U.S. patent application number 10/459564 was filed with the patent office on 2003-10-23 for apparatus and method for measuring pulse transit time.
This patent application is currently assigned to Empirical Technologies Corporation. Invention is credited to Adkins, Charles, Baruch, Martin C., Gerdt, David W..
Application Number | 20030199771 10/459564 |
Document ID | / |
Family ID | 29219583 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030199771 |
Kind Code |
A1 |
Baruch, Martin C. ; et
al. |
October 23, 2003 |
Apparatus and method for measuring pulse transit time
Abstract
In a method of measuring pulse transit time of a living subject,
first and second pulse wave signals are produced by sensing the
pulse at first and second pulse points, respectively, the first and
second pulse points being spaced from one another. The first and
second pulse wave signals are differentiated, and based on the
results, corresponding points of the first and second pulse wave
signals are selected (e.g., points of maximum slope). The time
delay between the selected points is determined, thus yielding the
pulse transit time. A preferred apparatus measures pulse transit
time using at least one fiberoptic pulse sensor including a
fused-fiber coupling region having at least a portion that can be
deflected without putting the coupling region under tension.
Inventors: |
Baruch, Martin C.;
(Charlottesville, VA) ; Gerdt, David W.;
(Charlottesville, VA) ; Adkins, Charles;
(Earlysville, VA) |
Correspondence
Address: |
Mitchell W. Shapiro
Miles & Stockbridge P.C.
1751 Pinnacle Drive, Suite 500
McLean
VA
22102-3833
US
|
Assignee: |
Empirical Technologies
Corporation
|
Family ID: |
29219583 |
Appl. No.: |
10/459564 |
Filed: |
June 12, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10459564 |
Jun 12, 2003 |
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09763657 |
Feb 26, 2001 |
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09763657 |
Feb 26, 2001 |
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PCT/US99/19258 |
Aug 24, 1999 |
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60097618 |
Aug 24, 1998 |
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60126339 |
Mar 26, 1999 |
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Current U.S.
Class: |
600/485 |
Current CPC
Class: |
A61B 5/0285 20130101;
A61B 5/02416 20130101; A61B 2562/168 20130101; A61B 5/7239
20130101; A61B 7/04 20130101; A61B 5/6892 20130101; A61B 2562/0204
20130101; A61B 2562/0266 20130101 |
Class at
Publication: |
600/485 |
International
Class: |
A61B 005/02 |
Claims
What is claimed is:
1. A method of monitoring heartbeat of a living subject,
comprising: coupling a pulse sensor to a pulse point of the subject
through a fluid contained in a fluid-filled structure, the
fluid-filled structure having a portion secured to the subject with
a surface positioned over the pulse point, such that pulsations are
transmitted from the pulse point to the pulse sensor via the
fluid-filled structure, the pulse sensor providing an output signal
that changes in accordance with the pulsations; and monitoring the
output signal from the pulse sensor over a period of time while
maintaining the coupling of the pulse sensor to the pulse point
through a set amount of the fluid, said period of time being of
sufficient duration that said output signal is subject to
modulation due to a plurality of breathing cycles.
2. A method according to claim 1, wherein said fluid-filled
structure includes a pair of surfaces coupled through a fluid
column, one of said pair of surfaces being the surface positioned
over the pulse point.
3. A method according to claim 2, wherein the pulse point is
associated with the radial artery.
4. A method according to claim 2, wherein the pulse point is
associated with the brachial artery.
5. A method according to claim 2, wherein the pulse sensor is
disposed at the other of said pair of surfaces.
6. A method according to claim 5, wherein said other surface is an
elastic surface, the pulse sensor has a portion engaged with said
elastic surface and which deflects in response to pulsations
transmitted by said elastic surface, said output signal changing in
accordance with the deflection of the pulse sensor.
7. A method according to claim 6, wherein the pulse sensor is a
fiberoptic coupler sensor in which at least one input optical fiber
is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
8. A method according to claim 6, wherein the pulse sensor is a
fiberoptic coupler sensor in which at least one input optical fiber
is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
9. A method according to claim 1, wherein said fluid-filled
structure includes a fluid-filled pillow.
