Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR MEASURING
PULSE TRANSIT TIME
S P E C I F I C A T I O N
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims. the benefit of U.S. Provisional
Application Nos. 60/097,618 filed August 24, 1998, and
60/126,339 filed March 26, 1999, both of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for
measuring pulse wave transmission, and more particularly
pulse transit time, of a human or mammalian subject.
The human (or mammalian) pulse is a traveling Wave
disturbance that emanates from the heart and travels
throughout the arterial system. Since the velocity of pulse
propagation in a liquid is directly proportional to the
pressure of the liquid, it is possible to detect blood
pressure by measuring the propagation velocity of the pulse
wave. The propagation velocity of the pulse wave can be
measured by detecting the pulse transit time, which is the
time period required for the pulse wave to travel between
two spaced arterial pulse points.
An example of a blood pressure monitoring system that
utilizes pulse transit time can be found in U.S. Patent No.
4,245,648 to Trimmer et al. This system includes a pair of
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piezoelectric sensors closely spaced (by about 3 cm.) along
the brachial artery to detect the traveling pulse wave.
Pulse transit time is determined as the difference between
arrival times of the pulse wave at the two sensors.
The use of piezoelectric sensors as described in the
aforementioned patent leads to several significant practical
limitations. For example, piezoelectric sensors commonly
exhibit limited sensitivity at frequencies below about 2 Hz.
The pulse rate of a human adult is ordinarily around 60
beats per minute, or 1 Hz. The pulse rate of a human infant
,is typically about 120 to 180 beats per minute, or 2 to 3
Hz. Thus, the practical requirements of a system using
piezoelectric sensors for monitoring human subjects may push
the limit of, or even exceed, the performance capabilities
of the sensors. Another practical limitation stems from the
fact that piezoelectric sensors require the presence of
electrically conductive material (e. g., electrodes and lead
wires) at the sensor location on the test subject. The
system consequently cannot be used in environments where the
presence of such materials would be problematical. For
example, electrically conductive materials have been known
to cause severe burning of patients undergoing MRI
examinations, due to the presence of strong radio frequency
fields generated by the MRI machine. Still another
limitation is imposed by the location of the sensors in
mutual proximity along the same artery: Locating the
sensors in mutual proximity means that the pulse transit
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time to be measured will be very short and inherently more
difficult to measure accurately. It will be appreciated
that a given amount of error becomes more significant as the
time period being measured becomes shorter.
SUMMARY OF THE INVENTION
In one of its aspects, the present invention provides a
method of measuring pulse transit time that is especially
useful (although not limited to use) with pulse sensors
located at substantially spaced pulse points. For example,
one of the sensors may be located over the brachial artery
near or on the upper arm, and the other sensor located over
the radial artery on the wrist. The method involves
differentiation of the respective pulse wave signals from
the sensors to determine corresponding points of the two
signals, such as the points of maximum slope. The time
delay between these points is then determined, thus yielding
the pulse transit time. Differentiating the two pulse wave
signals facilitates the identification of corresponding
points of the signals, even though the pulse waveforms may
differ somewhat when the sensors are substantially spaced
from one another as noted above. Further, it allows for the
selection of a consistent time marker (e.g., point of
maximum slope) upon which to base the pulse transit time
calculation from one pulse wave to the next. This is
particularly advantageous since the pulse waveform
ordinarily varies from one heartbeat to the next.
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In another of its aspects, the invention provides an
apparatus for implementing the foregoing method. The
apparatus includes a pair of pulse sensors and a signal
processing unit that processes the respective pulse wave
signals of the pulse sensors in accordance with the method.
In another of its aspects, the present invention
provides an apparatus for measuring pulse transit time
including at least one pulse sensor, and preferably two
pulse sensors, constituted by a variable coupler fiberoptic
sensor having an improved design to be described herein.
The apparatus further includes a signal processor and may be
used to implement the aforementioned method or to implement
other methods of measuring pulse transit time.
