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| United States Patent | 5494043 |
| Link to this page | http://www.wikipatents.com/5494043.html |
| Inventor(s) | O'Sullivan; Martin (Mountain View, CA);
Brendlen, Jr.; Lawrence W. (Sunnyvale, CA);
Dong; Donald Q. (San Jose, CA);
Moser; Jeffrey M. (Oakland, CA);
Mollenauer; Kenneth H. (Santa Clara, CA);
Kitlas; Kenneth C. (Fremont, CA);
Kaspari; William J. (Portola Valley, CA) |
| Abstract | A sensor that utilizes strips of piezoelectric material to noninvasively
measure the surface force/displacement resulting from a blood pressure
wave traveling through an artery and transmitted through the arterial wall
and overlying tissue, while canceling noise artifact signals is disclosed.
Piezoelectric elements create an electrical signal when pressure is
applied to their surface. In the preferred embodiment, the sensor is
constructed so that there are three sensing elements--a signal sensing
element in the center and one noise sensing element on each side of the
center element. The center element is placed over an individuals artery,
e.g., the radial artery in a persons wrist. When positioned this way, the
two noise sensing elements are positioned on each side of the artery.
The center element generates a signal that is a function of the pressure
wave in the artery, whereas this signal is highly attenuated in the noise
sensing elements. However, all three elements detect the noise artifact
signals in the general area of the sensor. The area of the noise elements,
when combined, is equal to that of the center element. This provides an
average of the noise detected by the center element.
The signals from the two noise sensing elements are subtracted from the
center element signal, thereby canceling the noise in the center element.
Also disclosed is a unique method that allows repositioning of the sensor
after it has been attached to the patient, a mechanism for maintaining the
appropriate hold down pressure, a wrist stabilization device for
stabilizing the wrist during monitoring of the blood pressure. The present
invention also allows blood pressure calibrations to be obtained using the
wrist and arm stabilizer assembly. |
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Title Information  |
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Drawing from US Patent 5494043 |
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Arterial sensor |
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| Publication Date |
February 27, 1996 |
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Title Information |
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Description |
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FIELD OF THE INVENTION
This invention relates generally to a noninvasive method of continuously
measuring an arterial pulse for use in monitoring arterial blood pressure
and/or pulse amplitude, and more particularly to a disposable sensor and
sensor attachment apparatus to be used for the measurement of the surface
force/displacement resulting from a blood pressure wave traveling through
an artery and transmitted through the arterial wall and overlying tissue.
BACKGROUND AND PRIOR ART
An accurate measurement of the pulsatile blood pressure in a person's
cardiovascular system is a necessary component in most medical procedures.
An individual's blood pressure can be represented in the form of an
arterial waveform representing the instantaneous pressure continuously.
Normally such a waveform can only be accurately measured with an
intraarterial catheter and pressure transducer. The catheter can
continuously monitor blood pressure, but suffers from the obvious drawback
of requiring surgical intervention and associated risks of complications.
Many devices allow blood pressure to be measured by sphygmomanometer
systems that utilize an inflatable cuff applied to a person's arm. The
cuff is inflated to a pressure level high enough to completely occlude a
major artery. When air is slowly released from the cuff, blood pressure
can be estimated by detecting well-known "Korotkoff" sounds using a
stethoscope or other detection means placed over the artery. A major
drawback with these devices is that they can only provide an intermittent
measure of blood pressure; and they do not provide a continuous arterial
pressure waveform.
Some relatively recent inventions have made possible noninvasive methods of
monitoring blood pressure. Some of these devices are also capable of
detecting the arterial waveform. One such method is presented in U.S. Pat.
No. 4,295,471 issued to William J. Kaspari. This patent discloses a sensor
which uses two piezoelectric elements placed in a housing, and facing
opposite directions. These piezoelements are acoustically and electrically
isolated from each other. The sensor is placed in a pocket at the distal
end of an inflatable cuff, which is then wrapped around a person's arm.