10. A method according to claim 9, wherein said fluid-filled pillow
is secured to an arm of the subject.
11. A method according to claim 10, wherein the pulse point is
associated with the radial artery.
12. A method according to claim 10, wherein the pulse point is
associated with the brachial artery.
13. A method according to claim 9, wherein said fluid-filled pillow
has an elastic surface, the pulse sensor has a portion engaged with
said elastic surface and which deflects in response to pulsations
transmitted by said elastic surface, said output signal changing in
accordance with the deflection of the pulse sensor.
14. A method according to claim 13, wherein the pulse sensor is a
fiberoptic coupler sensor in which at least one input optical fiber
is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
15. A method according to claim 13, wherein the pulse sensor is a
fiberoptic coupler sensor in which at least one input optical fiber
is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
16. Apparatus for monitoring heartbeat of a living subject,
comprising: a pulse sensor coupled to a pulse point of the subject
through a fluid contained in a fluid-filled structure, the
fluid-filled structure having a portion secured to the subject with
a surface positioned over the pulse point, such that pulsations are
transmitted from the pulse point to the pulse sensor via the
fluid-filled structure, the pulse sensor providing an output signal
that changes in accordance with the pulsations; and a system
operative to monitor the output signal from the pulse sensor over a
period of time in which the coupling of the pulse sensor to the
pulse point is maintained through a set amount of the fluid for a
sufficient duration that said output signal is subject to
modulation due to a plurality of breathing cycles.
17. An apparatus according to claim 16, wherein said fluid-filled
structure includes a pair of surfaces coupled through a fluid
column, one of said pair of surfaces being the surface positioned
over the pulse point.
18. An apparatus according to claim 17, wherein the pulse point is
associated with the radial artery.
19. An apparatus according to claim 17, wherein the pulse point is
associated with the brachial artery.
20. An apparatus according to claim 17, wherein the pulse sensor is
disposed at the other of said pair of surfaces.
21. An apparatus according to claim 20, wherein said other surface
is an elastic surface, the pulse sensor has a portion engaged with
said elastic surface and which deflects in response to pulsations
transmitted by said elastic surface, said output signal changing in
accordance with the deflection of the pulse sensor.
22. An apparatus according to claim 21, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
23. An apparatus according to claim 21, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
24. An apparatus according to claim 16, wherein said fluid-filled
structure includes a fluid-filled pillow.
25. An apparatus according to claim 24, wherein said fluid-filled
pillow is secured to an arm of the subject.
26. An apparatus according to claim 25, wherein the pulse point is
associated with the radial artery.
27. An apparatus according to claim 25, wherein the pulse point is
associated with the brachial artery.
28. An apparatus according to claim 24, wherein said fluid-filled
pillow has an elastic surface, the pulse sensor has a portion
engaged with said elastic surface and which deflects in response to
pulsations transmitted by said elastic surface, said output signal
changing in accordance with the deflection of the pulse sensor.
29. An apparatus according to claim 28, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
30. An apparatus according to claim 28, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
31. Apparatus for monitoring heartbeat of a living subject,
comprising: a structure constructed to contain a fluid and having a
portion constructed to be secured to the subject with a surface
positioned over a pulse point of the subject to provide coupling of
the pulse point to the fluid; a pulse sensor arranged to be coupled
to the pulse point through the fluid and providing an output signal
that changes in accordance with pulsations received from the pulse
point via the fluid; and a system operative to monitor the output
signal from the pulse sensor over a period of time in which the
coupling of the pulse sensor to the pulse point is maintained
through a set amount of the fluid for a sufficient duration that
said output signal is subject to modulation due to a plurality of
breathing cycles.
32. An apparatus according to claim 31, wherein said structure
includes a pair of surfaces, one of which is the surface to be
positioned over the pulse point, and a portion to define a fluid
column to couple said pair of surfaces via the fluid.
33. An apparatus according to claim 32, wherein the pulse point is
associated with the radial artery.
34. An apparatus according to claim 32, wherein the pulse point is
associated with the brachial artery.