Other aspects of the invention will become apparent
from a reading of the following detailed description with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of an apparatus for measuring
pulse transit time in accordance with the invention.
Fig. 2 is a flow diagram for explaining the operation
of the system in Fig. 1.
Fig. 3 is a block diagram showing another apparatus of
the invention.
Fig. 4 is a top view of a variable coupler fiberoptic
sensor useful in the apparatus of Figs. 1 and 3.
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Fig. 5 is a sectional side view of the sensor of Fig.
4.
Fig. 6 shows explanatory views (Views 6a - 6d) of
normal and deflected states of the fusion region of a
conventional pre-tensioned linear coupler.
Fig. 7 shows corresponding explanatory views (Views
7a - 7d) for a U-shaped fusion region.
Fig. 8 shows a variable coupler fiberoptic sensor
useful in apparatus according to the invention.
Fig. 9 is a graph depicting the response of the sensor
of Fig. 8 to pulsations of the wrist.
Fig. 10 is another graph of the sensor response at the
wrist.
Fig. 11 is an exploded view of another variable coupler
fiberoptic sensor useful in apparatus according to the
invention.
Fig. 12 is an end view of the Fig. 9 sensor in
assembled form.
Fig. 13 illustrates another variable coupler fiberoptic
sensor useful in apparatus according to the invention, shown
in section as worn on the wrist.
Fig. 14 is a perspective view of a carotid artery
sensor useful in apparatus according to the invention.
Fig. 15 is a fragmentary side elevation of the Fig. 14
sensor.
Fig. 16 is a perspective view showing the Fig. 14
sensor and its fiberoptic leads with installed connectors.
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Figs. 17-21 are plots showing pulse waveforms and
corresponding pulse transit times obtained using an
apparatus as shown in Fig. 3 performing the method shown in
Fig. 2.
Fig. 22 is a diagram illustrating a practical
arrangement of an apparatus according to Fig. 1 or Fig. 3.
Fig. 23 illustrates the basic construction of a
conventional variable coupler fiberoptic sensor.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 is a block diagram of an apparatus for measuring
pulse transit time in accordance with the invention. The
apparatus includes two arterial pulse sensors S1,S2 which
may be of any suitable form. For example, the sensors may
be piezoelectric, fiberoptic, or of any known design capable
of converting skin displacements due to the pulse (pressure)
wave to a corresponding output signal representative of the
pulse waveform. However, at least one and preferably both
of the sensors will be in the form of a variable coupler
fiberoptic sensor constructed in accordance with the
improved design principles to be described later.
The pulse sensors S1,S2 are connected to a signal
processing unit SPU which processes the output signals from
the sensors to determine the pulse transit time. The signal
processing unit may be of either digital or analog design as
desired. Of course, if digital processing is used, the
sensor outputs may be supplied to the signal processing unit
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via analog-to-digital converters, or the processing unit may
be provided with such converters internally.
Referring additionally to Fig. 2, the operation of the
signal processing unit SPU in accordance with the invention
will now be explained. At first, in Step 1, the signal
processing unit inputs the pulse wave signals from sensors
S1,S2. Next, in Step 2, the signal processing unit
differentiates (takes the derivative of) each pulse wave
signal. The derivative, of course, indicates the
instantaneous slope of the pulse wave signal. Next, in Step
3, the signal processing unit uses the results of Step 2 to
select points having corresponding slope characteristics
from the two pulse wave signals. For example, the
processing unit may select the respective points of maximum
slope in the two pulse wave signals. Finally, in Step 4,
the signal processing unit calculates the time delay between
the two selected points. The calculated time delay
constitutes the pulse transit time.
