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BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for monitoring the
level of a constituent in the blood of a living organism.
Certain constituents in the blood affect the absorption of light at various
wavelengths by the blood. For example, oxygen in the blood binds to
hemoglobin to form oxyhemoglobin. Oxyhemoglobin absorbs light more
strongly in the infrared region than in the red region, whereas hemoglobin
exhibits the reverse behavior. Therefore, highly oxygenated blood with a
high concentration of oxyhemoglobin and a low concentration of hemoglobin
will tend to have a high ratio of optical transmissivity in the red region
to optical transmissivity in the infrared region. The ratio of
transmissivities of the blood at red and infrared wavelengths can be
employed as a measure of oxygen saturation.
This principle has been used heretofore in oximeters for monitoring oxygen
saturation of the blood in the body of a living organism as, for example,
in patients undergoing surgery. As disclosed in U.S. Pat. No. 4,407,290,
oximeters for this purpose may include red light and infrared light
emitting diodes together with a photodetector. The diodes and
photodetector typically are incorporated in a probe arranged to fit on a
body structure such as an earlobe or a fingertip, so that light from the
diodes is transmitted through the body structure to the photodetector. The
infrared and red light emitting diodes are switched on and off in
alternating sequence at a switching frequency far greater than the pulse
frequency. The signal produced by the photodetector includes alternating
portions representing red and infrared light passing through the body
structure. These alternating portions are segregated by sampling devices
operating in synchronism with the red/infrared switching, so as to provide
separate signals on separate channels representing the red and infrared
light transmission of the body structure. After amplification and low-pass
filtering to remove signal components at or above the switching frequency,
each of the separate signals represents a plot of optical transmissivity
of the body structure at a particular wavelength versus time.
Because the volume of blood in the body structure varies with the pulsatile
flow of blood in the body, each such signal includes an AC component
caused only by optical absorption by the blood and varying at the pulse
frequency or heart rate of the organism. Each such signal also includes an
invariant or DC component related to other absorption, such as absorption
by tissues other than blood in the body structure. According to well known
mathematical formulae, set forth in said U.S. Pat. No. 4,407,290, the
oxygen saturation in the blood can be derived from the magnitudes of the
AC and DC components of these signals.
As also set forth in the '290 patent, the same general arrangement can be
employed to monitor constituents of the blood other than oxygen such as
carbon dioxide, carbon monoxide (as carboxyhemoglobin) and/or blood
glucose, provided that the other constituents have some effect on the
optical properties of the blood.
Measurement apparatus and methods of this type have been widely adopted in
the medical profession. However, the signal sampling devices must be
precisely synchronized with the switching devices used to provide the
alternating rods and infrared illumination. The circuitry required to
maintain this synchronization adds cost and complexity to the apparatus.
Moreover, the signals representing light transmission at each wavelength
are necessarily discontinuous.
Moreover, such apparatus and methods have been subject to interference from
ambient light falling on the photodetector. The devices used to recover
the meaningful signal components after amplification of the photodetector
signal have been provided with circuits for canceling components caused by
ambient light. Generally, these circuits operate by obtaining a "dark
current" signal representing the amplified photodetector signal during
intervals when both of the light emitting diodes are off and hence all of
the light falling on the photodetector represents ambient light. The dark
current signal value can be used to cancel the ambient light component in
signals representing infrared and red light.
This approach provides only a partial solution to the ambient light
interference problem. The dark current cancellation circuitry adds
complexity and cost to the apparatus. Also, the ambient light signals may
saturate or overload the initial or front end amplifier connected to the
photodetector, resulting in unpredictable fluctuations in the amplifier
output. To prevent saturation of the front end amplifier, its gain may be
limited, but this in turn requires higher gain in subsequent stages, more
amplification stages or both. Baffles can be used to limit ambient light
reaching the photodetector, but these add further complexity and cost, and
are only partially effective.
