EXTERNAL DEFIBRILLATORS FOR ANALYSIS OF MULTIPLE ARRHYTHMIA CONDITIONS

This invention relates to defibrillators and, in particular, to external defibrillators which analyze a patient's heart rhythm for both life-threatening and non life-threatening conditions.

Defibrillators deliver a high-voltage impulse to the heart in order to restore normal rhythm and contractile function in patients who are experiencing arrhythmia, such as ventricular fibrillation ("VF"), also referred to as sudden cardiac arrest, or ventricular tachycardia ("VT") . There are several classes of defibrillators, including manual defibrillators, implantable defibrillators, and automatic external defibrillators (AEDs) . AEDs differ from manual defibrillators in that AEDs are pre-programmed to automatically analyze the electrocardiogram ("ECG") rhythm to determine if defibrillation is necessary and to provide administration measures such as shock sequences of the appropriate energy level followed or succeeded by periods when CPR is administered to the patient. AEDs are thus suitable for use by rescuers with no extensive medical training. Manual defibrillators are used by clinicians trained and experienced in advanced cardiac life support (ACLS) who are capable of reading a patient's ECG waveform, determining whether a shock is appropriate, then setting up the defibrillator to deliver the proper shock sequence and energy level.

The purpose of an AED is to allow people with little, or in some cases no, medical training to deliver a life-saving defibrillation shock to a victim of sudden cardiac arrest (SCA) . Since studies

have shown that delivering a shock in the first few minutes is usually crucial to the survival of the victim, it is important to allow treatment when possible before trained rescuers are able to arrive at the scene. However, because inappropriate application of a defibrillation shock can cause severe injury or death, AEDs have generally been conservatively designed to be highly specific about the conditions that allow a shock, particularly since layperson rescuers lack advanced medical training to correct the results of an error.

The goal of defibrillation is to restore blood flow when the victim has ventricular fibrillation or a non-perfusing tachycardia through application of an electric shock to settle the inappropriate electrical activity of the heart. Although most AED algorithms identify ventricular fibrillation well, it is impossible to determine if a tachycardia is perfusing from the ECG signal alone. Ventricular tachycardia at a high rate is considered to give insufficient time for adequate filling of the heart chambers, and the mechanics of VT produce inefficient pumping. The American Heart Association has not given specific recommendations for when tachycardia should be shock- treated based on ECG, and the AHA indicates that all tachycardias should be considered ventricular in origin unless the rhythm can be determined to be supraventricular in origin. Even supraventricular tachycardia (SVT) can yield insufficient cardiac output, as the time between beats is too short for ventricular filling.

AED algorithms that shock ventricular tachycardia generally have more stringent criteria for "organized" (monomorphic) tachycardia exhibiting a consistent, repeating ECG than for "disorganized'

(polymorphic) tachycardia which are highly irregular. Some people can maintain a perfusing rhythm at very high heart rates, so while the rhythm of monomorphic tachycardia remains stable, the condition could be presumed to be perfusing. A non-perfusing tachycardia will usually degenerate to an unstable rhythm, which would usually be recognized as shockable by the analysis algorithm. By setting very stringent criteria for shocking a monomorphic tachycardia, the chance of delivering an inappropriate, and potentially dangerous, shock to a perfusing patient is reduced.

However, some cardiologists recommend a more aggressive treatment protocol. These physicians contend that, even though the type of tachycardia is questionable, if it is perfusing, there is only a small chance of inducing a non-perfusing rhythm with a shock. And if the applied shock does induce a non- perfusing rhythm, the most common rhythm would be fibrillation, which is readily converted in a recently perfused heart. Because the probability of successful conversion of a non-perfusing rhythm decreases as the length without perfusion increases, it is a desirable to restore a normal rhythm before reaching a non-perfusing state. In the situation where a more serious condition is induced, in a hospital or EMS setting, a trained rescuer has already been dispatched. Some cardiologists therefore conclude that a delay in delivering a shock to a non-perfusing rhythm has higher risks than shocking a questionable rhythm.

In these cases, this philosophy contends, delivery of a synchronized shock, a therapy recommended by AHA for several classes of tachycardia, may be more appropriate. However,

synchronized cardioversion is a treatment for medical professionals with ACLS training, beyond the level that would be expected for layperson users of AEDs. Synchronized cardioversion can be, and often is, applied to patients who are responsive and breathing, a contraindication for (unsynchronized) defibrillation. While defibrillation is for life- threatening conditions where the treatment protocol should start immediately, treatment with synchronized cardioversion should usually start as soon as practical .