The device disclosed in the '471 patent monitors the Korotkoff sounds as
the pressure in the inflatable cuff is increased to a level just greater
than that necessary to occlude the artery. One piezoelectric sensor
element is placed directly over the artery and detects the Korotkoff
frequencies, as well as motion artifact signals produced by muscle
movement and/or other external factors. The second sensor element is
placed directly against the air bag. Because the air bag highly attenuates
the Korotkoff frequencies, the second sensor essentially only detects an
artifact signal, which is substantially equivalent to the artifact portion
of the signal detected by the first sensor element. The signal from the
second sensor element is subtracted from the signal from the first sensor
element, providing an arterial waveform with reduced common-mode artifact.
The '471 patent teaches detecting the arterial waveform in a noninvasive
manner that mitigates artifactual contamination of the signal of interest.
However, the device disclosed in the '471 patent is not intended to be
disposable, and does not lend itself to application at body sites such as
the radial artery, and generally requires a pressure cuff applied over the
sensor. All other devices which externally measure the arterial waveform
also suffer from the common problem of including a large artifact signal
along with the arterial signal.
SUMMARY OF THE INVENTION
The present invention seeks to overcome the problems associated with the
prior art by not only reducing the effects of artifact on the measured
arterial pulse signal, but also by providing a means to properly apply and
maintain the sensor over a person's artery, thus providing a more
accurate, repeatable, and complete waveform representing a person's
arterial pulse.
In the present invention, a strip of piezoelectric material is utilized to
detect the pressure wave in an artery from the body surface. Piezoelectric
elements create an electrical signal when a displacement force is applied
to them. The piezoelectric sensor is placed on the skin over a person's
artery, and measures the intra-arterial pressure wave through the arterial
wall, overlying tissue, and skin. The varying force generated by the
pulsatile blood pressure within the artery causes the piezoelectric strip
to experience a pulsatile tensioning, and to generate a charge
proportional to this tension. The piezoelectric strip acts as a charge
source, and a conventional charge amplifier can be used to amplify the
charge signal and yield a voltage waveform proportional to the charge.
However, muscle movement in the arm (or other body part) on which the
piezoelectric sensor is placed adds motion artifact to the pressure wave
signal. Additionally, artifactual signals are generated from other sources
such as vibration, external bumping of the patient, etc. It is desirable
to attenuate these artifacts in the signal. A preferred embodiment of the
invention includes 3 elements--a center element and 2 smaller elements on
each side of the center element. The center element is placed directly
over the artery. The smaller side elements are positioned off the sites
directly over the artery, and are minimally sensitive to the pressure wave
signal from the artery; but because of their proximity to the center
element they have substantially the same sensitivity to the artifact
signal as the center piezoelectric strip that is positioned over the
artery. The side elements essentially average the artifact signals
measured on each side of the pulse site in order to more closely
approximate the artifact at the pulse site. To do this the total
charge-generating area of the 2 side elements are the same as that of the
center element. The signal from the sum of the 2 side elements contains
substantially the same noise or artifact as the center element but
contains minimal arterial signal. The signals from the side elements are
subtracted from the signal from the center element to substantially cancel
the noise or artifact, while maintaining the arterial signal.
The basic piezoelectric material is a polymer film, typically specially
processed polyvinylidene fluoride (PVDF) that exhibits the property of
creating an electrical charge when tension is applied to the film. The
term "piezofilm" will be used to indicate piezoelectric film.
