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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for monitoring one
or more parameters of 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, such apparatus and methods have been
subject to interference from ambient light falling on the photodetector.
The signal processing devices used to recover the AC and DC components
after amplification of the photodetector signal have been provided with
circuits for cancelling signal 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. Moreover, the ambient light
ordinarily flickers at about twice the local power line frequency (100 or
120 Hz), thus introducing a substantial component at these frequencies
into the photodetector signal. The low-pass filters must be arranged to
suppress these flicker components while passing the AC component at the
pulse frequency and also maintaining acceptable limits on phase distortion
of the filterd signals. The low-pass filters therefore may require
expensive hand-matched components to achieve proper performance.
Moreover, the ambient light signals may saturate the initial or front end
amplifier connected to the photodetector. Thus, The signals caused by
ambient light may cause the front end amplifier to exceed its maximum
rated output, 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 have been used to reduce the amount
of ambient light reaching the photodetector and thus prevent saturation.
These baffles add further complexity and cost, and are only partially
effective.
In addition, interference from sources other than ambient light can
saturate the front end amplifier or create spurious signals. In
particular, electromagnetic fields from electrosurgical devices or the
like may create substantial signals in the photodetector or its leads
through capacitive or inductive coupling. The shielding used to protect
the photodetector and leads from such interference adds further cost,
complexity and bulk.
Accordingly, there have been significant unmet 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, preferably 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 a
photodetector signal representing the intensity of the transmitted light.
The illumination means and photodetector means may include, respectively,
light emitting diodes and a photoconductor such as a photodiode.
Carrier frequency means are provided for varying the intensity of the light
emitted by the illumination means at a carrier frequency. The carrier
frequency desirably is far above the pulse frequency of the patient and
far above the flicker frequency of the ambient illumination. Because the
light emitted by the illumination means and transmitted through the body
varies at the carrier frequency, the photodetector signal will also vary
at the carrier frequency. The component of the photodetector signal at the
carrier frequency will represent light transmitted through the body
structure and hence will bear information relating to the optical
absorptivity or transmissivity of the body structure at each wavelength
emitted by the light emitting means.
Most preferably, modification means are provided for modifying the
photodetector signal to increase the ratio of carrier frequency component
to other components and thereby provide a modified signal. The
modification means preferably attenuates the other components, and may
also boost the carrier frequency component. The modification means
desirably includes a passive resonant circuit having a resonant frequency
substantially matched to the carrier frequency.
Amplification means are provided for amplifying the modified signal and
interpretation means are provided for determining a parameter of the blood
in the body structure from the amplified signal. The parameter so
determined may be the level of a constituent such as oxygen in the blood.
The amplification means may include a generally conventional front end
amplifier having an input node connected to an output node of the
photodetector, whereas the resonant circuit of the modification means may
include a capacitor and an inductor connected to the input node of the
amplifier so as to shunt signal components at frequencies other than the
carrier frequency away from the input node. In some embodiments according
to this aspect of the present invention, the interpretation means includes
means for separating components of the amplified signal to form separate
signals representing the optical transmissivity of the body structure at
the different wavelengths, and means for recovering the DC and AC
components of these signals to thereby determine the level of the blood
constituent.
Photodetector signal components caused by ambient light may be
substantially suppressed by the modification means without ever reaching
the amplification means or front end amplifier. The front end amplifier
therefore is substantially immune to saturation caused by ambient light.
The gain of the front end amplifier need not be restricted to avoid such
saturation. Requirements for shielding the photodetector from ambient
light can be significantly relaxed. Therefore, the probe or photodetector
mounting may be simpler, more compact and more convenient. The dark
current subtraction or cancellation circuits are not required in the
interpretation means, leading to further simplification and cost savings.
Because ambient light signals are substantially blocked by the
modification means, the lowpass filters in the interpretation means need
not be arranged to block ambient light flicker components. The low-pass
filters therefore can be simpler than those previously employed.
