U.S. patent application number 10/441933 was filed with the patent office on 2004-10-28 for processing pulse signal in conjunction with ecg signal to detect pulse in external defibrillation.
Invention is credited to Boucher, Donald R., Geheb, Frederick, Marcovecchio, Alan F..
Application Number | 20040215244 10/441933 |
Document ID | / |
Family ID | 33313120 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040215244 |
Kind Code |
A1 |
Marcovecchio, Alan F. ; et
al. |
October 28, 2004 |
Processing pulse signal in conjunction with ECG signal to detect
pulse in external defibrillation
Abstract
An external defibrillator for delivering a defibrillation
waveform to a patient. The defibrillator includes chest electrodes
for detecting at least one ECG signal on the patient and for
delivering the defibrillation waveform to the patient, a
defibrillation circuit for shaping and delivering the
defibrillation waveform, a pulse sensor for detecting the presence
of a cardiac pulse in the patient, wherein the pulse sensor has a
pulse signal as its output, and memory and processing circuits for
analyzing the pulse signal in conjunction with the ECG signal to
determine whether the patient has a pulse.
Inventors: |
Marcovecchio, Alan F.;
(Woodbury, MN) ; Geheb, Frederick; (Danvers,
MA) ; Boucher, Donald R.; (Andover, MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
33313120 |
Appl. No.: |
10/441933 |
Filed: |
May 20, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10441933 |
May 20, 2003 |
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10421652 |
Apr 23, 2003 |
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Current U.S.
Class: |
607/5 |
Current CPC
Class: |
A61B 5/024 20130101;
A61B 5/021 20130101; A61H 31/005 20130101; A61N 1/3925 20130101;
A61B 5/11 20130101; A61N 1/39044 20170801; A61N 1/3904 20170801;
A61H 31/00 20130101; A61B 5/0205 20130101 |
Class at
Publication: |
607/005 |
International
Class: |
A61N 001/39 |
Claims
1. An external defibrillator for delivering a defibrillation
waveform to a patient, the defibrillator comprising: chest
electrodes for detecting at least one ECG signal on the patient and
for delivering the defibrillation waveform to the patient; a
defibrillation circuit for shaping and delivering the
defibrillation waveform; a pulse sensor for detecting the presence
of a cardiac pulse in the patient, wherein the pulse sensor has a
pulse signal as its output; memory and processing circuits for
analyzing the pulse signal in conjunction with the ECG signal to
determine whether the patient has a pulse.
2. A method for delivering defibrillation therapy to a patient, the
method comprising: applying electrodes to the chest of the patient;
detecting at least one ECG signal from electrodes; applying a pulse
sensor to the patient; detecting a pulse signal from the pulse
sensor; analyzing the pulse signal in conjunction with the ECG
signal to determine whether the patient has a pulse.
3. The subject matter of claim 1 or 2 wherein the pulse rate of the
pulse signal is compared to the pulse rate of the ECG signal.
4. The subject matter of claim 1 or 2 wherein the pulse signal is
processed to isolate the pulsatile component of the signal.
5. The subject matter of claim 1 or 2 wherein the pulse signal is
examined for a peak during a window initiated at the occurrence of
an R-wave in the ECG signal.
6. The subject matter of claim 1 or 2 wherein a comparison is made
of the energy in discrete frequency bands of the pulse and the ECG
signals.
7. The subject matter of claim 1 or 2 wherein a peak frequency
corresponding to the peak of one of the pulse and ECG signals is
determined, and the other of the pulse and ECG signals is examined
for a peak within a frequency band surrounding the peak
frequency.
8. The subject matter of claim 1 or 2 wherein a frequency domain
transformation of the pulse signal is processed.
9. The subject matter of claim 1 or 2 wherein frequency domain
transformations of the pulse and ECG signal are processed.
10. The subject matter of claim 1 or 2 wherein a decision whether
to deliver a defibrillation waveform to the patient is based in
part on whether processing of the ECG and pulse signals determines
that the patient has a pulse.
