# Patent

## Title
CPR sensitive ECG analysis in an automatic external defibrillator

## Abstract

An automatic external defibrillator including: a sensor for detecting when a rescuer is delivering a CPR chest compression to the patient; electrodes for application to the thorax of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal; defibrillation circuitry for delivering a defibrillation shock to the electrodes; and a processor and associated memory for executing software that controls operation of the defibrillator. The software provides: ECG analysis for analyzing the ECG signal to determine if the cardiac rhythm is shockable; CPR detection for analyzing the output of the sensor to determine when a CPR chest compression has been delivered, and integration of the ECG analysis and CPR detection so that the determination of whether the cardiac rhythm is shockable is based only on time periods of the ECG signal during which there has not been a CPR chest compression delivered.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation application of and claims priority to application Ser. No. 10/370,036 filed Feb. 19, 2003, now U.S. Pat. No. 6,961,612 issued on Nov. 1, 2005.


TECHNICAL FIELD
This invention relates to automatic external defibrillators (AEDs), and particularly to signal processing performed by such defibrillators.
BACKGROUND
Automated External Defibrillators include signal processing software that analyzes ECG signals acquired from the victim to determine when a cardiac arrhythmia such as Ventricular Fibrillation (VF) or shockable ventricular tachycardia (VT) exists. Usually, these algorithms are designed to perform ECG analyses at specific times during the rescue event. The first ECG analysis is usually initiated within a few seconds following attachment of the defibrillation electrodes to the patient. Subsequent ECG analyses may or may not be initiated based upon the results of the first analysis. Typically if the first analysis detects a shockable rhythm, the rescuer is advised to deliver a defibrillation shock. Following the shock delivery a second analysis is automatically initiated to determine whether the defibrillation treatment was successful or not (i.e. the shockable ECG rhythm has been converted to a normal or other non-shockable rhythm). If this second analysis detects the continuing presence of a shockable arrhythmia, the AED advises the user to deliver a second defibrillation treatment. A third ECG analysis may then be initiated to determine whether the second shock was or was not effective. If a shockable rhythm persists, the rescuer is then advised to deliver a third defibrillation treatment.
Following the third defibrillator shock or when any of the analyses described above detects a non-shockable rhythm, treatment protocols recommended by the American Heart Association and European Resuscitation Council require the rescuer to check the patient's pulse or to evaluate the patient for signs of circulation. If no pulse or signs of circulation are present, the rescuer is trained to perform CPR on the victim for a period of one or more minutes. Following this period of cardiopulmonary resuscitation (that includes rescue breathing and chest compressions) the AED reinitiates a series of up to three additional ECG analyses interspersed with appropriate defibrillation treatments as described above. The sequence of 3 ECG analyses/defibrillation shocks followed by 1-3 minutes of CPR, continues in a repetitive fashion for as long as the AED's power is turned on and the patient is connected to the AED device. Typically, the AED provides audio prompts to inform the rescuer when analyses are about to begin, what the analysis results were, and when to start and stop the delivery of CPR.
One limitation associated with many AEDs is that the period between each set of ECG analyses and shocks is pre-programmed into the device and is fixed for all rescue situations. When the application of CPR due to lack of circulation is warranted, this pre-programmed period is consumed by the delivery of rescue breaths and chest compressions. When no CPR is warranted because the last shock was effective in converting the patient to a perfusing cardiac rhythm, this pre-programmed period is consumed by periodically monitoring the patient's pulse and assuring that no relapse or re-fibrillation has occurred. Under some out-of-hospital rescue protocols, the period between successive sets of ECG analyses can be as long as 3 minutes.
It is commonly known that victims of cardiac arrest who have been successfully defibrillated sometimes relapse into ventricular fibrillation shortly after a successful shock treatment. In such cases, the rescuer who verified the presence of a pulse immediately following defibrillation and thus decided not to perform CPR, may be unaware that the victim's condition has deteriorated until the next ECG analysis is performed some 1-3 minutes later. Under these conditions, the delivery of needed defibrillation treatments and CPR may be delayed.