The modification means or passive resonant circuitry also blocks spurious
signals caused by electromagnetic interference. Thus, preferred apparatus
according to this aspect of the invention can function effectively even in
the presence of relatively "noisy" interfering equipment such as
electrosurgical devices. Also, the requirements for electromagnetic
interference shielding of the photodetector and associated leads can be
significantly relaxed.
The carrier frequency means may be arranged to vary the amplitude of the
light of all of the different wavelengths emitted by the illumination
means at a single carrier frequency, and the modification means may
include resonant circuitry having a single resonant frequency
substantially equal to this carrier frequency. In this arrangement, the
apparatus may include timing means for actuating the illumination means to
emit light of each wavelength at different times according to a
predetermined time division schedule. Thus, bursts of light of the
different wavelengths may follow one another in alternating sequence at a
predetermined switching frequency lower than the carrier frequency but
higher than the pulse frequency of the subject. The interpretation means
may include means for sampling the amplified signal at the times
associated with the different wavelengths and directing the sampled
signals accordingly into different signal processing channels. In systems
of this nature, the resonant circuit preferably is at least critically
damped, and more preferably somewhat overdamped. Therefore, the resonant
circuit comes to steady state oscillation at the carrier frequency quickly
during each burst of light, allowing use of a relatively high switching
frequency.
According to the broadest concepts of the present invention, more than one
carrier frequency may be employed. The modification means may be arranged
to enhance photodetector signal components of all of these frequencies
while suppressing components at other frequencies. Different carrier
frequencies may be employed for light of different wavelengths. Light of
plural wavelengths may be emitted simultaneously. Preferably, light of all
of the different wavelengths is emitted continuously. In this arrangement,
the photodetector signal may include components at different carrier
frequencies representing transmissivity of the body structure at different
wavelengths. The modification means may include separate resonant
circuits, each resonant at the carrier frequency associated with a
different wavelength. The photodetector signal is fed to these resonant
circuits, and the separate modified signal output by each resonant circuit
is supplied to a separate signal channel via a separate front end
amplifier. Thus, the modification means or resonant circuits in this
arrangement serve to separate the signal components associated with the
different wavelengths of light as well as to provide the other advantages
noted above. In this arrangement, the need for switching, timing and
sampling circuitry associated with the time division multiplexing
arrangement is eliminated.
Further aspects of the present invention include methods of monitoring
blood constituents. In preferrred methods according to this aspect of the
invention, light at a plurality of wavelengths is emitted and passed
through a body structure, the intensity of the emitted light being varied
at a carrier frequency as discussed above. The light passsing through the
body structure is detected to provide a photodetector signal having a
carrier frequency component bearing information relating to the optical
transmissivity of the body structure at the plural wavelengths. The
photodetector signal is modified so as to selectively increase the ratio
of the carrier frequency component to other components. The modified
signal is amplified and the level of the blood constituent is determined
from the amplified signal. Methods according to this aspect of the present
invention provide advantages similar to those discussed above in
connection with the apparatus.
The foregoing and other objects, features and advantages of the present
invention will be more fully apparent from the detailed discussion of the
preferred embodiments set forth below taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic, partially block diagram of apparatus in accordance
with one embodiment of the invention.
FIGS. 2A through 2F are a schematic depictions of certain wave forms
occurring during operation of the apparatus depicted in FIG. 1, all on the
same time scale.
FIGS. 3A and 3B are depictions of other waveforms occurring during
operation of the apparatus depicted in FIG. 1 on a time scale different
from that of FIGS. 2A-2F.
FIG. 4 is a fragmentary schematic view of apparatus according to a further
embodiment of the invention.
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 finger tip 16. A light source including a red light emitting diode or
"LED" 18 and infrared LED 20 is mounted to clip 12. A photodiode 22, which
in this case is a photoconductive diode is also mounted to clip 12. Clip
12 is arranged so that light from LED's 18 and 20 is directed through the
body structure of fingertip 16 towards photodetector 22.
