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A two fiche microfiche appendix is included herein, consisting of a total
of 67 frames.
This invention relates to pulse oximeters and specifically to the
photoelectric determination of arterial oxygen saturation in humans with
techniques for initializing data, receiving data for processing, setting
and triggering alarms all being set forth.
BACKGROUND OF THE INVENTION
Electronic, non-invasive techniques for determination of oxygen content are
known. U.S. Pat. No. 2,706,927 to Wood disclosed the computation of oxygen
saturation from measurement of light absorption of body tissue at two
wavelengths. A "bloodless" measurement was first taken in which as much
blood as possible was squeezed from the area where measurement was taken.
Thereafter, arterial blood was allowed to flow into the tissue as the
condition of normal blood flow was restored. A comparison of the light
absorption in the two states provided information on the arterial oxygen
saturation of the subject. A series of devices and procedures have been
founded using this technology.
In procedures based on this technology, difficulty has been experienced in
reliably determining the "bloodless" parameters; due in part to
geometrical distortion due to the compression of the tissue; imperfect
measurement of this parameter gave imperfect results.
The transmission of light of each wavelength is a function of the
thickness, color, and structure of skin, flesh, bone, blood and other
material through which the light passes. This attenuation in transmission
has been asserted to have a logarithmic characteristic, in accordance with
the Lambert-Beers Law.
In a pulse oximeter, the primary material of interest is pulsatile arterial
blood. Arterial blood is the only material whose quantity in the tissue
varies with time in synchrony with the beating of the heart. Variations in
light transmission therefore indicate variations in blood flow permitting
direct optical recording of the pulsatile component of arterial blood
flow. This ability to separate out the light absorption of arterial blood
is especially convenient; since the oxyhemoglobin component of blood is a
substance for which the absorption coefficients can be determined, the
fraction of any oxyhemoglobin in arterial blood can be determined.
Optical plethysmographs are well known. Such instruments measure pulse rate
and provide information on the quantity of blood forced into the tissues
on each heart beat. These instruments generally utilize a light frequency
near or at the isobestic point where measurement of pulsatile flow is made
independent of oxygen saturation. Consequently, they intentionally
eliminate information on oxygen saturation.
Following the Wood U.S. Pat. No. 2,706,927 patent, numerous attempts have
been directed at eliminating the difficulties connected with arterial
saturation measurements using light absorption where the analysis requires
the comparing the "bloodless" measurement either artificially induced or
naturally occuring during the rest state of the heart cycle with the
measurement of fresh arterial blood when fresh arterial blood enters the
tissue. For example, the signal received has been divided into its "AC"
and "DC" components and passed through a log amplifier before digital
analysis of the signal occurs. See Koneshi et al., U.S. Pat. No.
3,998,550. Likewise, a generation at both wavelengths of subtraction
outputs has been utilized before digital analysis. Subtraction outputs
have been used to eliminate the DC component and to approximate the
logarithmic response of the prior art. See Hamaguri, U.S. Pat. No.
4,266,554. Simply stated, because the pulsatile component constitutes a
small portion of the total signal of transmitted light, numerous
manipulations based on logarithms have been attempted to screen out the
unchanging component of the resultant signal before analysis.
U.S. Pat. No. 3,704,706 to Herczfeld et al disclosed use of a single
coherent red light source, preferably a laser. Use of a single light
source is unable to separate information dealing with the arterial flow
component from that dealing with the arterial oxygen component. The output
of such a single red light source instrument can only be an indication of
the product of blood flow and the saturation level present. Neither blood
flow alone or saturation alone can be known.
In all of the above schemes for the measurement of pulse rate, pulse flow
and oxygen saturation, the variant or AC component is a small portion of
the total absorption occurring. In such circumstances, discrimination of
the signal from other possible sources must occur. When it is remembered
that measurements of unconscious, partially anesthetized and otherwise
non-responsive patients must occur, and such patients have random and
irregular movements (and heart beats), the establishment of thresholds for
the reception and analysis of data is critical.
