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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to the field of biomedical instrumentation and, in
particular, to reflectance-mode blood circulation monitors.
Measurements of skin pallor are particularly useful in physiological
diagnoses and the monitoring of patients. Changes in skin pallor are
recognized as cardinal signs of motion sickness, nausea, and shock, and
presumably are due to changes in the volume of blood in the
microcirculation of the skin. Assessing these changes in peripheral
vascular activity using a light source and a photodetector is known as
reflectance-mode photoplethsmography.
Attempts have been made to measure blood circulation through skin,
employing tungsten lamps and photoconductive cells but several problems
have prevented these devices from producing a signal proportional to the
total blood volume. Temperature sensitive photocell detectors produce a
hysteresis related to prior light exposure. Additionally, relative motion
between the transducer and the skin generates artifactual outputs as a
result of distance variations between the transducer and skin or
compression of the dermal blood vessels. As a result, prior art devices
have measured only blood volume pulse amplitude (BVP) rather than absolute
reflectance (pallor). Moreover, when visible light is utilized, the BVP
devices typically cannot be used on dark skinned individuals and
artifactual light components (arising from the ambient environment)
seriously degrade the signal.
The deficiencies of the tungsten lamp/photocell designs have led designers
to suggest the use of solid-state infrared LED/silicon phototransistor and
photodiode devices as photoplethysmographic instruments. Unfortunately,
such devices also are sensitive to the infrared components of room and
natural lighting. Moreover, the LED light source output can vary with
temperature, and motion artifacts can still be present.
Devices which can accurately measure skin pallor will find applications in
a variety of cardiovascular and neurological diagnoses. In particular,
devices which can provide monitoring of either acute or ambulatory
illnesses by detecting changes in skin pallor can satisfy a long-felt
need. Additionally, such devices can be useful in detecting and analyzing
particular circulatory ailments, such as Raynaud's syndrome which
apparently is caused by occupational exposure to vibration and results in
reduced vascular circulation in the extremities of those afflicted.
Therefore, there exists a need for devices to monitor skin pallor and
preferably, skin temperature, BVP and heart rate as well. Such devices
should provide ambient light cancellation and compensate for temperature
variations in the LED light source. Such devices should also minimize
motion artifacts.
SUMMARY OF THE INVENTION
A new type of infrared reflectance device is disclosed to provide a
quantitative evaluation of skin pallor. The instrument is designed to
produce a measurement proportional to the absolute infrared reflectance of
the skin. In one embodiment, it consists of a miniature GaAs light
emitting diode, which provides a pulsed infrared light source, a silicon
photodiode detector, and processing circuitry. The device electronics are
temperature compensated and are designed to respond only to the pulsed
component of the detector output and to reject components due to stray
light from external sources. The wavelength is chosen such that the
measurement is relatively insensitive to the level of blood oxygenation
and melanin pigment in the skin.
Additional features of the device include: circuits to extract average
blood volume pulse amplitude (BVP) and heart rate information,
radiometrically calibrated measurements in physical units, and an
adjustable source pulse amplitude to standardize the amount of light
reflected back to the detector so that the device works equally well on
both dark and light skinned individuals.
Motion sickness test data obtained from individual human subjects employing
the invention while making head movements in a rotating chair or while
wearing left-right vision reversing goggles revealed a consistent pattern
on onset and remission of facial pallor. Changes in pallor measured by the
invention were correlated with changes in skin temperature measured close
by on the face. Prototype devices according to the present invention have
also been flown on U.S. space shuttle missions.
The present invention will find practical use in hospital applications,
particularly during surgical operations and in recovery and intensive care
units to indicate the onset of shock, nausea, syncope and related
illnesses. Additionally, the invention may be useful as a diagnostic tool
in evaluating cases of Raynaud's syndrome and other circulatory disorders.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of the pulsed infrared reflectance
system for monitoring pallor;
FIG. 2 is a detailed side view of the probe components of the invention as
attached to the skin;
FIG. 3a is a detailed side view of the electro-optical transducer, the
temperature compensating thermistor, and the associated wiring
connections;
FIG. 3b is a detailed side view of the skin temperature thermistor and its
attachment block;
FIG. 3c is a detailed side view of the probe cable connector assembly;
FIG. 4 is a schematic diagram of the timing circuitry of the system shown
in FIG. 1;
FIG. 5 is a schematic diagram of the current regulator and pulsed LED of
FIG. 1;
FIG. 6a is a schematic diagram of the LED on sampling circuitry;
FIG. 6b is a schematic diagram of the LED off sampling circuitry;
FIG. 6c is a timing diagram for the various components of FIGS. 6a and 6b;
FIG. 7 is a schematic diagram of the synchronous demodulation and ambient
light cancellation circuitry;
FIG. 8 is a schematic diagram of the temperature compensating thermistor
circuitry;
FIG. 9 is a schematic diagram of the power supply circuitry;
FIG. 10 is a schematic diagram of the circuitry to produce the BVP signal
from the pallor signal;
FIG. 11 is a schematic diagram of the thermistor and associated components
to provide for the skin temperature signal;
FIG. 12 is a schematic diagram of the circuit to detect a low battery
voltage.