A carrier frequency generator 24 is connected to red LED drive 23 and to
infrared LED drive 25, which in turn are connected to LED's 18 and 20
respectively. Generator 24 is arranged to supply power varying in
accordance with a square waveform having a predetermined carrier
frequency, in this case 19.2 kHz. A master timing unit 28 is arranged to
generate a two state square switching wave having a frequency of 300 Hz,
or exactly 1/64 of the carrier frequency supplied by carrier frequency
generator 24. Timing unit 28 is arranged to actuate a high speed switch 26
and to provide an actuating signal through switch 26 alternately to red
LED drive 23 or to infrared LED drive 25 in accordance with the switching
wave. Each drive provides power to the associated LED only while that
drive receives the actuating signal through switch 26. While each drive is
active, it supplies power to the associated LED varying in accordance with
the carrier frequency waveform. Thus, red drive 23 and LED 18 are actuated
during one-half of the switching waveform whereas infrared drive 25 and
LED 20 are actuated during the other half. Carrier frequency generator 24
is also connected to timing unit 28 so that the 19.2 kHz carrier frequency
waveform is synchronized with the 300 Hz switching waveform. A cycle of
the carrier frequency waveform commences simultaneously with the start of
each cyle of the switching waveform.
As drives 23 and 25 and LED's 18 and 20 respond to the driving power from
carrier frequency generator 24 in alternating sequence according to the
switching waveform from timing unit 28, the LED's will produce alternating
bursts of red and infrared light as shown in FIG. 2A and 2B respectively.
Curve 30 shows the intensity of red light I.sub.R emitted by LED 18,
whereas curve 32 shows the intensity of infrared light I.sub.IR emitted by
LED 20 on the same time scale. As indicated, bursts 86 of red light
alternate with bursts 88 of infrared light, there being one burst of red
light and one burst of infrared light within each cyle of the switching
waveform. The bursts follow in close sequence, so that a burst of infrared
light follows immediately after each burst of red light, and vice versa.
Each burst occupies one-half of the 300 Hz switching cycle. Thus, each
burst lasts for 1/600 second. Within each burst, the intensity of light
varies in accordance with the carrier frequency waveform. As the swtiching
frequency of 300 Hz is exactly 1/64 of the carrier frequency, each burst
includes 32 cycles of the carrier frequency waveform.
Photodiode 22 has an input node 36 and an output node 38. Input node 36 is
connected to a DC voltage bias source 42 so that the photodiode is reverse
biased. As the conductance G of photodiode 22 in this reverse bias mode
varies with the intensity of light falling on the diode, the voltage at
ouput node 38 will also tend to vary in accordance with the amount of
light falling on the diode. Output node 38 is connected to the input node
44 of a front end amplifier 46. An inductor 48, capacitor 50 and resistor
52 are connected in parallel with one another between output node 38 and
ground so that the inductor, capacitor and resistor cooperatively define a
damped inductive/capacitive passive resonant circuit 47 in parallel with
the input node of the amplifier.
The output of amplifier 46 is connected to the input of an intermediate
amplifier 54, the output of intermediate amplifier 54 in turn being
connected to a pair of signal sampling units 56 and 58. Sampling units 56
and 58 are controlled by timing unit 28 so that each sampling unit samples
the amplified signal at predetermined times during the switching waveform
as further discussed hereinbelow. The samples taken by unit 56 are fed to
signal processing channel 60. Channel 60 includes a low-pass filter 62
having a top cutoff frequency of about 10 Hz. The filter is arranged to
pass signals below this frequency and to substantially reject signals
above this frequency. The top cutoff frequency of filter 62 is well below
the 300 Hz switching frequency and hence far below the 19.2 KHz carrier
frequency. The output of low-pass filter 62 is connected to an analog to
digital converter 70, and also to the input of a high pass filter 66. High
pass filter 66 is arranged to attenuate frequencies below about 0.5 Hz and
to pass frequencies above about 0.5 Hz. The 0.5 Hz value represents the
"corner" frequency of the high pass filter, i.e., the frequency at which
the filter provides 3 db attenuation. Frequencies below 0.5 Hz are
attenuated to a greater degree. The output of high pass filter 66 is
connected to the input of an amplifier 63 referred to herein as a "channel
amplifier." The output of channel amplifier 63 is connected to a low pass
filter 65 having a corner or 3 db attenuation frequency of 10 Hz. Low pass
filter 65 in turn is connected to sample and hold device 64, and device 64
is connected to a further analog to digital converter 68. Converters 68
and 70 are connected to a microprocessor 72.
