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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns pulse oximeters, and more particularly devices and
methods for calibrating pulse oximeters.
2. General Discussion of the Background
Continuous assessment of arterial oxygenation is important in the treatment
of critically ill or anesthesized patients. Such individuals are often
dependent upon artificial life support systems which sufficiently
oxygenate their blood to prevent permanent physical impairment, brain
damage, or death. Blood oxygen concentration levels in such patients must
be carefully maintained within a narrow range to prevent serious physical
consequences. Premature infants, for example, must have a blood oxygen
content below about 95% to prevent retinal damage and above about 90% to
prevent respiratory distress.
Several methods have been devised for continuously monitoring blood oxygen
levels. Many of these methods involve invasive removal of blood for
analysis, which provides only intermittent information. Transcutaneous
oxygen tension measurement provides continuous information but requires
special site preparation, airtight probe mantling and a potentially
harmful local heat source to induce arterialization. In addition to these
serious drawbacks, transcutaneous oxygen monitoring often fails accurately
to reflect true arterial oxygenation.
Pulse oximeters, such as those shown in U.S. Pat. No. 4,167,331, European
patent application publication Nos. 0 104 771, and 0 102 816 are most
convenient for monitoring blood oxygen concentrations. Such pulse
oximeters function by positioning a pulsating arterial vascular bed
between a two wavelength light source and a detector The pulsating
vascular bed, by expanding and relaxing, creates a change in the light
path length that modifies the length detected and results in a
plethysmograph waveform The amplitude of the varying detected light
depends on the size of the arterial pulse change, the wavelength of light
used, and the oxygen saturation of the arterial hemoglobin. The detected
pulsatile waveform is produced solely from arterial blood using the
amplitude of each wavelength and Beer's law. An exact beat-to-beat
continuous calculation of arterial hemoglobin oxygen saturation can
thereby be obtained with no interference from surrounding venuous blood,
skin, connective tissue, or bone.
A typical pulse oximeter includes a probe which is attached on either side
of a distal digit, such as the tip of a finger. The probe includes the
light source and the detector which are held in opposing relationship to
one another on either side of the finger such that the light source
directs a beam of light through the finger and towards the detector Most
pulse oximeters use simple detecting circuitry with diodes that have broad
spectral sensitivity. The light source emits wavelengths of red and
infrared light which correspond to absorption peaks of oxyhemoglobin and
deoxyhemoglobin in red blood cells entering the capillaries during
systoly. A background absorption occurs from the hemoglobin remaining in
small vessels during diastoly and from general tissue absorption. By
rapidly alternating the wavelengths of light transmitted through the
tissue, the difference in absorption for total hemoglobin and
oxyhemoglobin can be measured for each pulse of arterial capillary blood.
An estimated percentage of oxygenated hemoglobin in each pulse can then be
calculated from the difference in absorption.
Several pulse oximeters employing these principles are now available. The
Nellcor pulse oximeter Model N-100 is available from Nellcor Inc. of
Hayward, Calif. Other such devices include the Ohmeda Biox 3700 pulse
oximeter and the Novametrix pulse oximeter.
Although pulse oximeters are valuable tools in continuously monitoring
oxygenation levels of blood hemoglobin, there is presently no way for a
user to test whether a pulse oximeter is accurate. A careful and
convenient method of calibrating pulse oximeters is needed since the
oxygenation level of a patient's blood hemoglobin must be maintained
within a narrow and often critical range. Slight deviations from the range
must be accurately detected by the oximeter.
It is therefore an object of the present invention to provide a device and
method for calibrating pulse oximeters.
It is yet another object of the invention to provide such a calibrating
device which is convenient and simple to use.
The foregoing and other objects and advantages of the invention will become
more apparent from the following detailed description of the preferred
embodiments which proceed with reference to the accompanying drawings.
