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Description  |
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FIELD OF THE INVENTION
This invention relates to solid state monitors for photoelectric
determination of arterial oxygen saturation and of pulse rate in a human
or animal patient, more particularly to a disposable probe, calibrated
through a remote sensing apparatus including a transducer herein called a
information encoding component.
BACKGROUND OF THE INVENTION
A serious problem exists in operating rooms. Spcifically, the chemical
determination of oxygen level in blood consumes at least 3 to 5 minutes. A
patient deprived of blood oxygen for such a duration typically incurs
irreversible brain damage if not death.
U.S. Pat. No. 2,706,927 to Wood disclosed the computation of oxygen
saturation from measurements of light absorption of body tissue at two
wavelengths. A series of devices and procedures have been founded using
this technology.
A required peripheral device of such photoelectric oximeters is a
photoelectric probe. Typically, such a probe is clamped to an appendage of
a patient's body, such as an ear or a finger. Such probes require at least
one light source for directing light into the appendage and at least one
sensor for receiving light diffused out of the appendage. One method of
obtaining light of the desired frequency has been to use a light source of
indeterminate wavelength range in combination with a monochromatic filter
of known output. Such devices are inefficient, and result in unwanted
power demands and heat generation.
U.S. Pat. No. 3,704,706 to Herczfeld et al. disclosed the use of a solid
state red laser in an optical probe with a solid state photodetector.
Although lasers are useful for emitting monochromatic light of known
wavelength, thereby eliminating need for a filter, they remain expensive
and unwieldy.
U.S. Pat. No. 3,847,483 to Shaw et al. disclosed the use of light emitting
diodes to provide the necessary monochromatic light. The probe of Shaw
required expensive fiber optic cables.
A problem with all prior art devices is that they are too expensive to be
readily disposable. The need for a truly disposable probe is great, given
the many surgical applications in which sterility must be assured. The
prior art optical probes, being more or less permanent portions of their
respective oximeters, were subjected to a one time determination of the
wavelength of the light sources therein and the oximeter was then
programmed or adjusted to process light of the known wavelength.
A problem in developing disposable probes, therefore, has been the
necessity to avoid having to reprogram or adjust the oximeter for each new
probe or alternately to maintain probes within narrow limits of wavelength
variation, a clearly impractical task.
Re-calibration, perhaps necessitating return of the oximeters to the
factory, can become necessary even for prior art devices when, for
example, a probe is broken. Alternatively, a supply of light sources
having consistently identical wavelengths is required. In particular,
light emitting diodes are known to vary in wavelengths from unit-to-unit.
Other optical probes are shown in patents to Shaw, U.S. Pat. No. 3,638,640,
Neilsen, U.S. Pat. No. 4,167,331, and Konishi, U.S. Pat. No. 3,998,550.
SUMMARY OF THE INVENTION
The present invention provides an optical oximeter probe which includes at
least one narrow bandwidth light emitting diode and at least one
photoelectric sensor. An information encoding component such as a resistor
of known resistance is selected to correspond to the measured wavelength
of the LED and is provided with each probe. The elements are mounted on a
flexible fastening medium. The wires from the electrical elements
terminate at a connector for detachably connecting the probe to the
related oximeter. Coding in other manners, such as the wiring of a
multiconductor digital value or binary array or into a disposable memory
containing the color information is disclosed.
The primary object of this invention is to provide apparatus for directing
light onto a portion of a human body for the detection of oxygenated blood
flow which is inexpensive, replaceable, easily applied and which overcomes
the disadvantages and limitations of the prior art.
It is a further object of this invention to provide an optical probe whose
wavelength emission characteristics are readily ascertainable by the
attendant oximeter.
Another object of this invention is to enable factory calibration of LEDs
for use in such probes. Typically, LEDs are purchased in batches of one
general wavelength, but whose exact wavelength characteristics are unknown
and vary from piece to piece.
It is a further object of this invention to eliminate the necessity for
oximeters to be calibrated for new probes, other than the initial factory
calibration.
