A system for automatically positioning a blood pressure monitor
transducer in a desired position relative to an underlying artery.
The positioning system comprises a positioniong motor which drives
a gear assembly. The gear assembly is operatively engaged with a
lead screw having a follower mounted thereto in threaded engagement
therewith. The follower is secured to a strap which is routed through
a roller system. As the motor is activated in response to output
signals from the sensor mounted within transducer housing, the lead
screw will rotate, thereby effectuating lateral movement of the
transducer housing and sensor along a path defined by the strap.
1. A system for maintaining a blood pressure monitor transducer
properly positioned over an artery, comprising:
a transducer housing;
a sensor contained within said housing, said sensor comprising
a plurality of pressure sensing elements, each of said elements
producing an output signal proportional to the pulse amplitude in
said underlying artery;
processing means responsive to said sensor element output signals
for forming an ordered relation between said output signals and
for generating an output signal indicating the desired position
of said sensor; and
means for moving said housing in response to said output signal
from said processing means.
2. The system according to claim 1, said ordered relation between
said output signals from said elements being in the form of a tonogram.
3. The system according to claim 2, wherein said means for moving
said housing comprises a housing drive assembly secured to said
housing and a strap defining a path for housing travel.
4. The system according to claim 3, wherein said drive assembly
comprises a d.c. gear motor and a motor controller.
5. The system according to claim 4, said strap being routed through
a system of rollers in said housing.
6. The system according to claim 5, said strap comprising a portion
formed from a semirigid plastic material.
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.
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
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
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
FIG. 9c is a bottom plan view of the transducer assembly of the
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
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
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
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.