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This invention relates to a probe monitoring system for an oximeter, and is
particularly to the provision of a monitoring apparatus enabling the
automatic determination of proper operating conditions in an oximeter.
An oximeter of the type to which the present invention is concern is a
non-invasive blood constituent measuring device disclosed, for example, in
U.S. Pat. No. 4,407,290, Scott A. Wilber, assign to Biox Technology Inc.,
of Boulder, Colo. In a non-invasive oximeter of this type, a plurality of
light emitting diodes emitting light at different wavelengths are directed
toward a blood-containing tissue. The diodes are sequentially energized by
a timing circuit. A photodiode receives light from the tissue, the output
from the photodiode being normalized with respect to a standard voltage,
and processed in a pair of channels responsive to light of the two
different wavelengths respectively. The resultant signals, corresponding
to aborption by the blood of energy at the two wavelengths, are processed
in a microprocessor, in accordance with a given algorithm, in order to
ascertain the various constituents of the blood, such as oxygen.
The present invention is directed to the provision of a measuring system
for continuously determining the operability of various critical elements
of an oximeter of the above type, such as the light emitting diodes and
photodetector, as well as a reference voltage used by the oximeter for
normalizing the signals.
Briefly stated, in accordance with the invention, voltages derived from the
light emitting diode circuits and photodetector circuits, reference and
voltage source are time multiplexed and applied to an analog to digital
converter. The output of the analog to digital converter is applied to a
microprocessor programmed to provide suitable output messages
corresponding to the conditions of operation of these elements.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention will be more clearly understood it will now be
disclosed in greater detail with reference to the accompanying drawing,
wherein:
FIG. 1 is a block diagram of an oximeter incorporating the monitoring
arrangement of the invention; and
FIGS. 2A, 2B, 2C, 2D, 2E and 2F illustrate a flow diagram of the monitoring
system of the ionvention.
Referring now to FIG. 1, therein is illustrated an oximeter of the type
disclosed in U.S. Pat. No. 4,407,290, in simplified form, and modified to
incorporate the monitoring system of the invention. The elements of the
illustrated system, incorporating the oximeter, which are disclosed in
greater detail in U.S. Pat. No. 4,407,290, will first be briefly
described, in order to enable a fuller understanding of the functioning of
the monitor system of the invention. The disclosure of U.S. Pat. No.
4,407,290 is hence incorporated by reference herein.
BRIEF DESCRIPTION OF THE OXIMETER
The oximeter is comprised of a pair of light sources of different
wavelengths, in the form of a pair of LED's 10 emitting, for example, red
light, and LED 11 emitting, for example, infrared radiation. The two red
LED's 10 are connected in series, in a practical embodiment of the
oximeter, in order to provide red radiation of sufficient intensity. A
non-light emitting diode 12 may be connected in series with the infrared
emitting diode 11, for purposes that will be dislosed in greater detail in
the following disclosure. The red and infrared LED's are sequentially
pulsed in a timing circuit to be more fully discussed, by way of
amplifiers 14 and 15 respectively.
Radiation from the red and infrared diodes is sensed in a photodiode 16,
after passing through the tissue, the output of the diode being applied to
a current to voltage converter 17. The output of the current to voltage
converter 17 is applied to Resistor 62 and test circuit 63 which can
modulate incoming signals to verify overall circuit function. After 63 the
signal is applied to variable gain amplifier 64 and then to normalization
circuit 18 which serves the function, among others, of normalizing the
signals with respect to a reference source 19. The normalized output of
the circuit 18 is decoded in decoders 20, filtered in filters 21,
amplified in amplifiers 22 and multiplexed in multiplexer 23. The
decoders, filters and amplifiers constitute a pair of channels for
processing the red and infrared radiation signals separately, and hence
timing signals for properly processing the signals in the respective
channels are obtained from the timing circuits, for controlling the
normalization circuits, decoders and multiplexer. The output of the
multiplexer 23 is converted to digital form in the analog to digital
converter 24, for processing in accordance with the applicable algorithm
in the microprocessor 25, to energize a display 26 for displaying the
desired blood constituents. The analog to digital converter 24 and
microprocessor 25 are of course also synchronized by the timing circuit.
The timing circuit, as may comprise a 3.6864 MHz oscillator 30, the output
thereof being buffered by devices 31 and 32. The output of buffer 32 is
then input to divider 33 such as CMOS type 4040B. One output of 33, at
921.6 KHz is the clock for A/D converter 41. Another output of 33, at 7200
Hz is the input to another divider 34. The outputs of 34, CMOS type 4017B
are used with Flip-Flops 35, 36, 37 and 38 to generate timing signals for
the system. Various outputs of 34, 35, 36, 37 and 38 are also used to
synchronize multiplexer 23 and A/D converter 24 via additional timing
circuitry 39.