FIG. 13 is a schematic diagram of the meter driver and associated switch
for the various outputs available.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1, one embodiment of the invention consists of an
infrared light emitting diode (LED) 10 to provide a source of infrared
light which is reflected from within the tissues of a patient's skin 11
and onto a photodiode 12. The signal is pulsed to identify the signal
apart from ambient light. A temperature compensating thermistor 14 is
employed in the transducer housing 16 to correct for the effect of
temperature changes on the emitted power of the LED. A current-to-voltage
converter and synchronous demodulator circuit 18 amplifies the photodiode
signal utilizing an offset 20 to keep the pallor signal from saturating
subsequent stages of circuitry. A low pass filter 22 can be utilized to
reduce noise in the output pallor signal. This signal can also be used to
produce the average blood volume pulse amplitude (BVP) 30 signal by
utilizing a band pass filter 26 (i.e., between 0.5 and 2.5 Hz) and a
rectifier-averager 28.
FIG. 2 is a side view of a probe assembly which consists of the pallor
monitor transducer 32 and skin temperature sensor 34 joined together by
enameled wires 40. This assembly is linked by cable 42 to a connector 44,
by which the transducer and temperature signals are communicated to the
synchronous circuitry (discussed below). Motion artifacts due to pulling
and twisting of the cable are further minimized with an anchor 38 which
can be mounted, for example, below the patient's chin using double stick
tape 13. The transducer 32 and skin temperature sensor 34 can also be
joined to the skin 11 with double stick micropore tape 13 to further
minimize motion artifacts. Approximately one square centimeter of
double-stick tape is sufficient to secure each component with the
transducer preferably mounted at an edge rather than the center of the
tape to minimize blushing artifacts. The transducer 32 can be directly
applied to tape and will function satisfactorily because the tape is
transparent in the infrared region.
FIG. 3a is a detailed drawing of the pallor monitor transducer 32 and its
associated electrical components. In the illustrated embodiment, the
transducer 32 is a reflective electro-optical solid state switch, such as
the Spectronics SPX5004-1 transducer which emits and collects the pulsed
infrared light to produce the skin pallor and peak BVP signals. The
transducer 32 consists of a miniature GaAs light emitting diode (LED)
infrared light source 72 and a silicon photodiode detector 73 with red
filter in a plastic housing. A thermistor 46 is also employed to
compensate for LED temperature changes. A heater element 47 can be
employed to maintain nearly constant the temperature of the LED 72. The
pulse signals, reflected signals, and temperature signals for the
transducer 32 are carried to the device electronics by wires 40 (i.e., #36
AWG enameled wires). Each enameled wire 40 can be installed with a strain
relief loop 33 within the coating 48 to resist breakage. The wires should
also be flexible so as to minimize the mechanical compression of the
dermal microvessels and thereby to reduce "artifactual" pallor due to skin
motion. A silicon rubber coating 48, such as Dow Corning RTV3140, can be
used to enclose the connections.
FIG. 3b is a detailed drawing of a skin temperature sensor 34 which can be
used in conjunction with the pallor monitor. Again, a thermistor 50 is
employed to provide an appropriate skin temperature signal. The thermistor
50 is mounted in sensing port 52 within a plexiglass housing 51.
Preferably, the thermistor 50 should contact the skin directly and in the
illustrated embodiment using double stick tape 13, the thermistor extends
beyond the edge of the tape 13. The housing 51 shields the thermistor 50
from ambient air and radiation. A miniature coaxial wire 54 is secured to
the plexiglass housing 51, for example by epoxy 56 and, again, a silicon
rubber coating 48 can enclose sensor 34 and the end of the miniature
coaxial wire 54. In the illustrated embodiment, the photodiode leads are
connected to the coaxial cable 54 while the skin thermistor wires, the LED
temperature-compensating thermistor wires and the LED pulse signal wires
can be wrapped about the outside of the coaxial cable 54.