The output of sampling unit 58 is connected to a second signal processing
channel 74. Processing channel 74 is substantially identical to processing
channel 60. Thus, channel 74 incorporates a low pass filter 76, high-pass
filter 80, channel amplifier 77, low pass filter 81, sample and hold
device 78, and analog to digital converters 82 and 84. Each of these
components is substantially identical to the corresponding component of
first channel 60.
Microprocessor 72 is arranged to compute the oxygen saturation of the blood
in finger tip 16 based upon the data input to the microprocessor via the
information supplied through the signal channels 60 and 74 as further
described hereinbelow. Microprocessor 72 is connected to a display device
79 such as a numeric readout, printer or the like for displaying the
computed oxygen saturation. Microprocessor 72 is also arranged to control
sample and hold devices 64 and 78. The microprocessor is also arranged to
adjust the power output level of each LED drives 23 and 25, to vary the
gain of intermediate amplifier 54 and also to vary the gain of each
channel amplifier 63 and 77 as discussed below.
In a method according to one embodiment of the present invention, the
timing unit 28, carrier generator 24, switch 26, drives 23 and 25 and
diodes 18 and 20 are actuated as described above to produce the
aforementioned alternating bursts of red and infrared light 86 and 88,
varying within each burst at the 19.2 KHz carrier frequency as illustrated
by curves 30 and 32 (FIGS. 2A and 2B). The light from diodes 18 and 20
passing through fingertip 16 is attenuated by the tissues in fingertip 16,
including the blood present in the blood vessels within the fingertip. As
the amount of blood within the fingertip and hence the length of the light
path through the blood vessels vary with the patient's pulse cycle, the
attenuation of light by the fingertip will also vary in accordance with
the pulse cycle. However, this variation occurs at the frequency of the
pulse which is below about 5 Hz (300 pulses/min.), and hence far below the
300 Hz switching frequency. Accordingly, the attenuation of the red and
the infrared light passing through the fingertip may be regarded as
substantially constant during each cycle of the switching waveform or
during each burst of red or infrared light.
The intensity I.sub.Incident (FIG. 2C) of the light impinging on
photodetector 22, as shown by curve 90 (FIG. 2C) will include an ambient
component I.sub.Ambient and a transmitted component I.sub.Trans. The
transmitted component I.sub.Trans represents red or infrared light
transmitted from the diodes 18 or 20 to photodetector 22. The magnitude of
I.sub.Trans will, of course, depend upon the attenuation caused by the
fingertip. Also, I.sub.Trans will vary in accordance with the output
I.sub.Red or I.sub.IR of whichever LED is operating. Therefore,
I.sub.Trans and I.sub.Incident will vary in accordance with the 19.2 KHz
carrier frequency waveform.
In the condition illustrated by FIG. 2C, the attenuation of red light by
the fingertip is relatively mild, whereas attenuation of infrared light is
more severe. Therefore, during each red burst 86 (FIG. 2A) the magnitude
I.sub.Trans of the transmitted light, and hence the magnitude of the
carrier frequency variation in the incident light will be relatively
large. During each infrared burst 88 (FIG. 2B) the magnitude I.sub.Trans
of the transmitted light and the magnitude of the carrier frequency
variation in the incident light will be relatively small. For practical
purposes, the response time of photodiode 22 can be taken as zero or
instantaneous. The conductance G.sub.diode of diode 22 thus varies
according to substantially the same curve 90 as does the intensity of the
incident light. The amplitude of the variations in the conductance
G.sub.diode of photodiode 22 at the carrier frequency therefore represents
the transmissivity of the fingertip 16 for the particular wavelength being
employed. During each red burst 86, the amplitude of these variations in
curve 90 represents the transmissivity of the fingertip in the red region,
whereas during each infrared burst 88 this amplitude represents the
transmissivity in the infrared region. Stated another way, the output from
diode 22 appearing at output node 38 has a component at the carrier
frequency, and the amplitude of that component indicates the
transmissivity at the red or infrared frequency.
