U.S. patent number RE39,268 [Application Number 10/852,774] was granted by the patent office on 2006-09-05 for simulation for pulse oximeter.
This patent grant is currently assigned to Fluke Electronics Corp.. Invention is credited to John Tobey Clark, Kay Haas, Peter Haas, Michael William Lane, Edwin B. Merrick.
United States Patent |
RE39,268 |
Merrick , et al. |
September 5, 2006 |
Simulation for pulse oximeter
Abstract
A method and system for simulating living tissue which is to be
monitored by a pulse oximeter that provides red and infrared light
flashes, the system including structure for: converting the red and
infrared light flashes of the pulse oximeter into electrical
signals; modulating the converted electrical signals to provide
modulated electrical signals; and converting the modulated
electrical signals to light flashes and transmitting the converted
light flashes to the pulse oximeter for detection so that the pulse
oximeter responds to the converted light flashes as it would to
light flashes modulated by a living tissue.
Inventors: |
Merrick; Edwin B. (Stowe,
MA), Haas; Kay (Fairfield, VT), Clark; John Tobey
(Essex, VT), Lane; Michael William (Milton, VT), Haas;
Peter (Fairfield, VT) |
Assignee: |
Fluke Electronics Corp.
(Everett, WA)
|
Family
ID: |
22012546 |
Appl.
No.: |
10/852,774 |
Filed: |
May 25, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
08057752 |
May 7, 1993 |
05348005 |
Sep 20, 1994 |
|
|
Current U.S.
Class: |
600/323;
250/252.1; 356/243.1; 356/41; 600/330 |
Current CPC
Class: |
A61B
5/1495 (20130101); A61B 2560/0233 (20130101) |
Current International
Class: |
A61B
5/00 (20060101) |
Field of
Search: |
;600/310,322,323,330,331
;73/1.01 ;356/39-41,243.1 ;250/252.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Winakur; Eric F.
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A method for simulating living tissue which is to be monitored
by a pulse oximeter that provides red and infrared light flashes,
the method comprising: a. converting the red and infrared light
flashes of the pulse oximeter into electrical signals; b.
modulating the converted electrical signals to provide modulated
electrical signals; and c. converting the modulated electrical
signals to light flashes and transmitting the converted light
flashes to the pulse oximeter for detection so that the pulse
oximeter responds to the converted light flashes as it would to
light flashes modulated by a living tissue.
2. The method as defined in claim 1, further comprising the step of
blocking the red and infrared light flashes produced by the
oximeter from being directly detected by the pulse oximeter.
3. The method as defined in claim 1, further comprising the steps
of: converting the brightness of each light flash created in step
(c) into a proportional electrical signal; comparing said
proportional electrical signal to the amplitude of the modulated
electrical signal formed in step (b); and adjusting the light
intensity of the light flash formed in step (c) so as to null out
any difference between said proportional electrical signal and said
modulated electrical signal.
4. An apparatus for simulating living tissue which is to be
monitored by a pulse oximeter that provides red and infrared light
flashes, the apparatus comprising: first means for converting the
red and infrared light flashes of the pulse oximeter into
electrical signals; second means for modulating the converted
electrical signals to provide modulated electrical signals; and
third means for converting the modulated electrical signal to light
flashes and transmitting the converted light flashes to the pulse
oximeter for detection so that the pulse oximeter responds to the
converted light flashes as it would to light flashes modulated by a
living tissue.
5. The apparatus as defined in claim 4, further comprising means
for blocking the red and infrared light flashes produced by the
oximeter from being directly detected by the pulse oximeter.
6. The apparatus as defined in claim 4, further comprising: means
for converting the brightness of each light flash created by said
first means into a proportional electrical signal; means for
comparing said proportional electrical signal to the amplitude of
the modulated electrical signal formed by said second means; and
means for adjusting the light intensity of the light flash formed
in said third means so as to null out any difference between said
proportional electrical signal and said modulated electrical
signal.