Because corresponding points of the two pulse wave
signals can easily be identified from the differentiated
waveforms, the foregoing method readily accoaanodates
substantial separation of the sensors Sl, S2, even though
the pulse waveforms may be somewhat different at the two
sensor locations. Further, as noted earlier,
differentiation also allows for the selection of a
consistent time marker (e. g., point of maximum slope) upon
which to base the pulse transit time calculation from one
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pulse wave to the next. This is particularly advantageous
since the pulse waveform ordinarily varies from one
heartbeat to the next.
Fig. 3 illustrates another apparatus according to the
present invention. The apparatus includes a pair of
variable coupler fiberoptic sensors S1',S2' of an improved
design to be explained herein. But first, in order to fully
appreciate the advantages of the apparatus, some additional
background regarding variable coupler fiberoptic sensors
will be helpful.
Variable coupler fiberoptic sensors conventionally
employ so-called biconical fused tapered couplers
manufactured by a draw and fuse process in which a plurality
of optical fibers are stretched (drawn) and fused together
at high temperature. The plastic sheathing is first removed
from each of the fibers to expose the portions for forming
the fusion region. These portions are juxtaposed, usually
intertwisted one to several twists, and then stretched while
being maintained above their softening temperature in an
electric furnace or the like. As the exposed portions of
the fibers are stretched, they fuse together to form a
narrowed waist region-the fusion region-that is capable of
coupling light between the fibers. During the stretching
process, light is injected into an input end of one of the
fibers and monitored at the output ends of each of the
fibers to determine the coupling ratio. The coupling ratio
changes with the length of the waist region, and the fibers
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are stretched until the desired coupling ratio is achieved,
typically by a stretching amount at which the respective
fiber light outputs are equal. The coupler is drawn to such
an extent that, in the waist region, the core of each fiber
is effectively lost and the cladding may reach a diameter
near that of the former core. The cladding becomes a new
"core," and the evanescent 'field of the propagating light is
forced outside this new core, where it envelops both fibers
simultaneously and produces the energy exchange between the
fibers. A detailed description and analysis of the
biconical fused tapered coupler has been given by J. Bures
et al. in an article entitled "Analyse d'un coupleur
Bidirectional a Fibres Optiques Monomodes Fusionnes",
Applied Optics (Journal of the Optical Society of America),
Vol. 22, No. 12, June 15, 1983, pp. 1918-1922.
Biconical fused tapered couplers have the advantageous
property that the output ratio can be changed by bending the
fusion region. Because the output ratio changes in
accordance with the amount of bending, such couplers can be
used in virtually any sensing application involving motion
that can be coupled to the fusion region.
Because variable coupler fiberoptic sensors can be made
entirely from dielectric materials and optically coupled to
remote electronics, they are particularly advantageous for
applications in which the presence of electrically
conductive elements at the sensor location would pose the
risk of electrical shock, burns, fire, or explosion. In the
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medical field, for example, variable coupler fiberoptic
sensors have been proposed for monitoring patient heartbeat
during MRI examinations. See U.S. Patent 5,074,309 to
Gerdt, which discloses the use of such sensors for
monitoring cardiovascular sounds including both audible and
sub-audible sounds from the heart, pulse, and circulatory
system of a patient. Other~applications of variable coupler
fiberoptic sensors can be found in U.S. Patent 4,634,858 to
Gerdt et al. (disclosing application to accelerometers),
U.S. Patent 5,671,191 to Gerdt (disclosing application to
hydrophones), and elsewhere in the art.
Conventional variable coupler fiberoptic sensors have
relied upon designs in which the fiberoptic coupler is
pulled straight, secured under tension to a plastic support
member and, in the resulting pre-tensioned linear (straight)
form, encapsulated in an elastomeric material such as
silicone rubber. The encapsulant forms a sensing membrane
that can be deflected by external forces to cause bending of
the coupler in the fusion region. The bending of the fusion
region results in measurable changes in the output ratio of
the coupler. The displacement of the membrane can be made
sensitive to as little as one micron of movement with a
range of several millimeters.
