U.S. patent application number 09/846673 was filed with the patent office on 2002-11-07 for pulse sensors.
Invention is credited to Dupelle, Michael R., Prew, Paul F., White, Sheldon S..
Application Number | 20020165585 09/846673 |
Document ID | / |
Family ID | 25298611 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020165585 |
Kind Code |
A1 |
Dupelle, Michael R. ; et
al. |
November 7, 2002 |
Pulse sensors
Abstract
External defibrillators are provided that include a pulse
sensor. These defibrillators may be used to treat a patient showing
signs of possible cardiac arrest. ?
Inventors: |
Dupelle, Michael R.; (N.
Attleboro, MA) ; Prew, Paul F.; (South Attleboro,
MA) ; White, Sheldon S.; (Brookline, MA) |
Correspondence
Address: |
G. ROGER LEE
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Family ID: |
25298611 |
Appl. No.: |
09/846673 |
Filed: |
May 1, 2001 |
Current U.S.
Class: |
607/5 |
Current CPC
Class: |
A61N 1/39044 20170801;
A61B 5/6822 20130101; A61N 1/3904 20170801 |
Class at
Publication: |
607/5 |
International
Class: |
A61N 001/39 |
Claims
What is claimed is:
1. An external defibrillator comprising defibrillator electrodes,
and a piezoelectric polymer pulse sensor.
2. The external defibrillator of claim 1 further comprising
instrumentation for performing an ECG analysis.
3. The external defibrillator of claim 1 further comprising
instrumentation for analyzing a signal obtained from the pulse
sensor.
4. The external defibrillator of claim 1 wherein said pulse sensor
is self-shielded.
5. The external defibrillator of claim 1 further comprising a strap
for attaching said pulse sensor to a patient's neck.
6. The external defibrillator of claim 1 wherein said piezoelectric
pulse sensor is mounted on one of said defibrillator
electrodes.
7. The external defibrillator of claim 1 further comprising a
display constructed to display information to a user.
8. The external defibrillator of claim 7 further comprising
instrumentation for performing an ECG analysis, instrumentation for
analyzing a signal obtained from the pulse sensor, and
instrumentation for converting the results of the ECG analysis and
signal analysis into a message to be displayed to the user or
provided as an auditory prompt.
9. A medical device comprising a piezoelectric polymer pulse
sensor; and a strap constructed to allow the pulse sensor to be
attached to a patient's neck.
10. The medical device of claim 9 wherein the pulse sensor is
self-shielded.
11. The medical device of claim 9 wherein the strap comprises an
elastic material.
12. The medical device of claim 9 further comprising a cable to
connect the pulse sensor to instrumentation.
13. A medical device comprising a piezoelectric polymer pulse
sensor, and a foam pad having a first surface to which the pulse
sensor is attached, and a second surface constructed to be attached
to a patient.
14. The medical device of claim 13 wherein the second surface
includes a layer of pressure-sensitive adhesive.
15. A method of treating a patient showing signs of possible
cardiac arrest comprising: applying a piezoelectric pulse sensor to
the patient; applying electrodes of a defibrillator to the patient;
and using the pulse sensor to detect whether the patient has a
pulse.
16. The method of claim 15 further comprising monitoring the pulse
if present.
17. The method of claim 15 wherein the defibrillator has an ECG
function and the method further comprises using the ECG function of
the defibrillator to monitor the patient's heart rhythm.
18. The method of claim 15 further comprising analyzing the pulse
and heart rhythm to determine the appropriate treatment for the
patient.
19. The method of claim 18 wherein the analyzing step includes
determining whether the patient's pulse, if present, is correlated
with the R-wave of the patient's heart rhythm.
20. The method of claim 19 wherein, if the determination is
positive, no ECG analysis is performed.
21. The method of claim 18 wherein the analyzing step includes
determining whether the ECG rhythm is treatable with
defibrillation.
22. The method of claim 21 further comprising, if the determination
is positive, delivering a shock to the patient using the
defibrillator.
23. The method of claim 22 further comprising delivering a
predetermined number of shocks to the patient, and then
subsequently determining whether the patient's pulse, if present,
is correlated with the R-wave of the patient's heart rhythm.
24. The method of claim 23 further comprising, if the subsequent
determination is negative, administering CPR to the patient.
25. The method of claim 24 further comprising using the pulse
sensor to determine the efficacy of the CPR treatment.