Previous methods have approached the uncertainty of whether to treat a monomorphic tachycardia in one of four ways :

1. be highly specific to avoid shocking perfusing rhythms by setting a very high rate threshold;

2. be highly sensitive to provide earliest treatment, relying on the low risk of complications for unnecessary shocks;

3. choose an arbitrary point to balance sensitivity vs. specificity; or

4. switch from defibrillation to synchronized cardioversion automatically if ventricular tachycardia is suspected.

Each of these approaches has problems. Being overly specific can delay electrical therapy to conditions that could benefit from the intervention. While waiting for a definitively shockable rhythm, the patient's condition could degrade to a point where the probability of recovery is reduced. Being overly sensitive can cause shocks to be delivered unnecessarily, possibly causing side effects that less-trained first-responders may be unable to

handle. Intermediary defibrillation thresholds merely trade risks of one error for risks of the other. Automatically changing modes from (unsynchronized) defibrillation to synchronized cardioversion as described, for instance, in US Pat. 6,246,907 (Lin et al . ) , reduces much of the risk to the patient associated with applying an unsynchronized shock, but it does not reduce the probability of applying an inappropriate shock. In other words, changing modes does not change the likelihood of making an erroneous decision to apply a shock, but if an inappropriate shock is delivered, the chance of a conversion to a nonperfusing rhythm by application of the erroneous shock is reduced by switching to synchronized delivery instead of an unsynchronized shock. The treatment protocol described in Lin et al . uses a starting energy suitable for defibrillation, so that if a QRS waveform is not available to synchronize the therapy pulse within a determined time frame, unsynchronized defibrillation therapy can be applied instead. In general, energy thresholds for synchronized cardioversion are much lower than for rhythms needing defibrillation, and for monophasic shocks, AHA recommends a starting energy for synchronized cardioversion that is half of the starting energy for defibrillation. See "2005 Guidelines for Cardiopulmonary Resuscitation and Cardiovascular Care" by the American Heart Association. There are several other reasons not to immediately apply synchronized cardioversion if it can be delayed. First, the shocks are painful, and prior administration of anesthetics would aid patient comfort. Second, the starting energy level for cardioversion should be selected by an experienced

clinician based on the cardiac rhythm and the rhythm determination by the defibrillator should be confirmed before applying elective therapy. Third, preferred therapy electrode placement is often different for atrial and ventricular cardioversion. Fourth, extended atrial fibrillation tends to cause thrombosis, and cardioversion without prophylactic thrombolytics or an echocardiogram to confirm the absence of a thrombosis could dislodge a clot and lead to stroke. Finally, selecting an ECG lead for analysis that does not have a dominant QRS can cause the defibrillator to select the wrong synchronization point, possibly triggering the shock on a tall T- wave, which can cause the exact condition that synchronized cardioversion attempts to avoid. Most treatment protocols, including the AHA Guidelines, indicate an ACLS-trained rescuer should verify that the defibrillator is correctly identifying the QRS as the synchronization point before delivering the shock.

In accordance with the principles of the present invention, an external defibrillator enhances the ECG analysis for rhythms which could respond to shock therapy by adding an alert for an alternate electrotherapy such as cardioversion to complement the regimen for defibrillation, while not automatically applying the alternate electrotherapy in a possibly inappropriate setting. The inventive method enhances the general ECG-based, shock-decision analysis algorithm of an external defibrillator to suggest that the rescuer consider cardioversion as a treatment option if the analyzed rhythm meets certain inclusion criteria, but is not a confirmed candidate for defibrillation. This enhancement can be augmented to enable cardioversion to be applied by

the defibrillator for medical systems that have users trained to deliver cardioversion therapy, either electrical, synchronized cardioversion using a defibrillation device or, alternatively, drug therapy to induce cardioversion. During the analysis for rhythms suitable for defibrillation, a defibrillator of the present invention can be used to indicate the appropriateness of a cardioversion therapy in cases where defibrillation is not indicated and when the proper personnel and conditions exist.

Insufficiently trained personnel should not give the therapy inadvertently, but the prompt from the device can speed therapy by trained rescuers. The invention overcomes the problem of not alerting the clinician to a treatment that the defibrillator identified and could provide, while not automatically applying the therapy when additional conditions are not known or other intervention called for that might indicate delaying such therapy. In the drawings:

FIG. 1 is an illustration of a defibrillator operable in either a manual or an automatic (AED) mode in accordance with the principles of the present invention . FIG. 2 is an illustration of defibrillation electrodes for connection to the defibrillator of FIG. 1.