11. The subject matter of claim 1 or 2 wherein the defibrillator
comprises an automatic external defibrillator (AED).
12. The subject matter of claim 11 further comprising the
capability to provide the user with prompts for performing CPR, and
wherein the prompts are dependent at least in part on whether the
processing of the pulse signal determines that the patient has a
pulse.
13. The subject matter of claim 1 or 2 wherein a determination of
whether the patient has a pulse is undertaken after delivery of the
defibrillation waveform, and the user is informed of the outcome of
the determination.
14. The subject matter of claim 1 or 2 wherein the pulse sensor is
mechanically connected to at least one of the defibrillation
electrodes.
15. The subject matter of claim 14 wherein there are two
defibrillation electrodes, each supported on a substrate, and the
pulse sensor is supported on the same substrate.
16. The subject matter of claim 15 wherein the wires connected to
the pulse sensor are bundled with wires connected to the
defibrillation electrodes to form a combined bundle of wires
extending from the defibrillator to the electrodes and sensor.
17. The subject matter of claim 1 or 2 wherein the pulse sensor is
mechanically separate from the defibrillation electrodes.
18. The subject matter of claim 17 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
to be attached to the forehead of the patient.
19. The subject matter of claim 17 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
to be attached to the ear lobe of the patient.
20. The subject matter of claim 17 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
to be attached to the nasal septum.
21. The subject matter of claim 17 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
to be attached to the nasal bridge.
22. The subject matter of claim 17 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
to be attached to the finger.
23. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
so that its output may also be used for pulse oximetry.
24. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises an optical sensor, and the optical sensor is configured
so that its output is not useful for pulse oximetry.
25. The subject matter of claim 1 or 2 wherein determining whether
the patient has a pulse includes determining whether the pulse, if
present, is correlated with the R-wave of the patient's heart
rhythm.
26. The subject matter of claim 1 or 2 wherein a determination
whether to administer CPR to the patient is based on whether it is
determined that the patient has a pulse.
27. The subject matter of claim 1 or 2 further comprising using the
pulse sensor to determine the efficacy of CPR treatment of the
patient.
28. The subject matter of claim 1 or 2 wherein the pulse sensor is
an optical sensor.
29. The subject matter of claim 15 wherein the pulse sensor is an
optical sensor.
30. The subject matter of claim 1 or 2 wherein analyzing the pulse
signal in conjunction with the ECG signal comprises analyzing
whether there is a correlation between the two signals indicative
of the presence of a pulse.
31. The subject matter of claim 1 or 2 wherein the pulse sensor is
a non-optical sensor.
32. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises an acoustic (heart sounds) sensor.
33. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises an ultrasonic blood flow sensor.
34. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises a pressure sensor on a limb-compressing pneumatic
cuff.
35. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises a at least one mechanical or ultrasonic sensor configured
to detect arterial wall motion caused by blood flow in an
artery.
36. The subject matter of claim 35 wherein the sensor is
piezoelectric-based.
37. The subject matter of claim 1 or 2 wherein the pulse sensor
comprises one or more sensors for measuring body impedance
variation from blood flow.
Description
RELATIONSHIP TO OTHER APPLICATIONS
[0001] This application is a continuation in part of U.S. patent
application Ser. No. 10/421,652, entitled "Optical Pulse Sensor for
External Defibrillator," filed on Apr. 23, 2003.
TECHNICAL FIELD
[0002] This invention relates to pulse sensors and methods of using
pulse sensors in conjunction with external defibrillators.
BACKGROUND
[0003] 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.
[0004] 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).
[0005] 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. 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 is unaccompanied by sufficient cardiac
output (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
caregiver.
[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.
[0008] A wide variety of sensors have been employed for pulse
detection.
[0009] Optical sensors have been used in pulse detection. For
example, pulse detectors of the type used for measuring heart rate
during exercise typically rely on reflectance or transmission of an
infrared light beam. Blood pulsing in the user's capillaries
produces a corresponding variation in the absorption of light by
capillaries, and that variation produces a pulsation in the output
of an optical sensor.
