U.S. patent application number 10/013941 was filed with the patent office on 2003-06-12 for pulse detection method and apparatus using patient impedance.
This patent application is currently assigned to Medtronic Physio-Control Manufacturing Corp.. Invention is credited to Hampton, David R., Jayne, Cynthia P., Lank, Paula, O'Hearn, Patricia, Stickney, Ronald E., Taylor, James W..
Application Number | 20030109790 10/013941 |
Document ID | / |
Family ID | 21762617 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030109790 |
Kind Code |
A1 |
Stickney, Ronald E. ; et
al. |
June 12, 2003 |
Pulse detection method and apparatus using patient impedance
Abstract
The presence of a cardiac pulse in a patient is determined by
evaluating fluctuations in an electrical signal that represents a
measurement of the patient's transthoracic impedance. Impedance
signal data obtained from the patient is analyzed for a feature
indicative of the presence of a cardiac pulse. Whether a cardiac
pulse is present in the patient is determined based on the feature
in the impedance signal data. Electrocardiogram (ECG) data may also
be obtained in time coordination with the impedance signal data.
Various applications for the pulse detection of the invention
include detection of PEA and prompting PEA-specific therapy,
prompting defibrillation therapy and/or CPR, and prompting rescue
breathing depending on detection of respiration.
Inventors: |
Stickney, Ronald E.;
(Edmonds, WA) ; Taylor, James W.; (Sammamish,
WA) ; O'Hearn, Patricia; (Mercer Island, WA) ;
Jayne, Cynthia P.; (Redmond, WA) ; Lank, Paula;
(Renton, WA) ; Hampton, David R.; (Woodinville,
WA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Assignee: |
Medtronic Physio-Control
Manufacturing Corp.
|
Family ID: |
21762617 |
Appl. No.: |
10/013941 |
Filed: |
December 6, 2001 |
Current U.S.
Class: |
600/500 |
Current CPC
Class: |
A61B 5/7278 20130101;
A61N 1/36521 20130101; A61N 1/3712 20130101; A61B 5/363 20210101;
A61B 5/0809 20130101; A61N 1/3925 20130101; A61B 5/7239 20130101;
A61B 5/024 20130101; A61B 5/7246 20130101 |
Class at
Publication: |
600/500 |
International
Class: |
A61B 005/02 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of determining the presence of a cardiac pulse in a
patient, comprising: (a) obtaining impedance signal data from the
patient; (b) analyzing the impedance signal data for a feature
indicative of the presence of a cardiac pulse; and (c) determining
whether a cardiac pulse is present in the patient based on the
feature in the impedance signal data.
2. The method of claim 1, wherein analyzing the impedance signal
data includes evaluating an amplitude of the impedance signal
data.
3. The method of claim 2, wherein evaluating the amplitude of the
impedance signal data includes: (a) locating low and high peak
amplitude values in the impedance signal data; and (b) calculating
a peak-to-peak change in amplitude using the low and high peak
amplitude values.
4. The method of claim 3, wherein the peak-to-peak change in
amplitude is the feature indicative of the presence of a cardiac
pulse, the method further comprising comparing the feature to a
predetermined threshold to determine whether a cardiac pulse is
present in the patient.
5. The method of claim 1, wherein analyzing the impedance signal
data includes evaluating energy in the impedance signal data.
6. The method of claim 5, wherein evaluating the energy in the
impedance signal data includes: (a) selecting impedance signal data
for an energy calculation; and (b) calculating the energy in the
selected impedance signal data.
7. The method of claim 6, wherein the calculated energy is the
feature indicative of the presence of a cardiac pulse, the method
further comprising comparing the calculated energy to a
predetermined threshold to determine whether a cardiac pulse is
present in the patient.
8. The method of claim 1, wherein analyzing the impedance signal
data includes comparing impedance signal data to a previously
identified impedance signal pattern known to predict the presence
of a cardiac pulse.
9. The method of claim 8, wherein the comparison produces a pattern
match statistic that is the feature indicative of the presence of a
cardiac pulse, the method further comprising comparing the feature
to a predetermined threshold to determine whether a cardiac pulse
is present in the patient.
10. The method of claim 1, further comprising analyzing the
impedance signal data for two or more features indicative of the
presence of a cardiac pulse, the two or more features being
determined from two or more analyses that evaluate (a) an amplitude
of the impedance signal data, (b) energy in the impedance signal
data, or (c) a comparison of impedance signal data with a
previously identified impedance signal pattern known to predict the
presence of a cardiac pulse.
11. The method of claim 1, further comprising prompting application
of defibrillation electrodes to the patient if a cardiac pulse is
determined not present in the patient.
12. A method of determining the presence of a cardiac pulse in the
patient, comprising: (a) obtaining impedance signal data from the
patient; (b) obtaining electrocardiogram (ECG) data from the
patient; (c) determining the presence of a QRS complex in the ECG
data; (d) analyzing the impedance signal data for a feature
indicative of the presence of a cardiac pulse based on a
relationship between the presence of a QRS complex in the ECG data
and amplitude changes in the impedance signal data; and (e)
determining whether a cardiac pulse is present in the patient based
on the feature in the impedance signal data.
13. The method of claim 12, further comprising locating a QRS
complex in the ECG data and selecting a segment of the impedance
signal data for the analysis based on the located QRS complex.
14. The method of claim 12, wherein analyzing the impedance signal
data includes evaluating an amplitude of the impedance signal
data.
15. The method of claim 13, wherein evaluating the amplitude of the
impedance signal data includes: (a) locating low and high peak
amplitude values in the impedance signal data; and (b) calculating
a peak-to-peak change in amplitude using the low and high peak
amplitude values.
16. The method of claim 14, wherein the peak-to-peak change in
amplitude is the feature indicative of the presence of a cardiac
pulse, the method further comprising comparing the feature to a
predetermined threshold to determine whether a cardiac pulse is
present in the patient.
17. The method of claim 12, wherein analyzing the impedance signal
data includes evaluating energy in the impedance signal data.
18. The method of claim 17, wherein evaluating the energy in the
impedance signal data includes: (a) selecting impedance signal data
for an energy calculation; and (b) calculating the energy in the
selected impedance signal data.
19. The method of claim 18, wherein the calculated energy is the
feature indicative of the presence of a cardiac pulse, the method
further comprising comparing the calculated energy to a
predetermined threshold to determine whether a cardiac pulse is
present in the patient.
20. The method of claim 12, wherein analyzing the impedance signal
data includes comparing impedance signal data to a previously
identified impedance signal pattern known to predict the presence
of a cardiac pulse.
21. The method of claim 20, wherein the comparison produces a
pattern match statistic that is the feature indicative of the
presence of a cardiac pulse, the method further comprising
comparing the feature to a predetermined threshold to determine
whether a cardiac pulse is present in the patient.
22. The method of claim 12, further comprising analyzing the
impedance signal data for two or more features indicative of the
presence of a cardiac pulse, the two or more features being
determined from two or more analyses that evaluate (a) an amplitude
of the impedance signal data, (b) energy in the impedance signal
data, or (c) a comparison of impedance signal data with a
previously identified impedance signal pattern known to predict the
presence of a cardiac pulse.