Electromagnetic interference capacitively or inductively coupled to the
photodetector and/or leads can also saturate the front end amplifier or
create spurious signals. The shielding used to protect the photodetector
and leads from this interference adds further cost, complexity and bulk.
A new solution to the problems of electromagnetic and ambient light
interference is set forth in the co-pending, commonly assigned United
States patent application of Alan Dean Martin entitled "Blood Parameter
Monitoring Apparatus and Methods", filed on the same day as the present
application. The disclosure of said application of Martin is incorporated
by reference herein. As disclosed in said application of Martin, the light
emitted by the illuminating means such as a light emitting diode is varied
at one or more carrier frequencies. Therefore, the photodetector signal
will include one or more components at the carrier frequency or
frequencies, and these components will represent the light transmitted
through the patient's body structure from the light emitting means.
Modification means are provided for modifying the photodetector signal,
preferably prior to any amplification, so as to increase the ratio of the
carrier frequency component or components to other components of the
signal. Typically, the modification means include a filter such as a
passive resonant circuit, resonant at the carrier frequency or frequencies
employed. The resonant circuit is arranged to pass only signal components
at the carrier frequency or frequencies, while substantially attenuating
other components. The modification means effectively eliminates both
components of the signals due to ambient light, and also effectively
eliminates typical electromagnetic interference signals. Thus, the front
end amplifier cannot be overloaded by these spurious signal components.
Also, because the ambient light components are effectively eliminated by
the modification means, the device need not incorporate separate "dark
current" compensation circuitry.
The preferred apparatus set forth in the aforementioned Martin application,
however, utilizes a time division multiplexing scheme. Thus, light of
different wavelengths is applied in a sequence of alternating bursts at a
predetermined switching frequency, with the light within each burst
varying in amplitude at the carrier frequency or frequencies. The modified
photodetector signal from the modification means or resonant circuit is
sampled at predetermined times corresponding to the alternating bursts of
light at the different wavelengths, so that the sampling procedure
effectively separates signals representing transmissivity at each
wavelength. This arrangement thus requires switching, sampling and timing
circuitry.
Accordingly, there have been significant needs heretofore for still further
improvements in blood constituent monitoring apparatus such as medical
oximeters.
SUMMARY OF THE INVENTION
The present invention provides apparatus and methods which address these
needs.
Apparatus according to one aspect of the present invention includes
illumination means for emitting light at a plurality of wavelengths and
directing the emitted light through a body structure. Photodetector means
are provided for receiving light transmitted from the illumination means
through the body structure and producing one or more photodetector signals
representing the intensity of the transmitted light. The illumination
means and photodetector means may include, respectively, light emitting
diodes and a photodiode.
Carrier frequency means are provided for varying the intensity of the light
of each wavelength emitted by the illumination means at a different
carrier frequency so that a separate carrier frequency is associated with
each wavelength. As the light of each wavelength emitted by the
illumination means and transmitted through the body varies at the
associated carrier frequency, the photodetector signal or signals will
also include components at the different carrier frequencies. The
component of the photodetector signal at each carrier frequency will
represent light of the associated wavelength transmitted through the body
structure. Thus, the component at each carrier frequency will bear
information relating to the optical absorptivity or transmissivity of the
body structure at the associated wavelength emitted by the light emitting
means. Typically, the amplitude of the component at each carrier frequency
will vary in accordance with the transmissivity of the body structure at
the associated wavelength.
Separation means are provided for subdividing the photodetector signal or
signals by frequency so as to separate the components at the different
carrier frequencies from one another. Such separation of the different
carrier frequency components serves to separate the signals representing
transmissivity at the different wavelengths. Interpretation means are
provided for determining the level of the blood constituent to be
monitored from the separated signals. Because the signals representing
transmissivity at the different wavelengths are separated according to
their respective carrier frequencies, the time division multiplexing
arrangements heretofore utilized can be eliminated. According to preferred
aspects of the present invention, the illumination means emits light of
all of the different wavelengths simultaneously. Light of each wavelength
may be emitted substantially continuously during the measurement
operation, subject only to the repetitive amplitude or intensity
variations imposed by the carrier frequency means.
