More than 330,000 Americans die annually from coronary heart disease and associated sudden cardiac arrest (SCA). Approximately 250,000 of these deaths occur in the out-of-hospital setting. Early defibrillation is critical to survival from cardiac arrest for several reasons:

1) the most frequent initial rhythm witnessed SCA is ventricular fibrillation (VF); 2) definitive treatment for VF is electrical defibrillation; 3) success of defibrillation decreases rapidly over time; 4) VF tends to convert to asystole within a few minutes.^1,2,3

The probability of survival for a victim of SCA greatly increases with early defibrillation, which along with cardiopulmonary resuscitation has consistently been the American Heart Association's recommended action in the event of an SCA. For a victim to have the best chance of survival, three actions must occur within the first moments of a cardiac arrest: 1) activation of emergency medical services response system; 2) provision of effective CPR; 3) timely operation of an AED or use of a defibrillator. Health care providers treating cardiac arrest in hospitals or other facilities should provide immediate CPR and should use an AED or defibrillator as soon as it is available.⁴

Early CPR and defibrillation represent definitive intervention in cardiac arrest. Properly performed CPR preserves heart and brain function and extends the period of time that the heart is likely to respond to defibrillation. If defibrillation can be provided within the first five minutes of a cardiac arrest, the victim has a 50% chance or better of survival. With each passing minute in arrest, however, opportunity for successful resuscitation is reduced by 7% to 10%. Beyond a 10-minute window, little probability of a successful outcome can be expected. The automated external defibrillator (AED) has greatly improved the accessibility of defibrillation and has made it available when it can be most effective.^5-8

The AED has primarily been used outside the hospital where most cardiac arrests occur, but increasingly, early defibrillation programs are being established in hospitals and outpatient/ambulatory care settings. Registered nurses and other health care professionals with proper training can augment the assembly of rescuers prepared to perform early defibrillation on a larger group of in-hospital cardiac arrest patients with this life saving device.^6,7

Initial interventions for cardiac arrest

Interventions during the first few minutes following SCA are critical and the three basic life support actions are early access, early CPR, and early defibrillation. Brain death will occur in four to six minutes after someone has a cardiac arrest; however, immediate, well-performed CPR and timely defibrillation can reverse cardiac arrest in many victims. CPR has been found to prolong VF, preventing deterioration to asystole however, CPR only buys a few precious minutes. Defibrillation within the first few minutes of collapse can potentially achieve a survival rate as high as 90%, but high survival rates are not achieved when time to defibrillation is extended, and the possibility is reduced by approximately 10% for each passing minute. CPR alone will not usually eliminate VF and restore a perfusing rhythm.^4,5,7

The AED should only be used when a patient is found unresponsive, apneic, pulseless, or showing no signs of life. The majority of cardiac arrest victims remain in VF or pulseless VT during the first eight minutes following collapse.^9,10 The chaotic electrical activity of VF originates from multiple sites within the heart. As a result, the myocardium quivers, and the heart's chambers cannot fill or generate a normal contraction to produce cardiac output. Pulseless VT is a less frequently occurring dysrhythmia that also requires defibrillation. The rhythm is organized, but the rate is extremely rapid, causing a shortened filling time for the heart's chambers. The result is decreased cardiac output and organ perfusion. Untreated pulseless VT can deteriorate to VF or asystole.

Defibrillation is not indicated and is ineffective therapy for pulseless electrical activity (PEA) and asystole. PEA is mechanical failure of heart muscle in the presence of an electrical rhythm. Heart muscle is not contracting, although conduction through the heart's electrical system continues, producing a waveform on the cardiac monitor. Asystole is electrical and mechanical cardiac failure. Electrical impulses are no longer generated or conducted by the heart and no muscular contraction is apparent. In these situations, the patient is also unresponsive, apneic, and pulseless. Defibrillating asystole can stun the heart and produce a profound parasympathetic discharge, preventing natural pacemaker recovery and spontaneous cardiac activity.^9,10

How AEDs work

An AED is easy for personnel with varying levels of emergency training and experience to use, because proficiency in cardiac rhythm recognition is not required for its operation. The device incorporates a rhythm analysis and shock delivery system. The applied chest electrodes and cable transmit the patient’s cardiac electrical activity for analysis and deliver defibrillation current if the identified rhythm is VF or pulseless VT. An AED relies on its computer program to recognize VF or pulseless VT, charge the defibrillator, and either advise or automatically deliver an appropriate shock.^10,11

During the analytical phase, the AED’s programmed logic algorithm evaluates the patient’s rhythm according to rate, amplitude, QRS slope, morphology, and variation from the isoelectric line. Comparison is made with preset abnormal levels. The ECG is digitally scanned at 100 samples per minute and evaluations are made at approximately two-to four-second intervals. Detection of abnormal complexes during two intervals of these consecutive checks will prime the AED for shock delivery at 150 to 360 joules. The average time for rhythm analysis, charge, and shock delivery is five to 15 seconds, depending on the particular brand of AED.^10,12

AEDs are classified as either fully automated or semiautomated. Fully automated AEDs deliver the shock without operator assistance once the defibrillator is fully charged to its preset level of energy. The devise issues an audible order to ‘stand clear’ and then initiates the rhythm analysis phase. If defibrillation is indicated, the AED charges and then discharges the current.