7. The system of claim 4, wherein the means for converting the red
and infrared light flashes of the pulse oximeter into electrical
signals comprises photodiode detector means, amplification means,
signal coupling means which removes the dc component from the
amplified photodiode electrical signal, and dc restorer means to
reference said photodiode electrical signal to a fixed level,
regardless of photodiode output due to ambient light.
8. The system of claim 4, wherein said modulating means for
modulating said electrical signals comprises a plurality of
multipliers.
Description
FIELD OF THE INVENTION
The present invention generally relates to the field of pulse
oximeters, and more particularly, relates to a device and method
for testing or calibrating pulse oximeters.
BACKGROUND OF THE INVENTION
The non-invasive monitoring of arterial oxygen saturation
(SaO.sub.2) by pulse oximetry is used in many clinical
applications. For example, SaO.sub.2 monitoring is performed during
surgery, in critical care situations, for hypoxemia screening, in
the emergency room, and in the field. The instruments are small and
lightweight, making them ideal for neonatal, pediatric and
ambulatory applications. Because this instrument is capable of
providing continuous and safe measurements of blood oxygenation
non-invasively, the pulse oximeter is widely recognized as one of
the most important technological advances in bedside monitoring. In
1986, the American Society of Anesthesiologists recommended pulse
oximetry as a standard of care for basic intraoperative monitoring,
and in 1988, the Society for Critical Care Medicine recommended
that this method be used for monitoring patients undergoing oxygen
therapy. The mandatory or voluntary use of pulse oximetry by
regulatory agencies and professional organizations is likely to
continue.
Because pulse oximeters are small, easy-to-use and readily
available, they have become widespread in the last decade. The high
costs associated with health care make the use of non-invasive
pulse oximetry very attractive as it permits effective oxygen
monitoring without the expensive clinical laboratory analysis of
blood samples.
Oxygen saturation measurements rely on the difference in optical
absorbance of deoxyhemoglobin (Hb) and oxyhemoglobin (HbO.sub.2),
as shown in FIG. 1. HbO.sub.2 absorbs less light in the red region
(ca. 660 nm) than does Hb, but absorbs more strongly in the
infrared region (ca. 940 nm). If both wavelengths of light are
used, their opposite change in light absorbed as HbO.sub.2 varies
versus Hb produces a sensitive index of blood oxygen saturation.
The "functional hemoglobin saturation" is defined as: Functional
SaO.sub.2={[HbO.sub.2]/[HbO.sub.2+Hb]}.times.100% (1)
Pulse oximeters thus employ two discrete wavelengths of light,
which are passed through a given tissue (typically a finger). The
amount of transmitted light for each wavelength is detected and
subtracted from the incident light to determine the amount
absorbed. From the ratio (R/IR or "red/infrared") of the amount of
light absorbed at each wavelength, the blood oxygen saturation is
calculated from a predetermined algorithm. If these were the only
conditions of the measurement, the calculated saturation value
would in some degree reflect the mixture of arterial and venous
blood flowing through the tissue. However, in pulse oximetry the
time-variant photoplethysmographic signal, caused by increases in
arterial blood volume due to cardiac contraction, is used to
determine the arterial blood oxygen saturation (FIG. 2). The
advantage of this method is that the oxygen saturation values of
the relatively constant flow of arterial and venous blood, as well
as the constant absorption of light by the tissue, are
discarded.