SUMMARY OF INVENTION
A display monitor is disclosed for a pulse oximeter of the type wherein
light of two different wavelengths is passed through body tissue, such as
a finger, an ear or the scalp, so as to be modulated by the pulsatile
component of arterial blood therein and thereby indicate oxygen
saturation. The disclosed instrument first receives and compares signal to
parameters to check for a pulse like signal. Assuming that a pulse like
signal is detected a tonal signal is emitted having a pitch proportional
the ratio of oxygen saturation and a sequential repetition proportional to
pulse. A visual cue consiting of an array of strobed light emitting diodes
is flashed having the number of lights strobed increase with increasing
magnitude of the pulses and having a sequential flashing rate proportional
to pulse rate. A systematic rejection of extraneous or irregular detected
data prevents undue sounding of alarms.
OTHER OBJECTS, FEATURES, AND ADVANTAGES OF INVENTION
An object of this invention is to disclose an instrument which can
simultaneously trace and indicate the pulse as well as the degree of
oxygen saturation of the individual. According to this aspect of the
invention, at least one of the wavelengths of light, preferably infrared,
is monitored for slope change. A signal is emitted proportional to and
typically synchronous with the slope change rate to indicate heart rate. A
second signal is emitted containing pulse rate and oxygen saturation
information.
An advantage of this aspect of the invention is that each pulsatile
component is individually analyzed. The heart beat and arterial oxygen
level of the patient is continually monitored.
Yet another object of this invention is to disclose a series of audible
signals which convey pulse rate and oxygen saturation. Pulse rate is
indicated by emitting sequential tones at time intervals corresponding to
the rate of negative slope reversals (indicating pulse wave maximum).
Oxygen saturation is indicated by having pitch decrease proportional to
decreasing oxygen saturation.
An advantage of this aspect of the invention is that the human ear is
particularly sensitive to both changes in frequency of sequential sound
signals and tonal variations in sequential sound signals. A simple beating
signal can make all in the immediate vicinity well aware of both the pulse
rate and oxygen saturation of the patient.
Yet another aspect of this invention is to emit a visual signal conveying
similar information. According to this aspect of the invention, a column
of light emitting diodes flashes in height proportional to pulse magnitude
and flashes in frequency proportional to pulse rate. As the eye is
particularly sensitive to changes in both flashing rate and angular
dimension or height of the flashing LED array, an indication of pulse
quality is given.
Yet another object of this invention is to disclose a plurality of alarms,
which alarms can all be individually set in accordance with the current
condition of the patient. According to this aspect of the invention, high
pulse rate, low pulse rate and oxygen saturation levels can all be
utilized as an alarm limit.
An advantage of this aspect of the invention is that the parameter of the
patient's warning limits can be tailored by the anesthesiologist or other
attending physicians. Individual adjustment can be made to the particular
physiology present.
Yet another object of this invention is to disclose a regimen in
combination with the instrument for rejecting extraneous data. Remembering
that patients are often in an unconscious or semi-conscious state when
this instrument is used, it can be realized that the instrument does not
operate in a perfect environment. Shaking or moving the sensor head or
even local variations in the patients pulsatile profile could
unnecessarily trigger alarms. According to this aspect, incoming processed
data is compared to confidence factors. If the data falls within expected
levels, confidence factors remain unchanged or are upgraded to the highest
level. Where data falls without the anticipated confidence levels, the
data itself may be rejected. The confidence levels are eroded or opened in
the range of data that can be processed. This process occurs until data
consistent with the confidence limits is received. When data consistent
with the confidence factor is received, it is compared to the alarm limit.
An advantage of this aspect of the invention is that small local variations
in the received signal do not trigger the alarms.
Yet another aspect of this invention is to disclose a totality of data
utilized for tracking the pulse. According to this aspect of the
invention, the points of maximum light transmission (commencement of
inflowing pulse) and maximum light absorption (end of arterial pulse) are
tracked for at least one wavelength. A maximum negative slope intermediate
the maxima and minima is plotted for avoidance of the dicrotic notch.