In the preferred embodiment, a single strip of piezofilm is used to form
the sensing elements. A support mechanism creates three parallel channels
that are bridged by the single piezofilm strip. The strip is securely
attached to the edge of each channel and is thus divided into three
distinct portions forming the 3 sensing elements. The center element is
formed from the center of the piezofilm, and is typically long enough to
span the artery. The remaining two portions form the side elements that
are used to measure the signal while sensing only minimal arterial pulse
signal. The piezofilm strip dimensions are such that it is easily
deflected by each pulsation of the artery, and accurately reproduces the
frequency components of the pressure waveform as measured on the skin
surface. An area of the film that lies within each of these three
channels, on the side of the film away from the skin, is coated with a
silver filled ink to form an electrode for each element. The opposite side
of the film (the side that contacts the skin), is also coated with the
same material to form the second electrode for each of the elements. The
skin-side contact is common to all elements, thereby removing any unwanted
(common-mode) electrical signals picked up from the patients skin, such as
electromagnetic interference.
The individual element within each channel is coated with an insulating
material following application of the conductive coating to prevent
electrical shorting of these elements due to moisture or other conductive
material.
In the preferred embodiment, the signals from each element are amplified by
a separate charge amplifier, and electronic means are provided to process
these signals to provide other signals used for correctly positioning the
sensor over the artery. In an alternate embodiment, 2 strips of piezofilm
are used to form the noise-canceling subtraction of the outside elements
signals from the center element signal. This yields a single signal to
amplify, and precludes generation of the signals for positioning.
In order to accurately detect the pressure waveform, and to maintain
detection of the waveform in an undistorted manner, the sensor must be
appropriately applied to the site to be monitored. In addition, an
appropriate hold-down pressure which is an external force applied to the
sensor to hold it securely in position, must be determined and maintained.
To optimize the signal and artifact pick-up in each of the elements of the
sensor, it is desirable to provide uniform coupling between each element
and the site to which it is applied. The hold-down mechanism is designed
to provide substantially uniform pressures on the elements, perpendicular
to the surface of the skin.
Since the site initially chosen for application of the sensor is the radial
artery, the sensor is designed to easily access the artery without
requiring excessive pressure. Excessive pressure may partially or
completely occlude the artery. Furthermore, once the required access is
achieved, it is necessary to maintain the sensor in position.
In one embodiment of the three element sensor, each of the two noise
elements is mounted on an independent frame and is attached to the center
element in such a way that each of the outer segments is free to seek its
own angle with respect to the center segment to conform to the contour of
each individual wrist. The frame is attached to the frame adjacent to it
by means of a hinge. Hinge material can be a thin flexible plastic, a
suitable adhesive backed tape, a hinge molded into the frame structure, or
other means. The piezofilm is attached to the frame using suitable means.
To apply the appropriate hold-down pressure, an inflatable air compartment
or bubble is placed between the sensor frame opposite the film side and a
backing plate assembly that is attached directly or indirectly to the side
of the wrist opposite the sensor site to yield a sensor hold-down
mechanism. The inflation of the bubble forces the elements to conform to
the patient's wrist, while maintaining uniform and constant hold down
forces on the elements substantially perpendicular to the skin.
The site being monitored (e.g., the radial artery) is maintained in a fixed
position in order to minimize artifact and to maintain sensor positioning.
More specifically, wrist rotation and finger flexing is restricted. Means
for maintaining the proper wrist position is described.
Another feature of the present invention is that it can contain a means of
obtaining calibration and/or verification measurements of blood pressure
via an occlusive cuff that is built into the wrist stabilization device,
and can also utilize the disposable noise-canceling sensor for detection
of the Korotkoff sounds.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view of a preferred embodiment of the arterial
sensor of the present invention;
FIG. 1B is a side view of the embodiment shown in FIG. 1A, showing the
sensor in a flexed position with an air bubble inflated.
FIG. 1C is an edge view of the piezoelectric film shown in FIG. 1A and 1B.
FIGS. 1D and 1E are bottom and top views, respectively, showing the
piezoelectric film and conductive coatings.
FIG. 2A is an embodiment of the sensor of the present invention with a
rigid framework.
FIG. 2B is an embodiment of the sensor of the present invention with a
rigid framework and end rails for defining the contour of the elements in
configurations different than in FIG. 2A.