SUMMARY OF THE INVENTION
The foregoing objects are achieved by providing the detector probe of a
pulse oximeter with an artificial image of a pulse by periodically varying
the intensity of light received by the light detector of the probe. This
is accomplished by placing, in the light path of the probe, a filter
system having a light transmissivity which can be gradually and
periodically varied to simulate a desired pulse rate. A sample of material
which simulates hemoglobin of a known oxygen content is also placed in the
light path between the source and detector. The readouts for the pulse
rate and oxygenated hemoglobin content can then be compared with the known
reciprocation rate and oxygen concentration. If the pulse oximeter has
detected values different from the induced pulse and known oxygen content
of the standard sample, the oximeter can be replaced or adjusted to match
the known values.
In one embodiment of the calibrating device, the pulse is simulated by
means of a light-absorbing, wedge-shaped member located between the light
source and the light detector. The member is longitudinally reciprocated
at a known rate along an axis generally perpendicular to the light path.
The progressively increasing thickness of the wedge as it moves in one
direction reduces the amount of light transmitted through the wedge to the
detector, while the progressively decreasing thickness of the wedge as it
moves in the opposite direction increases the amount of light detected.
Reciprocal movement of the wedge therefore creates, for the detector, the
image of a pulse at a known rate. The wedge member is a vessel which
contains hemoglobin having a known oxygen content. The known values for
pulse rate and oxygen percentage are compared with the detected values
computed by the oximeter.
In another embodiment, the pulse images are created by a pair of parallel
polarization filters. The detector probe is positioned in relation to the
polarization filters such that the light source directs a beam of light
through both filters towards the detector. One of the filters is then
rotated with respect to the other to sequentially increase and decrease
the amount of light received by the detector. A stepping motor can be used
to rotate the rotatable filter at a programmed rate to mimic changes in
transmittance during a normal physiological pulse. A sample of material
which contains hemoglobin with a known oxygen content is placed in the
beam of light which passes between the light source and detector of the
probe.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of a pulse oximeter.
FIG. 2 is a top plan view of a calibration wedge for a pulse oximeter.
FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2.
FIG. 4 is a top plan view of an assembly including a frame and two wedges,
similar to the wedge shown in FIGS. 1 and 2, the wedges being superposed
and tapering in opposite directions.
FIG. 5 is a side elevation of the assembly shown in FIG. 4.
FIG. 6 is a front elevation of the assembly shown in FIG. 4.
FIG. 7 is a cross-sectional view taken along line 7--7 of FIG. 4.
FIG. 8 is a cross-sectional view of two superposed wedges of different
tapers, the wedges tapering in the same direction.
FIG. 9 is a front elevational view of a detector probe of a pulse oximeter
being tested by reciprocating movement of a wedge of the type shown in
FIGS. 2 and 3.
FIG. 10 is a cross-sectional view taken along line 10--10 of FIG. 9.
FIG. 11 is a top plan view of an optical bench device for calibrating a
pulse oximeter.
FIG. 12 is a cross-sectional view taken along line 12--12 of FIG. 11.
FIGS. 13 and 14 are schematic views of a detector probe of a pulse oximeter
connected to the optical bench calibration device of FIGS. 11 and 12.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIGS. 2-14 show several embodiments of devices for calibrating a pulse
oximeter, such as those oximeters shown and described in U.S. Pat. No.
4,167,331 and European patent application publication Nos. 0 102 816 and 0
104 771, all of which are incorporated herein by reference. Commercial
embodiments of the pulse oximeter with which the calibration device can be
used include the Nellcor pulse oximeter Model N-100, the Ohmeda Biox 3700,
and the Novametrix pulse oximeter.
FIG. 1 illustrates a pulse oximeter 12 which includes a box-like housing
14, display screen 16, push button controls 18, 20, 22, 24, control knob
26, and speaker 28 through which acoustic signals and alarms are sounded.
Oximeter 12 gathers data through a detector probe 30 which is connected to
oximeter 12 through lead line 32. Detector probe 30 includes jaw members
36, 38 which are spring biased toward each other and suitably dimensioned
to fit snugly around the tip of a human finger or earlobe with top member
36 resting, for example, on the flat surface of a human fingernail while
bottom member 38 is secured on the opposing side of the fingertip. Top
member 36 carries a light emitting diode 40 while bottom member 38 carries
a light sensor 42. A more detailed discussion of the structure and
operation of the pulse oximeter is not given here since it is more fully
explained in the above-cited patents and published patent applications.