Fig. 23 of the accompanying drawings illustrates the
basic principles of a sensing apparatus including a variable
coupler fiberoptic sensor 10 as described above. In the
form shown, the sensor 10 includes a 2 x 2 biconical fused
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tapered coupler 11 produced by drawing and fusing two
optical fibers to form the waist or fusion region 13.
Portions of the original fibers merging into one end of the
fusion region become input fibers 12 of the sensor, whereas
portions of the original fibers emerging from the opposite
end of the fusion region become output fibers 14 of the
sensor. Reference numbers 18 denote the optical fiber
cores. The fusion region 13 is encapsulated in an
elastomeric medium 15, which constitutes the sensing
membrane. The support member is not shown in Fig. 1.
In practice, one of the input fibers 12 is illuminated
by a source of optical energy 16, which may be an LED or a
semiconductor laser, for example. The optical energy is
divided by the coupler 11 and coupled to output fibers 14 in
a ratio that changes in accordance with the amount of
bending of the fusion region as a result of external force
exerted on the sensing membrane. The changes in the
division of optical energy between output fibers 14 may be
measured by two photodetectors 17 which provide electrical
inputs to a differential amplifier 19. Thus, the output
signal of differential amplifier 19 is representative of the
force exerted upon medium 15. It will be appreciated that
if only one of the input fibers 12 is used to introduce
light into the sensor, the other input fiber may be cut
short. Alternatively, it may be retained as a backup in the
event of a failure of the primary input fiber. It should be
noted that, for simplicity, the coupler 11 is shown without
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the aforementioned fiber twisting in the fusion region.
Such twisting is ordinarily preferred, however, to reduce
lead sensitivity, which refers to changing of the output
light division in response to movement of the input
fiber(s).
Despite their advantages, conventional variable coupler
fiberoptic sensors have been subject to certain limitations
inherent in the conventional pre-tensioned linear (straight)
coupler design. The conventional design imposes, among
other things, significant geometrical limitations. In
particular, the size of the sensor must be sufficient to
accommodate the fiberoptic leads at both ends of the sensor.
The fiberoptic lead arrangement also requires the presence
of a clear space around both ends of the sensor in use.
Especially in medical applications, such as when placing a
sensor on a patient's body for continuous monitoring, the
size and lead positions of the sensor are both important
issues. Another limitation results from the fact that any
displacement of the fusion region necessarily places it
under increased tension. At some point of displacement, the
tension in the fusion region will become excessive, causing
the fusion region to crack or break, with resulting failure
of the coupler.
Returning to the invention, the apparatus of Fig. 3
utilizes an improved variable coupler fiberoptic sensor
designed to overcome one or more disadvantages of the
conventional pre-tensioned linear sensor design. More
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particularly, the sensor used in the present apparatus may
have an improved design that permits deflection of the
coupler fusion region without accompanying tension. The
coupler fusion region is preferably arranged substantially
in a U-shape, but may more generally be configured as
disclosed in co-pending U.S. Application No. 09/316,143
filed May 21, 1999, which ig incorporated herein by
reference. With a substantially U-shaped configuration it
becomes possible to locate the fiberoptic leads of the
sensor adjacent to each other, rather than at opposite ends
of the sensor, thus avoiding the earlier discussed
geometrical limitations inherent in the conventional pre-
tensioned linear coupler design.
It will be appreciated that by using two such sensors,
the apparatus of Fig. 3 fully realizes the benefit of the
improved sensor design. It is permissible within the
broader scope of the invention, however, to use one such
sensor in combination with another pulse sensor that does
not utilize the improved design described above, such as a
conventional linear variable coupler fiberoptic sensor or
even a piezoelectric sensor.
As shown in Fig. 3, each of the sensors Sl',S2' is
coupled to a corresponding light source 40 (e. g., a laser)
and a corresponding photodetector/differential amplifier
circuit 42 as previously described. These circuits have
respective outputs connected to corresponding inputs of a
digital signal processor (DSP) 44, each through an analog-
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to-digital converter 43. The digital signal processor
processes the input signals to detect the pulse transit
time.