FIG. 3 shows a schematic diagram of the positioning of the sensor of FIG. 1
over a radial artery.
FIG. 4 is a diagram showing the apparatus of FIG. 1A mounted to an adhesive
strip that permits attachment to the skin and repositioning of the sensor.
FIG. 5 is a drawing of the wrist stabilizer of the present invention that
contains the sensor hold-down air bubble and the cuff assembly for
obtaining calibration readings.
FIG. 6 is a drawing of the position indicator with air bubble and cuff
inflate/deflate controls.
FIG. 7A is a perspective view of the sensor of the present invention with
an alternate hold-down means.
FIG. 7B is an embodiment of the sensor assembly shown in FIG. 7A, wherein
the sensor portion is disposable.
FIG. 8 is a block diagram of the sensor amplifier circuit.
FIGS. 9 and 10 are amplifier circuits for use with the described sensors.
FIG. 11 shows the piezoelectric film assembly for an embodiment of the
sensor that directly provides a noise-canceling signal from the sensor.
FIG. 12 shows an embodiment of the present invention whereby the charges
produced by separate segments of the sensor are input to separate
preamplifiers.
DETAILED DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS
FIG. 1A shows a perspective view of the preferred embodiment of a sensor 10
according to the present invention. The sensor 10 consists of a strip of
piezoelectric material 12 which is used to measure the force or
displacement resulting from a pressure wave created as blood is forced
through an artery. The piezofilm strip 12 is attached to support member
14, which forms a center channel 16A and two outer channels 16B and 16C as
shown in FIG. 1B. The support member 14 may be constructed of rigid
plastic or similar material and its primary purposes are to create the
channels 16 (A-C) which are bridged by piezofilm strip 12 and to isolate
the forces on each of the three sensing elements. In creating three
channels 16 (A-C), the support member 14 segments the piezofilm strip 12
into three sections 18 (A-C) respectively shown in FIG. 1E, a center
section and two outer sections, which are the three sensing elements. The
piezofilm is attached to the edges of each channel by, for example, glue,
and is aligned perpendicular to the channels, such that tensions developed
in the film are developed along its axis of greatest sensitivity.
In the preferred embodiment, support member 14 contains two rails (14A and
14B), which define the center channel. These rails serve to provide means
of attaching film 12, thereby creating the three independent sensing
regions, and these rails contain a groove that forms a hinge that permits
each of the outside elements to flex with respect to the center element.
They therefore act as "living" hinges, and provide the capability of the
sensor to conform to various shapes. Additional rails can be provided at
each end of the support member 14 for added film-attachment area and may
prevent the film from touching the base of the member when excessive force
is applied to the film.
The width and thickness of piezofilm strip 12 are also relevant to the
strip's ability to detect and accurately reproduce the arterial waveform.
Specifically, dimensions are selected so as to accurately reproduce all of
the frequency components contained in the blood pressure waveform over a
wide range of blood flow velocities, while also producing a sufficiently
large charge output. In the preferred embodiment, the strip 12 on the
bottom of the sensor (facing the skin) has the approximate dimension of 5
mm.times.20 mm with the sections 18B and 18C each being 5 mm.times.5 mm
and section 18A being 5 mm.times.10 mm. As shown in FIGS. 1D and 1E, the
piezofilm strip 12 has two sides: first side 12A, which faces outward from
the sensor 10 and is positioned against the tissue of a patient, and a
second side 12B, which faces the channels 16A, 16B and 16C. A first
conductive coating 20A is applied to the first side 12A of the piezofilm
strip 12. The conductive coating 20A can be of a conductive material such
as silver-filled ink. A first electrical contact 22A provides a means of
connection for the detection of a first signal from the first conductive
coating 20A.