The present invention employs a method and apparatus for simulating the
periodic light transmissivity shifts which result from the pulsing of
blood through an arterial vascular bed and a standard sample of a material
which has the optical characteristics of hemoglobin of a known oxygen
content.
Oxygen Content Standard
The most convenient and flexible way of creating a hemoglobin oxygenation
standard is to use a vessel containing blood of a known oxygen content.
FIGS. 2 and 3 show such a vessel in the form of a cuvette wedge 10 which
has a flat bottom 50 and a slanted top 52 interconnected by a sidewall 54.
Wedge 10 is preferably made of an optical quality quartz glass or similar
material of a known optical density and defines a chamber 56. Access to
the chamber is provided by a pair of capillary filling tubes 58, 60
through sidewall 54 at the base end 61 of wedge 10.
The chamber 56 flares from a minimal thickness at narrow end 62 of wedge 10
to a thickness of one millimeter at the base end 61 where capillary tubes
58, 60 enter wedge 10. In other embodiments of the wedge, the height of
chamber 56 can be different, for example, two millimeters, three
millimeters, four millimeters, and so on at the base end 61. The thickness
of each glass surface, such as surfaces 50, 52, 54 is one millimeter.
Each wedge 10 is filled with a material which has the optical
characteristics of blood of a known oxygen content. In the illustrated
embodiment, this material is microbiological media, consisting of 5%
sheep's blood in agar, that has been equilibrated with a known gas mixture
before being introduced into wedge 10 through capillary tubes 58, 60. Once
the media is introduced into chamber 56, tubes 58, 60 are sealed either by
a cap or heat sealing to provide an airtight sample. Wedge 10 can then be
stored in a refrigerator until use.
Although 5% sheep's blood is used as the known sample in this embodiment,
human blood suspended in a matrix such as agar and having hemoglobin of a
known oxygen concentration can also be used. It is preferable to use a
sample having about 5% blood suspended in the matrix since about 5% to 8%
of finger tissue is comprised of blood. Agar is a preferred matrix
material because it reasonably simulates tissue and can easily be flushed
from the chamber 56 so that the wedge can be reused. When blood is used in
the medium, the samples are best prepared just shortly before the time of
calibration. Otherwise, steps should be taken to stabilize the blood.
Wedges, such as wedge 10, can be combined with other wedges of the same
thickness or other thicknesses for their additive effect. FIGS. 4-7, for
example, show a top wedge 70 having a flat face 72, slanted face 74,
upright end wall 76, and capillary filling tubes 78. A bottom wedge 80
similarly includes a flat face 82, slanted face 84, upright wall 86, and
filling tubes 88. Wedges 70 and 80 are both filled with media which
contains 5% calf's blood. Medium 90 of wedge 70 contains hemoglobin which
is completely unsaturated with oxygen, while medium 92 of wedge 80
contains hemoglobin which is 100% saturated. Wedge 70 is placed on top of
wedge 80 with slanted faces 74, 84 abutting such that wedges 70, 80
cooperatively form a rectangular box with tubes 78, 88 extending from
opposite ends of the box. The wedges 70, 80 are held in precise alignment
by inserting them snugly in a rigid frame 94. The frame includes ribs 96.
Graduation indicia 98 are provided on the frame 94 for precise alignment
of the frame with other equipment.
When wedges 70, 80 are arranged as shown in FIGS. 4-7, they form a
calibrating device which has a progressively changing concentration of
oxygenated hemoglobin along the length of the box. For example, a light
beam which shines through the box from a location 99 and is detected at a
location 100, will shine through completely deoxygenated hemoglobin, while
light that shines through the box from location 101 can be detected at
location 102 as being 50% oxygenated. Light that shines through the box
from location 103 will be detected at location 104 as shining through
hemoglobin which is 100% saturated with oxygen.