FIG. 3 is an illustration of monitoring electrodes for connection to the defibrillator of FIG. 1.

FIG. 4 is an illustration of the defibrillator of FIG. 1 with the access door for the manual mode in an open position.

FIG. 5 is a simplified block diagram of the defibrillator of FIG. 1 according to the present

invention

FIG. 6 is a flowchart illustrating an operating method of an external defibrillator in accordance with the present invention. FIGS. 7A and 7B are diagrams of a user interface of the defibrillator of FIG. 1 in AED mode and in manual mode employing a graphical display device in accordance with the principles of the present invention . FIG. 1 is an illustration of an external defibrillator 10 constructed in accordance with the principles of the present invention. The defibrillator 10 has an access door 20 shown in a closed position on a housing 19. On a top surface of the defibrillator 10 is located a power-on button 22 labeled "1" which powers up the defibrillator 10 and begins the process of the prompting the first responder 12 to connect defibrillation electrodes 16 to a patient 14. Adjacent to the power-on button 22 is an analyze button 24 labeled "2" which initiates an automated analysis of the ECG signal acquired from the patient 24 which produces a shock advisory message. The ECG analysis may also be initiated automatically upon detection of patient contact across the defibrillation electrodes 16. Adjacent to the analysis button 24 is a shock button 26 labeled "3" which initiates delivery of the defibrillation shock to the patient 14 across the defibrillation electrodes 16 if a shock is recommended by the ECG analysis. The power-on button 22, analysis button 24, and shock button 26 labeled "1", "2", and "3" collectively are AED buttons 34 which are used when the defibrillator 10 is operated in the AED mode. Various ones of the buttons 34 may also be operable in the manual mode. The precise labeling of the AED

buttons 34 is not critical as long as the logical order of the AED user interface is maintained to minimize confusion.

A display 36 mounted adjacent to the AED buttons 34 is preferably a liquid crystal display (LCD) that is capable of displaying text such as labels and messages and graphics such as ECG traces. A printer 38 mounted on the top surface of the defibrillator 10 provides a hard copy printout of ECG signals and event markers gathered during treatment of a patient. A set of softkey controls 40 mounted adjacent to the display 36 provides for selection of functions according to softkey labels (shown in FIG. 7) on the display 36. A speaker 42 provides for audio prompts to the user, particularly when the defibrillator 10 is in the AED mode. A connector 44 provides for coupling to other sensors gathering patient parameters such as pulse oximetry (SpO ₂ ) .

An AED instruction label 46 is mounted on the top surface of the defibrillator 10. The AED instruction label 46 provides initial instructions to the first responder who must use the defibrillator 10 in its AED mode.

A manual mode warning label 48 is mounted on a top side of the access door 20. The manual mode warning label 48 contains text or graphics designed to warn a layperson first responder not to open the access door 20. For example, text that reads "MANUAL MODE" may be used if this label has sufficient meaning to a first responder to prevent confusion in an emergency situation. Access to the manual mode can be further protected with a mechanical key lock (not shown) on the access door 20 or a software access code to implement the manual mode that must be entered by means of a combination of button presses.

FIG. 2 is an illustration of the defibrillation electrodes 16 having a connector 18 for insertion into a patient connector socket (not shown) on the defibrillator 10. The defibrillation electrodes 16 are used both to acquire the ECG signal from the patient and to couple the defibrillation shock across the patient from the defibrillator 10. The defibrillation electrodes 16 are selected automatically with the defibrillator 10 in the AED mode and may also be selected manually with the defibrillator 10 set to operate in the manual mode.

FIG. 3 is an illustration of a set of monitoring electrodes 28 having a connector 30 for connection to a monitoring port 32 on the defibrillator 10. Three monitoring electrodes 28 are shown for purposes of illustration. Greater or fewer numbers of monitoring electrodes 28 may be chosen for various applications such as to implement 3, 5, or 12 lead ECG monitoring. FIG. 4 is an illustration of the defibrillator 10 according to the present invention having the access door 20 shown in an open position. A manual access button 50 controls access to the manual mode. In the manual mode, various manual functions including defibrillation, synchronized cardioversion or pacing may be selected using a manual function select control 51. Mounted on the bottom side of the access door 20 is a manual mode instruction label 52 that provides instruction to an ACLS user on manual mode operation. The manual access button 50, the manual function select control 51, and the manual mode instruction label 52 are visible to the user only when the access door 20 is in the open position.