On the second side 12B of the piezofilm strip 12 are a plurality of
conductive coatings 24A, B and C. Each of the conductive coatings 24 (A-C)
lies within the respective sections 18 (A-C) of the piezofilm strip 12,
forming the three sensing elements. These conductive coatings collect the
charge generated by the piezoelectric film between the coating on opposite
sides. No charge is collected from the film that does not have conductive
coatings on both sides, thereby providing the means to electrically
isolate the charges generated by the 3 sensing elements of the single
strip of piezoelectric film from each other. An electrical contact 26
(A-C) is connected to each of the conductive coatings 24 (A-C), and
provides an output signal therefrom referenced to the common conductive
coating on the opposite side of the film. In the preferred embodiment, the
dimensions of conductive coatings 24 (A-C) are as follows: 24B and C are
each approximately 4 mm.times.4 mm and 24A is approximately 4 mm.times.8
mm. Thus, the area of the conductive coating 24A in the center section 18A
is approximately equal to the sum of the areas of conductive coatings 24B
and 24C. An insulating layer 28 covers the conductive coatings 24 (A-C).
The support member 14 comprises two rails 14A and 14B. The piezofilm strip
12 is mounted on the supporting member 14 such that rails 14A and 14B
support a piezofilm strip 12 at a position between adjacent sections
16A-16B and 16A-16C. The ends of piezofilm strip 12 are attached to each
end of support member 14.
In the preferred embodiment, a pneumatically activatable balloon 30 as
shown in FIG. 1B is positioned on the other side of support 14. When the
balloon 30 is inflated via air tube 32, which is connected to a
pressurized air supply at the other end, it causes the support 14 to press
and hold piezofilm strip 12 against the tissue.
An additional key feature of the preferred embodiment is that it causes the
center element of the sensor, once properly applied to the wrist, to
maintain a position approximately centered over the artery.
FIG. 2A shows an embodiment of the sensor 10 with a rigid support member.
FIG. 2B is an embodiment that also contains a rigid support member on each
end, to which the piezofilm is attached, in addition to the rails
separating the center and outer channels. The relative height of the outer
rails with respect to the inner rails can be used to provide for various
sensor contours by varying the angle of the outer elements with respect to
the center element.
As shown in FIG. 3, radial artery 34 lies between two rigid members in the
wrist, tendon 36 located toward the center of the wrist and bone 38
located at the outside (thumbside) of the wrist. The radial artery in
humans lies within the channel created by these two members, and is
protected by them. The sensor configuration shown in FIG. 1A and its
dimensions are designed to allow the center to seat itself within this
region. The dimensions are such that the two rails 14A and 14B fit between
the tendon and the bone, thereby maintaining the center element over the
artery.
Center cavity 16A is placed directly over the artery. The side of the
piezofilm strip which comes in contact with the skin is coated with a
conductive material 20A and is connected to ground. This is necessary
because extraneous electrical signals may be present at the skin's surface
and may add noise to the pulse signal. There are also conductive coatings
24 (A-C) that form each of the three positive electrodes on the side of
the piezofilm strip facing away from the skin. These conductive coatings
collect the charge from each element, and provide means of electrical
contact to the amplifier circuitry. Charges will only flow from the
portions of the piezofilm strip coated with conductive material on both
sides.
A short circuit between the two layers of conductive coating on opposite
sides of the piezofilm would eliminate the pulse signal. A short circuit
between two conductive coating sections on the same side of the film would
reduce the noise-canceling capability of the sensor. Such electrical
shorts can be created by moisture on a patient's skin, adhesive material
left from certain medical adhesives, and other means. To prevent shorts,
the piezofilm is coated with an insulating material 28 over the conductive
coatings on the ungrounded side of the sensor. The grounded side is the
common electrode side that contacts the skin, and is left uncoated to
allow electrical contact and grounding to the patient.