26. The method of claim 15 wherein the pulse sensor comprises a
piezoelectric polymer film.
27. The method of claim 15 wherein the pulse sensor is mounted on
an elastic strap.
28. The method of claim 27 further comprising attaching the elastic
strap around the patient's neck.
29. The method of claim 15 wherein the pulse sensor is mounted on
one of the electrodes of the defibrillator.
30. The method of claim 15 wherein the pulse sensor further
comprises a foam layer.
31. The method of claim 15 wherein the pulse sensor is
self-shielded.
32. The method of claim 15 further comprising attaching the pulse
sensor to a patient using a clip, patch or suction device.
33. The method of claim 32 wherein the pulse sensor is attached to
the patient's neck.
34. The method of claim 32 wherein the pulse sensor is attached to
a pulse point other than on the patient's neck.
35. A method of treating a patient showing signs of possible
cardiac arrest comprising: applying a piezoelectric pulse sensor to
the patient; using the pulse sensor to detect whether the patient
has a pulse; and using the pulse sensor to determine whether to
apply electrodes of a defibrillator to the patient.
36. A method of treating a patient showing signs of possible
cardiac arrest comprising: applying a piezoelectric pulse sensor to
the patient; using the pulse sensor to detect whether the patient
has a pulse; and using the pulse sensor to determine whether to
perform CPR on the patient.
37. The external defibrillator of claim 1 further comprising a
clip, patch or suction device constructed to attach the pulse
sensor to a patient.
38. The external defibrillator of claim 1 wherein said external
defibrillator comprises an automated defibrillator.
Description
TECHNICAL FIELD
[0001] This invention relates to pulse sensors and methods of using
pulse sensors in conjunction with defibrillators.
BACKGROUND
[0002] The pulse is a very important parameter that is used to aid
users of automated external defibrillators in determining whether
or not to administer a defibrillation shock to and/or to perform
cardiopulmonary resuscitation (CPR) on a victim who appears to be
in cardiac arrest. Such a victim may actually be in need of cardiac
resuscitation (including defibrillation and/or CPR), or may be
suffering from a condition for which such treatment would be
unsuitable, e.g., a stroke, seizure, diabetic coma, or heat
exhaustion. It is very important to the safety of the victim that
the presence or absence of a pulse be determined quickly and
accurately. However, it is often difficult for trained medical
personnel to take a victim's pulse accurately in the field during a
crisis situation, and may be impossible for a minimally trained or
untrained lay rescuer to do so. In many cases, it will take the
person assisting the victim a considerable time (on the order of
one minute or more) to find the victim's pulse. If a pulse is not
found, the caregiver is left unsure as to whether the victim does
not have a pulse, or whether the caregiver simply cannot find the
victim's pulse.
[0003] Another parameter that is used in determining whether to
administer a defibrillation shock is an ECG analysis of the
victim's heart rhythm that is provided by the automated external
defibrillator. Based on the ECG analysis, many automated
defibrillators will provide the user with a message indicating
whether a shock should be administered (i.e., whether or not
ventricular fibrillation is present).
[0004] Generally, the ECG analysis systems in most commercially
available automated external defibrillators display only two
options to the user: "Shock Advised" or "No Shock Advised." When
"Shock Advised" is output, this means that the patient is in
ventricular fibrillation or wide complex ventricular tachycardia
above 150 BPM, conditions which are effectively treated by
defibrillation. When "No Shock Advised" is output, this means that
the patient's heart rhythm is not treatable by defibrillation
therapy.
[0005] If the message indicates that a shock is not appropriate,
this does not necessarily mean that the victim is not in danger.
There are two ECG rhythms, generally referred to as asystole and
pulseless electrical activity, which should not be treated with
defibrillation (and thus will trigger a message not to shock) but
nonetheless are extremely serious in that they suggest that the
patient's heart rhythm has deteriorated beyond fibrillation (i.e.,
the patient is close to death). These conditions are treated by
administering cardiopulmonary resuscitation (CPR), in an effort to
provide blood flow to the heart and vital organs in the hope that
with improved blood flow and oxygenation, the heart muscle will
recover from its near death state and possibly begin to fibrillate
again, thus making defibrillation treatment a viable option.
[0006] Thus, when a "No Shock Advised" analysis is output, the
caregiver does not know whether this result is caused by a normal
heart rhythm, an abnormal but perfusing heart rhythm (i.e., the
patient was never in cardiac arrest or the last shock treatment
returned the patient's heart rhythm to normal), or a grossly
abnormal (non-perfusing) ECG rhythm requiring CPR treatment.