23. A medical device, comprising: (a) electrodes adapted to
communicate an impedance-sensing signal through a patient; (b) a
conversion circuit in communication with the electrodes for
converting the impedance-sensing signal received from the patient
into digital impedance signal data; and (c) a processing unit in
communication with the conversion circuit for processing the
impedance signal data, wherein the processing unit is configured to
analyze the impedance signal data for a feature indicative of the
presence of a cardiac pulse in the patient and determine the
presence of a cardiac pulse in the patient based on the feature in
the impedance signal data.
24. A medical device, comprising: (a) electrodes adapted to
communicate an impedance-sensing signal through a patient and
further to sense electrocardiogram (ECG) signals in the patient;
(b) a conversion circuit in communication with the electrodes for
converting the impedance-sensing signal and ECG signal received
from the patient into digital impedance signal data and ECG data,
respectively; and (c) a processing unit in communication with the
conversion circuit for processing the impedance signal data,
wherein the processing unit is configured to analyze the ECG data,
determine the presence of a QRS complex in the ECG data, and select
a segment of impedance signal data corresponding in time with the
QRS complex, the processing unit further configured to analyze the
segment of impedance signal data for a feature indicative of the
presence of a cardiac pulse in the patient and determine the
presence of a cardiac pulse in the patient based on the feature in
the impedance signal data.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to the detection of cardiac
activity in a patient, and more specifically, to a method and
apparatus for cardiac pulse detection.
BACKGROUND OF THE INVENTION
[0002] The presence of cardiac pulse in a patient is presently
detected preferably by palpating the patient's neck and sensing
changes in the volume of the patient's carotid artery due to blood
pumped from the patient's heart. If a pulse can be felt at the
carotid artery, it is likely that the patient's heart is pumping
sufficient blood to support life. A graph representative of the
physical expansion and contraction of a patient's carotid artery
during two consecutive pulses, or heartbeats, is shown at the top
of FIG. 1. When the heart's ventricles contract during a heartbeat,
a pressure wave is sent throughout the patient's peripheral
circulation system. The carotid pulse shown in FIG. 1 rises with
the ventricular ejection of blood at systole and peaks when the
pressure wave from the heart reaches a maximum. The carotid pulse
falls off again as the pressure subsides toward the end of each
pulse.
[0003] An electrocardiogram (ECG) waveform describes the electrical
activity of a patient's heart. The middle graph of FIG. 1
illustrates an example of an ECG waveform for two heartbeats
corresponding in time with the carotid pulse. Referring to the
first shown heartbeat, the portion of the ECG waveform representing
depolarization of the atrial muscle fibers is referred to as the
"P" wave. Depolarization of the ventricular muscle fibers is
collectively represented by the "Q," "R," and "S" waves of the ECG
waveform. Finally, the portion of the waveform representing
repolarization of the ventricular muscle fibers is known as the "T"
wave. Between heartbeats, the ECG waveform returns to an
isopotential level.
[0004] Discussed herein with respect to the present invention is
the correlation of fluctuations in a patient's transthoracic
impedance with blood flow that occurs with each cardiac pulse wave.
The bottom graph of FIG. 1 illustrates an example of a filtered
impedance signal for a patient in which fluctuations in impedance
correspond in time with the carotid pulse and ECG waveform.
[0005] The lack of a detectable cardiac pulse in a patient is a
strong indicator of cardiac arrest. Cardiac arrest is a
life-threatening medical condition in which the patient's heart
fails to provide enough blood flow to support life. During cardiac
arrest, the electrical activity may be disorganized (ventricular
fibrillation), too rapid (ventricular tachycardia), absent
(asystole), or organized at a normal or slow heart rate (pulseless
electrical activity). A caregiver may apply a defibrillation shock
to a patient in ventricular fibrillation (VF) or ventricular
tachycardia (VT) to stop the unsynchronized or rapid electrical
activity and allow a perfusing rhythm to commence. External
defibrillation, in particular, is provided by applying a strong
electric pulse to the patient's heart through electrodes placed on
the surface of the patient's body. If a patient lacks a detectable
pulse but has an ECG rhythm of asystole or pulseless electrical
activity (PEA), an appropriate therapy includes cardiopulmonary
resuscitation (CPR), which causes some blood flow.
[0006] Before providing defibrillation therapy or CPR to a patient,
a caregiver must first confirm that the patient is in cardiac
arrest. In general, external defibrillation is suitable only for
patients that are unconscious, apneic (i.e., not breathing),
pulseless, and in VF or VT. Medical guidelines indicate that the
presence or absence of a pulse in a patient should be determined
within 10 seconds. See, "American Heart Guidelines 2000 for
Cardiopulmonary Resuscitation and Emergency Cardiovascular Care,
Part 3: Adult Basic Life Support," Circulation 102 suppl. I:
I-22-I-59, 2000.
[0007] Unfortunately, under the pressures of an emergency
situation, it can be extremely difficult for first-responding
caregivers with little or no medical training to consistently and
accurately detect a cardiac pulse in a patient (e.g., by palpating
the carotid artery) in a short amount of time such as 10 seconds.
See, Eberle B., et al., "Checking the Carotid Pulse Diagnostic
Accuracy of First Responders in Patients With and Without a Pulse"
Resuscitation 33: 107-116, 1996. Nevertheless, because time is of
the essence in treating cardiac arrest, a caregiver may rush the
preliminary evaluation, incorrectly conclude that the patient has
no pulse, and proceed to provide defibrillation therapy when in
fact the patient has a pulse. Alternatively, a caregiver may
incorrectly conclude that the patient has a pulse and erroneously
withhold defibrillation therapy. A need therefore exists for a
method and apparatus that quickly, accurately, and automatically
determines the presence of a pulse in a patient, particularly to
prompt a caregiver to provide defibrillation or CPR therapy, as
appropriate, in an emergency situation.
SUMMARY OF THE INVENTION
[0008] The present invention provides a method and apparatus that
determines the presence of a cardiac pulse in a patient by
evaluating fluctuations in an electrical signal that represents a
measurement of the patient's transthoracic impedance. By
physiologically associating impedance fluctuations with the
presence of a cardiac pulse, the presence or absence of a cardiac
pulse in the patient is determined.
[0009] In accordance with one aspect of the present invention,
impedance signal data obtained from a patient is analyzed for a
feature indicative of the presence of a cardiac pulse. Whether a
cardiac pulse is present in the patient is determined based on the
feature in the impedance signal data.
[0010] The feature in the impedance signal data may be obtained
from evaluating an amplitude of the impedance signal data, an
energy in the impedance signal data, and/or a pattern match
statistic resulting from comparing the impedance signal data with a
previously-identified impedance signal pattern known to predict the
presence of a cardiac pulse. Other determination and classification
techniques known in the art may be used for the evaluation.