In an alternative embodiment of the present invention, switching between the AED mode and the manual mode may be done simply by opening the access

door 20, thereby eliminating the need to press the manual access button 50 to place the defibrillator 10 in the manual mode. A switch or sensor (not shown) may be coupled to the access door 20 to sense the open or closed position of the access door 20 to place the defibrillator 10 in the manual or AED modes respectively. The manual function select 51 located underneath the access door 20 would still be used to select among various manual functions when the defibrillator 10 is in the manual mode.

FIG. 5 is a simplified block diagram of the defibrillator 10 according to the present invention. An ECG front end 100 is connected to the defibrillation electrodes 16 that are in turn connected across the chest of a patient. The ECG front end 100 operates to amplify, buffer, filter and digitize an electrical ECG signal generated by the patient's heart to produce a stream of digitized ECG samples. The digitized ECG samples are provided to a controller 102 which performs an ECG analysis to detect VF, VT or other shockable rhythm. If a shockable rhythm is detected, the controller 102 sends a signal to HV delivery 104 to charge up in preparation for delivering a defibrillation shock. Pressing the shock button 26 (shown in FIG. 1) then triggers the HV delivery 104 via the controller 102 to deliver a defibrillation shock to the patient through the electrodes 16.

Also connected to the ECG front end 100 is the set of monitoring electrodes 28. The ECG front 100 is capable of selecting among the defibrillation electrodes 16 and the monitoring electrodes 28, preferably under control of the controller 102.

A memory 106 may be implemented either as on- board RAM or ROM, a removable memory card, or a

combination of different memory technologies. The memory 106 contains an AED personality program 108 and a manual personality program 110. The AED personality program 108 defines the operation of the defibrillator 10 in AED mode, such as the operation of the AED buttons 34 and the information content that is displayed on the display 36. A manual personality program 110 defines the operation of the defibrillator 10 in the manual mode, such as the operation of the buttons 34, the softkey controls 40, the manual function select control 51, and the information content which is displayed on the display 36.

The AED buttons 34, softkeys 40, display 36, speaker 42, printer 38, manual access button 50, and manual function select 51 collectively form the user interface of the defibrillator 10 and are connected to the controller 102 to receive input from the user and provide audio and visible feedback to the user. Dedicated buttons, such as to control the printer operator or to adjust the ECG size, display contrast, and speaker volume, may be included to control common functions of the defibrillator 10 in both AED and manual modes. FIG. 6 is a flowchart of an operating sequence of the defibrillator 10 in accordance with the present invention. When the controller 102 begins to receive ECG samples from the ECG front end 100 the controller begins to execute its ECG analysis routine at 70 in order to decide at 72 whether a shockable rhythm is present. When the ECG analysis routine indicates that a life-threatening arrhythmia is present such as VF or polymorphic tachycardia, the user is informed at 74 that a shock is advised. The "shock advised" message is issued audibly through the

speaker 42 and may also or alternatively be displayed on the display 36. This message is followed by other messages such as a warning to the user not to touch the patient while the shock is being delivered. The HV delivery 104 then charges the capacitor of the high voltage circuitry and delivers a shock at 76, preferably a biphasic defibrillation pulse.

Alternatively the ECG analysis routine may conclude at 72 that a life-threatening arrhythmia is not present. In accordance with the present invention, the ECG analysis routine then evaluates the ECG data at 80 to determine whether cardioversion is appropriate. In the illustrated embodiment the ECG analysis routine analyzes for a cardiac condition for which synchronized or chemical cardioversion may be appropriate if defibrillation is determined to be in appropriate. For example, a ventricular tachycardia that exceeds 150 beats per minute may not meet all the indications for a defibrillation shock by the AED algorithm, but the AHA allows for this rhythm to be treated with synchronized electrical cardioversion if an ACLS-trained provider deems it appropriate. When the result of this analysis is that no other therapy is identified as appropriate, a "no shock advised" message is issued at 82 and the defibrillator continues to analyze the ECG at 70. When the result of the analysis is that cardioversion may be appropriate, the user is advised at 84 to consider cardioversion. This may be done through the visual display and/or sound generation capability of the device to inform the clinician of a suggested alternative treatment for cardiac rhythms that may respond to the cardioversion therapy. For example, after the ECG analysis determines that defibrillation is not indicated but the analysis determines that

synchronized or chemical cardioversion may be appropriate, the defibrillator displays the words "Consider Cardioversion" on the screen 36 and a voice prompt would be announced on the speaker 42 as, "Defibrillation not advised. Consider cardioversion." Exact phrasing, both visual and audible, would be determined by the desires of the users, and may be translated to languages appropriate for the location where the defibrillator is used. If a visual display or audio speaker is not part of the defibrillator, then the corresponding announcement is be omitted. Other forms of announcement, such as tones, lights, or other non-verbal or verbal alerts can be used. The defibrillator then resumes ECG analysis at 70, unless an ACLS-qualified operator begins use of the defibrillator to apply the alternate therapy as illustrated below.