Semiautomated or shock advisory AEDs are more commonly used to analyze the patient’s cardiac rhythm, but do not automatically deliver the shock. Some models may also require the user to press an analysis button to initiate the process. Prior to defibrillation, these devices advise personnel again to stand clear of the patient and prompt the operator to depress a shock discharge button. The semiautomated machine significantly increases the safety margin, permitting the operator to assess the scene for any personnel who may be in contact with the patient and to warn of imminent defibrillation before discharging the shock.

AED microprocessors and computer programs have benefited from more than 20 years of testing and refinement. They reliably distinguish between shockable and nonshockable rhythms with exceptional accuracy. Mechanical error is always a possibility, but is impressively limited with these devices. Error is further restricted when the operator establishes that the patient is unresponsive, apneic, and pulseless, and then uses the device correctly.^4,5,6,10,12

Monophasic vs. biphasic — what’s the difference?

The two classes of defibrillation waveforms are: monophasic and biphasic. Monophasic waveforms use escalating high levels of energy, delivered in one direction through the patient’s heart. Biphasic waveforms are delivered in two phases as the current reverses direction and passes through the heart a second time.

Monophasic AEDs deliver an initial shock at 200 joules and then escalate the energy to 360 joules on subsequent shocks to increase effectiveness. The newer AEDs are programmed to continually shock at energy levels of approximately 150 to 300 joules. The biphasic waveform has become the standard waveform used in implantable defibrillators. AHA guidelines, however, have not made an official recommendation for the use of high or low energy or one specific type of waveform for defibrillation.^10,11,12

Operating the AED

AEDs equipped with adult electrode pads may now be attached to unresponsive, pulseless, apneic adults, or pediatric patients between one year of age and the onset of adolescence or puberty (about 12-14 years of age). This ‘health care provider’ guideline is indicated by the development of secondary sex characteristics (development of underarm hair in males and breast development in females). No recommendations have been established for or against the use of AEDs for infants less than 1 year of age. Pediatric electrode pads that reduce the delivered energy to approximately 50 joules should be used for children 1 to 8 years of age ‘lay rescuer guideline.’ These pads should not be used for patients 8 years of age or older as the energy delivered will be inadequate for these individuals.^4,10,13,14

If the patient is unresponsive with no respirations and pulse, then the nurse should activate the facility’s emergency response system and immediately perform CPR until an AED is available.^4,5,10 Presence of agonal respirations in an individual has often caused confusion. Community and lay rescuers have failed to attach an AED when this activity was present because they did not believe a cardiac arrest had occurred.¹⁵ Although agonal respirations rarely present a problem for current AEDs, older models may not be able to complete the analytical phase if the patient is gasping. If a patient presents with agonal respirations, but is unresponsive and pulseless, the AED should be applied and rhythm analysis attempted. If the device cannot complete the analytical phase or does not advise a shock, CPR should be continued for two minutes until the order is given by the machine to start the analyze-shock sequence again.^4,5,10

The AED is best positioned near the patient’s left ear, permitting the nurse to easily reach the controls, apply the electrodes, and direct CPR. The AED is turned on; the electrodes are applied to the patient’s chest during CPR and connected to the machine. The most frequently recommended pad placement is a modified Lead II position (right-upper sternal border below the clavicle and left-lower ribs over the apex of the heart). Although other positions may be used such as anterior/posterior placement, this lead configuration facilitates cardiac rhythm analysis and delivers the current along the heart’s normal conduction axis.

The pediatric pads are smaller and have a rheostat in the plug or cable that attenuates the delivered energy. They can be identified by a child’s face on the pads or in some cases, animal shaped plugs or other illustrations. These pads may be placed in the anterior/posterior or modified Lead II position depending on the child’s size.^4,10

The electrode pads must be applied firmly on the chest to ensure complete contact and should be placed at least one inch away from an implanted cardioverter defibrillator (ICD) or permanent pacemaker. Placing electrodes over these implants will reduce and interfere with the flow and amount of energy that is discharged through the heart and damage the batteries and capacitors in these devices. Transdermal medication patches under an electrode can also increase impedance and may ignite and burn. These patches should be removed and the skin wiped dry prior to applying the AED electrode.¹⁰

