Rapidly delivered defibrillatory countershock increases survival rates in persons with cardiac arrest. Automated external defibrillators (AEDs) allow outpatient treatment of ventricular fibrillation (VF) and ventricular tachycardia (VT) by trained personnel. The accuracy of AED in detecting unstable heart rhythms and its efficacy when used by nonmedical personnel must be assessed as AEDs become more widely available, and units are placed in public locations. MacDonald and associates studied the efficacy of AEDs in detecting and delivering countershocks in the field.
Accuracy in detecting and defibrillating VF and VT was assessed using a review of three years of experience with AED performance in firefighting, airport rescue, and EMS units of the Boston Emergency Medical Service. Errors were identified and divided into two categories: (1) machine-dependent or (2) operator-dependent.
Sensitivity and specificity for identification of VF or VT were 81.0 percent and 99.9 percent, respectively. Sensitivity was higher in the presence of coarse VF (greater than 200 mV in amplitude) than in fine VF (200 mV in amplitude or less). Of the 3,448 AED management episodes, 132 errors occurred; slightly more than one half were operator-dependent. Most errors were caused by patient movement during cardiopulmonary resuscitation and artificial ventilation during the AED analysis of the cardiac rhythm, or premature halting of the rhythm analysis. These errors could be prevented by (1) teaching operators to know when a cardiac rhythm is being analyzed by the AED, (2) developing a lockout mechanism that clarifies the amount of time needed for accurate analysis, and (3) providing an audible or visible prompt during rhythm analysis.
The authors conclude that AEDs have high specificity and fairly high sensitivity in detecting and appropriately shocking unstable cardiac rhythms. Their results show that errors connected with the use of AEDs are low. In an editorial in the same journal, White points out that their evidence suggests that further study is needed to establish correct timing for shock because evidence confirms that a period of cardiopulmonary resuscitation before shock may prepare the heart to respond more positively to shock. He supports the American Heart Association algorithm for AED cardiac rhythm management that allows rapid shock. He agrees that, with the currently available evidence, it remains appropriate to attempt to quickly end VF.