[0011] As to evaluating the amplitude of the impedance signal data,
low and high peak amplitude values in the impedance signal data may
be located and the peak-to-peak change in the amplitude from the
low to the high peak amplitude value may be calculated. The
peak-to-peak change in amplitude, constituting a feature indicative
of the presence of a cardiac pulse, may be compared to a threshold
to determine the presence of a pulse.
[0012] As to evaluating energy in the impedance signal data, an
energy calculation may be performed using impedance signal data
obtained from the patient. The calculated energy, constituting
feature indicative of the presence of a cardiac pulse, is compared
to a predetermined threshold to determine whether a cardiac pulse
is present in the patient.
[0013] As to analyzing the impedance signal data using pattern
matching, the impedance signal data may be compared to a
previously-identified impedance signal pattern known to predict the
presence of a cardiac pulse. The comparison produces a pattern
match statistic, constituting the feature indicative of the
presence of a cardiac pulse, which is compared to a predetermined
threshold to determine whether a cardiac pulse is present in the
patient.
[0014] The impedance signal data may be obtained from the patient
using defibrillation electrodes placed on the patient or using
separate impedance-sensing electrodes placed on the patient. If
separate electrodes are used, the present invention includes
prompting application of the defibrillation electrodes to the
patient if a cardiac pulse is determined not present in the
patient.
[0015] In accordance with another aspect of the present invention,
electrocardiogram (ECG) data is obtained from the patient in time
coordination with the impedance signal data. A QRS complex located
in the ECG data is used to select a segment of the impedance signal
data for further analysis. If a pulse is present in the patient,
the pulse should be detectable following the located QRS
complexes.
[0016] In accordance with yet another aspect of the present
invention, pulseless electrical activity (PEA) may be detected when
the patient is determined pulseless and the patient is not
experiencing ventricular defibrillation (VF), ventricular
tachycardia (VT), or asystole. In circumstances where PEA is found
present, the present invention includes prompting delivery of
PEA-specific therapy to the patient. The present invention may be
employed in a variety of devices that provide monitoring and/or
therapy. If, for example, the patient is determined pulseless and
experiencing VT with a pulse rate greater than 100 beats per
minute, the present invention may prompt delivery of a
defibrillation pulse. If a cardiac pulse is later found in the
patient after delivery of the defibrillation pulse, the present
invention may report the return of spontaneous circulation in the
patient.
[0017] The present invention is further useful in evaluating
capture while delivering pacing stimuli to a patient. If a cardiac
pulse is not detected immediately following a pacing pulse, the
current level of the pacing pulse may be increased until capture by
the pacing stimuli is achieved.
[0018] Other applications and advantages of the present invention
are readily apparent. For example, the invention may be implemented
in an automated external defibrillator (AED) that prompts the user
to perform cardiopulmonary resuscitation (CPR) based on the absence
of a pulse in a patient. The AED may also prompt the user to
provide rescue breathing depending on detection of respiration. In
regard to the latter, the impedance signal data and other relevant
information, such as the patient's ECG, may be analyzed to detect
the presence of respiration in the patient.
[0019] Embodiments of the invention intended for trained medical
personnel may provide a display of the impedance signal data that
is representative of the presence or absence of a pulse in a
patient. In that regard, the impedance signal data may be shown as
a way form, as shown in FIG. 1. The impedance signal data may also
be displayed as a bar.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0021] FIG. 1 is a pictorial diagram of a carotid pulse waveform,
an electrocardiogram (ECG) waveform, and a filtered transthoracic
impedance signal for two consecutive heartbeats reflecting
fluctuations in transthoracic impedance that correspond with
pulsatile blood flow;
[0022] FIG. 2 is a pictorial diagram of a defibrillator and
electrodes constructed in accordance with the present invention and
attached to a patient;
[0023] FIG. 3 is a block diagram of major components of the
defibrillator shown in FIG. 2;
[0024] FIG. 4 is a flow diagram of a pulse detection process
performed in accordance with the present invention;
[0025] FIG. 5 is a flow diagram of a pulse rate analysis performed
in accordance with the present invention the pulse detection
process shown in FIG. 4;
[0026] FIG. 6 is a flow diagram of another pulse detection process
performed in accordance with the present invention in which an
impedance signal pattern analysis is performed without an ECG
signal analysis;
[0027] FIG. 7 is a flow diagram of a protocol implemented by a
defibrillator as shown in FIG. 2 that incorporates a pulse
detection process provided by the present invention;
[0028] FIG. 8 is a flow diagram of protocol implemented by the
defibrillator shown in FIG. 2 that incorporates a pulse detection
process provided by the present invention;
[0029] FIG. 9 is a flow diagram of still another protocol
implemented by the defibrillator shown in FIG. 2 that incorporates
a pulse detection process provided by the present invention;
[0030] FIG. 10 is a flow diagram of an auto-capture detection
process for cardiac pacing that uses a pulse detection process of
the present invention; and
[0031] FIG. 11 is a flow diagram of a patient condition advisory
process for use in a manual defibrillator or monitor which
incorporates a pulse detection process of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0032] A device constructed in accordance with the present
invention uses measurements of a patient's transthoracic impedance
to determine the presence of a cardiac pulse in the patient. As
will be appreciated from the description herein, the device may be
a stand alone unit or it may be incorporated into another
monitoring or therapy-providing device. In one suitable
application, the present invention is implemented in a
defibrillator, such as the defibrillator 10 shown in FIG. 2. A
patient 40 is connected to the defibrillator 10 via electrodes 12,
14 placed on the skin of the patient. The defibrillator 10 uses the
electrodes 12, 14 to deliver defibrillation pulses to the patient
40. The defibrillator 10 also uses the electrodes 12, 14 to obtain
ECG signals from the patient 40.
[0033] The electrodes 12, 14 are further configured to communicate
an impedance-sensing signal through the patient 40. The
impedance-sensing signal is used by the defibrillator 10 to observe
the patient's impedance. Alternatively, the defibrillator 10 may
use sensors 20, 22 that are separate from the electrodes 12, 14 for
communicating the impedance-sensing signal through the patient. The
sensors 20, 22 may be connected to the electrodes 12, 14, as shown
in FIG. 2, they may be attached to the patient 40 via separate
wires (not shown) connected to the defibrillator 10. In either
case, the sensors 20, 22 may be suitably constructed from standard
external patient electrodes known in the art.
[0034] The defibrillator 10 measures the impedance of a patient
between the electrodes 12, 14 (or between the sensors 20, 22, as
the case may be) when placed on the patient 40. An impedance
measuring component of the defibrillator 10 is preferably used to
measure the patient's impedance.
[0035] A preferred embodiment of the invention uses a
high-frequency, low-level constant current technique to measure the
patient's transthoracic impedance, though other known impedance
measuring techniques may be used. A signal generator included in
the defibrillator 10 produces a low-amplitude, constant current,
high-frequency signal (typically sinusoidal or square). The signal
is preferably generated having a frequency in the range of 10
kHz-100 kHz. The current flows between the electrodes 12 and 14.
The resulting current flow causes a voltage to develop across the
patient's body that is proportional to the product of the patient's
impedance and the applied current. To calculate the patient's
impedance, the impedance measuring component in the defibrillator
10 divides the measured sensing voltage by the applied current.