Because of this uncertainty, the normal medical protocol when "No
Shock Advised" is output is to check the patient for a pulse and if
no pulse is detected to start CPR. If a pulse is detected, then the
patient's heart is effectively pumping blood and neither CPR nor
defibrillation is warranted. If the victim does not have a pulse,
CPR should be started immediately; if a pulse is present, then CPR
should not be administered. Because CPR, even if properly
administered, can result in broken ribs or other injury to the
victim, it is undesirable to administer CPR if it is not actually
necessary. Thus, it is again vitally important that an accurate
determination of the presence or absence of a pulse be made by the
care giver.
[0007] A similar situation of uncertainty occurs after the third
defibrillation shock is delivered in the three-shock protocol
recommended by the American Heart Association. In this case, if the
patient's fibrillation has not been "cured" after delivery of three
shocks, the caregiver is instructed to perform CPR on the patient.
Because automated external defibrillators generally do not perform
an ECG analysis immediately after the third shock, the caregiver
does not know whether the third shock provided effective treatment.
Therefore, the caregiver must determine whether the patient has a
pulse in order to determine whether CPR is needed or whether the
patient is out of danger.
SUMMARY
[0008] The inventors have found that pulse sensors fabricated from
piezoelectric polymer films, when used in conjunction with
automated external defibrillator machines, enable users of such
machines to make a quick and accurate determination of the
appropriate treatment (defibrillation, CPR, or no cardiac-related
treatment) for a victim who appears to be suffering from cardiac
arrest. Such sensors provide an accurate determination of the
presence or absence a victim's pulse, even under adverse
conditions, thus significantly reducing the risk that an
inappropriate and even dangerous treatment will be given
erroneously to a victim. The accurate pulse determination thus
provided relieves the uncertainty experienced by caregivers in the
circumstances discussed above, and thus increases the likelihood of
the patient receiving prompt, safe and effective treatment.
[0009] The inventors have also found that piezoelectric polymer
pulse sensors can be used to determine whether CPR is necessary, in
the event that an automated defibrillator indicates that it is not
appropriate to shock a victim who appears to be suffering from
cardiac arrest.
[0010] The invention also features methods of using piezoelectric
polymer pulse sensors to measure the efficacy of CPR, when CPR is
used in conjunction with or instead of defibrillation.
[0011] The piezoelectric sensors of the invention, when applied to
areas of a human body where mechanical movement is present because
of blood flow, will produce a significant electrical signal that is
proportional to and representative of changes in blood flow
activity. This will be the case whether the blood flow is due to
normal or abnormal heartbeat action, or due to cardiopulmonary
resuscitation action. The sensor will operate in either a flexural
or a tension mode, and the produced electrical signal from the
sensor may be utilized as either a voltage (high impedance) or a
current (low impedance).
[0012] In one aspect, the invention features an automated
defibrillator comprising defibrillator electrodes and a
piezoelectric polymer pulse sensor.
[0013] Implementations may include one or more of the following
features. The automated defibrillator further includes
instrumentation for performing an ECG analysis. The automated
defibrillator further includes instrumentation for analyzing a
signal obtained from the pulse sensor. The pulse sensor is
self-shielded. The automated defibrillator further includes a strap
for attaching said pulse sensor to a patient's neck. The
piezoelectric pulse sensor is mounted on one of said defibrillator
electrodes. The automated defibrillator further includes a display
constructed to display information to a user. The automated
defibrillator further includes instrumentation for converting the
results of the ECG analysis and signal analysis into a message to
be displayed to the user or provided as an auditory prompt.
[0014] In another aspect, the invention features a medical device
including a piezoelectric polymer pulse sensor; and a strap
constructed to allow the pulse sensor to be attached to a patient's
neck.
[0015] Implementations may include one or more of the following
features. The pulse sensor is self-shielded. The strap includes an
elastic material. The device further includes a cable to connect
the pulse sensor to instrumentation.
[0016] In a further aspect, the invention features a medical device
including a piezoelectric polymer pulse sensor, and a foam pad
having a first surface to which the pulse sensor is attached, and a
second surface constructed to be attached to a patient.
[0017] In another aspect, the invention features a method of
treating a patient showing signs of possible cardiac arrest
including applying a piezoelectric pulse sensor to the patient;
applying electrodes of a defibrillator to the patient; and using
the pulse sensor to detect whether the patient has a pulse.