[0010] Optical sensors are also widely used in pulse oximetry, in
which a measurement is made of the percentage of hemoglobin
saturated with oxygen. An optical plethysmographic probe attached
to the patient's finger or ear lobe generates light at two
wavelengths (e.g., 650 nm and 805 nm). The light is partially
absorbed by hemoglobin, by amounts that differ depending on whether
or not the hemoglobin is saturated with oxygen. By calculating
absorption at the two wavelengths, a pulse oximetry device can
compute the proportion of hemoglobin that is saturated
(oxygenated).
[0011] Acoustic sensors have also been applied to pulse detection.
Typically, the acoustic sensor is configured to detect sounds
characteristic of a beating heart (e.g., the action of a heart
valve). E.g., Joo U.S. Pat. No. 6,440,082. Some acoustic sensors
used for pulse detection are based on piezoelectric devices.
[0012] Another use of a piezoelectric devices in pulse detection is
proposed in U.S. patent application Ser. No. 9/846,673, filed on
May 1, 2001. The piezoelectric device detects motion of the surface
of the body resulting from the pulse (e.g., motion resulting from
blood flowing in a blood vessel beneath the sensor).
SUMMARY
[0013] The inventors have found that an external defibrillator can
be configured to determine whether a patient has a pulse by
analyzing the output of a pulse sensor in conjunction with the
patient's ECG signal to determine whether a pulse is present. In
general the invention features applying electrodes to the chest of
the patient, detecting at least one ECG signal from electrodes,
applying a pulse sensor (of varying kinds) to the patient,
detecting a pulse signal from the pulse sensor, and analyzing the
pulse signal in conjunction with the ECG signal to determine
whether the patient has a pulse.
[0014] Preferred implementations of the invention may incorporate
one or more of the following: The pulse rate of the pulse signal
may be compared to the pulse rate of the ECG signal. The pulse
signal may be processed to isolate the pulsatile component of the
signal. The pulse signal may be examined for a peak during a window
initiated at the occurrence of an R-wave in the ECG signal. A
comparison may be made of the energy in discrete frequency bands of
the pulse and the ECG signals. A peak frequency corresponding to
the peak of one of the pulse and ECG signals may be determined, and
the other of the pulse and ECG signals may be examined for a peak
within a frequency band surrounding the peak frequency. A frequency
domain transformation of the pulse signal may be processed. The
frequency domain transformations of the pulse and ECG signal may be
processed. A decision whether to deliver a defibrillation waveform
to the patient may be based in part on whether processing of the
ECG and pulse signals determines that the patient has a pulse. The
defibrillator may be an automatic external defibrillator (AED). The
implementation may include the capability to provide the user with
prompts for performing CPR, wherein the prompts are dependent at
least in part on whether the processing of the pulse signal
determines that the patient has a pulse. A determination of whether
the patient has a pulse may be undertaken after delivery of the
defibrillation waveform, and the user may be informed of the
outcome of the determination. The pulse sensor (e.g., an optical
plethysmographic sensor) may be mechanically connected to at least
one of the defibrillation electrodes. There may be two
defibrillation electrodes, each supported on a substrate, and the
pulse sensor may be supported on the same substrate. The wires
connected to the pulse sensor may be bundled with wires connected
to the defibrillation electrodes to form a combined bundle of wires
extending from the defibrillator to the electrodes and sensor. The
pulse sensor may be mechanically separate from the defibrillation
electrodes. In the case of an optical pulse sensor, the sensor may
be configured to be attached to the forehead of the patient. to the
ear lobe, to the nasal septum, to the nasal bridge, or to the
finger. In the case of an optical pulse sensor, the sensor may be
configured so that its output may also be used for pulse oximetry,
or it may be configured so that its output is not useful for pulse
oximetry. Determining whether the patient has a pulse may include
determining whether the pulse, if present, is correlated with the
R-wave of the patient's heart rhythm. A determination whether to
administer CPR to the patient may be based on whether it is
determined that the patient has a pulse. The pulse sensor may be to
determine the efficacy of CPR treatment of the patient. Analyzing
the pulse signal in conjunction with the ECG signal may comprise
analyzing whether there is a correlation between the two signals
indicative of the presence of a pulse.