Piezofilm strip center portion 24A senses artifact signals mixed with the
arterial pressure wave signal. This artifact signal may be created by
muscle movement or other external forces and becomes part of the signal
measured by the piezofilm strip and sent as a first output signal to the
amplifier circuit. Muscle movement and other sources of artifact are also
measured by outer element piezofilm strip portions, but the pressure wave
signal is minimal. These signals are sent as a second and third output
signal to the amplifier circuit in the preferred embodiment.
FIG. 8 shows a block diagram of the general operation of the electronic
signal processing circuitry. Before describing all of the elements in the
circuit shown in FIG. 9 or 10 in detail, a brief description of the
general operating method of the pulse sensing device of FIG. 1 may be
useful.
The portion of the piezoelectric strip 12 spanning the center channel 16A
acts as a charge source when tension is applied to it. The charge source
generates a charge proportional to the displacement of the underlying
tissue which in turn is a function of the underlying pressure waveform
being measured. The charge signal also includes charges resulting from
other causes such as movement of the arm to which the pulse sensing device
is attached. These signals add artifact to the pressure wave signal. The
portions of the piezoelectric strip placed over the two outer channels
generate a charge proportional to the artifact just described. In the
preferred embodiment, the signals from charge sources 24 (B+C) are
amplified and converted to a voltage signal by amplifier stages before
being sent to a differential amplifier, where they are subtracted from the
signal generated by charge source 24 (A), which is amplified by another
amplifier stage. The output from differential amplifier stage may then be
sent through a filter stage to filter out unwanted high and low
frequencies, with the resulting signal being a waveform that is a function
of the pressure waveform inside the artery of the person being measured.
FIGS. 9 and 10 show two versions of amplifier circuit for use with the
pulse measuring device as embodied by FIG. 1. In the preferred embodiment
of the present invention, the circuit is external to device and may be
connected to it as shown in FIG. 1 through collection of wires and
connector(s). In an alternate embodiment of the present invention, part of
the circuit may be placed on a circuit board that is on the top of support
member 14.
The portion of piezofilm strip 12 located over center cavity 16A is
represented as charge source QA in FIGS. 9 and 10. The portions to each
side are represented as charge sources QB & QC. Charge source QA is
connected to the input of preamplifier A. Charge sources QB & QC are
connected in parallel to sum the charges, and connected to the input of
preamplifier (B+C). Various types of preamplifiers can be used. Two common
types of amplifier circuit are shown in FIG. 9 and FIG. 10. The type in
FIG. 9 uses an input amplifier commonly known as a charge amplifier. It
produces a voltage output proportional to the charge applied to the
summing node of the operational amplifier. The charge generated by the
piezoelectric film, when connected to the low impedance input of the
charge amplifier, is proportional to the change in tension and therefore
the change in the forces are measured. This charge needs to be
substantially integrated to yield a signal proportional to film tension,
which is essentially accomplished by the charge amp. The decay time of the
charge and hence the time constant of the circuit is set by the parallel
combination of R.sub.F and C.sub.F. This RC time constant sets the
low-frequency cutoff of its respective sensor element. In the preferred
embodiment, the RC time constant is approximately 2 sec. The high
frequency cutoff at this point of the circuit is set mainly by the
mechanical properties of the sensor. Capacitor C.sub.I is a DC blocking
capacitor and is optional. It may be necessary where stray, or parasitic
DC or low frequency current sources occur in the film side of the circuit.
Its value is selected to be much larger than the sensor element
capacitance so not to substantially change the frequency response of the
system.
The input amplifier circuit shown in FIG. 10 is a high input impedance
amplifier with a low output impedance. It may have a voltage gain
(Vout/Vin) of less than one. It will have a current gain (Iout/Iin) of
greater than one. A single field-effect transistor (FET) can be used as
the front-end amplifier. This type of amplifier is commonly called a
buffer. The charge generated by the piezofilm charge source is caused to
flow as a current through resistor R.sub.L, yielding a voltage drop across
it (Vq=Iq R.sub.L). This voltage is applied to the input of the buffer.