The values of inductor 48, capacitor 50 and damping resistor 52 are
selected so that the resonant frequency of the circuit incorporating these
elements is substantially equal to the carrier frequency, i.e., about 19.2
kHz. Therefore, the resonant circuit tends to reinforce the component of
the photodetector signal at the carrier frequency and to attenuate any
component of the photodetector signal at frequencies other than the
carrier frequency. DC or 100-120 Hz components caused by ambient lighting
and other low frequency components in the photodetector signal will be
directed away from the input node 44 of amplifier 46 to groud via inductor
48. Components at frequencies above the carrier frequency, such as the
high frequency components of electromagnetic interference and the like
will be shunted away from the amplifier to ground via capacitor 50.
Resonant circuit 47 does not respond instantaneously to changes in the
photodetector ouput signal or changes in the conductance of the diode.
Rather, after the start of each burst the resonant circuit comes to
equilibrium gradually. As indicated by curve 94, the voltage at the input
node 44, and hence the output voltage of front end amplifier 46 and the
output voltage of intermediate amplifier 54 as well, all vary according to
a sinusoidal waveform at the carrier frequency. Although the same curve 94
is used in FIG. 2D as indicating all of these voltages, the output
voltages of the amplifiers have the same waveform but greater amplitudes
than the voltage at input node 44. The amplitudes of all of these voltages
show a characteristic rising and falling pattern. Towards the end of each
burst 86 or 88, the amplitudes of the carrier frequency variations in
these voltages are substantially constant, whereas at the beginning of
each burst these amplitudes are in transition. During the early portion of
burst 88, the amplitudes of amplifier input and output voltages are
gradually decaying to the steady state values which prevail at the end of
the same burst. During the next succeeding red burst 96, the amplifier
input and output voltages gradually increase once again to new steady
state value. The value of damping resistor 52 is selected so that the
amplifier input and output voltages stabilize at their steady state values
during the first half of each burst, i.e., after about sixteen cycles of
the carrier frequency waveform or about 1/1200 sec. The amplitudes of the
amplifier signals during the first half of each burst are indeterminate
values, whereas steady state values during the last half of each burst are
directly related to the amplitude of the transmitted light waveform 90 for
the burst.
The output signal from amplifier 54 passes to the sampling units 56 and 58.
Sampling unit 56 is associated with the red light wavelength. That is,
timing unit 28 controls sampling unit 56 so that this unit samples the
amplitude of the amplified signal V.sub.54 during a predetermined interval
98 (FIG. 2E) within each red burst 86 (FIG. 2A). Each sampling interval 98
is delayed by a predetermined delay time 100 following the start of the
associated red burst 86, so that each sampling interval 98 corresponds to
the second half of the burst. Each sampling interval 98 thus corresponds
to the last sixteen oscillations of the carrier frequency waveform within
each red burst. Delay time 100 thus is sufficient for resonant circuit 47
to establish substantially steady state oscillations. During each sampling
interval 98, unit 56 is actuated by timing unit 28 to sample the amplified
signal V.sub.54 in synchronism with the carrier frequency waveform, at
times corresponding to the peaks of the sinusoidal amplified voltage
V.sub.54. Thus a plurality of individual voltage samples 99 are obtained
during each sampling interval 98. Sampling unit 58 is likewise actuated by
timing unit 28 to sample the amplitude of amplified signal V.sub.54 during
intervals 102 corresponding to the last half of each infrared burst 88.
Each such interval 102 is delayed after the start of the associated
infrared burst by an appropriate delay time 104, also corresponding to
one-half the duration of the burst. Sampling unit 58 likewise obtains a
plurality of individual samples 101 during each sampling interval 102.