FIG. 3c is a detailed drawing of a side view of the probe cable connector
assembly to the main housing. The enameled wires 40 are wrapped around the
miniature coaxial cable 54 and protected by the RTV coating 48. At the
connector interface, the enameled wires 40 are joined to the 6
teflon-coated wires 60 and surrounded by opaque heat shrink 58. (One wire
from each thermistor is joined together to a common line in connector 62).
The teflon-coated wires 60 are an integral part of the ITT Cannon
connector 62 which interfaces to the electronic circuitry of the
invention.
In FIG. 4 an illustrative timing circuit is shown comprising a standard
4060 National ("U1") integrated circuit chip 64 and a 16 kHz oscillator
crystal 66 which produces a 250 Hz rectangular pulse at pin 4 of the
integrated circuit U1, designated as U1-4, with the appropriate selection
of resistors R1 and R2 and capacitors C1 and C2. The 250 Hz rectangular
waveform from U1-4 has a 4 ms period pulse with a 50% duty cycle and a
peak voltage of 5.8 V. The source of this +5.8 V voltage is supplied to
U1-16 and a 2 Hz output at U1-2 is used to pulse the battery level
detection circuitry. (Both the power supply and the battery level
detection circuitry are discussed in detail below).
The ouput from U1-4 is received by a 4528 National ("U2A") integrated
circuit chip 68 which is a one-shot monostable multi-vibrator. The 4528 IC
chip 68 converts the rectangular waveform at U2A-4 from a 4 ms period, 2
ms pulse (50% duty cycle) to a 4 ms period, 0.2 ms pulse (5% duty cycle)
at the output U2A-6. The purpose of such a reduction in the duty cycle is
to reduce the power consumption when restrictions of size and power levels
exist. Resistor R3 and capacitor C3 are chosen to convert the 50% duty
cycle at U2A-4 to the 5% duty cycle driving pulse at U2A-6. It is
important to note that the 0.2 ms pulse is not critical and the selection
of the pulse duty cycle may be varied with the appropriate selection of
Resistor R3 and Capacitor C3 as long as the frequency of the driving pulse
is not too close to a harmonic of 60 Hz.
In FIG. 5 a schematic diagram of a current regulator and its relationship
to the LED 72 of transducer 32 is shown. The regulator comprises a Harris
HA2-2705-5 ("U3") integrated circuit chip 70 and appropriate capacitors
and resistors. Because the LED 72 typically has a temperature dependent
resistance of approximately -8 ohm/10.degree. C., a voltage source, if
used, would therefore produce a higher power output as the LED 72
increased in temperature over time. However, the current regulator
prevents small changes in the LED resistance from affecting the power
output from the LED 72. The resistors R4 and R5 reduce the 0.2 ms pulse
amplitude of the -5.8 V at the output U2A-6 in FIG. 4 to a +1.8 V 0.2 ms
pulse. The U3 chip 70 converts this 0.2 ms waveform into a 25 mA peak 0.2
ms waveform at the LED with the appropriate selection of Resistor R6.
Potentiometer P1 which sets peak LED current is utilized to adjust for
variation in skin pigment content thereby achieving a standardized output
and detection level regardless of melanin and baseline hemoglobin content
in the skin.
In FIG. 6a and 6b, schematic drawings of the switching and sampling
circuitry for the analog switches are shown. FIGS. 6a and 6b represent the
delay and sample stages for the LED on LED off pulses, respectively. Power
is supplied by +V (+5.8) at pin 16 on both one-shot monostable
multivibrators, U4 and U5. The delay circuits are necessary for sampling
the photodiode pulse at U6-6 during the middle of the LED on interval and
LED off interval. The arrow on each of the integrated circuits U4A, U4B,
U5A, U5B indicates on which polarity the one-shot is activated, either on
the leading edge of the waveform or on the trailing edge of the waveform.
Resistors R7-10 and capacitors C5-8 are used to provide the necessary time
delays and sampling periods to the analog switches. The time delays and
sampling periods to the analog switches are shown in FIG. 6c.