[0018] Implementations may include one or more of the following
features. The method includes monitoring the pulse if present. The
defibrillator has an ECG function and the method further includes
using the ECG function of the defibrillator to monitor the
patient's heart rhythm. The method further includes analyzing the
pulse and heart rhythm to determine the appropriate treatment for
the patient. The analyzing step includes determining whether the
patient's pulse, if present, is correlated with the R-wave of the
patient's heart rhythm. The analyzing step includes determining
whether the ECG rhythm is treatable with defibrillation. The method
further includes, if the determination is positive, delivering a
shock to the patient using the defibrillator. The method further
includes delivering a predetermined number of shocks to the
patient, and then subsequently determining whether the patient's
pulse, if present, is correlated with the R-wave of the patient's
heart rhythm. The method further includes, if the subsequent
determination is negative, administering CPR to the patient. The
method further includes using the pulse sensor to determine the
efficacy of the CPR treatment.
[0019] In a further aspect, the invention features a method of
treating a patient showing signs of possible cardiac arrest
comprising: (a) applying a piezoelectric pulse sensor to the
patient; (b) using the pulse sensor to detect whether the patient
has a pulse; and (c) using the pulse sensor to determine whether to
apply electrodes of a defibrillator to the patient.
[0020] In another aspect, the invention features a method of
treating a patient showing signs of possible cardiac arrest
including (a) applying a piezoelectric pulse sensor to the patient;
(b) using the pulse sensor to detect whether the patient has a
pulse; and (c) using the pulse sensor to determine whether to
perform CPR on the patient.
[0021] Other features and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a flow diagram illustrating steps in a method
according to one embodiment of the invention.
[0023] FIG. 2 is a flow diagram illustrating steps in a method
according to an alternate embodiment of the invention.
[0024] FIG. 3 is a diagrammatic view of a pulse sensor in use on a
patient.
[0025] FIG. 4 is an enlarged diagrammatic cross-sectional view of a
pulse sensor according to one embodiment of the invention.
[0026] FIGS. 4A and 4B are unfolded and folded top views of the
sensor element shown in FIG. 4; FIGS. 4C and 4D are unfolded and
folded bottom views of the sensor element.
[0027] FIG. 5 is a schematic of the equivalent electrical circuit
of the piezoelectric polymer film component of the pulse
sensor.
[0028] FIG. 6 is a circuit diagram of a complete pulse sensor
assembly.
[0029] FIG. 7 is an example of a data acquisition scan performed
using the pulse sensor of FIG. 4.
[0030] FIG. 8 is a diagrammatic representation of a pulse sensor
according to an alternate embodiment of the invention.
[0031] FIG. 9 is a diagrammatic representation of a sculpted sensor
with enhanced sensitivity.
[0032] FIG. 10 is a side view of a piezoelectric polymer film
sensor element with neutral plane inducer for optimized sensing of
bending moments of the thickness member.
[0033] FIG. 11 is a cross sectional diagram of a piezoelectric
polymer film pulse detector integrated into the fabrication of a
defibrillator pad.
[0034] FIG. 12 is a cross sectional diagram of a piezoelectric
polymer film pulse detector for use in a slightly concave body area
such as that encountered in the area of the neck at the carotid
artery.
[0035] FIG. 13 is a schematic representation of a piezoelectric
polymer film sensor element with a transimpedance (current to
voltage converter) amplifier.
[0036] FIG. 14 is a schematic representation of a piezoelectric
polymer film sensor element with an integrating amplifier (charge
amplifier).
[0037] FIG. 15 is a schematic representation of a piezoelectric
polymer film sensor element with a voltage amplifier.
DETAILED DESCRIPTION
[0038] FIGS. 1 and 2 show methods of using a pulse sensor and
defibrillator to determine the appropriate treatment for a victim
who appears to be suffering from cardiac arrest. Both methods
utilize a pulse sensor, described in further detail below, and an
automated defibrillator. The automated defibrillator includes (a) a
pair of electrodes that are placed on the victim's chest, (b)
instrumentation constructed to receive signals from the electrodes,
monitor the victim's ECG based on the received signals, and deliver
a shock to the electrodes, and (c) a display constructed to display
information and recommendations to the caregiver. Suitable
automated defibrillators are commercially available from Zoll
Medical Corp., Burlington, Mass., e.g., M-Series Automated External
Defibrillators. The instrumentation also includes the capability of
comparing the R-wave of the heart rhythm monitored by the ECG to
the victim's pulse to determine whether the two signals are
synchronized. Together, the pulse sensor and defibrillator
constitute a treatment system for victims who appear to be
suffering from cardiac arrest.