It is possible to combine the sensors S1', S2' by
arranging their respective fiberoptic components in mutual
proximity on a common support structure. But, as earlier
noted, locating the pulse sensors in mutual proximity leaves
little margin for error because the measured pulse transit
time will be short.
The digital signal processor 44 may be programmed to
determine the pulse transit time in any desired manner,
including but not limited to the manner explained in
connection with Fig. 2.
Figs. 4 and 5 of the accompanying drawings show a
specific example of an improved variable coupler fiberoptic
sensor 20 useful in the apparatus of the present invention.
The sensor is constructed for placement against a person's
body, such as on the chest, arm, or wrist, for sensing skin
displacements due to the pulse. The sensor is more
generally capable of sensing both audible and sub-audible
cardiovascular and breathing sounds that are manifested by
skin displacement.
The sensor 20 comprises a support member 22 having a
generally circular head portion 24, which is provided with a
central well or through hole 26, and a handle-like extension
28. A biconical fused tapered coupler 30 is mounted to the
support member with at least a portion (here, the entirety)
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of its fused coupling region 32 disposed in the space 26 and
arranged in a U-shape. Input fiber leads 34 and output
fiber leads 36 of the coupler are disposed beside one
another in a channel 29 formed in the extension 28. The
leads are manipulated so as to bend the coupling region 32
through 180° into the desired shape and then secured within
the channel by a suitable adhesive, such as an epoxy-based
glue. The coupling region, which is not under tension, may
be potted by filling the space 26 with elastomer to form a
sensing membrane 38 (not shown a.n Fig. 4) in the known
manner-for example, by filling with a silicone rubber such
as GE RTV 12. Alternatively, as will be seen hereinafter,
the coupling region may be coated with a layer of coating
material such as GE SS 4004 (polydimethylsiloxane with
methyl silsesquioxanes) to eliminate the need for potting.
This material is normally used as a primer for bonding room
temperature vulcanizing (RTV) materials to surfaces that
would otherwise form weak bonds. The advantage of
eliminating the potting is that the sensitivity is
increased, because the potting tends to reduce sensitivity
no matter how thinly it is applied. Support member 22 is
suitably formed of a moldable plastic, such as Plexiglass~,
polyvinyl chloride (PVC), or other suitable materials known
in the art.
As shown in Fig. 5, the upper portion of the membrane
38 has a convex surface 39 that protrudes from the plane of
the support structure for contacting a person's body. The
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convex configuration of the contact surface makes the sensor
more of a point probe to better localize the cardiovascular
sounds being monitored. In a practical embodiment of the
sensor, the maximum diameter of the membrane may be about
the same as that of a nickel coin with the contact surface
protruding by about half that amount, but the membrane may
be smaller or larger as desired to suit a particular
application. The support plate dimensions may be any
convenient size, so long as the coupler fusion region and
the fiber portions near the fusion region are securely
supported. The sensitivity of the device is dependent upon
the stiffness of the membrane, as in prior devices.
When the contact surface 39 is positioned upon a pulse
point, such as on a person's arm over the brachial artery or
radial artery, the membrane 38 couples skin displacements
associated with the pulse to the coupling region 32 of the
fiberoptic coupler 30. The coupling region is thereby
deflected, changing the light output ratio of the output
fibers 36 in accordance with the sounds being monitored.
Figs. 6 and 7 provide a pictorial comparison between
the deflection of a conventional pre-tensioned linear
fiberoptic coupler and the deflection of the U-shaped
coupler in the sensor of Figs. 4 and 5. Views 6a and 6c are
top and side views, respectively, showing the fusion region
of the conventional coupler in its normal state. Views 6b
and 6d are corresponding views of the fusion region being
deflected by a downward force F. Views 7a - 7d in Fig. 7
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are corresponding views to Fig. 6, but show the U-shaped
coupler employed in the present invention.