Yet another object of this invention is to provide flexible attachment
means for the probe which will allow rapid attachment to human or animal
appendages of varying sizes yet maintain the photoelectric sensor in
direct optical isolation from the LEDs.
Yet another object of this invention is to disclose wiring of a
multiconductor plug in a binary array to transmit probe calibration.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a part perspective, part schematic diagram of the optical probe
of the preferred embodiment of the present invention.
FIG. 2 is an end view of a patient's finger showing placement of the probe
of the present invention.
FIG. 3 is a side elevation of an embodiment of a photoelectric sensor of
the probe.
FIG. 4 is a simplified schematic circuit diagram illustrating the method in
which an oximeter microprocessor decodes the wavelength values of the
probe through use of a coded resistor.
FIG. 5 is a schematic of the probe of this invention calibrated by a
multiconductor plug and wired in a binary array; and
FIG. 6 is a circuit schematic of an oximeter utilizing the calibrated probe
of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 6, the pulse oximeter of this invention is illustrated.
Conventional microprocessor 116 has a bus 117 extending therefrom. Bus 117
has connected thereto conventional ROM 118 and RAM 119. An LED display 120
is schematically illustrated having a select latch 121 and a digit
designation latch 122.
Having set forth the more or less conventional portions of the
microprocessor, attention will now be directed to the analog portions of
the circuitry.
Finger 14 of a patient is illustrated with probe 101 having schematic
detection circuitry. First light emitting diode 132 in the red range and a
second light emitting diode 130 in the infrared range are sequentially
pulsed to emit light in their respective frequencies by amplifiers 131,
133. Typically, LED 132 is in the 660 nanometers range with LED 130 being
in the 940 nanometer range.
It is necessary that maximum light from the active light emitting diode go
through the flesh in finger 14. Therefore, a light impervious barrier 136
is placed between photosensor 138 and the paths to the light emitting
diodes 130 and 132 which are not through finger 14. Barrier 136,
terminating in contact with the flesh of finger 14, makes the path between
the respective light emitting diodes 130, 132 and the light receiving
diode 138 occur only through the flesh of finger 14.
Signal received from each respective light emitting diode first passes
through a pre-amplifier 140. This signal is thereafter amplified in
parallel at amplifiers 141, 142. As amplified, the signal is passed in
parallel from each amplifier through respective phase detectors 143, 144.
Passage through respective low pass filters 145, 146 thereafter occurs.
Amplification at offset amplifiers 147, 148 then takes place. The
pulsatile component is passed to multiplexer 150.
Multiplexer 150 has output to a comparator 152. Comparator 152 is ramped in
half steps by a 12 bit digital to analog converter (hereinafter DAC) 154.
DAC 154 places a comparison signal divided in one part from 4096 parts
with the comparator outputting to bus 117.
The reader will recognize that not all human fingers and appendages are the
same. Specifically, the difference between the races, skin pigment,
weight, age, maturity and other factors all can lead to different signals
being sensed at photosensor 138, even though the frequency and intensity
of the light signal output at each of the diodes 130, 132 is the same.
Accordingly, microprocessor 116 is programmed to receive a signal from
photosensor 138 within an optimum range. Utilizing a second operating
phase of DAC 154, and communicating a signal to a sample hold 157, the
individual LED's 130, 132 are given voltage outputs 160, 161. These
voltage outputs 160, 161 are adjusted so that in each case photosensor 138
looks at a signal well withing the range of the DAC.
Clock 170 controls the sequential output of light from the light emitting
diodes 130, 132 to a duty cycle of at least 1 in 4. This is schematically
illustrated by signals .phi.1 through .phi.4. Reception of signal at
detector 143 occurs during time periods .phi.1 and .phi.2 and reception of
signal occurs at detector 144 during time periods .phi.3 and .phi.4.
It can be immediately realized that during respective time periods .phi.1,
.phi.3 active signal from the light emitting diodes 130, 132 is being
received. During the time periods .phi.2 and .phi.4, no signal and only
noise is being received. As will hereinafter become apparent, by
amplifying the negative signal before passage through the low pass filter,
noise can be subtracted out utilizing the illustrated 1 in 4 duty cycle.