Preferably, the separation means includes a plurality of filters, one for
each carrier frequency, and each such filter is arranged to provide a
filtered output signal consisting principally of the signal component at
the associated carrier frequency. Thus, the filters serve to reject
ambient light and electromagnetic interference signals in addition to
separating the different carrier frequency components from one another.
According to the broad compass of the invention, the separation means can
act on the photodetector signal either before or after that signal has
been preliminarily amplified or otherwise processed. Preferably, however,
the filters in the separation means are connected between the
photodetector means and the initial amplification stage. Thus, the filters
receive the photodetector signal or signals directly from the
photodetector means without prior amplification. Separate amplifiers are
associated with the individual filters so that each amplifier receives the
output signal from one such filter, and hence receives the signal
component at one carrier frequency. In this arrangement, the filters
prevent overloading of the front end amplifiers by spurious signal
components. Although active or passive filters may be employed, passive
filters are preferred, and each filter preferably incorporates an
inductive/capacitive network. Preferably, tuning means are provided for
adjusting the carrier frequency means, the filters or both to match the
carrier frequencies with the resonant frequencies of the filters.
The interpretation means typically includes separate signal processing
channels, each associated with one carrier frequency and hence with one
wavelength. Each such channel typically incorporates means for
demodulating the carrier frequency signal so as to recover a base band or
unmodulated signal representing the variation in transmissivity at the
associated wavelength versus time. Appropriate means are provided for
recovering the AC and DC components of each such baseband signal. The
interpretation means preferably also includes means for computing the
level of a constituent in the blood from the AC and DC components of the
transmissivity signals for the different wavelengths.
Further aspects of the present invention include methods of monitoring the
level of a blood constituent. These methods preferably employ steps
corresponding to the functions described above.
These and other objects, features and advantages of the present invention
will be more readily understood from the detailed description of the
preferred embodiments set forth below taken in conjunction with the
accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of apparatus according to one embodiment of the
present invention together with certain waveforms utilized therein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Apparatus according to one embodiment of the present invention includes a
probe 10 incorporating a clip 12 adapted to mount on a body structure such
as a fingertip 14. Light emitting means including a red light emitting
diode or "LED" 18 and an infrared light emitting diode 20 are mounted to
clip 12. A photodetector 22, which in this case is a photodiode, is also
mounted to clip 12. These components are arranged so that when the clip 12
is secured to the body structure, light from the LED's 18 and 20 will be
directed through the body structure to impinge upon the photodiode 22.
A first carrier frequency generator 24 is arranged to provide a
substantially continuous signal at a first carrier frequency within a
first frequency range. Carrier frequency generator 24 is responsive to a
control signal applied at an input 25 to adjust the first carrier
frequency within the first frequency range. Carrier frequency generator 24
is connected to a red LED drive circuit 26, which in turn is connected to
red LED 18. The carrier frequency generator 24 and drive circuit 26 are
arranged so that the power applied by drive circuit 26 to LED 18, and
hence the intensity of the light emitted by LED 18 will vary in accordance
with the carrier frequency signal. Thus, the intensity of the red light
will repeatedly increase and decrease at the first carrier frequency.
A second carrier frequency generator 28 is connected to an infrared LED
circuit 30, which in turn is connected to infrared light emitting diode
20. These components are similar to first carrier frequency generator 24
and red LED drive circuit 26 respectively. However, second carrier
frequency generator 28 is arranged to operate within a second carrier
frequency range different from the first frequency range, and to adjust
the second carrier frequency within this second range responsive to a
control signal applied at an input 25. The intensity of the infrared light
emitted by LED 20 will vary at the second carrier frequency. Because the
first and second frequency ranges utilized by carrier frequency generators
24 and 28 do not overlap, the first and second carrier frequencies will
differ from one another.