Since the measured voltage is linearly related to the patient's
impedance, the impedance signal data used herein may either be a
calculated impedance signal or the measured voltage signal.
[0036] While embodiments of the invention specifically described
herein are shown implemented in a defibrillator 10, the present
invention is not limited to such specific type of application.
Those of ordinary skill in the art will recognize that the
advantages of the invention may similarly be achieved by
implementing the present invention in cardiac monitors and other
types of medical equipment that do not necessarily provide
defibrillation therapy.
[0037] Prior to discussing various pulse detection processes that
the defibrillator 10 may implement in accordance with the present
invention, a brief description of certain major components of the
defibrillator 10 is provided. Referring to FIG. 3, the
defibrillator 10 includes defibrillation electrodes 30 (e.g.,
electrodes 12, 14 described above in FIG. 2). An impedance-sensing
signal generator 56 communicates an impedance-sensing signal
through the patient via the electrodes 30. A signal amplifier 32
amplifies the impedance-sensing signal to a level appropriate for
digitization by analog-to-digital (A/D) converter 36. A bandpass
filter 34 filters the amplified impedance-sensing signal to isolate
the portion of the signal that most closely reveals fluctuations
due to blood flow from cardiac pulses. In one embodiment of the
invention, the bandpass filter 34 is a 1-10 Hz bandpass filter.
Fluctuations in the impedance signal below 1 Hz are more likely to
be caused by respiration in the patient, and not blood flow.
Accordingly, the bandpass filter attenuates that component of the
impedance signal. The portion of the impedance signal exceeding 10
Hz is more likely affected by surrounding noise and is likewise
filtered out.
[0038] The filtered impedance signal is delivered to the A/D
converter 36 that converts the impedance signal into digital
impedance data for further evaluation. The bandpass filter 34 or
other filter may be provided to reduce any aliasing introduced in
the impedance signal by the A/D converter 36. The parameters of
such filtering depend, in part, on the sampling rate of the A/D
converter. Bandpass and antialiasing filters, as well as A/D
converters, are well-known in the art, and may be implemented in
hardware or software, or a combination of both. For example, a
preferred embodiment uses a hardware lowpass filter on the
impedance signal before the A/D converter 36, and then a software
highpass filter on the digital impedance data after the A/D
conversion. Another preferred embodiment additionally uses a
software lowpass filter after the A/D conversion to further limit
the bandwidth of the impedance signal. The A/D converter 36
delivers the digital impedance signal data to the processing unit
38 for evaluation.
[0039] The processing unit 38 evaluates the impedance signal data
for the presence of a cardiac pulse. The processing unit 38 is
preferably comprised of a computer processor that operates in
accordance with programmed instructions stored in a memory 40 that
implement a pulse detection process 42, described in more detail
below. The processing unit 38 may also store in the memory 40 the
impedance signal data obtained from the patient, along with other
event data and ECG signal data. The memory 40 may be comprised of
any type or combination of types of storage medium, including, for
example, a volatile memory such as a dynamic random access memory
(DRAM), a nonvolatile static memory, or storage media such as a
magnetic tape or disk drive or optical storage unit (e.g.,
CD-RW).
[0040] The processing unit 38 may report the results of the pulse
detection process to the operator of the defibrillator 10 via a
display 48. The processing unit 38 may also prompt actions (e.g.,
CPR) to the operator to direct the resuscitation effort. The
display 48 may include, for example, lights, audible signals,
alarm, printer, or display screen. The processing unit 38 may also
receive input from the operator of the defibrillator 10 via an
input device 46. The input device 46 may include one or more keys,
switches, buttons, or other types of user input devices.
[0041] The defibrillation electrodes 30 may further be used to
sense the patient's electrocardiogram (ECG) signals. ECG signals
obtained from the patient may be amplified and filtered in a
conventional manner, and converted into digitized ECG data for
evaluation by the processing unit 38.
[0042] Preferably, the processing unit 38 evaluates the ECG signals
in accordance with programmed instructions stored in the memory 40
that carry out an ECG evaluation process 44 to determine whether a
defibrillation shock should be provided. A suitable method for
determining whether to apply a defibrillation shock is described in
U.S. Pat. No. 4,610,254, which is assigned to the assignee of the
present invention and incorporated by reference herein. If the
processing unit 38 determines that delivery of a defibrillation
pulse is appropriate, the processing unit 38 instructs a
defibrillation pulse generator 50 to prepare to deliver a
defibrillation pulse to the patient. In that regard, the
defibrillation pulse generator 50 uses an energy source (e.g.,
battery) to charge one or more defibrillation capacitors in the
defibrillator 10.
[0043] When the defibrillation charge is ready for delivery, the
processing unit 38 advises the operator via the display 48 that the
defibrillator 10 is ready to deliver the defibrillation pulse. The
processing unit 38 may ask the operator to initiate the delivery of
the defibrillation pulse. When the operator initiates delivery of
the defibrillation pulse (e.g., via the input device 46), the
processing unit 38 instructs the defibrillation pulse generator 50
to discharge through the patient the energy stored in the
defibrillation capacitors (via the electrodes 30). Alternatively,
the processing unit 38 may cause the defibrillation pulse generator
50 to automatically deliver the defibrillation pulse.
[0044] While FIG. 3 illustrates certain major components of the
defibrillator 10, those having ordinary skill in the art will
appreciate that the defibrillator 10 may contain more or fewer
components than those shown. The disclosure of a preferred
embodiment of the defibrillator 10 does not require that all of the
general conventional components be shown. It will further be
appreciated that the invention may be implemented in a cardiac
monitor having essentially the same components as the defibrillator
10 shown in FIG. 3, except that the cardiac monitor does not have
the components necessary for delivering a defibrillation pulse.
Furthermore, some or all of the programmed instructions 42, 44 may
be implemented in hardware as an alternative to software
instructions stored in the memory 40.
[0045] As noted above, the present invention uses a portion of the
impedance-sensing signal whose frequency range is most likely to
reveal fluctuations indicating the presence of a cardiac pulse in
the patient. The presence of characteristic fluctuations in patient
impedance associated with a cardiac pulse is used to identify the
presence of a cardiac pulse in the patient.
[0046] FIG. 4 illustrates a pulse detection process 60 conducted in
accordance with the present invention. The pulse detection process
60 uses an analysis of impedance signal data to determine the
presence of a pulse in a patient. Preferably, the impedance signal
data selected for analysis is obtained during time intervals
associated with QRS complexes in the patient's ECG.
[0047] Beginning at block 70 the pulse detection process 60
captures both ECG and impedance signal data, synchronized in time,
for a predetermined time interval (e.g., 10 seconds). Preferably,
at this time, persons around the patient are advised to not touch
the patient during this time interval (e.g., the device could
report "analyzing now . . . stand clear"). Alternatively, the ECG
and impedance capturing step may continue until the first or a
specified number of QRS complexes in the ECG have been identified,
or in the event of asystole or a low heart rate, a predetermined
maximum period of time (e.g., 10 seconds) has passed.