THE MONITOR SYSTEM
In accordance with the invention, the following 4 system signals are
applied to a multiplexer and voltage limiter circuit 50;
1. The voltage across the series connection of the two red LED's 10. This
voltage may be derived therefore at the collector of the transistor driver
51 for these light emitting diodes. When the red LED's are not energized,
the voltage at the collector of transistor 51 shoiuld be 5 volts (the
positive supply source voltage). Sincer a typical voltage drop of 1.65
volts is expected across each of these LED's when energized, a voltage of
approximately 1.7 volts should appear at the input A of the multiplexer
when the red LED's are properly energized.
2. The voltage drop across the infrared LED 11. This voltage is derived at
the collector of a transistor 52 that drives the infrared LED. In one
embodiment of the oximeter of the invention, the oximeter may be
selectively adapted for use at an individual's earlobe, or at an
individuals finger. In order to enable the instrument to determine the
instant application of the oximeter, so that it can be properly
calibrated, the series diodes 12 may be connected in series with the
infrared LED when the oximeter is to be employed for determining blood
constituents in a finger, the conventional diode 12 being omitted when the
oximeter is setup to determine blood constituents in an earlobe. Since the
voltage drop expected across an energized infrared LED is about 1.3 volts
and the expected voltage drop across the diode 12 is about 0.6 volts, it
is apparent that the voltage at the input B of the multiplexer 50 will be
5 volts when the LED is not energized, 3.1 volts when the LED is properly
energized and the oximeter is setup for measuring constituents of a finger
and 3.7 volts if the infrared LED is properly energized and the oximeter
is setup to measure blood constitents in an earlobe.
3. The output of the photodetector 16. This voltage may be obtained at the
output of the inverting amplifier 65 and applied to an input C of the
multiplexer 50.
4. The reference voltage of the source 19, which is connected to the input
D of the multiplexer 50.
The output of the multiplexer 50 is applied to an analog to digital
converter 41, the digital output thereof being applied to a microprocessor
25 for driving a display 26.
Suitable timing signals are applied, from the timing circuit, to the
multiplexer 50, analog to digital converter 41 and microprocessor 25.
Thus, it is necessary that the voltages across the LED's, and the output
of the photodiodes 16 be multiplexed to the analog to digital converter 51
at times that the signals are valid. The signal across the LED's and
photodiode are not valid upon initial energization, and hence, for
example, the synchronization signals applied to the decoders 20 are all
indicative of the valid state of the signals, and may be employed for
synchronizing the multiplexer 50. The input D of the multiplexer 50 is of
course valid at all times, and may be sampled at any desired time.
The analog to digital converter 51 may of course be synchronized with the
signals that synchronize the analog to digital converter 24.
In the monitoring of the red and infrared LED's, there are four points in
time that are of interest in determining possible probe and probe drive
failures, i.e., the interval during which the red LED should be lit, the
two intervals when the both LED's should be off, and the interval when the
infrared LED should be lit. If the voltages measured at the monitor points
are not within determined tolerances of the above discussed levels, the
resultant digital signal applied to the microprocessor will result in the
display of an error on the display device 26. Such errors may occur, for
example, if the probe has been disconnected from the oximeter system. In
this case the voltage measured at the LED's will be zero when the LED's
should be on, and 5.0 volts when the LED's should be off (due to the use
of the illustrated resistors 60, 61 respectively in parallel with the red
and infrared LED's.) If this condition occurs, "No Probe" message may be
displayed on the display device 26.
If, on the other hand, the red LED is open circuited, the voltage measured
when the red LED should be on will be zero, and the other voltages
monitored will be correct. In this case, for example, the error message
and the ranges that the monitor voltages fall within may be displayed.
Such displays enable the simple analysis of the fault.
If the voltage at the photodiode detector amplifier is less than 80
millivolts for an ear probe or less than 30 millivolts for a finger probe,
a "Probe Lo" message may be displayed, indicating that an insufficient
amount of either red or infrared light is being passed through the sample
tissue. If the detector amplifier voltage is greater than 1.6 volts, a
"Probe Off" message may be displayed, to indicate that the red or infrared
light is probably not being passed through any tissue.