[0039] FIG. 1 illustrates a method according to a first embodiment
of the invention. In this method, the pulse sensor and
defibrillator are used to control the initiation of ECG analysis
and to advise a caregiver whether it is appropriate to administer a
shock and/or CPR.
[0040] Referring to FIG. 1, when a victim shows signs of cardiac
arrest, e.g., fainting, a caregiver applies a pulse sensor and the
electrodes of a defibrillator to the victim (100). Generally, as
shown in FIG. 3, the pulse sensor is applied by fastening an
elastic strap 20 carrying the sensor element 22 around the victim's
neck 24, so that the sensor element 22 is held in close contact
with the victim's carotid artery by the elastic strap 20. The strap
is fastened snugly but not tightly about the neck. Generally, to
obtain an optimal signal the sensor should be placed on the left
side of the neck orthogonal to and centered on the carotid artery.
If desired, other locations on the patient's body where a pulse can
normally be detected may be used instead of the neck. The neck is
generally preferred since it is the measurement and sustenance of
blood flow to the brain which is of ultimate importance and the
carotid artery carrying this flow provides a strong signal if a
pulse is present. The sensor element 22 is connected, e.g., by a
coaxial cable, to instrumentation 26, which is preferably
incorporated in the instrumentation of the defibrillator. The
electrodes of the defibrillator are applied in a conventional
manner.
[0041] Referring again to FIG. 1, the victim's pulse is detected
and monitored by the pulse sensor, and the victim's heart rhythm is
monitored by the ECG function of the defibrillator via the
defibrillation pads. The instrumentation of the defibrillator
determines whether the victim's pulse is correlated with the
R-waves of the victim's heart rhythm (102). If the pulse and
R-waves are synchronized, then a display on the defibrillator
indicates to the caregiver that the victim does not appear to be in
cardiac arrest, and that the victim should not be shocked or
treated with CPR (104). In this case, the caregiver puts the victim
in a "rescue" position, according to standard first aid protocol,
and continues to monitor the victim's pulse and ECG (106) so as to
observe if a change in synchronization, a loss of pulse signal or
ventricular fibrillation should occur (108).
[0042] If the victim's pulse is or becomes unsynchronized with the
R-waves, the defibrillator then performs an ECG analysis (110) to
determine whether the victim's heart rhythm should be treated by
administering an electrical shock, i.e., whether the victim is
suffering from ventricular fibrillation or wide complex ventricular
tachycardia (112).
[0043] If a shockable rhythm is detected, the display of the
defibrillator advises the caregiver to administer a shock to the
victim (114). The caregiver delivers a shock to the victim, and the
defibrillator then performs another ECG analysis to determine
whether a shockable rhythm is detected (110). As long as a
shockable rhythm is detected, this process is repeated for three
shocks (116). Once three shocks have been administered, the system
checks again to see whether the victim's pulse is synchronized with
the R-waves (118). If synchronization is not detected, then the
display advises the caregiver to administer CPR for a specified
period of time (120) and then to discontinue CPR (122). After CPR
is discontinued, the ECG analysis (110) and following steps are
repeated. If synchronization is detected, then a display on the
defibrillator indicates to the caregiver that the victim does not
appear to be in cardiac arrest, and that the victim should not be
shocked or treated with CPR (104), and treatment proceeds as
discussed above with reference to steps 106-108.
[0044] If a shockable rhythm is not detected, this indicates that
either the victim is not suffering from cardiac arrest or that the
victim has a condition that is not treatable by defibrillation
(124). The system then checks again to see whether the victim's
pulse is synchronized with the R-waves (118), and treatment
proceeds, based on the results, as discussed above.
[0045] FIG. 2 illustrates an alternative method, in which the pulse
sensor is used only to advise the caregiver whether it is
appropriate to administer CPR. In this method, the ECG analysis is
used alone to advise the caregiver whether it is appropriate to
administer a defibrillation shock. This method may be used in cases
in which checking the patient's pulse prior to applying the
defibrillator electrodes is deemed to be unnecessary, e.g., the
protocol that is currently recommended by the American Heart
Association for lay caregivers. The other steps of the method shown
in FIG. 2 are as described above with reference to FIG. 1.