Finally, the percent of oxygen saturation is determined by comparison of
light transmission at both frequencies.
All of these data are analyzed against the confidence limits for reception.
Where three out of the six data values are outside the limits, the
entirety of the data is rejected. Where four or more of the data values
are within, the data is received and the confidence limits under the
acceptable catagories upgraded or maintained at the narrowest limit.
Confidence limits of unacceptable data are eroded or opened.
An advantage of this aspect of the invention is that interruption of data
often occurs at more than one parameter. With such interruption, the
entire data block may be averaged to prevent the premature sounding of
alarms.
A further object of this invention is to disclose a simplified control for
adjusting alarm limits. According to this aspect, adjustment occurs to a
shaft encoder directly coupled to an alarm limit adjustment knob. The
alarm limit to be set is selected by pressing at least one selector
button. Thereafter, turning of the alarm limit adjuster knob updates the
limit by sign--depending upon direction of turn--and in limit--depending
upon amount of turns. The current alarm limit being changed is shown in
the visual display. If the alarm limit does not change for a preset
period, that is, the knob is no longer being turned, the knob to the alarm
limit is disconnected and the knob is again connected to its original
connection and the display returns to its original status.
An advantage of this aspect of the invention is that alarm limit control is
easily and simply adjusted. The complex environment of the operating room
and intensive care unit is provided with a useful instrument having
simplified adjustment. In particular, the alarms can be controlled by one
hand, important in some aspects of patient care. It is not necessary to
manually reset the instrument to its original status of displaying
saturation or pulse rate.
Other objects, features and advantages of this invention will become more
apparent after referring to the following specifications and attached
drawings in which:
DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective view of the instrument of this invention
illustrating the instrument housing and attachment of a sensor to the
digit of a patient;
FIG. 2 is an overall circuit schematic of this invention;
FIG. 3 is a circuit schematic in the vicinity of the microprocessor;
FIG. 4. is a circuit schematic in the vicinity of the read only memory or
ROM of this invention;
FIG. 5 is a circuit schematic in the vicinity of the random access memory
or RAM of this invention;
FIG. 6 is a circuit schematic of the memory select;
FIG. 7 is a circuit schematic of the input/output select;
FIG. 8 is a circuit schematic of the counter of this invention;
FIG. 9 is a circuit schematic of the comparator circuit wherein 12 bit
digital to analog conversion occurs;
FIG. 10 is a circuit schematic of the sample-hold circuitry of this
invention;
FIG. 11 is a circuit schematic of the offset amplifier circuit of this
invention;
FIG. 12 is a circuit schematic of the detector of this invention;
FIG. 13 is a detail of a clock circuit having an output for powering the
light emitting diodes;
FIG. 14 is a detail of circuitry for powering the light emitting diodes,
the diodes being switched at a point proximate to the detector;
FIG. 15 is a circuit schematic illustrating the operation of the optically
coupled adjustment knob;
FIG. 16 is a view of the control button circuitry of this invention;
FIG. 17 is a view of L.E.D. circuitry outputs;
FIG. 18 is a view of the audio output circuitry; and
FIG. 19 is a block logic diagram of the numerical process steps which
result in the instrument output.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the instrument housing 26 of this invention is
illustrated. Outwardly, the housing includes a digit display 1, circuitry
select button array 2 through 5, alarm status lights 6 through 9, an
optically coupled adjustment knob 10, sync status light 11, LED digital
viewmeter 12, and power switch 13. A speaker 15 is placed under and in the
instrument housing.
From a connector (not shown) in housing 26 there extend leader wires 27.
Wires 27 extend to a detector probe 29. Detector 29 is placed upon the
finger 14 of a patient 28. Utilizing the placement of the detector 29 at
the finger 14, all of the readings in this invention are made possible.
Oximeter Operation
A broader view of the operation of this invention can be made by
considering carefully the circuit schematic of FIG. 2.