With respect to the reference voltage from the source 19, if this voltage
is less than 2.9 volts or greater than 3.1 volts the proper normalization
of the oximeter circuits will not occur, and hence an error message, such
as "err 999" may be displayed.
In the flow diagram of FIGS. 2A-2F, the data structure LTTAB indicates
samples taken during LED "on" times, as follows:
0=3.0 volt source
1=infrared LED driver sample
2=red LED driver sample
3=photodetector sample during dark time
4=photodetector sample during infrared time
5=photodetector sample during red time.
The data structure DKTAB refers to samples taken before LED on times, as
follows:
0=3.0 volts
1=infrared LED driver sample
2=red LED driver sample.
FIG. 2A shows the starting procedure for each monitoring cycle, which may
occur under synchronization from the timing circuit at about 33
millisecond intervals.
In block 70, an index count and an error count LPCNT are set to zero. In
blocks 71-74 the three volt source is tested during the LED on times and
LED off times, and if the voltage is out of range, the index stepping
subroutine NXCHN of FIG. 2B is called at block 75. If the index setting is
zero, at block 76, the subroutine of FIG. 2C is called to step the error
count LPCNT. In the event of continuous errors detected in the three volt
source, the circuit loops continuously until the value of LPCNT is equal
to at least 255, whereupon the program at block 77 of FIG. 2C calls an
endless loop display of "err 999". These routines thereby enable a
stabilization period for the three volt source, upon initially turning on
the oximeter, so that an error is not called until a reasonable time has
passed for the voltage level to become stable.
If the three volt source is within the proper range, the program of FIG. 2A
calls the index stepping subroutine at block 78, and then jumps to the
subroutine that commences at FIG. 2B. In block 80, the program jumps to
the subroutine of FIG. 2E to set a value for the variable A, corresponding
to the voltage across the infrared LED circuit, during LED on time. Thus,
in FIG. 2E, the value of RNGFND, and hence the value of the variable A, is
determined on the basis of voltage value thereof, with the variable A
being set to an integer from 0 to 5. Similarly, in block 81 the subroutine
of FIG. 2E is called to set a RNGFND value for the variable B
corresponding to the detected voltage at the infrared LED circuit during
off time. The variable C is set in block 82 to correspond to the output of
the amplifier 17 durig infrared LED on times. The variable D is set in
block 83, with a jump to RNGFND, to correspond to the voltage of the red
LED's during red LED on time, and the variable E is similarly set in block
85 to correspond to the voltage of the red LED's during red LED off time.
The variable F is set in block 86 to correspond to the output of the
amplifier 17 during on times of the red LED's. Upon setting such values,
the program jumps to the routione of FIG. 2F.
Referring to FIG. 2F, in block 90 the value DAEB is compared with the
number 1455. This variable DAEB is a numerical string formed of the
determined range values of the variables D, A, E and B respectively, in
that order. If the value of DAEB is equal to 1455, as tested in block 90,
then an ear probe must be provided in this system, and the finger probe
flag is hence cleared in block 91. This serves to advise the oximeter of
the fact that an ear probe is being employed and that the parameters
employed in the algorithm must be set in accordance with ear probe
measurements. On the other hand, if the value of DAEB is not equal to
1455, it is tested in block 92 to see if the value is 1355 thereby
indicating the present of a finger probe. If a finger probe is indicated,
block 93 ensures the setting of the finger probe flag to advise the
oximeter of the use of the figure probe. An error count PRVERR is set to
zero in block 94, followed by the testing in blocks 95-98 for out-of-range
values of the variables C and F. If these values are within range, the
program exits in block 99, to enable the oximeter to make the necessary
calculations. If the values of C and F are incorrect, error displays are
given at blocks 100 or 101 and the program jumps to the restart of the
routine of FIG. A, from block 102.
If it was found in block 92, that the value of DAEB was not equal to 1355,
it is evident that an error has been detected. In block 105 a test is made
to determine if the value of DAEB is 0055. In this case, an indication of
the absence of the probe is given in block 106, and, after a suitable
delay in block 107 to enable the connection of a probe, the program
returns to the start from block 102.
For other values of DAEB, a determination is made in block 110 of the
number of such errors, and if the number is less than 5, the error count
PRVERR is incremented in block 111 to return to the start of the program
at block 102. If, on the other hand, the number of detected errors is
equal to or greater than 5, a suitable delay and display endless loop 112
advises the user of such errors.
While the invention has been disclosed and described with reference to a
single embodiment, it will be apparent that variations and modifications
may be made therein, and it is therefore intended in the following claims
to cover each such variation and modification as falls within the true
spirit and scope of the invention.
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