[0048] In block 72, the pulse detection process 60 locates all of
the QRS complexes in the captured ECG signal. Identification of QRS
complexes can be done using methods published in the literature and
well-known to those skilled in the art of ECG signal processing.
For example see, Watanabe K., et al., "Computer Analysis of the
Exercise ECG: a Review," Prog Cardiovasc Dis 22: 423-446,1980.
[0049] In block 74, for each time that a QRS complex was identified
in the ECG signal, a segment of filtered impedance signal data
obtained from the captured impedance data is selected. In one
embodiment of the invention, the time window of each segment of
impedance data is approximately 600 milliseconds in length, and
commences prior to the end of the identified QRS complex. If no QRS
complexes were identified in the captured ECG signal in block 72
(as would happen for example, during asystole), there will be no
segments of impedance data selected in block 74.
[0050] In block 76, one or more measurements are made on a segment
of impedance signal data selected in block 74 to identify or
calculate a feature indicative of a cardiac pulse. The measurements
may include one or more of the following:
[0051] (1) peak-to-peak amplitude of the impedance signal in the
segment (measured in milliohms);
[0052] (2) peak-peak amplitude of the first derivative of the
impedance signal in the segment (measured in milliohms per
second);
[0053] (3) energy of the impedance signal in the segment
(preferably calculated by squaring and summing each of the
impedance data values in the segment); or
[0054] (4) a pattern matching statistic.
[0055] As to the latter measurement (i.e., pattern matching), the
segment of impedance signal data is compared with one or more
previously identified impedance signal patterns known to predict
the presence of a pulse. The comparison produces a pattern match
statistic. Generally, in this context, the greater the value of the
pattern match statistic, the closer the patient's impedance signal
matches a pattern impedance signal that predicts the presence of a
pulse. Other candidate measurements will be apparent to those
skilled in the art, and may be used instead of, or in addition to,
the aforementioned measurements. A measurement resulting from the
analysis in block 76 constitutes a feature of the impedance signal
data indicative of the presence of a pulse.
[0056] In decision block 78, the one or more features from block 76
are evaluated to determine the presence of a cardiac pulse in the
patient. The embodiment shown in FIG. 4 compares the one or more
features to predetermined thresholds to determine whether or not a
pulse is detected. For example, an impedance peak-to-peak amplitude
measurement would be consistent with the presence of a pulse if it
exceeded a certain threshold (e.g., 50 milliohms). Similarly, an
impedance energy measurement would be consistent with a pulse if
its magnitude exceeded a predetermined threshold. Likewise, a
pattern matching statistic would be consistent with a pulse if it
exceeded a predetermined threshold. If the feature exceeded the
specified threshold, the pulse detection process determines that a
pulse was detected, as indicated at block 80. If the feature did
not exceed the specified threshold, a pulse was not detected, as
indicated at block 82. If no segments of impedance signal data were
selected in block 74 (i.e., no QRS complexes were located in block
72 in the captured ECG), the pulse detection process 60 would
determine that a pulse was not detected, as indicated at block
82.
[0057] The embodiment shown in FIG. 4 uses thresholding in block 78
to determine whether a pulse was detected. However, those skilled
in the art will recognize other forms of classification and
determination that may suitably be used in the invention. For
example, multi-dimensional classifiers may be used in decision
block 78 to determine whether a pulse was detected. For example,
separate analyses of the amplitude and energy in the impedance data
segment, may be performed, with the resultant outcome of each
analysis constituting a detection statistic that is provided to a
multi-dimensional classifier. The detection statistics may be
weighted and compared in the classifier to determine an overall
conclusion whether a pulse is present in the patient. In other
embodiments, individual calculations of instantaneous and
background amplitudes and/or energies may be provided as detection
features for evaluation in a multi-dimensional classifier. Pattern
match statistics may also be evaluated in the multi-dimensional
classifier, as may other candidate measurements of the impedance
signal data. Techniques for constructing multi-dimensional
classifiers are well-known in the art. For an expanded description
of classifiers suitable for use with of the invention, see, e.g.,
R. Duda and P. Hart, Pattern Classification and Scene Analysis,
published by John Wiley & Sons, New York, and incorporated
herein by reference.
[0058] After determining whether a pulse was detected (block 80) or
not detected (block 82), the pulse detection process 60 determines
whether all of the segments of impedance signal data selected in
block 74 have analyzed. If not, the analysis and decision process
of block 76, 78, 80, and 82 is repeated for a new impedance data
segment. This continues until all of the impedance data segments
selected in block 74 have been analyzed.
[0059] It is recognized that the resulting determination (pulse
detected or no pulse detected) may not be the same for each
impedance data segment analyzed. An additional decision step is
used to determine the overall outcome of the pulse detection
process 60. As indicated at decision block 86, the pulse detection
process 60 may evaluate the determinations for each impedance data
segment and decide that a pulse is present in the patient if a
pulse was detected in a simple majority of the impedance segments
analyzed. Of course, other voting schemes may be used. If, in
decision block 86, a majority is found, the pulse detection process
concludes that a cardiac pulse is present in the patient, as
indicated at block 90. Otherwise, the pulse detection process 60
concludes that the patient is pulseless, as indicated at block
88.
[0060] Requiring a pulse to be found in more than a simple majority
of the impedance data segments would improve the specificity of the
detection, but decrease the sensitivity for detecting a pulse.
Conversely, requiring a pulse to be found for just one impedance
segment or for less than a majority of the impedance segments would
improve sensitivity for detecting a pulse but decrease specificity.
If the pulse detection process 60 concludes that a pulse is present
in the patient, the process 60 may optionally proceed to check the
pulse rate of the patient, as illustrated in FIG. 5. Turning to
FIG. 5, in block 92, the number of QRS complexes (located in block
72 in FIG. 4) are counted. Decision block 94 subsequently compares
the number of QRS complexes to a threshold. In one preferred
embodiment, the threshold is 5, corresponding to a heart rate of
approximately 30 bpm. If the number of QRS complexes is at least
equal to the threshold, the pulse detection process 60 proceeds to
block 96, concluding that the patient has a pulse and an adequate
pulse rate. If the number of QRS complexes is less than the
threshold, the pulse detection process 60 proceeds to block 98,
concluding that the patient has a pulse, but also severe
bradycardia.
[0061] While a preferred embodiment of the invention as shown in
FIG. 4 includes capturing both ECG and impedance signal data, and
selecting the segments of impedance signal data based on QRS
complexes located in the ECG, other embodiments of the invention
may not capture or use the ECG signal. In FIG. 6, an alternative
pulse detection process 100 begins by capturing only impedance
signal data from the patient, as indicated at block 102. Depending
on the length of the time interval in which impedance data is
captured, it may be advantageous to select a segment of the
impedance signal data for further analysis, as indicated at block
104. In that regard, one suitable selection process includes
scanning the impedance signal data for the maximum peak and
selecting a segment of data that surrounds the detected maximum
peak.
