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Claims  |
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What is claimed is:
1. A sensor for sensing blood pressure within an underlying artery of a
patient, the sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
a flexible diaphragm having an active portion for transmitting blood
pressure pulses of the underlying artery;
interface means coupled between the sensing surface of the transducer and
the flexible diaphragm for transmitting the blood pressure pulses within
the underlying artery from the flexible diaphragm to the sensing surface
of the transducer; and
means isolated from the interface means and surrounding the active portion
for conforming to the anatomy surrounding the underlying artery.
2. The sensor of claim 1 wherein the means for conforming includes means
for equalizing pressure around the active portion.
3. The sensor of claim 2 wherein the means for equalizing pressure includes
a compressible ring surrounding the active portion having a top surface
and a bottom surface, the ring being partially filled with a fluid so that
the distance between the top surface and the bottom surface varies along
the ring to conform to the anatomy of the patient surrounding the
underlying artery.
4. The sensor of claim 3 including:
a foam rubber ring surrounding the active portion and positioned between
the compressible ring and the anatomy of the patient.
5. The sensor of claim 1 including means isolated from the interface means
and at least partially surrounding the active portion for dampening forces
in a direction parallel to the underlying artery which are exerted by the
blood pressure pulses on the sensor so that a substantially zero pressure
gradient exists within the interface means across the pressure transducer.
6. The sensor of claim 5 wherein the means for dampening includes a
compressible sidewall surrounding the active portion.
7. The sensor of claim 1 wherein the tissue surrounding the underlying
artery exerts a force as the underlying artery is compressed, and wherein
the sensor includes means isolated from the interface means and
surrounding the active portion for neutralizing the force exerted by the
tissue surrounding the underlying artery.
8. The sensor of claim 7 wherein means for neutralizing includes a
compressible sidewall surrounding the active portion.
9. A sensor for sensing blood pressure within an underlying artery, the
sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
a flexible diaphragm spaced from the sensing surface, the flexible
diaphragm having an active portion for receiving forces exerted by the
blood pressure pulses;
interface means coupled between the sensing surface of the transducer and
the flexible diaphragm for transmitting the blood pressure pulses within
the underlying artery from the flexible diaphragm to the sensing surface
of the transducer; and
means isolated from the interface means and at least partially surrounding
the active portion for dampening forces in a direction parallel to the
underlying artery which are exerted by the blood pressure pulses on the
sensor so that a substantially zero pressure gradient exists across the
active portion of the flexible diaphragm.
10. A sensor for measuring blood pressure pulses within an underlying
artery surrounded by tissue as the underlying artery is compressed,
wherein the tissue surrounding the underlying artery exerts a force as the
underlying artery is compressed, the sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
a flexible diaphragm spaced from the sensing surface, the diaphragm having
an active portion for receiving forces exerted by the blood pressure
pulses;
interface means coupled between the sensing surface of the transducer and
the flexible diaphragm for transmitting the blood pressure pulses within
the underlying artery from the flexible diaphragm to the sensing surface
of the transducer; and
means isolated from the interface means and surrounding the active portion
for neutralizing the force exerted by the tissue surrounding the
underlying artery.
11. A sensor for sensing blood pressure within an underlying artery, the
sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
supporting means for supporting the transducer above the underlying artery;
a flexible diaphragm spaced from the sensing surface; and
interface means coupled between the sensing surface of the transducer and
the flexible diaphragm for transmitting the blood pressure pulses within
the underlying artery from the flexible diaphragm to the sensing surface
of the transducer, wherein the interface means are isolated from the
supporting means.
12. The sensor of claim 11 wherein the interface means comprises a fluid
coupling medium and wherein the flexible diaphragm partially defines an
interface chamber extending between the flexible diaphragm and the sensing
surface of the transducer, the interface chamber containing the fluid
coupling medium and being substantially isolated from the supporting means
so as to prevent pressure from the support means from being transmitted to
the fluid coupling medium and to the sensing surface of the transducer.
13. The sensor of claim 12 wherein the flexible diaphragm includes:
a first flexible member having an active portion encircled by the
supporting means, wherein the active portion receives forces exerted by
the blood pressure pulses; and
a second flexible member supported between the supporting means and the
first flexible member and bonded to the first flexible member, the second
flexible member having a portion above the active portion which is
isolated from the supporting means so that the isolated portion of the
second flexible member and the active portion of the first flexible member
at least partially define the interface chamber.