The SaO.sub.2 values are derived by analyzing only the changes in
absorbance caused by the pulsating arterial blood at a red
wavelength (e.g., 660 nm) where the absorbance of HbO.sub.2 is less
than that of Hb, and a second reference infrared wavelength (e.g.,
940 nm), where the absorbance of HbO.sub.2 is slightly larger than
Hb. Because the transmitted light intensities depend on the
sensitivity of the detector and the individual intensities of the
light sources (light-emitting diodes, or LEDs), and because tissue
absorption can vary a great deal between individuals, a
normalization procedure is commonly used. This normalization
involves dividing the pulsatile (AC) component of the red and
infrared photoplethysmograms (which is a result of the expansion
and relaxation of the arterial blood) by the corresponding
non-pulsatile (DC) component of the photoplethysmogram (which is
due to the absorption of light by tissue, non-pulsatile arterial
blood, and venous blood). This scaling process results in a
normalized red/infrared ratio (R/IR) which is virtually independent
of the incident light intensity. R/IR can thus be expressed as:
R/IR=[AC.sub.red/DC.sub.red]/[AC.sub.ir/DC.sub.ir] (2)
Pulse oximeters are calibrated empirically by correlating the
measured ratio of normalized AC/DC signals from the red and
infrared photoplethysmograms with blood SaO.sub.2 values obtained
from a standard in vitro oximeter. A typical relationship between
the normalized R/IR ratio and SaO.sub.2 is shown in FIG. 3. At
approximately 85% SaO.sub.2, the amount of light absorbed by Hb and
HbO.sub.2 is nearly the same, so the normalized amplitudes of the
red and infrared signals are equal, and R/IR is 1. For properly
functioning instruments, further calibration should not be required
in the field because the optical properties of blood are fairly
similar among different individuals.
Pulse oximeter probes consist of LEDs for two separate and discrete
wavelength (e.g., 660 and 940 nm) and a photodiode light detector.
Three different light levels are measured by the photodiode: the
red (660 nm) light level, the infrared (940 nm) light level, and
the ambient light level. These three light sources are detected
separately by a single photodiode by sequencing the red and
infrared light sources on and off, allowing an interval when both
are off in order to detect (and subtract out) ambient light. An
example from the commercially available Ohmeda model 3700 pulse
oximeter is shown in FIG. 4. Sequencing the red and infrared LEDs
at a frequency that is an integer multiple of the power line
frequency allows the system of operate synchronously with
flickering room lights. For example, fluorescent lights generate a
120 Hz flicker on 60 Hz power. The sequencing avoids potential
interference of light flickers on the photodiode that would distort
or disguise the tiny pulse signals of arterial pulse flow. The
light timing sequence shown in FIG. 4 cycles 480 times per second
at 60 Hz power; 16 of the red-infrared-off sequences are used to
calculate SaO.sub.2 every 0.033 second. These signals are used
differently by different pulse oximeter manufacturers, as described
below.
The response time of the instrument depends on the number of data
points averaged before a final SaO.sub.2 reading is displayed.
There are two basic approaches to this averaging, one of which
relies on the time average of the peak-to-peak amplitudes of each
pulse (FIG. 5A). This method depends on the patient's heart rate
and is relatively slow as the signals are available for averaging
only once every heartbeat. Another approach is to average a large
number of step changes along the steep slopes of the
photoplethysmogram (FIG. 5B). In this case, the response time in
the instrument is shorter because there are many more data points
between successive heartbeats; also, the accuracy and stability of
the measured SaO.sub.2 values are usually improved by this
approach. The accuracy of pulse oximeters has been extensively
studied and has been found to be generally acceptable for a large
number of clinical applications. Most manufacturers claim that
their instruments are accurate to within .+-.2% in the SaO.sub.2
range of 70-100% and within .+-.3% for SaO.sub.2 values between 50
and 70%, with no specified accuracy below 50% saturation.
Most pulse oximeters offer other display features in addition to
SaO.sub.2, such as the pulse rate and displays to indicate the
pulse waveform and relative pulse amplitude. These help the user to
partially assess the quality and reliability of the measurement.
For instance, if the patient's actual heart rate does not agree
with that displayed by the pulse oximeter, the displayed SaO.sub.2
value is brought into question. In addition, the shape and
stability of the photoplethysmographic waveform often serves as an
indication of possible motion artifacts.
Although pulse oximeters offer such advantageous features as
described above, are now mandatory for all anesthesias and tens of
thousand's of oximeters are in clinical use, doctors and hospitals
have no way of knowing if the oximeters are working correctly.