Photodiode 22 has an input node 32 and an output node 34. The input node of
the photodiode is connected to a bias voltage source 36 which maintains
the diode in a reverse biased condition. The output node 34 of the diode
is connected to the a node 38 of a filter network 40.
Filter network 40 incorporates a first resonant circuit or filter 42
including inductor 44, capacitor 46 and damping resistor 48, all of which
are connected in parallel between node 38 and a midpoint node 50. Filter
network 40 also includes a second, generally similar resonant circuit or
filter 52 composed of inductor 54, capacitor 56 and damping resistor 58,
these components being connected in parallel between midpoint node 50 and
a ground node 60.
The values of inductor 44, capacitor 46 and damping resistor 48 establish
the resonant frequency of first resonant circuit 42, and also establish
the width of the passband of the first resonant circuit extending above
and below its resonant frequency. Inasmuch as the inductor, capacitor and
resistor values are subject to finite tolerances, the resonant frequency
is subject to a finite tolerance. However, the component values and
tolerances are selected so that the resonant frequency of the first
resonant circuit is within the first frequency range associated with red
carrier frequency generator 24. Also, the passband of circuit 42 is
encompassed within the first frequency range. The passband of circuit 42
thus substantially excludes the second frequency range associated with IR
carrier frequency generator 28. Conversely, second resonant circuit or
filter 52 has a resonant frequency within the second frequency range
associated with generator 28, and a passband extending slightly above and
slightly below this resonant frequency. The passband of second resonant
circuit 52 substantially excludes the first frequency range. Where the
signals applied to filter network 40 include a composite of signals at
many different frequencies, the first filter output signal appearing
between nodes 38 and 50 will consist essentially of voltage signals at
frequencies within the passband of filter 42. Signals at other frequencies
are shunted between these output terminals and hence do not appear as
voltages across nodes 38 and 50. The output signal appearing as a voltage
between nodes 50 and 60 will consist essentially of frequencies within the
passband of the second filter 52.
A first differential amplifier 62 is connected across the output nodes 38
and 50 of first filter 42. Amplifier 62 is arranged to deliver an output
voltage substantially proportional to the voltage appearing between nodes
38 and 50. A first intermediate amplifier 64 is connected to the output of
amplifier 62, the output of intermediate amplifier 64 being connected to
the input of a demodulator 66. Demodulator 66 per se may be a conventional
AM demodulating circuit of the type normally utilized to recover audio
frequency or sub-audio frequency baseband signals form an amplitude
modulated signal. For example, demodulators of the types normally utilized
in AM radio reception can be employed. Alternately, demodulator 66 may be
a sampling device arranged to take successive, very brief samples of the
signal from intermediate amplifier 64 at times coordinated with the peaks
in the first carrier frequency signal from generator 24. The output of
demodulator 66 is connected to the input of a low pass filter 70 having a
top cutoff frequency of about 10 Hz, filter 70 being arranged to pass
signal components below this frequency and to substantially attenuate
signal components above the cutoff frequency. The output of low pass
filter 70 is connected to an analog to digital converter 72, which in turn
is connected to a microprocessor 74. The output of low pass filter 70 is
also connected to the input of a high pass filter 76. High pass filter 76
is arranged to attenuate frequencies below about 0.5 Hz and to pass
frequencies above that value. The 0.5 Hz value represents the "corner"
frequency of the high pass filter, i.e., the frequency at which the filter
provides about 3 db attenuation. Frequencies below about 0.5 Hz are
attenuated to a greater degree. The output of high pass filter 76 is
connected to an amplifier 78, referred to herein as a "channel amplifier",
and the output of channel amplifier 78 is connected to a low pass filter
80, having a corner or 3 db attenuation frequency of about 10 Hz. The
output of low pass filter 80 is connected to a sample and hold device 82,
which in turn is connected to an analog to digital converter 84. The
analog to digital converter 84 is connected to microprocessor 74. The
components connected between amplifier 64 and the microprocessor
constitute a first signal processing channel 86.