Referring to FIG. 2, conventional microprocessor 16 has a bus 17 extending
therefrom. Bus 17 has connected thereto conventional ROM 18 and RAM 19. An
LED display 20 is schematically illustrated having a select latch 21 and a
digit designation latch 22. The circuit select button array 2-5 and
optically coupled control knob 10 previously illustrated are gated through
controls generally denominated 24.
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 patient 28 is illustrated with detector 29 having schematic
detection circuitry. First light emitting diode 32 in the red range and a
second light emitting diode 30 in the infrared range are sequentially
pulsed to emit light in their respective frequencies by amplifiers 31,33.
Typically, LED 32 is in the 660 nanometer range with LED 30 being in the
940 nanometer range.
It is necessary that all the light from the active light emitting diode go
through the flesh in finger 14. Therefore, a light impervious barrier 36
is placed between photosensor 38 and finger 14. Barrier 36, terminating in
contact with the flesh of finger 14, makes the path between the respective
light emitting diodes 30, 32 and the light receiving diode 38 occur only
through the flesh of finger 14.
In the instrument herein we utilize two discrete wavelengths. These
wavelengths are 660 nanometers (red) and 940 nanometers (infrared). A
small amount of discussion related to these parameters is in order.
First, the wavelengths are chosen so that they are far enough apart so that
the transmission of light appreciably varies with changes in oxygen
saturation.
Secondly, the wavelength are chosen so that the same tissue is sampled. For
example, a wavelength in the ultraviolet would not sample the same tissue
due to scattering.
While wavelengths extremely close could be used, we have chosen not to do
so. We find that drifting of light source wavelengths can occur with
accompanying problems.
Signal received from the respective light emitting diodes first passes
through a pre-amplifier 40. This signal is thereafter amplified in
parallel at amplifiers 41, 42. As amplified, the signal is passed in
parallel from each amplifier through respective phase detectors 43, 44.
Passage through respective low pass filters 45, 46 thereafter occurs.
Amplification at offset amplifiers 47, 48 then takes place. The pulsatile
component is passed to multiplexer 50.
Multiplexer 50 has output to a comparator 52. Comparator 52 is ramped in
half steps by a 12 bit digital to analog converter (hereinafter DAC) 54.
DAC 54 places a comparison signal divided in one part from 4096 parts with
the comparator outputting to bus 17.
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 38, even though the wavelength and intensity
of the light signal output at each of the diodes 30, 32 is the same.
Accordingly, microprocessor 16 is programmed to receive a signal from
photosensor 38 within an optimum range. Utilizing a second operating phase
of DAC 54, and communicating signal to a sample hold 57, the individual
LED's 30, 32 are given voltage outputs 60, 61. These voltage outputs 60,
61 are adjusted so that in each case photosensor 38 looks at a signal well
withing the range of the DAC.
Clock 70 controls the sequential output of light from the light emitting
diodes 30, 32 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 43 occurs during time periods .phi.1 and .phi.2 and reception of
signal occurs at detector 44 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 30, 32 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.
Having given the reader an overview of the circuitry utilized with this
invention, the invention will now be discussed in detail.
Referring to FIG. 3, the microprocessor 100 is illustrated having an
attached crystal 104. This crystal, in combination with clock circuitry
incorporated within the microprocessor 100, generates the clock signals
required by the microprocessor chip itself as well as providing clock
pulses to the rest of the oximeter circuitry through output 102.
Microprocessor 100 is an 8085A CPU integrated circuit chip available from
Intel Corporation of Santa Clara, Calif. The family identification
suffixes of the remaining IC components are listed on the drawing and the
components are readily available from various manufacturers.
An address bus 103 includes address lines A0 through A15. To accommodate
the eight bit processor, lines A0 through A7 on the address bus are
latched from microprocessor pins AD0 through AD7 so that during the
address time state these lines may be read. During an alternate time
state, lines AD0 through AD7 become output data lines 104, OD0 through
OD7, which lines as here configured are only capable of outputting data.
