Cardiovascular disease remains the leading cause of death in the United States, and although many of these cardiovascular deaths are the result of progressive disease, most are sudden cardiac deaths. Sudden cardiac death most commonly refers to unexpected death in a person without known preexisting heart disease, usually less than one hour after the onset of symptoms. In 1999, there were more than 700,000 cardiac deaths in the United States, 63 percent of which were classified as sudden cardiac deaths.1 Nearly one half of sudden cardiac deaths occur outside of the hospital.1,2
Several cardiac conduction disturbances have been identified as the final rhythm in persons with sudden cardiac death, including ventricular fibrillation and pulseless ventricular tachycardia (25 to 35 percent of sudden cardiac deaths), pulseless electrical activity (about 25 percent), and cardiac asystole (40 to 50 percent).3,4 Asystole, which is often the terminal event after several minutes of ventricular fibrillation, has a particularly poor prognosis because of the hypoxia and acidosis that result from the prolonged absence of perfusion.
Survival rates following cardiac arrest are generally low regardless of the setting. Among inpatients, survival to discharge following cardiac arrest is about 15 percent.5 Out-of-hospital survival following cardiac arrest is related to several factors, but the most important of these is the electrical mechanism of arrest. Less than 10 percent of patients with asystole or pulseless electrical activity as the initial rhythm during out-of-hospital resuscitation are admitted to a hospital and survive until discharge.6,7 The survival rate is much better when the initial rhythm is ventricular fibrillation; between 25 and 40 percent of these patients survive until hospital discharge.8,9
Uniform reporting guidelines in large metropolitan areas have helped identify other factors associated with improved survival rates after sudden cardiac arrest. The promptness of action by witnesses in initiating cardiopulmonary resuscitation (CPR) and in activating the emergency medical system (EMS) response for early defibrillation has a significant effect on survival. In Seattle, bystander-initiated CPR nearly doubled rates of survival to hospital discharge in patients with sudden cardiac arrest compared with waiting for EMS personnel to initiate CPR.10
In an effort to optimize community/lay-person responses to persons experiencing unexpected cardiac events, the American Heart Association (AHA) has promoted the four-step “Chain of Survival”: (1) early access to EMS, (2) early initiation of CPR, (3) early defibrillation, and (4) early advanced cardiac life support.11 For this system to be effective, bystanders must first recognize the problem, activate EMS, initiate effective CPR, and, if trained, use an automated external defibrillator (AED) until the EMS team arrives and begins advanced cardiac life support. The AHA guidelines emphasize early defibrillation for persons with ventricular fibrillation to increase survival rates after sudden cardiac arrest.11
An AED is a lightweight, battery-operated, portable device that analyzes the cardiac rhythm and delivers a nonsynchronized shock to the patient’s chest wall with a monophasic or biphasic waveform. AEDs were introduced in 1979, but their use was limited primarily to EMS personnel. AED use has increased dramatically with the development of compact units, self-adhesive electrode pads, voice/text prompts, and automatic internal memory recordings to allow data retrieval, and the devices commonly are found in public places. However, the ready availability of AEDs has not been without controversy. Because chest compression must stop while intrinsic electrical activity is being evaluated by the AED, as well as during the countershock delivery, victims of cardiac arrest may receive suboptimal CPR.12 In addition, the cost of supplying and maintaining AEDs may be prohibitive, particularly in places where the risk of sudden cardiac death is low (e.g., private homes).11 The AHA recommends that the devices be used only by persons who have been trained properly and certified by a designated program.11
The goal of public-access defibrillation programs is to decrease mortality rates associated with ventricular fibrillation–induced cardiac arrest. Several studies12–14 have found a survival benefit when AEDs are readily accessible in public areas. A recent prospective, randomized, controlled trial10 compared survival rates after CPR with and without AED use by laypersons in 235 persons with witnessed cardiac arrests; 23 percent of persons in the CPR-AED group survived to hospital discharge compared with 14 percent in the CPR-only group.
Survival rates after out-of-hospital sudden cardiac arrest remain disappointingly low. However, the increased public availability of AEDs offers hope for the future. The cardiac rhythm in persons experiencing sudden cardiac arrest likely will respond to appropriate use of an AED if a witness is present who is appropriately trained to deliver the shock.