As will be appreciated from View 7d, the deflection of
the fusion region in the conventional coupler causes a
bowing that tends to stretch and thereby increase the
tension on the fusion region. By contrast, the deflection
of the U-shaped fusion region in View 7d, which is seen to
occur along a direction perpendicular to the plane of the U-
shape, merely causes a flexing of the U along its height
(horizontal dimension in View 7d), without subjecting the
fusion region to tension. Thus, even large displacements of
the fusion will not cause cracking or breaking.
Fig. 8 shows another variable coupler fiberoptic sensor
20' that may be used in the apparatus of the invention. The
sensor has the same basic structure as that of the previous
embodiment, except that the support member 22' is formed as
a substantially rectangular plate angled at about 30° to
conform to the human arm/wrist anatomy and facilitate
wearing of the sensor by the patient, as by strapping the
sensor to the arm/wrist. If appropriate to a particular
application, the support member may house the light source
40, the photodetection/differential amplifier circuit 42,
and a radio transmitting device (not shown) coupled to the
circuit 42 to provide for remote monitoring. Indeed, such
provision can be made in any of the sensor structures
described herein.
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Fig. 9 shows the wrist heartbeat/breathing signal
obtained from a human subject with the sensor 20' of Fig. 8.
The data stream in Fig. 9 was obtained at a sampling rate of
128 samples per second. It will be appreciated that the
pulse waveform, as read by the sensor, is a more complex
phenomenon than standard pulse readings. The pulse waveform
exhibits the amplitude structure of the pulse as a function
of time. The amplitude structure of the pulse is not what
is "felt" as an impulse function by a finger at a pulse
point, although that function is present. Within the
amplitude structure, there are all of the heart sounds as
well as information on breathing and other indicators of
physical condition. The sensitivity achieved with the
improved sensors described herein makes them very good at
sensing the complex pulse waveform.
Fig. 10 shows another wrist heartbeat/breathing signal
obtained from a human subject with the sensor 20'. Here,
the data stream was digitized using a 12-bit A/D converter
at a sampling rate of 64 samples per second. The heartbeat
signal is very well resolved, as the inset graph
demonstrates. In addition, the modulation introduced by the
breathing cycle is clearly visible over the course of the 84
second run.
Figs. 11 and 12 show another arm/wrist sensor 50 that
may be used in the apparatus of the invention. In this
sensor, the fusion region 62 of the fiberoptic coupler is
not potted, but coated as previously discussed. The fusion
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region 62 is coupled to pulsations of the arm/wrist (denoted
by arrow P) by a fluid- or gel-filled elastic pillow 68.
The fiberoptic coupler is mounted to a support plate 52
similar to that of Fig. 8, except that the support plate 52
is planar, not angled (the channel for the input and output
leads 64, 66 having been omitted from illustration for
simplicity). The support plate is secured to the top side
of pillow 68 and a cover 69 is attached to the top side of
the support plate to protect the fusion region 62 of the
coupler 60 at the hole 56. The hole 56 allows the hydraulic
pressure of the pulse activity to push on and deflect the
fusion region by virtue of the contact between the fusion
region and the upper surface of the pillow 68 which, due to
its flexibility, protrudes into the hole 56 to contact the
coupler fusion region. A strap 57 attached to the support
plate 52, as by glue, allows the sensor to be secured to the
arm/wrist. Reference numbers 64 and 66 denote the input
fibers and output fibers, respectively.
The unpotted sensor design of Figs. 11 and 12 is
advantageous over the potted designs previously described,
because the absence of the sensing membrane results in
greater sensitivity. Also, unlike the bent design in Fig.
8, the planar configuration of the support plate does not
require out-of-plane bending of the coupler leads, which
causes a reduction of light intensity. Instead, the coupler
is maintained in a planar configuration, which optimizes the
light intensity in the system.