14. The sensor of claim 13 wherein the supporting means defines an
expansion cavity so that the isolated portion of the second flexible
member is permitted to move into the expansion cavity without changing the
volume of the interface chamber when the sensor is conforming to the
anatomy of the patient surrounding the underlying artery.
15. The sensor of claim 12 wherein the means for supporting includes a
compressible sidewall at least partially surrounding the sensing surface
of the transducer.
16. The sensor of claim 15 wherein the compressible sidewall includes:
a fluid filled wall for conforming to the anatomy surrounding the
underlying artery, the fluid filled wall equalizing pressure around the
sensing surface of the transducer.
17. The sensor of claim 15 wherein the compressible sidewall includes:
a foam wall for dampening forces in a direction parallel to the underlying
artery which are exerted by the blood pressure pulses on the sensor so
that a substantially zero pressure gradient exists within the interface
means across the pressure transducer.
18. The sensor of claim 15 wherein the tissue surrounding the underlying
artery exerts a force as the underlying artery is compressed and wherein
the compressible sidewall includes:
a foam wall for neutralizing the force exerted by the tissue surrounding
the underlying artery.
19. A sensor for sensing blood pressure within an underlying artery, the
sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
a flexible diaphragm spaced from the sensing surface;
a sidewall coupled to the transducer, wherein the sidewall supports the
transducer above the underlying artery; and
a fluid coupling medium coupled between the sensing surface of the
transducer and the flexible diaphragm and isolated from the sidewall so
that blood pressure pulses within the underlying artery are transmitted
from the flexible diaphragm to the sensing surface of the transducer and
so that external forces from the sidewall are not transmitted through the
fluid coupling medium to the sensing surface of the transducer.
20. A sensor or claim 19 wherein the flexible diaphragm at least partially
defines an interface chamber isolated from the sidewall and wherein the
interface chamber contains the fluid coupling medium.
21. A sensor for sensing blood pressure within an underlying artery of a
patient, the sensor comprising:
a pressure transducer for sensing blood pressure pulses of the underlying
artery, the transducer having a sensing surface;
a flexible diaphragm spaced from the sensing surface, wherein the flexible
diaphragm at least partially defines an interface chamber in communication
with the sensing surface of the pressure transducer;
a fluid coupling medium within the interface chamber for transmitting blood
pressure pulses within the underlying artery from the flexible diaphragm
to the sensing surface of the transducer; and
a supporting structure for supporting the transducer above the underlying
artery, wherein the supporting structure defines an expansion cavity
adjacent the flexible diaphragm so that the interface chamber containing
the fluid coupling medium may undergo a change in shape without a
corresponding change in volume and pressure. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to systems for measuring arterial blood
pressure. In particular, the invention relates to a sensor assembly for
measuring arterial blood pressure in a relatively continuous and
non-invasive manner.
Blood pressure has been typically measured by one of four basic methods:
invasive, oscillometric, auscultatory and tonometric. The invasive method,
otherwise known as an arterial line (A-Line), involves insertion of a
needle into the artery. A transducer connected by a fluid column is used
to determine exact arterial pressure. With proper instrumentation,
systolic, mean and diastolic pressure may be determined. This method is
difficult to set up, is expensive and involves medical risks. Set up of
the invasive or A-line method poses problems. Resonance often occurs and
causes significant errors. Also, if a blood clot forms on the end of the
catheter, or the end of the catheter is located against the arterial wall,
a large error may result. To eliminate or reduce these errors, the set up
must be adjusted frequently. A skilled medical practitioner is required to
insert the needle into the artery. This contributes to the expense of this
method. Medical complications are also possible, such as infection or
nerve damage.
The other methods of measuring blood pressure are non-invasive. The
oscillometric method measures the amplitude of pressure oscillations in an
inflated cuff. The cuff is placed against a cooperating artery of the
patient and thereafter pressurized to different levels. Mean pressure is
determined by sweeping the cuff pressure and determining the mean cuff
pressure at the instant the peak amplitude occurs. Systolic and diastolic
pressure is determined by cuff pressure when the pressure oscillation is
at some predetermined ratio of peak amplitude.
The auscultatory method also involves inflation of a cuff placed around a
cooperating artery of the patient. Upon inflation of the cuff, the cuff is
permitted to deflate. Systolic pressure is indicated when Korotkoff sounds
begin to occur as the cuff is deflated. Diastolic pressure is indicated
when the Korotkoff sounds become muffled or disappear. The auscultatory
method can only be used to determine systolic and diastolic pressures.