Applicant herewith incorporates by reference his United States application
filed of even date herewith, entitled "pulse Oximeter" U.S. patent
application Ser. No. 414,174: filed Sept. 2, 1982 now abandoned in favor
of co-pending U.S. patent application Ser. No. 417,311 filed Sept. 13,
1982. FIG. 6 is a copy of the FIG. 2 from that application having been
renumbered to avoid confusion with the context herein.
Likewise, the Summary of Invention in the incorporated application is:
SUMMARY OF INVENTION
A pulse oximeter is disclosed of the type wherein light of two different
wavelengths is passed through any human or animal body pulsatile tissue
bed, such as a finger, an ear, the nasal septum or the scalp, so as to be
modulated by the pulsatile component of arterial blood therein, and
thereby allowing indication of oxygen saturation, blood perfusion and
heart rate. The level of incident light is continually adjusted for
optimal detection of the pulsatile component, while permitting
accommodation to variable attenuations due to skin color, flesh thickness
and other invariants. At significant slope reversal of the pulsatile
component to negative (indicating a wave maximum), wave form analysis of
blood flow occurs. A quotient of the pulsatile component of light
transmission over the constant component of light transmission is measured
for each of two wavelengths by direct digital tracking. The respective
quotients are thereafter converted to a ratio, which ratio may be
thereafter fitted to a curve of independently derived of oxygen saturation
for the purpose of calibration. The saturation versus ratio calibration
curve may be characterized by various mathematical techniques including
polynomial expansion whereby the coefficients of the polynomial specify
the curve. An output of pulse rate, pulsatile flow and oxygen saturation
is given. An incident light source duty cycle is chosen to be at least 1
in 4 so that noise, inevitably present in the signal, may be substantially
eliminated and filtered.
A representative claim is:
A pulse oximeter for determining arterial oxygen saturation and arterial
pulse amplitude in a patient, said oximeter comprising: first and second
light emitting sources for emitting sequential light pulses in the red and
infrared into the flesh of a human; a sensor sensitive to each of said
light sources having an indirect light path through the flesh of said
human from said first and second light sources; said sensor sequentially
outputting signals to an amplifier from each of said light sources; means
for digitally tracking the light absorption; means for dividing the change
of light transmission due to the pulsatile component of blood flow with
respect to the total light transmission to determine a quotient of light
absorption for each optical wavelength; means for making a ratio related
directly to the respective quotients of light transmission at each said
frequency and means for fitting the ratios of light transmission to
experimentally determined saturations at said ratio to enable the optical
determination of saturation.
Referring to FIG. 1, a part-schematic, part-perspective view of the optical
probe 1 is shown. A suitable length of adjustable, self-fastening tape 50
is provided, such as that sold under the trademark VELCRO, obtainable from
American Velcro, Inc. Incorporated into tape 50 at suitably spaced
intervals are the electrical components of probe 1. Photoelectric sensor
30 is attached to the outside of tape 50 and protrudes slightly from the
underside of tape 50. Sensor 30 has ground wire G and lead wire 31. Light
emitting diode 10, typically emitting frequencies in the infrared range of
the spectrum, is mounted to and pierces tape 50 in a similar manner to
sensor 30 and at a distance from sensor 30 selected upon the basis of the
typical appendage size expected to be encountered. LED 10 is connected to
ground wire G and has input lead wire 11. Placed in proximity to LED 10 is
a second LED 20, typically having wavelength emission characteristics in
the red range of the spectrum. LED 20 attaches to ground wire G and has
input lead wire 21.
Resistor 40 is shown mounted to tape 50 between sensor 30 and LED 10.
However, the physical location of resistor 40 is not important and it may
be mounted to probe 1 at any other convenient location. Resistor 40 has
input lead wire 41 and is connected to ground wire G.