Until the present invention, there has not been a simple method or
device for verifying oximeter operation despite a clear and
pressing need. Manufacturers sometimes provide simple electronic
simulators to test the electronic circuitry of their oximeters, but
these do not test the performance of the optical sensor and
therefore are inadequate. U.S. Pat. Nos. 4,968,137 and 5,166,517
are examples of prior art methods and devices for testing pulse
oximeters.
SUMMARY OF THE INVENTION
It is a general object of the invention to provide an apparatus and
method for fully determining the quality and reliability of
measurements made with pulse oximeters.
It is another object of the invention to provide an apparatus and
method which are suitable for testing most commercially available
pulse oximeters.
These and other objects of the invention are achieved in accordance
with the present invention which provides a system for simulating
living tissue which is to be monitored by a pulse oximeter which
provides red and infrared light flashes, the system including:
converting the red and infrared light flashes of the pulse oximeter
into electrical signals; modulating the converted electrical
signals to provide modulated electrical signals; and converting the
modulated electrical signals to light flashes and transmitting the
converted light flashes to the pulse oximeter for detection so that
the pulse oximeter responds to the converted light flashes as it
would to light flashes modulated by a living tissue.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-3 are graphs for explaining the principles of pulse
oximetry.
FIG. 4 is a graph for explaining the output of a photo-detector on
a known pulse oximeter.
FIGS. 5A and 5B are graphs for explaining response times of pulse
oximeter instrumentation.
FIGS. 6A-6C are schematic diagram of an oximeter test instrument
according to an embodiment of the invention.
FIG. 7 is a circuit diagram of an oximeter test instrument
according to an embodiment of the invention.
FIG. 8 is a circuit diagram showing elements of the circuit of FIG.
7 in greater detail.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 6 is a schematic diagram of a pulse oximeter detector or test
instrument according to an embodiment of the invention. The test
instrument shown in FIG. 6 is intended for use with pulse oximeters
employing sensors which clamp around the patient's finger. As shown
in FIG. 6, the test instrument has a finger-like shape which is
intended to mimic that of the patient. The test finger may be, for
example, 3.5'' long with a 0.75'' diameter. According to this
embodiment, the test instrument is fabricated from steel. Further,
two long sensing photodiodes are positioned in the lower
longitudinal slot 1, one diode having an infrared band pass filter
so as to only receive IR, and a red LED light bar is placed in the
upper longitudinal slot 2, with another photodiode placed so as to
partially cover the light bar. The long, narrow shape of the test
instrument (and the LED light bar) is intended to facilitate
positioning of the instrument within the grip of the pulse
oximeter, or the "unit-under-test" (UUT).
The flat section at the end of the "finger" provides a mechanical
connection point for an analog processing circuit board. The use of
a steel construction provides both opacity between the UUT light
source and the UUT detector, and electrical shielding between the
pulsing calibrator LED and the sensitive calibrator photodiode. It
has been found that such shielding is essential to provide accurate
measurements of the UUT. The round smooth sides will form a
reasonably good seal with the UUT finger grip (e.g., Nellcor).
Although the steel finger-shaped test instrument according to this
embodiment is attached directly to the circuit board, it can be
mounted at the end of a cable, much like a mouse. The electronics
could then be placed within a computer, with, for example, only an
photosensor pre-amplifier inside the "finger".
In an alternative embodiment, the steel finger-shaped test
instrument is replaced with a printed circuit board cut to
approximate finger width and length, with the two sensing
photodiodes on the bottom surface and the LED bar with its
associated photodiode mounted on the top surface. It should be
noted that with a PC board it is still essential to provide opacity
between the UUT light source and the UUT detector.