Because both the oscillometric and the auscultatory methods require
inflation of a cuff, performing frequent measurements is difficult. The
frequency of measurement is limited by the time required to comfortably
inflate the cuff and the time required to deflate the cuff as measurements
are made. Because the cuff is inflated around a relatively large area
surrounding the artery, inflation and deflation of the cuff is
uncomfortable to the patient. As a result, the oscillometric and the
auscultatory methods are not suitable for long periods of repetitive use.
Both the oscillometric and auscultatory methods lack accuracy and
consistency for determining systolic and diastolic pressure values. The
oscillometric method applies an arbitrary ratio to determine systolic and
diastolic pressure values. Similarly, the auscultatory method requires a
judgment to be made as to when the Korotkoff sounds start and when they
stop. This detection is made when the Korotkoff sound is at its very
lowest. As a result, the auscultatory method is subject to inaccuracies
due to low signal-to-noise ratio.
The fourth method used to determine arterial blood pressure has been
tonometry. The tonometric method typically involves a transducer including
an array of pressure sensitive elements positioned over a superficial
artery. Hold down forces are 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 typically have at least one
dimension smaller than the lumen of the underlying artery in which blood
pressure is measured. The transducer is positioned such that at least 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 pressure measured
by the selected pressure sensitive element is dependent upon the hold down
pressure used to press the transducer against the skin of the patient.
These tonometric systems measure a reference pressure directly from the
wrist and correlate this with arterial pressure. However, if a patient
moves, recalibration of the tonometric system is required because the
system may experience a change in gains. Because the accuracy of these
tonometric systems depends upon the accurate positioning of the individual
pressure sensitive element over the underlying artery, placement of the
transducer is critical. Consequently, placement of the transducer with
these tonometric systems is time-consuming and prone to error.
The oscillometric, auscultatory and tonometric methods measure and detect
blood pressure by sensing force or displacement caused by blood pressure
pulses as the underlying artery is compressed or flattened. The blood
pressure is sensed by measuring forces exerted by blood pressure pulses in
a direction perpendicular to the underlying artery. However, with these
methods, the blood pressure pulse also exerts forces parallel to the
underlying artery as the blood pressure pulses cross the edges of the
sensor which is pressed against the skin overlying the underlying artery
of the patient. In particular, with the oscillometric and the auscultatory
methods, parallel forces are exerted on the edges or sides of the cuff.
With the tonometric method, parallel forces are exerted on the edges of
the transducer. These parallel forces exerted upon the sensor by the blood
pressure pulses create a pressure gradient across the pressure sensitive
elements. This uneven pressure gradient creates at least two different
pressures, one pressure at the edge of the pressure sensitive element and
a second pressure directly beneath the pressure sensitive element. As a
result, the oscillometric, auscultatory and tonometric methods produce
inaccurate and inconsistent blood pressure measurements.
SUMMARY OF THE INVENTION
A transducer having a sensing surface for sensing blood pressure within an
underlying artery of a patient includes a transducer, a sidewall, a
flexible diaphragm and a fluid coupling medium. The sidewall supports the
transducer above the underlying artery. The flexible diaphragm is spaced
from the sensing surface of the transducer. The fluid coupling medium is
coupled between the sensing surface of the transducer and the flexible
diaphragm and transmits blood pressure pulses within the underlying artery
from the flexible diaphragm to the sensing surface of the transducer. The
fluid coupling medium is isolated from the sidewall so that forces are not
transmitted from the sidewall through the fluid coupling medium to the
transducer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a blood pressure monitoring system having a
sensor assembly mounted to the wrist of a patient.
FIG. 2 is a block diagram of the blood pressure monitoring system of FIG.
1.
FIG. 3 is a cross-sectional view of the sensor assembly of FIG. 1 mounted
to the wrist of the patient.
FIG. 4 is a cross-sectional view of the sensor assembly of FIG. 1 having a
sensor.
FIG. 5 is a cross-sectional view of an alternate embodiment of the sensor
of FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a blood pressure monitoring system 10 for measuring and
displaying blood pressure within an underlying artery (not shown) within
wrist 12 of a patient. Monitoring system 10 includes wrist assembly 13,
sensor assembly 14, cable 15 and monitor 16.
