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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to a method and apparatus for
continuous noninvasive measurement of blood pressure. More specifically,
the present invention provides a method and apparatus for maintaining a
continuous blood pressure monitor transducer properly positioned over an
underlying artery in order to ensure that at least one of the pressure
sensing elements in the transducer tracks the actual pulse waveform in the
underlying artery, thus providing the most accurate measurement of the
patient's blood pressure.
BACKGROUND
There has been considerable interest in recent years in the development of
a monitoring system for obtaining a continuous measurement of a patient's
blood pressure. One of the most promising techniques for obtaining such a
continuous measurement involves the use of an arterial tonometer
comprising an array of small pressure sensing elements fabricated in a
silicon "chip." The use of such an array of sensor elements for blood
pressure measurements is disclosed generally in the following U.S.
Patents.: U.S. Pat. Nos. 3,123,068 to R. P. Bigliano, 3,219,035 to G. L.
Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick,
4,269,193 to Eckerle, and 4,423,738 to P. M. Newgard, and in an article by
G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous
External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med.
Elec., April 1963, pp. 77-81).
In a typical tonometric technique for monitoring blood pressure, a
transducer which includes an array of pressure sensitive elements is
positioned over a superficial artery, and a hold-down force is applied to
the transducer so as to flatten the wall of the underlying artery without
occluding the artery. The pressure sensitive elements in the array have at
least one dimension smaller than the lumen of the underlying artery in
which blood pressure is measured, and the transducer is positioned such
that more than one of the individual pressure-sensitive elements is over
at least a portion of the underlying artery. The output from one of the
pressure sensitive elements is selected for monitoring blood pressure. The
element that is substantially centered over the artery has a signal output
that provides an accurate measure of intraarterial blood pressure.
However, for the other transducer elements the signal outputs generally do
not provide as accurate a measure of intraarterial blood pressure as the
output from the centered element. Generally, the offset upon which
systolic and diastolic pressures depend will not be measured accurately
using transducer elements that are not centered over the artery. One
method for selecting the pressure sensitive element for monitoring blood
pressure is disclosed in the above mentioned U.S. Pat. No. 4,269,193
issued to J. S. Eckerle. In addition, an improved method for selecting the
correct pressure sensitive element for measuring blood pressure is
disclosed in a patent application entitled "Active Element Selection for
Continuous Blood Pressure Monitor Transducer" filed on even date herewith.
One of the difficulties encountered in the development of tonometric blood
pressure monitoring systems is the correct placement of the transducer on
the patient's wrist such that the pressure sensing elements are centered
over the underlying artery. The positioning system of the present
invention, described in greater detail below, overcomes this difficulty.
SUMMARY OF THE INVENTION
The present invention provides an automatic positioning system for
maintaining a continuous blood pressure monitor transducer properly
positioned over an underlying artery in order to ensure that a plurality
of the pressure sensing elements in the transducer track the actual pulse
waveform in the underlying artery. In the preferred embodiment, the pulse
amplitude output signals from each of the pressure sensing elements in the
sensor array are monitored and used to generate a tonogram indicating the
approximate position of the sensor relative to the underlying artery. When
the sensor is properly centered over the artery, the tonogram will exhibit
a characteristic peak which is usually near the center of the tonogram.
However, when the transducer is offset to either side of the artery, the
tonogram will be a curve having primarily either a positive or a negative
slope without a clearly defined peak.
In the automatic positioning system of the present invention, the tonogram
of the pulse amplitudes is used to determine the position of the
transducer relative to the underlying artery and to maintain the
transducer in the optimal position. The preferred embodiment of the
positioning system comprises a gear motor which is operatively engaged
with a lead screw/follower mechanism by means of a pair of gears. The gear
motor effectuates rotation of the lead screw in response to appropriate
signals indicating the position of the sensor relative to the underlying
artery. Rotation of the lead screw causes the transducer housing to travel
or move laterally along a path defined by a transducer strap. Lateral
movement of the housing in response to the rotation of the lead screw is
caused by the threaded engagement of the follower with the lead screw and
the securement of the follower to the strap in a fixed position. The
travel of the transducer housing is further facilitated by a roller
mechanism through which the strap is routed. The movement of the housing,
in response to feedback from the sensor, continues until the sensor is
appropriately positioned over the underlying artery.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of the continuous blood pressure monitoring transducer of
the present invention attached to a patient's wrist at a position
overlying the radial artery.