The time constant of the circuit, and hence the low-frequency cutoff, is
set by the RC network formed by the combination of the film capacitance
and R.sub.L. The high frequency response, as before, is mostly a function
of the mechanical properties of the sensor.
The outputs of preamplifiers go to a differential amplifier, which
subtracts the output of preamplifier (B+C) from the output of preamplifier
A. The output of the differential amplifier represents the pulse signal
with suppressed or canceled artifact, and is sent to a bandpass filter for
attenuation of unwanted high and low frequencies.
FIG. 5 is a drawing of wrist stabilizer 50 containing a hold-down bubble
30, attached to a hold-down strap 52. Wrist stabilizer 50 is designed to
maintain a wrist at an angle of flexion that yields a large amplitude of
the sensed signal. It is also designed to limit the degree of wrist
rotation and finger movement.
Pressurized air is supplied by means of air tube 32 to hold-down bubble 30,
and the bubble is inflated after the sensor and bubble are positioned
correctly. During this inflation process, the signal from sensor 10 is
monitored, and a calculation is made by a separate electronic device to
determine the optimum hold-down pressure for each individual.
U.S. Pat. No. 4,799,491 by Eckerle describes a method for determining a
hold-down pressure by fitting a polynomial to "a set of at least one of
the diastolic pressure, systolic pressure and pulse amplitude versus
hold-down pressure values over a range of hold-down pressures between
which the underlying artery is unflattened and it is occluded."
The present invention differs from this technique in three very significant
ways:
1. As the hold-down pressure is varied from a low to a high value, the
waveform from the underlying sensor is analyzed, to determine a range of
hold-down pressures over which the morphology of the pressure waveform
does not change.
2. Simultaneously, a measure of the signal to noise ratio, as determined by
the ratio of the sensor output signal to the noise signal in the outside
elements of the sensor (previously described), is determined as a function
of hold-down pressure.
3. The hold-down pressures are not selected to provide flattening of the
artery.
In one embodiment, the hold-down pressure is determined as the minimum
pressure at which the signal waveform maintains its shape (i.e., the
morphology of the waveform does not change), and the signal to noise ratio
is maximum.
Alternatively, the criteria for determining the hold down pressure may be
that pressure which provides the maximum amplitude undistorted signal.
The optimum hold down pressure is then maintained automatically by
independent pressure servo means (not shown).
Protective cover 56 is attached to wrist stabilizer 50 over sensor 10 to
minimize contact by persons or objects that may cause movement of these
items during use, e.g. in an operating room or intensive care unit of a
hospital.
Cuff 58 provides means of occluding the radial artery during determination
of a calibration or confirmation pressure reading. During the process of
deflating cuff 58, sensor 10 of FIG. 1 may be monitored to determine the
cuff pressures at which Korotkoff sounds begin and end, in order to
determine systolic and diastolic pressures. For decreasing cuff pressure,
the Korotkoff sounds begin at the point where cuff pressure is just below
the peak intra-arterial pressure (systolic pressure) and end at the point
where cuff pressure is just below the minimum intra-arterial pressure
(diastolic pressure).
An additional feature of this embodiment as shown in FIG. 5 is the addition
of the forearm calibration scale on the cuff. When taking blood pressure
with an occlusive cuff, the degree of accuracy obtained is related to the
ratio of cuff width to arm circumference. A typical value for this ratio
is approximately 0.4. The smaller this ratio becomes, the more the
measurement tends to err toward the high pressure side, and vice versa.
The numbers shown printed on the cuff in FIG. 5 provide a means of
determining a relative measure of the circumference of the arm around
which it is wrapped. For example, if the arrow is pointing at number 2, it
indicates a much larger forearm than if it were pointing at number 7.
Various means can be used to utilize this information. In the preferred
embodiment, the indicated Arm Circumference Number is manually input to
the cuff controller uni | | |