Wrist assembly 13 includes sensor support 18 and strap 20. Sensor support
18 fits over wrist 12 above an underlying artery (not shown) and supports
sensor assembly 14 over the underlying artery. Sensor support 18 is
preferably rigid.
Strap 20 comprises a flexible band and is preferably made of nylon. Strap
20 latches to sensor support 18 and wraps around wrist 12 to maintain
sensor assembly 14 on wrist 12.
Sensor assembly 14 is electrically coupled to monitor 16 through cable 15
and generally includes motor assembly 22 and sensor 24. Motor assembly 22
is coupled to sensor support 18 and is mechanically coupled to sensor 24.
Motor assembly 22 applies a variable hold down pressure to sensor so that
blood pressure can be sensed and measured as varying hold down pressures
are applied to the underlying artery by sensor assembly 14.
Sensor 24 is coupled to motor assembly 22. When placed on wrist 12, sensor
24 is positioned over the underlying artery. Sensor 24 senses and measures
blood pressure pulses within the underlying artery.
Monitor 16 is coupled to motor assembly 22 and to sensor 24 by cable 15.
Monitor 16 includes control switches or various inputs 25a-25h, digital
displays 26a-26c, and display screen 28. Inputs 25a-25h control monitor
and permit monitor 16 to be calibrated. Inputs 25a-25c comprise hard keys
for controlling monitor 16. Inputs 25d-25h consist of software
programmable keys which are adaptable for various functions. Digital
displays 26a-26c continually display systolic, diastolic and mean blood
pressure, respectively. Display screen 28 displays the blood pressure
pulse, waveforms and prompts to guide the operator. Monitor 16 receives
the sensed blood pressure pulse signals taken by sensor 24 and calculates
the systolic, diastolic and mean blood pressures. Once these values are
determined, monitor 16 displays the corresponding values in both analog
and digital form. Monitor 16 also controls motor assembly 22.
In operation, sensor 24 is strapped to wrist 12 over the radial artery.
Motor assembly 22 moves sensor 24 to vary the pressure applied to wrist 12
above the radial artery. As this pressure is varied, an arterial pressure
waveform is sensed. An arterial pressure waveform or shape is obtained by
measuring amplitude of pressure versus time of an individual cardiac
cycle. The shape of the waveform is a function of the applied pressure and
is used by digital signal processing circuitry of monitor 16 to calculate
systolic, mean and diastolic pressure. The calculated pressures are
displayed by displays 26a-26c and display screen 28.
FIG. 2 shows a block diagram of blood pressure monitoring system 10. As
best shown by FIG. 2, monitor 16 further includes input signal processor
30, analog-to-digital converter 32, microprocessor 34, inputs 25a-25h,
motor drive 38, displays 26a-26c and 28, and power supply 42. In
operation, microprocessor 34 receives inputted signals from inputs
25a-25h. Inputs 25a-25h may also consist of a keyboard or other input
mechanisms. Inputs 25a-25h permit microprocessor 34 to be calibrated.
Microprocessor 34 controls motor drive 38 to vary hold down pressure
applied by motor assembly 22 on sensor 24. Hold down pressure is applied
to the anatomy of the patient directly above the artery by sensor 24. The
hold down pressure applied by motor assembly 22 on sensor 24 is increased
over time. As the force or hold down pressure applied by sensor 24
increases, the amplitude of the blood pressure pulse also increases until
a maximum amplitude results. Once the maximum amplitude or maximum energy
transfer results, the amplitude of the blood pressure pulse begins to
decrease as the artery begins to flatten out beyond the point of maximum
energy transfer.
Sensor 24 senses and detects the amplitude and shape of the blood pressure
pulses within the underlying artery. Sensor 24 creates electric sensor
signals representing the amplitude of the sensed blood pressure pulses.
The sensor signals are transmitted to input signal processor 30 of monitor
16. Input signal processor 30 processes the sensor signals and filters any
unwanted or undesirable noise and other effects. The sensor signals are
then transmitted from input signal processor 30 to analog-to-digital
convertor 32. Analog-to-digital convertor 32 converts the sensor signal
into digital form. A digital signal representing the amplitude of the
sensed blood pressure pulses is sent to microprocessor 34.