[0015] A wide variety of pulse sensors may be used. The pulse
sensor may include a non-optical sensor. The pulse sensor may be an
acoustic (heart sounds) sensor. The pulse sensor may include an
ultrasonic blood flow sensor. The pulse sensor may include a
pressure sensor on a limb-compressing pneumatic cuff. The pulse
sensor may include at least one mechanical or ultrasonic sensor
(e.g., a piezoelectric sensor) configured to detect arterial wall
motion caused by blood flow in an artery. The pulse sensor may
include one or more sensors for measuring body impedance variation
from blood flow.
[0016] Implementations of the invention have many and various
advantages. They can make it possible to perform a quick and
accurate determination of the appropriate treatment
(defibrillation, CPR, or no cardiac-related treatment) for a
patient who appears to be suffering from cardiac arrest. They can
provide an accurate determination of the presence or absence of a
pulse in a patient, even under adverse conditions, thus
significantly reducing the risk that an inappropriate and even
dangerous treatment will be given erroneously. 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. For example, a pulse sensor 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
patient who appears to be suffering from cardiac arrest.
[0017] In some implementations, in which the pulse sensor is an
optical sensor, the invention provides an optical sensor without
the complexity of pulse oximetry, in which two optical
measurements, one at each of two wavelengths, are made, and complex
signal processing is performed to estimate blood oxygen saturation
from the two measurements.
[0018] Other features and advantages of the invention will be
apparent from the drawings, detailed description, and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagrammatic, perspective view of an electrode
pad with built-in pulse sensor applied to the chest of patient.
[0020] FIGS. 2A-2F are diagrammatic views of other possible pulse
sensors applied to the patient.
[0021] FIG. 3A shows an unprocessed pulse signal from a pulse
sensor.
[0022] FIG. 3B shows a filtered pulse signal, preconditioned for
beat detection.
[0023] FIG. 4 is a block diagram of an algorithm used in one
implementation of the invention.
[0024] FIG. 5 illustrates how an ECG signal is used in conjunction
with the pulse signal from the pulse sensor.
[0025] FIG. 6 is a block diagram of an algorithm for using an ECG
signal in conjunction with the pulse signal.
[0026] FIGS. 7 and 8 illustrate other implementations of the
invention, in which the frequency transforms of the ECG and pulse
signals are compared.
[0027] FIG. 9 is a table summarizing a decision algorithm used in
one implementation of the invention.
[0028] FIG. 10A is a diagrammatic view of a transmittance optical
sensor.
[0029] FIG. 10B is a diagrammatic view of a reflectance optical
sensor.
DETAILED DESCRIPTION
[0030] There are a great many possible implementations of the
invention, too many to describe herein. Some possible
implementations that are presently preferred are described below.
It cannot be emphasized too strongly, however, that these are
descriptions of implementations of the invention, and not
descriptions of the invention, which is not limited to the detailed
implementations described in this section but is described in
broader terms in the claims.
[0031] The descriptions below are more than sufficient for one
skilled in the art to construct the disclosed implementations.
Unless otherwise mentioned, the processes and manufacturing methods
referred to are ones known by those working in the art
[0032] FIG. 1 shows a defibrillator 10 connected to a patient 20.
Cable 12 connects the defibrillator to an electrode assembly 14,
which supports two chest electrodes 16, 18, an accelerometer 20
(for measuring chest compression), and a pulse sensor 22 (e.g., an
optical plethysmographic sensor). All wiring for the electrodes,
accelerometer, and pulse sensor are bundled together in cable
12.