Some AEDs are designed to avoid this undesirable delay in treatment by continuously and automatically analyzing the victim's ECG whenever defibrillation electrodes are connected to the patient. These AEDs perform a continuous “background” analysis that evaluates the victim's ECG signals during the 1-3 minute CPR/monitoring period between analysis/shock sequences. As such, they are able to detect refibrillation of the victim's heart (should it occur) and promptly advise the rescuer of the patient's deteriorated condition. While these “improved” systems help prevent the delay in treatment that can result from undetected refibrillation of the patient's heart, they are also susceptible to misinterpreting the ECG artifact introduced by CPR related chest compressions as shockable arrhythmias. When the ECG analysis algorithm misinterprets CPR related ECG artifact as a shockable rhythm, it may advise the rescuer to prematurely stop performing CPR and to deliver a defibrillation treatment. While in some cases immediate defibrillation may be the appropriate treatment, some clinical research suggests that an appropriately long period of CPR between sequences of defibrillation treatments may be more beneficial to the patient than immediate defibrillation, particularly when VF has been of long duration or persistently recurring. Furthermore when the victim's cardiac rhythm is not treatable by defibrillation therapy (non-shockable) but incompatible with life such as in cases of asystole or electromechanical disassociation, the premature cessation of CPR in response to the erroneous detection of a shockable cardiac rhythm can reduce the patient's chances of survival.
For those AEDs that perform background ECG analysis during periods between analysis/shock sequences, a common strategy for ensuring that sufficient time is provided for effective CPR delivery even in the presence of shockable rhythms is to disable background ECG analysis or ignore the results of this analysis for a predetermined time period following the last shock in each treatment sequence. During this period, the rescuer is allowed to perform CPR without advice from the unit that a shockable rhythm is present. Following this period, ECG analysis results are used to prompt the user to stop CPR and thus allow a CPR artifact free ECG analysis to be performed.
Since most currently available AEDs are not equipped to detect or monitor the delivery of CPR related chest compressions, they are incapable of determining when CPR artifact is present in the ECG signals and when it is not. The automatic activation/deactivation of their background ECG analysis function, therefore, is based exclusively upon time since the last shock or completion of the last “foreground” ECG analysis. If the delivery of CPR is stopped during this period when background ECG analyses have been disabled, life threatening changes in the patient's ECG rhythm will remain undetected (even though it could be effectively analyzed) for at least some period of time during the rescue event.
SUMMARY
In general, the invention features an automatic external defibrillator including: a sensor for detecting when a rescuer is delivering a CPR chest compression to the patient; electrodes for application to the chest of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal; and defibrillation circuitry for delivering a defibrillation shock to the electrodes. ECG analysis is performed to determine if the cardiac rhythm is shockable (i.e., treatable by defibrillation therapy). The output of the sensor is detected to determine when a CPR chest compression has been delivered. The ECG analysis and CPR detection are integrated so that the determination of whether the cardiac rhythm is treatable by defibrillation therapy is based only on time periods of the ECG signal during which there has not been a CPR chest compression delivered.
The invention improves the specificity as well as the reliability of the ECG rhythm classification. The improved reliability of ECG rhythm classification (e.g. during ECG background analysis) enhances the survival chances of victims in at least two ways. It reduces the likelihood of premature cessation of needed CPR as the result of a false detection of a shockable cardiac rhythm when the victim's cardiac rhythm is actually not treatable by defibrillation therapy (e.g., cases such as asystole or electromechanical disassociation, for which the more appropriate therapy is CPR). It avoids undesirable delay in treatment of re-fibrillation when it occurs, by allowing for continuously and automatically analyzing the victim's ECG during the 1-3 minute CPR monitoring periods between analysis/shock sequences.
Preferred implementations of the invention may incorporate one or more of the following:
The ECG analysis may comprise analysis of the ECG signal over a minimum ECG analysis time to determine whether the cardiac rhythm is shockable, and wherein integration of the ECG analysis and CPR detection may be done so that a determination that the cardiac rhythm is shockable is only made if the time period over which the ECG signal has been analyzed to make that determination includes a time period at least as long as the minimum ECG analysis time during which there has not been a CPR chest compression delivered.
A timer may be reset when a CPR chest compression has been delivered, and the value of the timer may be examined to determine whether the minimum ECG analysis time has been exceeded.
The ECG analysis may be reinitialized when it is determined that a CPR chest compression has been delivered, and only after the ECG analysis has been conducted for at least the minimum ECG analysis time without being reset is a determination made as to whether the cardiac rhythm is shockable.
The ECG analysis and CPR detection may be performed continuously during the period in which determinations are being made as to whether the cardiac rhythm is shockable.
Other features and advantages of the invention will be apparent from the following drawings and detailed description, and from the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of one implementation of the invention.
FIG. 2 is a block diagram of the integrating task of the implementation of FIG. 1.
FIG. 3 is a block diagram of another implementation of the invention.
FIG. 4 is a block diagram of the ECG analysis of the implementation of FIG. 3.
FIG. 5 is a block diagram of another implementation of the invention.
FIG. 6 is a block diagram of the integrating task of the implementation of FIG. 5.



