This study is designed to examine the impact of an available technology within an automated external defibrillator (AEDs) to improve survival following out-of-hospital cardiac arrest for patients presenting in ventricular fibrillation.

The delivery of an electrical shock, termed defibrillation, has long been recognized as one of the critical "links" in the "Chain of Survival" following out-of-hospital cardiac arrest. This is particularly the case for patients who present in ventricular fibrillation (VF), a state of constant and yet uncoordinate firing of the lower portions of the heart (the ventricles), and the ability to treat these patients with defibrillation prior to their arrival in the hospital has remained one of the reasons why this group represents the patients who are most likely to survive an out-of-hospital cardiac arrest.

Though this technology has been successfully utilized in the prehospital setting for more than forty years, the long-held belief that "immediate defibrillation" was the optimal treatment for all patients has now come into question.

Following research done in locations such as Seattle, WA and Oslo, Norway, there came a recognition that some patients (particularly those who have been in cardiac arrest for 4-5 minutes prior to EMS arrival) may actually benefit from a period of CPR prior to defibrillation ("delayed defibrillation"). This has to do with the changes that take place within the heart and even at the level of the cells within the heart following the onset of VF. After several minutes of VF, the cells within the heart have been deprived and depleted of oxygen and other energy-containing molecules, and there has been a build-up of other substances such as acids and potassium. By providing CPR prior to defibrillation, it is thought that the patient's heart may be provided with enough oxygen and other energy-containing molecules, making it more likely that the heart will respond favorably to defibrillation.

Yet this is not necessarily true for all VF patients. Other data from patients whose collapse and cardiac arrest were witnessed and for whom defibrillation was able to be provided quite rapidly (i.e. those in airports, airplanes, and casinos) demonstrate a very high survival rate when compared to those patients who have been in arrest for a longer period. This suggests that there are patients who are best treated with immediate defibrillation and those who are treated with "delayed defibrillation."

The problems for modern emergency medical services (EMS) systems include determining just when the VF began, the impact of bystander CPR, the patient's overall condition at the time of the cardiac arrest, and the time interval from the 911 call until the arrival of the EMS providers (EMTs and paramedics) at the side of the patient.

By choosing to provide immediate defibrillation to all patients, in hopes of benefiting those who are most likely to respond to defibrillation and to survive, an EMS system would simultaneously be choosing to provide less than ideal treatment to those patients who are likely to benefit from "delayed defibrillation." Conversely, choosing to provide "delayed defibrillation" to all patients likely treats the larger percentage of VF patients in any EMS system appropriately, yet it potentially delays life-saving treatment from those who are most likely to survive (the patients who would benefit from immediate defibrillation).

Research involving the mathematic properties of the VF waveform (something that the human eye cannot calculate) have led to the development of computer algorithms that may predict, based on the calculated mathematical "score" of the VF, whether a patient is likely to respond more favorably to immediate defibrillation or delayed defibrillation. Such a technology could, therefore, seem to be able to recommend every patient to the treatment that is best for their individual condition, and it would follow that such individual treatment may improve survival from VF cardiac arrest overall.

This study is designed to examine the effect of just such a technology on VF patients presenting to EMS providers in New York, NY and London, England.

Inclusion Criteria:

• initial treatment includes application of a study AED
• complete initial waveform analysis
• presenting rhythm is ventricular fibrillation
• arrest of cardiac etiology

Exclusion Criteria:

• arrest of noncardiac etiology
• initial treatment with a non-study defibrillator
• missing AED data
• age <18 (London only)
• resuscitation terminated due to a DNR order / decision