[0033] FIGS. 2A-2F show alternative locations for the pulse sensor
in the event the sensor is an optical plethysmographic sensor 22
(some of the same locations may be useful with other types of pulse
sensors): anywhere on the forehead, the bridge of the nose, the
septum of the nose, the ear lobe, the sternum above the
accelerometer, and on any of the fingers or thumb. All of the
sensors shown are commercially available, except the nasal septum
and sternum sensor. The nasal septum sensor would be a
transmittance sensor (e.g., as shown in FIG. 10A), similar to that
available for the ear lobe. The sternum sensor would be a
reflectance sensor (e.g., as shown in FIG. 10B).
[0034] The optical plethysmographic sensor 22 detects transmitted
or reflected light, and provides a pulse signal 34, which
represents a parameter correlated with the patient's pulse. In the
case of an optical plethysmographic sensor, the pulse signal 34
would be an optical signal representing the brightness of light
transmitted through or reflected from a portion of the body through
which blood capillaries extend. The term "pulse signal" is simply a
shorthand for an electrical signal representative of a parameter
correlated with the pulse of the patient. E.g, in the case of an
optical sensor, the pulse signal could be the light sensed by the
optical receiver. If the patient has a pulse, there will generally
be a pulsing variation in the pulse signal. In the case of an
optical sensor, the variation will be in the absorption of light by
the blood capillaries.
[0035] A variety of signal processing techniques may be used to
process the pulse signal to determine whether a pulse is present or
absent. Several possibilities are described below. These may be
used alone or in combination.
[0036] One processing technique is to process the pulse signal 34
to isolate any pulsatile component. FIG. 3 shows a typical pulse
signal 34 that would be detected when a pulse is present. A time
domain analysis can be performed on the pulse signal to determine
whether it contains a pulsatile component. For example, the
non-pulsatile component can be estimated as the mean of the pulse
signal, and variation from the mean can be analyzed for the
presence of a pulsatile component indicative of a pulse. The
frequency and strength of the pulsatile component (i.e., the
variation from the mean) can be compared to predetermined frequency
ranges and strength ranges to decide whether a pulse is
present.
[0037] The pulse signal may be filtered so that pulses in the
waveform are enhanced and then detected with a beat detection
algorithm (e.g., the algorithm conventionally utilized for ECG
R-wave detection). The filtering may include a high pass filter, a
low pass filter, and also a notch filter to remove line noise if
necessary. The high pass filter with a -3 dB cutoff frequency near
0.5 Hz removes any DC component, thus enhancing any existing
pulsatile component. The low pass filter, with a -3 dB point in the
range of 5-15 Hz removes some signal components unrelated to
patients' pulse (i.e., noise). A smoothed or unsmoothed difference
operation may also be applied to the pulse signal as one method to
enhance or precondition the pulse signal for a time-domain beat
detection algorithm. FIG. 3A illustrates a raw pulse signal, and
FIG. 3B shows a filtered, preconditioned pulse signal ready to be
processed by a beat detection algorithm.
[0038] The beat detection algorithm of FIG. 4 may be applied to the
preconditioned pulse signal of FIG. 3B. The beat detection
algorithm is intended to identify individual pulses within the
pulse waveform. The arrows at the bottom of FIG. 3B indicate
detection of a beat by the algorithm. Time intervals may then be
computed between any two successive pulse detections. These time
intervals, derived from the pulse waveform, may then by analyzed to
determine the likelihood of a pulse. A moving average of these
intervals may also be computed and updated as new intervals are
measured. If the average interval is within a range (e.g. 35-185
beats/minute), then a pulse might be declared present.
[0039] Another processing technique is to use both a pulse signal
from the pulse sensor and an ECG signal from the electrodes.
Typically, both the pulse signal and the ECG signal will exhibit
periodicity when a pulse is present, because a true pulse
originates from a mechanically beating heart, and thus the same
periodicity observed on the pulse signal should be present on the
ECG signal when the heart is beating. However, a periodic ECG
signal is not always indicative of a pulse, and thus should only be
used to verify (or in combination with) periodicity detected in the
pulse signal.