DETAILED DESCRIPTION
There are a great many different implementations of the invention possible, too many to possibly 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.
FIG. 1 shows one preferred implementation, in which the CPR detection task 20 and ECG background analysis task 22 run simultaneously and continuously during a CPR preprogrammed interval of 1 to 3 minutes. The outputs of the CPR detection task (e.g., times at which CPR compressions are delivered, amplitude of compressions, rate of compressions) and the outputs of the ECG analysis task (e.g., times at which QRS is detected, heart rate, and rhythm classification) are passed to a higher level integrating task 24. The higher level integrating task analyzes the data from both ECG and CPR tasks and qualifies ECG rhythm classifications based on the presence or absence of the detection of CPR chest compressions during the ECG analysis period. For example, if a CPR chest compression is detected, the higher level task ignores background ECG rhythm classifications for a time interval of at least X seconds after the compression, where X seconds is the minimum time interval needed by the ECG background analysis algorithm to classify an ECG rhythm. This guarantees that for the duration of at least X seconds the ECG background analysis algorithm uses a noise free ECG signal to make its classification. As a result, the number of false shockable rhythm detections due to CPR related artifact in the ECG signal is reduced and the specificity as well as the reliability of the ECG rhythm classification is improved. This improved reliability of ECG background analysis enhances the survival chances of victims in two ways: (1) it eliminates premature cessation of needed CPR as the result of a false detection of a shockable cardiac rhythm when the victim's cardiac rhythm is actually not treatable by defibrillation therapy (e.g., cases such as asystole or electromechanical disassociation, for which the more appropriate therapy is CPR); (2) it avoids undesirable delays in treatment of re-fibrillation when it occurs, by continuously and automatically analyzing the victim's ECG during the 1-3 minute CPR monitoring period between analysis/shock sequences and informing the rescuer when a shockable rhythm has been detected.
FIG. 2 shows the process followed by the integrating task. CPR compression detection data from the CPR task and ECG rhythm classification data from the ECG task are received. The integrating task checks (30) if a compression has been detected. If a CPR compression is detected, the integrating task sets to zero (32) the elapsed time T0 since the last compression was detected and continues to execute and check for new compression detections. If no new compression is detected, the integrating task calculates (34) the elapsed time T0 since the last compression was detected. It then checks (36) to determine if T0 is greater than the time needed (X seconds) by the ECG analysis algorithm to generate a rhythm classification. If T0 is smaller than X seconds, the integrating task returns to checking for newly detected CPR compressions. Else if T0 is larger than X seconds, the integrating task moves to checking (38) the rhythm classifications being received from the ECG task. If a shockable rhythm is being reported, the integrating task alerts the user to the presence of a shockable rhythm (40) and causes the defibrillator to switch to the ECG foreground analysis state (42).
FIG. 3 shows another implementation. The CPR detection task 50 and ECG background analysis task 52 run simultaneously and continuously during a CPR preprogrammed interval. The outputs of the CPR task are passed to the ECG task, and the ECG task uses the CPR data to qualify ECG rhythm classification based on the presence or absence of the detection of CPR chest compression during the ECG analysis period. For example, if a CPR chest compression is detected, the ECG task will restart its ECG rhythm classification process, which usually requires a predefined time interval of X seconds to complete its classification of rhythms. This method guarantees that for the predefined time interval X seconds needed to classify a rhythm the ECG background analysis algorithm uses a noise free ECG signal to make its classification. As a result, the number of false shockable rhythm detections is reduced. Thus, the specificity as well as the reliability of the ECG rhythm classification is improved.
FIG. 4 shows the process followed by the ECG background analysis task in the implementation of FIG. 3. The ECG task receives CPR compression detection data from the CPR task and ECG waveform data from the defibrillator front end simultaneously and continuously. Upon receiving an ECG waveform data sample, the ECG task checks (62) if a compression has been detected. If a CPR compression is detected, the ECG task reinitializes (64) the ECG analysis by restarting it from time zero. The ECG analysis task analyzes an ECG segment of data of length X seconds to determine the ECG rhythm. By initializing the ECG analysis process, the ECG task restarts its analysis of a new ECG data segment of X seconds long resulting in no ECG rhythm classification for the next X seconds. If no new compression is detected, the ECG task checks (60) if the analysis process has been initialized. If no ECG analysis is in progress, the ECG task initializes (64) the ECG analysis process. If the ECG analysis has been initialized, the ECG task continues to analyze the ECG waveform data for duration of X seconds. If no shockable rhythm is detected (66), the ECG task returns to receive the next sample. If a shockable rhythm is being reported (66), the ECG task switches to the ECG foreground analysis state (68), and alerts (170) the user to the presence of a shockable rhythm.
FIG. 5 shows another implementation. The CPR detection task 70 and ECG background analysis 72 task run simultaneously and continuously during a CPR preprogrammed interval of 1 to 3 minutes. The outputs of the CPR task and the outputs of the ECG task are passed to a higher level integrating task 74. This higher level integrating task analyzes the data from both ECG and CPR tasks, and reinitializes the ECG rhythm classifications when a CPR chest compression is detected. The higher level integrating task alerts the user about a shockable rhythm when reported by the ECG task. For example, if a CPR chest compression is detected, the higher level integrating task will restart the ECG rhythm classification process, which usually requires a predefined time interval of X seconds to a complete its classification of rhythms. This method guarantees that for the predefined time interval X seconds needed to classify a rhythm the ECG background analysis algorithm uses a noise free ECG signal to make its classification.
FIG. 6 shows the process followed by the integrating task in the implementation of FIG. 5. The Integrating task receives CPR compression detection data from the CPR task and ECG rhythm classification data from the ECG task. The integrating task checks (80) if a compression has been detected. If a CPR compression is detected, the integrating task reinitializes (82) the ECG analysis by restarting it from time zero. The ECG analysis task analyzes an ECG segment of data of length X seconds to determine the ECG rhythm. By initializing the ECG analysis process, the ECG task restarts its analysis of a new ECG data segment of X seconds long resulting in no ECG rhythm classification for the next X seconds. If no new compression is detected, the integrating task checks (84) the rhythm classifications being received from the ECG task. If a shockable rhythm is being reported, the integrating task alerts the user (86) about the presence of a shockable rhythm, at which point the defibrillator switches to the ECG foreground analysis state.
Many other implementations of the invention other than those described above are within the invention, which is defined by the following claims.