FIG. 2 is a cross sectional side view taken along section lines 2--2 of
FIG. 1 illustrating the continuous blood pressure monitor positioned over
an artery with the artery being partially flattened in response to
pressure applied by a transducer piston assembly.
FIG. 3 is a perspective view of an array of pressure sensing elements,
etched in a monocrystaline silicon substrate, of the type employed in the
pressure transducer of the present invention.
FIG. 4 is a schematic diagram illustrating the force balance between the
artery and the multiple transducer elements (arterial riders), with the
artery wall properly depressed to give accurate blood pressure readings.
FIG. 5 is a simplified block diagram of the system components for
monitoring a plurality of force sensing elements to generate a tonogram
which can be used to control a positioning system to maintain the
transducer assembly at a desired position over an underlying artery.
FIG. 5a is a block diagram of the position controller.
FIG. 6 is a waveform of human blood pressure versus time of the type which
may be obtained using the present invention for illustrating systolic and
diastolic pressures and pulse amplitude of the blood pressure wave.
FIG. 7a is a graphical representation of a tonogram of pulse amplitudes
measured by the force sensing elements of the continuous blood pressure
monitor transducer when the transducer is properly positioned over the
underlying artery.
FIG. 7b is a graphical representation of a tonogram of pulse amplitudes
measured by the force sensing elements of the continuous blood pressure
monitor transducer when the transducer is improperly positioned over the
underlying artery.
FIG. 7c is another graphical representation of a tonogram of pulse
amplitudes measured by the force sensing elements of the continuous blood
pressure monitor transducer when the transducer is improperly positioned
over the underlying artery.
FIG. 8 is a perspective view of the transducer assembly of the present
invention showing the routing of the transducer strap through the
transducer roller system.
FIG. 9a is a cross-sectional side view showing the routing of the strap
over a system of rollers used in the automatic positioning system of the
preferred embodiment.
FIG. 9b is a top plan view of the transducer assembly of the preferred
embodiment.
FIG. 9c is a bottom plan view of the transducer assembly of the preferred
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference is now made to FIG. 1 wherein a continuous blood pressure monitor
transducer 10 is shown attached to a patient's wrist at a point overlying
the radial artery. The transducer is attached by means of a strap 12 in a
manner similar to a conventional wristwatch. A cable assembly 14 connected
to the transducer contains electrical cables for carrying electrical
signals to and from the transducer. The cable assembly 14 also contains a
pneumatic tube for providing pressurized air to a pressurizable bladder in
the interior of the transducer in order to bring a sensor into contact
with the patient's skin in a manner described in greater detail
hereinbelow.
For the transducer to properly measure blood pressure it is important that
the underlying artery be partially compressed. Specifically, it is
important that the artery be flattened by a plane surface so that the
stresses developed in the arterial wall perpendicular to the face of the
sensor are negligible. This generally requires that the blood pressure
measurement be taken on a superficial artery which runs over bone, against
which the artery can be flattened.
FIG. 2 is a cross sectional side view, taken along section lines 2--2 of
FIG. 1, showing the continuous blood pressure monitor positioned on the
patient's wrist at a point overlying the radial artery 24. A transducer
piston 16 including a sensor mounting platform 18 is shown protruding from
the bottom of the transducer to flatten the artery 24 against the radius
bone 28. A sensor 20 is mounted on the lower surface of the sensor
mounting platform 18. The sensor 20 comprises a plurality of pressure
sensing elements described below. In FIG. 2, the ends 12' and 12" of the
strap 12 are shown attached to ground symbols to illustrate that the strap
is firmly secured to the patient's wrist. In practice, the strap is
secured in generally the same manner as that for a conventional wrist
watch.
FIG. 3 is a perspective view of the sensor 20 used in the continuous blood
pressure monitor of the preferred embodiment. The sensor 20 comprises an
array of individual pressure sensing elements 22 which are formed in a
thin rectangular monocrystalline silicon substrate using conventional but
modern integrated circuit techniques. One method which can be used to form
such a silicon chip with regions of predetermined thickness in the chip is
described in U.S. Pat. No. 3,888,708 issued to Wise, et al. for "Method
for Forming Regions of Predetermined Thickness in Silicon." In the sensor
shown in FIG. 3, the individual pressure sensing elements 22 are shown
aligned in two rows. This particular arrangement is shown only for
purposes of illustration. In practice, various numbers of force sensitive
elements can be used, depending on the desired monitoring resolution, and
various patterns can be used for arranging the sensing elements within the
silicon substrate.