FIG. 7 is a circuit diagram of the oximeter test instrument
according to an embodiment of the invention. As shown in FIG. 7,
the circuitry includes a pair of photodiodes represented by the
reference numeral 10 which feed a pulse separator and edge timing
circuit 12, a pair of DC multipliers M1A, M1B which are coupled to
the pulse separator and edge timing circuit 12 via a pair of
switches S1A and S1B; respectively, a pair of AC multipliers M2A
and M2B which are connected to receive the outputs of DC
multipliers M1A and M1B, respectively, a multiplier M3A which is
coupled to receive one of the outputs of AC multipliers M2A and M2B
depending on the position of switch S2B, and a switch S2A which is
coupled to selectively pass one of the outputs of DC multipliers
M1A and M1B. As shown in FIG. 7, switches S1A and S1B are used
controlled, whereas switches S2A and S2B are controlled according
to an output of pulse separator and edge timing circuit 12. As will
be discussed in greater detail below, switches S2A and S2B are
controlled in accordance with detected IR flashes.
The circuitry shown in FIG. 7 further includes an amplifier A2
having an inverting terminal (-) which receives the signal passed
by switch S2A, as amplifier A3 having an inverting terminal coupled
to receive the output of amplifier A2 summed with the output of
multiplier M3A, a servo amplifier A4 having a non-inverting
terminal (+) coupled to receive the output of amplifier A3 and
coupled to the drain of FET Q1 which has its source connected to
ground and its gate coupled to receive an output of pulse separator
and edge timing circuit 12, and an inverting terminal of amplifier
A4 is coupled to receive an output of a pulse amplifier with
baseline restore circuit 14. The circuit 14 is coupled to a
photodiode 18 which detects light emitted from LED bar 16. In
addition, the circuit of FIG. 7 includes a driving transistor A2,
an LED bar 16, an ambient light simulation circuit 19 and a
computer 20 for controlling the DC multipliers M1A, M1B, the AC
multipliers M2A, M2B, multiplier M3A and the ambient light
simulation circuit 19 via a 12-bit data line bus 22. The ambient
light simulation circuit 19 includes a multiplier M3B which
attenuates a DC reference signal under control of computer 20, an
amplifier A5 having its non-inverting terminal connected to receive
an output of multiplier M3B, and a driving transistor Q3 coupled
between the LED BAR 16 and the output of amplifier A5.
The operation of the circuitry shown in FIG. 7 will now be
described.
In general, the circuitry of FIG. 7 uses one photodetector to
capture the red and infrared pulses from the UUT, and another
photodetector which is filtered such that it captures IR only, and
uses the timing of these pulses to generate modulated light pulses
to the UUT (i.e., pulse oximeter) via an LED bar.
The pulse separator and edge timing circuit 12 receives the outputs
of the photodiodes 10, and in response thereto outputs four
signals. A first signal IR Switch (represented by dotted lines) is
a switch control signal for IR. This signal controls switches S2A
and S2B, and is used to select the AC and DC corresponding to the
infrared transmission pulse wave. That is, when the pulse separator
and edge timing signal receives an IR, this signal is supplied to
switches S2A and S2B to select the AC and DC corresponding to the
infrared transmission pulse wave. At all other times, the red
values are selected so switches S2A and S2B are in the positions
shown in FIG. 7. A second signal output by circuit 12 is the red
plus infrared (R+IR) pulses. As shown in FIG. 7, this signal is
supplied to the gate of FET Q1. A third signal provided by circuit
12 is an electrical analog to the UUT red flash; this signal is
provided to multiplier M1A via switch S1A. The fourth signal
provided by circuit 12 is an electrical analog to the UUT infrared
flash; this signal is supplied to multiplier M1B via switch
S1B.
The circuit shown in FIG. 7 includes three multiplier chips M1A and
M1B, M2A and M2B, and M3A and M3B. Each of these chips contains
dual multiplying digital-to-analog converters (DACs) with internal
output amplifiers. This eliminates the amplifiers and their
associated components from the circuit board, and brings them
within desired multiplier accuracy specifications.