Yet another arrangement of wedges is shown in FIG. 8. A top wedge 110
includes a flat face 112, slanted face 114, a one millimeter high upright
end wall 116, and capillary filling tube 118. Bottom wedge 120 similarly
includes a flat face 122, slanted face 124, a four millimeter high upright
end wall 126, and filling tube 128. Top wedge 110 is filled with a
biologic medium 130 which contains hemoglobin which is completely
unsaturated with oxygen. Wedge 120, however, is filled with a biologic
medium 132 which contains 100% oxygen saturated hemoglobin. The wedges are
placed in a frame 134 with wedge 110 on top of wedge 120 with bottom face
112 of wedge 110 abutting slanted face 124 of wedge 120 such that tubes
118, 128 are aligned one above the other.
The combined wedge which results from the combination of wedges 110, 120 as
shown in FIG. 8 will appear to probe detector 30 to have hemoglobin with
an 80% concentration of oxygen along its entire length. Wedges of varying
heights can be similarly juxtaposed to provide a multilayered calibrating
wedge which appears, when scanned by a pulse oximeter probe, to have any
desired hemoglobin oxygen content. The following chart illustrates some
combinations of wedges which can be used to calibrate pulse oximeters at
differing oxygen concentrations.
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Oxygen Concentration
100% 80% 60 40% 20%
Detected
Height of 100%
4 mm 4 mm 3 mm 2 mm 1 mm
Concentration Wedge
Height of 0% 0 mm 1 mm 2 mm 3 mm 4 mm
Concentration Wedge
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Moving Cuvette Pulse Generator
In a first embodiment of the invention, a simulated pulse signal is
provided by reciprocating a wedge 10, filled with blood of a known oxygen
content, (or an assembly of the type shown in FIG. 8) between the diode 40
and sensor 42, in a plane generally perpendicular to the light path as
shown in FIG. 9.
Wedge 10 is first filled with a medium such as 90 or 92 having hemoglobin
of a known oxygen concentration. The wedge 10 is then inserted between
members 36, 38 such that a beam 138 of light emitted by LED 40 passes
through surfaces 52 and 50. For convenience the wedge 10 may be contained
in a channel member 142 that has a compression spring 144 positioned to
engage the end wall 61 of the wedge. The probe 30 can be clipped to the
channel member 142 such that the jaw 36 engages an upper edge of the
member and the jaw 38 engages the bottom of the member with the sensor 42
positioned in alignment with an opening 146 through the bottom of the
member. Wedge 10 is then periodically moved, by hand pushing the narrow
end 62 or by machine, along an axis of movement indicated by directional
arrow 140.
As wedge 10 is progressively moved (to the left in FIG. 9) through jaw 34,
the intensity of light from the LED 40 which is transmitted through the
wedge 10 and detected at sensor 42 grows progressively less as
progressively thicker portions of wedge 10 pass between the LED 40 and the
sensor 42. The resulting reduction in transmissivity of light through the
wedge mimics the reduction in transmissivity that occurs in human tissues
as blood surges through the tissues, engorges it with blood, and expands
the capillary bed and surrounding tissue. When wedge 10 is handheld, the
operator moves it through jaw 34 at a known rate that imitates a normal
human pulse. To ease operation, the length of the spring 144 can be
selected so that, as the spring travels between fully extended and fully
compressed positions, the wedge 10 is moved an appropriate distance to
simulate the degree of transmissivity shift observed in living subjects.
For even greater precision, the wedge can be reciprocated by a machine at
a preselected reciprocation rate and distance. Since the reciprocation
rate is known, it can be compared to the pulse rate displayed on screen 16
of oximeter 12. The oxygen saturation of hemoglobin within wedge 10 is
also known, and it can similarly be compared to the oxygen concentration
level shown on screen 16.
Polarizing Filter Pulse Generator
Another embodiment of the calibrating device, shown in FIGS. 11-14, employs
an optical bench which uses a pair of polarizing discs to simulate the
changing light transmissivities that imitate a human pulse. The optical
bench 150 includes a top, round steel plate 152, and bottom round steel
plate 154 which are held in parallel, spaced relationship to each other by
a plurality of plastic brackets 156. Bottom plate 154 is held above a
stepping motor 158 by four steel struts 160. The stepping motor can be,
for example, a Slo-Syn synchronous stepping motor manufactured by Superior
Electric Company of Bristol, Conn.