Based upon the digital sensor signals representing the sensed amplitude and
shape of the blood pressure pulses, microprocessor 34 determines wave
shape information by measuring amplitude and shape versus time of
individual cardiac cycles. The arterial wave shape information is
determined by sampling the arterial waves at a rate significantly above
heart rate so that a good definition of the arterial pressure wave is
measured. From this information, microprocessor 34 calculates systolic,
diastolic and mean blood pressures. When no pressure gradient exists
across the face of sensor 24, the hold down pressure corresponding to the
cardiac cycle having the peak pressure amplitude or the maximum energy
transfer is substantially equal to the mean arterial pressure. Based upon
the mean arterial pressure, microprocessor 34 calculates systolic and
diastolic blood pressure.
In the alternative, microprocessor 34 calculates blood pressure from the
relationship between the pressure amplitude of the individual cardiac
waveform and the applied hold down pressure of sensor 24. These results
may be derived from waveforms both before and after the waveform that has
maximum energy transfer.
In addition, microprocessor 34 may also calculate blood pressure from the
shape of individual cardiac waveforms. These results are based on the area
under part of the waveforms or they may be based on the shape of a rise
time on any number of parameters. The calculated blood pressures are
displayed on displays 26a-26c. Power supply 42 provides power to monitor
16 and motor assembly 22.
FIG. 3 is a cross-sectional view, taken along lines 3--3 of FIG. 1, showing
wrist assembly 13 and sensor assembly 14 placed upon wrist 12 of a patient
having an underlying artery 44. FIG. 3 shows sensor support 18 and strap
20 of wrist assembly 13 and sensor 24 of sensor assembly 14 in greater
detail. Sensor support 18 includes frame 46, spacer 48, latch 50,
stabilizing support 52 and screw 54. Frame 46 is a metal frame bent to
partially surround wrist 12. Frame 46 includes lower strap holes 56a, 56b
and adjustment slot 58. Strap holes 56a, 56b are located along the lower
end of frame 46. Strap hole 56b permits end loop 20a of strap 20 to be
secured to frame 46. Strap 20 is fed through strap hole 56a so that strap
20 is doubled back, and free end 20b is attached to latch 50. Adjustment
slot 58 extends upward from above strap hole 56b toward a top end of frame
46. Adjustment slot 58 permits stabilizing support 52 to be moved up and
down within slot 58 so that stabilizing support 52 may be adjusted for the
particular anatomy to which sensor assembly 14 is being secured. Frame 46
supports motor assembly 22 and sensor 24 above underlying artery 44 of
wrist 12. Frame 46 also supports strap 20 below wrist 12. As a result,
sensor 24 is held in a stable position with respect to wrist 12 while
blood pressure pulses are being sensed and measured.
Spacer 48 is mounted along a top horizontal portion of frame 46 and is
positioned between sensor assembly 14 and frame 46. Spacer 48 spaces
sensor assembly 14 from frame 46.
Latch 50 is fixedly coupled to frame 46 between strap hole 56b and
adjustment slot 58. Latch 50 releasably secures free end 20b of strap 20
to frame 46 so that wrist 12 is supported and positioned between frame 46,
sensor 24 and strap 20.
Stabilizing support 52 generally consists of a V-shaped bar having a first
leg 52a extending parallel to adjustment slot 58 of frame 46 and having a
second leg 52b extending over and above wrist 12. Stabilizing support 52
is slidably secured to frame 46 by screw 54. Screw 54 extends through
first leg 52a and adjustment slot 58. Screw 54 and slot 58 cooperate to
permit stabilizing support 52 to be vertically positioned with respect to
wrist 12. Thus, second leg 52b of stabilizing support 52 holds wrist 12
down against strap 20 to limit movement of wrist 12 while blood pressure
pulses within underlying artery 44 are being sensed and measured.
Strap 20 consists of an elongate flexible band. Strap 20 has a first end
looped through strap hole 56b and secured to itself to form end loop 20a.
Strap 20 has second free end 20b which is fed through strap hole 56a and
doubled back below wrist 12 to latch 50 where free end 20b is latched and
releasably secured to frame 46 by latch 50. Strap 20 supports wrist 12 in
position below frame 46 and sensor 24.
Also as best shown by FIG. 3, sensor 24 is coupled to cable 15 and includes
pivot block pin 62, pivot block 64, transducer 66, flange 68, sidewall 70,
restraining ring 72, diaphragm 74, pressure or fluid coupling medium 76
and fluid gel port 78. Pivot block pin 62 has a first end coupled to pivot
block 64 and a second end which is coupled to motor assembly 22. Pivot
block pin 62 couples sensor 24 to motor assembly 22.