## Claims

1. An automatic external defibrillator for delivering defibrillation shocks to a patient, comprising
a sensor for detecting when a rescuer is delivering a CPR chest compression to the patient;
electrodes for application to the thorax of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal;
defibrillation circuitry for delivering a defibrillation shock to the electrodes;
a processor and associated memory for executing software that controls operation of the defibrillator, the software providing
ECG analysis for analyzing the ECG signal to determine if the cardiac rhythm is shockable (treatable by defibrillation therapy);
CPR detection for analyzing the output of the sensor to determine when a CPR chest compression has been delivered, and
integration of the ECG analysis and CPR detection so that the determination of whether the cardiac rhythm is shockable is based on a plurality of time periods of the ECG signal spaced apart in time during which there has not been an artifact from a delivered CPR chest compression,
wherein at least one of the time periods of the ECG signal on which the determination is based is a time period prior to delivery of a CPR chest compression and at least a second of the time periods of the ECG signal on which the determination is based is a time period after delivery of the same CPR chest compression.

2. An automatic external defibrillator for delivering defibrillation shocks to a patient, comprising
a sensor for detecting when a rescuer is delivering a CPR chest compression to the patient;
electrodes for application to the chest of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal;
defibrillation circuitry for delivering a defibrillation shock to the electrodes;
ECG analysis means for analyzing the ECG signal to determine if the cardiac rhythm is shockable (treatable by defibrillation therapy);
CPR detection means for analyzing the output of the sensor to determine when a CPR chest compression has been delivered, and
integration means for integrating the ECG analysis and CPR detection so that the determination of whether the cardiac rhythm is shockable is based on a plurality of time periods of the ECG signal spaced apart in time during which there has not been an artifact from a delivered CPR chest compression,
wherein at least one of the time periods of the ECG signal on which the determination is based is a time period prior to delivery of a CPR chest compression and at least a second of the time periods of the ECG signal on which the determination is based is a time period after delivery of the same CPR chest compression.

3. A method for automatically delivering defibrillation shocks to a patient, comprising
applying electrodes to the chest of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal;
using a sensor to detect when a rescuer is delivering a CPR chest compression to the patient;
analyzing the ECG signal to determine if the cardiac rhythm is shockable (treatable by defibrillation therapy);
analyzing the output of the sensor to determine when a CPR chest compression has been delivered, and
integrating the ECG analysis and CPR detection so that the determination of whether the cardiac rhythm is shockable is based on a plurality of time periods of the ECG signal spaced apart in time during which there has not been an artifact from a delivered CPR chest compression,
wherein at least one of the time periods of the ECG signal on which the determination is based is a time period prior to delivery of a CPR chest compression and at least a second of the time periods of the ECG signal on which the determination is based is a time period after delivery of the same CPR chest compression.