[0040] FIG. 5 shows an ECG signal 38 in the upper half of the
figure and the filtered, pulsatile component 44 of the pulse signal
in the lower half of the figure. FIG. 6 shows the algorithm
followed in processing the signals. Upon detecting (50 in FIG. 6)
an ECG R-wave (ventricular depolarization) 40 a time window 42 is
initiated (52 in FIG. 6). The duration of a time window may change
as a function of the previous ECG cycle length or current average
ECG cycle length. This variable window length is intended to
shorten for shorter cycle lengths (high heart rates) and lengthen
for longer cycle lengths (lower heart rates). During that time
window, the pulsatile component 44 of the pulse signal is analyzed
for a corresponding pulse (54 in FIG. 6). The pulse signal is not
analyzed outside of this time window, thereby reducing the number
of false detections on the pulse signal. Alternatively, the entire
pulse signal is analyzed and pulse detects outside of an R-wave
initiated time window are ignored. Detection of a pulse on the
pulse signal within the time window could be sufficient to conclude
that a pulse is present. Greater confidence that a pulse is present
can be had using a confidence index that is increased each time a
pulse is detected in the pulse signal during the prescribed time
window following an R-wave. In FIG. 5, all of the pulse pulses,
except the seventh pulse 46, are detected and considered as valid
detects since they are within time windows initiated by R-wave
detections on the ECG signal. The portion of the pulse waveform
comprising the seventh pulse 46 is either not analyzed or detected
and ignored since it is outside of an pulse detect time window. One
possible confidence index would be the percentage of instances in
which a pulse is detected in a window following detection of an
R-wave. If the confidence index exceeds an empirically determined
threshold, the existence of a pulse is declared. Otherwise a pulse
is considered to be absent.
[0041] A simpler technique is to determine the pulse rate of each
of the ECG and pulse signals, and simply compare the two pulse
rates. If the difference between the two rates is within a range
(e.g., 1-5 beats/minute), the existence of a pulse is declared.
This technique may be used in conjunction with the pulse windowing
scheme to minimize the number of false positive detections on the
pulse signal. If the implementation does not employ a method (e.g.,
windowing scheme) to minimize false positive detections on the
pulse signal, then the comparison between pulse and ECG derived
pulse rates may be modified. In this case the absence of a pulse
would be declared if the pulse rate is less than the ECG rate. This
modification takes into consideration that the pulse rate may be
higher than the ECG rate due to false pulse detections, and such a
condition may be indicative of a pulse despite the difference
between the two rates being outside of a range.
[0042] Another technique is to compare band-limited versions of the
ECG and pulse signals. The signals can be band limited over a range
of likely pulse frequencies (e.g., 0.5-5 Hz). The band-limited
signals are compared to determine if a pulse or peak frequency of
the pulse signal compares well with a pulse or peak frequency of
the ECG signal. If the two compare well, the existence of a patient
pulse is supported. Comparisons can be made using several different
quantitative techniques. Cross correlation (convolution) of the two
filtered waveforms is one technique that can be used to quantify
the comparison of the two waveforms.
[0043] One technique for comparing the peak frequencies of the two
signals is shown in FIG. 7. Each of the ECG and pulse signals is
transformed into a frequency domain using, e.g., an FFT, wavelet,
or other transform. The frequency peak of the transformed pulse
signal is then compared with the ECG derived heart rate or peak
frequency. An association between the two frequency peaks supports
the existence of a pulse. A quantitative technique for determining
how well the two frequency peaks compare is suggested in FIG. 7.
The energy levels in discrete frequency bands are compared, e.g.,
the a-b band 60 in the ECG transform is compared to the a-b band 62
in the pulse signal transform.