[0046] Preferably, the pulse sensor is a piezoelectric polymer
sensor. Piezoelectric sensors are, in their simplest form,
capacitive electromechanical transducers that generate electrical
charge in proportion to applied stress. The primary purpose of
these sensors in the present invention is to generate an electrical
signal that is proportional to the force caused by blood flow
(pulse) in the area of the carotid artery or other areas of the
body where a pulse could be detected. The sensors of the invention
are not mechanically clamped at their periphery, and are primarily
sensitive to longitudinal stress as opposed to a sound pressure
wave front. Although the sensor material is somewhat sensitive to
stress applied normal to its thickness and width, the sensor is
designed to be most sensitive to stresses applied normal to its
length (or "machine direction").
[0047] As blood flows through the carotid artery, or other area
(either by normal heart action or as a result of CPR), the artery
expands and exerts a small amount of stress on the sensor, which is
in close contact with the exterior of the patient's neck, as
discussed above. The stress induced in the sensor thus reflects
changes in arterial blood flow.
[0048] As shown in FIG. 4, the sensor includes a piezoelectric
polymer film 30, a common metalization layer 32 and a signal
metalization layer 34. Because piezoelectric sensors are generally
of very high impedance, electrical interference is often
problematic. To minimize electrical interference at the sensor
surface, the sensor may be fabricated in a folded (self-shielding)
manner, as shown in FIGS. 4, 4B and 4D, so that the common
metalization layer 32 almost completely envelops the exterior of
the sensor. The sensor is shown prior to folding in FIGS. 4A and 4C
(top and bottom views, respectively) in which the dashed line
indicates the line about which the sensor is folded. The sensor is
held in its folded position by a layer of compliant adhesive 36
(FIG. 4). This shielding technique, in conjunction with coaxial
cable connections, greatly minimizes interference created by
undesirable stray electrical fields.
[0049] A data acquisition scan performed using the pulse sensor of
FIG. 4 is shown in FIG. 7. The voltage signal level is
approximately 100 mv p-p with most of the energy being spectrally
located at about 1.2 Hz, indicating a pulse rate of 72 beats/minute
(bpm).
[0050] Suitable piezoelectric polymer films include polyvinylidine
fluoride (PVDF) and co-polymers thereof. Such piezoelectric polymer
films are commercially available, e.g., from Measurement
Specialties, Inc. of Valley Forge, Pa. (e.g., Part No.
1-1004347-0). Preferably, the thickness of the polymer film is
between about 0.001 and 0.005 inch.
[0051] A preferred sensor material is polarized piezoelectric
polymer film sheet, about 28 micron (.about.0.001 inch) thick with
a silver conductor printed on both sides for a total thickness of
about 40 microns (.about.0.0015 inch). The sensor material may be
approximately 5 inches in length and folded back upon itself with
compliant adhesive, as shown in FIG. 4, to create a self-shielded
sensor that is approximately 1.0 inch in width and 2.5 inches in
length.
[0052] An electrical signal is produced (as measured between the
`signal` and the `shield` electrodes) when any stress is applied
(especially in the longitudinal axis) to the sensor. The circuit
shown in FIG. 5 consists of a voltage source in series with a
capacitor. This circuit is acceptable for visualization and
modeling of sensor elements of this type as long as the frequency
is below ultrasonic range.
[0053] The capacitance exhibited by these sensors is approximately
10 nF and loss tangent is less than 0.02. Because the leakage is so
low, it is generally necessary to eliminate long term buildup of
charge (DC drift) on the electrodes due to triboelectric,
pyroelectric or other phenomena especially when the sensor element
is to be connected to a high (>10 meg ohm) impedance instrument.
Eliminating this undesirable DC drift (charge buildup) is
accomplished (at the expense of low frequency response) by placing
a `termination` resistor (10 Meg ohm) across (in parallel with) the
sensor, as shown in FIG. 6. The termination resistor allows charge
to bleed off from the sensor thus eliminating long-term voltage
drift. The loss of low frequency response does not generally pose a
problem, as it is typically preferable to minimize low frequency
(<1 Hz) response so as to attenuate signals due to breathing or
other slow artifacts.