The AED may automatically begin the cardiac rhythm analysis or the nurse will be prompted to press the analysis button to start the process. CPR stops at this time and the patient must not be moved or touched during the machine’s analytical phase. All personnel must be told by the operator to stand clear of the patient. A fully automated AED, detecting a shockable rhythm, will issue a ‘stand clear’ order and immediately deliver the preset electric current. The semiautomated AED advises the nurse by a vocal, light, or printed screen prompt to press the ‘shock’ control to defibrillate the patient. In either case, the first monophasic or biphasic waveform defibrillation impulse of 150 to 300 joules is discharged through the electrode pads.¹⁰

The AHA defibrillation guideline has been modified, specifying delivery of one shock and immediate return to CPR beginning with chest compressions. No delay can be allowed to check for a rhythm or pulse. The rhythm analysis cycles for a potential three-shock sequence performed by commercially available AEDs delays the first post-shock chest compression by more than 37 seconds. If one shock does not convert VF, the effect of subsequent defibrillations has been found to be significantly low, and continuing CPR seems to have more benefit for the patient. Recurrent VF resulting from continued chest compressions does not appear to be an issue for concern. If a nonshockable rhythm is detected, CPR should again be immediately resumed beginning with chest compressions. CPR should be continued for five cycles (about two minutes) and the AED should be used to analyze the cardiac rhythm and deliver another shock if indicated. Reduction in the interval from compression to shock and from shock to compression by even a few seconds can increase the probability of a successful effort. CPR is continued until signs of circulation return. Then rescue breathing is provided as necessary. AED pads remain attached, and the machine continues to monitor the patient at frequent intervals, advising the nurse to check for signs of circulation or analyze the rhythm if VF/VT returns.^10,16,17,18

The majority of currently available AEDs will have to be reprogrammed or replaced to follow the new AHA guidelines. Rescuers using AED's that have not been updated to a single shock program should follow directions given by the machine in order to simplify performance skills and limit potential confusion. The AED should not be turned off, and there should be no interference with rhythm analysis.^4,19

Developing and maintaining competency

Although sudden cardiac arrest may not be an everyday event, it can occur unexpectedly in many nursing units. Nurses practicing in critical and noncritical care settings can learn to safely and competently use an AED. AED policies and procedures should be established by a facility before nurses and other health care personnel train and use the device. AED operation and maintenance, as well as training and continued competency issues, should be delineated within these policies and procedures.^4,10,15 As defibrillation may be delayed when patients develop SCA in unmonitored hospital settings, AEDs should be utilized with the goal of obtaining defibrillation within three minutes of collapse, particularly in locations where staff have limited rhythm recognition ability or defibrillators are not often operated.^4,10,20

Heartsaver AED training takes three to five hours and focuses on performance of one rescuer CPR the associated proper use of the AED. In comparison, operating a standard monitor-defibrillator requires about 12 hours of instruction and practice because cardiac rhythm recognition and interpretation must be learned in addition to congruent defibrillation skills. Developing and maintaining AED competency, which does not require knowledge of rhythm recognition, is more cost efficient from a time factor alone. Nurses can easily acquire and retain both CPR and AED skills.^4,15 Initial AED training is regarded by some learners as easier to master than the protocols for initiating basic CPR. Follow-up inservices and evaluation sessions are advised for maintaining and ensuring competence. These activities often include a review of treatment protocols, practice skills, and AHA guideline updates. Staging unannounced, simulated cardiac arrest scenarios serves as another method for evaluating continued competence.^4,10

Practice implications

AEDs have reduced a significant problem in the management of sudden cardiac arrest — delay in defibrillation — by making this essential therapy available to a patient in need. Clinical and experimental research continues to demonstrate that early commencement of CPR, followed by timely defibrillation, airway management and ventilation, and administration of intravenous medications increases survival.^4,5,9 Success of AEDs in early defibrillation has sparked the enactment of federal and state legislation promoting public access defibrillation (PAD) and ‘Good Samaritan’ protection for trained AED operators.²¹ Additional practice issues remain pertaining to recognition of an arrest that necessitates use of the device and continued quality monitoring by responders.

The Joint Commission on the Accreditation of Health care Organizations (JCAHO) continues to monitor system-wide response to resuscitation and consistently evaluates protocols, staff competency, equipment access and maintenance, outcomes, and data collection. The AHA maintains a National Registry of Cardiopulmonary Resuscitation that is developing a database for documenting resuscitation performance in hospitals and other facilities. This Registry can assist these organizations with systematic data collection pertaining to resuscitative efforts; and participation in the Registry facilitates an organization’s compliance with JCAHO standards. Many AEDs are capable of recording the cardiac rhythm and progress of an arrest using a tape or digital chip that can provide ongoing opportunities for case review and quality improvement.

Prolific placement of AEDs, now available for more than 20 years, makes the goal of early defibrillation a reality, significantly reduces the time to initiate defibrillation for cardiac arrest victims, and profoundly increases the prospects for survival.^4,10