The multipliers multiply by a computer-set value between 0 and -1;
that is, the multipliers are both attenuating and inverting. Dual
12-bit multipliers are used for setting the finger density (DC
attenuation) and creating the blood pressure wave from (AC
attenuation); multipliers M1A, M1B and M2A, M2B, respectively. A
single dual 8 bit multiplier is used to attenuate the AC wave
(multiplier M3A) and control simulated ambient light (multiplier
M3B). The switches S1A, S1B allow selection between the analogs of
the UUT flashes (i.e., IR or R) and a fixed voltage (e.g., -5 V) as
the DC references. When receiving the UUT light analogs, switches
S1A, S1B are in the position shown in FIG. 7, and the multipliers
M1A and M1B receive the R and IR analogs, respectively. However,
the user is able to set switches S1A and S1B such that each of
multipliers M1A and M1B receives the references signal (e.g., -5
V). This will cause the DC components of the R/IR equation (2) to
drop out, thereby simplifying the equation for diagnostic purposes.
The circuitry can be designed such that the selection of the UUT
light analogs by switches S1A and S1B is the default choice.
The attenuated DC reference voltage (i.e., the output of
multipliers M1A and M1B) becomes the reference for multipliers M2A
and M2B. Further, the attenuated DC reference voltage is inverted
by amplifier A2 into the range of 0 to -5 volts. The multipliers
M2A and M2B serve to create the R and IR waveforms. The IR waveform
has a peak multiplier setting of 1000, and the R waveform has a
peak multiplier setting which varies from 400 to 3500. Multiplier
M3A receives the output of either AC multiplier, depending on the
position of switch S2B, and attenuate the output passing through
switch S2B from its maximum value down to zero. This attenuation
simulates the strength of the blood pressure wave. For example, the
value zero would correspond to no heart beat. This attenuation is
also for the UUT pulse loss detection test and should allow
demonstration of the UUT output invariance from the highest to the
lowest non-alarm AC/DC ratio.
The first element of the output stage of the circuit is amplifier
A2, which inverts the positive DC levels out of multiplier M1. The
inverted DC, which is now negative, is then summed with the
positive AC from multiplier M3A. The DC is a negative voltage which
will be proportional to base brightness, and the AC is a positive
voltage representing attenuation of the blood pressure wave. The
R1/R2 resistor ratio at the input of amplifier A3 sets the maximum
AC at 25% of the DC applied this summing and inverting stage. The
actual AC is always less than this maximum, as the largest AC
signal is only 3500/4096 times the DC out of multiplier M1A. The
inverted and summed AC and DC from amplifier A3 are applied to
amplifier A4 through resistor R3 and are chopped by Q1. Q1 is
switched by the UUT R+IR light pulse; during the pulse, Q1 is off
and amplifier A4 is driven by amplifier A3. On the other hand, when
Q1 is on, the LED current (brightness) is commanded to be zero.
Amplifier A4 sets the brightness for the LED bar 16 to be
proportional to the input voltage of amplifier A4 when Q1 is turned
off. The LED bar 16 is coupled to photodiode 18 which detects the
light generated and feeds it back to amplifier A4. This is done to
ensure that the LED bar output is linear. The test instrument
controls the light output directly, rather than depending on the
linearity and temperature stability of the LED vs. the LED
current.
The ambient light simulation circuit 19 includes a multiplier M3B,
an amplifier A5 and a driving transistor Q2 and serves to generate
a fixed current to the LED bar in addition to the red and infrared
pulses in order to simulate ambient light.
As shown in FIG. 7, the multipliers M1A, M1B, M2A, M2B, M3A and M3B
are controlled by computer 20. This can be done using a simple
program for setting the fixed parameters and then manipulating the
R/IR ratio. The various control parameters for the multipliers are
described below.
In order to provide the DC, or non-pulsatile, level, the circuit
includes the multipliers M1A and M1B which cover the range from
opaque to transparent and is settable by the computer 20 over this
range in 4,096 steps. Also, computer 20 is able to set the red and
infrared DC attenuation (i.e., multipliers M1A and M1B)
separately.