The apparatus also includes a second differential amplifier 88 having its
input terminals connected to the output nodes 50 and 60 of the second
filter or resonant circuit 52. The output of amplifier 88 is connected to
a second intermediate amplifier 90 which in turn is connected to a second
signal processing channel 92. Amplifiers 88 and 90 are substantially
similar to amplifiers 62 and 64 respectively, whereas the second signal
processing channel 92 is substantially similar to the first signal
processing channel 86. Thus, second channel 92 includes a demodulator 94,
low pass filter 96, analog to digital converters 98 and 100, high pass
filter 102, channel amplifier 104, low pass filter 106 and sample and hold
unit 108. These components are substantially similar to the corresponding
components of the first signal processing channel 86.
The microprocessor 74 is arranged to receive digital voltage values or
representations from the various analog to digital converters. As
explained hereinbelow, the microprocessor is adapted to determine the
level of oxygen in the patient's blood from these values. Also, the
microprocessor 74 is connected via feedback connections, indicated in
broken lines in FIG. 1, to each of intermediate amplifiers 64 and 90 and
to channel amplifiers 78 and 104. Via these feedback connections, the
microprocessor 74 can control the gain of each of these amplifiers. The
microprocessor 74 is also connected to a display unit 110 so that the
calculated oxygen value from the microprocessor is displayed on unit 110.
A tuning circuit 120 is associated with red carrier frequency generator 24
and with first resonant circuit 40. Tuning circuit 120 includes a phase
detector 122 having one input connected to the output of intermediate
amplifier 64 and another input connected to the output of first or red
carrier frequency generator 24. Phase detector 122 is arranged to provide
a control signal varying with the difference in phase between the signals
applied at its two inputs. The output of phase detector 122 is connected
to one input terminal of a switch 124, whereas another input terminal of
the switch is connected to the output of a digital-to-analog converter
126. Converter 126 in turn is connected to microprocessor 74. Switch 124
is controlled by microprocessor 74 to feed signals from converter 126 or
detector 122 to the input of a low pass filter 128. The output of filter
128 is connected to the control input 25 of first carrier frequency
generator 24.
A similar tuning circuit 130 is associated with the second or IR carrier
frequency generator 28 and second resonant circuit 52. Tuning circuit 130
includes a phase detector 132, switch 134, digital-to-analog converter 136
and low pass filter 138 substantially identical to the corresponding
components of circuit 120. The inputs of phase detector 132 are connected
to the outputs of intermediate amplifier 90 and of second carrier
frequency generator 28. The output of low pass filter 138 is connected to
the control input 27 of generator 28.
In a method according to one embodiment of the present invention, clip 12
is mounted to the fingertip 14 of a human patient. Carrier frequency
generator 24, red LED drive 26 and red LED 18 are operated so that LED 18
substantially continuously emits red light with its intensity varying at a
first carrier frequency within the first range. Likewise, carrier
frequency generator 28, IR LED drive 30 and IR emitting LED 20 are
actuated to emit infrared light substantially continuously but with its
intensity varying at the second predetermined carrier frequency. Apart
from the variations in intensity at the carrier frequencies, the intensity
of the red and infrared light is substantially constant. The light from
diodes 18 and 20 passes through the fingertip to the photodiode 22. As the
light passes through the fingertip, portions of the liguht are absorbed
and only the remainder is transmitted to photodiode 22. Thus, the red and
infrared light passing through the fingertip will be amplitude-modulated
in accordance with the red and infrared transmissivities of the fingertip
respectively. The transmissivity at each wavelength will vary with the
specific optical absorbtivity of the blood at that wavelength and hence
with the oxygen concentration in the blood. The transmissivity at each
wavelength will also vary with the amount of blood in the patient' s
fingertip and hence with the patient's pulse.