Pivot block 64 receives the first end of pivot block pin 62. Pivot block 64
has a lower end which is coupled to transducer 66. Pivot block 64 couples
sensor 24 to motor assembly 22.
Transducer 66 is disc-shaped. Transducer 66 is coupled between pivot block
64 and flange 68. Transducer 66 contains a pressure-sensitive element such
as a piezoresistive sensor bridge (not shown) for sensing blood pressure
pulses within artery 44.
Flange 68 is annular and is slightly concave so that sensor 24 better
conforms to the anatomy or shape of wrist 12. Flange 68 is fixedly coupled
around an outer perimeter of transducer 66. Flange 68 supports side wall
70 and couples side wall 70 to transducer 66.
Side wall 70 is ring shaped and compressible, and is coupled to a lower
surface of flange 68. Side wall 70 is distant from transducer sensing
elements (not shown) of transducer 66, yet engages tissue surrounding
artery 44 to support transducer 66 above artery 44 and above tissue
surrounding artery 44. As a result, the exact positioning of transducer 66
over artery 44 is not required. At the same time, side wall 70 is not so
distant from transducer 66 so as to surround a large enough area of tissue
surrounding artery 44 to cause discomfort to the patient. Because sidewall
70 separates transducer 66 from the tissue surrounding artery 44, blood
pressure measurement errors caused by inadvertent patient movement are
lessened.
In addition, sidewall 70 creates a substantially zero pressure gradient
across sensor 24 so that sensor 24 more accurately measures blood
pressure. Sidewall 70 is constrained from expanding outward in a planar
direction away from the outer perimeter of transducer 66. Because side
wall 70 is compressible, side wall 70 dampens and absorbs forces or
pressure exerted by blood pressure pulses as the pulses cross side wall 70
along the perimeter or edge of sensor 24. Side wall 70 also applies force
to tissue surrounding artery 44. The force applied by side wall 70
substantially equals force exerted by the tissue surrounding artery 44 to
offset or neutralize the force exerted from the tissue. As a result, the
force applied by side wall 70 presses the tissue to a neutral position so
the pressure of artery 44 can be more accurately measured. The force of
side wall 70 that is applied to the tissue surrounding artery 44 is
coupled to flange 68. Flange 68 is coupled to transducer 66, but is not
coupled to transducer sensing elements (not shown) of transducer 66. Thus,
the force applied by side wall 70 which is used to press the tissue to a
neutral position is not sensed by the transducer sensing elements of
transducer 66. This neutralizing effect of sensor 24 allows blood pressure
monitoring system 10 to more accurately measure the arterial pressure of
artery 44 without inaccuracies introduced by forces from the surrounding
tissue. Consequently, side wall 70 reduces or eliminates uneven pressure
gradients within fluid coupling medium 76 across sensor 24 to create a
substantially zero pressure gradient across sensor 24. As a result, sensor
24 more consistently and more accurately measures blood pressure.
Preferably, side wall 70 is formed from closed cell foam. Alternatively,
side wall 70 may be formed from open cell foam or other compressible
materials or structural designs.
Because side wall 70 is compressible, side wall 70 better conforms to the
anatomy or shape of wrist 12. However, because side wall 70 is constrained
from expanding outward, side wall 70 does not stretch diaphragm 74 when
being pressed against wrist 12. By preventing tension across diaphragm 74,
sensor 24 further eliminates pressure gradients across transducer 66,
which results in more accurate and consistent blood pressure readings.
Restraining ring 72 normally consists of a flexible ring made of fiber or
other similar material. Restraining ring 72 encircles side wall 70 and
further prevents side wall 70 from expanding outward in a direction away
from the outer perimeter of transducer 66.
Diaphragm 74 is preferably formed from a thin flexible polymer or rubber.
Diaphragm 74 extends across side wall 70 to form chamber 79 in front of
transducer 66. Diaphragm 74 is preferably positioned across side wall 70
so as to be free of tension. Diaphragm 74 transmits blood pressure pulses
from a first side 80 to a second side 82 within chamber 79.
Fluid coupling medium 76 preferably is a gel, although fluid coupling
medium 76 may consist of any fluid or liquid capable of transmitting
pressure from diaphragm 74 to transducer 66. Fluid coupling medium 76 is
contained within chamber 79 between diaphragm 74, side wall 70, flange 68
and transducer 66. Fluid coupling medium 76 interfaces between diaphragm
74 and transducer 66 and transmits the blood pressure pulses from surface
82 of diaphragm 74 to transducer 66.