FIG. 7 describes the synchronous demodulator and ambient light cancellation
circuitry. The output at integrated circuit U6-6 is the amplified 5% duty
cycle reflectance signal from the photodiode in FIG. 5., and filtered by
resistor R14 and capacitor C9. FIG. 6c also shows the sampling and delay
periods to the analog switches U7A and U7B. The photodiode detector 73,
receives the LED radiation reflected off the skin. In the illustrated
embodiment a photodiode is utilized rather than a phototransistor because
the photo-transistor, despite its high gain, has a non-linear light to
output current relationship and this can produce an undesirable error in
an environment having fluctuating ambient light levels. The photodiode, on
the other hand, has a relatively linear light to output current
relationship.
In reflection-mode, a large signal attenuation is typically observed. At a
peak LED current of 25 mA, about 1.3 mW peak infrared power is radiated
into the skin while only a few microwatts peak typically is received by
the photodiode, which produces a peak signal current of less than one
microampere. In the illustrated embodiment of FIG. 7, the signal is first
amplified by current-to-voltage converter U6 using a low noise LM308
operations amplifier. Values for first Resistor R14 and capacitor C9 can
be chosen to attenuate the high frequency noise in the signal without
allowing integration of the signal pulse to take place. A potentiometer P2
can be employed to offset the signal so as to prevent the signal from
approaching the saturation voltage. In the illustrated circuit the
resulting output from U6-6 is a 4 ms pulse with a peak voltage of
approximately -40 mV which is input to the analog switches U7A-4 and
U7B-13.
The signal during the LED on interval is stored on capacitor C10 and the
signal during the LED off interval is stored on capacitor C11. A
differential amplifier U8A-C amplifies only the difference in voltage
between capacitors C10 and C11. An instrumentation differential amplifier
preferably is employed because of the high impedence characteristics which
prevents any significant discharge of the capacitors storing the waveform
pulses. Since ambient light level changes will produce only a level shift
in the pulse signal, such changes are effectively cancelled by the
synchronous detection and ambient light cancellation circuit in FIG. 7.
FIG. 8 is a schematic diagram of the temperature compensation circuitry for
the pallor signal. The pallor signal (the waveform from the synchronous
detection and ambient light cancellation circuit) is compensated with the
signal from the thermistor 46. The value of R70 should be selected to
cancel LED temperature effects as detected by the thermistor 46. The
temperature-compensated signal is then amplified and low pass filtered by
integrated circuit U17. The pallor signal can then be sent to a readout
device to display the degree of pallor (or blushing). This signal is also
used to produce the average BVP signal.
FIG. 9 shows the circuit diagram of the voltage regulators for the positive
and negative supply voltages. The operation of the positive regulator is
described first. U15A and U15B comprise a low current LM10CH integrated
circuit which contains an internal 0.2 volt reference available at pin 4
as symbolized by the battery 0.2 in FIG. 9. U15A is wired as a voltage
follower to buffer the +0.2 V reference for input to U15B. U15B is wired
as a non-inverting amplifier to boost the +02 V to +5.8 V nominal, by the
action of gain setting resistors R25 and R26. C15 acts to stabilize the
circuit. The +5.8 V output from U15B is buffered by op amp U10D and
transistor Q1, since neither U15B nor U10D alone supply the nominal 25 mA
peak current pulses drawn by the monitor circuitry. C16 and C31 filter out
noise on the +5.8 V supply caused by the LED current pulses.
The negative supply voltage regulator also employs an LM10CH integrated
circuit, but in a slightly different configuration. U16A buffers the +0.2
V reference and outputs it across R28, which acts as a current source for
transistor Q2. Q2 drives this current through R27 to produce the negative
supply voltage, -5.8 V nominal. U16B, an op amp voltage follower buffers
the output of Q2. C17 and C18 filter the negative supply voltage and
prevent the circuit from self oscillating.
FIG. 10 is a schematic diagram of a circuit for obtaining the average blood
volume pulse amplitude, BVP, from the pallor signal, through filtering and
rectifying of the pallor signal. In the illustrated embodiment the pallor
signal is high pass filtered at approximately 0.5 Hz by the first
operational amplifier, U9A, by selecting the appropriate resistors R52 and
R53 and the appropriate capacitors C22 and C23. The signal is then low
pass filtered at 2.5 Hz through the second operational amplifier, U9B,
again with the appropriate selection of capacitors C24 and C25 and
resistors R54 and R55. The filtered BVP signal is thus band pass filtered
to attenuate various ambient noise such as low frequency drift in the
pallor signal and 60 Hz harmonic signals from lighting. The potentiometer,
P6, is utilized to zero the signal from the filtered BVP so as to prepare
the signal to be rectified by the fourth operational amplifier, U10A, and
averaged with a 4 second time constant with the fifth operational
amplifier to produce the average Blood Volume Pulse signal.