In order to provide the AC, or pulsatile, level, the circuit
includes the multipliers M2A and M2B. As indicated above, these
multipliers create the R and IR waveforms, with the IR waveform
having a peak multiplier setting of 1000, and the R waveform having
a peak multiplier setting which varies from 400 to 3500.
As shown in FIG. 3, the red to infrared ratio (R/IR) ratio can
range from 0.4 to 3.4, corresponding to 100% and 0% SaO.sub.2,
respectively. Pulse oximeters have approximately 1% resolution; in
order to effectively calibrate such an instrument, the calibrator
should be several times better, preferably an order of magnitude.
Therefore, the circuit employs a 12-bit multiplying
digital-to-analog converter (DAC), which will provide 0.1% (or
better) resolution of the full wave amplitude over the range of
R/IR values from 0.4 to 3.5. The tracking accuracy between the two
sections of the DAC chip is one bit or better.
The AC to DC ratio corresponds to the strength of the blood
pressure wave, and this ratio is simulated by multiplier M3A. One
of the tasks of a pulse oximeter is to sound an alarm if the blood
pressure wave is lost. Therefore, an important question is: "At
what level of wave weakness is the alarm tripped?" The computer 20
is able to set the wave amplitude (i.e., multiplier M3A) from zero
up to approximately 20% of the DC level in 256 steps.
A blood pressure wave corresponding to one heartbeat is generated
by the computer 20 feeding the AC multipliers M2A, M2B a series of
64 numbers corresponding to blood pressure amplitude, starting at
zero and returning to zero. The series of 64 numbers then repeats
to form the next beat. The 64 numbers are selected such that if the
series of numbers were plotted against time, then the resulting
curve would be a blood pressure wave corresponding to one heart
beat. A simulated heart rate is established by the computer 20
setting the time between the presentation of each of the 64
numbers. For example, if they are presented to the multipliers
1/64th of a second apart, the full wave takes one second to
generate, corresponding to 60 beats/minute. The computer 20 can
readily set the time between multiplier settings so that any
reasonable simulated heart rate can be established. A simulated
heart rate range of between 30 and 240 bpm should be adequate for
most applications.
As indicated above, the ambient light simulation circuit 19 serves
to drive the LED bar 16 in order to simulate ambient light.
Computer 20 controls multiplier M3B of circuit 19 so as to allow
for a settable minimum dc current through the LED bar 16.
FIG. 8 shows the pulse separator and edge timing circuit 12 and the
pulse amplifier with baseline restore circuit 14 of FIG. 7 in
greater detail. As shown in FIG. 8, the photodiodes 10 include a
first diode for receiving both R and IR, and a second diode which
is filtered so as to receive only IR, and the outputs of the diodes
are supplied to the pulse separator and edge timing circuit 12. As
shown in FIG. 8, circuit 12 comprises several amplifiers,
comparators and buffers which are connected as shown so as to
output four different signals. Specifically, circuit 12 outputs a
signal representing R+IR, a signal representing IR only, a signal
representing R only and the IR switch control signal. As shown in
FIG. 8, the IR+R signal is supplied to the gate of chopping
transistor Q1 which has its drain connected to the non-inverting
terminal of servo amplifier A4 whose output drives the LED bar 16
via driving transistor Q2. As also shown in FIG. 8, the pulse
amplifier receives the output of photodiode 18 (which is disposed
so as to sit on the LED BAR 16) and includes several amplifiers and
buffers. As discussed above, the circuit 14 provides an output to
the inverting terminal of servo amplifier A4, thereby providing
closed loop control of the LED BAR 16.
As set forth above, the device and method according to the present
invention is able to simulate a living tissue, such as a finger,
thereby enabling testing of a pulse oximeter by comparing the
parameters of the simulated living tissue with the parameters
obtained from the pulse oximeter under test.
Although the present invention has been shown and described with
reference to particular embodiments, various changes and
modifications as apparent to those skilled in the art can be made
without departing from the true scope and spirit of the invention
as defined in the claims.
* * * * *