The output of sampling unit 56 will be a series of voltage spikes
corresponding to the transmissivity of the fingertip for red light,
whereas the output from sampling unit 58 will be a series of voltage
spikes corresponding to the transmissivity of the fingertip for infrared
light. Low-pass filter 62 smooths the outputs from sampling unit 56 to
provide a substantially continuously varying signal 106 (FIG. 3A)
representing the transmissivity of the fingertip in the red region. Signal
106 is depicted in FIG. 3A on a greatly compressed time scale relative to
the time scale employed in FIGS. 2A-2F. The oscillations in signal 106
correspond to the patient's pulse cylce, i.e., below 5 Hz and typically
about 1-2 Hz. Thus, the samples 99 taken by red wavelength sampling unit
56 during each sampling interval 98 correspond to a single point on curve
106. Low-pass filter 76 smoothes the discrete sample outputs from sampling
unit 58 into a substantially similar continuous signal 107 (FIG. 3B)
representing the transmissivity of the fingertip in the infrared region.
The continuous signal 106 from filter 62 is fed to analog to digital
converter 70. A digital representation of this signal is passed to
microprocessor 72. As shown in FIG. 3A, the AC component of signal 106 is
small compared to the DC component of this signal. Therefore, each signal
from converter 70 approximates the true DC value. Further, microprocessor
72 performs a digital low pass filtering or averaging on the
representations captured over a predetermined time so as to derive a more
accurate value of the DC component of transmissivity at the red
wavelength.
The signal from low pass filter 62 is also delivered to high pass filter
66. The high pass filter blocks the DC component, and delivers only the AC
component to channel amplifier 63. After amplification, the AC component
is treated by low pass filter 65 to remove residual noise and/or carrier
frequency components, and then sampled by sample and hold unit 64.
Microprocessor 72 actuates unit 64 to sample the treated AC component at a
sampling frequency of about 25-30 Hz. This sampling frequency is
substantially faster than the maximum pulse frequency of about 5 Hz, but
substantially slower then the switching frequency of 300 Hz. The sampled
values are converted to digital representations by converter 68 and
supplied to the microprocessor. From this succession of sampled values,
the microprocessor computes the AC component of signal 106 and hence the
AC component of the red-wavelength transmissivity.
In exactly the same way, the infrared signal channel 74 and microprocessor
72 cooperate to determine the DC and AC components of the infrared
transmissivity. All of these values are substantially free of "dark
current" or spurious DC components caused by ambient light falling on
detector 22, and hence no DC restoration or dark current subtraction
circuitry is required.
The microprocessor continually adjusts the power level of LED drives 23 and
25 and the gains of amplifiers 54, 63 and 77 to keep the signal supplied
to each analog to digital converter within the operating range of the
converter. For example, if the values of the DC component received through
converter 70 approach the upper bound of the converter's range, the
microprocessor will reduce the power level applied by red LED drive 23. If
the value of the red AC component drops below the operating range of
converter 68, the microprocessor will increase the gain of channel
amplifier 63, and so on. The microprocessor keeps track of these
adjustments, and applies appropriate multipliers to the values received
from the analog to digital converters so as to compensate for these
adjustments. For example, when the gain of amplifier 63 is increased, the
microprocessor applies a correspondingly smaller multiplier to the values
received from converter 68. In this way, the microprocessor can determine
the true values of the AC and DC transmissivity components.
Microprocessor 72 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 transmisivity 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 72 is displayed on
display unit 79. 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 pulsatile
signals provided by channel amplifiers 63 and 77. For example, the
microprocessor can be programmed to detect peaks in the AC component of
the red transmissivity signal supplied by amplifier 63 by monitoring the
sequence of digital representations delivered through analog to digital
converter 68. The pulse rate can be determined from the times between
successive peaks, whereas the presence or absence of a pulse can be
deduced from the occurrence or non-occurrence of peaks having at least a
predetermined magnitude. As used in this disclosure, the term "parameter
of the blood" includes information regarding the pulse as well as
information regarding the level of a | | |