35. An apparatus according to claim 32, wherein the pulse sensor is
disposed at the other of said pair of surfaces.
36. An apparatus according to claim 35, wherein said other surface
is an elastic surface, the pulse sensor has a portion disposed to
engage said elastic surface and which deflects in response to
pulsations transmitted by said elastic surface, said output signal
changing in accordance with the deflection of the pulse sensor.
37. An apparatus according to claim 36, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
38. An apparatus according to claim 36, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
39. An apparatus according to claim 31, wherein said structure
includes a pillow to contain a portion of the fluid.
40. An apparatus according to claim 39, wherein said structure
includes a portion constructed to secure said pillow to an arm of
the subject.
41. An apparatus according to claim 40, wherein the pulse point is
associated with the radial artery.
42. An apparatus according to claim 40, wherein the pulse point is
associated with the brachial artery.
43. An apparatus according to claim 39, wherein said pillow has an
elastic surface, the pulse sensor has a portion disposed to engage
said elastic surface and which deflects in response to pulsations
transmitted by said elastic surface, said output signal changing in
accordance with the deflection of the pulse sensor
44. An apparatus according to claim 43, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
fused-fiber coupling region configured such that at least a portion
of said coupling region can be deflected to change a distribution
of light from said input optical fiber to said output optical
fibers with said coupling region being subjected to substantially
no tension.
45. An apparatus according to claim 43, wherein the pulse sensor is
a fiberoptic coupler sensor in which at least one input optical
fiber is joined to a plurality of output optical fibers through a
substantially U-shaped fused-fiber coupling region which can be
deflected to change a distribution of light from said input optical
fiber to said output optical fibers.
46. An apparatus according to claim 1, wherein said portion of said
structure is constructed to be secured about the arm.
47. An apparatus according to claim 16, wherein said portion of
said structure is constructed to be secured about the wrist.
48. An apparatus according to claim 8, wherein said coupling region
is disposed substantially in a plane, and is deflected by said
elastic surface along a direction perpendicular to said plane.
49. An apparatus according to claim 15, wherein said coupling
region is disposed substantially in a plane, and is deflected by
said elastic surface along a direction perpendicular to said
plane.
50. An apparatus according to claim 23, wherein said coupling
region is disposed substantially in a plane, and is deflected by
said elastic surface along a direction perpendicular to said
plane.
51. An apparatus according to claim 30, wherein said coupling
region is disposed substantially in a plane, and is deflected by
said elastic surface along a direction perpendicular to said
plane.
52. An apparatus according to claim 38, wherein said coupling
region is disposed substantially in a plane, and is deflected by
said elastic surface along a direction perpendicular to said
plane.
53. An apparatus according to claim 45, wherein said coupling
region is disposed substantially in a plane, and is deflected by
said elastic surface along a direction perpendicular to said plane.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. application Ser. No.
09/763,657 filed Feb. 26, 2001, which is the National Stage of
International Application No. PCT/US99/19258 filed Aug. 24, 1999,
which claims the benefit under 35 U.S.C. .sctn. 119(e) of U.S.
Provisional Application Nos. 60/097,618 filed Aug. 24, 1998, and
60/126,339 filed Mar. 26, 1999, both of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 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.
[0003] 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.
[0004] An example of a blood pressure monitoring system that
utilizes pulse transit time can be found in U.S. Pat. No. 4,245,648
to Trimmer et al. This system includes a pair of 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.
[0005] 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 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
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
[0010] FIG. 1 is a block diagram of an apparatus for measuring
pulse transit time in accordance with the invention.
[0011] FIG. 2 is a flow diagram for explaining the operation of the
system in FIG. 1.
[0012] FIG. 3 is a block diagram showing another apparatus of the
invention.
[0013] FIG. 4 is a top view of a variable coupler fiberoptic sensor
useful in the apparatus of FIGS. 1 and 3.
[0014] FIG. 5 is a sectional side view of the sensor of FIG. 4.
[0015] FIG. 6 shows explanatory views (Views 6a-6d) of normal and
deflected states of the fusion region of a conventional
pre-tensioned linear coupler.