Reference now is made to FIG. 4 wherein a diagrammatic mechanical model is
shown which is representative of physical factors to be considered in
blood pressure measurements using tonometry techniques. The illustrated
model is adapted from that shown in the above-mentioned U.S. Pat. No.
4,269,193, issued to J. S. Eckerle, which by this reference is
incorporated for all purposes. An array 22 of individual pressure
sensitive elements or transducers 22-A through 22-E, which constitute the
arterial riders, is positioned so that one or more of the riders are
entirely over an artery 24. The individual riders 22-A through 22-E are
small relative to the diameter of the artery 24, thus assuring that a
plurality of the riders overlie the artery. The skin surface 26 and artery
underlying the transducer must be flattened by application of a hold-down
pressure to the transducer. One rider overlying the center of the artery
is identified as the "centered" rider, from which rider pressure readings
for monitoring blood pressure are obtained. Means for selecting the
centered rider are discussed general in the above mentioned U.S. Pat. No.
4,269,193. In addition, an improved means for selecting the best pressure
sensing element for measuring blood pressure is disclosed in a patent
application entitled "Active Element Selection for Continuous Blood
Pressure Monitor Transducer" filed on even date herewith. For present
purposes it will be understood that one of the riders, such as rider 22-E,
may be selected as the "centered" rider, in which case the remainder of
the riders, here riders 22-A through 22-D and 22-F through 22-J, comprise
"side plates" which serve to flatten the underlying skin and artery.
Superficial arteries, such as the radial artery, are supported from below
by bone which, in FIG. 4, is illustrated by ground symbol 28 under the
artery. The wall of artery 24 behaves substantially like a membrane in
that it transmits tension forces but not bending moments. The artery wall
responds to the loading force of the transducer array, and during blood
pressure measurements acts as if it is resting on the firm base 28. With
the illustrated system, the transducer assembly 10 and mounting strap 12,
together with air pressure applied to a pressurizable bladder in the
transducer assembly, supply the required compression force and hold the
riders 22-A through 22-J in such a manner that arterial pressure changes
are transferred to the riders which overlie the artery 24. This is
illustrated schematically in FIG. 4 by showing the individual riders 22-A
through 22-J backed by rider spring members 30-A through 30-J,
respectively, a rigid spring backing plate 32, and hold-down force
generator 36 between the backing plate 32 and the mounting strap system
38.
If, without force generator 36, the coupling between the mounting strap
system 38 and spring backing plate 32 were infinitely stiff to restrain
the riders 22-A through 22-J rigidly with respect to the bone structure
28, the riders would be maintained in a fixed position relative to the
artery. In practice, however, such a system is not practical, and
hold-down force generator 36, comprising (in the present example) a
pneumatic loading system, is included to keep constant the force applied
by the mounting strap system 38 to riders 22-A through 22-J. In the
mechanical model the spring constant, k (force per unit of deflection) of
the force generator, 36, is nearly zero. Pneumatic loading systems are
shown and described in the abovereferenced U.S. Pat. Nos. 3,219,035 and
4,269,193, and the Pressman and Newgard IEEE article. In addition, an
improved pneumatic loading system is disclosed in a patent application
entitled "Pressurization System for Continuous Blood Pressure Monitor
Transducer" filed on even date herewith.
In order to insure that the riders 22-A through 22-J flatten the artery and
provide a true blood pressure measurement, they must be rigidly mounted to
the backing plate 32. Hence, the rider springs 30-A through 30-J of the
device ideally are infinitely rigid (spring constant k=.infin.). It is
found that as long as the system operates in such a manner that it can be
simulated by rider springs 30-A through 30-J having a spring constant on
the order of about ten times the corresponding constant for the
artery-skin system, so that the deflection of riders 22-A through 22-J is
small, a true blood pressure measurement may be obtained when the correct
hold-down pressure is employed.