Wires G, 11, 21, 31, 41 lead to connector 52 so that probe 11 may be
readily disconnected from the oximeter 60 (schematically illustrated in
FIG. 4).
The probe 1 illustrated in FIG. 1 is designed for use in connection with an
oximeter 60 designed to operate in conjunction with two LEDs 10, 20
sequentially transmitting light to a single sensor 30. However, the
mechanism of the instant invention works equally well for oximeters
requiring only a single LED and single or multiple photo sensors.
Oximeters requiring more than two LEDs may be equally well accommodated by
the probe of the present invention.
FIG. 2 is an end elevation of a typical finger 51 of a human patient.
Finger 51 is encircled by probe 1 at its tip by overlapping the ends of
self-connecting tape 50. Light emitted from LEDs 10, 20 enter the flesh of
finger 51 and are subjected to diffusion and scattering. Sensor 30 picks
up only light which has been diffused through the flesh of finger 51.
FIG. 3 is a detailed side elevation of sensor 30, showing the manner in
which it is assured that no light emitted by LEDs 10, 20 is received by
sensor 30 without first passing through finger 51. Sensor element 32 is
recessed somewhat within metal cylinder wall 33 of the sensor housing.
Since tape 50 presses sensor 30 directly against the skin of finger 51, it
is readily seen that no light passes to sensor element 32 other than
through the flesh of finger 51.
Probe 1 is constructed in the following manner: LED's 10, 20 are selected
from batches of LEDs with generally known wavelength characteristics. The
exact wavelength characteristics of the specific LED's 10, 20 chosen are
determined at this time through readily available metering means. Resistor
40 or a similar impedance reference is then selected to have an impedance
or specifically a resistance whose amount is exactly specified by a table
made available to the factory technician for this purpose, of all possible
wavelength combinations which may be expected to be encountered from the
available supplies of LEDs. The following table is an example of how a
single resistor 40 might be selected for any hypothetical combination of
LED's 10, 20 in a case where each has only two possible wavelengths:
TABLE A
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Resistor 40 LED 10 LED 20
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150 ohms 940 nM 660 nM
160 ohms 950 nM 660 nM
170 ohms 940 nM 670 nM
180 ohms 950 nM 670 nM
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A typical probe will have an infrared LED 10 of wavelength 940 nanometers
and a red LED 20 of wavelength 660 nanometers. According to the above
table, a probe having such wavelength characteristics will be supplied at
the factory with a resistor 40 of one, and only one, resistance value, in
this case shown to be 150 ohms.
The value in having such a unique known resistance incorporated into probe
1 is shown by reference to FIG. 4. Oximeter 60 contains a microprocessor
61, and a read only memory 62 and random access memory 63. Table A (the
same table used for calibrating probe 1 at the factory) no matter how
extensive, may be easily programmed into ROM 62 at the time oximeter 60 is
fabricated. Current I from current source 69 is passed through resistor
40. The resulting voltage (per Ohm's law) is passed through multiplexor 66
through comparator 65, to microprocessor 61.
Microprocessor 61 may be programmed to calculate the resistance of resistor
40 and thereafter to look up the wavelengths of LED's 10, 20 from Table A
in ROM 62. Microprocessor 61 is also programmed to itself recalibrate the
optical comparison circuitry of oximeter 60 once the wavelengths of LEDs
10, 20 are known. By this means, it is not required to recalibrate by hand
oximeter 60 for each new probe 1 nor, alternatively, to require that LEDs
10, 20 be of precisely standardized wavelengths.
The specific function and design of the circuitry schematically illustrated
in FIG. 4 is seen as obvious when taken in combination with the general
description of its function. The functions of microprocessors and read
only memories are well known and understood and it is well within the
capability of a person with ordinary skill in the art to design and
program microprocessor 61 to calculate the resistance of resistor 40 and
thereby obtain the wavelengths of LEDs 10, 20 from a simple lookup table
in a ROM 62.