[0062] For exemplary purposes, the pulse detection process 100 is
shown evaluating the selected segment of impedance signal data
using a pattern match analysis. However, those skilled in the art
will recognize that other techniques (e.g., analysis of the
amplitude or energy in the impedance signal data, as discussed
above, may be used.) In block 106, the selected impedance data
segment is compared with previously identified impedance signal
patterns known to predict the presence of a pulse. The resulting
pattern match statistic is evaluated against a threshold in
decision block 108 to determine whether a pulse was detected in the
patient. If the pattern match statistic exceeded the threshold, the
pulse detection process 100 concludes in block 110 that a pulse was
detected in the patient. Otherwise, the pulse detection process 100
concludes that the patient is pulseless, as indicated in block 112.
At this point, the pulse detection process is finished.
Alternatively, if a pulse was detected in the patient, the pulse
detection process 110 may proceed to evaluate the patient's pulse
rate in a manner described in reference to FIG. 5.
[0063] As noted above, the transthoracic impedance signal can
contain fluctuations due to cardiac pulses, respiration, or patient
motion. To assess whether a patient has a pulse, it is desirable to
suppress fluctuations in the patient's impedance that are due to
causes other than cardiac pulses. Fluctuations due to noncardiac
causes may contain components at frequencies similar to those of
impedance fluctuations due to cardiac pulses. Consequently,
bandpass filtering may not always adequately suppress fluctuations
due to noncardiac causes.
[0064] Signal averaging of the impedance signal can be used to
suppress fluctuations that are due to noncardiac causes. Signal
averaging makes advantageous use of the fact that impedance
fluctuations due to cardiac pulses are synchronized to QRS
complexes in the ECG signal, whereas other impedance fluctuations
are asynchronous to QRS complexes. Pulse detection may be more
accurately accomplished using an averaged impedance signal.
[0065] A preferred method for signal averaging of the impedance
signal first stores the continuous ECG and transthoracic impedance
signals, synchronized in time, for a predetermined time interval
(e.g., ten seconds). The locations of the QRS complexes (if any) in
the stored ECG signal are determined. Using true mathematical
correlation (or an alternative correlation technique such as area
of difference), the QRS complexes are classified into types, where
all QRS complexes of the same type have high correlation with the
first occurring QRS complex of that type. The dominant QRS type is
selected as the type containing the most members, with a preference
for the narrowest QRS type when a two or more types tie for most
members. Using the first QRS of the dominant type as a reference
complex, the second QRS complex of the same type is shifted in time
until it is best aligned with the reference complex (i.e., it
achieves a maximum correlation value). The corresponding impedance
signal is also shifted in time to stay synchronized with the
time-shifted QRS complex. When the second QRS complex is optimally
aligned with the reference complex, the two QRS complexes are
averaged together. Their corresponding impedance signals, over a
time period from about the start of the QRS complex to about 600
milliseconds after the end of the QRS complex, are also averaged
together. The averaged QRS complex is then used as a new reference
complex and the process of averaging both the QRS complexes and the
corresponding impedance data is repeated with the remaining QRS
complexes of the dominant type.
[0066] Preferably, during the subsequent averaging of the QRS
complexes and impedance segments, the new QRS complex and impedance
segment carry a weight of one and the previous averaged QRS complex
and impedance segment carry a weight equal to the number of QRS
complexes that have been included in the averaged QRS complex. When
all of the QRS complexes of the dominant type have been processed
as described above, the averaged impedance segment is evaluated
using one or more of the techniques previously described (e.g.,
amplitude, energy, pattern matching), or by using another measuring
technique known in the art, to determine whether or not the patient
has a pulse.
[0067] During severe bradycardia, there will be few QRS complexes
in a 10-second period and signal averaging of the transthoracic
impedance signal will not be as effective as when the heart rate is
higher. However, at very low heart rates, there is unlikely to be
enough blood flow to support life. For that reason, below a certain
heart rate (e.g., 30 bpm), the patient may be considered
pulseless.
[0068] The pulse detection process of the present invention may be
used as part of a shock advisory protocol in a defibrillator for
determining whether to recommend defibrillation or other forms of
therapy for the patient. FIG. 7 illustrates a pulse
detection/defibrillation process 130, preferably for use in an
automated external defibrillator (AED) capable of providing a
defibrillation pulse if a patient is determined to be pulseless and
in VF or VT.
[0069] In the pulse detection/defibrillation process 200 in FIG. 7,
the AED initializes its circuits when it is first turned on, as
indicated at block 132. The defibrillation electrodes of the AED
are placed on the patient. When the AED is ready for operation, the
process 130 performs an analysis of the patient, as indicated at
block 134, in which the AED obtains selected parameters such as
impedance signal data and ECG data from the patient. During the
analysis performed in block 134, the AED preferably reports
"Analyzing now . . . stand clear" to the operator of the AED.
[0070] Using the information obtained in the patient analysis, the
process 130 determines in decision block 136 whether the patient is
experiencing ventricular fibrillation (VF). If VF is present in the
patient, the process 130 proceeds to block 142 where the AED
prepares to deliver a defibrillation pulse to the patient. In that
regard, an energy storage device within the AED, such as a
capacitor, is charged. At the same time, the AED reports "Shock
advised" to the operator of the AED.
[0071] Once the energy storage device is charged, the process 130
proceeds to block 144 where the AED is ready to deliver the
defibrillation pulse. The operator of the AED is advised "Stand
clear . . . push to shock." When the operator of the AED initiates
delivery of the defibrillation pulse, the process 130 delivers the
defibrillation shock to the patient, as indicated in block 146.
[0072] The AED preferably records in memory that it delivered a
defibrillation pulse to the patient. If the present pulse delivery
is the first or second defibrillation shock delivered to the
patient, the process 130 may return to block 134 where the patient
undergoes another analysis. On the other hand, if the pulse
delivery was the third defibrillation pulse to be delivered to the
patient, the process 130 may proceed to block 140 where the AED
advises the operator to commence providing CPR therapy to the
patient, e.g., by using the message "Start CPR." The "No shock
advised" prompt shown in block 140 is suppressed in this instance.
The AED may continue to prompt for CPR for a predetermined time
period, after which the patient may again be analyzed, as indicated
in block 134.
[0073] Returning to decision block 136, if VF is not detected in
the patient, the process 130 proceeds to decision block 138 and
determines whether a cardiac pulse is present in the patient. The
pulse detection performed in block 138 may be one of the pulse
detection processes 60 or 100 described above.
[0074] If, at decision block 138, a pulse is detected in the
patient, the process 130 proceeds to block 139 and reports "Pulse
detected . . . start rescue breathing" to the operator. The process
130 may also report "Return of spontaneous circulation" if a pulse
is detected in the patient any time after the delivery of a
defibrillation pulse in block 146. In any event, after a
predetermined time period for rescue breathing has completed, the
process 130 preferably returns to block 134 to repeat an analysis
of the patient.
[0075] If a cardiac pulse is not detected at decision block 138,
the process 130 determines whether the patient is experiencing
ventricular tachycardia (VT) with a heart rate of greater than a
certain threshold, e.g., 100 beats per minute (bpm), as indicated
at decision block 141. Other thresholds such as 120, 150, or 180
bpm, for example, may be used. If the determination at decision
block 141 is negative, the process 130 proceeds to block 140 and
advises the operator to provide CPR therapy. Again, at this point,
the AED reports "No shock advised . . . start CPR" to the operator.
