on a single photodetector.
[0092] While optical sensors with multiple photodetectors for
application in reflectance pulse oximetry have been described
before, their main limitation relates to the way the information
derived from these photodetectors is processed. Although the
primary purpose of utilizing multiple photodetectors is to collect
a larger portion of the backscattered light from the skin,
practically, summing the individual intensities of each
photodetector and using the resulting value to compute SaO.sub.2
can introduce large errors into the calculations. These errors can
be caused, for example, by situations where the sensor is placed
over inhomogeneous tissue structures such as when the sensor is
mounted on the chest. The case may be such that, when using a
continuous photodetector ring to collect the backscattered light, a
portion of the photodetector ring lies over a rib, which acts as a
strongly reflecting structure that contributes to a strong DC
component, and the remaining part of the photodetector is
positioned over the intercostals space, where the DC signal is much
smaller. In this case, the final calculation of SaO.sub.2 would be
inaccurate, if the current produced by this photodetector is used
indiscriminately to compute the DC value before the final
computation of SaO.sub.2 is performed. Therefore, in addition to
automatically correcting errors in the calibration curve as
outlined above using three different LEDs (one R and two different
IR wavelengths), the sensor 10 has the optional ability to track
automatically and compare changes in the R/IR ratios obtained from
each of the discrete photodiodes individually. For example, if some
of either the near or the far photodetectors in the two
concentrically arranged arrays detect larger than normal DC signals
during the operation of one of the photodiodes compared to the
other photodiodes in the sensor, it could be indicative of one of
the following situations: the sensor is positioned unevenly, the
sensor is partially covering a bony structure, or uneven pressure
is exerted by the sensor on the skin causing partial skin
"blanching" and therefore the blood-to-bloodless tissue ratio might
be too high to allow accurate determination of SaO.sub.2. If such a
situation is detected, the oximeter has the ability to selectively
disregard the readings obtained from the corresponding
photodetectors. Otherwise, if the DC and AC signals measured from
each photodetector in the array are similar in magnitude, which is
an indication that the sensor is positioned over a homogeneous area
on the skin, the final computation of SaO.sub.2 can be based on
equal contributions from every photodetector in the array.
[0093] Turning now to FIGS. 10A, 10B and 10C, there are illustrated
three main steps of the signal processing technique utilized in the
present invention. Here, TH.sub.1, and TH.sub.2 are two different
threshold values (determined experimentally) related respectively
to W.sub.2/W.sub.3 and (W.sub.1/W.sub.2-W.sub.1/W.sub.3).
[0094] During step 1 (FIG. 10A), measured data generated by the
"near" and "far" photodetectors indicative of the detected
(backscattered) light of wavelength .lambda.2 and .lambda.3 is
analyzed to calculate the two ratios W.sub.2/W.sub.3 (far and
near). If one of the calculated ratios (far or near) is not in the
range of 1.+-.TH.sub.1 (TH.sub.1 is for example 0.1), then this
data point is rejected from the SpO.sub.2 calculation, but if both
of them are not in the mentioned range, a corresponding alarm is
generated indicative of that the sensor position should be
adjusted. Only if there are calculated ratios which are in the
range of 1.+-.TH1, they are accepted and the process (data
analysis) proceeds by performing step 2.
[0095] Step 2 (FIG. 10B) consists of determining whether the
quality of each photoplethysmogram is acceptable or not. The
quality determination is based on the relative magnitude of each AC
component compared to its corresponding DC component. If the
quality is not acceptable (e.g., the signal shape detected by any
detector varies within a time frame of the measurement session,
which may for example be 3.5 sec), the data point is rejected and a
corresponding alarm signal is generated. If the AC/DC ratio of
W.sub.1, W.sub.2 and W.sub.3 are within an acceptable range, the
respective data point is accepted, and the process proceeds through
performing step 3.
[0096] In step 3 (FIG. 10C), the measured data is analyzed to
calculate ratios W.sub.1/W.sub.2 and W.sub.1/W.sub.3 from data
generated by far and near photodetectors, and to calculate the
differences (W.sub.1/W.sub.2-W.sub.1W.sub.3).
[0097] In a perfect situation, W.sub.1/W.sub.2 (far) is very close
to W.sub.1/W.sub.3 (far), and W.sub.1/W.sub.2 (near) is very close
to W.sub.1/W.sub.3 (near). In a practical situation, this condition
is not precisely satisfied, but all the ratios are close to each
other if the measurement situation is "good".
[0098] Then, the calculated differences are analyzed to determine
the values (corresponding to far and near photodetectors) that are
accepted and to use them in the SpO.sub.2 calculation. For each
detector that satisfied the condition
ABS(W.sub.1/W.sub.2-W.sub.1/W.sub.3)<TH.sub.2)- , where ABS
signifies the absolute value, its respective data point is accepted
and used to calculate the oxygen saturation value that will be
displayed. If the condition is not satisfied, the data point is
rejected. If all data points are rejected, another measurement
session is carried out.
[0099] It should be noted that, although the steps 1-3 above are
exemplified with respect to signal detection by both near and far
photodetectors, each of these steps can be implemented by utilizing
only one array of detection locations along the closed path. The
provision of two such arrays, however, provides higher accuracy of
measurements.
* * * * *