FIG. 11 is a circuit diagram for a skin temperature thermometer. U14A and
U14B comprise an LM10CLH integrated circuit. U14A buffers the +0.2 V
reference which is amplified by U14B which is wired as a non-inverting
amplifier. Gain setting resistors R29 and P4 boost the +0.2 V to precisely
+1.60 V, which is the supply voltage for a thermistor bridge circuit. The
bridge consists of resistors R30, R31, R32, and P5. The thermistor, T1 is
part of the probe assembly, and sense facial skin temperature. The output
voltage from the bridge is nearly linear over the 28 to 38 degree
centrigrade range typically encountered on the skin and is input to an
instrumentation amplifier consisting of op amps U8D, U9D, and U13. The
bridge is balanced for zero volts output at 28 degrees centrigrade by
adjustment of trimpot, P5. The gain of the instrumentation amplifier is
set by resistors R33 through R39 to be 12.2, so that the output range is 0
V to +2 V for the 28 to 38 degree centigrade range. Capacitors C20, C21,
C19 and C28 filter out noise pulses picked up from the probe cable and
monitor circuitry wiring.
FIG. 12 shows the circuit which illuminates the panel battery low indicator
when either the +9 V or -9 V battery output drops close to the minimum
value necessary to operate the voltage regulator circuitry. The thresholds
are set by R41 and R42 for the +9 V battery, and R44 and R43 for the -9 V
battery. Op amps U10B and U10C are wired as comparators. When the battery
voltages drop such that either the--input to U10B is less positive than
+1.6 V, or the + input to U10C is less negative than -5.8 V (nominal) then
the output of the respective op amp switches from about -5.8 V to about
+5.8 V, thus activating op amp U11. Diodes D3, D4, and D5 gate the
positive voltage from a triggered U10B or U10C to U11, and block the
negative voltage from inactivated U10B or U10C. When U11 is activated by
applying a positive voltage to its positive supply voltage input terminal,
the 2 Hz pulses from oscillator, U1, are gated to the panel LED battery
low indicator. Resistors R49, R50, and R51 effectively limit the LED
current.
FIG. 13 shows the meter driver circuit. The op amp, U18, acts as a current
source for the panel meter. In the illustrated embodiment the meter should
not be driven directly by the monitor circuitry since the maximum output
signal voltage, which is +2 V, will not set the meter to full scale, due
to its high coil resistance. Driving the meter from a current source also
has the advantage that temperature related coil resistance changes will
not affect the meter reading. Rotary switch, SW2, selects the desired
signal (pallor, BVP, or skin temperature) to be displayed on the panel
meter. An off position was supplied to eliminte the current drawn by the
meter from the battery drain when the meter is not being read. Resistor
R24 was selected to set the meter to a full scale reading for a +2 V input
from the pallor, BVP, or skin temperature circuits.
Representative values for the components discussed herein are provided in
Table I below:
TABLE I
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Capacitors, by component designation
Designation
Value Mfg. Part No. Mfg.