The prompt to provide CPR is provided for a defined period of time.
When the period of time for CPR is finished, the process 130
preferably returns to block 134 and performs another analysis of
the patient. If the determination at decision block 141 is positive
(i.e., the patient is experiencing VT with a heart rate greater
than the threshold), the process 130 performs the shock sequence
shown at blocks 142, 144, 146 to deliver a defibrillation
pulse.
[0076] Variations and additions to the process 130 within the scope
of the invention are recognized by the those having ordinary skill
in defibrillation and cardiac therapy. FIG. 8, for example,
illustrates an alternative pulse detection/defibrillation process
150 for use in an AED. As with the process 130 in FIG. 7, the AED
begins by initializing its circuits at block 152. At block 154, the
AED performs an analysis of the patient in a manner similar to that
described with respect to block 134 in FIG. 7. After completing the
analysis of the patient, the process 150 proceeds to decision block
156 to determine whether a pulse is present in the patient. The
pulse detection performed in block 156 may be, for example, any one
of the pulse detection processes 60 or 100 discussed above.
[0077] If a pulse is detected in the patient, the process 150 may
enter a monitoring mode at block 158 in which the patient's pulse
is monitored. The pulse monitoring performed at block 158 may use
any one or a combination of the pulse detection processes described
herein. Preferably, the process 150 is configured to proceed from
block 158 to block 154 after expiration of the predetermined
monitoring time period. If the pulse monitoring at block 158
determines that at any time a pulse is no longer detected, the
process 150 returns to block 154 to perform another analysis of the
patient. The process 150 also preferably reports the change in
patient condition to the operator.
[0078] If, at decision block 156, a pulse is not detected in the
patient, the process 150 proceeds to decision block 160 where it
determines whether the patient has a shockable cardiac rhythm
(e.g., VF or VT). As referenced earlier, U.S. Pat. No. 4,610,254,
incorporated herein by reference, describes a suitable method for
differentiating shockable from non-shockable cardiac rhythms.
[0079] If a shockable cardiac rhythm, such as VF or VT, is
detected, the process 150 proceeds to a shock delivery sequence at
blocks 162, 164, and 166, which may operate in a manner similar to
that described with respect to blocks 142, 144, and 146 in FIG. 7.
If the pulse delivery was the third defibrillation shock delivered
to the patient, the process 150 may proceed to block 168 and prompt
the delivery of CPR, as discussed with block 140 in FIG. 7.
[0080] If VF or VT is not detected at decision block 160, the
process 150 checks for asystole, as indicated at block 167. One
suitable process for detecting asystole is described in U.S. Pat.
No. 6,304,773, assigned to the assignee of the present invention
and incorporated herein by reference. If asystole is detected at
block 167, the process 150 proceeds to prompt the delivery of CPR,
as indicated at block 168. If asystole is not detected, the process
150 determines that the patient is experiencing pulseless
electrical activity (PEA), as indicated at block 169. PEA is
generally defined by the presence of QRS complexes in a patient and
the lack of a detectable pulse, combined with no detection of VT or
VF. As described above, detection of PEA in block 168 is achieved
by ruling out the presence of a pulse (block 156), detecting no VF
or VT (block 160), and detecting no asystole (block 167).
Alternatively, if the ECG signal is monitored for QRS complexes
(e.g., as shown at block 70 in FIG. 4), the process 150 may
conclude the patient is in a state of PEA if it repeatedly observes
QRS complexes without detection of a cardiac pulse associated
therewith. If a PEA condition is detected, the process 150 proceeds
to block 170 and prompts the operator to deliver PEA-specific
therapy to the patient. One suitable method of treating PEA is
described in U.S. Pat. No. 6,298,267, incorporated by reference
herein. The process 150 may prompt other therapies as well,
provided they are designed for a PEA condition. After a
PEA-specific therapy has been delivered to the patient, possibly
for a predetermined period of time, the process 150 returns to
block 154 to repeat the analysis of the patient.
[0081] FIG. 9 illustrates yet another pulse
detection/defibrillation process 200 that may be used in an AED. At
block 202, after the AED has been turned on, the AED initializes
its circuits. The defibrillation electrodes are also placed on the
patient. The AED is then ready to analyze the patient, as indicated
at block 204. This analysis may be performed in a manner similar to
that described with respect to block 134 in FIG. 7.
[0082] If at any point the AED determines that the defibrillation
electrodes are not connected to the AED, the process 200 jumps to
block 206 where the AED instructs the operator to "Connect
electrodes." When the AED senses that the electrodes are connected,
the process 200 returns to the analysis in block 204. Likewise, if
the AED finds itself in any other state where the electrodes are
not connected, as represented by block 208, the process 200 jumps
to block 206 where it instructs the operator to connect the
electrodes.
[0083] Furthermore, during the analysis performed in block 204, if
the AED detects motion on the part of the patient, the process 200
proceeds to block 210 where the AED reports to the operator of the
AED "Motion detected . . . stop motion." If the patient is moved
during the analysis process 204, the data obtained during the
analysis is more likely to be affected by noise and other signal
contaminants. Motion of the patient may be detected in the
impedance signal data collected by the present invention. A
suitable method for detecting motion of the patient is described in
U.S. Pat. No. 4,610,254, referenced earlier and incorporated by
reference herein. The AED evaluates the impedance measured between
the defibrillation electrodes placed on the patient. As noted
earlier, noise and signal components resulting from patient motion
cause fluctuations in the impedance signal, generally in a
frequency range of 1-3 Hz. If the measured impedance fluctuates
outside of a predetermined range, the AED determines that the
patient is moving or being moved and directs the process 200 to
proceed to block 210. When the motion ceases, the process 200
returns to the analysis in block 204.
[0084] The process 200 next proceeds to decision block 212 where it
determines whether a pulse is detected in the patient. Again, the
pulse detection processes performed in decision block 212 may be,
for example, one of the pulse detection processes 60 or 100
described above.
[0085] If a pulse is not detected in the patient, the process 200
proceeds to decision block 214 where it determines whether the
patient has a shockable cardiac rhythm (e.g., VF or VT or a
non-shockable cardiac rhythm (such as asystole and bradycardia). As
referenced earlier, one suitable method for differentiating
shockable from non-shockable cardiac rhythms is disclosed in U.S.
Pat. No. 4,610,254, incorporated herein by reference. If the
patient's cardiac rhythm is determined to be shockable (e.g., VF or
VT is found), the process 200 proceeds to blocks 216, 218, and 220
to deliver a shock to the patient. The shock delivery may be
performed as described earlier with respect to block 142, 144, 146
in FIG. 7.
[0086] If the pulse delivery was the third defibrillation pulse to
be delivered to the patient, the process 200 proceeds to block 222
where the AED advises the operator to commence providing CPR
therapy to the patient. The CPR prompt may continue for a defined
period of time, at which the process 200 returns to block 204 and
performs another analysis of the patient.