[0044] The peak frequencies can also be compared by examining one
signal for a peak located within a predetermined frequency band
centered on the peak frequency of the other signal. For example, as
shown in FIG. 8, the peak ECG frequency 70 could be measured and
updated at regular intervals (which could be constant or variable
as a function of heart rate), with the measurement being done
either in the time domain or in a transformed frequency domain. The
pulse signal 74 would then be examined for a corresponding peak
frequency within a band 72 centered on the continuously updated
peak ECG frequency (e.g., using an adaptive filter). The existence
of a peak on the pulse signal within the ECG determined band
supports the existence of a pulse. In FIG. 8, the peak in the pulse
signal is outside the band, thus suggesting that a pulse is not
present.
[0045] The autocorrelation of the ECG signal can also be compared
with the autocorrelation of the pulse signal. Comparing
autocorrelation signals improves the signal to noise ratio and may
be particularly useful if the periodic component of the signals is
small. The two autocorrelation signals may be compared using a
cross-correlation. The comparison determines if the periodicity of
the two autocorrelated signals are similar. Similar periodicities
support the existence of a pulse.
[0046] The addition of a pulse detection system adds significant
improvements to automatic and semi-automatic external
defibrillators. The addition of a pulse detection system will
reduce the number of inappropriate shock advisements. Particularly
in cases where patients have a pulse, but have been incorrectly
classified by the ECG analysis algorithm as having a shockable
rhythm. As shown in the table of FIG. 9, the addition of the pulse
detection system will override the shock advisement from the ECG
analysis algorithm thereby appropriately inhibiting defibrillation
therapy. The addition of a pulse detection system will also enable
the defibrillator to advise the rescuer when CPR is appropriate and
inappropriate. In the case where the ECG analysis algorithm advises
"No Shock" and the pulse detection algorithm indicates "No Pulse",
the defibrillator may indicate to the rescuer that CPR should be
administered. Conversely, detection of a pulse by the pulse
detection system may be used to indicate to the rescuer that CPR is
no longer necessary.
[0047] The pulse detection system may also be used to improve the
defibrillator system by providing feedback to the rescuer during
the administration of CPR. In this scenario, the pulse detection
system may be utilized to monitor the effect that chest
compressions have on a patients pulse. The detected magnitude and
frequency of a pulse by the pulse detection system may be used
independently or in conjunction with accelerometer data to aid the
rescuer in delivering optimal CPR to the victim. Whereas an
accelerometer may be used to measure the rate and depth of
delivered chest compressions, a pulse detection system may be used
to determine if these chest compressions actually result in the
movement of blood thus resulting in a pulse.
[0048] A wide variety of pulse sensors may be used. For example,
the sensor could be optical or non-optical. It could include an
acoustic sensor (e.g., amplified stethoscope signals) for detecting
heart sounds characteristic of a beating heart. It could include a
sensor capable of mechanical or ultrasonic measurement (e.g.,
piezoelectric) of arterial wall motion--e.g., in locations such as
the neck (carotid arteries), arms (radial and brachial arteries),
and legs (femoral artery) where the arteries are relatively close
to the surface. It could include an ultrasonic measurement of blood
flow (e.g., such as the ultrasonic blood flow detectors used to
detect carotid and/or femoral artery stenosis). It could include a
pressure sensor that measures variation in the pressure in a
limb-compressing pneumatic cuff. The sensor could employ impedance
techniques for monitoring blood flow into and out of an arterial
bed, e.g., as now used to non-invasively measure pulsatile cardiac
output by measuring impedance across the chest. The same approach
could be applied to other part of the body where pulsatile blood
flow exists.
[0049] Both invasive (e.g., direct measurements of a parameter) and
non-invasive (indirect measurements of a parameter) sensors can be
used The examples given above are generally non-invasive. But
invasive sensors could also be used, including, for example,
pressure sensors coupled to a patient's vascular pressure via a
liquid filled catheter, or intravascular pressure sensors, in which
the sensor is incorporated onto the tip of a catheter placed in the
vascular system.
[0050] Many other implementations of the invention other than those
described above are within the invention, which is defined by the
following claims.
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