Referring to FIG. 5, a simplified illustration of the transducer assembly
10 is shown to include a transducer piston 16, a pressurizable chamber 40
and a position controller 60. In FIG. 5a, the position controller can be
seen to comprise a motor controller 61 and a D.C. gear motor 68. The
output of the individual pressure sensors (not shown) on the sensor 20 are
connected by appropriate electrical wiring 42 to the input of a
multiplexer 44. From the multiplexer, the signals are digitized by an
analog-to-digital (A-D) converter 46, and the digitized signals are
supplied to a microprocessor 48. Output from the microprocessor 48 is
supplied to data display and recorder means 50 which may include a
recorder, cathode ray tube monitor, a solid state display, or any other
suitable display device. Also, the output from the microporcessor 48 is
provided to the pressure controller 52 which controls a pressure source 54
to maintain the appropriate pressure in the pressurizable chamber 40, thus
ensuring the proper hold down pressure for the transducer piston 16.
Operation of the microprocessor 48 can be controlled by a program
contained in program storage 56 or by user input from the user input
device 58, which can be in the form of a keyboard or other interface
device, such as a "joystick," etc. The program storage 56 or the user
input device 58 can be used to cause the microprocessor 48 to control
operation of the position controller 60 as described in greater detail
below.
Reference is now made to FIG. 6 which illustrates the signal waveform of
the output from one of the pressure sensitive elements 22-A through 22-J
which overlies artery 24. Other elements of the transducer array which
overlie the artery will have waveforms of similar shape. With a correct
hold-down pressure and correct selection of the "centered" arterial rider
(i.e., the rider substantially centered over the artery) the waveform is
representative of the blood pressure within the underlying artery.
Systolic, diastolic and pulse amplitude pressures are indicated on the
waveform, wherein pulse amplitude is the difference between the systolic
and diastolic pressures for a given heartbeat.
A graphical illustration, or "tonogram," of the pulse waveforms produced by
the portion of the artery underlying the sensor 20 can be formed by
displaying the respective pulse pressure output signals produced by each
of the pressure sensing elements 22. The tonogram can then be used to
determine whether the transducer assembly 10 is properly positioned over
the artery. FIGS. 7a-7c illustrate tonograms for sensors which are
properly centered over an artery, as well as for sensors which are offset
to either side from the center of the artery. For the tonograms
illustrated in FIGS. 7a-7c, pulse pressures have been illustrated for a
sensor having 15 pressure sensing elements. It is to be understood that
the element no. 1 would occupy the approximate position illustrated for
element 22-A in the model shown in FIG. 4 and that element no. 15 would
occupy the approximate position illustrated for element 22-J. FIG. 7a is a
typical tonogram for a sensor which is properly centered over an
underlying artery. The tonogram produced by the plurality of pressure
sensing elements exhibits a peak at approximately the center of the
tonogram. FIGS. 7b and 7c, however, illustrate tonograms which are
produced by sensors which are offset either in direction A or direction B,
as illustrated in FIG. 4, from the center of the underlying artery. FIG.
7b is a typical tonogram which is produced by a sensor which is offset in
direction A shown in FIG. 4. FIG. 7c is a typical tonogram which is
produced by a sensor which is offset in direction B shown in FIG. 4.
Referring to FIG. 8 and FIG. 9a, the positioning system of the present
invention will be described in greater detail. The positioning system is
utilized to correctly position the transducer assembly so that the sensor
20 is properly centered over an underlying artery. As illustrated in FIG.
8 and FIG. 9a, the strap 12 is routed around a roller system comprising
upper rollers 62a and 62 b and lower rollers 64 a and 64b. Rollers 62a,
62b, 64a, and 64b are appropriately mounted to outer housing 11, as
illustrated in FIG. 9b and FIG. 9c, so as to permit rotation of the
rollers. The portion of the strap 12 which is in contact with the rollers
is formed of a strip 12a of semirigid plastic which is attached to
portions 12' and 12" of the strap by stitched seams 13' and 13",
respectively, as shown in FIG. 8. An aperture 63 in the strap portion 12a
is adapted to receive a connector screw 78 (illustrated in FIG. 9b) to
secure the transducer assembly to the strap 12 as the outer housing 11
moves laterally along the path defined by strap portion 12a.