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Fig. 13 shows still another sensor 70 that may be used
in the apparatus of the invention, the sensor being shown in
cross-section as worn on the wrist. The sensor includes a
frame member 72 having an inner configuration which conforms
generally to the wrist, as shown. The frame member may be
constructed from any suitable material, preferably a plastic
such as Delrin~, PVC, acrylic, Lucite, Plexiglass~,
styrene, or other polymers.
An upper portion of the frame provides a chamber 77 for
housing the fiberoptic coupler 80 and its support plate 81.
Since the coupler is housed by the frame member, the support
plate, which is channeled to receive the input and output
leads, need not include an opening (e. g., a well or through
hole) to house the fusion region 82 of the coupler as in
earlier discussed sensors. The fusion region is coated,
rather than potted, as previously described. The support
plate 81, which may be of the same material as the frame 72,
and the coupler are assembled as a module and glued in place
in the chamber 77. The chamber is closed by a protective
cover plate (not shown).
To couple the fusion region to the pulsations of the
radial artery, a fluid column 74 is provided. The column
has a pair of resilient membranes 73 and 75 provided at its
inner and outer ends, respectively, and extends through the
thickness of the frame 72 between the chamber 77 and the
frame inner surface. The coupler module is installed with
the coupler fusion region 82 in contact with the outer
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membrane 75 of the fluid column. The outer membrane is
attached to an annular boss 76 to raise the height of the
fluid column for contact with the coupler fusion region.
The contact with the outer membrane may subject the fusion
region to a slight pre-load. The coupler may be
manufactured such that the pre-loading of the fusion region
will produce a substantially equal division of light between
the output fibers, thus providing a more linear dynamic
range. The inner portion (lower portion in Fig. 13) of the
fluid column is stepped as shown, so as to increase the
diameter of the coupling area at the wrist.
The membranes constitute an important part of the fluid
colunui. Since the arterial pulsations are weak, the
membranes should be light, thin, and of low durometer and
high extensibility for optimum performance. At the same
time, at least the inner membrane should be rugged enough to
endure continuous contact with the skin. A material found
to have excellent characteristics for the membrane is
FlexChem, an FDA-approved, highly durable, vinyl based
material available in pellet form from Colorite. FlexChem
is also thermo-moldable, which permits the inner sensing
membrane 73 to be molded to provide maximum coupling area
with the radial artery and to protrude from the inner
surface of the frame member 72 for better coupling with the
wrist. A compatible fluid for use with FlexChem membranes
is medical grade 1~M silicone fluid available from Applied
Silicone Corp. Water, incidentally, is not preferred for
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use with FlexChem membranes since the membranes are
permeable to water vapor.
Several inner membrane sizes were tested to determine
the effect on sensor response. In particular, membrane
diameters of 4 mm, 7 mm, and 10 mm were tasted for response
to driven-oscillator stimuli calibrated using a commercial
accelerometer. The response was examined over a frequency
range of 0 to about 11 Hz (cardiovascular and breathing
signals are typically in the range from 0.1 to 4 Hz). Each
of the membranes provided acceptable response, With the 10
mm membrane providing the best response.
Returning to Fig. 13, the present construction also
demonstrates how ancillary components, such as the light
source and output circuitry (e.g., photodetectors and
differential amplifier circuitry) may be incorporated into
the sensor unit. More particularly, such components may be
housed in one (as shown) or more internal chambers 79 of the
frame 72.
Figs. 14 - 16 illustrate another sensor 80, designed
for application to the carotid artery. This sensor uses a
planar, channeled support plate 82 and coupler arrangement
similar to that of Fig. 11, except that the fusion region is
potted to provide a sensor membrane. The membrane area may
be made sufficiently large (e.g., about the size of a
quarter dollar) to allow for the addition of a spherical cap
99' over the convexly protruding surface of the sensing
membrane 98. The addition of the spherical cap renders the
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sensor less sensitive to any rocking motion caused by the
hand when the sensor is manually pressed against the neck.