[0087] If, at decision block 214, the patient's cardiac rhythm is
determined not shockable, the process 200 preferably proceeds to
block 222 and advises the operator to provide CPR therapy, as
discussed above.
[0088] Returning to decision block 212, if a pulse is detected in
the patient, the process 200 proceeds to decision block 224 where
it determines whether the patient is breathing. In that regard, the
AED may again use the impedance signal for determining whether a
patient is breathing. As noted earlier, fluctuations in impedance
of the patient below 1 Hz are largely indicative of a change in
volume of the patient's lungs. The breathing detection at block 224
(and at blocks 226 and 228, discussed below) may monitor the
impedance signal for characteristic changes that indicate patient
breathing, e.g., as described in Hoffmans et al., "Respiratory
Monitoring With a New Impedance Plethysmograph," Anesthesia 41:
1139-42, 1986, and incorporated by reference herein. Detection of
breathing may employ a process similar to that described above for
detection of a pulse (i.e., evaluating impedance amplitude, energy,
or pattern), though a different bandpass filter would be used to
isolate the frequency components that more closely demonstrate
patient breathing. If automatic means for detecting breathing in
the patient are not available, the AED may ask the operator of the
AED to input information (e.g., by pressing a button) to indicate
whether the patient is breathing.
[0089] If, at decision block 224, the process 200 determines that
the patient is not breathing, the process 200 proceeds to a block
226 where the operator of the AED is advised to commence rescue
breathing. In that regard, the AED reports to the operator "Pulse
detected . . . start rescue breathing." The AED also continues to
monitor the patient's cardiac pulse and returns to block 204 if a
cardiac pulse is no longer detected. If, at any point during the
provision of rescue breathing, the AED detects that the patient is
breathing on his own, the process 200 proceeds to block 228 where
the AED monitors the patient for a continued presence of breathing
and a cardiac pulse.
[0090] Returning to decision block 224, if the process 200
determines that the patient is breathing, the process 200 proceeds
to block 228 where the AED monitors the pulse and breathing of the
patient. In that regard, the AED reports "Pulse and breathing
detected . . . monitoring patient." If, at any time during the
monitoring of the patient the process 200 determines that the
patient is not breathing, the process 200 proceeds to block 226
where the operator of the AED is advised to commence rescue
breathing. If a cardiac pulse is no longer detected in the patient,
the process 200 proceeds from block 228 to block 204 to commence a
new analysis of the patient.
[0091] Lastly, as noted in FIG. 9, during the rescue breathing
procedure in block 226 or the monitoring procedure performed in
block 228, the AED may assess whether CPR is being administered to
the patient. If the AED finds that CPR is being performed, the AED
may prompt the operator to cease providing CPR. If, during the CPR
period of block 222, the AED determines that CPR is not being
administered to the patient, the AED may remind the operator to
provide CPR therapy to the patient. One method for determining
whether CPR is being administered is to monitor patient impedance
to observe patterns of impedance fluctuation in the patient that
are indicative of CPR. During CPR, repetitive chest compression
typically causes repetitive fluctuations in the impedance
signal.
[0092] FIG. 10 illustrates yet another application in which the
pulse detection process of the present invention may be used. The
process described in FIG. 10 pertains to auto-capture detection in
cardiac pacing.
[0093] Specifically, the auto-capture detection process 250 begins
at block 252 in which pacing therapy for the patient is initiated.
A counter N, described below, is set to equal 0. At block 254, a
pacing pulse is delivered to the patient. Thereafter, filtered
impedance signal data is obtained from the patient, as indicated at
block 256. The impedance data is used in block 258 to detect the
presence of a cardiac pulse in the patient. The pulse detection
process used in block 258 may be one of the pulse detection
processes 60 or 100, discussed above.
[0094] The sequence of delivering a pacing pulse and determining
the presence of a cardiac pulse in blocks 254, 256, 258 is repeated
a predetermined number of times. With respect to FIG. 10, for
example, the sequence is repeated five times. At block 260, the
counter N is evaluated, and if not yet equal to 5, the counter is
incremented by 1 (block 262), following which the process 250
returns to deliver another pacing pulse to the patient.
[0095] If, at decision block 260, the counter N equals 5, the
process 250 determines at decision block 264 whether a cardiac
pulse occurred consistently after each pacing pulse. The process
250 requires that some portion or all of the pacing pulses result
in a detectable cardiac pulse before pronouncing that capture has
been achieved. If the presence of a cardiac pulse is determined
consistently follow the pacing pulses, the process 250 determines
that capture has been achieved, as in indicated at block 266.
Otherwise, the current of the pacing pulses is increased by a
predetermined amount, e.g., 10 milliamperes, as indicated at block
268. At block 270, the counter N is set back to equal 0 and the
process 250 returns to the pacing capture detection sequence
beginning at block 254. In this manner, the pacing current is
increased until capture has been achieved.
[0096] In FIG. 10, the presence of a pulse is used to determine
whether the pacing stimulus has been captured by the ventricles.
Detection of QRS complexes in the patient's ECG may also be used to
identify pacing capture. In that regard, the patient's ECG would be
monitored along with, or in place of, the impedance data collection
in block 256. A QRS complex will occur immediately following the
pacing stimulus if capture has been achieved. If QRS complexes are
not observed, the current of the pacing pulses may be increased, as
discussed above, until capture has been achieved.
[0097] FIG. 11 illustrates still another application in which the
pulse detection process of the present invention may be used. The
process 280 described in FIG. 11 is particularly suited for use in
a manual defibrillator or patient monitor. Beginning at block 282,
the process 280 monitors the patient's ECG for QRS complexes. At
block 284, the process 280 also obtains filtered impedance signal
data from the patient. The process 280 uses the ECG and impedance
signals in decision block 286 to determine the presence of a pulse.
The pulse detection implemented in block 286 may be one of the
pulse detection processes 60 or 100.
[0098] If a pulse is detected, the process 280 determines whether a
defibrillation pulse has been provided to the patient and if so,
reports the return of spontaneous circulation to the operator, as
indicated at block 298. The process 280 then returns to block 282
to repeat the pulse detection analysis. If a pulse is not detected,
the process 280 evaluates the ECG signal to determine whether the
patient is experiencing ventricular fibrillation or ventricular
tachycardia with a heart rate greater than 100 bpm. If so, then a
the process identifies the patient's condition and sounds a VT/VF
alarm, as indicated at block 290. If not, the process 280 then
proceeds to block 292 to check for an asystole condition.
[0099] Detection of asystole may be accomplished as noted earlier
and described in greater detail in U.S. Pat. No. 6,304,773,
incorporated herein by reference. If asystole is detected, the
process 280 identifies the patient's condition and sounds an
asystole alarm, as indicated at block 294. Otherwise, the patient
is experiencing PEA and the patient's condition is so identified,
with the sound of a PEA alarm, as indicated at block 296. In this
manner, the operator of the manual defibrillator or monitor is kept
advised of the patient's condition.
[0100] While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
* * * * *