Probe 1 may be used with any number of prior art oximeters, the method of
operation of which is well understood and beyond the scope of the teaching
of the present invention. Basically, for each heart beat, fresh arterial
blood is pumped into the arteries of finger 51, thereby causing a periodic
increase and decrease in light intensity observed by sensor 30. The oxygen
saturation of hemoglobin in the pulsatile blood
For any known wavelength, there is a known extinction coefficient B. Given
B and measuring the intensity of diffused light received by sensor 30 the
oxygen saturation can be computed and displayed. In fact, the coefficients
B of the various wavelengths of Table A can be substituted for the
wavelengths directly when the table is programmed into ROM 62, thereby
eliminating a computational step.
Microprocessor 61, through LED control circuitry 67, operates LEDs 10, 20.
Light from LEDs 10, 20 results in current in sensor 30 which passes
through amplification and filtration circuitry 68 to multiplexor 66.
Comparator 65, and a digital to analog converter 70 are operative as an
analog to digital converter means to present a digital signal to the
microprocessor 61 thereby to allow the microprocessor 61 to determine
oxygen saturation and/or pulse rate. Results are shown on display 64.
Referring to FIG. 5, an alternate way of coding a probe of this invention
is illustrated. Specifically, an eight pin connector 52 similar to the
connector 52 of FIG. 4 is illustrated having respective lead lines 201,
202, 203 respectively communicating to light-emitting diode 130, light
emitting diode 132 and photodetector 138. Conductor 204 is illustrated
providing the ground connection.
It will be noted that the eight pin connector 52 of FIG. 5 has four empty
channels. These channels can be provided to communicate the coded value of
the probe.
For example, assuming that the connectors when provided with a common
potential provide a true binary value and when independent of any
potential (i.e., a high impedance or open circuit) provide a false binary
value. Thus, the four conductors of plug 52, as illustrated in FIG. 5,
would communicate the binary value 1100. Thus, communication of the
resistance value of the connected probe would be possible by coding the
connector to a value of 1 part in 16.
Those skilled in the art will appreciate that other binary connections
could as well be made. For example, by expanding the number of connectors
on the probe relatively large expansions can occur.
Those skilled in the art will realize that in determining the variable
transmission of light in human flesh the frequency at which the flesh is
integrated by a substantially monochromatic light source is critical. If
the frequency varies the results of the instrument can be inaccurate with
such variation. Simply stated, at different points in the spectral
frequency, oxygenated hemoglobin and reduced hemoglobin transmit varying
amounts of light.
Commercially produced light emitting diodes do have variation in their
spectral frequency from diode to diode. Therefore if such commercially
produced diodes are going to be used as replaceable probes in an
instrument it has been found that provision must be made for a probe by
probe calibration of the instrument. Thus, effectively disposable probes
can be readily used even though they are affecting integration at
differing frequencies from probe to probe.
Some comment can be made directed specifically at calibrating the
disposable probe of the instrument herein. As a practical matter, the
blood of a human is interrogated through the skin by light transmission
utilizing red and infrared. The rate of change of constants in the
infrared is relatively flat. Therefore a variance in the frequency of the
infrared diode has little effect.
Not so in the red range. It has been found that the attenuation of light in
oxygenated and unoxygenated hemoglobin has a rapidly changing slope in the
red range. This being the case, it is of primary concern to calibrate in
the particular instrument illustrated in the red range.
Those skilled in the art will realize that there are many ways in which
change of instrument calibration can occur. Specifically, separate look-up
tables can be generated for various grouped relationships. Alternately,
and perhaps more productively, incremental alternation to the constants of
curvature between the saturation level S and the ratio of quotients R of
light transmission can be determined.
Although the foregoing invention is described in some detail by way of
illustration and example for purpose of clarity of understanding, it is
understood that certain changes and modifications may be practiced and
equivalents employed within the spirit of the invention as limited only by
the scope of the appended claims. For example, two resistors may be used
in place of one, each resistor coded to the wavelength of a separate LED.
Other components could be used in place of resistors, e.g., capacitors or
the like. Therefore, the above description and illustrations should not be
construed as limiting the scope of the invention which is defined by the
appended claims.
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