Referring to FIG. 4, the ROM configuration is seen to be standard. The ROM
is addressed with a conventional address bus including lines A0 to A10
addressing in parallel ROMs 106, 107 and 108. These respective ROMs are
enabled by three decoded address bits from lines A11-A13 (see FIG. 6). As
will hereinafter be set forth with respect to FIG. 6, enabling outputs for
reading of the ROMs include read enable 110 (see FIGS. 3, 4) and specific
ROM addresses including ROM 0 address 111, ROM 1 address 112, and ROM 2
address 114. The particular ROMs here utilized are of the optically
erasable programmable read only memory variety and include an output data
bus 115.
Referring to FIG. 5, two conventional RAMs 120, 121 are shown addressed in
parallel at address bits A0 through A9 at bussing 125. These RAMs write
and read over eight bits with four bus lines AD0 to AD3 at bus 127
addressing RAM 120 and AD4-AD7 addressing RAM 121 at bussing 128. RAMs
120, 121 are read when enabled through enable ports 129 in the absence of
a write signal on port 130. These RAMs are written when enabled by port
129 in the presence of a write signal through write ports 130. As each of
the RAMs connect to four separate data bits, individual enabling of each
of the RAMs is not required.
Referring to FIG. 6, the memory select circuit of this invention is
illustrated. The memory select has a three bit input 140 at lines A11-A13.
Output occurs when memory is selected at ROM 0 enable 111, ROM 1 enable
112, ROM 2 enable 114. A RAM enable 141 passes through an inverter and
NAND gate to enable reading of RAMs 120, 121 for either reading or
writing.
Referring to FIG. 8, a counter used as a divider is illustrated. Referring
briefly back to FIG. 3, it will be seen that the microprocessor 100 is
provided with a clock running at 2.5 MHz generally denominated 102. The
CPU clock outputs at 102 to a counter 172 (see FIG. 8.) Counter 172
divides signal 102 by the number 171 and outputs to binary counter 173 in
order to generate an LED clock frequency of 1.827 kHz, which is unrelated
to room light frequencies. Counter 173 outputs signals LED A 191, LED B
192, LED CLK 190 and DCLK 189. This circuit in cooperation with the
circuit of FIG. 13, effects light and detector switching to enable signal
phasing.
Having set forth in generality the microprocessor, it will be realized that
much of that disclosed is already known in the art. Specifically, complete
descriptions of the wiring of this microprocessor can be found in the
MCS-8085 Family Users Manual, published October 1979 by Intel Corporation.
Those having skill in the art are referred to this publication should
question arise about the circuitry thus far described.
Referring briefly back to FIG. 8, LED clock outputs 190, 191, 192 are
inputed to the clock divider 194 of FIG. 13. Divider 194 outputs four
sequential duty cycle states denominated .phi.1' through .phi.4'.
Complements of signals .phi.1' and .phi.3' are outputed directly at clock
driver outputs 196. It will be noted that all four signals .phi.1'-.phi.4'
are outputed at 198 for timing purposes hereinafter discussed.
Having set forth the timer, the remainder of this disclosure will be broken
down into five discrete parts. First, timing for the light emission of the
LED's will be discussed. Emphasis will be placed on the fact that the
diodes are switched locally.
Second, light reception will be set forth. With respect to the reception,
emphasis will be made to the fact that the signal is digitally extracted
without any analog treatment whatsoever. The pure digital signal is
thereafter processed and utilized to create the light curves herein.
Effort is made to eliminate all variables present, including those in the
flesh analyzed as well as ambient light noise.
Thirdly, and in view of variant light transmission qualities of human
flesh, the light level adjustment circuit of this invention will be
traced. It will be pointed out that the adjustment of the emitted light
occurs so that the sensor receives an amount appropriate for the
amplification circuitry.
Fourth, setting of the alarm limits will be analyzed. Illustration will be
made.
Fifth, and finally, the program alarm will be discussed. Specifically, the
utilization of "confidence limits" and a totality of data received in the
monitoring program will be disclosed as screening extraneous data yet
permitting a timely alarm to ward off catastrophe.