FIG. 7A and 7B are illustrations of portions of the user interface of the defibrillator 10 in the AED mode and in the manual mode respectively. In FIG. 7A, with the defibrillator 10 in AED mode, the AED buttons 34 for operating the defibrillator 10 are labeled by a set of soft labels 54 on the display including "ON", "ANALYZE", and "SHOCK" which define their functions according to the AED personality 108. A graphical trace 56 of the ECG signal may be displayed in the center portion of the display 36. A message block 58 containing selected user prompts such as "NO SHOCK ADVISED CHECK PATIENT" may be placed on the lower portion of the display 36. In this example the defibrillator has just concluded its analysis of the advisability of alternate therapies and concluded that cardioversion may be appropriate for the patient, in which case the message block 58 advises the user to "CONSIDER CARDIOVERSION." The

softkey controls 40 may be configured to be on or off with the defibrillator 10 depending on the particular version of the AED personality program 108 that has been chosen. In some EMS systems, none of the softkey controls 40 may be enabled while in other EMS systems, a selected set of functions may be made available with the defibrillator in the AED mode.

In FIG. 7B, the manual access button 50, which is revealed by opening the access door 20, may be pressed to place the defibrillator 10 into the manual mode. In the manual mode, the set of soft labels 54 are changed as functions controllable by the more advanced ACLS user are added to the menu structure in manual mode according to the manual personality program 110. As shown, the set of soft labels 54 on the display now read "ENERGY SELECT", "CHARGE", and "SHOCK" which define the functions of the buttons 34 in their operation of the defibrillator in the manual mode. Various manual functions such as synchronized cardioversion or pacing may be selected with the manual function select 51 which is also located under the access door 20. In addition, the softkey controls 40 may be activated to reveal a set of soft labels 60 including, for example, "ALARM SETTINGS", "LEAD SELECT", and "SpO ₂ " which are added to label the functions of each of the softkey controls 40 for additional user control of manual functions according to the manual personality program 110. Menu trees may be implemented and logically arranged to provide ready access to the desired manual functions available according to the manual personality program 110.

In a hospital setting in non-critical wards, highly trained clinicians are generally arriving at the scene of a cardiac event at about the time that

an AED has been applied to the patient and the rhythm is being analyzed. Many hospitals use advanced defibrillators in the AED mode in these non-critical areas because the first-responding nurses may not be fully trained in advanced cardiac life support skills. The AED mode then immediately provides the initial care that is needed during this early response period. The follow-up ACLS-qualified rescuers can be shown the prompt for consideration of cardioversion as a treatment option and can respond accordingly. Alternatively, more basic-level AEDs can be enhanced to optionally deliver synchronized cardioversion by those with the proper training. In another embodiment, the algorithm advising cardioversion can be used as an additional monitoring alert of a cardiac monitor, independent of analysis for shockable rhythms .

For advanced defibrillators, the application of the alternate therapy can be achieved as is known for application of a synchronized cardioversion pulse, which is device-dependent. One such defibrillator achieves this by exiting the AED mode and entering the manual mode of operation. A button is depressed to start a "synchronized cardioversion mode, " an appropriate energy for the pulse is selected, and the "charge" button is depressed. After the device charges, the "shock" button is depressed to deliver the cardioverting shock. The clinician is responsible for determining that the patient is properly prepared for electrical cardioversion, e.g., sedated/unconscious; thrombolytics if recommended, and that the ECG indication is appropriate for the therapy .

As a further example, an advanced defibrillator can more simply apply synchronized cardioversion from

within the AED mode, such as by depressing a button to start "synchronized cardioversion mode" without exiting the AED application. The defibrillator is charged to a predetermined cardioversion level automatically, with the application of the pulse triggered by depressing the "shock" button. The protocol of an alternate method can be determined by the clinical service which has deployed the defibrillator, depending on the level of training of the staff. The defibrillator can be configured for different therapy protocols.

For basic AEDs, it is generally advisable to prevent their use for cardioversion by those not ACLS-trained for the therapy. For instance, the application of synchronized cardioversion may only be conducted after the operator depresses buttons on the defibrillator in a pattern known only to trained users, or by entry of a code enabling advanced operation of the defibrillator by some other method. Restricted use of the cardioversion mode prevents a layperson user from administering a therapy which should only be performed by properly trained specialists .