The conductance of photodiode 22 will vary substantially instantaneously in
accordance with the total intensity of all light impinging on the
photodiode. The light impinging upon the photodiode will include both the
red and infrared light transmitted through the patient's fingertip, in
addition to some ambient light. The ambient light typically will include a
constant or DC component together with a flicker frequency component,
typically at about 100-120 Hz. The conductance of the photodiode, and
hence the photodetector output signal appearing at node 34 thus will
include a component at the first carrier frequency, the amplitude of the
this component varying in accordance with the red wavelength
transmissivity of the fingertip, and also will include a corresponding
component at the second carrier frequency, the amplitude of this component
varying with the infrared transmissivity. In addition, the photodetector
output signal will include DC and flicker frequency components
representing ambient light. The signal may also include components at
other frequencies representing electromagnetic interference and the like.
As the photodetector output signal is applied to filter network 40, the
filtered output signal or voltage appearing between nodes 38 and 50
corresponds essentially to only those components of the photodetector
output signal within the passband of the first resonant circuit or filter
42. The filtered output signal is amplified by front end amplifier 62 and
by intermediate amplifier 64.
Because of the tolerances in the resonant circuit components, there may be
some mismatch between the first carrier frequency applied by generator 24
and the resonant frequency of filter 42 when the system is first started.
However, any such mismatch is corrected by tuning circuit 120. At startup,
switch 124 connects digital-to-analog converter 126 to filter 128 and
hence to control input 25 of the generator. As further discussed below,
microprocessor 74 receives a baseband signal through analog-to-digital
converter 72. The amplitude of this signal is directly related to the
amplitude of the signals passing through filter 40 and through amplifiers
62 and 64. By returning a feedback signal through converter 126 to control
input 25, the microprocessor adjusts the first or red carrier frequency
supplied by generator 24 to maximize the amplitude of the signals passing
through filter 42. This maximum occurs when the red carrier frequency is
substantially equal to the resonant frequency of filter 42 and hence
within the passband of the filter. At this point, the microprocessor
actuates switch 124 to connect phase detector 122 with control input 25
via filter 128, and to disconnect converter 126. The signal from the phase
detector represents the phase shift caused by filter 42. The phase shift
signal applied to the control input of generator 24 causes further
adjustment of the red carrier frequency, until the phase shift is reduced
to zero. This zero phase shift or "phase lock" condition occurs when the
red carrier frequency applied by generator 24 is precisely equal to the
resonant frequency of filter 42. Thus, the phase shift signal provides a
final, precise turning of the red carrier frequency to the resonant
frequency of the filter. The tuning circuit maintains phase lock, with
continued feedback of the phase shift signal from detector 122, during
continued operation.
Tuning circuit 130 acts in substantially the same way to adjust the second
or IR carrier frequency applied by generator 28 to match the resonant
frequency of filter 52, and to maintain the IR carrier frequency signal in
"phase lock" with the output signal from filter 52. Thus, in steady state
operation, each carrier frequency is precisely matched to the resonant
frequency of one of the filters. As the resonant frequency of each filter
lies at the center of the filter's passband, each carrier frequency lies
within the passband of one filter. In this steady state condition the
signal from filter 42 applied to amplifier 62 consists essentially of the
signal component at the first carrier frequency, representing the red
wavelength transmissivity. Conversely, the signal component passing
through second filter 52 to amplifier 88 consists essentially of the
signal component at the second carrier frequency, representing the
infrared transmissivity of the fingertip. The filter network 40 thus
serves both to reject the spurious ambient light and electromagnetic
interference components and also to separate the two carrier frequency
signals from one another.
As indicated by curve F.sub.1, the separated, amplified signal as fed into
the first signal processing channel 86 through amplifiers 62 and 64 has
amplitude varying in accordance with the varying red wavelength
transmissivity of the fingertip, i.e., it is a signal at the first carrier
frequency amplitude-modulated with the red wavelength transmissivity.