Referring to FIG. 14, and assuming that sufficient voltage is present
across leads 301, 302, current of an appropriate level will be emitted to
each of the light emitting diodes 30, 32. The diodes here are illustrated
schematically across a connector 305 and are shown being switched by
respective transistors 307, 309. Specifically, when a negation pulse is
received at each of the transistors, the transistors open, voltage appears
across the respective diodes 30, 32, and light is emitted.
Assuming light is transmitted, it is passed to the flesh of the digit 14
and is thereafter received at the receiving photosensor 38.
Referring to FIG. 12, photosensor 38 is illustrated. It is coupled across a
connector 305. Connector 305 in turn passes its signal through amplifier
40. The signal is then split and passed to voltage amplifiers 41, 42, the
amplification here occurring in parallel, allowing differences in gain
between red and infrared signal processing. Respective phase detectors 43,
44 are clocked at inputs .phi.1'-.phi.4' from the clock circuit of FIG.
13. Remembering that a 1 in 4 duty cycle is here utilized with each of the
signals .phi.1', .phi.2', .phi.3', .phi.4' being clock periods, it is seen
that the signal is gated. Specifically, and during the .phi.1' time
period, negative amplification of the total light signal, including
pulsatile component and noise, occurs at amplifier 201 with passage of the
resultant signal through the low pass filter 45.
Referring to FIG. 14, in the next sequential time period, and due to the
signal .phi.1' no longer appearing to close transistor 309, transistor 309
will be shunted to ground. At the same time, during time period 02' gate
43 will open to amplify the positive component received. This component
received, however, will have no light emission whatsoever; it instead will
represent pure electronic or optical noise. The timing of this circuit
will therefore yield on equal bases first light containing the pulsatile
component and noise and thereafter just noise. Amplifier 201 amplifies one
signal positively and the other signal negatively in equal amounts. It
will be seen that integrated over the full four periods of the clock,
through amplifier 201 the instrument sees equal components of noise which
cancel and unequal components of signal which do not cancel. By the
expedient of taking the respective intermittent pulses and passing them
through the low pass filter 45, there results a signal out containing
valid signal only; noise cancels.
The remaining channel is analogous. Specifically, during time period
.phi.3', noise and light signal are amplified negatively and passed
through low pass filter 46. During time period .phi.4', noise only is
positively amplified and cancelled in passage through the low pass filter
46.
The emitted signal V.sub.A and V.sub.B can be described as having two
components. The first component is constant. It is that element of light
which remains essentially invariant. This signal includes an absorption
component because of skin pigment, bone, flesh and venous blood. The
second component represents the pulsatile inflow of arterial blood.
The ratio of that second component to the first component is what is sought
by the instrument. What is sought is the ratio of the arterial and
pulsatile component of the blood to that of the total absorbing tissue.
The color of the arterial component of the blood produces the differential
light absorption that is dependant upon the oxygen saturation of the
hemoglobin. The instrument must isolate this component.
Referring to FIG. 11, amplification of the signal to an idealized state is
illustrated. Specifically, in taking respective signals VA', VB', an
offset voltage VOFF introduced. This signal is a constant voltage which
subtracts out part of the constant portion of the received light signal
which relates to passage through the nonvariant portions of the flesh.
Since it is known that the pulsatile component is always very small with
respect to the total signal, an improvement on the accuracy of digital
conversion can be obtained by this subtraction. It is necessary, however,
for the microprocessor program to mathematically reinsert this subtracted
voltage prior to processing the signal. This subtraction and amplification
occurs at the respective amplifiers 330, 331 with passage of the signals
VA' and VB' from the network.
With digital to analog conversion of these signals, a combination of the
pulsatile component and the remainder of the constant component is then
required. This can best be seen through the circuitry of FIG. 9.