[0054] FIG. 6 also shows the capacitance associated with the cable,
which is used to connect the sensor to the instrumentation or other
apparatus. The capacitance of the connecting cable appears in
parallel with the capacitance of the sensor. This should not
significantly impair the operation of the sensor as long as the
cable capacitance is not appreciably large as compared to the
capacitance of the sensor. If the sensor were operated in the
`current` mode (connected to a transimpedance amplifier, as shown
in FIG. 13, or a charge amplifier, as shown in FIG. 14) the effect
of the cable capacitance on the entire sensor system operation
would be eliminated since there would be no appreciable voltage for
the cable capacitance to charge to.
[0055] The sensor electronics shown in FIGS. 13-15 may be
integrated onto the sensor, or may be connected as a remote sensing
instrument.
[0056] Other embodiments are within the scope of the following
claims.
[0057] In alternative embodiments, a piezoelectric polymer sensor
element 22' may be bonded to the surface of a compliant thickness
member, e.g., a foam pad 50 as shown in FIGS. 8 and 9, to form a
sensor assembly. A coaxial cable and terminating resistor are
affixed to the foam pad and electrically connected to the sensor
element. An epoxy 48 is applied to the resistor/cable/sensor area
for protection. This sensor is designed to be most sensitive to
forces which are normal to the length of the sensor. If desired, a
second foam pad (not shown) may be adhered over the
resistor/cable/sensor area. The foam pad is preferably about 1 to 2
mm thick, more preferably about 1.5 mm.
[0058] As shown in FIG. 9, if desired the foam pad 50 may be
contoured in regions 51 to enhance signal sensitivity by allowing
more stress to be felt by the sensor element 22.
[0059] As shown in FIG. 10, the piezoelectric sensor may also be
bonded to the surface of a neutral plane inducer 52, i.e., a
compliant but non-stretchable member that converts any flexure in
the assembly into a tensile or compressive motion normal to the
sensor's length. Piezoelectric polymer sensors may be bonded to
both opposing surfaces of a foam pad (not shown). Because any
flexure in this assembly will cause tensile force in one sensor
while simultaneously causing compressive force in the other sensor,
the combined electrical signal will be differential in nature with
advantages in increased sensitivity and improved common mode
rejection of unwanted signals such as noise or pyroelectric
response due to temperature transients.
[0060] As shown in FIG. 11, the sensor element 22 (shown in FIG. 4
and described above) and foam pad 50 may be bonded to a
defibrillator electrode 53, so that the sensor element may be
attached to the chest as part of the defibrillator electrode. The
defibrillator electrode includes a pressure-sensitive adhesive 56
and a conductive hydrogel 58, as is well known in the defibrillator
field.
[0061] As shown in FIG. 12, a stiff low mass object 54 may be
bonded to the adhesive side of the foam pad to aid in making
intimate contact with the carotid artery in cases where there are
areas of depression in the neck surface.
[0062] The pulse sensor assembly may be used anywhere on the human
body that a pulse can be detected, e.g., the neck, chest, wrist,
arm or ankle. The unamplified voltage signal level output from the
sensor should be in the approximate tenths of a volt range when
properly affixed to an area of the body that exhibits a moderate
pulse strength. The pulse sensor may be attached in a different
manner, e.g., by a clip, as a patch, or using a suction device.
[0063] Small surface mount type electrical components such as
operational amplifiers, resistors and capacitors may be integrated
onto the sensor element and used to amplify the signal, provide
noise immunity, mitigate the effects of cabling (parasitic
capacitance and/or equivalent series resistance), filter certain
frequencies, integrate the sensor electrical charge over time,
double integrate the sensor electrical charge over time, scale
and/or offset signal output, or otherwise accomplish signal
conditioning functions for the electrical signals produced by the
sensor element.
[0064] While automated defibrillators have been discussed above,
the pulse sensor may be used in conjunction with a non-automated
defibrillator. In this case the pulse sensor would be used by a
trained caregiver as an aid in determining the proper course of
treatment. The caregiver would observe displayed pulse and ECG
signals and perform an independent analysis to determine the proper
mode of treatment.
[0065] While the step of determining whether the patient's pulse
signal and R-wave are synchronized is used in the methods described
above, in many cases this step is not necessary. This step is a
signal processing technique that is sometimes used to improve
performance when one or both of the signals are contaminated with
artifacts. Other suitable techniques include autocorrelation
processing, matched filter processing, and other pattern
recognition schemes. In some cases, e.g., if the signals are
relatively uncontaminated, none of these techniques are
required.
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