Demodulator 66 recovers the base band or information content of this
modulated signal and passes the base band signal through low pass filter
70. Low pass filter 70 removes residual carrier frequency or noise
component and smoothes the base band signal to provide a varying base band
signal voltage as indicated by curve BB.sub.R. The base band signal
voltage BB.sub.R represents the red wavelength transmissivity of the
fingertip. This signal has a DC component and an AC component at a
frequency equal to the patient's pulse frequency or heart rate, viz.,
typically about 1-2 Hz, and never more than about 5 Hz (300 beats/min.).
Analog to digital converter 72 provides digital representations of the red
transmissivity signal BB.sub.R to microprocessor 74. As indicated by curve
BB.sub.R, the AC component or variation with time in the red
transmissivity is typically much smaller than the DC component. Therefore,
each digital representation or value delivered by analog to digital
converter 72 to microprocessor 74 provides a reasonable approximation of
the DC component of transmissivity at the red wavelength. Moreover,
microprocessor 74 is arranged to perform a digital low pass filtering or
averaging on successive values captured through analog to digital
converter 72, so as to derive a more accurate value for the DC component
of red wavelength transmissivity.
The base band red wavelength transmissivity signal is also delivered to
high pass filter 76, which effectively strips the DC component from the
signal and passes only the AC component to channel amplifier 78. After
amplification in the channel amplifier and further filtering by low pass
filter 80 to remove stray noise components and the like, the amplified AC
component passes to sample and hold unit 82. Sample and hold unit 82 is
actuated by microprocessor 74 to sample the AC signal at a sampling rate
of about 25-30 Hz or more, and to deliver successive samples to analog to
digital converter 84. The analog to digital converter delivers a stream of
successive digital representations or values each representing one sample
of the AC component. The microprocessor is arranged to calculate the AC
component or peak to peak value of the red transmissivity signal from the
successive values.
In exactly the same way, the amplitude-modulated signal F.sub.2 at the
second carrier frequency representing IR transmissivity is delivered by
filter 52 through amplifiers 88 and 90. This signal is processed by the
second signal processing channel 92 to recover the base band infrared
transmissivity signal BB.sub.IR and further processed in microprocessor 74
to recover the AC and DC components of this signal.
Microprocessor 74 is arranged to calculate the oxygen saturation of the
blood within fingertip 16 according to the formula:
Oxygen Saturation=AR.sup.2 +BR+C
where:
##EQU1##
AC.sub.R and DC.sub.R are the AC and DC components, respectively, of the
red transmissivity signal;
AC.sub.IR and DC.sub.IR are the AC and DC components respectively of the
infrared transmissivity signal; and
A, B and C are constants determined by empirical curve fitting in design of
the system, against the results of standard blood oxygen determinations.
The oxygen saturation calculated by microprocessor 74 is displayed on
display unit 110. As will be appreciated, apparatus in accordance with the
present invention may also include other well known features commonly
found in oximeters as, for example, testing devices for checking operation
of the system and devices for deriving information concerning the presence
or absence of a pulse and the pulse rate from one or both of the baseband
transmissivity signals. The microprocessor can be programmed to detect the
pulse rate by monitoring peaks in the AC component of the red
transmissivity signal supplied by channel amplifier 78, as by monitoring
the sequence of digital representations delivered through analog to
digital converter 84. As set forth in U.S. Pat. No. 4,407,290, the levels
of more than one constituent in the blood may be detected by using light
at three or more different wavelengths. That technique may be employed
according to the present invention. A separate carrier frequency would be
utilized for each of these different wavelengths, i.e., three different
carrier frequencies for three different wavelengths to detect the levels
of two different constituents.
