ABSTRACTAutomatic external defibrillators (AEDs) may have advantages over manual defibrillation in managing prehospital cardiac arrest, particularly in rural communities. We conducted a two-part evaluation of a commercially available AED. We first established the diagnostic accuracy of the AED's rhythm recognition algorithm by challenging it with 205 cardiac arrest rhythms previously recorded from actual patients in the field. The AED demonstrated 100% specificity and 92% sensitivity for ventricular fibrillation (VF) in this nonclinical setting. We then compared the clinical efficacy of AEDs in 18 small communities (study group) with that of manual defibrillation in 18 additional communities (control group) of similar size. Ambulance technicians using manual defibrillators correctly diagnosed VF more frequently than the AEDs (98% vs 83%; p <.025). Specificity for VF was similar in the two groups (100% for AEDs vs 94% for technicians; p > .10). AEDs were able to deliver shocks more quickly than was possible with the manual defibrillators (1.56 vs 2.77 min; p <.001). The ability of the AEDs to terminate VF was excellent, converting VF in 28 of 29 (97%) patients to some other rhythm compared with only 37 of 53 (70%) patients in the control group (p < .01). Hospital admission and discharge rates were similar for the two groups. Ten of the 35 (29%) patients managed with AEDs achieved admission and six (17%) were ultimately discharged. In the control group 17 of 53 (33%) patients with VF were admitted and seven (13%) were discharged (p > .75). AEDs are an effective alternative to manual defibrillation in small communities.