Relationship between battery capacity and the delivery of shocks in prehospital defibrillators.

STUDY OBJECTIVE: Automatic external defibrillators (AED) have enabled the medical act of defibrillation to be performed in the community by a number of non-physician providers. However, these portable, battery-powered units are costly to maintain and service. This study examines the life of AED batteries and provides a battery replacement protocol. DESIGN: Prospective diagnostic testing of 191 field batteries to determine their ability to deliver shocks at 360 joule. SETTING: Ottawa General Hospital Paramedic Program. OUTCOMES: Using a battery analyzer, battery capacity and the number of shocks delivered were determined for each battery (at room temperature and in a controlled, refrigerated setting). In addition, the reliability of the testing method was assessed using the interclass correlation coefficient (ICC). RESULTS: High reliability of blinded technical assessment of the batteries was achieved (ICC = 0.85). A strong correlation between the battery's capacity and the number of shocks it can deliver was obtained. For example, a battery with a measured capacity of 75% is capable of delivering more than 30 consecutive 360 joule shocks. This compares to a battery with a capacity of 20%, which is capable of delivering only 12 consecutive 360 joule shocks. CONCLUSIONS: While manufacturers' recommendations on battery replacement always have been based on an assumed technical threshold, these recommendations are not based on individual battery performance. The system for testing batteries described in this paper, should provide significant cost savings and improve quality assurance within a prehospital AED program.