Referring to FIG. 9, a multiplexor 50 is illustrated. During the analytical
operation here shown, this multiplexor samples signals VA' and VB'. Signal
is passed to the negative side of comparator 52. Signal for driving the
multiplexor passes through lines OD4-OD6 in the DAC high latch 360. The
DAC low latch 362 is thereafter actuated in sequence responsive to
enabling signals on enabling line 363. Output occurs to a digital to
analog converter 54 on a twelve bit basis. Division to one part in 4096
occurs.
Typically, the signal is compared in halves. Output of DAC 54 occurs over
lead 365 to comparator 52. The comparator output 366 is passed to the
microprocessor. Depending upon whether a high or low signal is received,
stepping of the twelve bit DAC 54 occurs in halves, enabling the twelve
bit division to occur rapidly. Consequently, the output level of the
voltage of the receiving photosensor is rapidly determined with the result
that the pulsatile component can be rapidly followed. This process is
repeated for both signals VA' and VB' at a rate that allows the
microprocessor to faithfully track both signals.
Having set forth the light reception circuitry of this invention, attention
will now be directed to the level of light adjustment.
It will be remembered that each of the patients, due to flesh, skin
pigment, skin thickness, bone, venous blood present and other invariants,
will present his own factor of constant light absorption at both
wavelengths. This being the case, it is necessary to adjust the level of
current applied. This is done through the DAC circuit of FIG. 9 and the
sample hold circuit of FIG. 10.
The sampling of the light signals by the microprocessor was described
above. In the case where the signals are not within the useful range of
the conversion circuitry, the light level must be adjusted up or down as
required to restore the signal level to the voltage range acceptable to
the analog to digital conversion. Referring to FIG. 9, the program will
output a code corresponding to the desired voltage level through its data
bus into latches 362 and 360, setting the DAC 54 output to a voltage
corresponding to desired LED current. Note that this is only done during a
time period when the DAC is not used for input conversion. The program
will then output using the same bus a bit corresponding to the selected
LED into latch 370 of FIG. 10. This bit, or selection signal, is converted
to a compatible voltage by voltage converter 371 and applied to one of
eight analog switches 372 and 374. These have the effect of applying the
voltage from the DAC, corresponding to the desired LED current level, to a
storage capacitor which will latch this voltage after the input has been
removed. This voltage is buffered by amplifiers 375 and 376 and applied to
the LED circuitry. Thus, dependant upon the intensity of the signal
received by the photosensor, the respective light emitting diodes can be
driven with greater or lesser voltage to produce the optimum voltage
output.
It is noted that only two of the available eight channels of this sample
hold circuitry are required to adjust the LED intensities. The remaining
channels provide a general purpose analog output from the microprocessor
for a variety of unrelated functions. The output of amplifier 377 provides
the fixed offset for the offset amplifiers described above; VVOL, the
output of amplifier 378, provides a volume control for the alarm; outputs
of amplifiers 601, 602, and 603 provide external outputs for an optional
chart recorder; and the output of amplifier 604 provides a control for the
pitch of the alarm.
Monitor Operation
The manner in which the signal information derived by the oximeter
apparatus is presented to the attendant physician through the oximeter
monitor of this invention will now be discussed.
Referring to FIG. 1, when the instrument power is turned on via power
switch 13, digit display 1 and LED digital viewmeter 12 both flash
momentarily until microprocessor 16 begins its operation. Speaker 15 also
emits a beep. As soon as the microprocessor 16 takes control of the
instrument, which is on the order of a millisecond, the digit display 1 is
cleared, with zeros flashing on digits 794-796. On power up, the oximeter
default is for the audio alarm to be inhibited so that LED alarm inhibit
light 9 begins flashing. Synchronization of the pulse rate of patient 28
through detector probe 29 is not yet established. Therefore, the sync
status light 11 flashes, indicating no sync. Microprocessor 16 begins to
sample the signals from photosensor 38 until it determines that valid
pulses are being received, at which point digits 794-796 of digit display
1 indicate in decimal numbers the percentage of oxygen saturation in the
patient's 28 blood. Digits 797-799 numerica | | |