[0016] FIG. 7 shows corresponding explanatory views (Views 7a-7d)
for a U-shaped fusion region.
[0017] FIG. 8 shows a variable coupler fiberoptic sensor useful in
apparatus according to the invention.
[0018] FIG. 9 is a graph depicting the response of the sensor of
FIG. 8 to pulsations of the wrist.
[0019] FIG. 10 is another graph of the sensor response at the
wrist.
[0020] FIG. 11 is an exploded view of another variable coupler
fiberoptic sensor useful in apparatus according to the
invention.
[0021] FIG. 12 is an end view of the FIG. 9 sensor in assembled
form.
[0022] FIG. 13 illustrates another variable coupler fiberoptic
sensor useful in apparatus according to the invention, shown in
section as worn on the wrist.
[0023] FIG. 14 is a perspective view of a carotid artery sensor
useful in apparatus according to the invention.
[0024] FIG. 15 is a fragmentary side elevation of the FIG. 14
sensor.
[0025] FIG. 16 is a perspective view showing the FIG. 14 sensor and
its fiberoptic leads with installed connectors.
[0026] 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.
[0027] FIG. 22 is a diagram illustrating a practical arrangement of
an apparatus according to FIG. 1 or FIG. 3.
[0028] FIG. 23 illustrates the basic construction of a conventional
variable coupler fiberoptic sensor.
DETAILED DESCRIPTION OF THE INVENTION
[0029] 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.
[0030] 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 via analog-to-digital
converters, or the processing unit may be provided with such
converters internally.
[0031] 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.
[0032] Because corresponding points of the two pulse wave signals
can easily be identified from the differentiated waveforms, the
foregoing method readily accommodates substantial separation of the
sensors S1, 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 pulse wave to the next. This is
particularly advantageous since the pulse waveform ordinarily
varies from one heartbeat to the next.
[0033] 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.
[0034] 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 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, Jun. 15, 1983, pp. 1918-1922.
[0035] 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.
[0036] 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 medical field, for example, variable
coupler fiberoptic sensors have been proposed for monitoring
patient heartbeat during MRI examinations. See U.S. Pat. No.
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. Pat. No. 4,634,858 to Gerdt et al.
(disclosing application to accelerometers), U.S. Pat. No. 5,671,191
to Gerdt (disclosing application to hydrophones), and elsewhere in
the art.
[0037] 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.
[0038] 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.times.2 biconical fused. 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.
[0039] 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 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).
[0040] 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.
[0041] 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 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 Ser. No. 09/316,143 filed May 21, 1999, which is
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.
[0042] 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.
[0043] As shown in FIG. 3, each of the sensors S1', 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-to-digital converter 43. The
digital signal processor processes the input signals to detect the
pulse transit time.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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) 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 in 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.RTM.,
polyvinyl chloride (PVC), or other suitable materials known in the
art.
[0048] 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 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.
[0049] 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.
[0050] 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 are corresponding views to FIG. 6, but show the U-shaped
coupler employed in the present invention.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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 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.
[0056] 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.
[0057] 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.RTM., PVC,
acrylic, Lucite.RTM., Plexiglass.RTM., styrene, or other
polymers.
[0058] 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).
[0059] 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 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.
[0060] The membranes constitute an important part of the fluid
column. 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 MDM silicone fluid
available from Applied Silicone Corp. Water, incidentally, is not
preferred for use with FlexChem membranes since the membranes are
permeable to water vapor.
[0061] 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 tested 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.
[0062] 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.
[0063] 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 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.
[0064] 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.
[0065] 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 programmed 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.
[0066] FIG. 17 shows data for a supine adult male breathing
normally. The pulse transit time is seen run about 50 msec. on
average.
[0067] 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 msec.
on average.
[0068] 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 msec. on average.
[0069] 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.
[0070] 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 msec. on average, indicating higher blood
pressure.
[0071] 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.
[0072] 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 S1, 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.
[0073] 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.
[0074] 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 used in
applications where cost is a major concern and a lower noise level
is not.
[0075] 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.
* * * * *