Sensor 24 continuously and accurately senses blood pressure pulses within
the underlying artery. Because sidewall 70 is compressible, sensor 24
dampens forces parallel to the underlying artery which are extended upon
sensor 24 by blood pressure pulses crossing beneath the edge of sensor 24.
In addition, sensor 24 better conforms to the anatomy of wrist 12. Because
sidewall 70 and diaphragm 74 are constrained from expanding outward and
are free of tension, pressure gradients across transducer 66 are
eliminated. Moreover, sensor 24 also neutralizes the tissue surrounding
artery 44. Consequently, more accurate and consistent blood pressure
readings are taken. Moreover, sidewall 70, diaphragm 74 and fluid coupling
medium 76 form a large sensing area through which blood pressure pulses
may be transmitted to transducer 66. As a result, sensor 24 is not as
dependent upon accurate positioning of transducer 66 over the underlying
artery. Sensor 24 quickly and accurately provides a continuous measurement
of blood pressure over long periods of use without discomfort to the
patient.
Fluid port 78 extends into flange 68 and communicates with chamber 79. Port
78 permits chamber 79 to be filled with fluid coupling medium 76.
Cable 15 electrically couples sensor 24 to the monitor 16. Cable 15
includes transducer leads 82, ground wire 84, connector 86, cable 88 and
clamp 90. Transducer leads 82 consist of wires electrically coupled to
transducer 66. Transducer leads 82 transmit signals representing the
sensed blood pressure pulses from transducer 66.
Grounding wire 84 consists of a wire having a grounding clip 92 at one end.
Grounding clip 92 mounts onto fluid port 78. An opposite end of grounding
wire 84 is electrically coupled to cable 88. Grounding wire 84
electrically grounds sensor assembly 24.
Connector 86 electrically couples transducer leads 82 to cable 88. Cable 88
has first end coupled to connector 88 and a second end which is coupled to
monitor 16 (shown in FIG. 1). Cable 88 permits sensor 24 to transmit
signals representing the sensed blood pressure pulses to monitor 16 where
the signals are measured. Clamp 90 couples cable 88 to frame 46 of sensor
support 18 and relieves strain within cable 88.
FIG. 4 is a cross-sectional view of sensor assembly 14 and sensor support
18 of wrist assembly 13 taken along lines 4--4 of FIG. 1. FIG. 4 shows
sensor 24 and motor assembly 22 in greater detail. Portions of sensor
support 18 are omitted for clarity. FIG. 4 best shows pivot block pin 62
and pivot block 64 of sensor 24. As shown in FIG. 4, upper end 62a of
pivot block pin 62 is bifurcated and is frictionally coupled to motor
assembly 22. Because pivot block pin 62 is bifurcated, the diameter of
pivot block pin 62 may be enlarged or reduced. As a result, pivot block
pin 62 and sensor 24 may be removed from motor assembly 22 without the use
of tools by compressing pivot block pin 62 to reduce its diameter. Pivot
block 64 includes a cavity for receiving the lower end 62b of pivot block
pin 62. Pivot block 64 rotates or pivots about lower end 62b of pivot
block pin 62 to permit accurate orientation of sensor 24.
As best shown by FIG. 4, transducer 66 is generally disc-shaped and
includes transducer holder 94 and transducer element 96. Transducer holder
94 includes central bore 98 extending into a lower end of transducer
holder 94. Transducer holder 94 has a top end which is coupled to pivot
block 64. The lower end of transducer holder 94 is fixedly secured to
flange 68. Transducer element 96 is supported and mounted within bore 98
of transducer holder 94.
Transducer element 96 is well known in the art and includes sensing surface
100. Sensing surface 100 is preferably sensitive to pressure changes
within chamber 79 transmitted through fluid coupling medium 76. Transducer
element 96 is positioned within bore 98 of transducer holder 94 so that
sensing surface 100 of transducer element 96 faces downward out of bore
98. Transducer element 96 preferably comprises a piezoresistive sensor
bridge. Transducer element 96 senses blood pressure pulses from an
underlying artery of a patient.
Motor assembly 22 presses sensor 24 against skin overlying the underlying
artery so that the amplitudes of blood pressure pulses may be sensed over
a range of various hold down pressures. As best shown by FIG. 4, motor
assembly 22 includes base plate 106, outer sleeve 108, upper and lower
outer races 110a and 110b, bearing balls 112a and 112b, upper and lower
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