Microprocessor 74 controls the gains of amplifiers 64, 78, 90 and 104 to
maintain the signals supplied to each analog to digital converter 72, 84,
98 and 100 within the optimum operating range of the converter. For
example, if the digital values or representations applied by converter 72
indicate that the signal voltage applied to this converter is approaching
or exceeding the operating range of the converter, microprocessor 74 will
apply an appropriate feedback signal to amplifier 64 to reduce the gain of
that amplifier. Conversely, if the digital values supplied by converter 84
indicate that the signals arriving at that converter are at or below the
minimum operating voltage of the converter, the microprocessor will
increase the gain of amplifier 78. The microprocessor keeps track of the
gain adjustments made with respect to each amplifier and applies
appropriate multipliers to the digital values supplied through each
digital converter. For example, as the gain of amplifier 64 is increased,
the multiplier applied to the values from converters 72 and 84 will be
decreased. Thus, the microprocessor will obtain accurate values for the AC
and DC components of the various transmissivity signals despite changes in
the gains of the amplifiers.
The carrier frequencies utilized in apparatus and methods according to the
present invention should, desirably, be far above the fundamental flicker
frequency of the ambient lighting, and above the harmonics of the flicker
frequency. However, it is desirable to maintain the carrier frequencies
below the frequencies predominating in electromagnetic interference such
as the interference caused by electrosurgical equipment. Carrier
frequencies between about one KHz and about ten KHz are preferred. The
carrier frequencies must differ from one another by an amount sufficient
to permit separation of the respective signals according to the carrier
frequencies. The necessary separation in turn will depend upon the
bandwidth of the filters. Although relatively narrow filter bandwidths are
desirable for signal separation and interference rejection, there is a
lower bound on filter bandwidth set by phase distortion considerations.
Each filter should pass signals within about .+-.15 Hz of the associated
carrier frequency, without substantial phase error. For a phase error of
less than about 1.degree. over this .+-.15 Hz range using
inductive-capacitive networks as shown, the half-power bandwidth of each
filter should desirably be about 1.7 KHz or more, assuming that each
carrier frequency is accurately matched to the resonant frequency of the
associated filter. With the continuous phase lock tuning used in the
preferred embodiments discussed above, this assumption is justified.
The waveform of the carrier frequency signal, and hence the waveform of the
power applied to each LED, most preferably is sinusoidal. Other waveforms
have significant harmonic components. The harmonic components of one
carrier frequency may fall within the passband of the filter associated
with the other carrier frequency, and hence may cause errors.
Numerous variations and combinations of the features set forth above can be
utilized without departing from the invention as defined by the claims.
For example, the photodetector may be arranged to operate in a
photovoltaic or photoamperic mode, and hence the photodetector bias supply
36 shown above may be eliminated. Also, the center node 50 of the filter
network may be grounded. Filters of configurations other than that shown
may be utilized.
In a variant of the tuning arrangement, the filters 42 and 52 may be
provided with variable resistors in place of the fixed resistors 48 and
58. These may be set at startup to provide a broad bandwidth, and adjusted
to the normal value under control of the microprocessor once the system is
in phase lock. In this arrangement, the digital-to-analog converters 126
and 136 may be omitted. In a less preferred arrangement, tuning can be
accomplished without phase lock, as by amplitude maximization alone. Also,
the tuning circuits can operate by adjusting the resonant frequencies of
the filters rather than the carrier frequencies. The result--precise
matching of carrier and resonant frequencies--is the same. In a less
preferred variant, the tuning circuitry can be omitted, provided that the
filters are built to very close tolerances to provide an acceptable match
between carrier and resonant frequencies.
In the embodiment discussed above, the filters serve both to separate the
two carrier frequency signals and also to reject interference upstream of
the front end amplifiers 62 and 88. In a less preferred embodiment, the
photodetector signal could be amplified prior to separation, as by
connecting the photodetector to a single front end amplifier and
delivering the output signal from the front end amplifier to a filter
network substantially as discussed above. According to the broad compass
of the invention, it is not essential to employ analog domain filters.
Thus, the components of the photodetector signal can be separated from one
another according to their frequencies by digital filtering techniques. As
these and other variations and combinations of the features described
above can be utilized without departing from the present invention as
defined in the claims, the foregoing description of the preferred
embodiments should be taken by way of illustration rather than by way of
limitation of the invention as defined in the claims.
* * * * *
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