Referring to FIG. 9b and FIG. 9c, the apparatus for moving the housing 11
laterally along the path or track defined by strap portion 12a comprises a
DC gear motor 68 which is operatively engaged with a first gear 70 so as
to appropriately drive or rotate gear 70. Motor 68 is mounted within motor
housing 66 (illustrated in FIG. 9a), which is mounted to and within outer
housing 11. Gear 70 is appropriately engaged with a second gear 72 so as
to effectuate rotation of gear 72 upon rotation of gear 70. A lead screw
74 is appropriately mounted to and within housing 11 and engaged with gear
72 so that rotation of gear 72 will effectuate rotation of lead screw 74.
Lead screw 74 has a follower 76 mounted thereon in threaded engagement
therewith. Follower 76 has an upstanding screw 78 connected thereto which
extends through aperture 63 to secure follower 76 to strap 12.
Referring again to FIG. 9b and FIG. 9c, the operation of the positioning
system will be described in greater detail. The gear motor 68 responds to
output signals from the motor controller 61 so as to effectuate clockwise
or counter clockwise rotation of gear 70, thereby effectuating rotation of
gear 72 and lead screw 74. Due to the securement of follower 76 to strap
portion 12a and the threaded engagement of follower 76 with lead screw 74,
the rotation of lead screw 74 will cause housing 11 and sensor 20 to move
laterally in direction A or B, depending upon the direction of rotation of
lead screw 74. As the gear motor 76 appropriately activates or rotates
lead screw 74 by means of gears 70 and 72, the sensor 20 is thereby
accurately positioned and properly centered over an underlying artery.
Referring again to FIG. 9a, details relating to the pressurizable chamber
40 will be described in greater detail. A flexible silicon rubber bellows
or diaphragm 41 is shown with its perimeter attached to the lower surface
of the motor housing 66 and is further secured to the top of the sensor
piston 16 by means of a plate 43. The sealed perimeter portion of the
diaphragm is illustrated by reference number 41' in FIG. 9a. Both of the
above mentioned attachments of the diaphragm 41 provide air tight seals.
With the diaphragm 41 attached to the lower face of the motor housing 66
and the upper surface of the transducer piston assembly as described
above, a pressurizable chamber 40 is formed within the transducer housing
assembly. Since the flexible rubber bellows is sealed both to the
transducer piston 16 and to the lower face of the motor housing 66,
pressurized air introduced into the pressurizable cavity 40 causes the
transducer piston 16 to be pneumatically loaded. As the pressure in the
cavity 40 is increased the transducer piston assembly 16 will be forced
downward to the position shown in FIG. 9a. Pressurized air is introduced
by means of a pneumatic tube 14b which is in fluid communication with
chamber 40 and may extend through terminator housing 65, as illustrated in
FIG. 9c. The pneumatic pressure applied inside the rubber bellows 41 may
be adjusted to supply the compressional force required to provide the
necessary flattening of the artery wall, thus allowing the device to meet
the flattening criteria described above in connection with FIG. 2.
Furthermore, the pressure source of the present invention can be used to
provide a constant pressure to maintain the artery in an optimally
flattened condition. When the transducer case is held in place on the
wrist, generally over the radial artery, as shown in FIG. 1, the
transducer piston 16 is thus supported over the radial artery by the
rubber bellows, air pressure inside the bellows holds the sensor 20 and
its supporting structure, against the skin surface with sufficient force
to achieve the desired degree of flattening of the wall of the artery.
Therefore, the individual force sensing elements in the sensor will
produce output signals which accurately track the pulse wave form in the
underlying artery.
In operation, the positioning system of the present invention can be
operated manually by a user who views the display 50 and inputs
directional commands via the user input device, e.g., arrow keys on a
computer keyboard, to cause the transducer to move in the desired
direction. Alternatively, a program stored in program storage 56 can be
used to cause the microprocessor to provide an appropriate control signal
to the motor controller 61, thus moving the D.C. gear motor to cause
repositioning of the transducer assembly. Such a program would include
curve analysis techniques which are well known in the art and can be
implemented by a programmer of ordinary skill.
Although the method and apparatus of the present invention has been
described in connection with the preferred embodiment, it is not intended
to be limited to the specific form set forth herein, but on the contrary,
it is intended to cover alternatives and equivalents as may reasonable be
included within the spirit and scope of the invention as defined by the
appended claims.
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