The coupler is protected at the back side (bottom in Figs.
14 and 15) of the sensor by a plastic cover plate 97. The
sensor may be secured to the neck by any suitable means,
such as adhesive tape.
The input and output fibers are encased as pairs in
respective protective sheaths 102 and 104, which in turn are
encased in an outer protective sheath 106. Fiberoptic
connectors 108 are provided at the ends of the leads to
interface the sensor with external components.
Figs. 17-21 are plots showing brachial and radial
artery pulse waveforms and corresponding pulse transit times
obtained with an apparatus according to Fig. 3 using two
variable coupler fiberoptic sensors of the improved type
described herein. The digital signal processor was
programaned in accordance with the method described in
connection with Fig. 2. It will be appreciated,
incidentally, that the apparatus of Figs. 1 and 3 are not
mutually exclusive. For example, when programmed in
accordance with Fig. 2, the apparatus of Fig. 3 will
constitute a particular form of the structure generally
represented in Fig. 1. Conversely, when provided with an
improved variable coupler fiberoptic sensor of the type
described, the apparatus of Fig. 1 will constitute a
particular form of the structure generally represented in
Fig. 3.
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Fig. I7 shows data for a supine adult male breathing
normally. The pulse transit time is seen run about 50 cosec.
on average.
Fig. 18 is a similar plot except that the breathing
pattern was changed to simulate sleep, inhaling for two
seconds and exhaling for 3 seconds. The pulse transit time
runs about 35 cosec. on average.
Fig. 19 used a similar breathing pattern as just
described, but breathing was constricted by pinching the
nose. Blood pressure falls under these circumstances since
the thoracic cavity is under more negative pressure (pulsus
paradoxus). This is evidenced by the increase in pulse
transit time to about 50 cosec. on average.
Fig. 20 again used a similar breathing pattern, but
with complete obstruction of airflow. To simulate an apnea
event, no air was admitted to the lungs over the entire 16
sec. test period. As is apparent, the pulse transit time
increased substantially, indicating a further fall in blood
pressure relative to Fig. 19.
Fig. 21 shows another plot for a 16 sec. period of no
breathing, but with a full lung. The pulse transit time
values decreased to about 30 cosec. on average, indicating
higher blood pressure.
The results of Figs. 17-21 are consistent with the
known fact that negative lung pressure causes blood pressure
to fall whereas increasingly positive lung pressure causes
blood pressure to rise.
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Fig. 23 depicts a practical arrangement using variable
coupler fiberoptic sensors for implementing an apparatus
according to Fig. 1 or Fig. 3. In the form shown, the
sensors Sl,S2 (S1',S2') are strapped to the arm over
brachial and radial artery pulse points, respectively. The
light sources and signal processing electronics are
contained in a module M also strapped to the arm. The
sensors and the module M are connected through corresponding
sets of fiberoptic leads 34,36. The module M may include a
radio transmitting device (not shown) to communicate with
external electronics.
It should be noted that the optical fiber used in the
above-described sensors is most preferably of very high
quality, such as Corning SMF28 which exhibits an optical
loss of about 0.18 dB per Km. The photodetectors may be
gallium-aluminum-arsenide or germanium detectors for light
wavelengths above 900 nm and silicon detectors for shorter
wavelengths.
The photodetectors may be connected in either a
photovoltaic mode or a photoconductive mode. In the
photovoltaic mode, transimpedance amplifiers (which convert
current to voltage) may be used to couple the detectors to
the differential amplifier inputs. The transimpedance
amplifier outputs may also be filtered to eliminate
broadband noise. In the photoconductive mode, the detector
outputs can be connected to a conventional voltage
amplifier. This approach results in more noise, but may be
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used in applications where cost is a major concern and a
lower noise level is not.
It is to be understood, of course, that the foregoing
embodiments of the invention are merely illustrative and
that numerous variations of the invention are possible in
keeping with the invention as more broadly described herein.
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