Sandwiched between plates 152, 154 are polarizing discs 162, 164. Top disc
162 is mounted stationary below plate 152, while lower plate 164 is
attached to and rotatably driven by drive shaft 166 of motor 158.
Plate 152 is provided with a plurality of openings 168 along the peripheral
edge of plate 152. Each of these openings is aligned with an identical
opening 170 through bottom plate 154.
In the optical bench embodiment, a wedge or plurality of juxtaposed wedges
can be used as the sample of hemoglobin material. For example, a pair of
wedges 70, 80, juxtaposed as shown in FIGS. 4-7, can be positioned over an
opening 168 as shown in FIGS. 11 and 14. FIG. 14 shows a detailed view
wherein the ribs 96 of the frame 94 are slidably received in alignment
channels 178 of a track 80 adjacent the opening 168. As shown in FIG. 14,
a transparent plastic or glass cover 181 may be present over the track 180
so that the wedge assembly can be moved after the probe is attached. The
tracks 180 can be mounted in a fixed position on the plate 152 or can be
attached by a gear drive mechanism (not shown) of the type used to
position slides on a microscope table. Such a gear drive mechanism would
allow very precise adjustment of the position of the wedge assembly. Each
pair of wedges is calibrated to indicate the known oxygen concentration of
the sample at various points along the pair of wedges. Indicia 98 on the
frame 94 can be aligned with indicia 182 on the plate 152 to indicate that
when a particular portion of the wedge assembly is aligned between
openings 168, 170. By positioning the wedge assembly at different
locations, different known oxygen concentrations can be presented in the
light path from the LED 40 to the sensor 42.
In operation, as shown in FIGS. 13 and 14, jaw members 36, 38 of detector
probe 30 are secured to the optical bench by placing top member 36 on top
of plate 152 with LED 40 shining through one of openings 168. Bottom
member 38 is correspondingly positioned against bottom plate 154 with
light sensor 42 being exposed through an opening 170 immediately below one
of the openings 168.
The assembly wedges 70, 80 in frame 94 is then moved to a position in
alignment with the pair of opposing holes 168, 170 such that hemoglobin of
a known oxygen saturation will be detected by light shining through the
pair of opposing holes 168, 170 and the wedge assembly. Stepping motor 158
is next actuated to reciprocatingly rotate polarizing disc 164 relative to
stationary polarizing disc 162 at a programmed rate. As disc 164 rotates,
the amount of light transmitted from the LED 40 to the sensor 42 varies at
a rate that mimics the variation of light transmissivity through a finger
during normal human pulsation. The angular velocity of rotating disc 164
can be preselected to be of a known value such that the pulse reading of
oximeter 12 can be compared to the preselected known value of the pulse
and the accuracy of the oximeter's pulse reading thereby determined.
Similarly, the known oxygen saturation of hemoglobin encountered by a beam
of light as it passes from LED 40 to sensor 42 can be compared to the
concentration percentage that appears on screen 16 of oximeter 12.
In the illustrated embodiment, eight openings 168 are provided through
plate 152, while eight corresponding openings 170 are provided through
bottom plate 154. Up to eight pulse oximeters can be therefore be
calibrated simultaneously by attaching the probe sensors of each of the
eight oximeters to one of the pairs of corresponding openings through
optical bench 150.
Having illustrated and described the principles of the invention in
preferred embodiments, it should be apparent to those skilled in the art
that the invention can be modified in arrangement and detail without
departing from such principles. For example, rather than using wedge
cuvettes containing blood, some other type of filter can be used to
simulate hemoglobin of a known oxygen saturation, provided the filter has
proper optical characteristics. Also, in the moving cuvette embodiment, a
filter of progressively dimminishing light transmissivity might be
constructed from a flat material, which could be juxtaposed on a
parallel-sided cuvette filled with blood or some other parallel-sided
filter that mimics hemoglobin of a known oxygen saturation. I claim all
modifications coming within the spirit and scope of the following claims.
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