______________________________________
C1 33 pF CK05BX330K Centralab
C2 10 pF CK05BX100K "
C3 0.01 uF CK05BX103K "
C4 100 pF CK05BX101K "
C5 0.001 uF CK05BX102K "
C6 0.001 uF CK05BX102K "
C7 0.01 uF CK05BX103K "
C8 0.001 uF CK05BX102K "
C9 220 pF CK05BX221K "
C10 0.1 uF CK05BX104K "
C11 0.1 uF CK05BX104K "
C12 0.22 uF CK06BX224K "
C13 0.22 uF CK06BX224K "
C14 1.0 uF CK06BX105K "
C15 0.01 uF CK05BX103K "
C16 10 uF 150D106X9010B2
Sprague
C17 2 .times. 1.0 uF
CK06BX105K .times. 2
Centralab
C18 47 uF 150D476X9010R2
Sprague
C19 0.22 uF CK06BX224K Centralab
C20 1.0 uF CK06BX105K "
C21 1.0 uF CK06BX105K "
C22 1.0 uF CK06BX105K "
C23 1.0 uF CK06BX105K "
C24 0.1 uF CK05BX104K "
C25 0.047 uF CK05BX473K "
C26 2 .times. 1.0 uF
CK06BX105K .times. 2
"
C27 0.22 uF CK06BX224K "
C28 0.22 uF CK06BX224K "
C29 33 pF CK05BX330K "
C30 0.1 uF CK05BX104K "
C31 1.0 uF CK05BX105K "
C32 1.0 uF CK05BX105K "
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Resistors, By Component Designation:
Designation Value
______________________________________
R1 470K
R2 22 M
R3 56K
R4 100K
R5 47K
R6 27
R7 120K
R8 150K
R9 750K
R10 470K
R11 47K
R12 47K
R13 20K
R14 100K
R15 1K
R16 1K
R17 1K
R18 100K
R19 100K
R20 100K
R21 100K
R22 1K
R23 100K
R24 3.9K
R25 1.96K*
R26 56.2K*
R27 56.2K*
R28 1.96K*
R29 4.99K
R30 10.0K
R31 10.0K
R32 3.40K
R33 1.47K
R34 8.25K
R35 8.25K
R36 100K
R37 100K
R38 100K
R39 100K
R41 1 M
R42 287K
R43 1 M
R44 100K
R45 10 M
R46 1K
R47 1K
R48 10 M
R49 100K
R50 100K
R51 4.7K
R52 220K
R53 430K
R54 910K
R55 910K
R56 47K
R57 47K
R58 1K
R59 10K
R60 4.7K
R61 1.1K
R62 2.7K
R63 470K
R64 470K
R65 10K
R66 100K
R67 2 M
R68 91K
R70 34.8K*
R71 4.99K
R72 100K
R73 100K
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Potentiometers and Trimpots:
Designation Value Vendor
______________________________________
P1 100 ohm, 10 turn
Allied 521-3101
P2 10K trim Allied 822-2241
P3 50K trim Allied 822-2253
P4 2K trim Allied 822-2232
P5 5K trim Allied 822-2238
P6 10K trim Allied 822-2241
P7 50K trim Allied 822-2253
______________________________________
Designation Type Manufacturer
______________________________________
Integrated Circuits, by component designation:
U1 CD4060BMJ National (RCA,F)
U2 CD4528BMJ National (F)
U3 HA2-2705-5 Harris
U4 CD4528BMJ National
U5 CD4528BMJ National
U6 LM308H National
U7 HI1-0305-5 Harris
U8 OP-420HY PMI
U9 OP-420HY PMI
U10 OP-420HY PMI
U11 HA2-2705-5 Harris
U12 LM308 National
U13 LM308 National
U14 LM10CLH National
U15 LM10CH National
U16 LM10CH National
U17 LM308 National
U18 HA2-2705-5 Harris
U19 HA2-2705-5 Harris
Diodes and Transistors by Component Designation
Q1 2N1304 Etco
D1 1N4148 F
Q2 2N2222 Etco
D2 1N4148 F
D3 1N3592 ITT
D4 1N3592 ITT
D5 1N3592 ITT
D6 LED TLR-107 TI
S1 SPX-5004-1 Spectronics
Thermistors
46 UUA35J1 Fenwal
50 UUA35J1 Fenwal
Electronic Oscillator
66 GX1V Statek
Double Stick Tape
13 2181 3 M
______________________________________
*selected component value
The invention has been described in connection with certain preferred
embodiments which include simultaneous measurements of BVP and skin
temperature; however, it should be clear that various changes and
modifications can be made without departing from the spirit or scope of
the invention. For example, various electronic modifications can be made
to reduce the overall chip count. The demodulation scheme for subtracting
artifact signals can be simplified and active temperature compensation in
some circumstances can be avoided altogether by carefully selecting the
components. Additionally, various changes can be made in the temperature
compensation scheme such as incorporating a compensating resistance into
the probe assembly, itself, rather than the monitor electronic hardware.
Alternatively, a heater circuit, built into the transducer, can be used to
stabilize the temperature.
Although the described embodiment is battery-powered and designed to have
low power consumption for monitoring an ambulatory patient, similar
devices can also be designed to make use of public utility voltages in
hospital and home settings.
Moreover, the LED transmitters employed in the invention can be modified to
emit at particularly useful wavelengths. For example, a properly-doped LED
can be chosen to emit a wavelength of light corresponding to one of the
isobestic wavelengths of hemoglobin, thus rendering the instrument
insensitive to the blood oxygen content in the vascular system.
* * * * *
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